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polymer composite hybrid structures
Accepted Manuscript A New Refill Friction Spot Welding Process for Aluminum/Polymer Composite Hybrid Structures Hossein Karami Pabandi, Mojtaba Movahedi, Amir Hossein Kokabi PII: DOI: Reference:
S0263-8223(17)30547-0 http://dx.doi.org/10.1016/j.compstruct.2017.04.053 COST 8484
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
16 February 2017 28 March 2017 20 April 2017
Please cite this article as: Pabandi, H.K., Movahedi, M., Kokabi, A.H., A New Refill Friction Spot Welding Process for Aluminum/Polymer Composite Hybrid Structures, Composite Structures (2017), doi: http://dx.doi.org/10.1016/ j.compstruct.2017.04.053
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A New Refill Friction Spot Welding Process for Aluminum/Polymer Composite Hybrid Structures Hossein Karami Pabandi, Mojtaba Movahedi*, Amir Hossein Kokabi
Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran * Corresponding author: Tel: +98 21 66165224; Fax: +98 21 66005717; E-mail: [email protected] Abstract: A new refill friction spot welding process called Threaded Hole Friction Spot Welding (THFSW) was introduced to join AA5052 aluminum to short-carbon-fiber-reinforced polypropylene (PP-SCF) composite sheets. The process was based on filling of the pre-threaded hole by melted and re-solidified polymer. The results showed that THFSW was successful to join aluminum to polymer sheets and the hole was completely filled with melted polymer. Formation of a reaction layer composed mostly of Al, C and O as well as interlocking between the threaded hole and the resolidified polymer were recognized as main bonding mechanisms. Maximum shear-tensile strength of the joints reached to ~80 percent of the composite base strength. Moreover, Mechanical strength and fracture energy of the joints increased with enhancement of tool rotational speed. Variation of the joint strength was explored in light of the fracture surface features as well as crystallinity percent of the re-solidified polymer inside the hole. Keywords: Friction spot welding; Refill; Polymer; Aluminum; Joint strength; Fracture energy
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1. Introduction Successful joint between polymers and metallic alloys is a necessity for the production of light weight polymer/metal hybrid structures in various industries such as automotive, aeronautics and shipbuilding industries [13]. Together with traditional methods such as adhesive bonding [4,5] and mechanical fastening [6], new joining techniques have been patented and developed in recent years for “spot” joining of polymers to metallic alloys. Ultrasonic staking [7] and Injection Clinching Joining (ICJ) [1,8] are two methods with relatively the same principles used for spot joining of polymers and metals. Abibe et al. [1] compared ultrasonic staking and ICJ methods for joining polyetherimide and AA6082 aluminum alloy. Their results indicated that both processes show similar mechanical properties. However, ICJ had a higher strength-to-weight ratio due to the presence of hole formed in the stake. Moreover, ICJ showed longer joining time in comparison to ultrasonic staking. Abibe et al. [8] also employed ICJ process to join short-glass-fiber-reinforced polyamide and AA2024 aluminum alloy. They reported that volume of the rivet head in contact with the tool system and efficiency of cavity filling are key factors controlling the strength of the joints. Welding processes have also been used for spot joining of polymers to metals. Friction Stir Spot Welding (FSSW) is a solid state process with high potential for these types of joints. However, because of leaving an exit-hole at the center of the nugget, the Friction Spot Joining (FSpJ) process was developed by GKSS of Germany in order to refill the formed exit-hole [9]. FSpJ has been used successfully for joining of metals to glass and carbon-fiber-reinforced composites [3,10-13]. Goushegir et al. [3] utilized FSpJ process for joining AA2024 aluminum and reinforced poly (phenylene sulfide) with 50 vol% carbon fibers (CF-PPS). They concluded that by enhancement of the tool plunge depth, the shear fracture load of the joint as well as the extension to fracture improved due to increase in the joint area at the sheets interface. Esteves et al. [11] employed FSpJ process for joining AA6181-T4 aluminum and carbon-fiber-reinforced poly (phenylene sulfide) composite. Their results showed that tool rotational speed was the most effective parameter on the joint strength. In addition, they reported that at high tool rotational speed (1600 rpm), reduction of the metal viscosity led to decrease in the heat generated during the process. Therefore, joint area between the aluminum and polymer sheets at the interface was reduced. In the present work, a new friction spot welding process called “Threaded Hole Friction Spot Welding (THFSW)” was used to join AA5052 aluminum alloy and short-carbon-fiber-reinforced polypropylene composite sheets. In this 2
joining process, a pre-threaded hole in the aluminum sheet is filled by the melted and re-solidified polymer during the process. Thus, no hole remains at the joint area after completing the process. In comparison to ultrasonic staking and ICJ, THFSW does not require an initial polymeric stud on the polymer sheet; however, the stud is formed in the hole by molten polymer during the process. On the other hand, the same as the ultrasonic staking and ICJ, a hole must be created in the aluminum sheet before joining process. Furthermore, in FSSW a keyhole remains at the joint center due to pin penetration, while the suggested joining process is able to refill the threaded hole during the process. In FSpJ, a relatively complex mechanical and electronic system is required for joining. Although, in THFSW, the joint is produced using a simple cylindrical tool without pin. After joining process, mechanisms of bonding, as well as the effects of the tool rotational speed on the mechanical behavior of the joints, were studied.
2. Materials and experimental procedures
2.1.
Materials
As-received AA5052 aluminum alloy with a thickness of 2 mm was used as the metallic part of the joint. Chemical composition, mechanical and physical properties of AA5052 aluminum alloy used in this study are presented in Table 1. Polymeric sheets used in this research were short-carbon-fiber-reinforced polypropylene Z30S composite (PP-SCF) with 2 mm thickness. Dimensions of carbon fibers were ~3 mm in length and ~17 µm in diameter. Chemical composition and mechanical properties of PP-SCF composite and physical properties of polypropylene Z30S are listed in Tables 2 and 3, respectively. Fig. 1 shows distribution of carbon fibers in Z30S polypropylene matrix.
2.2.
Joining process
Aluminum sheet with a threaded hole and composite sheet, both with demotions of 70×30 mm2 were used for joining process. To produce the threaded hole, aluminum sheet was drilled with 4 mm diameter and then threaded by M4 reamer (with the aim of increasing the joint strength through formation of mechanical interlocks). A cylindrical tool from heat treated H13 steel composed of a shoulder with a diameter of 20 mm was employed as the welding tool. Joining process was performed in lap configuration with an overlap of 30 mm, while aluminum (as the upper sheet) was in contact with the tool shoulder. Tool rotational speeds of 500, 1000, 1500 and 2000 rpm and dwell time of 5 s were selected for joining. 3
Schematic of different steps of new joining process introduced in this research (THFSW) is illustrated in Fig. 2. At the first step of the process, rotating tool is located coaxially with threaded hole on top of the aluminum sheet surface as shown in Fig. 2-a. In the next step, the rotating tool is moved downward and the shoulder penetrates ~0.3 mm in the aluminum surface (Fig. 2-b). As a result of contact between the rotating tool and aluminum sheet, frictional heat is generated. This heat reaches to the aluminum/polymer interface via conduction and melts the polymer at the interface. However, since the polymer is a heat insulation material, melting of the polymer is limited to the surface of the composite sheet at the interface. Then, by the downward axial pressure of the tool, melted polymer flows towards the periphery of the joint location at the interface. The molten polymer around the hole in aluminum sheet fills the hole and mechanical interlocks are formed between polymer and aluminum due to the existence of threads inside the hole (Figs. 2-c and d). After 5 s dwell time, the tool is retracted upward (Fig. 2-e), and the joint is completed with the solidification of the molten polymer inside the hole (Fig. 2-f). It should be noted that by enhancement of the hole diameter, the required volume of the molten polymer for filling the hole increases. On the other hand, raising the shoulder plunging depth leads to increase in the volume of the molten polymer flowed from the interface of the base sheets toward the threaded hole. Therefore, larger diameter of the threaded hole needs higher shoulder plunging depth.
2.3.
Temperature measurement
In order to measure temperature variations during the joining process, K-type thermocouples with 0.5 mm diameter were embedded into the interface of sheets at a distance of 10, 20 and 30 mm from the joint center. The maximum temperature obtained during the process was recorded at each rotational speed.
2.4.
Microscopic observations
The stereomicroscope was used for observation of cross-section and fracture surface of the joints. Moreover, for investigation of the joints microstructure and the fracture surfaces, Field Emission Scanning Electron Microscope (FE-SEM) equipped with Energy Dispersive Spectroscopy (EDS) was used. For increasing the quality of SEM images, a gold layer was coated on the samples surfaces.
2.5.
Differential scanning calorimetry analysis
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The crystallinity of PP-SCF composite and re-solidified polymer inside the hole was evaluated by differential scanning calorimetry (DSC) analysis at the temperature of 250 ºC for 5 min. The measurements were performed on a 8 mg sample under N2 atmosphere. The specimens were heated and cooled at a constant rate of 10 ºC/min. The exothermic crystallization peak was recorded as a function of temperature.
