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Knowledge based design advisory system for multi-material joining
Journal of Manufacturing Systems 52 (2019) 253–263
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Journal of Manufacturing Systems journal homepage: www.elsevier.com/locate/jmansys
Knowledge based design advisory system for multi-material joining a,⁎
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Ji Hoon Kim , Lyang Suan Wang , Kaushalya Putta , Payam Haghighi , Jami J. Shah , Pete Edwardsb a b
Digital Design Manufacturing Laboratory, The Ohio State University, Columbus, OH, USA Honda Engineering North America, Inc, Marysville, OH, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Design advisory system Multi material joining Database Knowledge based advisory system
Multi-Material Joining Design Explorer is discussed in this paper, which is a knowledge-based advisory system to help structural designers at the early design phase to select the potential joining methods. Data mining on various joining methods was conducted from any available sources, such as experts from academia and industry, handbooks, and vendors. Collected data was organized in a concept map which is an informal way of representing the data structure. The data were arranged into several categories according to their characteristics which include joinable materials, mechanical and design requirements, geometry, and so on. Common parameters and unique parameters were extracted from deep investigation of the gathered data to create a formalized data structure. A database using a general tree structure was then created to be fed into the advisory system. Searching algorithm using SQL query was implemented to navigate through the database to find the joining methods that match the requirements defined by the user. Two test cases were generated to validate the function of the knowledge-based system.
1. Introduction For the past several decades, the automotive industry has been following multiple parallel paths to achieve energy efficient and ecofriendly vehicles. This includes hybrid and electric drivetrains, new battery technologies and overall weight reduction of their products without sacrificing crash-worthiness [1–4]. Driven by the goal to reduce the overall weight of the vehicles, steel manufacturers developed many new steel grades that have a higher strength to weight ratios, such as HSLA and AHSS formulations that go by designations of DP, TRIP, MS, and PHS. It is now apparent that the possibilities of getting more out of steel have been exhausted. As a result, non-ferrous alloys, particularly aluminum and magnesium, and even composites are being considered for “light-weight” materials. This brings a new challenge: how to join structural components of different materials in a cost-effective manner while integrating it into a mass-production environment. Many new technologies are emerging for multi-material joining: they include thermal, adhesive, and mechanical methods, such as laser welding, flow driven screws, rivets, and high-speed impact nails, respectively. There are also hybrid methods for combining adhesives with mechanical and/or thermal elements to get synergetic structural integrity. As shown in Fig. 1, various joining methods are being used
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currently in cars that have multi-materials components. European luxury car manufacturers, BMW, Mercedes and Porsche have been leading the way in adopting these emerging technologies due to their low production volumes and higher product cost. BMW has reduced the weight of the i3 [5] by using Carbon Fiber Reinforced Polymers parts joined with adhesives, which have fewer components compared to metallic structures. Mercedes is using high-speed impact nails to join the different types of aluminum and steel grades in S-Class vehicles [11]. Ford is now using adhesive bonding in conjunction with selfpiercing rivets and flow-driven screws [12]. Studies on various joining methods are being actively conducted, but they are scattered around in academic literature, OEM proprietary, supplier information and more. There is a need to integrate these into one structure so that the knowledge on various joining methods are available to the designers in the early design stage. Performance and processing information of the joints that are relatively new is disparate; design and analysis methods and tools are not available to body structure designers. Multi-Material Joining (MMJ) Design Explorer was developed to consolidate this scattered information and to streamline them to the designers in the early designing phase. This paper discusses the components of the advisory system which are a comprehensive database of the joining methods, attributes composing the database library and the
Corresponding author. E-mail address: [email protected] (J.H. Kim).
https://doi.org/10.1016/j.jmsy.2019.03.003 Received 30 November 2018; Received in revised form 8 February 2019; Accepted 8 March 2019 Available online 22 April 2019 0278-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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ideal joint without causing too much damage to the material and find optimized pitch length to reduce the required number of the fasteners to overcome such challenges. There are several failure modes in fastener joints. Depending on the type of loading, failure modes can vary. Major failure modes are bending, fastener shear, net tension, and bearing. Pull-through and fastener failure can happen in the fasteners as well. Abe et al. have investigated joinability of self-piercing rivets (SPR) to mild steel and aluminum, high-strength steel, and aluminum substrates [17,18]. The authors varied the parameters such as substrate thicknesses and insertion speed to identify the failure modes using finite element analysis and experimental methods. Gotlib, Sundeep and Luscher have studied the shear performance of high-speed impact nails in aluminum joints with different substrate thickness [19,20]. They have successfully developed a model that replicates the strength performance and failure modes from their experimental results. They concluded that the ridged nails can be a good solution for multi-material joining under lap shear loading. Sønstabø et al. studied flow driven screws for joining aluminum substrates [21]. The authors derived force-displacement responses and the failure trend of the joint under various types of loadings. They have compared their results with self-piercing rivets and found that the overall performances to be very similar. Min et al. investigated the new blind riveting method to join dissimilar materials [22]. The difference between the new blind riveting method and the conventional blind riveting method is the elimination of the pre-hole process by self-penetration with a high rotational speed. Al6111 and Al6022 were used in this work, with the conclusions made by the authors that the tensile strength is 20% higher compared to the conventional blind rivets. Clinching is another common fastening method. Clinching deforms the materials by stamping with a special tool and forms an interlock between them as shown in Fig. 2d. The main difference from the methods described above is that there is no fastener inserted through the materials, so no additional weight is added to the joint, thus making it suitable for lightweight structures. But the method can only be used with ductile materials, since deformation of the materials during the joining process is large. Lambiase et al. have investigated the feasibility of joining various aluminum alloys to titanium alloys [23]. The authors have developed a numerical model to understand the formability of the joint and stress flow inside the joint. The model was verified with experiments, which led to a conclusion that AL7075 formed successful joints with titanium alloys. Zhang et al. have studied the failure mechanism of clinching on the aluminum sheets [24]. The authors identified various possible failure modes and investigated failure modes on the lower plate. Lambiase and Paoletti have studied the effect of rotating the stamping tool in the clinching process to reduce the high stamping force required [25]. The authors have found that by rotating the stamping tool, the stamping force was reduced to 1/10 of the conventional clinching method.
Fig. 1. Various joining methods used in recent cars [6–10].
working mechanism of the searching algorithm. In-depth insight has been provided on the decision of the attributes of the joining methods and how the database is structured with the attributes.
2. Survey on the state of the art 2.1. Mechanical fasteners Mechanical fasteners are the joining elements that fix the substrates through deformation of substrates and/or fasteners themselves. Shown in Fig. 2, self-piercing rivets (SPR), flow drill screws (FDS), and impact nails are examples of fasteners currently being used in sheet metal joining. Joining processes of the mechanical fasteners that involve tightening or deforming the substrate, create a pre-stress field. Joining materials are then joined by the pre-load generated by the field. Bolted joints create pre-tension by the tightening torque of the bolt and nut. SPRs flare as they are inserted and engage with the materials and create residual stress through the deformed geometry of the rivet. High-speed impact nails “open-up” the joining material during insertion, and as the material closes back in, it is engaged at the ridges of the nail. One of the advantages of joining processes of these kinds does not accompany thermal or chemical manipulation of the joining materials, allowing the joining of dissimilar materials without causing incompatibility issue between the different material characteristics with high shear and tensile strength. However, there are limitations and challenges, such as the possibility of excessive material damage during the joining process and weight addition due to the fasteners themselves. Numerous studies have been done to define the process parameters that would form an
2.2. Adhesive joints Adhesives can be categorized into two main groups: structural or sealing. Structural adhesives are used to join the materials with high strength whereas sealing adhesives are used to lock the thread and prevent moisture penetration. Depending on the component, adhesives can also be categorized into 1-part and 2-part. 1-part adhesives usually have triggered curing mechanisms, such as elevated temperature or UV light. 2-part adhesives have an initiator, so they are cured at room temperature once the components are applied. Once the adhesive is cured, joining materials are bonded with the adhesive layer through Van der Waals bonding. Van der Waals bonding is a secondary bond due to the attractions between atoms and/or molecules. Secondary bonds are weaker than primary bonds, such as covalent and metallic, which share the electrons, but this has a considerable role in various fields including the adhesive bonding.
Fig. 2. Examples of mechanical fasteners, (a) SPR: Rivet with hollow center, two-sided access with interlock formulation through flare of the rivet; (b) FDS: One-sided access, joint set with deformation due to friction generated while rotation of the screw; (c) Impact nails: One-sided access, with ridges on the nail shank, inserted by specific gun with high speed; (d) Clinching: Two-sided access, with stamping tool stamping to form an interlock without any fasteners inserted [13–16]. 254
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methods is that the individual joining methods act as reinforcements to each other, to form a more efficient joint. Spot welds and mechanical fasteners are point elements, so the load being transferred through the joint tends to be high and concentrated. On the other hand, adhesives are applied throughout the joining interface, so the load flow is uniform, but is vulnerable to peeling. By combining these methods, positive synergetic performance can be expected from them. Several studies have been carried out on the load transfer and the performance of bonded-bolted single-lap joints. Kelly used both finite element analysis (FEA) data and experimental data to investigate the load transfer of the joint for different joint design parameters [31]. Furthermore, the comparison between pure adhesive joint and hybrid joint was made on the possibility of improvements on the structural performance. Moroni et al. performed design of experimental analysis on weld-bonded and clinch-bonded joints [32]. The authors have performed DOE studies on conventional joints and hybrid joints to compare the performance with different environment conditions. It was found out by the authors that hybrid joints, weld-bonded and clinchbonded joints show an overall higher strength and energy absorption, and less variance to the environmental or geometrical change. Numerous studies have been performed in rivet-bonded joining methods for the sheet metal joining purpose. Solmaz and Topkaya compared the failure modes of the riveted, adhesive bonded, and rivet bonded double lap joints for composite materials [33]. The authors investigated the failure modes with overlap length, and how the failure of respective joints initiate. Franco et al. performed parametric studies on the effect of pitch on SPR-bonded joints to join aluminum and carbon fiber [34]. Fatigue life of different rivet pitch for SPR-bonded joints have been measured, and 60 mm pitch showed the highest fatigue compared to 30 mm and 45 mm pitch joints. By comparing the hybrid joints to their individual counterpart, studies have shown that hybrid joints performed better. Friction element welding (FEW), resistance element welding (REW), and friction self-piercing riveting (F-SPR) are combinations of thermal and mechanical joining methods. Common characteristics of FEW and REW are that there is a steel rivet/element that is inserted, and it is welded to the lower material. This avoids the material incompatibility of welding process because non-ferrous metals are placed on the upper layer which are penetrated through friction or pre-hole, and steel products are placed on the lower side to be welded with the insertion element. These methods do not require any pre-hole or forming required on the steel, thus making the method highly suitable for joining the high strength materials with tensile strength up to 1800 MPa [35]. However, heat affected zone is generated at the welded region that could change the material property of the materials that can affect the overall strength of the joint. Absar et al. have performed a parametric study to form a successful joint using FEW, and the temperature response in different joining stage using thermocouples have been investigated [36]. The authors found that the applied force and generated torque have the largest effect on the resultant FEW joints and have successfully profiled the temperature of the joint during the joining process. On the other hand, friction self-piercing rivet penetrates through all the materials. Li et al. have investigated the joinability of Al 6061-T6 and AZ31B [37]. The authors claimed that the riveting process is improved through rotating the rivet, and the strength of the joint formed by the F-SPR was higher than conventional SPR joints.
