Effects of Refill Friction Stir Spot Weld Spacing and Edge Margin on Mechanical Properties of Multi-Spot-Welded Panels

Journal of Manufacturing and Materials Processing

Article Eﬀects of Reﬁll Friction Stir Spot Weld Spacing and Edge Margin on Mechanical Properties of Multi-Spot-Welded Panels

Guruvignesh Lakshmi Balasubramaniam 1, Enkhsaikhan Boldsaikhan 1,* , Shintaro Fukada 2, Mitsuo Fujimoto 2 and Kenichi Kamimuki 3

1 Industrial, Systems & Manufacturing Engineering Department, Wichita State University, Wichita, KS 67260, USA; [email protected] 2 Corporate Technology Division, Kawasaki Heavy Industries, Ltd., Akashi, Hyogo 673-8666, Japan; [email protected] (S.F.); [email protected] (M.F.) 3 Aerospace Division, Kawasaki Heavy Industries, Ltd., Kakamigahara, Gifu 504-8710, Japan; [email protected] * Correspondence: [email protected]; Tel.: +1-316-978-6323

Received: 30 April 2020; Accepted: 2 June 2020; Published: 7 June 2020

Abstract: Refill friction stir spot welding (RFSSW) is an emerging technology for joining aerospace aluminum alloys. The aim of the study is to investigate the effects of the refill friction stir spot weld spacing and the edge margin on the mechanical properties of multi-spot-welded AA7075-T6 panels. AA7075-T6 is a baseline aerospace aluminum alloy used in aircraft structures. The study employs an innovative robotic RFSSW system that is designed and developed by Kawasaki Heavy Industries (KHI). The experimental strategy uses Design of Experiments (DoE) to characterize the failure loads of multi-spot-welded panels in terms of the spot weld spacing, edge margin, and heat-affected zone (HAZ) of the spot weld. The RFSSW process leaves behind a thermal “imprint” as HAZ in heat-treatable aluminum alloys. According to the DoE results, larger spot weld spacings with no HAZ overlap produce higher failure loads of multi-spot-welded panels. On the other hand, edge margins that are equal to or less than the spot weld diameter demonstrate abnormal plastic deformations, such as workpiece edge swelling and weld crown dents, during the RFSSW process. The larger edge margins do not demonstrate such abnormal deformations during the welding process.

Keywords: reﬁll friction stir spot welding; aerospace aluminum alloy; robotic spot welding; spot weld spacing; edge margin

1. Introduction The consumption of aluminum alloys is growing in many industries, as they are lightweight and easy to recycle. This growth creates the need for better ways of welding aluminum alloys with satisfactory mechanical and metallurgical properties [1]. Aluminum alloys welded with conventional fusion welding techniques exhibit the formation of brittle intermetallic compounds that usually cause severe intermetallic cracking [2]. It is diﬃcult or almost impossible to weld high-strength aluminum alloys with conventional fusion welding techniques. Hence, solid-state welding techniques, such as ultrasonic welding and friction stir welding, are better alternatives to welding aluminum alloys. Zhang et al. [3] reported their research ﬁndings on the segregation and intermetallic precipitation of alloying elements at the material interface during the dissimilar ultrasonic welding of AA6111 and Ti6Al4V. Macwan et al. [4] characterized the mechanical and microstructural properties of ultrasonic spot welds of a rare earth-containing ZEK100 magnesium alloy and AA5754.

J. Manuf. Mater. Process. 2020, 4, 55; doi:10.3390/jmmp4020055 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 55 2 of 17

Iwashita et al. [5] and Fujimoto et al. [6] documented a friction stir spot welding method for joining aluminum alloys for automotive applications. Friction Stir Spot Welding (FSSW) is a solid-state welding technique that is a variant of friction stir welding (FSW) invented by The Welding Institute in 1991 [7]. Li et al. [8] documented the current status and the research progress of Al-Cu FSSW in terms of weld tool features, mechanical and microstructural properties, and weld defects. Su et al. [9] demonstrated FSSW of dissimilar AA5754 and a magnesium alloy. FSSW leaves behind a keyhole, which is a tool exit hole at the center of the spot weld. A keyhole causes stress concentrations that degrade the structural integrity of the assembly. Additionally, a keyhole is susceptible to corrosion, as its inner surfaces are hard to reach during painting. Therefore, refilling strategies for FSSW have been explored by numerous researchers, including Schilling and Dos Santos [10] as well as Okada et al. [11,12]. Refill friction stir spot welding (RFSSW) is an emerging technology for joining aerospace aluminum alloys. Currently, aerospace industries use riveting to assemble aircraft components, as riveting bears well-established standards and specifications. Unlike riveting, RFSSW does not require any filler or foreign materials for joining and, hence, no additional weight is added to the assembly. RFSSW thermo-mechanically produces a molecular level bond between workpieces. There is no lack of fusion or material deterioration exhibited by RFSSW, as it does not involve major phase transitions during the welding process. Unlike FSSW, RFSSW produces a spot joint with a near-flush surface finish that is free from a key or exit hole. Schmal et al. [13] explored the influences of the RFSSW process parameters on the joint formation and load-bearing capacities. Okada et al. [11,12] and Boldsaikhan et al. [14] successfully demonstrated the use of RFSSW for joining high-strength aerospace aluminum alloys. The robotic RFSSW system used in this study is shown in Figure1. This innovative robotic RFSSW system was designed and developed by Kawasaki Heavy Industries (KHI) [11,12]. The C-frame RFSSW end effector in Figure1b is equipped with a clamp tip, a backing anvil, and a retractable weld tool. The weld tool consists of a retractable probe enclosed in a cylindrical shoulder. The sizes of the refill spot weld are defined by the tool shoulder diameter and the tool plunge depth. During the RFSSW process, the weld tool goes through five different stages [12,15].

Stage 1: Stage 1 involves workpiece clamping and preheating. The workpieces are firmly clamped • between the backing anvil and the clamp tip. Then, the rotating weld tool touches down and resides on the top surface of the workpiece for a certain time period to preheat the material via friction. Stage 2: In this stage, the shoulder plunges into the workpiece and the probe retracts. The retraction • of the probe opens a cavity to accommodate the material displaced by the plunging shoulder. When the targeted plunge depth is reached, the weld tool dwells in that position for a certain time period. The shoulder-plunging process requires a higher plunge force to stir a higher volume of material to achieve a stronger weld [12,15]. This study employed the shoulder-plunging process. Stage 3: In the third stage, the rotating probe and the shoulder move in the reverse directions • to re-inject the displaced material back into the weld chamber. Furthermore, to improve the flushness of the surface, the probe and the shoulder continue to move in the reverse directions until they slightly exceed their aligned position. Stage 4: In Stage 4, the probe and the shoulder are aligned with each other right on the top surface • of the workpiece to leave behind a near-flush surface finish. Stage 5: In the fifth stage, the weld tool is removed. • J. Manuf. Mater. Mater. Process. Process. 20202020,, 4,, x 55 FOR PEER REVIEW 33 of of 17

(a) (b)

