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Temperature measurement in friction element welding process with micro thin film thermocouples
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Procedia Manufacturing 26 (2018) 485–494 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA 46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA
Temperature measurement in friction element welding process with Temperature measurement in friction element welding process with micro thin film thermocouples Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June micro thin film thermocouples 2017, Vigo (Pontevedra), Spain
Saheem Absaraa, Brandt J. Ruszkiewiczbb, Jamie D. Skovronbb, Laine Mearsbb, Tim Abkecc, Saheem Absar , Brandt J. Ruszkiewicz , Laine Mears , Tim Abke , a , Jamie D. Skovron a, Xin Zhao , Hongseok Choi Costing models for capacity optimization ina,**Industry 4.0: Trade-off a Xin Zhao , Hongseok Choi Department of Mechanical Engineering, Clemson University,Clemson, SC 29634, USA between used capacity and operational efficiency DepartmentofofAutomotive MechanicalEngineering, Engineering,Clemson ClemsonUniversity, University,Clemson, Department Greenville,SC SC29634, 29607,USA USA a
ba
c Department of Automotive Engineering, Clemson University, Greenville, Honda R&D Americas, Raymond, OH, 43067, USA SC 29607, USA c a R&D Americas,a,* b USA b Honda Raymond, OH, 43067,
b
A. Santana , P. Afonso , A. Zanin , R. Wernke
a * Corresponding author. Tel.: +1-864-656-5642. University of Minho, 4800-058 Guimarães, Portugal * Corresponding Tel.: +1-864-656-5642. bUnochapecó, 89809-000 Chapecó, SC, Brazil E-mail address:author. [email protected] E-mail address: [email protected]
Abstract Abstract Abstract The friction element welding (FEW) process is a thermo-mechanical approach toward joining of dissimilar materials using an The friction element welding (FEW)is4.0", process is thermo-mechanical approach toward joining of dissimilar materials using an auxiliary consumable element which welded to aa base material whichwill is harder than its counterpart. It is necessary to understand Under the concept of "Industry production processes be pushed to be increasingly interconnected, auxiliary consumable element which is welded avarious base material which isprocess harder than itsIninsight counterpart. It isinfluence necessary to understand the transient thermal response during thenecessarily, stages of FEW to gain into the ofoptimization FEW process information based on a realgenerated time basis and,to much more efficient. this context, capacity the transient thermal response generated duringalong the various stagescontributing of FEW process gain insight intointerface. the profitability influence of FEW process parameters on the traditional physics of the FEW process with microstructural changes theorganization’s weld zone However, due to the goes beyond the aim of capacity maximization, alsotooffor and value. parameters associated on the physics of the FEW process along with microstructural changes of the the weld zone interface. However, of due challenges with accessibility of the welding interface during the FEW process, experimental investigation thermal Indeed, lean management and continuous improvement approaches suggest capacity optimization insteadto the of challengesisassociated accessibility of the welding interface during FEW the experimental investigation of thermal behavior rendered towith be highly complicated. In this work, an array of the micro thinprocess, film thermocouples (TFTCs) were fabricated on maximization. The study of capacity optimization and costing models is an important research topic that deserves behaviorsteel is rendered to bewhich highlywere complicated. In this work, an array temperatures of micro thin generated film thermocouples (TFTCs) were fabricated on JSC980 substrates, used to measure the transient at the vicinity of the friction element contributions from bothwhich the practical and theoretical This paper presents and discusses mathematical JSC980the steel substrates, were to measure the perspectives. transient temperatures generated at of the vicinity thea friction during FEW joining of Al6005 andused JSC980 sheets. The high spatial and temporal resolution the TFTCsofenabled us to element observe model for FEW capacity management based on sheets. different models (ABC and TDABC). ATFTCs generic model been during the joining of Al6005response and JSC980 Thecosting high and temporal resolution of –thepenetration, enabled us has to welding observe the overall transient temperature generated during thespatial four stages of the FEW process cleaning, developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s the overall transient temperature generated during stages of the FEW – penetration, cleaning,due welding and compression. Analysis of the response applied compressive forcesthe on four the friction element and process the resulting torque generated to the value. Thefrictional trade-off capacity maximization vs operational is highlighted and ittorque istemperatures shown thataredue capacity and compression. Analysis of the applied compressive forces on theefficiency friction and resulting generated to the increased resistance of the element tip against the JSC980 surfaceelement showed thatthe peak process reached increased of the element against the JSC980 surface showed that peak process temperatures arethis reached optimization mightresistance hidewelding operational inefficiency. during thefrictional cleaning and steps of the tip FEW process. The experimental temperature measurements obtained in work during the cleaning and welding steps of analytical the FEW and process. The experimental obtained in This this work © 2017be The Authors. Published Elsevier B.V. could utilized to validate andby improve numerical models of thetemperature temperaturemeasurements evolution FEW process. couldlays be the utilized toresponsibility validate and of improve analytical and numerical models of the Engineering temperature evolution FEW This work Peer-review under the committee of thethe Manufacturing Society International also necessary groundwork for scientific further investigation into microstructural characteristics of the weld process. zone Conference dependent on alsotemperatures lays the necessary groundwork investigation 2017. the reached at each stepforoffurther the FEW process. into the microstructural characteristics of the weld zone dependent on the temperatures reached at each step of the FEW process. Keywords: CostAuthors. Models; Published ABC; TDABC; CapacityB.V. Management; Idle Capacity; Operational Efficiency © 2018 The by Elsevier © 2018 The Authors. Published by Elsevier B.V. © 2018 The under Authors. Published by Elsevier B.V. committee of NAMRI/SME. Peer-review responsibility of the scientific Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference. Peer-review under responsibility of the scientific committee of NAMRI/SME.
1. Introduction Keywords: Thin film thermocouple; Friction element welding. Keywords: Thin film thermocouple; Friction element welding.
The cost of idle capacity is a fundamental information for companies and their management of extreme importance in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018responsibility The Authors. Published by Elsevier B.V.hours Peer-review of the scientific committee of NAMRI/SME. in several©under ways: tons of production, available of manufacturing, etc. The management of the idle capacity Peer-review underTel.: responsibility the761; scientific committee NAMRI/SME. * Paulo Afonso. +351 253 of 510 fax: +351 253 604of741 E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference. 10.1016/j.promfg.2018.07.057
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1. Introduction Thin film thermocouples (TFTCs) are highly sensitive devices capable of providing extremely fast measurements of surface temperatures. Compared to bulk thermocouples, TFTCs provide more intimate thermal contact with the surface along with low thermal inertia and high spatial resolution due to the ability to scale down in size [1]. Fabrication of TFTCs directly on various surfaces allows them to be noninvasive in applications where the presence of bulk thermocouples or sensors might interfere with the physical process being monitored. Tougas et al have shown the application of non-invasive metallic and ceramic TFTCs in temperature measurement of the hot gas flow in gas turbine engines [2]. Other successful applications of TFTCs directly embedded into a metal structure for in-situ process monitoring have been investigated for ultrasonic joining [3], laser micromachining [4] and tool-chip contact temperatures in conventional machining [5-7]. It has been shown that when the film thickness is comparable to the mean free path of the charge carriers in a metallic film, all transport processes exhibit the size effects [1]. The size effects in Seebeck coefficient of TFTCs are dependent on the predominant type of electron scattering. The scattering mode in turn affects the relaxation time of the thin film. Boundary scattering starts to dominate over bulk scattering when the film thickness is of an order or smaller than the electron mean-free path, typically for films thinner than several hundreds of nanometers. Studies of Pt/Au TFTCs showed that for a film thickness greater than 20 nm, the difference between the electronic thermopower of bulk metal and thin films, ΔSF, has an inversely linear relationship with the film thickness. Film thicknesses lower than this threshold exhibit quantum size effects, leading to a non-linear relationship of ΔSF with film thickness. Assuming that the electrons are free and taking into account size effects, the electronic thermopower of a thin metallic film is given by [1,8]: 𝑈𝑈 3 𝑙𝑙 𝑆𝑆𝐹𝐹 = 𝑆𝑆𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 ( ) (1 − ) (1 − 𝑝𝑝) 1 + 𝑈𝑈 8 𝑡𝑡
where,
(1)
𝑈𝑈 = [
𝑑𝑑ln𝜎𝜎(𝐸𝐸) ] 𝑑𝑑ln𝐸𝐸 𝐸𝐸=𝜉𝜉
(2)
Sbulk is the thermopower of the bulk metal, E is the Fermi energy level, l is the length of the mean free path of electrons, t is the thickness of the film and p is the scattering coefficient. The scattering coefficient is the fraction of the energy lost by charge carriers when reflected from the surface of the film. For t > 150 nm, the Seebeck coefficients assume a maximum value, independent of film thickness which is slightly below the Seebeck coefficient of the bulk thermocouple [9]. The friction element welding (FEW) process offers the possibility of joining dissimilar materials such as lightweight aluminum alloys and high strength steels. This process combines thermal and mechanical welding principles using an auxiliary consumable joining element made of steel. In the FEW process, the workpieces or sheets to be joined are stacked together with the softer material being placed on top of the harder material. The FEW joining process consists of four main steps – penetration, cleaning, welding and compression, which are shown in a schematic in Fig. 1. The penetration process is initiated by rotating the friction element to high speeds using a spindle, which is pressed against the surface of the upper sheet with a high axial force while rotating. The resulting heat generated from the friction between the rotating element and the upper sheet causes the material to be plasticized and allows penetration without the requirement of a pre-hole or melting of the material. The rotating element then contacts the surface of the harder base sheet material, resulting in a significant increase in the temperature of the element due to greater amount of friction. The element material is also plasticized at this stage and its shape is deformed generating a characteristic ‘upset’. The sliding surface of the tip of the element cleans and activates the surface of the base sheet. The cleaning step is essentially pre-heating step prior to welding, which cleans any coatings present on the surface of the base metal sheet, such as galvanneal coating in the case of advanced high strength steel (AHSS) sheets. During the welding step, a friction weld is formed between the element and the base sheet. After the welding step, the rotation of the element is stopped and a large axial compressive force is applied to close any cracks that may have been formed when the element rotation is stopped. The head of the element is also fully seated
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against the upper sheet in the compression step. Throughout the entire FEW process, a hollow cylindrical tube surrounding the spindle (the downholder) keeps the sheets compressed while an anvil below the base sheet provides support [10-12].
