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Materials Science and Engineering A 431 (2006) 114–117

Microstructure and properties of friction stir butt-weldedAZ31 magnesium alloy

Wang Xunhong a,∗, Wang Kuaishe a,b

a Department of Metallurgy Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, Chinab State Key Laboratory for Mechanical Behavior of Materials, Xi’an JiaoTong University, Xi’an, Shaanxi 710048, China

Received 15 March 2006; received in revised form 18 May 2006; accepted 23 May 2006

bstract

Friction stir welding (FSW) is a relatively new joining technique particularly for magnesium and aluminum alloys that are difficult to fusion

eld. In this paper, an excellent friction stir weld of AZ31 magnesium alloy was obtained at proper parameter. In the friction stir zone (FSZ), theicrostructure of the base material (BM) is replaced by fine grains and small particles of intermetallic compounds. The average microhardness of

he friction stir zone is higher than that of the base material. The maximum tensile strength of joint can reach 93% that of the base material. Andhe failure locations are almost at the heating affected zone.

2006 Elsevier B.V. All rights reserved.

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eywords: Friction stir welding; Magnesium alloys; Microstructure; Grain refi

. Introduction

Friction stir welding (FSW) is a solid-state joining processhat was invented by The Welding Institute, UK, in 1991 [1].he technique is a variant of the friction welding processes,ut utilizes a rotating tool with a shoulder and a profiled probehat is plunged into the work pieces and traversed along theeld centerline. The motion of the tool generates frictional heatithin the work pieces, extruding the softened plasticized mate-

ial around it and forging the same in place so as to form aolid-state seamless joint. No melting takes place in the process,hus keeping the temperatures relatively low and producing gooduality welds with significantly lower residual stresses [2]. Theiggest advantage of the method, however, is its capability toeld light structural materials like certain aluminum alloys andagnesium alloy that used to be considered unweldable or dif-cult to weld by conventional fusion welding techniques [3].

Magnesium alloys have many attractive properties, such asow density and high specific strength. It is predicted thathe application of magnesium alloys will grow rapidly in the

ear future; Park et al. [4] discussed the microstructural evolu-ion of Mg alloy AZ91D during FSW. However, the effects ofSW parameters, especially travel speed and rotation speed on

∗ Corresponding author. Tel.: +86 29 8186 5594.E-mail address: honghong1981312@yahoo.com.cn (W. Xunhong).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.05.128

nt; Mechanical properties

he microstructure changes and mechanical properties of FSWelded Mg alloy have not rigorously studied. The present author

xamined mechanical properties, such as hardness and tensileroperties in defect-free FSW welded AZ31 Mg alloy to evaluatehe effects of the FSW parameter on the microstructure changesnd mechanical properties [5–9].

. Experimental procedures

The plate of AZ31 magnesium alloy was friction stirelded. Friction stir welded plates with nominal composition

3.0–4.0)Al–(0.2–0.8)Zn–(0.15–0.5)Mn remaining Mg. Nomi-al dimensions of the plates were 250 mm (length) × 250 mmwidth) × 4 mm (thick). The tensile strength of base material is75 MPa.

Tool of friction stir welding was made on CNC lathe from5 Mn steel, varying welding parameters such as tool rotationpeed and travel speed was used in this investigation. Rotationpeed was 375–2250 rpm, and travel speed was 20–375 mm/min.he probe diameter of the tool measured 4 mm and the shoulder2 mm. The penetration depth was adapted to fully penetratedutt joint in a material of 4 mm thickness.

Microstructural examination was carried out on cross sec-ions perpendicular to the welding direction. After being

echanically ground and polished, the specimens were etchedn a solution of 5 g picric acid + 5 ml acetic acid + 20 ml

ence and Engineering A 431 (2006) 114–117 115

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W. Xunhong, W. Kuaishe / Materials Sci

ater + 150 ml ethanol. Microhardness profiles were measuredn the specimen. Tensile test and SEM scanning on fractureurface of friction stir weld in a tensile test were carried out.

. Results and discussion

.1. Macrostructure and microstructure of friction stir weld

Fig. 1 shows the macrostructure of the cross section in frictiontir weld at a rotation speed 1200 rpm, a travel speed 90 mm/minnd a welding pressure 150 N. The width of the friction stir zoneFSZ) of the experimental material was about 4 mm, equal to theiameter of the pin. Fig. 2 shows the optical microstructure ofhe FSW welded Mg alloy at rotation speed 1500 rpm, travelpeed 90 mm/min and a 150 N welding pressure. In addition,he tool advance vector in line with tangent vector of a shoulderringe calls advancing side (AS), and the contrary vector calls

etreating side (RS). The macrostructure of the cross section inetreating side and advancing side is different, which is relativehe flow field of friction stir welds. The tunnel defect usuallyan be seen near the RS zone.

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Fig. 2. SEM images showing the microstructures of cross sections

Fig. 1. Cross section of friction stir weld in AZ31 magnesium alloy.

The microstructure of FSZ is greatly refined due to dynamicecrystallization. As going from base metal to welding bound-ry part and the central part of welding, crystal grain becomesmall. Besides, FSZ is a thermomechanically affected zoneTMAZ) in which magnesium grains are severely deformed.he microstructure of BM, which is shown in Fig. 2(a), is mainlyomposed of granular phase of magnesium solid solution grains.

ome tiny eutectic zones, which are shown by arrows in Fig. 2(a),ould also be seen in the microstructure. A typical microstructuref TMAZ is shown in Fig. 2(b). It can be seen that magnesiumrains in TMAZ show an elongated shape due to plastic defor-

of: (a) BM; (b) TMAZ; (c) FSZ; (d) TMAZ/FSZ interface.

