M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 1 5 8 3 – 1 5 9 0

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Effect of heat input on the microstructure and mechanicalproperties of tungsten inert gas arc butt-welded AZ61magnesium alloy plates

Dong Min, Jun Shen⁎, Shiqiang Lai, Jie ChenCollege of Material Science and Engineering, Chongqing University, Chongqing 400044, People's Republic of China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 13883111150E-mail address: shenjun2626@163.com (J.

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.09.010

A B S T R A C T

Article history:Received 20 June 2009Received in revised form11 September 2009Accepted 16 September 2009

In this paper, the effects of heat input on the microstructures and mechanical properties oftungsten inert gas arc butt-welded AZ61 magnesium alloy plates were investigated bymicrostructural observations, microhardness tests and tensile tests. The results show thatwithan increaseof theheat input, thegrainsboth in the fusionzoneand theheat-affectedzonecoarsen and the width of the heat-affected zone increased. Moreover, an increase of the heatinput resulted inadecreaseof the continuousβ-Mg17Al12 phase andan increaseof the granularβ-Mg17Al12 phase in both the fusion zone and the heat-affected zone. The ultimate tensilestrength of thewelded joint increasedwith an increase of the heat input,while, too high a heatinput resulted in a decrease of the ultimate tensile strengthof thewelded joint. In addition, theaverage microhardness of the heat-affected zone and fusion zone decreased sharply with anincrease of the heat input and then decreased slowly at a relatively high heat input.

© 2009 Elsevier Inc. All rights reserved.

Keywords:AZ61 magnesium alloyTungsten inert gas arc weldingMicrostructureMechanical propertiesHeat input

1. Introduction

Magnesiumand itsalloyshave theprospectofwideapplication inthe automotive, aircraft and electronic consumer industriesbecause of their low density in combinationwith a high strength,an excellent castability, a perfect electromagnetic interferenceshielding property, a high thermal conductivity and a highdamping capability [1,2]. However, the production of complicatedpieces from magnesium alloys is usually difficult and expensivebecause of their poor ductility and cold processability at roomtemperature. This is becausemagnesium has a hexagonal close-packed (HCP) crystal structure, which has insufficient slipsystems at room temperature [3]. Therefore, welding technologyof magnesium alloys plays an important role in the boardapplication of magnesium alloy structural parts. Up to now,

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several welding methods, such as laser beam welding (LBW),tungsten inert gas (TIG) arcwelding, electronbeamwelding (EBW)and frictionstirwelding (FSW)havebeenapplied to theweldingofmagnesium alloys [3–7]. Compared with other welding methods,TIG welding technology is themain weldingmethod adopted formagnesium alloys because of its advantages of utility andeconomy [4,5]. Liu and Dong found that the difference of thegrain size in the heat-affected zone (HAZ) between a TIG fillerwelded joint and a TIG welded joint without a filler resulted in avariety of the fracture location and the ultimate tensile strength(UTS) value ofwelded joints [8]. Zhu et al. found that a high rate ofmeltingofα+βeutecticphasesanda lowrateofdissolutionof theβ-Mg17Al12 phase led to the formation of a partially melted zone(PMZ) [9,10]. Also, the formation mechanisms of pores [11] andliquation cracking [12] in the PMZwere investigated by Baseslack

.

Table 1 – TheTIGwelding parameters in these experiments.

Samples Current(A)

Welding speed v(mm s−1)

Heat input E(J mm−1)

a 70 10 63b 80 10 72c 86 10 77.4d 90 10 81e 100 10 90

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et al. and Shen et al. Liu et al. found that thewelded seammainlyconsisted of dendritic crystals while the parts near the weldedseamwere composed of columnar crystals through a study of themicrostructure of Mg/Al dissimilar materials TIG welded joints[13]. The above-mentioned investigations mainly focused onmicrostructural characteristics andmechanismsof the formationof defects in welded seams of TIG welded magnesium alloys.However, the influence of the welding parameters (in particular,heat input) on the microstructure (such as the grain size and themorphology of the eutectic phase) and themechanical propertiesof TIG weldedmagnesium alloys is a valuable problem for an in-depth study.