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consideration. The diameter of the shaft of the friction element was 4.55 mm, and the diameter of the head of the friction element was 9.8 mm. 8 TFTCs were placed in a linear arrangement such that 4 TFTCs would be situated directly under the area covered by the friction element, while the rest of the TFTCs were situated outside the body of the element. This design allowed us to obtain the temperature profile of different zones as the friction element progressively travels through the substrate. The locations of the TFTCs on the substrate is shown in a schematic in Fig. 2.
Fig. 1. Schematic of the different stages of the FEW process – (1) Penetration, (2) Cleaning, (3) Welding, and (4) Compression [13].
Previous work on the study of process parameters showed that the applied force and generated torque during cleaning and welding steps play a dominant role on the outcome of the FEW process in terms of joint quality. The influence of various FEW process parameters on temperature was also studied by recording temperature profiles of the top surface of the upper sheet using a thermal camera, at a distance of 8 mm from the center of the downholder [11]. However, since the workpieces are bonded together through the friction element during the welding process and also due to the presence of the large downholder component on the top sheet, conventional methods of temperature measurement are infeasible due to the inaccessibility of the weld zone. In this work, the temperatures generated during the various stages of the friction element welding of Al6005 (Al-0.9Si-0.6Mg) and JSC980 (Fe-0.1C0.02Si-2.0Mn) steel sheets were measured using an array of thin film thermocouples fabricated directly on the surface of the JSC980 steel sheet. The effect on temperature response due to the torques and applied compressive forces of the various stages of the FEW process were also investigated. 2. Design, fabrication and calibration of micro TFTCs In order to accommodate a multitude of TFTCs on the substrate adjacent to the welding zone, the dimensions of the friction element were taken into
Fig. 2. Top-view schematic showing locations of TFTCs relative to the center of the friction element.
The TFTCs were fabricated on top of an insulating dielectric multilayer deposited on a JSC980 steel substrate with dimensions of 40×40 mm. Standard Ktype thermocouple materials composed of Chromel (90Cr/10Ni) and Alumel (95Ni/2Al/2Mn/1Si) films were used to fabricate the TFTC legs. An insulating dielectric layer was deposited on the TFTC films to cover and protect the sensors and also allow electrical isolation. 2.1. Mask design Two separate photomasks for the Chromel and Alumel legs of the TFTCs were designed using AutoCAD and printed on a 4×4 inch transparent quartz plate. The layout of the sensor with TFTCs is shown in Fig. 3. There are 8 pairs of TFTCs with a narrowest track width of 50 μm. The overlapping area of each of the Chromel-Alumel junctions of the TFTCs is 50×50 μm. The width of the junction plays an important role in the dynamic response of a thermocouple. Previous studies on TFTC films having different junction widths showed that a response time of about 50 ns can
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be achieved for a film thickness of 100 nm [14]. The spacing between each TFTC junction is 0.875 mm.
peak-to-valley height of the original surface of the substrate was 22.80 ± 2.04 μm and the average surface roughness (Ra) was 0.92 ± 0.04 μm. After polishing, the average peak-to-valley height of the substrate surface was reduced to 0.94 ± 0.70 μm and the average Ra value was reduced to 0.03 ± 0.01 μm. The surface roughness profile maps of the substrate before and after polishing are shown in Fig. 4.
Fig. 3. Layout of TFTCs.
2.2. Preparation of JSC980 steel substrate Extensive preparation of the substrate was necessary to facilitate microfabrication of the TFTCs to be performed in a cleanroom environment. Multiple layers of thin films of the dielectric and metallic materials are required to be precisely deposited on the surface of the substrate during the microfabrication process. It is essential to reduce the height of surface asperities present on the substrate to ensure uniform coverage of thin films, and also to ensure reliable patterning of microscale junction widths of the TFTCs. Manual grinding and polishing steps were used to reduce the surface roughness using a Buehler grinder/polisher machine. Ethanol was used as the coolant for grinding due to the JSC980 surface being reactive toward water. To hold the substrate flat against the grinding wheel, the substrate was attached to a stainless steel mount with a recessed pocket. Grinding was performed successively with 320, 400, 800 and 1200 grit SiC abrasive papers. The substrate was then polished with 6 and 1 μm diamond suspensions. For the final polishing step, the substrate was polished in a 0.05 μm alumina suspension contained in a Buehler VibroMet vibratory polishing machine. During vibratory finishing, the substrates are entrained in the fluidized polishing media with a slow relative velocity against the polishing cloth due to the application of a continuous horizontally oscillating motion. A superior degree of flatness is achieved due to multiple modes of multi-body contact which promotes gentle plastic deformation of the substrate surface by continuous sliding or burnishing. The final surface roughness of the substrate was measured with a Zygo optical surface profilometer. The average
Fig. 4. Surface roughness profile maps of JSC980 substrate – (a) before and (b) after polishing.
2.3. Microfabrication process of TFTCs The entire microfabrication process of the TFTCs was performed in the cleanrooms at the Georgia Institute of Technology’s Marcus Institute of Nanotechnology. A schematic of the microfabrication process is shown in Fig. 5. To ensure removal of any contaminants or residues on the substrate, initial cleaning of the metal substrate was performed in acetone, followed by methanol and then isopropanol accompanied by ultrasonic agitation. The substrate was finally rinsed with deionized water and thoroughly dried with a dry nitrogen spray gun.
Fig. 5. TFTC microfabrication process.
