Microstructure and Joint Strength of Friction Stir Spot ...

Microstructure and Joint Strength of Friction Stir Spot

Welded 6022 Aluminum Alloy Sheets and Plated Steel Sheets*1

Keyan Feng1;*2, Mitsuhiro Watanabe2 and Shinji Kumai1

1Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan2Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo 152-8552, Japan

Friction stir spot welding (FSSW) was performed for joining of an aluminum alloy sheet to a steel sheet. A 6022 aluminum alloy sheet, anon-plated steel and four kinds of plated steel sheets were prepared. They were plated by pure zinc (GI), zinc alloy (ZAM), Al-Si alloy (AS) andzinc alloy including Fe (GA). The melting temperature of each plated layer was 420, 330, 640 and 880�C. The aluminum alloy sheet wasoverlapped on the steel sheet. A rotating tool was inserted from the aluminum alloy sheet side and the probe tip was kept at the position of0.2 mm above the lapped interface for 3 s. Temperature change at the welding interface was measured during FSSW by using thermocoupleswhich were located at the joint interface below the rotating tool. The maximum operating temperature was 430�C. It was found that interfacemorphology, strength and joining area of the joint varied depending on whether melting temperature of the plate layer was higher or lower thanthe maximum operating temperature. Large joint strength and joining area were obtained for the steel sheet with the low melting temperature ofplated layer. In this case, the original plated layer was removed from the interface during FSSW and aluminum/steel interface with a thinintermediate layer was observed. [doi:10.2320/matertrans.L-M2011811]

(Received October 5, 2010; Accepted March 2, 2011; Published June 15, 2011)

Keywords: friction stir spot welding, microstructure, joint strength, aluminum alloy, plated steel

1. Introduction

Solid-state welding methods for joining of dissimilarmetals are attracting a lot of interests.1–3) This is because theycan avoid mechanical property degradation of original metalsdue to melting and re-solidification and they can beapplicable to metal combinations with a large meltingtemperature difference. Friction stir spot welding (FSSW),which is one of the solid-state welding methods, has beenapplied for joining of dissimilar materials. In particular, it hasbeen applied for the combination of aluminum alloy sheets tosteel sheets from the strong industrial interests.4–7)

A number of studies have been made so far for joining ofthe aluminum alloy sheets to the steel sheets by the frictionstir welding methods (FSW) and FSSW. These workssuggested the existence of the plated layer significantlyimproved their weldability.8–10)

The previous reports, however, did not mention the reasonwhy the plate metals could improve the weldability and thebehavior of the plated layers during FSW and FSSWoperation.

In the present study, we focus on the relationship betweenthe operation temperature and the melting temperature of theplate metals or alloys on the steel sheets. An aluminum alloysheet and several kinds of plated steel sheets were FSSWed.Microstructures of the joint interface, joint strength andfracture behavior of the joints were investigated and the roleof plated layers was discussed.

2. Experimental Procedure

2.1 MaterialsA 1.1 mm-thick 6022 aluminum alloy sheet (called Al

alloy, hereafter), four kinds of 1.2 mm-thick plated steelsheets and a 1.2 mm-thick non-plated low carbon steel sheetwere prepared. These materials were supplied from theMitsubishi Motors Corporation. The four kinds of platedlayers were as follows: (1) Zinc alloy (Zn-6%Al-3%Mg (inmass%)) (called ZAM), (2) pure Zinc (called GI), (3) Al-Sialloy (called AS) and (4) Zinc alloy including Fe (called GA).They have different melting temperatures, 330, 420, 640 and880�C for ZAM, GI, AS and GA, respectively. The micro-structure of plated layer for each steel sheet is shown inFigs. 1(a)–(d). The thicknesses of plated layers were almostthe same and about 10 mm. It should be mentioned that Al-Feintermetallic compound layer (IMC) with about 3 mm-thickwas observed at the interface of steel and Al-Si alloy for ASplated sheet, an indicated by a black arrow in Fig. 1(c).