Fig. 3. Possible failure modes of multi-material adhesive joints [26].
Adhesive bonding is a straight-forward process. It is applied on the surface of the material desired to be bonded, and a joint will be formed after the adhesive has been cured. There is no mechanical/thermal interaction between the adhesive and the joining material, so there are few limitations on the type of the materials the adhesives can join. It is also possible to join different materials, because the adhesive layer prevents the galvanic corrosion and other issues coming from the material incompatibility. However, surface preparation of the materials is essential to obtain high quality bonding and curing process such as, thermal and UV light can be very time consuming as well. Adhesives are widely used despite these limitations in the industry due to their merits such as fast cure types and adhesives that can withstand some amount of dust and grease. There are three failure modes shown in Fig. 3 and mix of these modes are in adhesive joints. Cohesive failure is the fracture of the adhesive layer itself. This occurs mainly when the applied load exceeds the strength of the adhesive and indicates that the joint was a wellformed one. Adhesion failure is a separation between the adhesive and the joining material. The main cause of this failure is the existence of impurities such as dust and oil on the joining material. Inappropriate or insufficient surface preparation leaves debris on the material surface, so adequate surface treatment is necessary. Adherend failure, or plate failure is the failure of the joining material. This occurs when the load exceeds the material strength. Banea and Silva surveyed the adhesive bonding in composite materials [27]. Thorough review of the effect of various factors such as surface preparation and joint configuration has been done, along with numerical methods to simulate the behavior of the adhesively bonded joint. Silva et al. investigated the shear strength of different types and thickness of adhesive joints [28]. The authors used adhesives with three different characteristics, which are ductile, brittle, and intermediate. The authors claimed that the shear strength of adhesive joints increases as the thickness of the adhesive layer decreases for all three types of adhesives. Zhu el al. investigated the effect of the distortion caused during curing process on the strength of multi-material adhesive joints [29,30]. Thermal expansion coefficients of different materials are not equal: the elongation the materials during the curing process under elevated temperature is different one another. Thus, the residual stress is developed in the cooling stage, leading to distortion of the joint or debonding and fracture of the adhesive layer. The authors have investigated the influence of this phenomena on the strength of adhesive joints and have found that the shear strength and energy absorption are strongly affected while peeling force was relatively less influenced. Additionally, the authors have performed a parametric study to understand the parameters that cause the distortion and fracture of adhesive joints. They found that the peak curing temperature and plate thickness were the most influential, while the material properties of the adhesive and plates, and constraint types were less influential ones.
2.4. Joining methods in finite element analysis packages Commercial FEA software packages such as ANSYS, LS-DYNA, and ABAQUS have various ways of modeling process and performance of joining methods [38,39]. There are simulation packages that are dedicated to simulate material forming, welding and mechanical joining as well, such as Simufact, SORPAS, and DEFORM [40–42]. Generally, modeling the geometry of joints is a very complex task. For thermal joining, modeling the heat affected zone and the exact shape of
2.3. Hybrid joints Hybrid joining methods are combinations of two or more joining processes. Weld-bonded, rivet-bonded, and mixed mode adhesives are the generic examples of hybrid joints. The advantage of hybrid joining 255
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structure, and consumer appliances.
the weld nugget is very difficult. Also, the geometry of mechanical fasteners is very complex due to the existence of the threads and material engagement during the joining process. Furthermore, describing the exact material properties of adhesive layers is highly challenging as well. Residual stress formed after the joining process is another difficulty in joint modeling. To overcome these extensive challenges, various contact formulations and element formulations have been developed to simulate the behavior of the joints. Mechanical fasteners with threads have complex geometry and require very small size elements to model the actual geometry. ANSYS has the capability of creating virtual threads using contact formulations. Fastener shank which is flat without threads are used as the geometry, and contact elements re-align on the shank face according to the given dimensions of the thread [43]. Joining methods that have small size joining elements such as spot welds and SPRs are expressed as point connectors [38,39]. Single beam element or solid element is generally used with corresponding material properties such as normal, shear, and torsional load associated with failure criteria. Adhesive layers are substituted with cohesive elements that is based on the fracture mechanics. These alternate methods are advantageous on reducing the size of the model by avoiding the use of very small elements, thus applicable to large scale models. However, these methods require material properties that can only be acquired through specific experiments.
4. Problem formulation The main objective of the Design Explorer is defined as “Deliver the knowledge of the joining methods to the structural designers”. The advisory system is developed to establish the bridge to connect the gap between the structural designers and the sparse knowledge of the joining methods. New and exotic materials are being used to achieve lightweight and high strength of the structure, new joining methods other than spot welds and other conventional joining methods should be considered. Under the objective, there are functions that would operate as a team to fulfil the main objective. Database, user interface, and searching algorithm are the example functions in the advisory system. Each function has a unique task and operation phase. Databases store comprehensive data on various materials and joining methods. Searching algorithm is a way of finding appropriate joining methods that correspond to the requirements the user has specified. The user interface is an interactive platform where the user would set the criterion for various factors that would act as constraints in the advisory system. Constraints on the advisory system are any type of requirements of the joint that the designers have in selecting the appropriate joining methods. Joint strength, location of the joint, environment, and economics are good examples of the constraints. There are requirements on joint strength in structures to maintain the structural integrity under certain loadings, either those are shear strength, tensile strength, or both. Location of the joint is also an important constraint because depending on how important the location is in the structure would lead to selecting the different joining methods. For example, in automotive manufacturing laser welding is preferred in joining the A-surface, because A-surface is exposed directly to consumers, and the laser weld does not leave any visible trace. On the other hand, internal structures such as B-pillars and shock towers require high strength, so various mechanical fasteners are used to achieve high joint strength. Also, external environment determines what joining methods to use as well. For locations in parts where they encounter water frequently, joining methods that are resistant to corrosion and have good water sealing capability are preferred. Furthermore, some joining methods require two-sided access while others require one-sided access. Depending on the geometry of the location, two-sided access joining methods may not be possible to use. Economics is also an influential factor in joint selection. The cost of joining method and time taken to set the joint are very important considerations for potential users of a mass-manufacturing environment, such as automotive.
2.5. Software tools for selecting joining methods Storing and providing the experimental and simulation data describing the behavior and the characteristics of the joining methods into the designing phase is a very challenging task. Several researchers have discussed on how to represent those data for design purposes. LeBacq et al. proposed a methodology for the selection of joining methods as software interface [44]. By initial elimination of an infeasible process and use of a fuzzy logic algorithm to multi-criteria evaluation, the authors have developed a software that recommends a list of joining methods. Esawi and Ashby considered geometry, loads, material properties, and material combinations as the factors for selecting the joining process. The authors composed the database using linked-list: the matrices containing different types of information are linked to one another to how they are related. The software could provide scores of different joining methods based on cost and production rate [45]. Swift considered performance requirements, design aspects, and quality of various joining processes that can be referred to as guidelines in design tools [46]. However, the software lacks the type of joining methods that are recently developed, such as friction element welding, flow driven screws, and hybrid joints which are the combinations of mechanical, thermal, and adhesives. Thus, the software developed by the authors is not suitable for assisting in decision making on joints for multi-materials. 3. Scope of the research
5. Overview of the MMJ Design Explorer
MMJ Design Explorer, discussed in this paper, is a knowledge-based advisory system to help structural designers at the early design phase to select the potential joining methods. Thus, this can be formulated as a design problem to utilize and integrate the sparsely spread knowledge on the joining methods from and establish a bridge to the designers under well-formulated objectives and constraints. The scope of the research summarized below:
Multi-material joining (MMJ) Design Explorer is a comprehensive advisory system that provides information on various joining methods. The main objective of this advisory system is to aid the automotive structural designers to select appropriate joining methods for multimaterial joining. The focus of this paper is on a preliminary design where This paper discusses the development of the database on joining methods searching algorithm to go through those data to find adequate candidates. The user interface of the advisory system is also dealt here. An in-depth discussion on the component of the advisory system, a comprehensive database of the joining methods, attributes composing the database library and the working mechanism of the searching algorithm are presented. The rationale in selecting the attributes to be included in the software and how the database structured with these attributes are also discussed here.