The multilayer insulating dielectric layer was composed of three layers – a 500 nm aluminum oxide (Al2O3) film, followed by a 1 μm silicon nitride
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(Si3N4) film and finally a 500 nm Al2O3 film. Deposition of Al2O3 film can be performed using three approaches – electron beam evaporation, radio frequency (RF) sputtering, and atomic layer deposition (ALD). Although Al2O3 deposition can be successfully performed using ALD process with precise control of layer growth, it is impractical to deposit film of micron-scale thickness due to the extremely slow deposition rate compared to physical vapor deposition methods. On the other hand, RF sputtering of Al2O3 requires effective control of the stoichiometric ratio of aluminum and oxygen content to allow the formation of Al2O3 films with high dielectric properties, and a detailed optimization study is needed to be performed to find the right set of parameters. Therefore, e-beam evaporation was selected for deposition of sufficiently thick Al2O3 films from alumina pellets due to its higher deposition rates. Si3N4 film was deposited using plasmaenhanced chemical vapor deposition (PECVD), which can produce low stress Si3N4 films using chemical precursor gases in a plasma environment. The materials and structure of the multilayer dielectric films were chosen based on extensive investigations related to mutual compatibility of their coefficient of thermal expansions with the substrate and metal layers, including their ability to maintain their dielectric strength at elevated temperatures [15]. It is known that evaporated thin films generally result in poor conformal coverage of surface features and is also prone to developing pinhole defects. Therefore, if the Al2O3 film was used alone as a dielectric layer, presence of pinholes and surface irregularities on the substrate would result in the shorting of the metal layers of the sensors to the substrate [15]. Deposition of a Si3N4 film using the PECVD process on the initial Al2O3 film allows us to achieve conformal coverage of the pinholes and remaining surface asperities on the substrate. However, it is possible that the Si3N4 film might also possess pinhole defects, thus a third layer of Al2O3 was also deposited to complete the multilayer dielectric structure. A dielectric pad test was performed to verify the functionality of the dielectric multilayer. For this test, a square grid of Ti/Chromel pads was deposited by DC sputtering on the dielectric multilayer as shown in Fig. 6. The electrical resistance was measured between each Chromel pad and the exposed areas of the metal substrate was measured with a multimeter. The mutual resistance between each of Chromel pads were also
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measured. The resistance readings for both these cases showed an open circuit, thereby confirming the electrical isolation of the insulating films. In the event that the resistance readings showed a short circuit, it would suggest the presence of pinhole defects in the dielectric films.
Fig. 6. Dielectric pad test.
To initiate the sensor microfabrication process, a 25 nm layer of Ti was initially deposited on the metal substrate by DC sputtering prior to the deposition of the initial Al2O3 layer to promote its adhesion to the substrate. During the deposition of the Ti and dielectric multilayers, a stainless steel shadow mask was attached to the substrate using miniature stainless steel clips to cover areas of the substrate where deposition on films was not desired. Al2O3 was deposited on the substrate using an electron beam evaporation unit with a deposition rate of 18 nm/min, which was reached at a working pressure of 5×10 -5 Torr. Si3N4 was deposited using a PECVD system with a mixture of SiH4 and NH3 gases; N2 and He were additionally used for controlling film stress. A deposition rate of 7.78 nm/min was achieved at a chamber pressure of 1.1 Torr and a substrate temperature of 250°C. For fabrication of TFTCs, Chromel and Alumel legs were patterned using standard photolithography and liftoff processes on top of the insulating films. First, Microposit S1813 positive photoresist was spin coated on the substrate, followed by patterning of the Chromel legs of the sensor using a Karl Suss MA6 mask aligner equipped with a 350 W mercury arc lamp. The exposed photoresist was baked at 115°C for 60 seconds. A 120 nm layer of Chromel was then deposited on the patterned substrate. Liftoff of the Chromel layer was performed in acetone under ultrasonic agitation. The Alumel legs were then patterned after alignment of the second photomask with the Chromel legs in the mask aligner, followed by deposition and liftoff of a 180 nm Alumel layer. A 10 nm adhesion promoting layer of Ti was deposited
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prior to the deposition of the Chromel and Alumel films. The Chromel and Alumel layers were deposited by DC sputtering at a rate of 12-15 nm/min from 3 inch alloy targets with a power of 350 W. After fabrication of the Chromel and Alumel legs, a 500 nm thick Al2O3 cover layer was deposited on the completed sensor by e-beam evaporation. A shadow mask was attached to the substrate during deposition of the cover layer to ensure deposition on selected areas of the fabricated sensor. This insulation layer covered the TFTC junctions and lines except the pad areas. 2.4. Calibration of TFTC Calibration of a TFTC was performed in a temperature-controlled tube furnace from room temperature to 1000°C. Continuous Argon as flow was maintained in the tube furnace during the calibration process. Each exposed pad of the TFTC was bonded with its corresponding fine gage (70 μm) compensation wire with high temperature silver paste (Pyro-Duct 597-A). The wires were then secured onto the substrate using high temperature ceramic cement (Omega CC High Temp). The compensation wires were connected to the differential analog input channels of an NI USB-6003 data acquisition system (DAQ). The Alumel wire was referenced to the ground of the analog input channel of the DAQ device using a bias resistor. Voltage signals from the TFTC from room temperature to 1000°C were recorded with LabVIEW software. A plot of the voltage output of the TFTC including that of a commercial thermocouple corresponding to different temperatures is shown in Fig. 7.
Fig. 7. Calibration plot of TFTC.
The calibration results indicate that the TFTC has a linear response within the measured temperature range. The thermoelectric sensitivity of the TFTC was determined to be 41.3 μV/°C. The TFTC sensitivity was found to closely match the sensitivity of commercial K type thermocouples, 41.5 μV/°C which was calculated from standard voltage-temperature tables [16]. 3. Friction element welding experiment and results The stackup for the friction element welding (FEW) experiment consisted of an Al6005 sheet and a JSC980 steel sheet with embedded TFTCs. The dimensions of both the sheets were 40×40 mm. The thickness of the Al6005 sheet was 1.7 mm and that of the JSC980 sheet was 1.16 mm. An EJOWELD® FEW machine has a maximum spindle speed of 9000 RPM, maximum element endload of 9 kN and maximum downholder force of 6 kN. An EJOWELD® 4.55×4.5-Z-8 steel element was used as the friction element, having a cylindrical tip (Z) with a shaft diameter of 4.55 mm and length of 4.5 mm. The element is composed of a grade 8 steel which has a strength class of 800-1000 MPa. The data related to spindle speed, torque, applied force, spindle position and element way were recorded by the FEW machine controller at a sampling rate of 1 kHz during the entire welding process. Plots of the recorded data generated from the FEW machine are shown in Fig. 8.
Fig. 8. Plots of spindle speed, torque, force, spindle travel and element way generated during the FEW process. The steps involved in the FEW process are marked on the plot with solid blue lines.
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For positioning of the center of the friction element relative to the location of TFTC1 on the JSC980 sheet, a laser alignment pointer was used to mark the center of the desired welding point. The laser pointer is aligned with the longitudinal axis of the FEW machine spindle. The location of this point was situated 1.4 mm from the TFTC1 junction. In order to record voltage signals from the TFTCs on the JSC980 sheet, Chromel and Alumel compensation wires were attached to the corresponding TFTC pads with Kapton tape. The compensation wires were then connected to a NI-USB 6003 DAQ system. The voltage signals generated during the FEW process were recorded at a sampling rate of 25 kHz using LabVIEW software. Postprocessing and analysis of the recorded sensor voltage signals was performed using OriginPro software. The voltage signals were filtered using applying a low pass Fast Fourier Transform (FFT) filter to remove the high frequency noise components in the signal, as shown in Fig. 9.
Fig. 9. Raw voltage signal obtained from TFTC3. A 50 Hz FFTbased low pass filter is applied to attenuate noisy components in the raw signal.
The filtered signals were then smoothed by applying a locally weighted linear regression function with a span value of 0.02. The smoothed voltage signals were then converted to temperature using the calibration relationship of the TFTC described in Section 2. The temperature profile of the FEW process obtained from the TFTC located at a distance of 3.15 mm from the center of the element is shown in Fig. 10. In order to identify the temperature transitions between occurring between the various stages of the FEW process, the second derivative of the temperature profile was used. The second derivative was used in
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this case because using the first derivative of the temperature profile was too noisy to be used to identify the transitions.