2.6.
Mechanical properties
For evaluation of the mechanical behavior of the joints, shear-tensile and cross-tension tests were carried out with the cross head speed of 0.5 mm/min. Dimensions of the sheets and the overlap area were selected as 70×30 mm2 and 30×30 mm2, respectively (Fig. 3). The threaded hole was located at the center of the overlap area. At each set of joining parameters, three samples were tested for tensile tests and the average of maximum forces were reported. The hardness of the composite sheet and re-solidified polymer inside the hole was measured in Shore D scale to investigate the effect of temperature changes during the joining process on the hardness and joint strength.
3. Results and discussion
3.1.
Macro-profile of joints
The surface appearance of the joints for different rotational speeds is presented in Fig. 4. The re-solidified polymer in the hole and the surface of aluminum affected by the rotating tool are shown in the figure. As can be seen, surface appearance of the joints is smooth especially at higher rotational speeds due to more heat generation and improved flow of the aluminum under the tool shoulder. Fig. 5 shows the cross-section of the joints and various macrostructural zones including base materials, threaded hole and re-solidified polymer inside the threaded hole. It can be seen that the rotational speed of the tool affected filling behavior of the threaded hole by the molten polymer. The higher was the rotational speed, the more was the frictional heat generated during welding. The maximum temperature experienced at different distances from the joint center (R) are shown in Fig. 6. It was observed that by enhancement of the tool rotational speed, the maximum temperature produced during the process increased. Moreover, the area experiencing temperatures higher than the melting point of the polymer was broader. Therefore, the temperature and volume of the molten polymer at the interface of the base sheets increased. At the rotational speed of 500 rpm, the volume of the molten polymer is not enough to fill completely the hole. Fig. 5-a shows porosity and unfilled zone at the joint center and separation occurred at the interface of sheets due to lack 5
of adhesion force in this area. By enhancement of the tool rotational speed, more heat was generated and as a result, more polymers melted and entered into the hole. In the cross-section of the joints produced at 1000, 1500 and 2000 rpm, all areas of the hole were filled completely with molten polymer and mechanical interlocks were formed between the threads of the hole and re-solidified polymer. Furthermore, due to the melting of polymer at the interface of the aluminum and composite sheets, an adhesion force was also formed at the periphery of the hole [11]. Referring to Fig. 5-d, as a result of high frictional heat at a rotational speed of 2000 rpm, the excess melted polymer re-solidified as a convex surface on the threaded hole. This convexity was formed in effect of the pressure of holding clamps after retracting of the welding tool.
3.2.
Reaction layer formation and joining mechanisms
Fig. 7-a shows SEM image from a typical cross-section of the joint. Magnified SEM image of the interface between aluminum and re-solidified polymer inside the hole (area “A” in Fig. 7-a) is given in Fig. 7-b. It is obvious that a reaction layer was formed with a thickness of approximately 15 µm at the interface. The term of “reaction layer” refers to a layer formed between the aluminum and the molten polymer and atoms of both materials exist in this layer. It seems that this layer has been formed by erosion mechanism (a well-known phenomenon in the brazing and soldering) [16] in which due to the collision between molten polymer and the hole wall at high temperatures, aluminum atoms were separated from the aluminum surface and entered into the molten polymer and formed the reaction layer. Comparison between Figs. 7-b and 7-c indicate that the thickness of the reaction layer did not change significantly by variation of the tool rotational speed. However, the chemical composition of the layer was affected by the tool rotation speed. Table 4 shows EDS analysis of the reaction layer for samples welded at the tool rotational speed of 500 rpm (minimum heat input) and 2000 rpm (maximum heat input). Enhancement of the tool rotational speed led to increase in weight percent of Al and slightly decrease in weight percent of C. In the explanation of this phenomenon, it can be mentioned that the higher was the temperature of the molten polymer entered into the hole, the more was the aluminum surface erosion inside the hole and consequently the presence of aluminum atoms in the reaction layer. Moreover, the presence of oxygen in the reaction layer was probably attributed to degradation of the molten polymer in effect of reaction with oxygen [2]. Formation of a reaction layer with 8 wt% of Al was also mentioned by Shahmiri et al. [16] in FSW of Polypropylene C30S to AA5052 aluminum alloy. 6
In addition to the reaction layer, there was also a gap between the reaction layer and re-solidified polymer some regions inside the hole (Figs. 7-b and 7-c). It seems that this gap was formed during the cooling process of the joint due to the large difference between the coefficients of thermal expansion (CTE) of the base materials [1]. The CTE of aluminum and polypropylene are 23.8×10-6 K-1 and 100-150×10-6 K-1, respectively [1,15]. Compared to aluminum, higher CTE of polypropylene resulted in more shrinkage of the molten polymer during the cooling cycle of the weld and thus, a gap was formed between the polymer and the reaction layer. On the other hand, shrinkage of the molten polymer during solidification depended on the melt temperature. The higher was the temperature of the molten polymer, the more was its shrinkage during cooling. Since the temperature of the molten polymer at the rotational speed of 2000 rpm was higher (Fig. 6), a wider gap was expected in this tool rotational speed. As can be seen in Fig. 7-c, the wide gap was filled with epoxy resin (cold mounting material) in the preparation stage of the sample for microstructural studies. From what mentioned above, the obtained joint between aluminum and composite sheets using THFSW process is the result of two phenomena: i) Mechanical interlocks between re-solidified polymer and threads inside the hole, ii) Formation of reaction layer between aluminum and polymer. It is crucial of importance that the results of a research carried out by Khodabakhshi et al. [17] on FSW of aluminum to polymer sheets proved the formation of Van der Waal's bonds between the atoms of aluminum oxide layer on the surface and polymer. However, formation of a gap between the reaction layer and the re-solidified polymer inside the hole reduces the effectiveness of Van der Waal's bonds and reaction layer for the joint strength. This mechanism can be helpful for joint strength at the interface of the base sheets and periphery of the hole (a region with adhesion force in Fig. 5).
3.3.
Distribution of carbon fibers
Fig. 8 presents the carbon fibers distribution in the re-solidified polymer matrix inside the hole. As can be seen, variation of the tool rotational speed had no effect on the fibers distribution in the re-solidified polymer matrix. However, comparison between Fig. 1 (base composite sheet) and Fig. 8 shows that in general, the carbon fibers in the re-solidified polymer were shorter than those in the base composite sheet. This may be due to the fragmentation 7
of the carbon fibers within the molten polymer in the effect of their flowing from the interface of the base sheets toward the threaded hole during the welding process. On the other hand, the carbon fibers in the base composite sheet had a relatively preferred orientation which is a well-known phenomenon in the injected polymer matrix composites [18,19]. However, carbon fibers in the re-solidified polymer show a random orientation.
3.4.