2.2 Welding toolA conventional vertical milling machine was used for

FSSW. The welding tool was made of SKH51 steel. Theshoulder diameter and probe diameter of the tool were 10 mmand 5 mm, respectively. The probe length was 0.8 mm.Vertical grooves were introduced to the probe surface.

2.3 Welding procedurePrior to welding, the Al alloy sheet surface and the non-

plated low carbon steel sheet surface were polished by awaterproof abrasive paper (# 4000) and cleaned with acetoneand dried. No special surface treatment was made. The Alalloy sheet was overlapped on the steel sheet and they aretightly clamped by the fixture. The tool with a rotationalspeed of 3000 rpm was inserted from the Al alloy sheet side ata rate of 4 mm/min. The probe tip was kept at the position of0.2 mm above the lapped interface for 3 s, and the rotatingtool was retracted from the Al alloy sheet.

2.4 Microstructure observationThe joints were sectioned along the normal direction to the

*1The Paper Contains Partial Overlap with the ICAA12 Proceedings by

USB under the Permission of the Editorial Committee.*2Graduate Student, Tokyo Institute of Technology

Materials Transactions, Vol. 52, No. 7 (2011) pp. 1418 to 1425#2011 The Japan Institute of Light Metals

http://dx.doi.org/10.2320/matertrans.L-M2011811

lapped sheets for the microstructure observation of the jointinterface. The polished specimens were observed using bothan optical microscope (OM) and a scanning electron micro-scope (SEM).

2.5 Temperature measurement during FSSWTemperature change at the welding interface was meas-

ured during FSSW with a series of thermocouples. Thethermocouples were set at the interface of Al alloy sheet andsteel sheet below the probe center and at the location 2.5 mmaway from the center (probe surface).

2.6 Joint strength evaluationJoint strength was evaluated using a modified cross tension

test (called M-CTT, hereafter). Figure 2 shows how toproduce the test specimen. Two Al alloy sheets wereoverlapped on the steel sheet and the three sheets werewelded together. The sandwiched middle Al alloy sheet waswelded to both the upper Al alloy and the steel sheet. Theupper Al alloy and the steel sheet were fixed to the loadingapparatus using the bolts which were inserted to the holes ofthe sheet.

In the present specimen configuration, the upper sheet actsas a part of the specimen grip system. Deformation of theupper Al alloy sheet during loading is available to apply avertical tensile load to the joint interface between the middleAl alloy sheet and the steel sheet. This is quite important toexamine the strength of the Al alloy sheet/steel sheet jointinterface. The tests were performed using an Instron-typetensile testing machine under a crosshead speed of 0.5 mm/min at room temperature.

In some cases, M-CTT was interrupted in order to obtainthe joint specimen in the course of fracture and to examinethe fracture process.

3. Results

3.1 Temperature change during FSSWFigure 3 shows the temperature change at the joint

interface during the FSSW. Melting temperatures of platedlayer of ZAM, GI, AS and GA were shown by dashed lines inFig. 3, respectively. Those are temperature-time curves forbeneath of the probe-center and for outer-region of the probe.The temperature increased gradually as increasing theplunging depth. The temperature became almost constantwhen the probe tip reached the target position (0.2 mm abovethe lapped interface). The temperature was stable for 3 s ofthe dwell time. The maximum temperature was 430�C for theprobe-center and it was 350�C for the outer-region of theprobe. The important finding here is the maximum tem-perature at the joint interface is higher than the meltingtemperature of the two plated layers, that is, ZAM and GIcan be melted at the joint interface during the welding.

(d)

Plated layer

Steel

Steel

Plated layer

(b)

(c)

Plated layer

Steel IMC layer

(a)

Steel

Plated layer

Fig. 1 Plated layers on the plated steel sheets. (a) ZAM, (b) GI, (c) AS, (d) GA.

Al alloy

Steel

Al alloy

Fig. 2 Schematic illustration of the FSSW method and the modified cross

tension test (M-CTT) for the lap joint.

