Refill friction stir spot welding of dissimilar aluminum ...

Author's Personal Copy Journal of Manufacturing Processes 30 (2017) 353–360

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Refill friction stir spot welding of dissimilar aluminum alloy and AlSi coated steel Y. Ding, Z. Shen ∗ , A.P. Gerlich Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

a r t i c l e

i n f o

Article history: Received 29 June 2017 Received in revised form 6 September 2017 Accepted 5 October 2017 Keywords: Refill friction stir spot welding Microstructures Dissimilar welding AlSi coating Boron steel

a b s t r a c t AA5754 Al alloy and coated steel were successfully joined by refill friction stir spot welding through the eutectic Al-Si coating. The overlap shear strength increased with plunge depth and reached a high strength plateau which was attributed to maximum bonding size and strong Al/AlSi coating joint interface. The AlSi coating in the as-received coated steel consisted of AlSi eutectic phase and two main intermetallic phases (Al7 Fe2 Si and Al5 Fe2 (Si)) with an average width of 8.5 ␮m and around thickness of 1 ␮m, respectively. The former played a key role in dissimilar welding, the latter effectively restrained the formation of additional IMCs and the growth of exiting IMCs during welding. Microstructural evolution showed the high strength of joints is also associated with fine silicon particles, elimination of defects and control of intermetallics at the joint interface. Fracture of full bonding welds initiated from the partial metallurgical bonds (PMBs) or kissing bonds, then propagated through the AlSi coating and AlSi/Al7 Fe2 Si interface, final fracture occurred at Al7 Fe2 Si/Al5 Fe2 (Si)/steel substrate interfaces. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

1. Introduction Two major methods have been developed to reduce vehicle weight over the last few decades: sheet thickness reduction using advanced high and ultrahigh-strength steels instead of conventional low-strength steels [1,2]; and using light-weight metals by relying on dissimilar material joints [3–5]. For the latter, the two most commonly used metals, steel and aluminum, have been joined for various applications [6,7]. The most common approaches of dissimilar joining of Al-steel combinations include a) direct welding [8–12], b) utilizing a filler material or a transition metal layer [13–18], c) hybrid welding [19–23], and d) joining aluminum to galvanized steel by brazing through melting of the Zn coating [24–30]. These approaches rely on either fusion or frictional heating to melt the Zn coating. Direct welding of Al-steel most typically involves fusion welding, including laser [12], resistance spot [8,9,11,12,31], electron beam [32] and arc [13] welding technologies. For fusion welding Al-steel joints, one of the main problems is the formation of complex intermetallic compounds (IMCs) at the material interface, which significantly influences mechanical properties of dissimilar joints [32,33]. Controlling intermetallic formation and growth is still a difficult task during welding [34–36].

∗ Corresponding author. E-mail address: [email protected] (Z. Shen).

When a third element, such as zinc or silicon, is added to the AlFe system, the number of intermetallic compounds will increase and the interface microstructure will become much more complex [37–39]. Although many publications have also described welding Al-steel with a filler material, a transition metal layer, or hybrid welding, this increases processing complexity and is cumbersome for many industrial applications. Welding Al-coated steel may instead be done with an alternative approach by solid state welding. Based on current commercial trends, one should also note Zn-based and AlSi-coated steels are the most popular for industrial applications [40]. Previous investigations on Al alloy/Zn coated steel demonstrated different joining mechanisms compared to uncoated steels [27,41–43]. Since the melting point of Zn is only 420 ◦ C, it is likely that liquid Zn from the coating will be present, and this can cause severe porosity or expulsion and produce complex IMCs with other metals. Friction stir spot welding (FSSW) is a variation of linear Friction Stir Welding (FSW), which was invented as an alternative to resistance spot welding (RSW) and riveting of lightweight alloys in the automobile, high speed train, shipbuilding and aerospace industries. Recently, the application of FSSW has rapidly extended to a variety of dissimilar materials, including uncoated steel [44–47] and coated steels [44,48,49]. Refill FSSW was invented by researchers in Germany in 1999 [50] as a further modification of FSSW. The main advantage of refill FSSW is that no keyhole is left on the surface as in the case of conventional friction spot weld-

https://doi.org/10.1016/j.jmapro.2017.10.006 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