Fracture surface analysis
Fig. 9 shows the fracture surfaces of the samples after cross-tension and shear-tensile tests. In the samples welded at the tool rotational speeds of 500, 1000 and 1500 rpm, fracture occurred at the interface of the composite sheet and re-solidified polymer inside the hole. Indeed, fracture surface of the joint on the composite sheet was a circle in which unmelted polymer surface at the center of this circle was surrounded by the melted polymer surface. The melted surface was formed due to entry of the melted polymer from the sheets interface toward inside the hole resulting in surface melting of the primary polymer at the periphery of the hole and combination of the two mentioned melts with each other. In other words, the surface of the composite sheet that is at the base of the hole had no direct contact with the aluminum sheet. Therefore, an increase in the temperature of the aluminum sheet as a result of the frictional heat did not directly melt the polymer surface at this region. The existence of the unmelted surface in the center of the circular base of the threaded hole at the interface of the sheets means that re-solidified polymer in the hole was not joined to the primary polymer at this region. Therefore, if the re-solidified polymer inside the hole is considered as a small cylinder with a diameter of 4 mm and a height equal to the thickness of aluminum sheet (2 mm), there was a crack in the base of this cylinder (interface of the composite sheet and resolidified polymer) due to lack of melting of the composite sheet. Huang et al. [20] utilized self-riveting friction stir lap welding for joining of AA6082 to steel with filing the prefabricated holes in the bottom steel sheet by plasticized aluminum. They reported that downward flow of the plasticized aluminum occurred because of two origins: i) with progress of the welding tool, aluminum alloy ahead of the pin is plasticized and then, threads of the rotating pin pushes the plasticized metal downwards; ii) imposed pressure by the pin bottom extrudes the plasticized aluminum alloy into the prefabricated hole. Since the tool used for welding in the present work is pin-less, there is no flow of the molten polymer by the threads of the tool pin. Therefore, the molten polymer formed at the interface of the base sheets flows upward with back extrusion into the threaded hole just in the effect of the downward pressure of the tool. It should be mentioned that the molten polymer has high fluidity and can easily flow and fill the threads of the hole with the pressure applied by the welding tool. High fluidity of the molten polymer is caused by the elevated 8
temperature experienced by it during the welding process. Fig. 6 confirms that the temperature of the molten polymer at the distance of 10 mm from the joint center is significantly higher than the melting point of polypropylene (from ~30 oC above the melting point at 500 rpm to ~175 oC above the melting point at 2000 rpm). Fig. 10 shows the flow model of the molten polymer inside the threaded hole. The molten polymer moves upward after entering into the threaded hole from the periphery of the hole. However, upward movement of the molten polymer is restricted by the shoulder surface. As a result, the molten polymer moves horizontally and then continues its path downward. Therefore, a vortex structure is formed in the re-solidified polymer inside the hole center. By comparing Figs. 9-a to 9-c, it is evident that enhancement of the tool rotational speed reduced the area of the unmelted surface of the polymer. The higher was the temperature of the molten polymer, the more was the melting of the polymer surface. Figs. 9-d and 9-e show the fracture surfaces of the samples welded at 2000 rpm after crosstension and shear-tensile tests, respectively. It is observed in fracture surface of the shear-tensile sample (Fig. 9-e) that there is no unmelted surface at the rotational speed of 2000 rpm. Therefore, the obtained joint failed in crosstension test with the exit of the re-solidified polymer from the threaded hole rather than fracture from the interface of the composite sheet and re-solidified polymer inside the hole (Fig. 9-d). SEM images of the fracture surface of the joint processed at 500 rpm rotational speed clearly indicate the difference between the internal and marginal regions of the fracture surface at the hole base (Fig. 11). The internal region of the joint surface (area “A” in Fig. 11-a) showed a rough structure, while the structure of the marginal region was relatively smooth (area “B” in Fig. 11-a). The rough structure formed in area “A” may be the result of partial plastic deformation during tensile test [21]. In other words, the region in which the re-solidified polymer and the initial polymer were combined and joined to each other during welding undergoes plastic deformation before fracture. However, there was no evidence of plastic deformation in area “B”. It seems that this area with brittle fracture characteristics was formed as a result of crack rapid growth before final fracture. Figs. 11-d to 11-f show the SEM images from the fracture surface of the sample welded at 2000 rpm after shear-tensile test. As can be seen, the surface included ductile (region C) and brittle (region D) fracture, relatively similar to the fracture surface of the cross-tension sample welded at 500 rpm. However, locations of the brittle and ductile fracture regions in sheartensile test are different from those in cross-tension test, maybe due to difference in loading mode between two types of the tests. 9
3.5.
Hardness variation of re-solidified polymer
Since fracture in the samples welded at the tool rotational speed of 500 to 1500 rpm occurred from the melted and re-solidified polymer at the interface of the base sheets (as mentioned in section 3.3), the hardness of the resolidified polymer may play an important role in the mechanical strength of the joints. Fig. 12-a gives the hardness variation of the re-solidified polymer inside the hole versus tool rotational speed. The results showed that the hardness of re-solidified polymer inside the hole decreased slightly (about 4-6 in Shore D scale) compared to primary composite sheet. In order to investigate the reasons for this phenomenon, DSC analysis was carried out on the primary composite and re-solidified polymer inside the hole. Fig. 12-b presents DSC curve of the primary composite sheet and the effect of the tool rotational speed on the crystallinity percent of the re-solidified polymer. Given Fig. 12, by enhancement of the tool rotational speed, the hardness and crystallinity percent of the re-solidified polymer decreased. This may be related to molecular weight reduction of the resolidified polymer, since the molten polymer was affected by the frictional heat during the process [21,22]. The more was the tool rotational speed, the higher was the temperature of the molten polymer.
3.6.
Strength and fracture energy of joints
3.6.1.
Cross-Tension test
Fig. 13 shows the force-extension curves and the effect of tool rotational speed on the fracture force and fracture energy of the joints in cross-tension tests. As can be seen, the fracture force increased by enhancement of the tool rotational speed. In this joining process, two factors compete with each other in order to determine the fracture force by enhancement of the tool rotational speed: 1) Increase in fracture force due to the increase in the load bearing area caused by the reduction of the unmelted surface of the polymer (as mentioned in section 3.4). 2) Decrease in the fracture force due to the reduction of the hardness at the fracture location, i.e. re-solidified polymer (as mentioned in section 3.5). Results of cross-tension tests confirm that increase in the load bearing area overcomes hardness reduction of the resolidified polymer and thus, fracture force improves with enhancement of the tool rotational speed. During tensile test, the unmelted surface of the polymer at the base of the hole (which mentioned in Section 3.5) not only reduces 10
the load bearing area, but also acts as a stress concentration location and consequently decreases the joint strength. As mentioned before, the samples joined at the tool rotational speed of 500 to 1500 rpm failed from the interface of re-solidified polymer and primary composite sheet. By increasing the tool rotational speed, the polymer surface at the base of the hole was melted more and the joint strength improved. At the tool rotational speed of 2000 rpm, there was no unmelted surface or this surface was very small. Therefore, failure occurred by the exit of the re-solidified polymer from the threaded hole rather than fracture from the interface of the composite sheet and re-solidified polymer and the fracture load increased to higher values. It is worth mentioning that distribution of carbon fibers in the threaded hole can also be effective on the joint strength. However, as discussed in section 3.3, distribution of carbon fibers did not change with variation of the tool rotational speed. Therefore, from this point of view, enhancement of the tool rotational speed has no effect on the joint strength. Given the curves in Fig. 13-a, due to a decrease in the crack length (i.e. unmelted surface) at the interface of the primary composite sheet and re-solidified polymer, the extension of the joints at the maximum fracture load increased by enhancement of the tool rotational speed. Fig. 13-b shows fracture energy of the joints calculated by measuring the area under the load-extension curve until maximum fracture force obtained from cross-tension test. It is obvious that enhancement of the tool rotational speed led to increasing the fracture energy due to the simultaneous improvement of the fracture force and extension of the joints. 3.6.2.
Shear-Tensile test
The effect of tool rotational speed on the fracture force and energy in shear-tensile tests is shown in Fig. 14-a. All samples in this test failed from the interface of the primary composite sheet and the re-solidified polymer. According to the results, the relationship between the fracture force and energy with tool rotational speed in shear-tensile test is similar to that of in cross-tension test, i.e., by enhancement of the tool rotational speed, both fracture force and energy increased. This may be explained again by an increase in the load bearing area in the joints due to a decrease in the unmelted surface of the polymer at the base of the threaded hole. The maximum fracture load was ~350 N obtained at the tool rotational speed of 2000 rpm. If the joint area is considered as a circle with a diameter of 4 mm (base of the threaded hole), the maximum shear strength of the joint may be calculated as 28 MPa, i.e., ~80% of the composite sheet strength. 11
Fig. 14-b shows that the fracture energy in the cross-tension test was significantly higher than that in shear-tensile test. Although the fracture loads of the joints in cross-tension test were relatively lower due to opening-mode of fracture [23], extensions of the joints at the maximum fracture force in cross-tension tests (5-7 mm) were noticeably higher than that in shear-tensile test (less than 1 mm). The higher extension of the joints in cross-tension test was attributed to the elastic bending and deformation of the composite base sheet in the tensile direction during crosstension test. Since the fracture energy is directly related to the area under the force-extension curve up to maximum fracture load, fracture energy in the cross-tension test was obtained higher than that in shear-tensile test.