Microstructure and Joint Strength of Friction Stir Spot Welded 6022 Aluminum Alloy Sheets and Plated Steel Sheets 1419

3.2 Joint strengthFigure 4 shows the fracture load of joints. Under the

present welding condition (3 s of the dwell time), lap joiningof Al alloy to non-plated steel was not achieved sufficiently.In contrast, joining was successful for all the plated steelsheets. The obtained fracture load was low for AS and GA.The joints of ZAM and GI showed relatively high joint loadvalues.

3.3 Joint interface morphologyFigure 5 shows the optical micrograph of the cross-section

of joint interface for ZAM. The upper side is the Al alloysheet and the lower side is the steel sheet. The Al alloy sheetwas given a hollow by insertion of the welding tool with theprobe. About 0.2 mm-thick Al alloy region observed on thesteel sheet corresponds to the area beneath the probe tip.

The macroscopic cross-sectional view like this wascommon for all the joints. Careful observation, however,found that joining area was not common for all the joints.

Figures 6(a) and (b) are schematic illustrations showingthe difference in joining area. Joining was achieved in thelimited area close to the periphery of the probe for ZAM andGI, as indicated by ‘‘c’’ in Fig. 6(a). There was a gap betweenthe sheets in the region ‘‘d’’ in Fig. 6(a). While, the joiningarea encompassed through the lapped-interface under theprobe, in the area ‘‘c’’, as shown in Fig. 6(b) for AS and GA.

The interface microstructures of all joints were shown inFigs. 7(a)–(f).

Figures 7(a)–(f) show the back-scattered electron images(BEIs) in the areas at the joint interface corresponding to ‘‘c’’and ‘‘d’’ in Figs. 6(a) and (b). Bright and dark contrastscorrespond to steel and Al alloy, respectively. The inter-mediate layers with a medium contrast are clearly observed atthe joint interface for all joints. The thickness of intermediatelayer was about 1 mm for ZAM and GI joints, as shown inFigs. 7(a) and (b) in the areas at the joint interface ‘‘c’’ inFig. 6(a). These intermediate layers were different from theoriginal plated layers of ZAM and GI, as shown in Figs. 1(a)and (b). Cavities (un-bonded area) were often observed underthe probe center region for ZAM, GI joint, as indicated bywhite arrows in Figs. 7(c) and (d).

The joint interface of AS is shown in Fig. 7(e). Nointermediate layer was observed. The important finding isthat the IMC layer at the original Al-Si alloy/plated steelinterface remained after FSSW.

Figure 7(f) is the joint interface of GA. The thick layer wasclearly observed between Al alloy and steel sheets. Thethickness of layer was about 20 mm and thicker than theoriginal plated layer as shown in Fig. 1(d). In addition, manyvoids and cavities were observed at the bottom side of thelayer. Formation mechanism of the layer will be reportedprecisely in the next paper.

3.4 Macroscopic appearance of the fracture surfaceafter M-CTT

Figures 8(a)–(d) are optical micrographs of the fracturesurface on the steel sheet side after M-CTT. In the picture,Dp represents the probe diameter and Ds does the shoulderdiameter. As shown in Fig. 8(a), for ZAM, a doubledoughnut-like area is a slightly larger than the probediameter, Dp. We can also find the dark contrast area at thecentral region of the round fracture surface.

The fracture surface of GI (Fig. 8(b)) is also characterizedby the doughnut-like bright contrast area surrounding the

Fig. 3 Temperature change at the joint interface during FSSW. Melting

temperatures of plated layer of ZAM (330�C), GI (420�C), AS (640�C)

and GA (880�C) are shown by dashed lines.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Frac

ture

load

, P /

kN

ZAM GI AS Non-plated steelGA

Fig. 4 Fracture loads of four kinds of plated steel sheet joints and Non-

plated steel sheet joint.

Al alloy

Steel

Fig. 5 Macroscopic view of the cross section of joint interface (optical

micrograph).

Plated steel

Joining area

Plated steel

Al alloy

Joining area

c

c

d

(b)

(a)

Al alloy

c

Fig. 6 Schematic illustration of the joint interface. (a) ZAM and GI, (b) AS

and GA.