Author's Personal Copy 354

Y. Ding et al. / Journal of Manufacturing Processes 30 (2017) 353–360

ing. Literature on friction stir welding of Al alloys and AlSi coated steel is very scarce [49,51], although AlSi coating offers excellent resistance to both corrosion and elevated temperature oxidation. Numerous studies on refill FSSW have been published, but none of these involve AlSi-coated steel [52–54]. No reports on refill FSSW of Al alloys and AlSi coated steel have been published. It should be noted that this combination may undergo interesting transformations, since the minimum melting point or eutectic temperature of Al-Si alloys is 577 ◦ C, which is very close to the solidus temperatures for most aluminum alloys. In the present work, refill FSSW technology is applied to attain sound joints of dissimilar Al-AlSi coated steel, and the mechanisms controlling microstructure, mechanical properties and fracture and joining mechanisms are discussed.

2. Experimental The base materials examined during refill FSSW tests comprised 1.6 mm thick AA 5754 and 2 mm thick Usibor 1500P steel (or 22MnB5 alloy) as the substrate, which had a hot dipped AlSi coating, and used in the as-received condition with no hot stamping thermal cycle applied. 22MnB5 steel grade is the most commonly used steel grade in hot stamping processes. The chemical compositions and mechanical properties of the base materials show in Table 1. Test specimen dimensions were cut to 25 × 100 mm. According to the American Welding Society (AWS) standard D 17.2, the tensile shear tests were conducted using a universal testing machine with a constant crosshead of 5 mm/min and three-five specimens were tested [55]. These were welded in an overlap configuration (with overlapping area of 25 × 25 mm) with the aluminum alloy on top, and a non-consumable welding tool made from H13 steel heattreated to 48 HRC was used. The welding tests carried out using a displacement-controlled machine, model RPS100, manufactured by Harms & Wende, Germany, with appropriate clamping fixtures to prevent specimen movement during welding. Fig. 1 presents a schematic illustration of the sleeve plunge processing sequence applied during joining. As shown in Fig. 1, the joining equipment consists of a tool with two movable parts, a 6.4 mm diameter pin (Dp ) and a 9.0 mm diameter sleeve (Ds ), mounted coaxially to a clamping ring with an inner diameter of 14.5 mm (Dc ), with sleeveplunge-depth control employed during processing [54]. Based on previous study [56],the parameters, including tool rotation rate (2000 RPM) and welding schedule (1.5 s plunge, 1.5 s dwell and 0.5 s refill times) were unchanged during testing. Sleeve plunge depths ranged from 0.9 to 1.4 mm so that the welding tool always remains only in contact with the top aluminum sheet. After welding, the joints were transversely or longitudinally sectioned perpendicularly or parallelly to the sheet rolling direction for metallographic studies. The cross-section of each sample was ground and polished with standard procedures, with final polishing using 0.05 ␮m colloidal silica suspension. The welded samples were as-polished and etched with modified Poulton’s regent for AA5457 alloy. Optical microscopy was performed using an Olympus BX51, and electron microscopy relied on a JSM 6460 SEM attached with an Oxford instruments INCA-350 energy dispersive spectroscopy (EDS) to observe the material microstructures. The overlap shear tests on at least three specimens conducted using a Tinius Olsen HK10 tester. A Hystron Tribo nanoindentation system with Berkovich indenter (50 nm tip diameter) used to perform nanoindentation, operated at a 1000–4000 ␮N load used to perform material hardness. The Young’s moduli of the phase calculated from the slope of the upper third portion of the unloading part of the load-penetration of the load-penetration depth curve.