4. Conclusion In this research, a new refill friction spot welding process named Threaded Hole Friction Spot Welding (THFSW) was used to join AA5052 aluminum alloy and short-carbon-fiber-reinforced polypropylene composite sheets. The microstructure and mechanical properties of the joints were investigated and the following results were obtained: 1) The results indicated that THFSW was used successfully in order to join aluminum and polymer sheets and the hole refilled completely by the molten polymer at appropriate tool rotational speeds (higher than 1000 rpm). 2) A reaction layer composed mostly of Al, C and O was formed between aluminum and re-solidified polymer inside the hole. While enhancement of the tool rotational speed did not affect significantly the reaction layer thickness, the weight percent of Al in this layer increased. 3) Mechanical interlocks between threads and re-solidified polymer inside the hole and formation of the reaction layer between aluminum and polymer were recognized as two main bonding mechanisms in this process. Formation of a gap between the reaction layer and the re-solidified polymer inside the hole reduced the effectiveness of Van der Waal's bonds and reaction layer for the joint strength. 4) The samples joined at the rotational speed of 500, 1000 and 1500 rpm failed from the interface the composite sheet and the re-solidified polymer inside the hole. However, at the rotational speed of 2000 rpm, the joint failed by the exit of the re-solidified polymer from the threaded hole. The fracture surface was composed of two distinct regions of melted and unmelted polymer. The internal part of the melted region showed evidence of plastic deformation during a tensile test, while at the marginal region, brittle fracture occurred due to the rapid growth of the crack. 12
5) Two factors of load bearing area and the hardness of the fracture location competed with each other for determination of the fracture force. The strength and fracture energy of the joints improved by enhancement of the tool rotational speed due to a decrease in the unmelted surface of the polymer and increase in load bearing area. The maximum shear-tensile strength of the joints reached to ~80 percent of the composite base strength at the tool rotational speed of 2000 rpm. Fracture energy in cross-tension test was significantly higher than that in shear-tensile test. References [1] Abibe AB, Sônego M, Dos Santos JF, Canto LB, Amancio-Filho ST. On the feasibility of a friction-based staking joining method for polymer-metal hybrid structures. Materials & Design 2016; 92:632–642. [2] Seong MS, Kim TH, Nguyen KH, Kweon JH, Choi JH. A parametric study on the failure of bonded single-lap joints of carbon composite and aluminum. Composite Structures 2008;86:135-145. [3] Goushegir SM, Dos Santos JF, Amancio-Filho ST. Friction spot joining of aluminum AA2024/carbon-fiber reinforced poly (phenylene sulfide) composite single lap joints: microstructure and mechanical performance. Materials & Design 2014;54:196–206. [4] Teixeira de Freitas S, Sinke J. Failure analysis of adhesively-bonded metal-skin-to-composite-stiffener: Effect of temperature and cyclic loading. Composite Structures 2017;166:27-37. [5] Kang SG, Kim MG, Kim CG. Evaluation of cryogenic performance of adhesives using composite–aluminum double-lap joints. Composite Structures 2007;78:440-446. [6] Lambiase F, Ko D-C. Two-steps clinching of Aluminum and Carbon Fiber Reinforced Polymer Sheets. Composite Structures 2016, doi: http://dx.doi.org/10.1016/j.compstruct.2016.12.072. [7] Amancio Filho ST, Dos Santos JF. Joining of polymers and polymer–metal hybrid structures: recent developments and trends. Polymer Engineering & Science 2009;49(8):1461–1476. [8] Abibe AB, Amancio-Filho ST, Dos Santos JF, Hage E. Mechanical and failure behaviour of hybrid polymermetal staked joints. Materials & Design 2013;46:338–347. [9] Schilling C, Dos Santos JF. Method and device for joining at least two adjoining work pieces by friction welding. US Patent No. 6722556 B2, 2004. [10] Martinsen K, Hu SJ, Carlson BE. Joining of dissimilar materials. CIRP Annals- Manufacturing Technology 13
2015;64(2):679–699. [11] Esteves JV, Goushegir SM, Dos Santos JF, Canto LB, Hage E, Amancio-Filho ST. Friction spot joining of aluminum AA6181-T4 and carbon fiber-reinforced poly(phenylene sulfide): Effects of process parameters on the microstructure and mechanical strength. Materials & Design 2015;66:437–445. [12] Ratanathavorn W. Hybrid joining of aluminum to thermoplastics with friction stir welding. MSc Thesis, Department of Materials Science and Engineering, KTH-Royal Institute of Technology, Stockholm, 2012. [13] André NM, Goushegir SM, Dos Santos JF, Canto LB, Amancio-Filho ST. Friction Spot Joining of aluminum alloy 2024-T3 and carbon-fiber-reinforced poly (phenylene sulfide) laminate with additional PPS film interlayer: Microstructure, mechanical strength and failure mechanisms. Composites Part B: Engineering 2016;94:197–208. [14] ASM International Handbook Committee, Metals Handbook: Vol. 2, Properties and selection–nonferrous alloys and pure metals, Tenth Edition, pp. 3-215, New York: ASM International Handbook, 1990. [15] C. Hindle. Polypropylene (PP), Accessed on 14 February 2017; http://www.bpf.co.uk/plastipedia/polymers/pp.aspx. [16] Shahmiri H, Movahedi M, Kokabi AH. Friction stir lap joining of aluminium alloy to polypropylene sheets. Science and Technology of Welding and Joining 2016;22(2):120–126. [17] Khodabakhshi F, Haghshenas M, Chen J, Shalchi Amirkhiz B, Li J, Gerlich AP. Bonding mechanism and interface characterisation during dissimilar friction stir welding of an aluminium/polymer bi-material joint. Science and Technology of Welding and Joining 2016, Article in press. [18] Mahajan C, Cormier D. 3D printing of carbon fiber composites with preferentially aligned fibers. Proceedings of the 2015 Industrial and Systems Engineering Research Conference, Nashville, 30 May – 2 June 2015. p. 2953-2962. [19] Hannesschläger C, Revol V, Plank B, Salaberger D, Kastner J. Fibre structure characterisation of injection moulded short fibre-reinforced polymers by X-ray scatter dark field tomography. Case Studies in Nondestructive Testing and Evaluation 2015;3:34-41. [20] Huang Y, Wang J, Wan L, Meng X, Liu H, Li H. Self-riveting friction stir lap welding of aluminum alloy to steel. Materials Letters 2016;185:181-184. [21] Nagatsuka K, Yoshida S, Tsuchiya A, Nakata K. Direct joining of carbon-fiber–reinforced plastic to an 14
aluminum alloy using friction lap joining. Composites Part B: Engineering 2015;73:82–88,. [22] Kiss Z, Czigány T. Applicability of friction stir welding in polymeric materials. Periodica polytechnica, Mechanical Engineering 2007;51(1):15–18. [23] Hertzberg RW. Deformation and fracture mechanics of engineering materials, New York: Wiley, 1989;261-288.
15
Figure Captions Fig. 1- Distribution of carbon fibers in PP-Z30S matrix. Fig. 2- Schematic of the different steps for the joining process. Fig. 3- Schematic of tensile test specimens and their dimensions: a) Shear-Tensile specimen and b) Cross-Tension specimen. Fig. 4- Joint surface appearance at different rotational speeds: a) 500 rpm, b) 1000 rpm, c) 1500 rpm, d) 2000 rpm. Fig. 5- Stereograph images from cross-section of the joints at the rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm and d) 2000 rpm. Fig. 6- The effect of tool rotational speed on the maximum temperature obtained at different distances from the joint center. Fig. 7- a) SEM image from a typical cross-section of the joint, b) Reaction layer and the gap formed between aluminum and re-solidified composite in the threaded hole at the tool rotational speed of 500 rpm (magnified of area “A”) and c) Reaction layer and the gap refilled by epoxy resin at the tool rotational speed of 2000 rpm. Fig. 8- Carbon fibers distribution in the re-solidified polymer matrix at different rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm and d) 2000 rpm. Fig. 9- Fracture surface of the polymeric composite sheet; variation in the area of the unmelted surface and fracture location at different rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm, d) 2000 rpm (cross-tension) and e) 2000 rpm (shear-tensile). Fig. 10- Flow model of the molten polymer inside the hole. Fig. 11- SEM image from fracture surfaces: a) two different areas on the fracture surface of the sample welded at 500 rpm, b) ductile fracture surface (area A), c) brittle fracture surface (area B), d) two different areas on the fracture surface of the sample welded at 2000 rpm, e) ductile fracture surface (area C) and f) brittle fracture surface (area D). Fig. 12-a) Hardness and crystallinity percent of the primary and re-solidified composite inside the hole versus the tool rotational speed and b) DSC curve of the primary composite sheet. Fig. 13-a) Force-extension curves of cross-tension tests related to the specimens with maximum fracture force at different tool rotational speeds and b) effect of the tool rotational speed on the average cross-tension force and fracture energy of the joints. Fig. 14-a) Effect of tool rotational speed on the average shear-tensile load and fracture energy of the joints and b) comparison of fracture energy of the joints in shear-tensile and cross-tension tests.