Fig. 10. Temperature profile and its second derivative obtained from TFTC3, positioned 3.15 mm from center of element. The different steps of the FEW process on the temperature profile are marked with dashed lines (A: contact of element tip and downholder with top sheet, 1: Penetration, 2: Cleaning, 3: Welding, and 4: Compression)
At the beginning of the FEW process the downholder and spindle assembly holding the friction element travels down towards the top surface of the Al6005 sheet. When the tip of the friction element comes in contact with the Al6005 sheet surface, an axial force of 1 kN is applied on the head of the element. The pressure applied by the element tip generates an indent of 0.2 mm on the Al6005 sheet surface, which also generates a small instant fluctuation in the voltage signal generated from the TFTCs, which is observed as an initial spike on the second derivative of the temperature profile shown in Fig. 10. The ‘element way’ is then set to 0 by the FEW machine controller. The friction element begins to penetrate through the Al6005 sheet while the spindle speed is gradually ramped up to 5000 rpm. As the force applied on the element is increased at this time, the torque rises up to 5.47 Nm, signifying the increase in frictional resistance of the rotating element tip against the Al6005 sheet. The softening of the Al6005 sheet from plasticization then results in a decrease in the torque. From the temperature profile shown in Fig. 10, the first transition in the temperature profile occurs after 0.30 seconds from the contact of the element with the Al6005 sheet, where the temperature rises up to 148°C during the penetration of the Al6005 sheet by
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the element tip. This transition has a duration of 0.15 seconds, following which the temperature profile shows a sharp rise to 239°C during the cleaning step. The continuous sliding of the element tip against JSC980 surface leads to repeated occurrence of joining and separation at the interface, which causes temperature fluctuations for a period of 0.153 seconds. The increase in temperature can be attributed to the fact that the JSC980 sheet is harder than the Al6005 sheet, facilitating a higher amount of frictional resistance on the element tip. The significant rise in friction and temperature results in the deformation of the tip of the element due to plasticization of the element material. The second transition in the temperature profile corresponds to the welding step, where the temperature keeps rising until a peak temperature of 414°C is reached. The compressive force and rotational speed are increased to 7.6 kN and 6000 rpm respectively, resulting in a significant increase in frictional torque to 7.78 Nm. These conditions result in the localized softening of the interface between the JSC980 surface and the element, leading to further temperature increase from extensive plastic deformation. The element is rigidly bonded to the JSC980 sheet at the end of the welding step. After completion of the welding step, the axial force is further increased to 9 kN and the spindle rotation is stopped. The forging force applied in the compression step essentially allows pressure-welding of any gaps or cracks present in the weld zone. The head of the element is also firmly seated against the Al6005 sheet at the end of the compression step. The heat generated at the end of the welding step is gradually dissipated as shown by the decrease of the temperature profile in step 4. Representative temperature profiles from TFTC5 and TFTC6, located 4.9 mm and 5.78 mm from the center of the element respectively, are shown in Fig. 11. No significant rise in temperature is observed during the FEW process at these locations until the end of the compression stage, where a peak temperature of up to 130°C is reached. The delayed temperature profile obtained from TFTCs 5 and 6 suggest the effect of gradual heat dissipation propagating radially outward from the center of the weld zone.
Fig. 11. Representative temperature profiles obtained from TFTCs 3, 5 and 6.
In order to correlate the temperature profiles obtained from the different TFTCs to the locations of the TFTCs present on the JSC980 surface during the friction element welding process, the cross-section of the bonded sheets was observed under an optical microscope. The bonded sheets were sectioned along the center of the friction element using a high speed sectioning saw. The cross-section was mounted in epoxy followed by grinding and polishing. The surface of the cross-section was then etched using a 2% Nital solution to reveal the microstructure. An image of the cross-section of the welded joint, and locations of the TFTCs 1-6 are shown in Fig. 12. A distinct interface between the friction element and the JSC980 sheet is visible in the image of the cross-section. The deformation of the top surface of the JSC980 sheet during the welding process possibly resulted in the relocation of the TFTC junctions. As shown in Fig. 12, TFTC1 was located at a distance of 1.4 mm from the center of the element, while the other junctions were located 0.875 mm away from each other. The location of the TFTC1 junction within the weld zone resulted in its failure during the frictional shearing of interface between the JSC980 sheet and the friction element.
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Fig. 12. Optical image of the cross-section of the welded region between the friction element, JSC980 and Al6005 sheets. The locations of the TFTCs numbered 1-3 are shown with red circles.
The position of the TFTC2 junction can be observed to be at the meeting point between the interfaces of the deformed Al6005 and JSC980 sheets and the element shaft. The position of TFTC2 did not allow sufficient contact with the element during the welding process. The extensive deformation of the weld zone resulted in the delamination of the insulation and thermocouple films at the location of TFTC2 junction, contributing to its failure. The TFTC3 junction is located at a distance of 3.15 mm from the center of the element, where there was sufficient contact between the JSC980 and Al6005 sheets. The distance of TFTC3 from the center of the weld zone was sufficient to avoid early failure during the FEW process. However, the TFTC4 junction was located at a distance of 4.025 mm from the center of the element, which was situated close to the periphery of the downholder which presses down firmly on top of the Al6005 sheet during the entire duration of the FEW process. The extremely high contact forces exerted on the sheets at the vicinity of the downholder contributed to the failure of TFTC4 during the welding process. The TFTC5 and TFTC6 junctions were located 4.9 mm and 5.78 mm respectively from the center of the element. Due to the further distance of these junctions compared to TFTCs 1-4 from the weld zone, the temperatures detected at these locations do not reach their peak values rapidly during the FEW process.
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of Al6005 and JSC980 sheets. The TFTCs were composed of standard K-type thermocouple materials from overlapping junctions (50×50 μm) of 120 nm Chromel and 180 nm Alumel thin films. Calibration of the TFTCs showed good linear sensitivity of 41.3 μV/°C to a temperature of 1000°C. The overall transient temperature profile of during each stage of the FEW process, namely – penetration, cleaning, welding and compression were obtained from the TFTCs. The evolution of the temperature profiles were correlated with the torque generated and applied compressive forces during each stage of the FEW process. Increased frictional resistance due to plasticization of the element tip against the JSC980 substrate during the cleaning and welding steps of the FEW process allowed the process temperature to reach up to 239 - 414°C, at a distance of 3.15 mm from the center of the friction element. Future work will include individual measurements of the transient temperature response of each step of the FEW process, for a fixed combination of applied force and spindle rotation. These studies will enable the validation of analytical and computational models of the temperature evolution of the FEW process. Microstructural characterization of the weld zone will also be investigated in order to study the influence of temperatures reached during each step of the welding process on the microstructure of the bonding interface. Acknowledgements The authors would like to extend their thanks to EJOT Fastening Systems L.P. for their expertise and facilitating access to the EJOWELD® Friction Element Welding system. References [1]
[2]
4. Conclusions
[3]
An array of micro thin film thermocouples (TFTCs) were fabricated on top of a multilayer insulating dielectric film on the surface of a JSC980 steel substrate for measurement of temperatures generated at the interface between the friction element welding
[4]
M. Cattani, M.C. Salvadori, A.R. Vaz, F.S. Teixeira, I.G. Brown, Thermoelectric power in very thin film thermocouples: quantum size effects, Journal of Applied Physics 100 (2006) 114905. I.M. Tougas, M. Amani, O.J. Gregory, Metallic and ceramic thin film thermocouples for gas turbine engines, Sensors, 13 (2013) 15324–15347. H. Li, H. Choi, C. Ma, J. Zhao, H. Jiang, W. Cai, J. a. Abell, X. Li, Transient temperature and heat flux measurement in ultrasonic joining of battery tabs using thin-film microsensors, Journal of Manufacturing Science and Engineering, 135 (2013) 51015. H. Choi, X. Li, Fabrication and application of micro thin film thermocouples for transient temperature measurement in
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nanosecond pulsed laser micromachining of nickel, Sensors and Actuators, A: Physical, 136 (2007) 118–124. D. Werschmoeller, X. Li, K. Ehmann, Measurement of transient tool-internal temperature fields during hard turning by insert-embedded thin film sensors, Journal of Manufacturing Science and Engineering, 134 (2012) 61004. L. Li, B. Li, X. Li, K.F. Ehmann, Experimental investigation of hard turning mechanisms by pcbn tooling embedded micro thin film thermocouples, Journal of Manufacturing Science and Engineering, 135 (2013) 41013. N. Sugita, K. Ishii, T. Furusho, K. Harada, M. Mitsuishi, Cutting temperature measurement by a micro-sensor array integrated on the rake face of a cutting tool, CIRP Annals Manufacturing Technology, 64 (2015) 77–80. Y.Z. Chen, H.C. Jiang, W.L. Zhang, X.Z. Liu, S.W. Jiang, Film thickness influences on the thermoelectric properties of NiCr/NiSi thin film thermocouples, Modern Physics Letters B, 27 (2013) 1350103. A.R. Varrenti, C. Zhou, A.G. Klock, S.H. Chyung, J. Long, S.O. Memik, M. Grayson, Thermal sensing with lithographically patterned bimetallic thin-film thermocouple, IEEE Electron Device Letters, 32 (2011) 818–820. The Aluminum Automotive Manual - Joining, European Aluminium Association, 2015. J.D. Skovron, B.J. Ruszkiewicz, Investigation of the cleaning and welding steps from the friction element welding process, In Proceedings of the ASME 2017 12th International Manufacturing Science and Engineering Conference, MSEC2017 (2017) 1–14. G. Meschut, V. Janzen, T. Olfermann, Innovative and highly productive joining technologies for multi-material lightweight car body structures, Journal of Materials Engineering and Performance, 23 (2014) 1515–1523. EJOT GmbH & Co. KG, EJOT Technology, Ejoweld.com, Available: http://www.ejoweld.com/en/technology.html. X. Zhang, H. Choi, A. Datta, X. Li, Design, fabrication and characterization of metal embedded thin film thermocouples with various film thicknesses and junction sizes, Journal of Micromechanics and Microengineering, 16 (2006) 900–905. A. Datta, H. Choi, X. Li, Batch fabrication and characterization of micro-thin-film thermocouples embedded in metal, Journal of The Electrochemical Society, 153 (2006), H89-H93. M.C. Croarkin, W.F. Guthrie, G.W. Burns, M. Kaeser, G.F. Strouse, Temperature-Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90, National Institute of Standards and Technology, Gaithersburg, MD, 1993.