3. Results and discussion Fig. 2a shows the typical overall structure in an AA5754/steel refill FSSW joint produced using a 1.3 mm sleeve plunge depth. In the overall joint, a recrystallized structure is produced in the aluminum side, and no deformation can be observed on the steel side. Based on the microstructural characteristics associated with the grain size and deformation structure, the weld appears to be symmetrical with respect to the tool axis. As shown in Fig. 2(a,b), three distinct zones, i.e. stir zone (SZ) or nugget, thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) as well as base metal (BM), could be identified in sequence from the center of the weld outwards. The SZ corresponds to the large zone underneath the both sleeve and pin which contains a typical fine equiaxed grains resulting from the dynamic recovery and recrystallization processes during welding, leading to drastically refined grain sizes. Fig. 2b indicates that smaller grain sizes occur on the outer perimeter of the SZ region (indicating more extensive intermixing) than those at the middle region (Fig. 2c) in the SZ. For comparison the grain size in the BM is much larger than that in the SZ, see Fig. 2d. Three enlarged SEM images present the interfacial regions between the Al alloy and the steel at the left, middle and right side regions in Fig. 2(e–g), where the original Al alloy and AlSi coating are found to be completely joined without evidence of cracking at the interfaces. Partial metallurgical bonds (PMBs) containing surface oxides, also known as kissing bonds [57], are observed at the outer periphery of the joint, and are directed downwards and then arrested at the boundary of the SZ, see Fig. 2(a, e, g). As shown in Fig. 2(h, i), the two additional optical images show that the PMBs terminate the intermixing zone and indicate that this material including fine silicon-rich grains was displaced upwards into the top sheet. Macrocopic deformation of the steel substrate is not observed, which accounts for why the PMB propagated downwards to the AlSi coating layer and no ‘hook’ type defects are formed. Two optical images show material flow with PMBs at the h) left and i) right sides which are directed downwards and then arrested at the boundary of the SZ, namely at the fine silicon particle area. Later this will be discussed in detail. Fig. 3(a–c) show the detailed microstructure of the AlSi coating and the coating/steel substrate interface for the as-received coated (hot-dipped) steel. The steel substrate below the weld SZ has the same pearlitic-ferritic microstructure as in the as-received steel [58]. The AlSi layer of the coated steel sheet comprises a eutectic AlSi structure at the coating surface and a continuous layer of intermetallic compounds (IMCs) at the steel interface with an average total thickness of 28 ␮m. The microstructure of the eutectic AlSi coating consists mainly of acicular or spindly flakes of silicon. It can be noted from Fig. 3a that the distribution of silicon flakes in the coating is not homogeneous and the average Si flake size was about 10 × 2 ␮m. Fig. 3(b, c) are SEM images of the ascoated AlSi steel sample at higher magnification. The IMC phases appear to match well with Al7 Fe2 Si and Al5 Fe2 (Si) (as indicated in Table 2) with an average width of 8.5 ␮m and around thickness of 1 ␮m, which is in agreement with the phases formed at the interface of Al–10 wt.% Si coating and steels during hot dipping reported by Cheng & Wang [59] and Shi et al. [60]. Si in Al5 Fe2 has a maximum solubility of 6 at%, and above this limit Al2 Fe3 Si3 may precipitate from the Al5 Fe2 (Si) matrix [58]. Fig. 3(d) shows a gradual transition from the original interface of the AA5754 matrix towards the AlSi coating at the middle nugget region. It is clear from the image that the microstructure is very different from that of Fig. 3a. The fine equiaxed Si particles replacing the acicular Si flakes homogeneously dispersed in the original Al-Si coating. The stirring action of the tool during welding resulted in breakup of silicon flakes, thereby causing extensive refinement of the flaketype eutectic silicon which can simultaneously improve strength