• Joining methods considered are those used to join thin-walled components such as sheet metal or composite panels • The loads applied on the joints are tensile and/or shear loads. • Joints may need to join multi-materials such as steel, non-ferrous •
alloys such as aluminum and magnesium, and fiber reinforced polymers. Targeted industry fields are automotive body frames, aerospace 256
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6. Data collection using concept map Once these categorizations have been established, data was collected from various sources. The sources include: joining experts in OEM, academia, joint and material vendors, and handbooks dealing with diverse joining methods. In order to understand the semantic relationship, the data was represented as a concept map, rather than a formal E-R diagram [47]. Hence, the collected data were first arranged in Concept map by hand. A concept map is a graphical description of the information or data that are inter-related within a specific knowledge of interest. A concept map is composed of boxes or circles and labeled arrows, which indicate the data and information and the connection between them. Generally, one-way branching or downward hierarchical structure is utilized in creating concept maps. A concept map allows more informal annotations than a formal computer data structure. By doing so, it is easy to recognize what kind of relationship the boxes have and figure out the hierarchy of the database in a glance. Concept map goes well with the general tree structure which is the database structure chosen for the MMJ Design Explorer. Five concept maps were created: materials, thermal joining methods, mechanical fasteners, adhesives, and hybrid joints. As depicted in Fig. 4, the boxes represent the information of the various joining methods and material types. Additionally, texts inside the arrows indicate the topic of the lower level nodes connected from the upper. Components of data under each individual joining methods have been consistent using the parameters defined to be transferred easily to the actual database later.
Fig. 5. High level classification of joining methods.
in a manufacturing environment. There are various factors such as tool geometry, production rate, and cost on the respective process for different joining methods that describe the joining procedures. Whether it is thermal joining, mechanical joining, and adhesive bonding, there are specific and unique process parameters that have to be defined in order to obtain a successful joint. Process parameters are further split into two categories: unique and common parameters. Unique parameters are directly related to the tools and operation sequence. Common parameters are the ones that can be used to compare with other joining methods. The main purpose of searching for parameters for the database is to find common parameters that can provide a comparison between two or more different joining methods. Whereas, non-overlapping parameters will be displayed to the designers instead of being implemented in the software as one of the filters.
7. Attribute selection In order to represent knowledge about different methods in a uniform way, we need to identify key attributes. As there are many joining methods that are capable of joining metals, parameters describing the attributes of the joining methods also vary. However, the number of parameters describing the attributes should be reasonable and at the same time contains all the sufficient information to determine potential viability. Through careful investigation of different joining methods, these parameters were consolidated into two categories: process and performance attributes.
7.1.1. Unique attributes Process parameters of the joining method are different from other joining methods. Examples of such process parameters for resistance spot welds are electrode force, clamping force, electrical current, current time, etc. For flow driven screws, the process parameters are rotation speed, hold time, and insertion force. Process parameters of adhesive bonding are work time, cure time and temperature, single or two
7.1. Process attributes Shown in Fig. 5, attributes of the joining methods were initially split into two different groups that are: process and performance. The process group is made up of attributes related to the installation of a joint
Fig. 4. Concept map generated for various joining methods and material types. 257
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Fig. 6. Structure of the database of MMJ Design Explorer.
the joint has been formed, so they are all applicable to all the joining methods. Parametrizing the strength of the joints can be done in two ways: using the absolute joint strength or the joint efficiency. Absolute joint strength is an actual value of the strength of the joint when installed on the joining materials expressed in Pascal and Newton units. Using the absolute value provides direct information on the joint strength on different materials and whether or not that particular joining method is suitable for that application. However, the pitfall of making a decision based solely on the joint strength is the overlooking of joint efficiency. Joint efficiency is a percentage ratio of the strength of the joint to the strength of the base material and by just looking at joint strength, the joint efficiency could not be deduced. This could lead to unnecessary high material cost for low joint efficiency. Although the exact value of the strength of the joint is not shown in this format, the ratio is sufficient enough to be one of the deciding factors on whether or not the joining method is suitable for the selected base materials. Also, it is obvious that the strength of the joint may differ significantly with different base materials. By organizing the strength parameter with efficiency which is the percentage ratio, the designer can evaluate how well the selected joining methods and base materials can perform. This will help designers to decide on the most optimum joining method in terms of cost saving.
part, and so on. These parameters are unique in each method and cannot be applied to the other methods because the procedure of the joining is totally different. Not only these parameters vary with different joining methods, but the values also vary significantly with different joining material combinations. Same rotational speed used to join low carbon steel with flow driven screw is less likely to be successful in joining high strength steel. Thus, it is very challenging to collect and contain all the information in the database structure. Such parameters should not be treated as the dynamic variables in the database. Instead, they are stored in the database as text information without any numerical values. This information is displayed at the very final stage with a list of all the joining methods satisfying all the filters set by the designer. 7.1.2. Common parameters Although intuitively, parameters of different joining methods are different from each other, there are some process parameters that share common characteristics. Material combinations, geometry, cycle time, accessibility, and robustness are some examples of overlapping aspects in different joining methods. Material combinations are the list of the materials that a joining method is capable of joining. This is determined by metallurgy and material properties such as brittleness, electrical conductivity, and melting temperature, some materials cannot withstand the mechanical and/or thermal load applied during the joining process. Cycle time is the required time to install a joint, accessibility is whether the joining methods require single or two-sided access, and robustness is the measure of ability to produce joints under manufacturing variances. These type of process parameters are important in making decisions for the design and the manufacturing because these parameters are directly related to the possibility of applying the joining process into the manufacturing line. If the line is for mass-production, high robustness with fast cycle time, as well as, cost are the important factors to consider because production rate is high, and each joint should be successfully installed on the body in a very short time.
8. Structuring the database Since the MMJ Design Explorer is strictly based on the database on various joining methods, constructing a well-organized and robust database is the most important task on the successful development of this advisory system. Sorting out the collected parameters into detailed categories is a must in order to accomplish the task. The structure of the database is shown in Fig. 6. As can be seen from the structure, there is little relationship between different joining methods. Therefore, constructing a graph-style and linked-list database structure is deemed unnecessary. Instead, the structure should have different levels, distinguishing parent and children node to contain a different type of information in each level. Additionally, nodes should have the capabilities to be added and deleted easily without having to manipulate the entire connections within the database to accommodate new emerging joining technologies and further developments in current joining methods. Therefore, a general tree database structure was selected. The root nodes of the database consist of the major type of joining
7.2. Performance parameters Performance parameter group is composed of attributes that determine the functional efficiency of the joining methods. The group is further split into two subcategories which are mechanical and design aspects. Mechanical aspects include normal and shear strength, fatigue, corrosion. Examples of design aspects are the aesthetics, available space, and the location of the joint. These attributes are measured after 258
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methods: thermal, mechanical, adhesive, or hybrid. Under the type of joining methods, various joining methods are listed under the corresponding group as children nodes. Then, the parameters of the various joining methods are split into more detailed categories compared to such as joinable materials, mechanical / design requirements, geometry, and so on as shown in Fig. 6. Joinable materials describe list of materials that can be joined using that particular joining method. This could be a single material or multi-material combination. Mechanical requirements are the parameters that were initially in the performance parameter group. All the strength-wise parameters are included in this category. Every joining method requires a specific configuration of the base materials. For example, self-piercing rivets (SPRs) can only be joined via lap-joint, while adhesives can be used to bond any configurations. Geometry category deals with the possible configurations that the joining methods can handle such as lap joint, butt joint, scarf joint, etc. Design requirements are the factors that are related to design and workability, which are robustness, available real estate, cost, and so on. Process category includes attributes that are associated with joining process of each method. Characteristics consist of all other information that was included in the aforementioned factors. General description of the joining methods such as advantages and limitations are stored as text, and other minor attributes that could not fit into the major ones are included in this group as well. 8.1. Transforming the concept map into ACCESS database Concept maps generated during the collection process cannot be used directly in the advisory system, the data in the map is transferred to the database structure in the computer. There are several database platforms, such as MySQL, Oracle, and Microsoft Access. Among those choices, Microsoft Access has been selected as the initial database management system due to its simplicity of facilitating the environment and accessibility. The data structure translated into the computerformat database is in a way of matrix form with each row and column having different meanings. In this case, rows represent the joining methods within the category of joints and columns indicate characteristics and attributes. All the concept maps are transformed into this format, so that it is possible to develop a code that would implement a searching algorithm.
Fig. 7. Work flow of the MMJ Design Explorer; (a) Phase 1: material filtering; (b) Phase 2: attribute filtering process.
database as initial results. When the attributes are defined in the second phase, the joining methods satisfying the filter requirement are called the final results. This filtering process is done through the SQL query command, which would navigate through the designated rows or columns of the database and call all the data that matches or exceeds the requirement established by the user. This method visits the entire nodes in the database one by one, so no information is lost or missed during the searching process. While there is a concern that as the size of the database gets larger, the time required in searching through the database may grow proportionally. However, the increase in the number of joining methods would not be a big impact in time increase of searching because it is only adding some amount of the innermost loop in phase 2. As long as new attributes are not being added in a large number into the database which would create numerous outer loops in phase 2 shown in Fig. 7b, the exhaustive searching algorithm is the most suitable method for searching this database structure.