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46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA 46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA
Temperature measurement in friction element welding process with Temperature measurement in friction element welding process with micro thin film thermocouples Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June micro thin film thermocouples 2017, Vigo (Pontevedra), Spain
Saheem Absaraa, Brandt J. Ruszkiewiczbb, Jamie D. Skovronbb, Laine Mearsbb, Tim Abkecc, Saheem Absar , Brandt J. Ruszkiewicz , Laine Mears , Tim Abke , a , Jamie D. Skovron a, Xin Zhao , Hongseok Choi Costing models for capacity optimization ina,**Industry 4.0: Trade-off a Xin Zhao , Hongseok Choi Department of Mechanical Engineering, Clemson University,Clemson, SC 29634, USA between used capacity and operational efficiency DepartmentofofAutomotive MechanicalEngineering, Engineering,Clemson ClemsonUniversity, University,Clemson, Department Greenville,SC SC29634, 29607,USA USA a
ba
c Department of Automotive Engineering, Clemson University, Greenville, Honda R&D Americas, Raymond, OH, 43067, USA SC 29607, USA c a R&D Americas,a,* b USA b Honda Raymond, OH, 43067,
b
A. Santana , P. Afonso , A. Zanin , R. Wernke
a * Corresponding author. Tel.: +1-864-656-5642. University of Minho, 4800-058 Guimarães, Portugal * Corresponding Tel.: +1-864-656-5642. bUnochapecó, 89809-000 Chapecó, SC, Brazil E-mail address:author. [email protected] E-mail address: [email protected]
Abstract Abstract Abstract The friction element welding (FEW) process is a thermo-mechanical approach toward joining of dissimilar materials using an The friction element welding (FEW)is4.0", process is thermo-mechanical approach toward joining of dissimilar materials using an auxiliary consumable element which welded to aa base material whichwill is harder than its counterpart. It is necessary to understand Under the concept of "Industry production processes be pushed to be increasingly interconnected, auxiliary consumable element which is welded avarious base material which isprocess harder than itsIninsight counterpart. It isinfluence necessary to understand the transient thermal response during thenecessarily, stages of FEW to gain into the ofoptimization FEW process information based on a realgenerated time basis and,to much more efficient. this context, capacity the transient thermal response generated duringalong the various stagescontributing of FEW process gain insight intointerface. the profitability influence of FEW process parameters on the traditional physics of the FEW process with microstructural changes theorganization’s weld zone However, due to the goes beyond the aim of capacity maximization, alsotooffor and value. parameters associated on the physics of the FEW process along with microstructural changes of the the weld zone interface. However, of due challenges with accessibility of the welding interface during the FEW process, experimental investigation thermal Indeed, lean management and continuous improvement approaches suggest capacity optimization insteadto the of challengesisassociated accessibility of the welding interface during FEW the experimental investigation of thermal behavior rendered towith be highly complicated. In this work, an array of the micro thinprocess, film thermocouples (TFTCs) were fabricated on maximization. The study of capacity optimization and costing models is an important research topic that deserves behaviorsteel is rendered to bewhich highlywere complicated. In this work, an array temperatures of micro thin generated film thermocouples (TFTCs) were fabricated on JSC980 substrates, used to measure the transient at the vicinity of the friction element contributions from bothwhich the practical and theoretical This paper presents and discusses mathematical JSC980the steel substrates, were to measure the perspectives. transient temperatures generated at of the vicinity thea friction during FEW joining of Al6005 andused JSC980 sheets. The high spatial and temporal resolution the TFTCsofenabled us to element observe model for FEW capacity management based on sheets. different models (ABC and TDABC). ATFTCs generic model been during the joining of Al6005response and JSC980 Thecosting high and temporal resolution of –thepenetration, enabled us has to welding observe the overall transient temperature generated during thespatial four stages of the FEW process cleaning, developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s the overall transient temperature generated during stages of the FEW – penetration, cleaning,due welding and compression. Analysis of the response applied compressive forcesthe on four the friction element and process the resulting torque generated to the value. Thefrictional trade-off capacity maximization vs operational is highlighted and ittorque istemperatures shown thataredue capacity and compression. Analysis of the applied compressive forces on theefficiency friction and resulting generated to the increased resistance of the element tip against the JSC980 surfaceelement showed thatthe peak process reached increased of the element against the JSC980 surface showed that peak process temperatures arethis reached optimization mightresistance hidewelding operational inefficiency. during thefrictional cleaning and steps of the tip FEW process. The experimental temperature measurements obtained in work during the cleaning and welding steps of analytical the FEW and process. The experimental obtained in This this work © 2017be The Authors. Published Elsevier B.V. could utilized to validate andby improve numerical models of thetemperature temperaturemeasurements evolution FEW process. couldlays be the utilized toresponsibility validate and of improve analytical and numerical models of the Engineering temperature evolution FEW This work Peer-review under the committee of thethe Manufacturing Society International also necessary groundwork for scientific further investigation into microstructural characteristics of the weld process. zone Conference dependent on alsotemperatures lays the necessary groundwork investigation 2017. the reached at each stepforoffurther the FEW process. into the microstructural characteristics of the weld zone dependent on the temperatures reached at each step of the FEW process. Keywords: CostAuthors. Models; Published ABC; TDABC; CapacityB.V. Management; Idle Capacity; Operational Efficiency © 2018 The by Elsevier © 2018 The Authors. Published by Elsevier B.V. © 2018 The under Authors. Published by Elsevier B.V. committee of NAMRI/SME. Peer-review responsibility of the scientific Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference. Peer-review under responsibility of the scientific committee of NAMRI/SME.
1. Introduction Keywords: Thin film thermocouple; Friction element welding. Keywords: Thin film thermocouple; Friction element welding.