Figure 1. Refill friction stir spot welding work-cell: (a) refill friction stir spot welding robot; (b) refill Figure 1. Refill friction stir spot welding work-cell: (a) refill friction stir spot welding robot; (b) refill friction stir spot welding end effector. friction stir spot welding end effector. The RFSSW process leaves behind three distinct weld zones in the workpiece, which are a The RFSSW process leaves behind three distinct weld zones in the workpiece, which are a heat- heat-affected zone (HAZ), a Thermo-Mechanically-Affected Zone (TMAZ), and a weld nugget. HAZ is affected zone (HAZ), a Thermo-Mechanically-Affected Zone (TMAZ), and a weld nugget. HAZ is a a region in the heat-treatable base metal that spans around the weld as its outermost shell. It is affected region in the heat-treatable base metal that spans around the weld as its outermost shell. It is affected by the process heat but not the mechanical stirring. TMAZ is an interlayer shell between the HAZ and by the process heat but not the mechanical stirring. TMAZ is an interlayer shell between the HAZ the weld nugget. It is affected by the process heat as well as the mechanical stirring involved in the and the weld nugget. It is affected by the process heat as well as the mechanical stirring involved in weld nugget. The weld nugget is the core of the weld, where the tool forges and stirs the plasticized the weld nugget. The weld nugget is the core of the weld, where the tool forges and stirs the material together during the welding process. It experiences the greatest heat and plastic deformations plasticized material together during the welding process. It experiences the greatest heat and plastic that lead to significant grain refinement and re-precipitation. The size of the RFSSW weld nugget is deformations that lead to significant grain refinement and re-precipitation. The size of the RFSSW defined by the outer diameter of the weld tool shoulder and its plunge depth. The metal grain size weld nugget is defined by the outer diameter of the weld tool shoulder and its plunge depth. The usually coarsens in TMAZ and HAZ during and after the welding process. However, the grain size metal grain size usually coarsens in TMAZ and HAZ during and after the welding process. However, becomes fine and equiaxed in the weld nugget due to grain refinement and re-precipitation involved the grain size becomes fine and equiaxed in the weld nugget due to grain refinement and re- in the weld nugget. precipitation involved in the weld nugget. As RFSSW and FSSW are variants of FSW, they demonstrate similar weld properties. Fonda and As RFSSW and FSSW are variants of FSW, they demonstrate similar weld properties. Fonda and Bingert [16] investigated TMAZ and HAZ produced by FSW of AA2519 to determine their effects on the Bingert [16] investigated TMAZ and HAZ produced by FSW of AA2519 to determine their effects on mechanical properties of the weld. A micrographic map revealed that the location of the fracture was the mechanical properties of the weld. A micrographic map revealed that the location of the fracture at the boundary between TMAZ and HAZ, where the metal grains are coarser [16]. According to Paglia was at the boundary between TMAZ and HAZ, where the metal grains are coarser [16]. According and Buchheit [17], the sensitization in HAZ of friction-stir-welded aluminum alloys is responsible for to Paglia and Buchheit [17], the sensitization in HAZ of friction-stir-welded aluminum alloys is the corrosion susceptibility. Khodir and Shibayanagi [18] studied the microstructure and the mechanical responsible for the corrosion susceptibility. Khodir and Shibayanagi [18] studied the microstructure properties of friction-stir-welded dissimilar AA2024-T3 and AA7075-T6. They found that the hardness and the mechanical properties of friction-stir-welded dissimilar AA2024-T3 and AA7075-T6. They values of FSW increase with the increasing tool travel speed [18]. Zhao et al. [19], Arul et al. [20], found that the hardness values of FSW increase with the increasing tool travel speed [18]. Zhao et al. Freeney et al. [21], and Tozaki et al. [22] documented that FSSW produced higher weld strengths with [19], Arul et al. [20], Freeney et al. [21], and Tozaki et al. [22] documented that FSSW produced higher the decreasing tool spindle speed. However, low spindle speeds may induce an insufficient heat input weld strengths with the decreasing tool spindle speed. However, low spindle speeds may induce an that leads to the formation of lack-of-consolidation defects and/or weld tool failures. Tran et al. [23] insufficient heat input that leads to the formation of lack-of-consolidation defects and/or weld tool demonstrated that the failure loads of friction stir spot welds grow with the increasing process time. failures. Tran et al. [23] demonstrated that the failure loads of friction stir spot welds grow with the The literature on FSW, FSSW, and RFSSW mostly addresses relationships between the process increasing process time. parameters and the resultant weld properties. The novel contribution of this study is the investigation The literature on FSW, FSSW, and RFSSW mostly addresses relationships between the process into the effects of the RFSSW design parameters, such as the spot weld spacing and the edge margin, parameters and the resultant weld properties. The novel contribution of this study is the investigation on the mechanical properties of multi-spot-welded aircraft structures. A spot weld spacing refers to into the effects of the RFSSW design parameters, such as the spot weld spacing and the edge margin, a center-to-center distance between two adjacent spot welds. An edge margin is an edge-to-center on the mechanical properties of multi-spot-welded aircraft structures. A spot weld spacing refers to distance between the material edge and a refill spot weld. A refill spot weld usually starts to fail in a center-to-center distance between two adjacent spot welds. An edge margin is an edge-to-center TMAZ / HAZ, where the metal grains are coarser due to the thermo-mechanical processing of the distance between the material edge and a refill spot weld. A refill spot weld usually starts to fail in welding process. Therefore, the extent of HAZ/TMAZ of the refill spot weld is critical to properly TMAZ / HAZ, where the metal grains are coarser due to the thermo-mechanical processing of the arranging the design parameters of RFSSW. welding process. Therefore, the extent of HAZ/TMAZ of the refill spot weld is critical to properly arranging the design parameters of RFSSW.

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The aim of the study is to investigate the effects of the refill friction stir spot weld spacing and edge margin on mechanical properties of multi-spot-welded panels with an emphasis on aerospace applications. The study uses a baseline aerospace aluminum alloy, AA7075-T6, used in aircraft structures [24]. A spot weld spacing (pitch) is a center-to-center distance between two adjacent spot welds in the same row. An edge margin (EM) is a center-to-edge distance between a spot weld and the material edge. The experimental strategy uses Design of Experiments (DoE) to characterize the failure loads of multi-spot-welded AA7075-T6 panels in terms of the spot weld spacing, the edge margin, and the spot weld HAZ. The multi-spot-welded panels are subjected to static lap-shear pull tests and Vickers microhardness tests to identify their failure loads and the spot weld HAZ patterns, respectively. The spot weld spacing (pitch) and the edge margin are studied in relation to the size of the HAZ. As mentioned earlier, the RFSSW process leaves behind a thermal “imprint” as HAZ in heat-treatable aluminum alloys. The remaining sections of this article are organized as follows. Section2 presents the workpiece materials, experimental conditions, and research methods used in this study. Section3 contains the test results and a discussion of the analysis results. Section4 summarizes the conclusions supported by the research findings.

2. Materials and Methods Sheet metals of bare 1.6 mm-thick AA7075-T6 were refill friction stir spot welded in a lap joint configuration to produce multi-spotwelded panels. AA7075-T6 is a baseline aerospace aluminum alloy used in aircraft structures [24]. The properties of AA7075-T6 are shown in Table1. The RFSSW tool consists of a retractable probe with a diameter of 4 mm and a sleeve shoulder with an outer diameter of 7 mm. The RFSSW process employed the shoulder-plunging method. The outer diameter of the sleeve shoulder defines the diameter of the refill spot weld. The study used the following RFSSW process parameters to produce all the multi-spot-welded AA7075-T6 panels.