Author's Personal Copy Y. Ding et al. / Journal of Manufacturing Processes 30 (2017) 353–360

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Table 1 ® Nominal chemical composition and mechanical properties of AA5754 aluminum alloy, Usibor 1500P (22MnB5) steel and the AlSi coating studied (wt%). Chemical composition, wt%

AA5754-O 22MnB5 AlSi coating

Mechanical properties

Al

Si

Mn

Mg

Fe

Bal. 0.03 86.2

0.4 0.015 12.5

0.5 2.2

2.6–3.2

0.4 Bal 1.3

Cr 0.16

B 0.004

P 0.02

Ti 0.035

C

TS, MPa

Ss, MPa

Elong%

HV

140

0.22

215 474

25 30

67.2 (HV100 ) 190.1 (HV100 ) 75.1 (HV10 )

Fig. 1. A schematic illustration of a refill FSSW process showing (a) the tool initiative position, (b) the plunging and (c) the retreating stages of welding. Dp -pin, Ds -sleeve and Dc -clamping ring diameters.

Fig. 2. a) A typical macroscopic transverse cross-section of AA5754 alloy and AlSi coated steel lap joint produced using a plunge depth of 1.3 mm. Three enlarged optical micrographs showing b) an HAZ-TMAZ-SZ intermixing transition region in rectangle region denoted by “A” in Fig. 2a, and microstructures of c) SZ at middle and d) BM, and the two enlarged SEM images of the both PMB structures e) at the left, and g) the right, and one image f) at the middle showing a very good bonding between Al alloy and AlSi coating.

and ductility [61]. Enlarged SEM images (Fig. 3(e,f)) exhibit the presence of IMCs between steel substrate and AlSi coating for the spot welded sample. Microanalysis results indicate that they have the same interfacial microstructures and phases as the as-received and welded steel sample, where Table 2 presents their detailed chemical analysis. Due to higher invariant reaction temperatures for Al7 Fe2 Si (855 ◦ C) [37] and for Al5 Fe2 (Si) (1030 ◦ C) [62], they effectively restrain additional IMC formation and growth. In order to provide more detail regarding the IMC phases between the AlSi coating and steel substrate, measurement of their

nanoscale mechanical properties, hardness and Young’s modulus, were made at the interface by nanoindentation. Fig. 4a presents an SEM cross-sectional image of the as-polished welded sample showing the intermetallic layers. An enlarged AFM image from the rectangular frame of Fig. 3a shows the Berkovich indents on the steel substrate and intermetallic layers, see Fig. 3b. The loadpenetration depth curves for the two intermetallic compounds (Al7 Fe2 Si and Al5 Fe2 (Si)) and the steel substrate in the crosssection sample are shown in Fig. 3c. Each curve in the figure represents an average of 3–5 measurements taken from each phase.



Fig. 3. Optical and SEM images of as-received AlSi-coated steel (Fig. 3a–c) and dissimilar AA5754 aluminum/AlSi-coated steel (Figs. 3d–f) sheets, in welds produced using a 1.3 mm plunge depth.

Table 2 Quantification analysis of interfaces measured by EDS analysis and possible phases for as-received and refill FSSW samples. Region

Al Fe Si Phase

Chemical composition of as received sample (at%)

Chemical composition of welded sample (at%)

1

2

3

4

5

6

1

2

3

4

5

6

58.4 37.2 4.4 Al5 Fe2 (Si)

60.7 32.3 7.0

58.8 36.2 5.0

65.2 20.2 14.6 Al7 Fe2 Si

93.2 0.6 6.2 Al-Si

89.3 0.3 10.4 Al-Si

61.0 32.5 6.5 Al5 Fe2 (Si)