The cost of idle capacity is a fundamental information for companies and their management of extreme importance in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018responsibility The Authors. Published by Elsevier B.V.hours Peer-review of the scientific committee of NAMRI/SME. in several©under ways: tons of production, available of manufacturing, etc. The management of the idle capacity Peer-review underTel.: responsibility the761; scientific committee NAMRI/SME. * Paulo Afonso. +351 253 of 510 fax: +351 253 604of741 E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference. 10.1016/j.promfg.2018.07.057
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1. Introduction Thin film thermocouples (TFTCs) are highly sensitive devices capable of providing extremely fast measurements of surface temperatures. Compared to bulk thermocouples, TFTCs provide more intimate thermal contact with the surface along with low thermal inertia and high spatial resolution due to the ability to scale down in size [1]. Fabrication of TFTCs directly on various surfaces allows them to be noninvasive in applications where the presence of bulk thermocouples or sensors might interfere with the physical process being monitored. Tougas et al have shown the application of non-invasive metallic and ceramic TFTCs in temperature measurement of the hot gas flow in gas turbine engines [2]. Other successful applications of TFTCs directly embedded into a metal structure for in-situ process monitoring have been investigated for ultrasonic joining [3], laser micromachining [4] and tool-chip contact temperatures in conventional machining [5-7]. It has been shown that when the film thickness is comparable to the mean free path of the charge carriers in a metallic film, all transport processes exhibit the size effects [1]. The size effects in Seebeck coefficient of TFTCs are dependent on the predominant type of electron scattering. The scattering mode in turn affects the relaxation time of the thin film. Boundary scattering starts to dominate over bulk scattering when the film thickness is of an order or smaller than the electron mean-free path, typically for films thinner than several hundreds of nanometers. Studies of Pt/Au TFTCs showed that for a film thickness greater than 20 nm, the difference between the electronic thermopower of bulk metal and thin films, ΔSF, has an inversely linear relationship with the film thickness. Film thicknesses lower than this threshold exhibit quantum size effects, leading to a non-linear relationship of ΔSF with film thickness. Assuming that the electrons are free and taking into account size effects, the electronic thermopower of a thin metallic film is given by [1,8]: 𝑈𝑈 3 𝑙𝑙 𝑆𝑆𝐹𝐹 = 𝑆𝑆𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 ( ) (1 − ) (1 − 𝑝𝑝) 1 + 𝑈𝑈 8 𝑡𝑡
where,
(1)
𝑈𝑈 = [
𝑑𝑑ln𝜎𝜎(𝐸𝐸) ] 𝑑𝑑ln𝐸𝐸 𝐸𝐸=𝜉𝜉
(2)
Sbulk is the thermopower of the bulk metal, E is the Fermi energy level, l is the length of the mean free path of electrons, t is the thickness of the film and p is the scattering coefficient. The scattering coefficient is the fraction of the energy lost by charge carriers when reflected from the surface of the film. For t > 150 nm, the Seebeck coefficients assume a maximum value, independent of film thickness which is slightly below the Seebeck coefficient of the bulk thermocouple [9]. The friction element welding (FEW) process offers the possibility of joining dissimilar materials such as lightweight aluminum alloys and high strength steels. This process combines thermal and mechanical welding principles using an auxiliary consumable joining element made of steel. In the FEW process, the workpieces or sheets to be joined are stacked together with the softer material being placed on top of the harder material. The FEW joining process consists of four main steps – penetration, cleaning, welding and compression, which are shown in a schematic in Fig. 1. The penetration process is initiated by rotating the friction element to high speeds using a spindle, which is pressed against the surface of the upper sheet with a high axial force while rotating. The resulting heat generated from the friction between the rotating element and the upper sheet causes the material to be plasticized and allows penetration without the requirement of a pre-hole or melting of the material. The rotating element then contacts the surface of the harder base sheet material, resulting in a significant increase in the temperature of the element due to greater amount of friction. The element material is also plasticized at this stage and its shape is deformed generating a characteristic ‘upset’. The sliding surface of the tip of the element cleans and activates the surface of the base sheet. The cleaning step is essentially pre-heating step prior to welding, which cleans any coatings present on the surface of the base metal sheet, such as galvanneal coating in the case of advanced high strength steel (AHSS) sheets. During the welding step, a friction weld is formed between the element and the base sheet. After the welding step, the rotation of the element is stopped and a large axial compressive force is applied to close any cracks that may have been formed when the element rotation is stopped. The head of the element is also fully seated
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against the upper sheet in the compression step. Throughout the entire FEW process, a hollow cylindrical tube surrounding the spindle (the downholder) keeps the sheets compressed while an anvil below the base sheet provides support [10-12].
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consideration. The diameter of the shaft of the friction element was 4.55 mm, and the diameter of the head of the friction element was 9.8 mm. 8 TFTCs were placed in a linear arrangement such that 4 TFTCs would be situated directly under the area covered by the friction element, while the rest of the TFTCs were situated outside the body of the element. This design allowed us to obtain the temperature profile of different zones as the friction element progressively travels through the substrate. The locations of the TFTCs on the substrate is shown in a schematic in Fig. 2.
Fig. 1. Schematic of the different stages of the FEW process – (1) Penetration, (2) Cleaning, (3) Welding, and (4) Compression [13].
Previous work on the study of process parameters showed that the applied force and generated torque during cleaning and welding steps play a dominant role on the outcome of the FEW process in terms of joint quality. The influence of various FEW process parameters on temperature was also studied by recording temperature profiles of the top surface of the upper sheet using a thermal camera, at a distance of 8 mm from the center of the downholder [11]. However, since the workpieces are bonded together through the friction element during the welding process and also due to the presence of the large downholder component on the top sheet, conventional methods of temperature measurement are infeasible due to the inaccessibility of the weld zone. In this work, the temperatures generated during the various stages of the friction element welding of Al6005 (Al-0.9Si-0.6Mg) and JSC980 (Fe-0.1C0.02Si-2.0Mn) steel sheets were measured using an array of thin film thermocouples fabricated directly on the surface of the JSC980 steel sheet. The effect on temperature response due to the torques and applied compressive forces of the various stages of the FEW process were also investigated. 2. Design, fabrication and calibration of micro TFTCs In order to accommodate a multitude of TFTCs on the substrate adjacent to the welding zone, the dimensions of the friction element were taken into
Fig. 2. Top-view schematic showing locations of TFTCs relative to the center of the friction element.
The TFTCs were fabricated on top of an insulating dielectric multilayer deposited on a JSC980 steel substrate with dimensions of 40×40 mm. Standard Ktype thermocouple materials composed of Chromel (90Cr/10Ni) and Alumel (95Ni/2Al/2Mn/1Si) films were used to fabricate the TFTC legs. An insulating dielectric layer was deposited on the TFTC films to cover and protect the sensors and also allow electrical isolation. 2.1. Mask design Two separate photomasks for the Chromel and Alumel legs of the TFTCs were designed using AutoCAD and printed on a 4×4 inch transparent quartz plate. The layout of the sensor with TFTCs is shown in Fig. 3. There are 8 pairs of TFTCs with a narrowest track width of 50 μm. The overlapping area of each of the Chromel-Alumel junctions of the TFTCs is 50×50 μm. The width of the junction plays an important role in the dynamic response of a thermocouple. Previous studies on TFTC films having different junction widths showed that a response time of about 50 ns can
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be achieved for a film thickness of 100 nm [14]. The spacing between each TFTC junction is 0.875 mm.
peak-to-valley height of the original surface of the substrate was 22.80 ± 2.04 μm and the average surface roughness (Ra) was 0.92 ± 0.04 μm. After polishing, the average peak-to-valley height of the substrate surface was reduced to 0.94 ± 0.70 μm and the average Ra value was reduced to 0.03 ± 0.01 μm. The surface roughness profile maps of the substrate before and after polishing are shown in Fig. 4.
Fig. 3. Layout of TFTCs.
2.2. Preparation of JSC980 steel substrate Extensive preparation of the substrate was necessary to facilitate microfabrication of the TFTCs to be performed in a cleanroom environment. Multiple layers of thin films of the dielectric and metallic materials are required to be precisely deposited on the surface of the substrate during the microfabrication process. It is essential to reduce the height of surface asperities present on the substrate to ensure uniform coverage of thin films, and also to ensure reliable patterning of microscale junction widths of the TFTCs. Manual grinding and polishing steps were used to reduce the surface roughness using a Buehler grinder/polisher machine. Ethanol was used as the coolant for grinding due to the JSC980 surface being reactive toward water. To hold the substrate flat against the grinding wheel, the substrate was attached to a stainless steel mount with a recessed pocket. Grinding was performed successively with 320, 400, 800 and 1200 grit SiC abrasive papers. The substrate was then polished with 6 and 1 μm diamond suspensions. For the final polishing step, the substrate was polished in a 0.05 μm alumina suspension contained in a Buehler VibroMet vibratory polishing machine. During vibratory finishing, the substrates are entrained in the fluidized polishing media with a slow relative velocity against the polishing cloth due to the application of a continuous horizontally oscillating motion. A superior degree of flatness is achieved due to multiple modes of multi-body contact which promotes gentle plastic deformation of the substrate surface by continuous sliding or burnishing. The final surface roughness of the substrate was measured with a Zygo optical surface profilometer. The average
Fig. 4. Surface roughness profile maps of JSC980 substrate – (a) before and (b) after polishing.