Plunge depth: 1.9 mm. • Probe speed: 4 mm/s. • Spindle speed: 1400 rpm. • It is assumed that the HAZ geometry of a refill spot weld is radial. To identify the diameter of the HAZ, a single spot weld coupon was produced with bare 1.6 mm-thick AA7075-T6 sheets using the same RFSSW tool and the same process parameters. Figure2 depicts a cross-section image of the single spot weld coupon, where the top and bottom sheets are 1.6 mm-thick AA7075-T6 sheets. Figure3 exhibits a cross-sectional microhardness map of the single spot weld coupon. Figure3 suggests that the diameter of HAZ is about 3 times the diameter of the refill spot weld. The diameter of the refill spot weld is 7 mm, which is the outer diameter of the tool shoulder.

Table 1. Properties of AA7075-T6.

AA7075-T6 Hardness Value, Vickers 175 HV Ultimate Tensile 572 MPa Strength Tensile Yield Strength 503 MPa Elongation 11% J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 16 J. Manuf. Mater. Process. 2020, 4, 55 5 of 17 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 16

Figure 2. Cross-section image of a refill friction stir spot weld. The top and bottom sheets are bare 1.6

mm-thick AA7075-T6. The refill spot weld diameter is 7 mm. Figure 2. Cross-sectionCross-section image image of of a arefill reﬁll friction friction stir stir spot spot weld. weld. The The top top and and bottom bottom sheets sheets are arebare bare 1.6 1.6mm-thick mm-thick AA7075-T6. AA7075-T6. The The refill reﬁll spot spot weld weld diameter diameter is 7 ismm. 7 mm.

MC4-TopAA7075-T6 AA7075-T6 Top Sheet MC4-BottomAA7075-T6 Bottom AA7075-T6 Sheet

200 MC4-TopAA7075-T6 AA7075-T6 Top Sheet MC4-BottomAA7075-T6 Bottom AA7075-T6 Sheet 190 Heat Affected Zone (HAZ) 200 180 Heat Affected Zone (HAZ) 190 Weld Nugget 170 180 160 Weld Nugget 170 150 160 140 150 130 140

Hardness Value, HV 120 130 110

Hardness Value, HV 120 100 110 -20 -15 -10 -5 0 5 10 15 20 100 Distance from the weld center, mm -20 -15 -10 -5 0 5 10 15 20 Figure 3. Cross-sectional microhardnessDistance map from of the the refill weld spot weld.center, The mm hardness values are taken Figurefrom the 3. midplaneCross-sectional lines ofmicrohardness the top sheet map and theof the bottom refill sheet spot ofweld. a weld The cross-section. hardness values The are top taken sheet fromand thethe bottommidplane sheet lines are of 1.6 the mm-thick top sheet AA7075-T6. and the bo Thettom base sheet metal of a hardnessweld cross-section. value is 175 The HV. top The sheet refill andspotFigure the weld 3.bottom Cross-sectional diameter sheet is are 7 mm. 1.6microhardness mm-thick AA7075-T6. map of the Threfeill base spot metal weld. hardness The hardness value valuesis 175 areHV. taken The refillfrom spot the midplaneweld diameter lines isof 7the mm. top sheet and the bottom sheet of a weld cross-section. The top sheet Theand the experimental bottom sheet method are 1.6 mm-thick of this study AA7075-T6. was based The base on Design metal hardness of Experiments value is 175 (DoE). HV. AThe 32 full factorialTherefill designexperimental spot weld was diameter employed method is 7 mm. withof this two study design was factors, based whichon Design are the of spotExperiments weld spacing (DoE). (pitch) A 32 full and factorialthe edge design margin was (EM). employed The DoE with response two design variable factors, was the which ultimate are the lap-shear spot weld load spacing (the failure (pitch) load) and of thea refill edgeThe spot marginexperimental weld. (EM). A spot Themethod weld DoE spacing ofresponse this (pitch)study variable iswas a center-to-center basedwas the on ultimate Design distance lap-shearof Experiments between load (the two (DoE). adjacentfailure A 3load)2 spotfull ofweldsfactorial a refill within designspot theweld. was same Aemployed spot row. weld An with edge spacing two margin design(pitch) is a factors, center-to-edgeis a center-to-center which are distance the distance spot between weld between aspacing spot two weld (pitch) adjacent and and the spotmaterialthe edgewelds edge.margin within The (EM). the 32 fullsame The factorial DoErow. responseAn design edge consistsvariablemargin is ofwa a nine scenter-to-edge the DoE ultimate runs withlap-shear distance the configurations loadbetween (the afailure spot listed weldload) in andTableof a therefill2. Ninematerial spot DoE weld. edge. runs A Thespot produced 3 weld2 full ninefactorialspacing 9-spot-welded (pitch)design is consists a center-to-center panels, of whereasnine DoE distance the runs pitch with between and the the configurations edgetwo adjacent margin listedwerespot weldssystematicallyin Table within 2. Nine the varied DoE same runs according row. produced An to edge Table nine margin2.D( 9-spot-welded= is7 mm)a center-to-edge is the panels, spot weldwhereas distance diameter. the between pitch The and edgea spot the margin edgeweld marginwasand variedthe were material between systematically edge. 1D, The 1.5D, 3varied2 full and factorial 2D.according The pitchdesign to Table (the consists spot 2. D weld of (=7 nine mm) spacing) DoE is theruns was spot with varied weld the between diameter.configurations 2D, The 3D, edgeandlisted 4D.margin in TheseTable was 2. variations Nine varied DoE between were runs studied produced 1D, 1.5D, inrelation nine and 9-spot-welded 2D. to the The size pitch of panels, the(the HAZ. spot whereas weld the spacing) pitch and was the varied edge betweenmarginIt is were2D, assumed 3D, systematically and that 4D. the These HAZsvaried variations ofaccording the 9-spot-weldedwere to st Tableudied 2.in panels Drelation (=7 mm) should to theis the looksize spot of similar the weld HAZ. todiameter. the HAZ The in FigureedgeIt margin 3is. Withassumed was that varied beingthat the said,between HAZs the 1D-pitch of1D, the 1.5D, 9-spot-welded allows and 2D. the HAZsThe panels pitch of adjacent should(the spot spotlook weld weldssimilar spacing) to to overlap, the was HAZ varied as the in Figurediameterbetween 3. With2D, of the 3D, that HAZ and being is4D. about said, These 3D. the variations The1D-pitch 2D-pitch wereallows allows st udiedthe theHAZs in HAZs relation of adjacent of adjacent to the spot size spot welds of welds the to HAZ. overlap, to barely as touch the diametereachIt other. is ofassumed Thethe HAZ 4D-pitch that is theabout allows HAZs 3D. no Theof HAZ the 2D-pitch overlaps.9-spot-welded allo Thews spotthe panels HAZs weld should HAZsof adjacent look and thesimilar spot spot welds to weld the to spacingsHAZ barely in touchareFigure illustrated each 3. With other. inthatFigure The being 4D-pitch4. said, the allows 1D-pitch no HAZ allows overlaps. the HAZs The of adjacentspot weld spot HAZs welds and to theoverlap, spot asweld the spacingsdiameter are of illustratedthe HAZ is in about Figure 3D. 4. The 2D-pitch allows the HAZs of adjacent spot welds to barely touch each other. The 4D-pitch allows no HAZ overlaps. The spot weld HAZs and the spot weld spacings are illustrated in Figure 4.