1420 K. Feng, M. Watanabe and S. Kumai

dark contrast region at the center of fracture surface. Thesize of the bright contrast area is a slightly larger than Dp inthis case, too.

In contrast, the fracture surface of AS is round pancake-like and the diameter of the bright contrast area is also aslightly larger than Dp, as shown in Fig. 8(c).

As for GA (Fig. 8(d)), the round pancake-like area is alsoobserved, but the contrast of this area is relatively dark andcomplicated. The diameter of this area is slightly larger thanDp, too.

3.5 Microscopic view of the fracture surface andfracture path at the joint interface

In order to investigate fracture surface more precisely,SEM observation was performed for each region marked withframes in Fig. 8: a1 to a4 in ZAM, b1 to b4 in GI, c1 in AS andd1 in GA.

Figures 9(a)–(j) show SEM images for these regions.Figures 9(a)–(d) correspond to areas ‘‘a1’’–‘‘a4’’ for ZAM.The area ‘‘a1’’ shows the flat featureless surface with manysmall voids and sharp cracks. The area ‘‘a2’’ is covered withrelatively flat fracture surface with dark contrast. A brightcontrast network-like pattern is also observed. The area ‘‘a3’’exhibits dimple fracture morphology. Rough and compli-

cated appearance is observed for the area ‘‘a4’’. However, thedoughnut-like area including the area ‘‘a4’’ is not fracturesurface but the surface of solidified ZAM plate metals whichonce melted and being squashed out from the probe centerregion during FSSW.

These features are common for GI, as shown in Figs. 9(e)–(h) corresponding to the areas ‘‘b1’’–‘‘b4’’ in Fig. 8(b),respectively.

Figure 9(i) is the fracture surface of AS for the area ‘‘c1’’.This is characterized by the relatively flat appearance withmany cracks forming ‘‘mosaic’’ structure. Figure 9(j) corre-spond to the area ‘‘d1’’ in GA exhibits more complicatedfracture surface covered with the flat area with sharp cracksand the relatively rough area.

3.6 Measurement of substantial joining area in theFSSW joints

Based on both optical micrographs of fracture surface andcross sectional view of the joint interface, the substantialjoining area was estimated for the joints after M-CCT.

Relationship between the obtained fracture load and theestimated joining area is summarized as shown in Fig. 10.Fracture load increased as increasing the joining area in alljoints.

Al alloy

GI steel

Al alloy

ZAM steel

Al alloy

AS steel

(b)(a)

(d)(c)

(f)(e)

Al alloy

GA steel

Al alloy

ZAM steel

CavityAl alloy

GI steel

Cavity

Fig. 7 Back-scattered electron images (BEIs) of the joint interface. (a) ‘‘c’’ in Fig. 6(a) for ZAM, (b) ‘‘c’’ in Fig. 6(a) for GI, (c) ‘‘d’’ in

Fig. 6(a) for ZAM, (d) ‘‘d’’ in Fig. 6(a) for GI, (e) ‘‘c’’ in Fig. 6(b) for AS, (f) ‘‘c’’ in Fig. 6(b) for GA.


3.7 Fracture path at the joint interface during M-CTTFracture path at the joint interface was examined with the

help of the interrupted M-CTT. The loading was interruptedat the maximum load point and the unbroken, but partiallycracked specimens were obtained. Figures 11(a)–(d) showcross sectional views of the cracked joint interfaces observedin the unbroken specimens. Figure 11(a) is BEI in the areacorresponding to the area ‘‘a2’’ in Fig. 8(a) and Fig. 9(b).A crack is clearly observed along the interface between thesteel surface and the intermediate layer. See Fig. 7(a) forcomparison.

Figure 11(b) is for the area ‘‘a3’’ in Fig. 8(a) and Fig. 9(c).In this case, we can see a large cavity in the Al matrix close tothe joint interface. This suggests that the crack propagated inthe Al alloy matrix. Please note that the intermediate layer onthe steel sheet is thin in this area and no fracture took placeat this interface.