Hence, in this paper, the effects of heat input on themicrostructure (grain size and morphology of the β-Mg17Al12phase) in both the HAZ and the fusion zone (FZ) of welded

Fig. 1 – A typical microstructural image of a TIG welded joint of Awelded joint, (b) BM of hot-extruded AZ61 magnesium alloy, (c) F

seams of TIG filler welded AZ61magnesium alloy and themechanical properties (microhardness and UTS) were inves-tigated. In addition, the morphologies of the tensile fracturedsurfaces and the fracturedmechanisms of thewelded joints ofTIG filler welded AZ61magnesium alloy plates are discussed.

2. Experimental Procedures

Hot-extruded AZ61magnesium alloy plates (provided by theChongqing Magnesium Company, China) with a size of30 mm×120 mm×3mm were used for TIG filler welding tests.Prior to welding, in order to avoid the influence of impurities(suchas surface contaminatesand theoxide film) on the surfaceof themagnesium alloy plates and weldingwires on the resultsof the welding tests, the surface of the samples were polishedlightlywith diamond powders and then cleanedwith a solution(99 vol.% C2H5OH+1 vol.% HCl). The samples were butt-weldedon top of a copper-backing strip containing a semi-circulargroovewith a dimension of 6 mm inwidth and 1.5 mm indepth.An AC penetrating square-wave welding procedure wasadopted for the welding tests. The detailed welded parametersin these experiments are shown in Table 1. Here, the flux ofshielding gas (argon), welding voltage and welding speed areconstant (the flux of shielding gas was10 l/min, welding voltagewas 10 V and welding speed was 10 mm s−1). The variation of

Z61 magnesium alloy at a heat input of 90 J mm−1, (a) wholeZ and (d) HAZ.

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the heat input was achieved by adjusting the welding current.The relationship between the parameters is as follows:

L = ηUIv

ð1Þ

where L is the heat input during the TIG welding process, U isthe welding voltage, v is the welding speed, I is the weldingcurrent and η is the efficiency of TIG welding (η=0.90 [14]).

After the welding tests, the samples were sectioned and thecross-sections of the welded seams were prepared usingstandard metallographic procedures (grinding, polishing andetching with a solution of 4 vol.% HNO3+96 vol.% C2H5OH for

Fig. 2 –Microstructural images of TIG welded joints with differe(d) 81 J mm−1 and (e) 90 J mm−1.

20 s–40 s). In addition, tensile test specimenswithagauge lengthof 15 mm and a width of 4 mmwere sectioned from the weldedseam parts by a numerically controlled linear cutting machine.The tensile tests were carried out with a tensile test machine atroom temperature and the tensile direction was perpendicularto the weld seams. Three tensile test results were collected insamples with the same heat input and the average values ofthem were adopted for discussion. The microhardness testswere performed with a Vickers hardness tester (V-1000) with aperiod of 20 s on, a load of 1000 g and a step size of 0.5 mm. Thevalues of themicrohardness of the FZ andHAZweremade froman average value of five data points. An optical microscope

nt heat inputs, (a) 63 J mm−1, (b) 72 J mm−1, (c) 77.4 J mm−1,

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(MDJ200) and a scanning electron microscope (TESCAN, Inc.VegaIILMU SEM) were used for microstructural observations.EnergydispersiveX-ray spectroscopy (OXFORD, Inc. ISIS300)wasused to determine the phases formed in the welded seam.

Fig. 3 – The effect of heat input on the width of the HAZ andthe average grain size of α-Mg in the HAZ.