16
Tables Table 1- Chemical composition, mechanical and physical properties of AA5052 used in this research Composition (wt%)
Mechanical and physical properties
Al
Base
Yield Strength, MPa
214
Mg
2.34
UTS, MPa
265
Cr
0.20
Elongation, %
13.5
Fe
0.23
Thermal Conductivity, W/m.K
138 [14]
Si
0.06
Melting Point, oC
644 [14]
Mn
0.006
Coefficient of Thermal Expansion, 10-6/K
23.8 [14]
Table 2-Chemical composition and mechanical properties of PP-SCF used in this research Composition of PP-SCF (wt%) PP-Z30S 92 PP-g-MA
5
SCF
3
Mechanical properties of PP-SCF Hardness, Shore D 72 UTS, MPa
35
Table 3-Physical properties of polypropylene Z30S [15] Properties Density Melt Flow Index (MFI) Coefficient of Thermal Expansion (CTE) Thermal Conductivity (Solid) Thermal Conductivity (Molten) Softening Point
Unit g/cm3 Dg/min
Amount 0.9 25
10-6/K
100-150
W/m.K W/m.K o C
0.17-0.22 0.16 165
Table 4- Effect of tool rotational speed on the composition of the reaction layer Elements Al C O
tool rotational speed (rpm) 500 2000 21 38 37 35 33 22
17
Fig. 1
Fig. 2 18
Fig. 3
Fig. 4
19
Fig. 5
Fig. 6
20
Fig. 7
21
Fig. 8
22
Fig. 9
Fig. 10
23
Fig. 11
Fig. 12
24
Fig. 13
25
a)
b)
Fig. 14
26
S0263-8223(17)30547-0 http://dx.doi.org/10.1016/j.compstruct.2017.04.053 COST 8484
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
16 February 2017 28 March 2017 20 April 2017
Please cite this article as: Pabandi, H.K., Movahedi, M., Kokabi, A.H., A New Refill Friction Spot Welding Process for Aluminum/Polymer Composite Hybrid Structures, Composite Structures (2017), doi: http://dx.doi.org/10.1016/ j.compstruct.2017.04.053
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A New Refill Friction Spot Welding Process for Aluminum/Polymer Composite Hybrid Structures Hossein Karami Pabandi, Mojtaba Movahedi*, Amir Hossein Kokabi
Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran * Corresponding author: Tel: +98 21 66165224; Fax: +98 21 66005717; E-mail: [email protected] Abstract: A new refill friction spot welding process called Threaded Hole Friction Spot Welding (THFSW) was introduced to join AA5052 aluminum to short-carbon-fiber-reinforced polypropylene (PP-SCF) composite sheets. The process was based on filling of the pre-threaded hole by melted and re-solidified polymer. The results showed that THFSW was successful to join aluminum to polymer sheets and the hole was completely filled with melted polymer. Formation of a reaction layer composed mostly of Al, C and O as well as interlocking between the threaded hole and the resolidified polymer were recognized as main bonding mechanisms. Maximum shear-tensile strength of the joints reached to ~80 percent of the composite base strength. Moreover, Mechanical strength and fracture energy of the joints increased with enhancement of tool rotational speed. Variation of the joint strength was explored in light of the fracture surface features as well as crystallinity percent of the re-solidified polymer inside the hole. Keywords: Friction spot welding; Refill; Polymer; Aluminum; Joint strength; Fracture energy
1
1. Introduction Successful joint between polymers and metallic alloys is a necessity for the production of light weight polymer/metal hybrid structures in various industries such as automotive, aeronautics and shipbuilding industries [13]. Together with traditional methods such as adhesive bonding [4,5] and mechanical fastening [6], new joining techniques have been patented and developed in recent years for “spot” joining of polymers to metallic alloys. Ultrasonic staking [7] and Injection Clinching Joining (ICJ) [1,8] are two methods with relatively the same principles used for spot joining of polymers and metals. Abibe et al. [1] compared ultrasonic staking and ICJ methods for joining polyetherimide and AA6082 aluminum alloy. Their results indicated that both processes show similar mechanical properties. However, ICJ had a higher strength-to-weight ratio due to the presence of hole formed in the stake. Moreover, ICJ showed longer joining time in comparison to ultrasonic staking. Abibe et al. [8] also employed ICJ process to join short-glass-fiber-reinforced polyamide and AA2024 aluminum alloy. They reported that volume of the rivet head in contact with the tool system and efficiency of cavity filling are key factors controlling the strength of the joints. Welding processes have also been used for spot joining of polymers to metals. Friction Stir Spot Welding (FSSW) is a solid state process with high potential for these types of joints. However, because of leaving an exit-hole at the center of the nugget, the Friction Spot Joining (FSpJ) process was developed by GKSS of Germany in order to refill the formed exit-hole [9]. FSpJ has been used successfully for joining of metals to glass and carbon-fiber-reinforced composites [3,10-13]. Goushegir et al. [3] utilized FSpJ process for joining AA2024 aluminum and reinforced poly (phenylene sulfide) with 50 vol% carbon fibers (CF-PPS). They concluded that by enhancement of the tool plunge depth, the shear fracture load of the joint as well as the extension to fracture improved due to increase in the joint area at the sheets interface. Esteves et al. [11] employed FSpJ process for joining AA6181-T4 aluminum and carbon-fiber-reinforced poly (phenylene sulfide) composite. Their results showed that tool rotational speed was the most effective parameter on the joint strength. In addition, they reported that at high tool rotational speed (1600 rpm), reduction of the metal viscosity led to decrease in the heat generated during the process. Therefore, joint area between the aluminum and polymer sheets at the interface was reduced. In the present work, a new friction spot welding process called “Threaded Hole Friction Spot Welding (THFSW)” was used to join AA5052 aluminum alloy and short-carbon-fiber-reinforced polypropylene composite sheets. In this 2
joining process, a pre-threaded hole in the aluminum sheet is filled by the melted and re-solidified polymer during the process. Thus, no hole remains at the joint area after completing the process. In comparison to ultrasonic staking and ICJ, THFSW does not require an initial polymeric stud on the polymer sheet; however, the stud is formed in the hole by molten polymer during the process. On the other hand, the same as the ultrasonic staking and ICJ, a hole must be created in the aluminum sheet before joining process. Furthermore, in FSSW a keyhole remains at the joint center due to pin penetration, while the suggested joining process is able to refill the threaded hole during the process. In FSpJ, a relatively complex mechanical and electronic system is required for joining. Although, in THFSW, the joint is produced using a simple cylindrical tool without pin. After joining process, mechanisms of bonding, as well as the effects of the tool rotational speed on the mechanical behavior of the joints, were studied.
2. Materials and experimental procedures
2.1.
Materials
As-received AA5052 aluminum alloy with a thickness of 2 mm was used as the metallic part of the joint. Chemical composition, mechanical and physical properties of AA5052 aluminum alloy used in this study are presented in Table 1. Polymeric sheets used in this research were short-carbon-fiber-reinforced polypropylene Z30S composite (PP-SCF) with 2 mm thickness. Dimensions of carbon fibers were ~3 mm in length and ~17 µm in diameter. Chemical composition and mechanical properties of PP-SCF composite and physical properties of polypropylene Z30S are listed in Tables 2 and 3, respectively. Fig. 1 shows distribution of carbon fibers in Z30S polypropylene matrix.
2.2.
Joining process
Aluminum sheet with a threaded hole and composite sheet, both with demotions of 70×30 mm2 were used for joining process. To produce the threaded hole, aluminum sheet was drilled with 4 mm diameter and then threaded by M4 reamer (with the aim of increasing the joint strength through formation of mechanical interlocks). A cylindrical tool from heat treated H13 steel composed of a shoulder with a diameter of 20 mm was employed as the welding tool. Joining process was performed in lap configuration with an overlap of 30 mm, while aluminum (as the upper sheet) was in contact with the tool shoulder. Tool rotational speeds of 500, 1000, 1500 and 2000 rpm and dwell time of 5 s were selected for joining. 3
Schematic of different steps of new joining process introduced in this research (THFSW) is illustrated in Fig. 2. At the first step of the process, rotating tool is located coaxially with threaded hole on top of the aluminum sheet surface as shown in Fig. 2-a. In the next step, the rotating tool is moved downward and the shoulder penetrates ~0.3 mm in the aluminum surface (Fig. 2-b). As a result of contact between the rotating tool and aluminum sheet, frictional heat is generated. This heat reaches to the aluminum/polymer interface via conduction and melts the polymer at the interface. However, since the polymer is a heat insulation material, melting of the polymer is limited to the surface of the composite sheet at the interface. Then, by the downward axial pressure of the tool, melted polymer flows towards the periphery of the joint location at the interface. The molten polymer around the hole in aluminum sheet fills the hole and mechanical interlocks are formed between polymer and aluminum due to the existence of threads inside the hole (Figs. 2-c and d). After 5 s dwell time, the tool is retracted upward (Fig. 2-e), and the joint is completed with the solidification of the molten polymer inside the hole (Fig. 2-f). It should be noted that by enhancement of the hole diameter, the required volume of the molten polymer for filling the hole increases. On the other hand, raising the shoulder plunging depth leads to increase in the volume of the molten polymer flowed from the interface of the base sheets toward the threaded hole. Therefore, larger diameter of the threaded hole needs higher shoulder plunging depth.
2.3.
Temperature measurement
In order to measure temperature variations during the joining process, K-type thermocouples with 0.5 mm diameter were embedded into the interface of sheets at a distance of 10, 20 and 30 mm from the joint center. The maximum temperature obtained during the process was recorded at each rotational speed.
2.4.
Microscopic observations
The stereomicroscope was used for observation of cross-section and fracture surface of the joints. Moreover, for investigation of the joints microstructure and the fracture surfaces, Field Emission Scanning Electron Microscope (FE-SEM) equipped with Energy Dispersive Spectroscopy (EDS) was used. For increasing the quality of SEM images, a gold layer was coated on the samples surfaces.
2.5.
Differential scanning calorimetry analysis
4
The crystallinity of PP-SCF composite and re-solidified polymer inside the hole was evaluated by differential scanning calorimetry (DSC) analysis at the temperature of 250 ºC for 5 min. The measurements were performed on a 8 mg sample under N2 atmosphere. The specimens were heated and cooled at a constant rate of 10 ºC/min. The exothermic crystallization peak was recorded as a function of temperature.
2.6.