Note that the fracture manners are common for GI.Figure 11(c) is for the area ‘‘c1’’ in Fig. 8(c) and Fig. 9(i)

for AS. It was found that the crack propagated in the originalIMC layer in Al-Si plate alloy. See Fig. 1(c) and Fig. 7(e) forcomparison. Figure 11(d) shows the fractured area corre-sponding to the area ‘‘d1’’ in Fig. 8(d) and Fig. 9(j) for GA.We can see that the fracture took place in the layer betweenAl alloy and steel sheets. See Fig. 7(f) for comparison.

4. Discussion

4.1 Effect of melting temperature of the plated layer onthe Al alloy/steel joint interface in FSSW

The joint interface morphology as shown in Figs. 5 to 7can be classified into two patterns. One is the case that nojoining takes place below the probe center but joining occursat the lapped interface close to the periphery of the probe.The other one is the case that joining occurs in the area underthe probe almost evenly. The former is for ZAM and GI, inwhich melting temperature of the plated layer is lower thanoperating temperature of the present FSSW, as shown inFig. 3. The latter is for AS and GA, which have the platedlayer with high melting temperature. Such difference injoining area is considered to cause the characteristic macro-scopic difference in fracture surface morphology: doughnut-like and pancake-like one.

Let us discuss the formation mechanism of the jointinterface. When the melting temperature of the plated layer islower than the operating temperature, the plated layer willmelt and the molten plated layer can be squeezed outward.This facilitates direct contact between the aluminum platesurface and the refreshed steel surface. At the probe-centerregion, a thin liquid film of the plated layer possibly stays andhinders the direct contact of aluminum and steel surface.

(a)

a 4

D p

D s

a 2

a 1

a 3

(b)

b 2

b 4

b 1

D p

D s

b 3

D p

D s

c 1

D p

D s

d 1

(c) (d)

Fig. 8 Macroscopic view of the fracture surface of the plated steel side (optical micrograph). (a) ZAM (b) GI (c) AS (d) GA.


(f)(e)

(h)(g)

(a) (b)

(c) (d)

(j)(i)

Fig. 9 Secondary electron images (SEIs) of fracture surface. (a) ‘‘a1’’ in Fig. 8(a) for ZAM, (b) ‘‘a2’’ in Fig. 8(a) for ZAM, (c) ‘‘a3’’ in

Fig. 8(a) for ZAM, (d) ‘‘a4’’ in Fig. 8(a) for ZAM, (e) ‘‘b1’’ in Fig. 8(b) for GI, (f) ‘‘b2’’ in Fig. 8(b) for GI, (g) ‘‘b3’’ in Fig. 8(b) for GI,

(h) ‘‘b4’’ in Fig. 8(b) for GI, (i) ‘‘c1’’ in Fig. 8(c) for AS, (j) ‘‘d1’’ in Fig. 8(d) for GA.


Since the temperature decrease occurs by pulling out of theprobe, the isolated liquid film will solidify and this will formshrinkage cavity. This may result in the gap formation asshown in Fig. 7(c) for ZAM and Fig. 7(d) for GI.

At the outer region close to the periphery of the rotatingprobe, corresponding to the area ‘‘c’’ in Fig. 6, the aluminummatrix above the lapped surface must be subjected to arelatively severe plastic flow condition. This is considered topromote solute diffusion between aluminum and steel.

It should be mentioned that thickness of the intermediatelayer decreases as increasing the distance from the probecenter. The effect of this thickness difference on fracturebehavior will be discussed later.

The joining area spreads over entirely under the probe forAS, as shown in Fig. 6(b). In this case, no intermediate layerwas observed as shown in Fig. 7(e). The original IMC layerformed on the steel sheet in Al-Si alloy plated layer wasclearly remained in the welded joint interface. The Al-Sialloy will be mixed to the 6022 aluminum alloy sheet by

plastic flow and both mechanical and metallurgical alloyingmust be achieved by FSSW.