2.3. Microfabrication process of TFTCs The entire microfabrication process of the TFTCs was performed in the cleanrooms at the Georgia Institute of Technology’s Marcus Institute of Nanotechnology. A schematic of the microfabrication process is shown in Fig. 5. To ensure removal of any contaminants or residues on the substrate, initial cleaning of the metal substrate was performed in acetone, followed by methanol and then isopropanol accompanied by ultrasonic agitation. The substrate was finally rinsed with deionized water and thoroughly dried with a dry nitrogen spray gun.
Fig. 5. TFTC microfabrication process.
The multilayer insulating dielectric layer was composed of three layers – a 500 nm aluminum oxide (Al2O3) film, followed by a 1 μm silicon nitride
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(Si3N4) film and finally a 500 nm Al2O3 film. Deposition of Al2O3 film can be performed using three approaches – electron beam evaporation, radio frequency (RF) sputtering, and atomic layer deposition (ALD). Although Al2O3 deposition can be successfully performed using ALD process with precise control of layer growth, it is impractical to deposit film of micron-scale thickness due to the extremely slow deposition rate compared to physical vapor deposition methods. On the other hand, RF sputtering of Al2O3 requires effective control of the stoichiometric ratio of aluminum and oxygen content to allow the formation of Al2O3 films with high dielectric properties, and a detailed optimization study is needed to be performed to find the right set of parameters. Therefore, e-beam evaporation was selected for deposition of sufficiently thick Al2O3 films from alumina pellets due to its higher deposition rates. Si3N4 film was deposited using plasmaenhanced chemical vapor deposition (PECVD), which can produce low stress Si3N4 films using chemical precursor gases in a plasma environment. The materials and structure of the multilayer dielectric films were chosen based on extensive investigations related to mutual compatibility of their coefficient of thermal expansions with the substrate and metal layers, including their ability to maintain their dielectric strength at elevated temperatures [15]. It is known that evaporated thin films generally result in poor conformal coverage of surface features and is also prone to developing pinhole defects. Therefore, if the Al2O3 film was used alone as a dielectric layer, presence of pinholes and surface irregularities on the substrate would result in the shorting of the metal layers of the sensors to the substrate [15]. Deposition of a Si3N4 film using the PECVD process on the initial Al2O3 film allows us to achieve conformal coverage of the pinholes and remaining surface asperities on the substrate. However, it is possible that the Si3N4 film might also possess pinhole defects, thus a third layer of Al2O3 was also deposited to complete the multilayer dielectric structure. A dielectric pad test was performed to verify the functionality of the dielectric multilayer. For this test, a square grid of Ti/Chromel pads was deposited by DC sputtering on the dielectric multilayer as shown in Fig. 6. The electrical resistance was measured between each Chromel pad and the exposed areas of the metal substrate was measured with a multimeter. The mutual resistance between each of Chromel pads were also
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measured. The resistance readings for both these cases showed an open circuit, thereby confirming the electrical isolation of the insulating films. In the event that the resistance readings showed a short circuit, it would suggest the presence of pinhole defects in the dielectric films.
Fig. 6. Dielectric pad test.
To initiate the sensor microfabrication process, a 25 nm layer of Ti was initially deposited on the metal substrate by DC sputtering prior to the deposition of the initial Al2O3 layer to promote its adhesion to the substrate. During the deposition of the Ti and dielectric multilayers, a stainless steel shadow mask was attached to the substrate using miniature stainless steel clips to cover areas of the substrate where deposition on films was not desired. Al2O3 was deposited on the substrate using an electron beam evaporation unit with a deposition rate of 18 nm/min, which was reached at a working pressure of 5×10 -5 Torr. Si3N4 was deposited using a PECVD system with a mixture of SiH4 and NH3 gases; N2 and He were additionally used for controlling film stress. A deposition rate of 7.78 nm/min was achieved at a chamber pressure of 1.1 Torr and a substrate temperature of 250°C. For fabrication of TFTCs, Chromel and Alumel legs were patterned using standard photolithography and liftoff processes on top of the insulating films. First, Microposit S1813 positive photoresist was spin coated on the substrate, followed by patterning of the Chromel legs of the sensor using a Karl Suss MA6 mask aligner equipped with a 350 W mercury arc lamp. The exposed photoresist was baked at 115°C for 60 seconds. A 120 nm layer of Chromel was then deposited on the patterned substrate. Liftoff of the Chromel layer was performed in acetone under ultrasonic agitation. The Alumel legs were then patterned after alignment of the second photomask with the Chromel legs in the mask aligner, followed by deposition and liftoff of a 180 nm Alumel layer. A 10 nm adhesion promoting layer of Ti was deposited
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prior to the deposition of the Chromel and Alumel films. The Chromel and Alumel layers were deposited by DC sputtering at a rate of 12-15 nm/min from 3 inch alloy targets with a power of 350 W. After fabrication of the Chromel and Alumel legs, a 500 nm thick Al2O3 cover layer was deposited on the completed sensor by e-beam evaporation. A shadow mask was attached to the substrate during deposition of the cover layer to ensure deposition on selected areas of the fabricated sensor. This insulation layer covered the TFTC junctions and lines except the pad areas. 2.4. Calibration of TFTC Calibration of a TFTC was performed in a temperature-controlled tube furnace from room temperature to 1000°C. Continuous Argon as flow was maintained in the tube furnace during the calibration process. Each exposed pad of the TFTC was bonded with its corresponding fine gage (70 μm) compensation wire with high temperature silver paste (Pyro-Duct 597-A). The wires were then secured onto the substrate using high temperature ceramic cement (Omega CC High Temp). The compensation wires were connected to the differential analog input channels of an NI USB-6003 data acquisition system (DAQ). The Alumel wire was referenced to the ground of the analog input channel of the DAQ device using a bias resistor. Voltage signals from the TFTC from room temperature to 1000°C were recorded with LabVIEW software. A plot of the voltage output of the TFTC including that of a commercial thermocouple corresponding to different temperatures is shown in Fig. 7.
Fig. 7. Calibration plot of TFTC.
The calibration results indicate that the TFTC has a linear response within the measured temperature range. The thermoelectric sensitivity of the TFTC was determined to be 41.3 μV/°C. The TFTC sensitivity was found to closely match the sensitivity of commercial K type thermocouples, 41.5 μV/°C which was calculated from standard voltage-temperature tables [16]. 3. Friction element welding experiment and results The stackup for the friction element welding (FEW) experiment consisted of an Al6005 sheet and a JSC980 steel sheet with embedded TFTCs. The dimensions of both the sheets were 40×40 mm. The thickness of the Al6005 sheet was 1.7 mm and that of the JSC980 sheet was 1.16 mm. An EJOWELD® FEW machine has a maximum spindle speed of 9000 RPM, maximum element endload of 9 kN and maximum downholder force of 6 kN. An EJOWELD® 4.55×4.5-Z-8 steel element was used as the friction element, having a cylindrical tip (Z) with a shaft diameter of 4.55 mm and length of 4.5 mm. The element is composed of a grade 8 steel which has a strength class of 800-1000 MPa. The data related to spindle speed, torque, applied force, spindle position and element way were recorded by the FEW machine controller at a sampling rate of 1 kHz during the entire welding process. Plots of the recorded data generated from the FEW machine are shown in Fig. 8.
Fig. 8. Plots of spindle speed, torque, force, spindle travel and element way generated during the FEW process. The steps involved in the FEW process are marked on the plot with solid blue lines.