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TableTable 2. 2.DoE DoE runsruns forfor makingmaking 9-spot-welded9-spot-welded panels. panels. DD ((= =77 mm) mm) is is the the spot spot weld weld diameter. diameter.

RunRun # # Edge Edge Margin Margin Spot WeldWeld Spacing Spacing (Pitch) (Pitch) 1 1 1D 1D (=7 (=7 mm) mm) 2D 2D (= (=1414 mm) mm) 2 21D 1D 3D 3D 3 31D 1D 4D 4D 4 41.5D 1.5D 2D 2D 5 1.5D 3D 5 61.5D 1.5D 4D 3D 6 71.5D 2D 2D 4D 7 82D 2D 3D 2D 8 92D 2D 4D 3D 9 2D 4D

HAZ

4D Pitch 3D Pitch 2D Pitch No HAZ overlaps Partial HAZ overlaps Major HAZ overlaps

Figure 4. Heat-affected zone (HAZ) and spot weld spacing (pitch). D (=7 mm) is the spot weld diameter. Figure 4. Heat-affected zone (HAZ) and spot weld spacing (pitch). D (=7 mm) is the spot weld diameter. The 1D edge margin puts the workpiece edge inside the HAZ. The 1.5D edge margin puts the workpiece edge on the edge of the HAZ. The 2D edge margin puts the workpiece edge outside of the The 1D edge margin puts the workpiece edge inside the HAZ. The 1.5D edge margin puts the HAZ. The spot weld HAZs and the edge margins are illustrated in Figure5. workpiece edge on the edge of the HAZ. The 2D edge margin puts the workpiece edge outside of the Sheets of metal were welded in a lap joint configuration, as specified in Figure6. Each welded HAZ. The spot weld HAZs and the edge margins are illustrated in Figure 5. panel had nine spot welds. The spot welds were numbered from left to right, as shown in Figure6. Sheets of metal were welded in a lap joint configuration, as specified in Figure 6. Each welded The 1-9-5-3-7-2-8-4-6 welding sequence was used for producing every welded panel. During the panel had nine spot welds. The spot welds were numbered from left to right, as shown in Figure 6. welding process, in order to avoid heat accumulation in the weld tool and the workpieces the weld The 1-9-5-3-7-2-8-4-6 welding sequence was used for producing every welded panel. During the tool was cooled down with compressed air for at least 90 s after every weld run. This 90 s delay welding process, in order to avoid heat accumulation in the weld tool and the workpieces the weld between two successive weld runs was enough for bringing the tool temperature and the workpiece tool was cooled down with compressed air for at least 90 s after every weld run. This 90 s delay temperature down to their normal levels at room temperature. Therefore, it was assumed that HAZs between two successive weld runs was enough for bringing the tool temperature and the workpiece of refill spot welds in a 9-spot-welded panel should look similar to the HAZ in Figure3. temperature down to their normal levels at room temperature. Therefore, it was assumed that HAZs After producing all the welded panels, they were naturally aged for at least two weeks. of refill spot welds in a 9-spot-welded panel should look similar to the HAZ in Figure 3. After natural aging, three single-spot-welded coupons including Spot Weld 3, Spot Weld 4, and Spot After producing all the welded panels, they were naturally aged for at least two weeks. After Weld 5 were extracted from each panel according to Figure6. The width of a single-spot-welded natural aging, three single-spot-welded coupons including Spot Weld 3, Spot Weld 4, and Spot Weld coupon was the spot weld spacing. Unguided static lap-shear pull tests were carried out on the 5 were extracted from each panel according to Figure 6. The width of a single-spot-welded coupon single-spot-welded coupons to identify their ultimate lap-shear loads (the failure loads). The pull rate was the spot weld spacing. Unguided static lap-shear pull tests were carried out on the single-spot- of the static test was 1.27 mm/min. welded coupons to identify their ultimate lap-shear loads (the failure loads). The pull rate of the static test was 1.27 mm/min.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 16 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 16 J. Manuf.J. Manuf. Mater. Mater. Process. Process.2020 2020, 4, ,4 55, x FOR PEER REVIEW 7 of7 16 of 17

HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ

1D EM 1.5D EM 2D EM 2D EM EM1D inside EM HAZ EM on1.5D the edgeEM of HAZ EM outside HAZ EM inside1D EM HAZ EM on the1.5D edge EM of HAZ EM outside2D EM HAZ EM inside HAZ EM on the edge of HAZ EM outside HAZ Figure 5. Heat-affected zone (HAZ) and edge margin (EM). D (=7 mm) is the spot weld diameter. FigureFigure 5.5. Heat-affectedHeat-aﬀected zone zone (HAZ) (HAZ) and and edge edge margin margin (EM). (EM). D D(=7 ( =mm)7 mm) is the is thespot spot weld weld diameter. diameter. Figure 5. Heat-affected zone (HAZ) and edge margin (EM). D (=7 mm) is the spot weld diameter.

Coupons Pitch

95mm Coupons 28mm Pitch 28mm Cross-Section 95mm 28mm 28mm Coupons Cross-Section Pitch Specimen

95mm Specimen 28mm 28mm Cross-Section EM 6 7 EM Specimen 1 4 5 6 7 8 1 2 3 4 5 67 8 9 2 3 9 1 EM EM4567 8 1 234 5 6 7 8 9 23 9 1 EM 4 5 6 7 8 1 2 3 4 5 8 9 9 2 3 EM Pitch 95mm Pitch 95mm Pitch 95mm

FigureFigure 6. 6.Top Top views views of of 9-spot-welded 9-spot-welded panels: panels: ((leftleft)) p panelanel configuration; conﬁguration; (r (ight)right c)ut cut plan. plan. The The refill reﬁll spot spot Figure 6. Top views of 9-spot-welded panels: (left) panel configuration; (right) cut plan. The refill spot weldswelds are are numbered numbered from from left left to to right. right. Notes:Notes: EM = EdgeEdge Margin; Margin; p pitchitch = =SpotSpot Weld Weld Spacing. Spacing. weldsFigure are 6. numbered Top views from of 9- spotleft to-weld right.ed Notes:panels: EM (left) = Edgepanel Margin; configuration; pitch = ( Spotright) Weld cut plan. Spacing. The refill spot InweldsIn addition addition are numbered to to the the single-spot-welded singlefrom left-spot to right.-weld Notes:ed coupons, EM = Edge Spot Spot Margin; Weld Weld p 6itch 6and and = Spot Spot Spot Weld Weld Weld Spacing. 7 7were were extracted extracted In addition to the single-spot-welded coupons, Spot Weld 6 and Spot Weld 7 were extracted fromfrom each each panel panel as as two-spot-welded two-spot-welded specimens specimens forfor cross-sectionalcross-sectional microhardness microhardness measurements measurements from eachIn addition panel as to thetwo-spot-welded single-spot-weld specimensed coupons, for cross-sectional Spot Weld 6 and microhardness Spot Weld 7measurements were extracted accordingaccording to to Figure Figure6. 6. Figure Figure7 depicts7 depicts the the microhardness microhardness measurementmeasurement lines of the cross cross-section-section accordingfromspecimen. each to Thepanel Figure microhardness as 6. twoFigure-spot 7 -valuesdepictswelded were thespecimen micr takenohardnesss frforom cross the measurement -midplanesectional microhardnesslines lines of theof thetop measurements cross-sectionsheet and the specimen.specimen.according The to Figuremicrohardness microhardness 6. Figure values values7 depicts were were the taken takenmicrohardness from from the the midplane measurement midplane lines lines of lines the of theoftop the topsheet cross sheet and-section andthe the bottombottom sheet sheet of of the the cross-section cross-section specimen.specimen. A diamond diamond indenter indenter with with a a0. 0.55 kg kg load load was was used used for for bottomspecimen.Vickers sheet hardness The of themicrohardness measurements. cross-section values specimen. were Ataken diamond from theindenter midplane with linesa 0.5 of kg the load top was sheet used and for the VickersVickersbottom hardnesshardness sheet of measurements. measurements.the cross-section specimen. A diamond indenter with a 0.5 kg load was used for Vickers hardness measurements. Cross-Section Specimen Cross-Section Specimen