Mechanical properties
For evaluation of the mechanical behavior of the joints, shear-tensile and cross-tension tests were carried out with the cross head speed of 0.5 mm/min. Dimensions of the sheets and the overlap area were selected as 70×30 mm2 and 30×30 mm2, respectively (Fig. 3). The threaded hole was located at the center of the overlap area. At each set of joining parameters, three samples were tested for tensile tests and the average of maximum forces were reported. The hardness of the composite sheet and re-solidified polymer inside the hole was measured in Shore D scale to investigate the effect of temperature changes during the joining process on the hardness and joint strength.
3. Results and discussion
3.1.
Macro-profile of joints
The surface appearance of the joints for different rotational speeds is presented in Fig. 4. The re-solidified polymer in the hole and the surface of aluminum affected by the rotating tool are shown in the figure. As can be seen, surface appearance of the joints is smooth especially at higher rotational speeds due to more heat generation and improved flow of the aluminum under the tool shoulder. Fig. 5 shows the cross-section of the joints and various macrostructural zones including base materials, threaded hole and re-solidified polymer inside the threaded hole. It can be seen that the rotational speed of the tool affected filling behavior of the threaded hole by the molten polymer. The higher was the rotational speed, the more was the frictional heat generated during welding. The maximum temperature experienced at different distances from the joint center (R) are shown in Fig. 6. It was observed that by enhancement of the tool rotational speed, the maximum temperature produced during the process increased. Moreover, the area experiencing temperatures higher than the melting point of the polymer was broader. Therefore, the temperature and volume of the molten polymer at the interface of the base sheets increased. At the rotational speed of 500 rpm, the volume of the molten polymer is not enough to fill completely the hole. Fig. 5-a shows porosity and unfilled zone at the joint center and separation occurred at the interface of sheets due to lack 5
of adhesion force in this area. By enhancement of the tool rotational speed, more heat was generated and as a result, more polymers melted and entered into the hole. In the cross-section of the joints produced at 1000, 1500 and 2000 rpm, all areas of the hole were filled completely with molten polymer and mechanical interlocks were formed between the threads of the hole and re-solidified polymer. Furthermore, due to the melting of polymer at the interface of the aluminum and composite sheets, an adhesion force was also formed at the periphery of the hole [11]. Referring to Fig. 5-d, as a result of high frictional heat at a rotational speed of 2000 rpm, the excess melted polymer re-solidified as a convex surface on the threaded hole. This convexity was formed in effect of the pressure of holding clamps after retracting of the welding tool.
3.2.
Reaction layer formation and joining mechanisms
Fig. 7-a shows SEM image from a typical cross-section of the joint. Magnified SEM image of the interface between aluminum and re-solidified polymer inside the hole (area “A” in Fig. 7-a) is given in Fig. 7-b. It is obvious that a reaction layer was formed with a thickness of approximately 15 µm at the interface. The term of “reaction layer” refers to a layer formed between the aluminum and the molten polymer and atoms of both materials exist in this layer. It seems that this layer has been formed by erosion mechanism (a well-known phenomenon in the brazing and soldering) [16] in which due to the collision between molten polymer and the hole wall at high temperatures, aluminum atoms were separated from the aluminum surface and entered into the molten polymer and formed the reaction layer. Comparison between Figs. 7-b and 7-c indicate that the thickness of the reaction layer did not change significantly by variation of the tool rotational speed. However, the chemical composition of the layer was affected by the tool rotation speed. Table 4 shows EDS analysis of the reaction layer for samples welded at the tool rotational speed of 500 rpm (minimum heat input) and 2000 rpm (maximum heat input). Enhancement of the tool rotational speed led to increase in weight percent of Al and slightly decrease in weight percent of C. In the explanation of this phenomenon, it can be mentioned that the higher was the temperature of the molten polymer entered into the hole, the more was the aluminum surface erosion inside the hole and consequently the presence of aluminum atoms in the reaction layer. Moreover, the presence of oxygen in the reaction layer was probably attributed to degradation of the molten polymer in effect of reaction with oxygen [2]. Formation of a reaction layer with 8 wt% of Al was also mentioned by Shahmiri et al. [16] in FSW of Polypropylene C30S to AA5052 aluminum alloy. 6
In addition to the reaction layer, there was also a gap between the reaction layer and re-solidified polymer some regions inside the hole (Figs. 7-b and 7-c). It seems that this gap was formed during the cooling process of the joint due to the large difference between the coefficients of thermal expansion (CTE) of the base materials [1]. The CTE of aluminum and polypropylene are 23.8×10-6 K-1 and 100-150×10-6 K-1, respectively [1,15]. Compared to aluminum, higher CTE of polypropylene resulted in more shrinkage of the molten polymer during the cooling cycle of the weld and thus, a gap was formed between the polymer and the reaction layer. On the other hand, shrinkage of the molten polymer during solidification depended on the melt temperature. The higher was the temperature of the molten polymer, the more was its shrinkage during cooling. Since the temperature of the molten polymer at the rotational speed of 2000 rpm was higher (Fig. 6), a wider gap was expected in this tool rotational speed. As can be seen in Fig. 7-c, the wide gap was filled with epoxy resin (cold mounting material) in the preparation stage of the sample for microstructural studies. From what mentioned above, the obtained joint between aluminum and composite sheets using THFSW process is the result of two phenomena: i) Mechanical interlocks between re-solidified polymer and threads inside the hole, ii) Formation of reaction layer between aluminum and polymer. It is crucial of importance that the results of a research carried out by Khodabakhshi et al. [17] on FSW of aluminum to polymer sheets proved the formation of Van der Waal's bonds between the atoms of aluminum oxide layer on the surface and polymer. However, formation of a gap between the reaction layer and the re-solidified polymer inside the hole reduces the effectiveness of Van der Waal's bonds and reaction layer for the joint strength. This mechanism can be helpful for joint strength at the interface of the base sheets and periphery of the hole (a region with adhesion force in Fig. 5).
3.3.
Distribution of carbon fibers
Fig. 8 presents the carbon fibers distribution in the re-solidified polymer matrix inside the hole. As can be seen, variation of the tool rotational speed had no effect on the fibers distribution in the re-solidified polymer matrix. However, comparison between Fig. 1 (base composite sheet) and Fig. 8 shows that in general, the carbon fibers in the re-solidified polymer were shorter than those in the base composite sheet. This may be due to the fragmentation 7
of the carbon fibers within the molten polymer in the effect of their flowing from the interface of the base sheets toward the threaded hole during the welding process. On the other hand, the carbon fibers in the base composite sheet had a relatively preferred orientation which is a well-known phenomenon in the injected polymer matrix composites [18,19]. However, carbon fibers in the re-solidified polymer show a random orientation.
3.4.
Fracture surface analysis
Fig. 9 shows the fracture surfaces of the samples after cross-tension and shear-tensile tests. In the samples welded at the tool rotational speeds of 500, 1000 and 1500 rpm, fracture occurred at the interface of the composite sheet and re-solidified polymer inside the hole. Indeed, fracture surface of the joint on the composite sheet was a circle in which unmelted polymer surface at the center of this circle was surrounded by the melted polymer surface. The melted surface was formed due to entry of the melted polymer from the sheets interface toward inside the hole resulting in surface melting of the primary polymer at the periphery of the hole and combination of the two mentioned melts with each other. In other words, the surface of the composite sheet that is at the base of the hole had no direct contact with the aluminum sheet. Therefore, an increase in the temperature of the aluminum sheet as a result of the frictional heat did not directly melt the polymer surface at this region. The existence of the unmelted surface in the center of the circular base of the threaded hole at the interface of the sheets means that re-solidified polymer in the hole was not joined to the primary polymer at this region. Therefore, if the re-solidified polymer inside the hole is considered as a small cylinder with a diameter of 4 mm and a height equal to the thickness of aluminum sheet (2 mm), there was a crack in the base of this cylinder (interface of the composite sheet and resolidified polymer) due to lack of melting of the composite sheet. Huang et al. [20] utilized self-riveting friction stir lap welding for joining of AA6082 to steel with filing the prefabricated holes in the bottom steel sheet by plasticized aluminum. They reported that downward flow of the plasticized aluminum occurred because of two origins: i) with progress of the welding tool, aluminum alloy ahead of the pin is plasticized and then, threads of the rotating pin pushes the plasticized metal downwards; ii) imposed pressure by the pin bottom extrudes the plasticized aluminum alloy into the prefabricated hole. Since the tool used for welding in the present work is pin-less, there is no flow of the molten polymer by the threads of the tool pin. Therefore, the molten polymer formed at the interface of the base sheets flows upward with back extrusion into the threaded hole just in the effect of the downward pressure of the tool. It should be mentioned that the molten polymer has high fluidity and can easily flow and fill the threads of the hole with the pressure applied by the welding tool. High fluidity of the molten polymer is caused by the elevated 8
temperature experienced by it during the welding process. Fig. 6 confirms that the temperature of the molten polymer at the distance of 10 mm from the joint center is significantly higher than the melting point of polypropylene (from ~30 oC above the melting point at 500 rpm to ~175 oC above the melting point at 2000 rpm). Fig. 10 shows the flow model of the molten polymer inside the threaded hole. The molten polymer moves upward after entering into the threaded hole from the periphery of the hole. However, upward movement of the molten polymer is restricted by the shoulder surface. As a result, the molten polymer moves horizontally and then continues its path downward. Therefore, a vortex structure is formed in the re-solidified polymer inside the hole center. By comparing Figs. 9-a to 9-c, it is evident that enhancement of the tool rotational speed reduced the area of the unmelted surface of the polymer. The higher was the temperature of the molten polymer, the more was the melting of the polymer surface. Figs. 9-d and 9-e show the fracture surfaces of the samples welded at 2000 rpm after crosstension and shear-tensile tests, respectively. It is observed in fracture surface of the shear-tensile sample (Fig. 9-e) that there is no unmelted surface at the rotational speed of 2000 rpm. Therefore, the obtained joint failed in crosstension test with the exit of the re-solidified polymer from the threaded hole rather than fracture from the interface of the composite sheet and re-solidified polymer inside the hole (Fig. 9-d). SEM images of the fracture surface of the joint processed at 500 rpm rotational speed clearly indicate the difference between the internal and marginal regions of the fracture surface at the hole base (Fig. 11). The internal region of the joint surface (area “A” in Fig. 11-a) showed a rough structure, while the structure of the marginal region was relatively smooth (area “B” in Fig. 11-a). The rough structure formed in area “A” may be the result of partial plastic deformation during tensile test [21]. In other words, the region in which the re-solidified polymer and the initial polymer were combined and joined to each other during welding undergoes plastic deformation before fracture. However, there was no evidence of plastic deformation in area “B”. It seems that this area with brittle fracture characteristics was formed as a result of crack rapid growth before final fracture. Figs. 11-d to 11-f show the SEM images from the fracture surface of the sample welded at 2000 rpm after shear-tensile test. As can be seen, the surface included ductile (region C) and brittle (region D) fracture, relatively similar to the fracture surface of the cross-tension sample welded at 500 rpm. However, locations of the brittle and ductile fracture regions in sheartensile test are different from those in cross-tension test, maybe due to difference in loading mode between two types of the tests. 9
3.5.