3. Results and Discussion

3.1. The Typical Microstructure of TIG Welded AZ61Magnesium Alloy Plates

Fig. 1 showsa typicalmicrostructure of a TIGwelded jointwith aheat input of 90 J mm−1. It was found that the welded joint wascomposedof a FZ, awideHAZand a basematerial (BM) area (seeFig. 1 (a)). The BM is characterized by a fine and uniformequiaxed structure (the average grain size is about 39 μm), asshown in Fig. 1 (b). The presence of this structure is due to therecrystallization of grains during hot rolling. The FZ mainlyconsisted of the α-Mg phase with a surrounding eutectic β-Mg17Al12 phase (see Fig. 1 (c)). Compared with the grains both inthe FZ and BM, the grains had coarsened markedly in the HAZ(see Fig. 1 (d)). This is because during the welding process, thetemperature in the HAZ may be up to 527 K [15], which is morethan the recrystallization temperature of the AZ series magne-sium alloys (about 478 K [16]). Therefore, recrystallization andgrain coarsening may easily occur in the HAZ.

3.1.1. Effects of Heat Input on the Microstructures of HAZFig. 2 illustrates the evolution ofmicrostructure in theHAZwithan increase of the heat input. In agreement with the literature[3], the results of EDX analysis indicated that the HAZ mainlyconsisted of a white α-Mg phase (marked with an arrow A inFig. 2 (a)) and a black β-Mg17Al12 phase (marked with arrow B inFig. 2 (a)). The average grain size of α-Mg in the HAZ and thewidth of HAZ were quantitatively analyzed by an imageanalyzer (UTHSCSA Image Tool 3.0) and the results are shownin Fig. 3. It can be seen that the width of the HAZ and the grainsize of α-Mg in the HAZ increased with an increase of the heatinput. When the heat input was low (63 J mm−1), the averagegrain size ofα-Mg (about 34.9 μm) in theHAZwas close to that ofthe BM. This indicated that when the heat input was relativelylow (63 J mm−1), although recrystallization occurred in the HAZ,the coarsening of grains in the HAZ was not obvious. When theheat input increased to the maximum value (90 J mm−1), thegrains in the HAZ grew up to an average grain size of 72.9 μm,whichwas about double the size of that in the BM. The increaseofheat input led toa coarseningof thegrains in theHAZbecausethe increase of heat input provided more driving force for grainboundary migration which then speeded the growth of grains.Fig. 3 also shows thatwithan increaseofheat input, thewidthofthe HAZ also increased due to the extra heat input to the BM.Moreover, it is worth noticing that when the heat input was 63 Jmm−1, a continuous β-Mg17Al12 phase formed in the HAZ (seeFig. 2 (a)). However,withan increaseofheat input, theamountofthecontinuousβ-Mg17Al12phasedecreasedwhile theamountofthe granular β-Mg17Al12 phase increased. When the heat inputwas 90 J mm−1, only a small area of the continuous β-Mg17Al12phase can be observed in the HAZ (see Fig. 2 (e)). This is becausethe increase of temperature led to more of the continuous β-Mg17Al12 phase dissolving into the α-Mg matrix and the

precipitation of the solute Al in the interior of α-Mg matrix toform granular β-Mg17Al12 [17].

3.1.2. Effects of Heat Input on the Microstructures of FZThe effect of heat input on the microstructures of the FZ isshown in Fig. 4. It can be seen that α-Mg in the FZ ischaracterized by fine equiaxed dendrites with a surroundingβ-Mg17Al12 phase. The results for the quantitative analysis ofthe effect of heat input on the average grain size of α-Mg areshown in Fig. 5. When the heat input was relatively low(63 J mm−1), the average grain size of α-Mg in the FZ was about17.5 μm, which is smaller than that in the BM. The formationof a fine grain structure in the FZ is due to the relatively highcooling rate. When the heat input was 90 J mm−1, the averagegrain size of α-Mg in the FZ was about 45.5 μm. Hence, the α-Mg grains in the FZ coarsened with an increase of the heatinput. By careful observation (Fig. 4 (a)–(e)), it can be seen thatmore continuous β-Mg17Al12 phase formed in the FZ at thelowest heat input (63 J mm−1). However, with the increase ofheat input, the amount of continuous β-Mg17Al12 phasedecreased while the amount of discontinuous/granular β-Mg17Al12 phase increased. This is because a high cooling ratedue to a low heat input in the FZ restrained the growth of theα-Mg grain.