61.0 33.6 5.4

59.4 33.3 7.3

65.8 20.8 13.4 Al7 Fe2 Si

96.2 0.80 3.0 Al-Si

91.1 0.4 8.5 Al-Si

The hardness (H) of Al5 Fe2 (Si), Al7 Si2 Si and steel at the maximum load Pmax and projected area A were determined using the equation (i.e. H = Pmax /A) [63] as follows: 220.9 GPa and 14.54 GPa for Al5 Fe2 (Si), 202.15 GPa and 12.35 GPa for Al7 Si2 Si, and 196.01 GPa and 3.54 GPa for the steel substrate. Nanoindentation studies reveal that the IMCs have much higher hardness and Young’s modulus than the steel substrate, which is consistent with previous published reports [3,64,65]. This higher hardness plus fine silicon particle size may improve the interface and shear tensile strengths for the dissimilar joints [65–67]. Fig. 5 presents results of the tensile shear strength of friction stir spot welded joints at different plunge depths, three fracture features and EDS analysis of corresponding fracture surfaces. The fracture load values indicate an initial linearly increasing trend with plunge depth, and then a stable plateau region for plunge depths at 1.2 −1.4 mm. Based on the nugget sizes and fracture surface features, three regimes can be distinguished as: i) initial debonding at 0.9 mm plunge depth, ii) partial bonding at a 1.1 mm depth, and finally iii) full bonding at plunge depths of 1.2-1.4 mm, see Fig. 5a. Fig. 5b–d reveal SEM images of the fracture surfaces produced on the Al sheet side after shear testing, with insets showing the corresponding steel side. Since the fractures in joints achieving partial and full bonding initiated from the PMBs terminated at the intermixing zone (rather than the original faying surface), all failures exhibit a shear mode. The fracture path associated with debonding at low plunge depths occurred through the faying surfaces due to a lack of metallurgical bonding between the Al alloy and the AlSi-coated steel. As shown in Fig. 5h, fracture surface shows very low Fe content

and no nugget can be created. The shear strength is mainly correlated to weld nugget size (or bonded area) and joint interface strength [66]. At a plunge depth of 1.1 mm, the average effective nugget size (De ) tends to be equivalent to the pin diameter (Dp ), see Fig. 5c, and fracture propagation to the AlSi coating, and failed through Al7 Fe2 Si and Al5 Si2 (Si) phases, see Fig. 5(c,f,i). As shown in Fig. 5(d,g,j), the samples made using a plunge depth between 1.2–1.4 mm exhibited full bonding at the nugget, contributing to the high lap shear strength. Based on composition analysis of the full bonding sample (Fig. 5j), fracture propagation likely occurred from the intermixing region, through the AlSi coating, then the AlSi-Al7 Fe2 Si interface, and finally through the Al7 Fe2 Si-Al5 Si2 (Si) or/and Al5 Si2 (Si)/steel interfaces. The metallographic cross-sections in the full bonding condition reveals two distinguished structures: intermixing at the joint periphery, and friction weld or good bonding is formed between the Al alloy and AlSi coating materials near the SZ, see Figs. 2f and 3e. When a fully bonded nugget is formed at the maximum size, these two joining mechanisms operate simultaneously to reinforce each other, with the resulting weld interface being comparable to a traditional rotary dissimilar friction weld. Fig. 6a shows fracture topography on the Al side for a welded sample at 1.4 mm plunge depth. This fractured sample was sectioned through a diameter along the red line, see Fig. 6a. A cross-section of the fractured surface is shown in Fig. 6b. Fracture exhibits a full bonding with a maximum weld size which approximate the sleeve diameter (Ds). The fracture initiated from the PMB at the right edge of the weld and the microanalysis from the edge to center presents in Fig. 6c which is similar to Fig. 5j.


357

Fig. 4. a) SEM image, b) AFM image of the Berkovich indents and c) penetration depth-load force curves of nanoindentation along the interface of AlSi coating and steel substrate for the cross-sectional sample of the dissimilar welding joint produced using a 1.3 mm plunge depth.