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For positioning of the center of the friction element relative to the location of TFTC1 on the JSC980 sheet, a laser alignment pointer was used to mark the center of the desired welding point. The laser pointer is aligned with the longitudinal axis of the FEW machine spindle. The location of this point was situated 1.4 mm from the TFTC1 junction. In order to record voltage signals from the TFTCs on the JSC980 sheet, Chromel and Alumel compensation wires were attached to the corresponding TFTC pads with Kapton tape. The compensation wires were then connected to a NI-USB 6003 DAQ system. The voltage signals generated during the FEW process were recorded at a sampling rate of 25 kHz using LabVIEW software. Postprocessing and analysis of the recorded sensor voltage signals was performed using OriginPro software. The voltage signals were filtered using applying a low pass Fast Fourier Transform (FFT) filter to remove the high frequency noise components in the signal, as shown in Fig. 9.
Fig. 9. Raw voltage signal obtained from TFTC3. A 50 Hz FFTbased low pass filter is applied to attenuate noisy components in the raw signal.
The filtered signals were then smoothed by applying a locally weighted linear regression function with a span value of 0.02. The smoothed voltage signals were then converted to temperature using the calibration relationship of the TFTC described in Section 2. The temperature profile of the FEW process obtained from the TFTC located at a distance of 3.15 mm from the center of the element is shown in Fig. 10. In order to identify the temperature transitions between occurring between the various stages of the FEW process, the second derivative of the temperature profile was used. The second derivative was used in
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this case because using the first derivative of the temperature profile was too noisy to be used to identify the transitions.
Fig. 10. Temperature profile and its second derivative obtained from TFTC3, positioned 3.15 mm from center of element. The different steps of the FEW process on the temperature profile are marked with dashed lines (A: contact of element tip and downholder with top sheet, 1: Penetration, 2: Cleaning, 3: Welding, and 4: Compression)
At the beginning of the FEW process the downholder and spindle assembly holding the friction element travels down towards the top surface of the Al6005 sheet. When the tip of the friction element comes in contact with the Al6005 sheet surface, an axial force of 1 kN is applied on the head of the element. The pressure applied by the element tip generates an indent of 0.2 mm on the Al6005 sheet surface, which also generates a small instant fluctuation in the voltage signal generated from the TFTCs, which is observed as an initial spike on the second derivative of the temperature profile shown in Fig. 10. The ‘element way’ is then set to 0 by the FEW machine controller. The friction element begins to penetrate through the Al6005 sheet while the spindle speed is gradually ramped up to 5000 rpm. As the force applied on the element is increased at this time, the torque rises up to 5.47 Nm, signifying the increase in frictional resistance of the rotating element tip against the Al6005 sheet. The softening of the Al6005 sheet from plasticization then results in a decrease in the torque. From the temperature profile shown in Fig. 10, the first transition in the temperature profile occurs after 0.30 seconds from the contact of the element with the Al6005 sheet, where the temperature rises up to 148°C during the penetration of the Al6005 sheet by
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the element tip. This transition has a duration of 0.15 seconds, following which the temperature profile shows a sharp rise to 239°C during the cleaning step. The continuous sliding of the element tip against JSC980 surface leads to repeated occurrence of joining and separation at the interface, which causes temperature fluctuations for a period of 0.153 seconds. The increase in temperature can be attributed to the fact that the JSC980 sheet is harder than the Al6005 sheet, facilitating a higher amount of frictional resistance on the element tip. The significant rise in friction and temperature results in the deformation of the tip of the element due to plasticization of the element material. The second transition in the temperature profile corresponds to the welding step, where the temperature keeps rising until a peak temperature of 414°C is reached. The compressive force and rotational speed are increased to 7.6 kN and 6000 rpm respectively, resulting in a significant increase in frictional torque to 7.78 Nm. These conditions result in the localized softening of the interface between the JSC980 surface and the element, leading to further temperature increase from extensive plastic deformation. The element is rigidly bonded to the JSC980 sheet at the end of the welding step. After completion of the welding step, the axial force is further increased to 9 kN and the spindle rotation is stopped. The forging force applied in the compression step essentially allows pressure-welding of any gaps or cracks present in the weld zone. The head of the element is also firmly seated against the Al6005 sheet at the end of the compression step. The heat generated at the end of the welding step is gradually dissipated as shown by the decrease of the temperature profile in step 4. Representative temperature profiles from TFTC5 and TFTC6, located 4.9 mm and 5.78 mm from the center of the element respectively, are shown in Fig. 11. No significant rise in temperature is observed during the FEW process at these locations until the end of the compression stage, where a peak temperature of up to 130°C is reached. The delayed temperature profile obtained from TFTCs 5 and 6 suggest the effect of gradual heat dissipation propagating radially outward from the center of the weld zone.
Fig. 11. Representative temperature profiles obtained from TFTCs 3, 5 and 6.
In order to correlate the temperature profiles obtained from the different TFTCs to the locations of the TFTCs present on the JSC980 surface during the friction element welding process, the cross-section of the bonded sheets was observed under an optical microscope. The bonded sheets were sectioned along the center of the friction element using a high speed sectioning saw. The cross-section was mounted in epoxy followed by grinding and polishing. The surface of the cross-section was then etched using a 2% Nital solution to reveal the microstructure. An image of the cross-section of the welded joint, and locations of the TFTCs 1-6 are shown in Fig. 12. A distinct interface between the friction element and the JSC980 sheet is visible in the image of the cross-section. The deformation of the top surface of the JSC980 sheet during the welding process possibly resulted in the relocation of the TFTC junctions. As shown in Fig. 12, TFTC1 was located at a distance of 1.4 mm from the center of the element, while the other junctions were located 0.875 mm away from each other. The location of the TFTC1 junction within the weld zone resulted in its failure during the frictional shearing of interface between the JSC980 sheet and the friction element.
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Fig. 12. Optical image of the cross-section of the welded region between the friction element, JSC980 and Al6005 sheets. The locations of the TFTCs numbered 1-3 are shown with red circles.
The position of the TFTC2 junction can be observed to be at the meeting point between the interfaces of the deformed Al6005 and JSC980 sheets and the element shaft. The position of TFTC2 did not allow sufficient contact with the element during the welding process. The extensive deformation of the weld zone resulted in the delamination of the insulation and thermocouple films at the location of TFTC2 junction, contributing to its failure. The TFTC3 junction is located at a distance of 3.15 mm from the center of the element, where there was sufficient contact between the JSC980 and Al6005 sheets. The distance of TFTC3 from the center of the weld zone was sufficient to avoid early failure during the FEW process. However, the TFTC4 junction was located at a distance of 4.025 mm from the center of the element, which was situated close to the periphery of the downholder which presses down firmly on top of the Al6005 sheet during the entire duration of the FEW process. The extremely high contact forces exerted on the sheets at the vicinity of the downholder contributed to the failure of TFTC4 during the welding process. The TFTC5 and TFTC6 junctions were located 4.9 mm and 5.78 mm respectively from the center of the element. Due to the further distance of these junctions compared to TFTCs 1-4 from the weld zone, the temperatures detected at these locations do not reach their peak values rapidly during the FEW process.
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of Al6005 and JSC980 sheets. The TFTCs were composed of standard K-type thermocouple materials from overlapping junctions (50×50 μm) of 120 nm Chromel and 180 nm Alumel thin films. Calibration of the TFTCs showed good linear sensitivity of 41.3 μV/°C to a temperature of 1000°C. The overall transient temperature profile of during each stage of the FEW process, namely – penetration, cleaning, welding and compression were obtained from the TFTCs. The evolution of the temperature profiles were correlated with the torque generated and applied compressive forces during each stage of the FEW process. Increased frictional resistance due to plasticization of the element tip against the JSC980 substrate during the cleaning and welding steps of the FEW process allowed the process temperature to reach up to 239 - 414°C, at a distance of 3.15 mm from the center of the friction element. Future work will include individual measurements of the transient temperature response of each step of the FEW process, for a fixed combination of applied force and spindle rotation. These studies will enable the validation of analytical and computational models of the temperature evolution of the FEW process. Microstructural characterization of the weld zone will also be investigated in order to study the influence of temperatures reached during each step of the welding process on the microstructure of the bonding interface. Acknowledgements The authors would like to extend their thanks to EJOT Fastening Systems L.P. for their expertise and facilitating access to the EJOWELD® Friction Element Welding system. References [1]
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4. Conclusions
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An array of micro thin film thermocouples (TFTCs) were fabricated on top of a multilayer insulating dielectric film on the surface of a JSC980 steel substrate for measurement of temperatures generated at the interface between the friction element welding
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