9. Work flow and searching algorithm This section discusses the overall work flow of the Design Explorer first and then followed by the searching algorithm. This is because the search algorithm is determined from the work flow of the Design Explorer. Potential users of the MMJ Design Explorer are the structural designers in industry. Base materials are typically known prior to making decisions on joining methods. Thus, the very first question asked by the system is the material combinations of the joining parts as plotted as phase 1 in Fig. 7a. The initial sort-out process is performed based on the selected materials the designers have specified. Completing this process allows the designers to input the additional requirements of the joining methods. Additional requirements are the attributes that were discussed previously and is shown in Fig. 7b. Once the second filtering is done, final results are displayed for the designers to browse through. As can be seen from how the two work flows operate, searching through the entire members in the database is required for each attribute that the designers have selected as filtering parameters. In other words, multiple loops are required within the searching algorithm to perform repetitive filtering process. Additionally, attributes do not have any relationship with each other. Therefore, each attribute must be investigated one by one because they cannot be generalized to be expressed by equations in the form of describing relationships with other attributes. Shown in Fig. 7, as the user defines the material combination, joining methods corresponding to the criteria are called from the
10. User interface MMJ Design Explorer has vast amount of data on various joining methods for thin walled structures. However, structural designers would appreciate the automated searching scheme and user-friendly interface to make use of the database. Software interface has been created to undertake the task of searching through the database and provide filtered results to the designers. As the work flow is composed of two phases, the software interface has two parts as well. It is discussed in previous section that the designers know the single or multi materials that need to be joined together, so the software starts with the selection of the joining materials, as shown in Fig. 8a. The designer can choose specific type of the materials that are under steel, aluminum, and carbon fiber polymers. Once materials are specified, initial filtering results are displayed at the right side of the interface, under individual categories: thermal, mechanical, adhesive, and hybrid. The designer can selectively choose the joining methods they have in favor, or just 259
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Fig. 8. Software interface of (a) Phase 1; (b) Phase 2.
with the requirements. The quality of the result is purely determined by the inputs the designers provide to the system. It is clear that the more attribute qualifications the designers define, the narrower the results will be given. Fig. 8b is the actual interface of phase 2. As designers select and define the attributes in the left side of the interface, corresponding results are published in the center and right side. There is a very useful feature in this phase to help designers make a better decision on selecting joining methods. That is, the results of the final joining methods can be ranked up to three ranking criteria which were initially selected by the designers. This allows the designers to compare the performance of the possible joints with respect to the attributes that are
select all of them and proceed to the next phase. After the initial filtering with joinable materials in phase 1, detailed attributes can be selected and defined according to the requirements and specifications that the joining section needs to obtain. Phase 2 is responsible for receiving the designer’s selection of attributes and requirements and further filter the initially filtered joining methods in consonance with such requirements. Different filtering scheme applies to different type of attributes. Attributes such as strength, cycle time, and cost are treated satisfactory if the values exceed requirements defined by the designer. Other attributes, such as tool accessibility and form of the adhesive are qualified only when the values match exactly 260
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more important to the designer. 11. Test case To demonstrate the advisory system, two test cases are presented here. The case studies have been created using AHSS – aluminum and AHSS- CFR combinations and locations with different emphasis. It is very challenging to join such combinations using conventional thermal joining methods, so the system is expected to provide alternate joining methods that are capable of joining dissimilar materials. Describing the strengths and limitations of the current advisory system, and what factors of improvements can be made to further advance the system. 11.1. AHSS-Aluminum combination High strength steel (DP 780) and aluminum (6000 series) combination is often used in hoods, front and rear panels of automotive to obtain the light-weight structure. Aluminum and steel have very different material properties, so joining methods that accompany material fusion would be difficult to join these materials. Thus, non-fusion methods such as mechanical fasteners, adhesives, and hybrid joints should be showing up on the list. Table 1 shows the example requirements and constraints that the designer has in joining these materials. Based on these requirements, first the materials and configurations are specified in phase 1. The results are shown in Fig. 9a, where several joining methods for each category are listed. For the demonstration purpose, all the methods are selected and phase 2 was initiated. After selecting the desired requirements in the interface, final results are displayed shown in Fig. 9b. It can be seen that hybrid joints are placed in higher ranks compared to other joining methods in shear strength. On the other hand, joining methods that does not accompany deformation of the materials such as adhesives and friction spot welding methods are high ranked in aesthetics part, although the aesthetics is not a big issue in this case. To compare with extreme cases, let the location of the joint be the roof of the automotive, which is an A-surface where the consumer sees the parts. In this case, laser beam welding and refill friction spot welding, and adhesives are the only case for joining multi materials with very good aesthetics as shown in Fig. 9c. 11.2. AHSS-CFR combination Carbon-fiber reinforce polymers generally are very brittle, so additional caution must be taken in selecting the joints. Obviously, joining methods that deform the materials too much would not be suitable for this case. Given all other conditions are same except the material combinations, the design requirements are listed in Table 2. As the results shown in Fig. 9d, fasteners that may cause excessive deformation are excluded compared to the previous combination. That is, SPRs, clinching, impact nails, and hybrid joints that are combined with those methods are eliminated from the previous result. Other methods that are still listed are the adhesives, laser brazing, and mechanical fasteners that generate heat due to friction to penetrate the composite and join with AHSS using conventional methods. For Table 1 Example design requirements and constraints of the automotive design I. Material
DP 780 and AL 6000 series
Configuration of the joint Thickness of the plates Service temperature Location of the joint
Lap joint 2–3 mm 0–40 °C Internal structure of the automotive – not visible to consumers Normal weather with moderate rainy days Moderate
Environment Strength requirement
Fig. 9. Results of (a) Phase 1; (b) Phase 2; (c) extreme case of case study I; and (d) Phase 1; (e) Phase 2; (f) extreme case of case study II.
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Dr. Luscher and Dr. Benatar at The Ohio State University, and Pete Edwards, Honda Engineering North America.
Table 2 Example design requirements and constraints of the automotive design II. Material
DP 780 and CFR Epoxy
Configuration of the joint Thickness of the plates Service temperature Location of the joint
Lap joint 2–3 mm 0–40 °C Internal structure of the automotive – not visible to consumers Normal weather with moderate rainy days Moderate
References
Environment Strength requirement
[1] Rizzoni G, Guzzella L, Baumann BM. Unified modeling of hybrid electric vehicle drivetrains. IEEE ASME Trans Mechatron 1999;4(3):246–57. [2] Meissner E, Richter G. The challenge to the automotive battery industry: the battery has to become an increasingly integrated component within the vehicle electric power system. J Power Sources 2005;144(2):438–60. [3] Casadei A, Broda R, Ricardo Inc. Impact of vehicle weight reduction on fuel economy for various vehicle architectures. 2008. p. 1–60. [4] Cheah LW. Cars on a diet: the material and energy impacts of passenger vehicle weight reduction in the U.S. Massachusetts institute of technology. Massachusetts Institute of Technology; 2010. [5] E. A. BMW i3 review: the best electric car this side of a Tesla – and hale the price. The Telegraph. 2016. [6] Self piercing rivets. National Rivet; 2019. [7] Sprovieri John. Resistance spot riveting. 2019. [8] Laser welding. 2019. [9] Flow drilling screw technology. 2019. [10] A. S. Inc. A look into epoxy and other types of adhesives used in the automotive industry. 2019. [11] Michalak F, Hoefer M, Ebner M. Body in white of the new S-Class, EuroCarBody. Automotive Circle International; 2013. [12] Sprovieri J. Flow-drilling screws help carmakers shed weight 2016-02-01 Assembly magazine. 2016. [13] L. WUHAN IRIVET MACHINERY CO. Self-piercing riveting machine. 2019. [14] Pellettieri J. High-Strength lightweight joints using ‘flow drill screw’ technology. 2019. [15] Bollhoff. RIVTAC® technology. 2019. [16] Varis J. Ensuring the integrity in clinching process. J Mater Process Technol 2006;174(1–3):277–85. [17] Abe Y, Kato T, Mori K. Joinability of aluminium alloy and mild steel sheets by self piercing rivet. J. Mater. Process. Technol. 2006;177(1–3):417–21. [18] Abe Y, Kato T, Mori K. Self-piercing riveting of high tensile strength steel and aluminium alloy sheets using conventional rivet and die. J Mater Process Technol 2009;209(8):3914–22. [19] Gotlib I. An analysis of high-speed impact nailing for lightweight automotive structures. The Ohio State University; 2014. [20] Siripurapu SK, Luscher AF. Modeling shear performance of high-speed ridged nail in aluminum joints. International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. 2017. [21] Sonstabo JK, Holmstrom PH, Morin D, Langseth M. Macroscopic strength and failure properties of flow-drill screw connections. J Mater Process Technol 2015;222:1–12. [22] Min J, Li Y, Carlson BE, Hu SJ, Li J, Lin J. A new single-sided blind riveting method for joining dissimilar materials. CIRP Ann Manuf Technol 2015;64(1):13–6. [23] Lambiase F, Ilio ADi. Joining Aluminum with Titanium alloy sheets by mechanical clinching. J Manuf Process 2018;35:457–65. [24] Zhang Y, He X, Wang Y, Lu Y, Gu F, Ball A. Study on failure mechanism of mechanical clinching in aluminium sheet materials. Int. J. Adv. Manuf. Technol. 2018;96(9–12):3057–68. [25] Lambiase F, Paoletti A. Friction-assisted clinching of Aluminum and CFRP sheets. J Manuf Process 2018;31:812–22. [26] Bischof P, Suter R, Chatzi E, Lestuzzi P. On the use of CFRP sheets for the seismic retrofitting of masonry walls and the influence of mechanical anchorage. Polymers (Basel) 2014;6(7):1972–98. [27] da Silva LFM, Banea MD. Adhesively bonded joints in composite materials: an overview. Proc Inst Mech Eng Part L J Mater Des Appl 2009;223(1):1–18. [28] Augusto M, et al. Effect of adhesive type and thickness on the lap shear strength article in the journal of adhesion · November. J Adhes 2006;82(11):1091–115. [29] Zhu X, Li Y, Chen G, Wang P-C. Curing-induced distortion mechanism in adhesive bonding of aluminum AA6061-T6 and steels. J Manuf Sci Eng 2013;135(5):051007. [30] Zhu X, Li Y, Ni J, Lai X. Curing-Induced debonding and its influence on strength of adhesively bonded joints of dissimilar materials. J Manuf Sci Eng 2016;138(6):061005. [31] Kelly G. Load transfer in hybrid (bonded/bolted) composite single-lap joints. Compos Struct 2005;69(1):35–43. [32] Moroni F, Pirondi A, Kleiner F. Experimental analysis and comparison of the strength of simple and hybrid structural joints. Int J Adhes Adhes 2010;30(5):367–79. [33] Solmaz MY, Topkaya T. Progressive failure analysis in adhesively, riveted, and hybrid bonded double-lap joints. J Adhes 2013;89(11):822–36. [34] Di Franco G, Fratini L, Pasta A. Influence of the distance between rivets in selfpiercing riveting bonded joints made of carbon fiber panels and AA2024 blanks. Mater Des 2012;35:342–9. [35] Meschut G, Janzen V, Matzke M, Olfermann T. Comparison of innovative thermal joining technologies for joining ultra-high strength steels in multi-material structures. Du¨sseldorf, Germany. 2013. [36] Absar S, et al. Temperature measurement in friction element welding process with micro thin film thermocouples. Procedia Manuf 2018;26:485–94. [37] Li Y, Wei Z, Wang Z, Li Y. Friction self-piercing riveting of aluminum alloy AA6061T6 to magnesium alloy AZ31B. J Manuf Sci Eng 2013;135(6):061007.