In the present welding condition, the probe tip is awayfrom the IMC layer and there is almost no chance the rotatingtool breaks the IMC layer. Consequently, joining between6022 aluminum alloy sheet and Al-Si alloy plate takes place,but the IMC layer remains almost intact after FSSW in spitethat some small broken pieces were observed in the Al alloymatrix.

4.2 Relationship among joint interface structure, frac-ture surface and joint strength

Figure 9(a) shows SEM image of the area ‘‘a1’’ in Fig. 8(a)for ZAM. Flat featureless surface with many small voids andsharp cracks is observed. As shown in Fig. 7(c), the crosssectional view of this area revealed that there is a cavitybetween Al alloy sheet and the intermediate layer on the steelsheet. Therefore, this is not the fracture surface produced byM-CTT but the image of original interfacial morphology ofthis region after FSSW.

Careful observation found that this is the surface ofintermediate layer formed during FSSW. This is partiallycovered with a thin film which was the once-melted andre-solidified Zn alloy plate.

Figure 9(b) is for the area ‘‘a2’’ in Fig. 8(a). The crosssectional view of the joint interface for this area is shown inFig. 7(a). The thin and continuous intermediate layer isformed at the joint interface. Figure 9(c) is for the area ‘‘a3’’in Fig. 8(a). This thickness difference is considered to affectfracture manner. It is considered that when the intermediatelayer is thick, the crack propagates along the interfacebetween the steel surface and the intermediate layer and/or inthe intermediate layer, as shown in Fig. 11(a). It is generallyrecognized that fracture and/or peeling of the intermediatelayer from the matrix seldom occurs when the intermediate

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0

Joining area, A / mm2

Frac

ture

load

, P

/ kN

ZAM GI AS GA

80604020

Fig. 10 Relationship between joining area and fracture load.

Al alloy

Al alloy

ZAM steel

Al alloy

ZAM steel

Al alloy

AS steel

Al alloy

GA steel

(a) (b)

(d)(c)

Fig. 11 Cross-sectional views of the cracked joint interface after the interrupted M-CTT. (a) ZAM, (b) ZAM, (c) AS, (d) GA.


layer is very thin. In this case, fracture tends to take place inthe matrix with weaker strength. This corresponds to theappearance of dimple fracture shown by Fig. 11(b), which isfor the area ‘‘a3’’ in Fig. 8(a) and Fig. 9(c). The crackpropagates in the Al alloy matrix.

As described in 4.1, at the joint interface in ZAM,thickness of the intermediate layer decreases as increasingthe distance from the probe center. Therefore, we can say thattransition of fracture manner from the type ‘‘a2’’ to the type‘‘a3’’ is reasonable. This interpretation can be applicable tothe case of GI.

In contrast, fracture of AS occurred mainly in the originalIMC layers in the plated Al-Si alloy. This was welldemonstrated by the interrupted M-CTT as shown inFig. 11(c). Relatively flat fracture surface with many cracks,which is like a forming ‘‘mosaic’’ structure, represents thebroken surface of IMC layer.

As shown in Fig. 11(d), the thick layer at the joint interfaceprovides a preferential crack growth path in GA. Fracturesurface of GA as shown in Fig. 9(j) consists of two differentfracture surfaces, flat and rough, this reflects the crack growthpath very well.

Figure 10 indicates that relatively large joining area isobtained for ZAM and GI. This is because the direct bondingbetween 6022 aluminum alloy and steel was promoted by theappearance of refreshed steel surface due to melting of theplate layer.

Fracture strength was controlled by the strength of originalIMC layer and that of intermediate layer for AS and GA,respectively.

5. Summary

Lap joining of aluminum alloy sheet/four kinds of plated

steels was carried out using the friction stir spot weldingmethod. Sound joining was achieved for all plated steelsheets.

The interface morphology, strength and joining area of thejoint varied depending on whether melting temperature of theplate layer was higher or lower than the maximum operatingtemperature.

Large joint strength and joining area were obtained forthe steel sheet with the low melting temperature of platedlayer. In this case, the original plated layer was removed fromthe interface during FSSW and aluminum/steel interfacewith a thin intermediate layer was observed for ZAM andGI joints.

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