Micro-Hardness Measurement Lines (Dotted Lines)

Figure 7. Cross-sectionMicro specimen-Hardness and Measurement microhardness Lines measurement (Dotted Lines) lines. The microhardness measurementFigure 7. Cross lines-section are the midplane specimen lines and of microhardness the top sheet and measurement the bottom lines. sheet. The microhardness Figuremeasurement 7. Cross-section lines are thespecimen midplane and lines microhardnes of the top sheets measurement and the bottom lines. sheet. The microhardness SpotmeasurementFigure Weld 7. 1Cross lines and- aresection Spot the Weld midplane specimen 9 in line Figure ands of microhardnessthe6 have top sheet only and one measurement the neighboring bottom sheet. lines. spot The each, microh butardness the rest of measurement lines are the midplane lines of the top sheet and the bottom sheet. the spots have two symmetric neighboring spots each. Hence, the thermal “imprints” of Spot Weld 1 and Spot Weld 9 can be asymmetric. Therefore, Spot Weld 1 and Spot Weld 9 were excluded from this study so that they can be studied in the future work.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 16

Spot Weld 1 and Spot Weld 9 in Figure 6 have only one neighboring spot each, but the rest of the spots have two symmetric neighboring spots each. Hence, the thermal “imprints” of Spot Weld 1 and Spot Weld 9 can be asymmetric. Therefore, Spot Weld 1 and Spot Weld 9 were excluded from J.this Manuf. study Mater. so Process.that they2020 can, 4, 55 be studied in the future work. 8 of 17

3. Results and Discussion The DoE runsruns producedproduced ninenine nine-spot-weldednine-spot-welded panelspanels with systematicallysystematically varied spot weldweld spacings and edge margins. Figure8 8 depicts depicts somesome ofof thethe weldedwelded panels.panels. AllAll thethe weldedwelded panelspanels werewere naturally agedaged for for at leastat least two two weeks. weeks. After After natural natural aging, single-spotaging, single-spot coupons andcoupons two-spot and specimenstwo-spot werespecimens extracted were from extracted every from welded every panel welded according panel acco to therding cut planto the shown cut plan in Figureshown6 in. Each Figure DoE 6. Each run DoE run had three single-spot coupons and one two-spot specimen. Static lap-shear pull tests were had three single-spot coupons and one two-spot specimen. Static lap-shear pull tests were carried out carried out on the single-spot coupons to identify their failure loads as the DoE response. Vickers on the single-spot coupons to identify their failure loads as the DoE response. Vickers microhardness microhardness tests were carried out on the two-spot specimens to identify their HAZ patterns. tests were carried out on the two-spot specimens to identify their HAZ patterns. The thermo-mechanical processing of RFSSW produced edge swelling and noticeable dents in The thermo-mechanical processing of RFSSW produced edge swelling and noticeable dents in the weld crowns of the 1D-edge-margin panels, where D (=7 mm) is the spot weld diameter. Figure the weld crowns of the 1D-edge-margin panels, where D (=7 mm) is the spot weld diameter. Figure9 9 depicts the edge swelling of the 1D edge margin (EM). None of the panels with 1.5D EM and 2D depicts the edge swelling of the 1D edge margin (EM). None of the panels with 1.5D EM and 2D EM EM exhibited edge swelling. Edge swelling was observed when the workpiece edge was exposed to exhibited edge swelling. Edge swelling was observed when the workpiece edge was exposed to the the heat-affected zone (HAZ) during the welding process. Furthermore, edge swelling caused a heat-aﬀected zone (HAZ) during the welding process. Furthermore, edge swelling caused a noticeable noticeable dent in the weld crown, as depicted in Figure 10. The panels with 1D EM exhibited 0.6 dent in the weld crown, as depicted in Figure 10. The panels with 1D EM exhibited 0.6 mm-deep dents mm-deep dents in the weld crowns. However, the panels with 1.5D EM and 2D EM exhibited 0.2 in the weld crowns. However, the panels with 1.5D EM and 2D EM exhibited 0.2 mm-deep dents in mm-deep dents in the weld crowns. the weld crowns.

Figure 8. Nine-spot-weldedNine-spot-welded panels panels with with 3D-pitch. 3D-pitch. EM EM is the is the edge edge margin. margin. D (=7 D mm) (=7 mm)is the is spot the weld spot weld diameter. diameter.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 1D EM & 4D Pitch 9 of 16 J.J. Manuf. Manuf. Mater. Mater. Process. Process. 20202020,, 44,, x 55 FOR PEER REVIEW 1D EM & 4D Pitch 99 of of 16 17 1.5D Top 1.5D Top

0.6 mm

Bottom < 1D 0.6 mm

Bottom 1D EM < 1D

Figure 9. Edge1D swellingEM exhibited by 1D-edge-margin panels during the RFSSW process. Larger edge margin panels did not exhibit edge swelling. D (=7 mm) is the spot weld diameter. Figure 9. EdgeEdge swelling swelling exhibited exhibited by by 1D 1D-edge-margin-edge-margin panels during the RFSSW process. Larger Larger edge edge margin panels did not exhibit edge swelling. D (= (=77 m mm)m) is is the the spot spot weld weld diameter. diameter. 0.6 mm 0.2 mm 0.6 mm 0.2 mm