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16 W. Xunhong, W. Kuaishe / Materials Sci

ation during FSW. The large eutectic zones were arrangedlong grain boundaries (shown by arrows). Fig. 2(c) shows theicrostructure of FSZ, which is characterized by the fine grains.arefully examining the microstructure of FSZ, some narrowands of particles could be seen, indicating material flow pat-erns. Fig. 2(d) shows the interface between TMAZ and FSZ. Itould be observed that in TMAZ, the deformation of the grainsncreased with decreasing distance to FSZ. Moreover, the net-ork structure of the compound phase was broken into particlesear the interface while it remained continuous far away fromhe interface.

In this investigation, a relatively fine magnesium grain struc-ure is produced by FSW. After FSW, the microstructures of FSZonsist almost entirely of dynamic recrystallization magnesiumrains and intermetallic compounds tend to disappear due toissolution at elevated temperature [10].

According to the Mg–Al binary phase diagram, Mg17Al12ntermetallic compounds in AZ31 will dissolve into the magne-ium matrix completely when the heating temperature is higherhan a certain value. Therefore, the peak temperature of FSZ inhis study is high enough for Mg17Al12 to dissolve into the mag-esium matrix. It is well recognized that second phase particlesill influence the stress and strain distribution during plasticeformation, and thus particles will strongly affect the recrys-allization process. As to the present material, the behavior ofecond phases is much more complicated since their size andistribution will change during FSW. During FSW process, mag-esium grains will plastically deform with rotation of the tool,ut the second phase, which is a brittle phase, is hard to deform.his causes stress concentration on the second phase. Whenpplied stress is higher than the fracture strength of the secondhase, fracture takes place and large second phases change intomaller ones gradually.

.2. Hardness

Fig. 3 shows the Vickers microhardness profiles measuredlong the mid-thickness line of the cross section. The Vickersest was done with 8-second at 100 g load and a 1 mm step. Thehree ranges are travel speed of 90, 75 and 37.5 mm/min at the

ig. 3. Microhardness distribution in the same rotation speed of 1500 rpm.

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nd Engineering A 431 (2006) 114–117

ame rotation speed 1500 rpm. It could be found that hardnessalue of FSZ is higher than that of BM. There are two main rea-ons for the improved hardness of FSZ. Firstly, since the grainize of FSZ is much finer than that of BM, grain refinementlays an important role in material strengthening. According tohe Hall–Petch equation, hardness increases as the grain sizeecreases. Secondly, the small particles of intermetallic com-ounds are also a benefit to hardness improvement, accordingo the Orowan hardening mechanism.

It can be known that microhardness in the weld zone waslmost same with the base material form the value. It should beoticed that the microhardness of weld center was little lowerhan base material and the maximum value was all off the cen-er about 1–1.5 mm. It can be known that the hardness of FSWelded AZ31 was considered with the grain size distribution in

he weld zone. There is a considerably softened region whoseotal range is 5 mm and appears near the heat affected zoneHAZ) located 2–3 mm away from the weld center. The elon-ated and deformed grain probably causes the softness. And theeason causing the dissymmetrically softened region is that theifferent transformation between the advancing side and retreat-ng side. And the material flow was unsteady around the weldingin. On the advancing side, the microstructure was compact andhe transmutation grain had a great strain and higher energy ofistortion. The work hardening was more notable than retreatingn the advancing side. The value of microhardness is growingith the increasing the travel speed.

.3. Mechanical properties of friction stir welds

The samples were carefully machined with coolant to mini-ize heating. Tensile test samples were tested at room tempera-

ure, with the weld transverse to welding direction. During test,ost specimens failed at heat affected zone. Fig. 4 shows the

ensile results of the AZ31 Mg alloy welded at same rotationpeed and welding pressure. It can be concluded that the higher

ravel speed, the larger tensile strength of the joint in certainxtension. When travel speed was 90 mm/min, the joint strengthas close to the base material. But when the travel speed passedfixed value (90 mm/min), the joint strength dropped as upping

Fig. 4. Tensile strength of friction stir weld.

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ig. 5. SEM of friction stir weld exhibiting ductile fracture in a tensile test.

he travel speed. It can be known that the mechanical propertyould be close to base material strength when joint welded inhe proper parameter (rotation speed: 1500 rpm, travel speed:0 mm/min, welding pressure: 150 N). The main reason causedhat should be the grain refinement. The value of tensile resultss growing with the increase in the rotation speed (Fig. 4).

Fig. 5 showed the SEM of friction stir weld exhibiting ductileracture in a tensile test. The dimples are traces of the microvoidsroduced during fracture. Normally, these microvoids round, orquiaxed, when a normal tensile stress produces the fracture.owever, on the shear lip, the dimple are oval-shaped, or elon-ated, with the ovals pointing toward the origin of the fracture.

. Conclusions

. FSW welded AZ31 Mg alloy joint with excellent appearanceand no distortion can be obtained when the process parameterwas proper.

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nd Engineering A 431 (2006) 114–117 117

. The weld zone was divided into three regions (weld nugget,thermomechanically affected zone and heat affected zone)based on the microstructure; the weld nugget had fine andequiaxed grains due to dynamic recrystallization; the HAZzone had a lower microhardness than base material. Com-pared with the base material, magnesium grains in FSZ weregreatly refined due to dynamic recrystallization. In FSZ,the average grain size of the present alloy was finer thanmost of the FSW magnesium alloys structures reported pre-viously. Microhardness of FSZ was higher than that of BM.The thermally stable intermetallic compounds in the presentalloy played an important role in microstructural evolutionby influencing the dynamic recrystallization process duringFSW.

. When process parameter was proper, the tensile strengthof the FSW welded AZ31 plate could get 93% that ofthe base material at rotation speed: 1500 rpm, travel speed:90 mm/min, welding pressure: 150 N.

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