Hardness variation of re-solidified polymer
Since fracture in the samples welded at the tool rotational speed of 500 to 1500 rpm occurred from the melted and re-solidified polymer at the interface of the base sheets (as mentioned in section 3.3), the hardness of the resolidified polymer may play an important role in the mechanical strength of the joints. Fig. 12-a gives the hardness variation of the re-solidified polymer inside the hole versus tool rotational speed. The results showed that the hardness of re-solidified polymer inside the hole decreased slightly (about 4-6 in Shore D scale) compared to primary composite sheet. In order to investigate the reasons for this phenomenon, DSC analysis was carried out on the primary composite and re-solidified polymer inside the hole. Fig. 12-b presents DSC curve of the primary composite sheet and the effect of the tool rotational speed on the crystallinity percent of the re-solidified polymer. Given Fig. 12, by enhancement of the tool rotational speed, the hardness and crystallinity percent of the re-solidified polymer decreased. This may be related to molecular weight reduction of the resolidified polymer, since the molten polymer was affected by the frictional heat during the process [21,22]. The more was the tool rotational speed, the higher was the temperature of the molten polymer.
3.6.
Strength and fracture energy of joints
3.6.1.
Cross-Tension test
Fig. 13 shows the force-extension curves and the effect of tool rotational speed on the fracture force and fracture energy of the joints in cross-tension tests. As can be seen, the fracture force increased by enhancement of the tool rotational speed. In this joining process, two factors compete with each other in order to determine the fracture force by enhancement of the tool rotational speed: 1) Increase in fracture force due to the increase in the load bearing area caused by the reduction of the unmelted surface of the polymer (as mentioned in section 3.4). 2) Decrease in the fracture force due to the reduction of the hardness at the fracture location, i.e. re-solidified polymer (as mentioned in section 3.5). Results of cross-tension tests confirm that increase in the load bearing area overcomes hardness reduction of the resolidified polymer and thus, fracture force improves with enhancement of the tool rotational speed. During tensile test, the unmelted surface of the polymer at the base of the hole (which mentioned in Section 3.5) not only reduces 10
the load bearing area, but also acts as a stress concentration location and consequently decreases the joint strength. As mentioned before, the samples joined at the tool rotational speed of 500 to 1500 rpm failed from the interface of re-solidified polymer and primary composite sheet. By increasing the tool rotational speed, the polymer surface at the base of the hole was melted more and the joint strength improved. At the tool rotational speed of 2000 rpm, there was no unmelted surface or this surface was very small. Therefore, failure occurred by the exit of the re-solidified polymer from the threaded hole rather than fracture from the interface of the composite sheet and re-solidified polymer and the fracture load increased to higher values. It is worth mentioning that distribution of carbon fibers in the threaded hole can also be effective on the joint strength. However, as discussed in section 3.3, distribution of carbon fibers did not change with variation of the tool rotational speed. Therefore, from this point of view, enhancement of the tool rotational speed has no effect on the joint strength. Given the curves in Fig. 13-a, due to a decrease in the crack length (i.e. unmelted surface) at the interface of the primary composite sheet and re-solidified polymer, the extension of the joints at the maximum fracture load increased by enhancement of the tool rotational speed. Fig. 13-b shows fracture energy of the joints calculated by measuring the area under the load-extension curve until maximum fracture force obtained from cross-tension test. It is obvious that enhancement of the tool rotational speed led to increasing the fracture energy due to the simultaneous improvement of the fracture force and extension of the joints. 3.6.2.
Shear-Tensile test
The effect of tool rotational speed on the fracture force and energy in shear-tensile tests is shown in Fig. 14-a. All samples in this test failed from the interface of the primary composite sheet and the re-solidified polymer. According to the results, the relationship between the fracture force and energy with tool rotational speed in shear-tensile test is similar to that of in cross-tension test, i.e., by enhancement of the tool rotational speed, both fracture force and energy increased. This may be explained again by an increase in the load bearing area in the joints due to a decrease in the unmelted surface of the polymer at the base of the threaded hole. The maximum fracture load was ~350 N obtained at the tool rotational speed of 2000 rpm. If the joint area is considered as a circle with a diameter of 4 mm (base of the threaded hole), the maximum shear strength of the joint may be calculated as 28 MPa, i.e., ~80% of the composite sheet strength. 11
Fig. 14-b shows that the fracture energy in the cross-tension test was significantly higher than that in shear-tensile test. Although the fracture loads of the joints in cross-tension test were relatively lower due to opening-mode of fracture [23], extensions of the joints at the maximum fracture force in cross-tension tests (5-7 mm) were noticeably higher than that in shear-tensile test (less than 1 mm). The higher extension of the joints in cross-tension test was attributed to the elastic bending and deformation of the composite base sheet in the tensile direction during crosstension test. Since the fracture energy is directly related to the area under the force-extension curve up to maximum fracture load, fracture energy in the cross-tension test was obtained higher than that in shear-tensile test.