1D EM 2D EM 1D EM 2D EM

(a) (b) 0.2 mm (b) Figure 10. Surface dents(a) in the weld crown: (a) 0.6 mm dent in a 1D-edge-margin panel; (b) 0.2 mm Figure 10. Surface dents in0.2 the weldmm crown: (a) 0.6 mm dent in a 1D-edge-margin panel; (b) 0.2 mm dent in a 2D-edge-margin panel. dent in a 2D-edge-margin panel. Figure 10. Surface dents in the weld crown: (a) 0.6 mm dent in a 1D-edge-margin panel; (b) 0.2 mm dentResearchers in a 2D-edge [9,25-margin–27] explain panel. possible causes of the mechanical failures of friction stir spot welds 2D EM underResearchers external loads. [9,25 Su–27 et] explain al. [9] observed possible thatcauses the of energy the mechanical input of the failures welding of friction process stir influences spot welds the 2D EM underfailureResearchers external mode of loads. the [9,25 friction –Su27 ]et explain stiral. [9] spot possibleobserved weld. Badarinarayancauses that the of theenergy mechanical et input al. [25 of] failures documented the welding of friction thatprocess factorsstir spotinfluences such welds as underthe failure stress external distributions, mode loads. of the Su friction material et al. [9]stir properties, observed spot weld. that and Badarinarayan the joint energy geometry input et al determined. of[25 the] documented welding the crack process that initiation factors influences such and theaspropagation the failure stress mode distributions, in friction of the stirfriction material spot-welded stir spotproperties, weld. assemblies. Badarinarayanand joint Mitlin geometry et al.et al [26 .determined []25 studied] documented friction the crack that stir initiation spot-weldedfactors such and aspropagationaluminum the stress alloys distributions, in friction and found stir material spot that-welded the properties, tool assemblies. plunge and depth joint Mitlin geometry has et a al significant. [26 determined] studied effect friction the on crack the stir failure initiation spot- modewelded and of propagationaluminumthe spot weld. alloys in friction and found stir spot that-welded the tool assemblies. plunge depth Mitlin has et a alsignificant. [26] studied effect friction on the stir failure spot -modewelded of aluminumthe spotIn thisweld. alloys study, and during found thethat unguidedthe tool plunge static depth lap-shear has a pullsignificant tests single effect on spot-welded the failure couponsmode of thedemonstrated spotIn thisweld. study, rotational during deformation the unguided [27 ] static due to lap the-shear asymmetric pull tests pull single loads spot applied-welded to coupons the top demonstratedandIn bottom this study, sheets. rotational during The deformation pull the rate unguided was [27 1.27] due static mm to / lapmin.the- shearasymmetric The pull failure tests pull load singleloads was appliedspot measured-weld to edthe in coupons thetop pulland demonstratedbottomload direction. sheets. rotational Single-spot-weldedThe pull deformation rate was 1.27 coupons [27 mm/min.] due demonstrated to theThe asymmetric failure di ff loaderent pull was mechanical loads measured applied failures in to the the during pull top loadand the bottomdirection.unguided sheets. staticSingle The lap-shear-spot pull-weld rate pulled was tests.coupons 1.27 The mm/min. spot demonstrated weld The failures failure different observed load mechanical was in this measured study failures were in the as during follows. pull load the direction.unguided staticSingle lap-spot-shear-weld pulled tests.coupons The spot demonstrated weld failures different observed mechanical in this study failures were as during follows. the The nugget pullout (NP) failure involves the weld nugget that is pulled out of the parent metal [27]. unguided• static lap-shear pull tests. The spot weld failures observed in this study were as follows.  SuchThe nugget failures pullout usually (NP) occur failure in TMAZ involves/HAZ the around weld nugget the weld that nugget, is pulled where out of the the metal parent grain metal is  Thecoarser.[27]. nugget Such A failures metal pullout region usually (NP) with failure occur a coarser involvesin TMAZ/HAZ grain the demonstrates weld around nugget the athat weld weaker is pullednugget, mechanical out where of the strength.the parent metal metalgrain is coarser. A metal region with a coarser grain demonstrates a weaker mechanical strength. [The27]. topSuch sheet failures breakdown usually (TSB)occur failurein TMAZ/HAZ is a through-thickness around the weld transverse nugget, crack where that the occurs metal acrossgrain • isthe coarser. top sheet A metal [27]. Theregion failure with originates a coarser ingrain TMAZ demonstrates/HAZ and propagatesa weaker mechanical into the parent strength. metal.

J. Manuf. Mater. Process. 2020, 4, 55 10 of 17

Figure 11 displays the DoE main eﬀects plot and Figure 12 displays the DoE response surface for the failure load. Each DoE run produced three coupon repetitions. All the coupons, except the coupons of DoE Run 2 and DoE Run 3, demonstrated nugget pullouts during the unguided static lap-shear pull tests. Figure 13 depicts a nugget pullout failure. The coupons of DoE Run 2 and DoE Run 3 demonstrated top sheet breakdown failures during the mechanical tests. Figure 14 depicts a top sheet breakdown failure. The failure loads of the DoE coupons were used as the DoE response. A response surface analysis [28] was performed with the use of the Statgraphic® software. The statistics of the response surface analysis are as follows.

R-squared = 94.5595%. • Standard Error of Estimation = 244.139 N. • Mean absolute error = 169.568 N. • p-value = 0.002. • The R-squared statistic of 94.5595% indicates that the response surface as fitted explains 94.5595% of the variability in the failure load. The standard error of the estimate shows the standard deviation of the residuals to be 244.139 N. The mean absolute error (MAE) of 169.568 N is the average value of the residuals. A p-value of 0.002 implies that the estimated response surface is statistically significant, as it is less than 0.05. Unlike the 1.5D-EM panels and the 2D-EM panels, the 1D-EM panels produced edge swelling and surface dents in the weld crown during the welding process. In Figure 11, the edge margin exhibits an insignificant effect on the failure load, as the material edge was transverse to the loading direction. If the material edge is parallel to the loading direction, it is intuitive to assume that larger side edge margins would offer a better strength, which could be a good topic for the future work. In addition, the main effects plotted in Figure 11 suggest that the panels with larger spot weld spacings were stronger than the panels with smaller spot weld spacings. Perhaps a smaller spot weld spacing induces a higher heat input, as a higher or excess heat input usually degrades metal properties. Overall, the spot weld spacing (pitch) had an inverse relationship with the failure load of the multi-spot-welded panel. During the welding process, in order to avoid heat accumulation in the weld tool and the workpieces, the weld tool was cooled down with compressed air for at least 90 s after every weld run. This 90 s delay between two successive weld runs was enough to bring the tool temperature and the workpiece temperature down to their normal levels at room temperature. Therefore, it was assumed that the HAZs of refill spot welds in a nine-spot-welded panel should look similar to the HAZ in Figure3. These findings can be verified with cross-sectional microhardness maps. Figure 15 displays two-spot cross-section specimens extracted from nine-spot-welded panels according to the cut plan in Figure6. The cross-section specimens were polished and then etched with a chemical reagent. The hardness values of a cross-section specimen were measured along the midplane lines of the top sheet and the bottom sheet according to Figure7. The measurement line indents were spaced 0.5 mm apart. Figures 16–18 display the microhardness maps of the cross-section specimens. According to the microhardness maps, the hardness values of the 2D-pitch specimens and the 3D-pitch specimens are about 30 HV lower than the base metal hardness value between the two spot welds. However, the bottom sheet hardness values of the 3D-pitch and 2D-EM specimen in Figure 17 are elevated in the midsection between the two spot welds. Perhaps the larger edge margin contributed to the heat dissipation in the 3D-pitch and 2D-EM panel during the welding process so that the bottom sheet hardness values are elevated in the midsection between the two adjacent spot welds. Both the top and bottom sheets of all the 4D-pitch specimens retained the base metal hardness value in the midsection between the two adjacent spot welds. This verifies that the 4D-pitch can prevent HAZ overlaps between two adjacent spot welds, as anticipated. The microhardness test results indicate that the 4D-pitch panels were able to retain the base metal hardness in the midsection between two adjacent spot welds, whereas the smaller pitch panels could not. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 16 results indicate that the 4D-pitch panels were able to retain the base metal hardness in the midsection J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 16 between two adjacent spot welds, whereas the smaller pitch panels could not. Main Effects Plot for Failure Load results indicate that the 4D-pitch panels were able to retain the base metal hardness in the midsection J.between Manuf. Mater. two Process.adjacent2020 spot, 4, 55 welds, whereas the smaller pitch panels could not. 11 of 17 Main Effects Plot for Failure Load 7500