composite materials, the excluded methods can be used to join this material combination if there is a pre-hole operation prior to the joining process. Currently the system does not account for such variation, so the methods are just excluded. For the extreme case shown in Fig. 9f, the results are the same as the test study 1 because adhesives and nondeforming methods are left as the results for good appearance of the joint. 11.3. Summary of the case studies Studies show that the advisory system is capable of providing adequate joining methods according to the designers’ requirements and constraints on the joints. It provides several possible joining methods for the designers, from thermal to hybrid joining methods. It gives explanations about unfamiliar joints and displays ranks of the joining methods for the selected attributes. While the system has such capabilities, more sophisticated query to the designers and accounting for process variations such as existence and/or possibility of pre-hole operation would be a very promising way to go. Obtaining more data on joining methods is a work that must be done continuously. More material combinations, detailed process information, and other beneficial information are to be consistently collected to further enrich the database on joining methods. 12. Conclusion Intensive research is being conducted to find ways to join multimaterials in an efficient way. However, there is a gap between the research and the application side, so there is a need to establish a bridge to provide the knowledge on joining to the designers. MMJ Design Explorer has been developed to bridge this gap. This tool consists of four levels namely feasibility, parametrization, configuration, and integration. Feasibility was discussed in this paper, which deals with finding the appropriate joining methods that satisfy the criteria defined by the designer. A database containing comprehensive information on various joining methods have been constructed. The work flow of the feasibility level and the searching algorithm used in the database have been discussed in detail. The upcoming task is to develop a simulation template to perform parametric studies to populate more data points for meta-model generation. Ultimately, MMJ Design Explorer is envisioned to be linked with a commercial CAD and FEA packages such as CATIA and LS-DYNA, as a knowledge-ware to aid the structural designers in their early design phase. Acknowledgments This work was partially supported under contract # 06-49-06019 with the Center for Design and Manufacturing Excellence (CDME) at The Ohio State University with financial support from the Economic Development Administration, Department of Commerce. The content reflects the views of the authors and does not necessarily reflect the views of the Economic Development Administration or the Department of Commerce. The authors are grateful for the providing the knowledge and reviewing the database by the experts in academia and industry, 262
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[44] LeBacq C, Brechet Y, Shercliff HR, Jeggy T, Salvo L. Selection of joining methods in mechanical design. Mater Des 2002;23(4):405–16. [45] Esawi AM, Ashby M. Computer-based selection of joining processes. Mater Des 2004;25(7):555–64. [46] Swift KG, Brooker JD. Process selection: from design to manufacture. 2014. [47] Novak JD. Learning, creating, and using knowledge: concept maps as facilitative tools in schools and corporations. 2009.
[38] Hallquist JO. LS-DYNA theory manual. 2006. [39] Hibbitt D, Karlsson B, Sorensen P. ABAQUS analysis user’s manual Pawtucket, USA 2004. [40] Simufact. SIMUFACT. 2019. [41] Swantec. SORPAS. 2019. [42] Analysis W. DEFORM. 2019. [43] Release Ansys Inc Usa. ANSYS theory manual. 2011.
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Contents lists available at ScienceDirect
Journal of Manufacturing Systems journal homepage: www.elsevier.com/locate/jmansys
Knowledge based design advisory system for multi-material joining a,⁎
a
a
a
T a
Ji Hoon Kim , Lyang Suan Wang , Kaushalya Putta , Payam Haghighi , Jami J. Shah , Pete Edwardsb a b
Digital Design Manufacturing Laboratory, The Ohio State University, Columbus, OH, USA Honda Engineering North America, Inc, Marysville, OH, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Design advisory system Multi material joining Database Knowledge based advisory system
Multi-Material Joining Design Explorer is discussed in this paper, which is a knowledge-based advisory system to help structural designers at the early design phase to select the potential joining methods. Data mining on various joining methods was conducted from any available sources, such as experts from academia and industry, handbooks, and vendors. Collected data was organized in a concept map which is an informal way of representing the data structure. The data were arranged into several categories according to their characteristics which include joinable materials, mechanical and design requirements, geometry, and so on. Common parameters and unique parameters were extracted from deep investigation of the gathered data to create a formalized data structure. A database using a general tree structure was then created to be fed into the advisory system. Searching algorithm using SQL query was implemented to navigate through the database to find the joining methods that match the requirements defined by the user. Two test cases were generated to validate the function of the knowledge-based system.
1. Introduction For the past several decades, the automotive industry has been following multiple parallel paths to achieve energy efficient and ecofriendly vehicles. This includes hybrid and electric drivetrains, new battery technologies and overall weight reduction of their products without sacrificing crash-worthiness [1–4]. Driven by the goal to reduce the overall weight of the vehicles, steel manufacturers developed many new steel grades that have a higher strength to weight ratios, such as HSLA and AHSS formulations that go by designations of DP, TRIP, MS, and PHS. It is now apparent that the possibilities of getting more out of steel have been exhausted. As a result, non-ferrous alloys, particularly aluminum and magnesium, and even composites are being considered for “light-weight” materials. This brings a new challenge: how to join structural components of different materials in a cost-effective manner while integrating it into a mass-production environment. Many new technologies are emerging for multi-material joining: they include thermal, adhesive, and mechanical methods, such as laser welding, flow driven screws, rivets, and high-speed impact nails, respectively. There are also hybrid methods for combining adhesives with mechanical and/or thermal elements to get synergetic structural integrity. As shown in Fig. 1, various joining methods are being used
⁎
currently in cars that have multi-materials components. European luxury car manufacturers, BMW, Mercedes and Porsche have been leading the way in adopting these emerging technologies due to their low production volumes and higher product cost. BMW has reduced the weight of the i3 [5] by using Carbon Fiber Reinforced Polymers parts joined with adhesives, which have fewer components compared to metallic structures. Mercedes is using high-speed impact nails to join the different types of aluminum and steel grades in S-Class vehicles [11]. Ford is now using adhesive bonding in conjunction with selfpiercing rivets and flow-driven screws [12]. Studies on various joining methods are being actively conducted, but they are scattered around in academic literature, OEM proprietary, supplier information and more. There is a need to integrate these into one structure so that the knowledge on various joining methods are available to the designers in the early design stage. Performance and processing information of the joints that are relatively new is disparate; design and analysis methods and tools are not available to body structure designers. Multi-Material Joining (MMJ) Design Explorer was developed to consolidate this scattered information and to streamline them to the designers in the early designing phase. This paper discusses the components of the advisory system which are a comprehensive database of the joining methods, attributes composing the database library and the
Corresponding author. E-mail address: [email protected] (J.H. Kim).
https://doi.org/10.1016/j.jmsy.2019.03.003 Received 30 November 2018; Received in revised form 8 February 2019; Accepted 8 March 2019 Available online 22 April 2019 0278-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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ideal joint without causing too much damage to the material and find optimized pitch length to reduce the required number of the fasteners to overcome such challenges. There are several failure modes in fastener joints. Depending on the type of loading, failure modes can vary. Major failure modes are bending, fastener shear, net tension, and bearing. Pull-through and fastener failure can happen in the fasteners as well. Abe et al. have investigated joinability of self-piercing rivets (SPR) to mild steel and aluminum, high-strength steel, and aluminum substrates [17,18]. The authors varied the parameters such as substrate thicknesses and insertion speed to identify the failure modes using finite element analysis and experimental methods. Gotlib, Sundeep and Luscher have studied the shear performance of high-speed impact nails in aluminum joints with different substrate thickness [19,20]. They have successfully developed a model that replicates the strength performance and failure modes from their experimental results. They concluded that the ridged nails can be a good solution for multi-material joining under lap shear loading. Sønstabø et al. studied flow driven screws for joining aluminum substrates [21]. The authors derived force-displacement responses and the failure trend of the joint under various types of loadings. They have compared their results with self-piercing rivets and found that the overall performances to be very similar. Min et al. investigated the new blind riveting method to join dissimilar materials [22]. The difference between the new blind riveting method and the conventional blind riveting method is the elimination of the pre-hole process by self-penetration with a high rotational speed. Al6111 and Al6022 were used in this work, with the conclusions made by the authors that the tensile strength is 20% higher compared to the conventional blind rivets. Clinching is another common fastening method. Clinching deforms the materials by stamping with a special tool and forms an interlock between them as shown in Fig. 2d. The main difference from the methods described above is that there is no fastener inserted through the materials, so no additional weight is added to the joint, thus making it suitable for lightweight structures. But the method can only be used with ductile materials, since deformation of the materials during the joining process is large. Lambiase et al. have investigated the feasibility of joining various aluminum alloys to titanium alloys [23]. The authors have developed a numerical model to understand the formability of the joint and stress flow inside the joint. The model was verified with experiments, which led to a conclusion that AL7075 formed successful joints with titanium alloys. Zhang et al. have studied the failure mechanism of clinching on the aluminum sheets [24]. The authors identified various possible failure modes and investigated failure modes on the lower plate. Lambiase and Paoletti have studied the effect of rotating the stamping tool in the clinching process to reduce the high stamping force required [25]. The authors have found that by rotating the stamping tool, the stamping force was reduced to 1/10 of the conventional clinching method.
Fig. 1. Various joining methods used in recent cars [6–10].
working mechanism of the searching algorithm. In-depth insight has been provided on the decision of the attributes of the joining methods and how the database is structured with the attributes.