4. Conclusion In this research, a new refill friction spot welding process named Threaded Hole Friction Spot Welding (THFSW) was used to join AA5052 aluminum alloy and short-carbon-fiber-reinforced polypropylene composite sheets. The microstructure and mechanical properties of the joints were investigated and the following results were obtained: 1) The results indicated that THFSW was used successfully in order to join aluminum and polymer sheets and the hole refilled completely by the molten polymer at appropriate tool rotational speeds (higher than 1000 rpm). 2) A reaction layer composed mostly of Al, C and O was formed between aluminum and re-solidified polymer inside the hole. While enhancement of the tool rotational speed did not affect significantly the reaction layer thickness, the weight percent of Al in this layer increased. 3) Mechanical interlocks between threads and re-solidified polymer inside the hole and formation of the reaction layer between aluminum and polymer were recognized as two main bonding mechanisms in this process. Formation of a gap between the reaction layer and the re-solidified polymer inside the hole reduced the effectiveness of Van der Waal's bonds and reaction layer for the joint strength. 4) The samples joined at the rotational speed of 500, 1000 and 1500 rpm failed from the interface the composite sheet and the re-solidified polymer inside the hole. However, at the rotational speed of 2000 rpm, the joint failed by the exit of the re-solidified polymer from the threaded hole. The fracture surface was composed of two distinct regions of melted and unmelted polymer. The internal part of the melted region showed evidence of plastic deformation during a tensile test, while at the marginal region, brittle fracture occurred due to the rapid growth of the crack. 12
5) Two factors of load bearing area and the hardness of the fracture location competed with each other for determination of the fracture force. The strength and fracture energy of the joints improved by enhancement of the tool rotational speed due to a decrease in the unmelted surface of the polymer and increase in load bearing area. The maximum shear-tensile strength of the joints reached to ~80 percent of the composite base strength at the tool rotational speed of 2000 rpm. Fracture energy in cross-tension test was significantly higher than that in shear-tensile test. References [1] Abibe AB, Sônego M, Dos Santos JF, Canto LB, Amancio-Filho ST. On the feasibility of a friction-based staking joining method for polymer-metal hybrid structures. Materials & Design 2016; 92:632–642. [2] Seong MS, Kim TH, Nguyen KH, Kweon JH, Choi JH. A parametric study on the failure of bonded single-lap joints of carbon composite and aluminum. Composite Structures 2008;86:135-145. [3] Goushegir SM, Dos Santos JF, Amancio-Filho ST. Friction spot joining of aluminum AA2024/carbon-fiber reinforced poly (phenylene sulfide) composite single lap joints: microstructure and mechanical performance. Materials & Design 2014;54:196–206. [4] Teixeira de Freitas S, Sinke J. Failure analysis of adhesively-bonded metal-skin-to-composite-stiffener: Effect of temperature and cyclic loading. Composite Structures 2017;166:27-37. [5] Kang SG, Kim MG, Kim CG. Evaluation of cryogenic performance of adhesives using composite–aluminum double-lap joints. Composite Structures 2007;78:440-446. [6] Lambiase F, Ko D-C. Two-steps clinching of Aluminum and Carbon Fiber Reinforced Polymer Sheets. Composite Structures 2016, doi: http://dx.doi.org/10.1016/j.compstruct.2016.12.072. [7] Amancio Filho ST, Dos Santos JF. Joining of polymers and polymer–metal hybrid structures: recent developments and trends. Polymer Engineering & Science 2009;49(8):1461–1476. [8] Abibe AB, Amancio-Filho ST, Dos Santos JF, Hage E. Mechanical and failure behaviour of hybrid polymermetal staked joints. Materials & Design 2013;46:338–347. [9] Schilling C, Dos Santos JF. Method and device for joining at least two adjoining work pieces by friction welding. US Patent No. 6722556 B2, 2004. [10] Martinsen K, Hu SJ, Carlson BE. Joining of dissimilar materials. CIRP Annals- Manufacturing Technology 13
2015;64(2):679–699. [11] Esteves JV, Goushegir SM, Dos Santos JF, Canto LB, Hage E, Amancio-Filho ST. Friction spot joining of aluminum AA6181-T4 and carbon fiber-reinforced poly(phenylene sulfide): Effects of process parameters on the microstructure and mechanical strength. Materials & Design 2015;66:437–445. [12] Ratanathavorn W. Hybrid joining of aluminum to thermoplastics with friction stir welding. MSc Thesis, Department of Materials Science and Engineering, KTH-Royal Institute of Technology, Stockholm, 2012. [13] André NM, Goushegir SM, Dos Santos JF, Canto LB, Amancio-Filho ST. Friction Spot Joining of aluminum alloy 2024-T3 and carbon-fiber-reinforced poly (phenylene sulfide) laminate with additional PPS film interlayer: Microstructure, mechanical strength and failure mechanisms. Composites Part B: Engineering 2016;94:197–208. [14] ASM International Handbook Committee, Metals Handbook: Vol. 2, Properties and selection–nonferrous alloys and pure metals, Tenth Edition, pp. 3-215, New York: ASM International Handbook, 1990. [15] C. Hindle. Polypropylene (PP), Accessed on 14 February 2017; http://www.bpf.co.uk/plastipedia/polymers/pp.aspx. [16] Shahmiri H, Movahedi M, Kokabi AH. Friction stir lap joining of aluminium alloy to polypropylene sheets. Science and Technology of Welding and Joining 2016;22(2):120–126. [17] Khodabakhshi F, Haghshenas M, Chen J, Shalchi Amirkhiz B, Li J, Gerlich AP. Bonding mechanism and interface characterisation during dissimilar friction stir welding of an aluminium/polymer bi-material joint. Science and Technology of Welding and Joining 2016, Article in press. [18] Mahajan C, Cormier D. 3D printing of carbon fiber composites with preferentially aligned fibers. Proceedings of the 2015 Industrial and Systems Engineering Research Conference, Nashville, 30 May – 2 June 2015. p. 2953-2962. [19] Hannesschläger C, Revol V, Plank B, Salaberger D, Kastner J. Fibre structure characterisation of injection moulded short fibre-reinforced polymers by X-ray scatter dark field tomography. Case Studies in Nondestructive Testing and Evaluation 2015;3:34-41. [20] Huang Y, Wang J, Wan L, Meng X, Liu H, Li H. Self-riveting friction stir lap welding of aluminum alloy to steel. Materials Letters 2016;185:181-184. [21] Nagatsuka K, Yoshida S, Tsuchiya A, Nakata K. Direct joining of carbon-fiber–reinforced plastic to an 14
aluminum alloy using friction lap joining. Composites Part B: Engineering 2015;73:82–88,. [22] Kiss Z, Czigány T. Applicability of friction stir welding in polymeric materials. Periodica polytechnica, Mechanical Engineering 2007;51(1):15–18. [23] Hertzberg RW. Deformation and fracture mechanics of engineering materials, New York: Wiley, 1989;261-288.
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Figure Captions Fig. 1- Distribution of carbon fibers in PP-Z30S matrix. Fig. 2- Schematic of the different steps for the joining process. Fig. 3- Schematic of tensile test specimens and their dimensions: a) Shear-Tensile specimen and b) Cross-Tension specimen. Fig. 4- Joint surface appearance at different rotational speeds: a) 500 rpm, b) 1000 rpm, c) 1500 rpm, d) 2000 rpm. Fig. 5- Stereograph images from cross-section of the joints at the rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm and d) 2000 rpm. Fig. 6- The effect of tool rotational speed on the maximum temperature obtained at different distances from the joint center. Fig. 7- a) SEM image from a typical cross-section of the joint, b) Reaction layer and the gap formed between aluminum and re-solidified composite in the threaded hole at the tool rotational speed of 500 rpm (magnified of area “A”) and c) Reaction layer and the gap refilled by epoxy resin at the tool rotational speed of 2000 rpm. Fig. 8- Carbon fibers distribution in the re-solidified polymer matrix at different rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm and d) 2000 rpm. Fig. 9- Fracture surface of the polymeric composite sheet; variation in the area of the unmelted surface and fracture location at different rotational speed of: a) 500 rpm, b) 1000 rpm, c) 1500 rpm, d) 2000 rpm (cross-tension) and e) 2000 rpm (shear-tensile). Fig. 10- Flow model of the molten polymer inside the hole. Fig. 11- SEM image from fracture surfaces: a) two different areas on the fracture surface of the sample welded at 500 rpm, b) ductile fracture surface (area A), c) brittle fracture surface (area B), d) two different areas on the fracture surface of the sample welded at 2000 rpm, e) ductile fracture surface (area C) and f) brittle fracture surface (area D). Fig. 12-a) Hardness and crystallinity percent of the primary and re-solidified composite inside the hole versus the tool rotational speed and b) DSC curve of the primary composite sheet. Fig. 13-a) Force-extension curves of cross-tension tests related to the specimens with maximum fracture force at different tool rotational speeds and b) effect of the tool rotational speed on the average cross-tension force and fracture energy of the joints. Fig. 14-a) Effect of tool rotational speed on the average shear-tensile load and fracture energy of the joints and b) comparison of fracture energy of the joints in shear-tensile and cross-tension tests.
16
Tables Table 1- Chemical composition, mechanical and physical properties of AA5052 used in this research Composition (wt%)
Mechanical and physical properties
Al
Base
Yield Strength, MPa
214
Mg
2.34
UTS, MPa
265
Cr
0.20
Elongation, %
13.5
Fe
0.23
Thermal Conductivity, W/m.K
138 [14]
Si
0.06
Melting Point, oC
644 [14]
Mn
0.006
Coefficient of Thermal Expansion, 10-6/K
23.8 [14]
Table 2-Chemical composition and mechanical properties of PP-SCF used in this research Composition of PP-SCF (wt%) PP-Z30S 92 PP-g-MA
5
SCF
3
Mechanical properties of PP-SCF Hardness, Shore D 72 UTS, MPa
35
Table 3-Physical properties of polypropylene Z30S [15] Properties Density Melt Flow Index (MFI) Coefficient of Thermal Expansion (CTE) Thermal Conductivity (Solid) Thermal Conductivity (Molten) Softening Point
Unit g/cm3 Dg/min
Amount 0.9 25
10-6/K
100-150
W/m.K W/m.K o C
0.17-0.22 0.16 165
Table 4- Effect of tool rotational speed on the composition of the reaction layer Elements Al C O
tool rotational speed (rpm) 500 2000 21 38 37 35 33 22
17
Fig. 1
Fig. 2 18
Fig. 3
Fig. 4
19
Fig. 5
Fig. 6
20
Fig. 7
21
Fig. 8
22
Fig. 9
Fig. 10
23
Fig. 11
Fig. 12
24
Fig. 13
25
a)
b)
Fig. 14
26