71007500

67007100

63006700

59006300

Failure Load, N

5900

Failure Load, N 5500

51005500 2D 3D 4D 1D 1.5D 2D 5100 Pitch EM 2D 3D 4D 1D 1.5D 2D Figure 11. Main effects plot of thePitch pitch and edge margin. The materialEM edge was transverse to the

loading direction. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing; EM = Edge Figure 11. Main eﬀects plot of the pitch and edge margin. The material edge was transverse to the loading Margin.Figure 11. Main effects plot of the pitch and edge margin. The material edge was transverse to the direction. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing; EM = Edge Margin. loading direction. Notes: D = 7 mm (Spot WeldEstimated Diameter); Response pitch = Spot Surface Weld Spacing; EM = Edge Margin. Estimated Response Surface

7800 7300 68007800 63007300 6800 5800 2 53006300 1.8

Failure Load, N 48005800 1.6 2 5300 1.4 1.8

Failure Load, N 2 2.4 1.2 EM, xD 4800 2.8 3.2 1.6 3.6 4 1 1.4 2 2.4 Pitch, xD 1.2 EM, xD 2.8 3.2 3.6 4 1 Figure 12. Estimated response surfacePitch, of pitch xD and edge margin (EM). Notes:Notes: D = 7 m mmm (Spot Weld Diameter); pitchpitch = SpotSpot Weld Spacing. Figure 12. Estimated response surface of pitch and edge margin (EM). Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing.

J. Manuf. Mater. Process. 2020, 4, 55 12 of 17 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 12 of 16 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 12 of 16

(a) (b) (a) (b) Figure 13. Nugget pullout failure of 3D-Pitch 2D-EM coupon: (a) top view of nugget pullout; (b) Figure 13. NuggetNugget pullout pullout failure failure of of 3D 3D-Pitch-Pitch 2D 2D-EM-EM coupon: coupon: (a) (taop) topview view of nugget of nugget pullout; pullout; (b) bottom view of nugget pullout. Notes: D = 7 mm (Spot Weld Diameter); EM = Edge Margin; pitch = (bbottom) bottom view view of nugget of nugget pullout. pullout. Notes: Notes: D = 7 Dmm= 7(Spot mm Weld (Spot Diameter); Weld Diameter); EM = Edge EM =Margin;Edge Margin;pitch = Spot Weld Spacing. pitchSpot Weld= Spot Spacing. Weld Spacing. For riveted structures, a rule-of-thumb spacing between two rivet joints must be at least three For riveted structures, a rule-of-thumbrule-of-thumb spacing between two rivet joints must be at least three times the rivet shaft diameter [24]. In contrast, for refill-spot-welded structures there is no lower limit times the the rivet rivet shaft shaft diameter diameter [24 [24]. ].In Incontrast, contrast, for forrefill reﬁll-spot-welded-spot-welded structures structures there there is no islower no lower limit for the spot weld spacing, as refill spot welds can be made right next to each other. However, a smaller forlimit the for spot the weld spot spacing weld spacing,, as refill as spot reﬁll welds spot can welds be made can be right made next right to each next other. to each However, other. However,a smaller spot weld spacing causes HAZ overlaps that weaken the material’s strength. In addition, the edge spota smaller weld spotspacing weld causes spacing HAZ causes overlaps HAZ that overlaps weaken that the weaken material the’s material’s strength. strength.In addition, In addition,the edge margin for RFSSW should not be less than 1.5D, as a smaller edge margin is susceptible to edge marginthe edge for margin RFSSW for should RFSSW not should be less not than be less 1.5D than, as 1.5D, a smaller as a smaller edge margin edge margin is susceptible is susceptible to edge to swelling. swelling.edge swelling.

(a) (b) (a) (b) Figure 14. Top-sheet-breakdown failure of the 2D-Pitch 1.5D-EM coupon: (a) top view of nugget Figure 14. Top-sheet-breakdown failure of the 2D-Pitch 1.5D-EM coupon: (a) top view of nugget pullout;Figure 14. (b )Top bottom-sheet view-breakdown of nugget failure pullout. of the Notes: 2D-Pitch D = 1.5D7 mm-EM (Spot coupon: Weld (Diameter);a) top view EM of = nuggetEdge pullout; (b) bottom view of nugget pullout. Notes: D = 7 mm (Spot Weld Diameter); EM = Edge pullout;Margin; pitch (b) bottom= Spot view Weld of Spacing. nugget pullout. Notes: D = 7 mm (Spot Weld Diameter); EM = Edge Margin; pitch = Spot Weld Spacing. Margin; pitch = Spot Weld Spacing. The research ﬁndings suggested that the 4D-pitch and the 1.5D edge margin can oﬀer an The research findings suggested that the 4D-pitch and the 1.5D edge margin can offer an acceptableThe research performance findings for welding suggested 1.6 mm-thick that the 4D AA7075-T6-pitch and sheet the metals 1.5D edge with the margin 7 mm can RFSSW offer tool. an acceptable performance for welding 1.6 mm-thick AA7075-T6 sheet metals with the 7 mm RFSSW Theacceptable microhardness performancetest results for welding supported 1.6 m them- factthick that AA70 the75 4D-pitch-T6 sheet panels metals demonstrate with the 7 higher mm RFSSW failure tool. The microhardness test results supported the fact that the 4D-pitch panels demonstrate higher tool.loads The than microhardness those of the smaller test results pitch supported panels, as thethe 4D fact spot that weld the 4D spacing-pitch allowed panels demonstrate the 4D-pitch higher panels failure loads than those of the smaller pitch panels, as the 4D spot weld spacing allowed the 4D-pitch failureto partially loads retain than thethose base of metalthe smaller hardness pitch along panels the, as row the of 4D spot spot welds. weld The spacing smaller allowed pitch panelsthe 4D- couldpitch panels to partially retain the base metal hardness along the row of spot welds. The smaller pitch panelsnot retain to part theially base retain metal hardnessthe base metal along hardness the row ofalong spot the welds row as of well spot as welds. the 4D-pitch The smaller panels pitch did. panels could not retain the base metal hardness along the row of spot welds as well as the 4D-pitch panelsHence, could the smaller not retain pitch the panels base demonstratedmetal hardness lower along failure the row loads of thanspot thosewelds ofas the well 4D-pitch as the 4D panels.-pitch panels did. Hence, the smaller pitch panels demonstrated lower failure loads than those of the 4D- panels did. Hence, the smaller pitch panels demonstrated lower failure loads than those of the 4D- pitch panels. pitch panels.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 13 of 16

J.J. Manuf. Manuf. Mater. Mater. Process. Process.2020 2020,, 44,, 55x FOR PEER REVIEW 1313 of of 16 17 5mm

5mm

Figure 15. Two-spot cross-section specimens: (top) 2D-pitch; (middle) 3D-pitch; (bottom) 4D-pitch. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing. Figure 15. Two-spot cross-section specimens: (top) 2D-pitch; (middle) 3D-pitch; (bottom) 4D-pitch. Figure 15. Two-spot cross-section specimens: (top) 2D-pitch; (middle) 3D-pitch; (bottom) 4D-pitch. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing. 1D EM Top Sheet 1D EM Bottom Sheet 1.5D EM Top Sheet 1.5D EM Bottom Sheet 2D1D EMEM TopTop SheetSheet 1D2D EM Bottom SheetSheet 1.5D EM Top Sheet 1.5D EM Bottom Sheet weld nugget2D EMregion Top Sheet weld2D nugget EM Bottom region Sheet 190.00 weld nugget region weld nugget region 190.00 180.00 Base Metal Hardness 170.00180.00 Base Metal Hardness 160.00170.00