2. Survey on the state of the art 2.1. Mechanical fasteners Mechanical fasteners are the joining elements that fix the substrates through deformation of substrates and/or fasteners themselves. Shown in Fig. 2, self-piercing rivets (SPR), flow drill screws (FDS), and impact nails are examples of fasteners currently being used in sheet metal joining. Joining processes of the mechanical fasteners that involve tightening or deforming the substrate, create a pre-stress field. Joining materials are then joined by the pre-load generated by the field. Bolted joints create pre-tension by the tightening torque of the bolt and nut. SPRs flare as they are inserted and engage with the materials and create residual stress through the deformed geometry of the rivet. High-speed impact nails “open-up” the joining material during insertion, and as the material closes back in, it is engaged at the ridges of the nail. One of the advantages of joining processes of these kinds does not accompany thermal or chemical manipulation of the joining materials, allowing the joining of dissimilar materials without causing incompatibility issue between the different material characteristics with high shear and tensile strength. However, there are limitations and challenges, such as the possibility of excessive material damage during the joining process and weight addition due to the fasteners themselves. Numerous studies have been done to define the process parameters that would form an
2.2. Adhesive joints Adhesives can be categorized into two main groups: structural or sealing. Structural adhesives are used to join the materials with high strength whereas sealing adhesives are used to lock the thread and prevent moisture penetration. Depending on the component, adhesives can also be categorized into 1-part and 2-part. 1-part adhesives usually have triggered curing mechanisms, such as elevated temperature or UV light. 2-part adhesives have an initiator, so they are cured at room temperature once the components are applied. Once the adhesive is cured, joining materials are bonded with the adhesive layer through Van der Waals bonding. Van der Waals bonding is a secondary bond due to the attractions between atoms and/or molecules. Secondary bonds are weaker than primary bonds, such as covalent and metallic, which share the electrons, but this has a considerable role in various fields including the adhesive bonding.
Fig. 2. Examples of mechanical fasteners, (a) SPR: Rivet with hollow center, two-sided access with interlock formulation through flare of the rivet; (b) FDS: One-sided access, joint set with deformation due to friction generated while rotation of the screw; (c) Impact nails: One-sided access, with ridges on the nail shank, inserted by specific gun with high speed; (d) Clinching: Two-sided access, with stamping tool stamping to form an interlock without any fasteners inserted [13–16]. 254
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methods is that the individual joining methods act as reinforcements to each other, to form a more efficient joint. Spot welds and mechanical fasteners are point elements, so the load being transferred through the joint tends to be high and concentrated. On the other hand, adhesives are applied throughout the joining interface, so the load flow is uniform, but is vulnerable to peeling. By combining these methods, positive synergetic performance can be expected from them. Several studies have been carried out on the load transfer and the performance of bonded-bolted single-lap joints. Kelly used both finite element analysis (FEA) data and experimental data to investigate the load transfer of the joint for different joint design parameters [31]. Furthermore, the comparison between pure adhesive joint and hybrid joint was made on the possibility of improvements on the structural performance. Moroni et al. performed design of experimental analysis on weld-bonded and clinch-bonded joints [32]. The authors have performed DOE studies on conventional joints and hybrid joints to compare the performance with different environment conditions. It was found out by the authors that hybrid joints, weld-bonded and clinchbonded joints show an overall higher strength and energy absorption, and less variance to the environmental or geometrical change. Numerous studies have been performed in rivet-bonded joining methods for the sheet metal joining purpose. Solmaz and Topkaya compared the failure modes of the riveted, adhesive bonded, and rivet bonded double lap joints for composite materials [33]. The authors investigated the failure modes with overlap length, and how the failure of respective joints initiate. Franco et al. performed parametric studies on the effect of pitch on SPR-bonded joints to join aluminum and carbon fiber [34]. Fatigue life of different rivet pitch for SPR-bonded joints have been measured, and 60 mm pitch showed the highest fatigue compared to 30 mm and 45 mm pitch joints. By comparing the hybrid joints to their individual counterpart, studies have shown that hybrid joints performed better. Friction element welding (FEW), resistance element welding (REW), and friction self-piercing riveting (F-SPR) are combinations of thermal and mechanical joining methods. Common characteristics of FEW and REW are that there is a steel rivet/element that is inserted, and it is welded to the lower material. This avoids the material incompatibility of welding process because non-ferrous metals are placed on the upper layer which are penetrated through friction or pre-hole, and steel products are placed on the lower side to be welded with the insertion element. These methods do not require any pre-hole or forming required on the steel, thus making the method highly suitable for joining the high strength materials with tensile strength up to 1800 MPa [35]. However, heat affected zone is generated at the welded region that could change the material property of the materials that can affect the overall strength of the joint. Absar et al. have performed a parametric study to form a successful joint using FEW, and the temperature response in different joining stage using thermocouples have been investigated [36]. The authors found that the applied force and generated torque have the largest effect on the resultant FEW joints and have successfully profiled the temperature of the joint during the joining process. On the other hand, friction self-piercing rivet penetrates through all the materials. Li et al. have investigated the joinability of Al 6061-T6 and AZ31B [37]. The authors claimed that the riveting process is improved through rotating the rivet, and the strength of the joint formed by the F-SPR was higher than conventional SPR joints.
Fig. 3. Possible failure modes of multi-material adhesive joints [26].
Adhesive bonding is a straight-forward process. It is applied on the surface of the material desired to be bonded, and a joint will be formed after the adhesive has been cured. There is no mechanical/thermal interaction between the adhesive and the joining material, so there are few limitations on the type of the materials the adhesives can join. It is also possible to join different materials, because the adhesive layer prevents the galvanic corrosion and other issues coming from the material incompatibility. However, surface preparation of the materials is essential to obtain high quality bonding and curing process such as, thermal and UV light can be very time consuming as well. Adhesives are widely used despite these limitations in the industry due to their merits such as fast cure types and adhesives that can withstand some amount of dust and grease. There are three failure modes shown in Fig. 3 and mix of these modes are in adhesive joints. Cohesive failure is the fracture of the adhesive layer itself. This occurs mainly when the applied load exceeds the strength of the adhesive and indicates that the joint was a wellformed one. Adhesion failure is a separation between the adhesive and the joining material. The main cause of this failure is the existence of impurities such as dust and oil on the joining material. Inappropriate or insufficient surface preparation leaves debris on the material surface, so adequate surface treatment is necessary. Adherend failure, or plate failure is the failure of the joining material. This occurs when the load exceeds the material strength. Banea and Silva surveyed the adhesive bonding in composite materials [27]. Thorough review of the effect of various factors such as surface preparation and joint configuration has been done, along with numerical methods to simulate the behavior of the adhesively bonded joint. Silva et al. investigated the shear strength of different types and thickness of adhesive joints [28]. The authors used adhesives with three different characteristics, which are ductile, brittle, and intermediate. The authors claimed that the shear strength of adhesive joints increases as the thickness of the adhesive layer decreases for all three types of adhesives. Zhu el al. investigated the effect of the distortion caused during curing process on the strength of multi-material adhesive joints [29,30]. Thermal expansion coefficients of different materials are not equal: the elongation the materials during the curing process under elevated temperature is different one another. Thus, the residual stress is developed in the cooling stage, leading to distortion of the joint or debonding and fracture of the adhesive layer. The authors have investigated the influence of this phenomena on the strength of adhesive joints and have found that the shear strength and energy absorption are strongly affected while peeling force was relatively less influenced. Additionally, the authors have performed a parametric study to understand the parameters that cause the distortion and fracture of adhesive joints. They found that the peak curing temperature and plate thickness were the most influential, while the material properties of the adhesive and plates, and constraint types were less influential ones.
2.4. Joining methods in finite element analysis packages Commercial FEA software packages such as ANSYS, LS-DYNA, and ABAQUS have various ways of modeling process and performance of joining methods [38,39]. There are simulation packages that are dedicated to simulate material forming, welding and mechanical joining as well, such as Simufact, SORPAS, and DEFORM [40–42]. Generally, modeling the geometry of joints is a very complex task. For thermal joining, modeling the heat affected zone and the exact shape of
2.3. Hybrid joints Hybrid joining methods are combinations of two or more joining processes. Weld-bonded, rivet-bonded, and mixed mode adhesives are the generic examples of hybrid joints. The advantage of hybrid joining 255
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structure, and consumer appliances.
the weld nugget is very difficult. Also, the geometry of mechanical fasteners is very complex due to the existence of the threads and material engagement during the joining process. Furthermore, describing the exact material properties of adhesive layers is highly challenging as well. Residual stress formed after the joining process is another difficulty in joint modeling. To overcome these extensive challenges, various contact formulations and element formulations have been developed to simulate the behavior of the joints. Mechanical fasteners with threads have complex geometry and require very small size elements to model the actual geometry. ANSYS has the capability of creating virtual threads using contact formulations. Fastener shank which is flat without threads are used as the geometry, and contact elements re-align on the shank face according to the given dimensions of the thread [43]. Joining methods that have small size joining elements such as spot welds and SPRs are expressed as point connectors [38,39]. Single beam element or solid element is generally used with corresponding material properties such as normal, shear, and torsional load associated with failure criteria. Adhesive layers are substituted with cohesive elements that is based on the fracture mechanics. These alternate methods are advantageous on reducing the size of the model by avoiding the use of very small elements, thus applicable to large scale models. However, these methods require material properties that can only be acquired through specific experiments.
4. Problem formulation The main objective of the Design Explorer is defined as “Deliver the knowledge of the joining methods to the structural designers”. The advisory system is developed to establish the bridge to connect the gap between the structural designers and the sparse knowledge of the joining methods. New and exotic materials are being used to achieve lightweight and high strength of the structure, new joining methods other than spot welds and other conventional joining methods should be considered. Under the objective, there are functions that would operate as a team to fulfil the main objective. Database, user interface, and searching algorithm are the example functions in the advisory system. Each function has a unique task and operation phase. Databases store comprehensive data on various materials and joining methods. Searching algorithm is a way of finding appropriate joining methods that correspond to the requirements the user has specified. The user interface is an interactive platform where the user would set the criterion for various factors that would act as constraints in the advisory system. Constraints on the advisory system are any type of requirements of the joint that the designers have in selecting the appropriate joining methods. Joint strength, location of the joint, environment, and economics are good examples of the constraints. There are requirements on joint strength in structures to maintain the structural integrity under certain loadings, either those are shear strength, tensile strength, or both. Location of the joint is also an important constraint because depending on how important the location is in the structure would lead to selecting the different joining methods. For example, in automotive manufacturing laser welding is preferred in joining the A-surface, because A-surface is exposed directly to consumers, and the laser weld does not leave any visible trace. On the other hand, internal structures such as B-pillars and shock towers require high strength, so various mechanical fasteners are used to achieve high joint strength. Also, external environment determines what joining methods to use as well. For locations in parts where they encounter water frequently, joining methods that are resistant to corrosion and have good water sealing capability are preferred. Furthermore, some joining methods require two-sided access while others require one-sided access. Depending on the geometry of the location, two-sided access joining methods may not be possible to use. Economics is also an influential factor in joint selection. The cost of joining method and time taken to set the joint are very important considerations for potential users of a mass-manufacturing environment, such as automotive.