Table 3 Quantification analysis of the three regions (three areas each region) measured by EDS and possible phases of fracture propagation for the refill FSSW cross-sectional sample at 1.4 mm plunge depth, see Figs. 6d–f. Region

C (Underneath pin:center) 1

Al Fe Si Phase

2

68.8 68.8 20.1 18.7 11.1 12.5 Al7 Fe2 Si/Al5 Fe2 (Si)

B (Underneath: pin-off center) 3

1

70.5 19.7 9.8

66.5 67.0 19.4 19.4 14.1 13.6 Al7 Fe2 Si/Al5 Fe2 (Si)

Also, the three interface regions A, B and C marked in Fig. 6a and b were selected to evaluate their microstructure and microanalysis. From Fig. 6f and Table 3, an enlarged SEM image and three area micro-analysis present a change of fracture surface topography and chemical composition. Area 3 shows very low Fe, while Areas 1 and 2 have very high Fe. Fracture propagation from area 3 to areas 1 and 2 had a jump, namely from AlSi coating to Al7 Fe2 Si/Al5 Fe2 Si. As shown in Fig. 6d, e and Table 3, fracture crack propagated along Al7 Fe2 Si/Al5 Fe2 Si and/or Al5 Fe2 Si/Steel substrate interfaces.

4. Conclusions This paper reports refill FSSW welding of dissimilar Al alloy and AlSi coated steel for the first time. AA5754 Al alloy and coated steel were successfully joined through the eutectic Al-Si coating. In this study, a rather different mechanism is explored in the present work since it does not involve melting or brazing. The refill FSSW overlap shear strength increased with plunge depth and reached a high strength plateau which was attributed to

2

A (underneath sleeve) 3

1

2

68.6 19.0 12.4

67.6 66.2 19.2 20.2 13.2 13.3 Al7 Fe2 Si/Al5 Fe2 (Si)

3 96.2 0.9 2.9 Al/AlSi

maximum bonding size and strong Al/AlSi coating joint interface. The AlSi coating in the as-received coated steel consisted of AlSi eutectic phase and two main intermetallic phases (Al7 Fe2 Si and Al5 Fe2 (Si)) with an average width of 8.5 ␮m and around thickness of 1 ␮m, respectively. The former played a key role in dissimilar welding, the latter effectively restrained the formation of additional IMCs and the growth of exiting IMCs during refill friction stir spot welding. Microstructural evolution showed the high strength of joints is also associated with fine silicon particles, elimination of defects and control of intermetallics at the joint interface. Also, the intermixing structure at the weld nugget periphery terminated the PMB propagation, and high hardness of Al7 Fe2 Si and Al5 Fe2 (Si) improved interface strength, which could improve tensile shear strength of the welds. Fracture of full bonding welds initiated from the partial metallurgical bonds (PMBs) or kissing bonds, then propagated through the AlSi coating and AlSi/Al7 Fe2 Si interface, final fracture occurred at Al7 Fe2 Si/Al5 Fe2 (Si)/steel substrate interfaces.



Fig. 5. (a) Results of the tensile shear strength of friction stir spot welded joints of AA5754/steel at different plunge depths, showing three fracture features: debonding, partial bonding and full bonding. Fracture surfaces at (b) 0.9 mm, (c) 1.1 mm, (d) 1.3 mm plunge depths, and EDS analysis in Figs. 5(h-j) of corresponding fracture surfaces in Figs. 5(e-g). The insets on Figs. 5 (b-d) show the opposite fracture surfaces of the steel side. Dp and Ds − outer diameters of the pin and sleeve, De - effective nugget diameter.


359

Fig. 6. a) the fractured surface, b) the cross-section sectioned through a diameter along the red line in Fig. 6a for a welded sample at 1.4 mm plunge depth, c) EDS analysis of the fracture surface from edge to center on Fig. 6a, and EDS analysis of the cross-section at three regions: d) “C”, e) “D” and f) “A” on Fig. 6b, three measurement areas each region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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