150.00160.00

140.00150.00 Hardness Value, HV Value, Hardness

130.00140.00 Hardness Value, HV Value, Hardness 120.00130.00 120.00 0 5 10 15 20 25 30 0 5 10Distance, mm15 20 25 30

Figure 16. Microhardness values of 2D-pitchDistance, cross-section mm specimens. Notes: D = 7 mm (Spot Weld

FigureDiameter); 16. Microhardness pitch = Spot Weld values Spacing; of 2D EM-pitch= Edge cross Margin.-section specimens. Notes: D = 7 mm (Spot Weld Diameter);Figure 16. pMicrohardnessitch = Spot Weld values Spacing; of 2D EM-pitch = Edge cross Margin.-section specimens. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing; EM = Edge Margin.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 14 of 16

J. Manuf. Mater. Process. 2020, 4, x FOR PEER1D EM REVIEW Top Sheet 1D EM Bottom Sheet 14 of 16 J. Manuf. Mater. Process. 2020, 4, 55 1.5D EM Top Sheet 1.5D EM Bottom Sheet 14 of 17 2D EM Top Sheet 2D EM Bottom Sheet

weld nugget1D regionEM Top Sheet weld 1Dnugget EM Bottom region Sheet 190.00 1.5D EM Top Sheet 1.5D EM Bottom Sheet 2D EM Top Sheet 2D EM Bottom Sheet 180.00 Base Metal weld nugget region weld nugget region Hardness 170.00190.00

180.00 160.00 Base Metal Hardness 150.00170.00

140.00160.00 Hardness Value, HV Value, Hardness 130.00150.00

120.00140.00 Hardness Value, HV Value, Hardness 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 130.00 Distance, mm

120.00 Figure 17. 0.0Microhardness5.0 values10.0 of the 3D15.0-pitch cross20.0-section25.0 specimens.30.0 Notes:35.0 D = 7 mm40.0 (Spot Weld45.0 Diameter); pitch = Spot Weld Spacing; EM = EdgeDistance, Margin. mm

Figure 17. Microhardness values of the 3D-pitch cross-section specimens. Notes: D = 7 mm (Spot Weld FigureDiameter); 17. Microhardness pitch = Spot Weld values Spacing; of the 3D EM-pitch= Edge cross Margin. -section specimens. Notes: D = 7 mm (Spot Weld Diameter); pitch = Spot Weld Spacing; EM = Edge Margin. 1D EM Top Sheet 1D EM Bottom Sheet 1.5D EM Top Sheet 1.5D EM Bottom Sheet 2D EM Top Sheet 2D EM Bottom Sheet

weld nugget region1D EM Top Sheet 1D EMweld Bottom nugget Sheet region 190.00 1.5D EM Top Sheet 1.5D EM Bottom Sheet 2D EM Top Sheet 2D EM Bottom Sheet 180.00 Base Metal weld nugget region weld nugget region Hardness 170.00190.00

160.00180.00 Base Metal Hardness 150.00170.00

140.00160.00 Hardness Value, HV Value, Hardness 130.00150.00

120.00140.00 Hardness Value, HV Value, Hardness 0 5 10 15 20 25 30 35 40 45 130.00 Distance, mm

Figure120.00 18. Microhardness values of the 4D-pitch cross-section specimens. Notes: D = 7 mm (Spot Weld FigureDiameter); 18. Microhardness0 pitch = Spot5 Weld values Spacing;10 of the 4D EM15-pitch= Edge cross20 Margin.-section 25specimens.30 Notes: D35 = 7 mm (Spot40 Weld 45 Diameter); pitch = Spot Weld Spacing; EM = EdgeDistance, Margin. mm 4. Conclusions The aim of this article is to chronicle an investigation into the design parameters of RFSSW for Figure 18. Microhardness values of the 4D-pitch cross-section specimens. Notes: D = 7 mm (Spot Weld producingDiameter); multi-spot-welded pitch = Spot Weld Spacing; AA7075-T6 EM = panels. Edge Margin. The design parameters are the reﬁll spot weld spacing (pitch) and the edge margin. The spot spacing or pitch is the center-to-center distance

J. Manuf. Mater. Process. 2020, 4, 55 15 of 17 between two adjacent spot welds within the same row. An edge margin is the center-to-edge distance between the spot weld and the material edge. AA7075 is a baseline aerospace aluminum alloy used in aircraft structures. RFSSW produces three distinct weld zones in the workpiece, which are the heat-affected zone (HAZ), the Thermo-Mechanically Affected Zone (TMAZ), and the weld nugget. A refill spot weld usually starts to fail in TMAZ/HAZ, where the metal grains are coarser. Therefore, the extent of HAZ/TMAZ must be considered for finding appropriate design parameters for producing multi-refill-spot-welded structures. The experimental strategy used a DoE method to characterize the failure loads of multi-spot-welded panels in terms of the spot weld spacing, edge margin, and spot weld HAZ. According to the DoE results, the larger spot weld spacings with no HAZ overlap produced higher failure loads in multi-spot-welded panels. On the other hand, the 1D edge margin demonstrated abnormal plastic deformations, such as workpiece edge swelling and weld crown dents, during the RFSSW process. The larger edge margins did not exhibit such abnormal deformations during the welding process. All the refill spot welds demonstrated two types of mechanical failures, including nugget pullouts and top sheet breakdowns, during the static lap-shear pull tests. The larger spot weld spacings were mostly associated with nugget pullouts, and the smaller spot weld spacings were mostly associated with top sheet breakdowns. The research findings suggested that the 4D-pitch and 1.5D edge margin can offer an acceptable performance for welding 1.6 mm-thick AA7075-T6 sheet metals with the 7 mm RFSSW tool. The microhardness test results supported the fact that the 4D-pitch panels demonstrated higher failure loads than those of the smaller pitch panels, as the 4D spot weld spacing allowed the 4D-pitch panels to partially retain the base metal hardness along the row of spot welds. The smaller pitch panels could not retain the base metal hardness along the row of spot welds as well as the 4D-pitch panels had. Hence, the smaller pitch panels demonstrated lower failure loads than those of the 4D-pitch panels.

Author Contributions: Conceptualization, E.B., S.F., M.F., K.K., and G.L.B.; methodology, E.B. and G.L.B.; validation, G.L.B., E.B., S.F., M.F. and K.K.; formal analysis, G.L.B. and E.B.; investigation, G.L.B. and E.B.; resources, S.F., M.F. and K.K.; data curation, G.L.B. and E.B.; writing—original draft preparation, G.L.B.; writing—review and editing, E.B., S.F., M.F. and K.K.; visualization, G.L.B. and E.B.; supervision, E.B.; project administration, E.B.; funding acquisition, S.F., M.F. and K.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Kawasaki Heavy Industries (KHI) through the Center for Friction Stir Processing (CFSP). Acknowledgments: The authors express their gratitude to National Institute for Aviation Research (NIAR) of Wichita State University for administrative and technical support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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