2.5. Software tools for selecting joining methods Storing and providing the experimental and simulation data describing the behavior and the characteristics of the joining methods into the designing phase is a very challenging task. Several researchers have discussed on how to represent those data for design purposes. LeBacq et al. proposed a methodology for the selection of joining methods as software interface [44]. By initial elimination of an infeasible process and use of a fuzzy logic algorithm to multi-criteria evaluation, the authors have developed a software that recommends a list of joining methods. Esawi and Ashby considered geometry, loads, material properties, and material combinations as the factors for selecting the joining process. The authors composed the database using linked-list: the matrices containing different types of information are linked to one another to how they are related. The software could provide scores of different joining methods based on cost and production rate [45]. Swift considered performance requirements, design aspects, and quality of various joining processes that can be referred to as guidelines in design tools [46]. However, the software lacks the type of joining methods that are recently developed, such as friction element welding, flow driven screws, and hybrid joints which are the combinations of mechanical, thermal, and adhesives. Thus, the software developed by the authors is not suitable for assisting in decision making on joints for multi-materials. 3. Scope of the research
5. Overview of the MMJ Design Explorer
MMJ Design Explorer, discussed in this paper, is a knowledge-based advisory system to help structural designers at the early design phase to select the potential joining methods. Thus, this can be formulated as a design problem to utilize and integrate the sparsely spread knowledge on the joining methods from and establish a bridge to the designers under well-formulated objectives and constraints. The scope of the research summarized below:
Multi-material joining (MMJ) Design Explorer is a comprehensive advisory system that provides information on various joining methods. The main objective of this advisory system is to aid the automotive structural designers to select appropriate joining methods for multimaterial joining. The focus of this paper is on a preliminary design where This paper discusses the development of the database on joining methods searching algorithm to go through those data to find adequate candidates. The user interface of the advisory system is also dealt here. An in-depth discussion on the component of the advisory system, a comprehensive database of the joining methods, attributes composing the database library and the working mechanism of the searching algorithm are presented. The rationale in selecting the attributes to be included in the software and how the database structured with these attributes are also discussed here.
• Joining methods considered are those used to join thin-walled components such as sheet metal or composite panels • The loads applied on the joints are tensile and/or shear loads. • Joints may need to join multi-materials such as steel, non-ferrous •
alloys such as aluminum and magnesium, and fiber reinforced polymers. Targeted industry fields are automotive body frames, aerospace 256
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6. Data collection using concept map Once these categorizations have been established, data was collected from various sources. The sources include: joining experts in OEM, academia, joint and material vendors, and handbooks dealing with diverse joining methods. In order to understand the semantic relationship, the data was represented as a concept map, rather than a formal E-R diagram [47]. Hence, the collected data were first arranged in Concept map by hand. A concept map is a graphical description of the information or data that are inter-related within a specific knowledge of interest. A concept map is composed of boxes or circles and labeled arrows, which indicate the data and information and the connection between them. Generally, one-way branching or downward hierarchical structure is utilized in creating concept maps. A concept map allows more informal annotations than a formal computer data structure. By doing so, it is easy to recognize what kind of relationship the boxes have and figure out the hierarchy of the database in a glance. Concept map goes well with the general tree structure which is the database structure chosen for the MMJ Design Explorer. Five concept maps were created: materials, thermal joining methods, mechanical fasteners, adhesives, and hybrid joints. As depicted in Fig. 4, the boxes represent the information of the various joining methods and material types. Additionally, texts inside the arrows indicate the topic of the lower level nodes connected from the upper. Components of data under each individual joining methods have been consistent using the parameters defined to be transferred easily to the actual database later.
Fig. 5. High level classification of joining methods.
in a manufacturing environment. There are various factors such as tool geometry, production rate, and cost on the respective process for different joining methods that describe the joining procedures. Whether it is thermal joining, mechanical joining, and adhesive bonding, there are specific and unique process parameters that have to be defined in order to obtain a successful joint. Process parameters are further split into two categories: unique and common parameters. Unique parameters are directly related to the tools and operation sequence. Common parameters are the ones that can be used to compare with other joining methods. The main purpose of searching for parameters for the database is to find common parameters that can provide a comparison between two or more different joining methods. Whereas, non-overlapping parameters will be displayed to the designers instead of being implemented in the software as one of the filters.
7. Attribute selection In order to represent knowledge about different methods in a uniform way, we need to identify key attributes. As there are many joining methods that are capable of joining metals, parameters describing the attributes of the joining methods also vary. However, the number of parameters describing the attributes should be reasonable and at the same time contains all the sufficient information to determine potential viability. Through careful investigation of different joining methods, these parameters were consolidated into two categories: process and performance attributes.
7.1.1. Unique attributes Process parameters of the joining method are different from other joining methods. Examples of such process parameters for resistance spot welds are electrode force, clamping force, electrical current, current time, etc. For flow driven screws, the process parameters are rotation speed, hold time, and insertion force. Process parameters of adhesive bonding are work time, cure time and temperature, single or two
7.1. Process attributes Shown in Fig. 5, attributes of the joining methods were initially split into two different groups that are: process and performance. The process group is made up of attributes related to the installation of a joint
Fig. 4. Concept map generated for various joining methods and material types. 257
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Fig. 6. Structure of the database of MMJ Design Explorer.
the joint has been formed, so they are all applicable to all the joining methods. Parametrizing the strength of the joints can be done in two ways: using the absolute joint strength or the joint efficiency. Absolute joint strength is an actual value of the strength of the joint when installed on the joining materials expressed in Pascal and Newton units. Using the absolute value provides direct information on the joint strength on different materials and whether or not that particular joining method is suitable for that application. However, the pitfall of making a decision based solely on the joint strength is the overlooking of joint efficiency. Joint efficiency is a percentage ratio of the strength of the joint to the strength of the base material and by just looking at joint strength, the joint efficiency could not be deduced. This could lead to unnecessary high material cost for low joint efficiency. Although the exact value of the strength of the joint is not shown in this format, the ratio is sufficient enough to be one of the deciding factors on whether or not the joining method is suitable for the selected base materials. Also, it is obvious that the strength of the joint may differ significantly with different base materials. By organizing the strength parameter with efficiency which is the percentage ratio, the designer can evaluate how well the selected joining methods and base materials can perform. This will help designers to decide on the most optimum joining method in terms of cost saving.
part, and so on. These parameters are unique in each method and cannot be applied to the other methods because the procedure of the joining is totally different. Not only these parameters vary with different joining methods, but the values also vary significantly with different joining material combinations. Same rotational speed used to join low carbon steel with flow driven screw is less likely to be successful in joining high strength steel. Thus, it is very challenging to collect and contain all the information in the database structure. Such parameters should not be treated as the dynamic variables in the database. Instead, they are stored in the database as text information without any numerical values. This information is displayed at the very final stage with a list of all the joining methods satisfying all the filters set by the designer. 7.1.2. Common parameters Although intuitively, parameters of different joining methods are different from each other, there are some process parameters that share common characteristics. Material combinations, geometry, cycle time, accessibility, and robustness are some examples of overlapping aspects in different joining methods. Material combinations are the list of the materials that a joining method is capable of joining. This is determined by metallurgy and material properties such as brittleness, electrical conductivity, and melting temperature, some materials cannot withstand the mechanical and/or thermal load applied during the joining process. Cycle time is the required time to install a joint, accessibility is whether the joining methods require single or two-sided access, and robustness is the measure of ability to produce joints under manufacturing variances. These type of process parameters are important in making decisions for the design and the manufacturing because these parameters are directly related to the possibility of applying the joining process into the manufacturing line. If the line is for mass-production, high robustness with fast cycle time, as well as, cost are the important factors to consider because production rate is high, and each joint should be successfully installed on the body in a very short time.
8. Structuring the database Since the MMJ Design Explorer is strictly based on the database on various joining methods, constructing a well-organized and robust database is the most important task on the successful development of this advisory system. Sorting out the collected parameters into detailed categories is a must in order to accomplish the task. The structure of the database is shown in Fig. 6. As can be seen from the structure, there is little relationship between different joining methods. Therefore, constructing a graph-style and linked-list database structure is deemed unnecessary. Instead, the structure should have different levels, distinguishing parent and children node to contain a different type of information in each level. Additionally, nodes should have the capabilities to be added and deleted easily without having to manipulate the entire connections within the database to accommodate new emerging joining technologies and further developments in current joining methods. Therefore, a general tree database structure was selected. The root nodes of the database consist of the major type of joining
7.2. Performance parameters Performance parameter group is composed of attributes that determine the functional efficiency of the joining methods. The group is further split into two subcategories which are mechanical and design aspects. Mechanical aspects include normal and shear strength, fatigue, corrosion. Examples of design aspects are the aesthetics, available space, and the location of the joint. These attributes are measured after 258
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methods: thermal, mechanical, adhesive, or hybrid. Under the type of joining methods, various joining methods are listed under the corresponding group as children nodes. Then, the parameters of the various joining methods are split into more detailed categories compared to such as joinable materials, mechanical / design requirements, geometry, and so on as shown in Fig. 6. Joinable materials describe list of materials that can be joined using that particular joining method. This could be a single material or multi-material combination. Mechanical requirements are the parameters that were initially in the performance parameter group. All the strength-wise parameters are included in this category. Every joining method requires a specific configuration of the base materials. For example, self-piercing rivets (SPRs) can only be joined via lap-joint, while adhesives can be used to bond any configurations. Geometry category deals with the possible configurations that the joining methods can handle such as lap joint, butt joint, scarf joint, etc. Design requirements are the factors that are related to design and workability, which are robustness, available real estate, cost, and so on. Process category includes attributes that are associated with joining process of each method. Characteristics consist of all other information that was included in the aforementioned factors. General description of the joining methods such as advantages and limitations are stored as text, and other minor attributes that could not fit into the major ones are included in this group as well. 8.1. Transforming the concept map into ACCESS database Concept maps generated during the collection process cannot be used directly in the advisory system, the data in the map is transferred to the database structure in the computer. There are several database platforms, such as MySQL, Oracle, and Microsoft Access. Among those choices, Microsoft Access has been selected as the initial database management system due to its simplicity of facilitating the environment and accessibility. The data structure translated into the computerformat database is in a way of matrix form with each row and column having different meanings. In this case, rows represent the joining methods within the category of joints and columns indicate characteristics and attributes. All the concept maps are transformed into this format, so that it is possible to develop a code that would implement a searching algorithm.