3.2. Effect of Heat Input on Mechanical Properties ofWelded Joints

3.2.1. Tensile StrengthThe effect of heat input on the UTS of the TIG welded joint isdepicted in Fig. 6. In this study, at the lowest heat input (63 Jmm−1), a partial penetration and pores were observed in thewelded seam. During the welding process, the shielding gasusually protected the surface of themolten pool, while the backof the sample failed to be protected. Hence, airmay intrude intothemolten pool easily through the gap between two plates andresult in the formationofpores in theweldedseam(seeFig. 6). Inthis condition, the UTS of the welded seam is 135 MPa, which

Fig. 4 – SEM imagesof themicrostructure in theFZwithdifferentheat inputs, (a) 63 Jmm−1, (b) 72 Jmm−1, (c) 77.4 Jmm−1, (d) 81 Jmm−1

and (e) 90 J mm−1.

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only accounted for 47% of that of BM (285 MPa). This resultilluminates that during the TIG welding of magnesium alloyAZ61, too low a heat input easily resulted in the presence ofwelding defects, which seriously decreased the tensile strengthof the welded joint. With an increase of the heat input, the UTSof the welded joints increased. The highest UTS of a welded

joint, with 90% of that of the BM, was obtained at a heat input of81 J mm−1 due to less welding defects (such as partialpenetration andpores) in thewelded seam (see Fig. 6). However,when the heat input was increased to 90 J mm−1, the UTS of thewelded joint decreased slightly due to the different volatility ofelements in theAZ61magnesiumalloy, suchasmagnesiumand

Fig. 5 – The effect of heat input on the average grain size ofα-Mg in FZ.

Fig. 7 – SEM images show typical tensile fracture surfaces ofthe magnesium alloy, (a) BM of hot-extruded AZ61

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zinc.Duringaweldingprocess,magnesiumandzinc evaporatedeasily at a high temperature due to the lower melting points/boiling points and the higher vapor pressures of these elementscompared with that of aluminum (the melting points/boilingpoints of aluminum, magnesium and zinc are: 2333 K/933 K,1380 K/923 K and 1023 K/693 K [18]). In general, the strengthen-ing effect of zinc on Mg–Al–Zn alloys is both by solutionstrengthening and by increasing the solubility of aluminum inthemagnesium alloy [19,20]. So, the vaporization of zinc due toan over high heat input weakens the effect of solutionstrengthening by aluminum and zinc and this decreased theUTS of the welded joint. This result is in agreement with theresult reported by Quan et al., in which dissimilar magnesiumalloys (AZ31, AM60 and ZK60) were welded by a CO2 laser [21].

SEM images of typical tensile fracture surfaces of AZ61magnesium alloy (BM hot extruded) and the TIG welded joint(90 J mm−1) are shown in Fig. 7(a) and (b), respectively. Thefracture surface of the BM (hot extruded) mainly exhibited thefeatures of a ductile fracture, which was characterized by moretearing fibers and ridges (see Fig. 7 (a)). Cleavage surfaces(markedwith an arrowA in Fig. 7 (b)), secondary cracks (marked

Fig. 6 – The effect of heat input on the UTS of the TIG weldedjoints.

magnesium alloy and (b) TIG welded joint (90 J mm−1).

with an arrow B in Fig. 7 (b)) and secondary phase particles(marked with an arrow C in the Fig. 7 (b)), which were taken astypical brittle fracture features, could be seen on the fracturesurface of the welded joint. The features of the fracture of thewelded joint indicated that theTIGwelded jointunderwentbothductile deformation and brittle deformation during the tensiletest. It should be pointed out that the fracture of the TIGweldedjoint occurred in theHAZbecause this is theweakest zone in theTIG welded joint of the magnesium alloy. This is because thegrain coarsening in the HAZ due to the effect of the thermalcycling resulted in a decrease of the tensile strength of thewelded joint. In addition, the secondary phase particles (β-Mg17Al12) along the grain boundaries in the HAZ produced localstress concentrations [4]. Hence, the cracks may initiate at thebrittle β-Mg17Al12 phase along the grain boundaries.

Fig. 9 – The effect of heat input on the averagemicrohardnessof both the HAZ and FZ.

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3.2.2. MicrohardnessFig. 8 shows theVickersmicrohardness profilesmeasured alongthemid-thickness line of the cross-section of a TIGwelded jointof the AZ 61 magnesium alloy at heat input of 81 J mm−1. Therelationships among the microhardnesses of the BM, the HAZand the FZ of welded joints were as follows: FZ>BM>HAZ.During a process of TIGwelding in themagnesiumalloys, the FZconsisted of fine equiaxed grains due to the high cooling ratewhile grain coarseningoccurred in theHAZbecause of the effectof the thermal cycling. Therefore, the grain sizes of the threezoneswere FZ<BM<HAZ.According to theHall–Petchequation,the smaller the grain size is, the higher is the microhardness.Hence, the change of the microhardness should be inverselyproportional to the square root of the grain size.

The effects of heat input on themicrohardness of the FZ andtheHAZareshowninFig. 9. It canbeseenthat the lower theheatinput was, the higher was the microhardness of the FZ and theHAZ. Ingeneral, the increaseof thehardness canbeattributed tothe grain refinement and the strengthening effect of the brittleand hard β-Mg17Al12 phase [22]. During a welding process, graincoarsening occurred in both the HAZ and the FZ with anincrease of the heat input. Hence, a highermicrohardness valuewas achievedwith a lower heat input. In addition, it can be seenthat with a further increase of heat input, themicrohardness ofthe FZ and HAZ changed slightly. This is because with arelatively high heat input, more granular β-Mg17Al12 phase wasformed in theHAZand in FZ,which partially offsets the effect ofthe grain coarsening on the decrease of the microhardness ofthe HAZ and FZ of the welded joint.

4. Conclusions

In this study, the effect of heat input on the microstructureand mechanical properties of TIG butt-welded AZ61 magne-sium alloy plates was investigated by microstructural obser-vations, tensile tests and microhardness tests. The mainconclusions may be summarized as follows:

1) An increase of heat input resulted in an increase of thewidth of HAZ and the grain coarsening of α-Mg in both the

Fig. 8 –A typical microhardness profile across the TIGweldedjoint at a heat input of 81 J mm−1.

HAZ and FZ. Moreover, the continuous β-Mg17Al12 phasedeceased while the granular β-Mg17Al12 phase increased inboth the HAZ and the FZwith an increase of the heat input.

2) In general, the UTS of welded joints increased with anincrease of the heat input because too low a heat input led tothe presence of partial penetration and pores. However, toohigh a heat input decreased theUTS ofwelded joints slightlydue to the evaporation of the zinc from theAZ61magnesiumalloy. The tensile fracture of the welded joints usuallyoccurred in the HAZ and the fracture surfaces of the weldedjoints were characterized by brittle and ductile components.

3) The microhardness of the HAZ was lower than that of theBM and FZ due to the grain coarsening of α-Mg in the HAZ.With an increase of heat input, the microhardness of boththe HAZ and FZ decreased sharply at first and thendecreased slightly due to the formation of the granular β-Mg17Al12 phase when a relatively high heat input was used.

Acknowledgements

This research was financially supported by a Research Fundfor the Doctoral Program of Higher Education of China (ProjectNo. 20070611029) and Key Scientific and Technological Projectof Chongqing (Project No. CSTC, 2009AC4046).

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