Fig. 7. Work flow of the MMJ Design Explorer; (a) Phase 1: material filtering; (b) Phase 2: attribute filtering process.
database as initial results. When the attributes are defined in the second phase, the joining methods satisfying the filter requirement are called the final results. This filtering process is done through the SQL query command, which would navigate through the designated rows or columns of the database and call all the data that matches or exceeds the requirement established by the user. This method visits the entire nodes in the database one by one, so no information is lost or missed during the searching process. While there is a concern that as the size of the database gets larger, the time required in searching through the database may grow proportionally. However, the increase in the number of joining methods would not be a big impact in time increase of searching because it is only adding some amount of the innermost loop in phase 2. As long as new attributes are not being added in a large number into the database which would create numerous outer loops in phase 2 shown in Fig. 7b, the exhaustive searching algorithm is the most suitable method for searching this database structure.
9. Work flow and searching algorithm This section discusses the overall work flow of the Design Explorer first and then followed by the searching algorithm. This is because the search algorithm is determined from the work flow of the Design Explorer. Potential users of the MMJ Design Explorer are the structural designers in industry. Base materials are typically known prior to making decisions on joining methods. Thus, the very first question asked by the system is the material combinations of the joining parts as plotted as phase 1 in Fig. 7a. The initial sort-out process is performed based on the selected materials the designers have specified. Completing this process allows the designers to input the additional requirements of the joining methods. Additional requirements are the attributes that were discussed previously and is shown in Fig. 7b. Once the second filtering is done, final results are displayed for the designers to browse through. As can be seen from how the two work flows operate, searching through the entire members in the database is required for each attribute that the designers have selected as filtering parameters. In other words, multiple loops are required within the searching algorithm to perform repetitive filtering process. Additionally, attributes do not have any relationship with each other. Therefore, each attribute must be investigated one by one because they cannot be generalized to be expressed by equations in the form of describing relationships with other attributes. Shown in Fig. 7, as the user defines the material combination, joining methods corresponding to the criteria are called from the
10. User interface MMJ Design Explorer has vast amount of data on various joining methods for thin walled structures. However, structural designers would appreciate the automated searching scheme and user-friendly interface to make use of the database. Software interface has been created to undertake the task of searching through the database and provide filtered results to the designers. As the work flow is composed of two phases, the software interface has two parts as well. It is discussed in previous section that the designers know the single or multi materials that need to be joined together, so the software starts with the selection of the joining materials, as shown in Fig. 8a. The designer can choose specific type of the materials that are under steel, aluminum, and carbon fiber polymers. Once materials are specified, initial filtering results are displayed at the right side of the interface, under individual categories: thermal, mechanical, adhesive, and hybrid. The designer can selectively choose the joining methods they have in favor, or just 259
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Fig. 8. Software interface of (a) Phase 1; (b) Phase 2.
with the requirements. The quality of the result is purely determined by the inputs the designers provide to the system. It is clear that the more attribute qualifications the designers define, the narrower the results will be given. Fig. 8b is the actual interface of phase 2. As designers select and define the attributes in the left side of the interface, corresponding results are published in the center and right side. There is a very useful feature in this phase to help designers make a better decision on selecting joining methods. That is, the results of the final joining methods can be ranked up to three ranking criteria which were initially selected by the designers. This allows the designers to compare the performance of the possible joints with respect to the attributes that are
select all of them and proceed to the next phase. After the initial filtering with joinable materials in phase 1, detailed attributes can be selected and defined according to the requirements and specifications that the joining section needs to obtain. Phase 2 is responsible for receiving the designer’s selection of attributes and requirements and further filter the initially filtered joining methods in consonance with such requirements. Different filtering scheme applies to different type of attributes. Attributes such as strength, cycle time, and cost are treated satisfactory if the values exceed requirements defined by the designer. Other attributes, such as tool accessibility and form of the adhesive are qualified only when the values match exactly 260
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more important to the designer. 11. Test case To demonstrate the advisory system, two test cases are presented here. The case studies have been created using AHSS – aluminum and AHSS- CFR combinations and locations with different emphasis. It is very challenging to join such combinations using conventional thermal joining methods, so the system is expected to provide alternate joining methods that are capable of joining dissimilar materials. Describing the strengths and limitations of the current advisory system, and what factors of improvements can be made to further advance the system. 11.1. AHSS-Aluminum combination High strength steel (DP 780) and aluminum (6000 series) combination is often used in hoods, front and rear panels of automotive to obtain the light-weight structure. Aluminum and steel have very different material properties, so joining methods that accompany material fusion would be difficult to join these materials. Thus, non-fusion methods such as mechanical fasteners, adhesives, and hybrid joints should be showing up on the list. Table 1 shows the example requirements and constraints that the designer has in joining these materials. Based on these requirements, first the materials and configurations are specified in phase 1. The results are shown in Fig. 9a, where several joining methods for each category are listed. For the demonstration purpose, all the methods are selected and phase 2 was initiated. After selecting the desired requirements in the interface, final results are displayed shown in Fig. 9b. It can be seen that hybrid joints are placed in higher ranks compared to other joining methods in shear strength. On the other hand, joining methods that does not accompany deformation of the materials such as adhesives and friction spot welding methods are high ranked in aesthetics part, although the aesthetics is not a big issue in this case. To compare with extreme cases, let the location of the joint be the roof of the automotive, which is an A-surface where the consumer sees the parts. In this case, laser beam welding and refill friction spot welding, and adhesives are the only case for joining multi materials with very good aesthetics as shown in Fig. 9c. 11.2. AHSS-CFR combination Carbon-fiber reinforce polymers generally are very brittle, so additional caution must be taken in selecting the joints. Obviously, joining methods that deform the materials too much would not be suitable for this case. Given all other conditions are same except the material combinations, the design requirements are listed in Table 2. As the results shown in Fig. 9d, fasteners that may cause excessive deformation are excluded compared to the previous combination. That is, SPRs, clinching, impact nails, and hybrid joints that are combined with those methods are eliminated from the previous result. Other methods that are still listed are the adhesives, laser brazing, and mechanical fasteners that generate heat due to friction to penetrate the composite and join with AHSS using conventional methods. For Table 1 Example design requirements and constraints of the automotive design I. Material
DP 780 and AL 6000 series
Configuration of the joint Thickness of the plates Service temperature Location of the joint
Lap joint 2–3 mm 0–40 °C Internal structure of the automotive – not visible to consumers Normal weather with moderate rainy days Moderate
Environment Strength requirement
Fig. 9. Results of (a) Phase 1; (b) Phase 2; (c) extreme case of case study I; and (d) Phase 1; (e) Phase 2; (f) extreme case of case study II.
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Dr. Luscher and Dr. Benatar at The Ohio State University, and Pete Edwards, Honda Engineering North America.
Table 2 Example design requirements and constraints of the automotive design II. Material
DP 780 and CFR Epoxy
Configuration of the joint Thickness of the plates Service temperature Location of the joint
Lap joint 2–3 mm 0–40 °C Internal structure of the automotive – not visible to consumers Normal weather with moderate rainy days Moderate
References
Environment Strength requirement
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composite materials, the excluded methods can be used to join this material combination if there is a pre-hole operation prior to the joining process. Currently the system does not account for such variation, so the methods are just excluded. For the extreme case shown in Fig. 9f, the results are the same as the test study 1 because adhesives and nondeforming methods are left as the results for good appearance of the joint. 11.3. Summary of the case studies Studies show that the advisory system is capable of providing adequate joining methods according to the designers’ requirements and constraints on the joints. It provides several possible joining methods for the designers, from thermal to hybrid joining methods. It gives explanations about unfamiliar joints and displays ranks of the joining methods for the selected attributes. While the system has such capabilities, more sophisticated query to the designers and accounting for process variations such as existence and/or possibility of pre-hole operation would be a very promising way to go. Obtaining more data on joining methods is a work that must be done continuously. More material combinations, detailed process information, and other beneficial information are to be consistently collected to further enrich the database on joining methods. 12. Conclusion Intensive research is being conducted to find ways to join multimaterials in an efficient way. However, there is a gap between the research and the application side, so there is a need to establish a bridge to provide the knowledge on joining to the designers. MMJ Design Explorer has been developed to bridge this gap. This tool consists of four levels namely feasibility, parametrization, configuration, and integration. Feasibility was discussed in this paper, which deals with finding the appropriate joining methods that satisfy the criteria defined by the designer. A database containing comprehensive information on various joining methods have been constructed. The work flow of the feasibility level and the searching algorithm used in the database have been discussed in detail. The upcoming task is to develop a simulation template to perform parametric studies to populate more data points for meta-model generation. Ultimately, MMJ Design Explorer is envisioned to be linked with a commercial CAD and FEA packages such as CATIA and LS-DYNA, as a knowledge-ware to aid the structural designers in their early design phase. Acknowledgments This work was partially supported under contract # 06-49-06019 with the Center for Design and Manufacturing Excellence (CDME) at The Ohio State University with financial support from the Economic Development Administration, Department of Commerce. The content reflects the views of the authors and does not necessarily reflect the views of the Economic Development Administration or the Department of Commerce. The authors are grateful for the providing the knowledge and reviewing the database by the experts in academia and industry, 262
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