Effect of heat input on the microstructure and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates

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1 MATERIALS CHARACTERIZATION 60 (2009) available at Effect of heat input on the microstructure and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates Dong Min, Jun Shen, Shiqiang Lai, Jie Chen College of Material Science and Engineering, Chongqing University, Chongqing , People's Republic of China ARTICLE DATA Article history: Received 20 June 2009 Received in revised form 11 September 2009 Accepted 16 September 2009 Keywords: AZ61 magnesium alloy Tungsten inert gas arc welding Microstructure Mechanical properties Heat input ABSTRACT In this paper, the effects of heat input on the microstructures and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that with an increase of the heat input, the grains both in the fusion zone and the heat-affected zone coarsen and the width of the heat-affected zone increased. Moreover, an increase of the heat input resulted in a decrease of the continuous β-mg 17 Al 12 phase and an increase of the granular β-mg 17 Al 12 phase in both the fusion zone and the heat-affected zone. The ultimate tensile strength of the welded joint increased with an increase of the heat input, while, too high a heat input resulted in a decrease of the ultimate tensile strength of the welded joint. In addition, the average microhardness of the heat-affected zone and fusion zone decreased sharply with an increase of the heat input and then decreased slowly at a relatively high heat input Elsevier Inc. All rights reserved. 1. Introduction Magnesium and its alloys have the prospect of wide application in the automotive, aircraft and electronic consumer industries because of their low density in combination with a high strength, an excellent castability, a perfect electromagnetic interference shielding property, a high thermal conductivity and a high damping capability [1,2]. However, the production of complicated pieces from magnesium alloys is usually difficult and expensive because of their poor ductility and cold processability at room temperature. This is because magnesium has a hexagonal closepacked (HCP) crystal structure, which has insufficient slip systems at room temperature [3]. Therefore, welding technology of magnesium alloys plays an important role in the board application of magnesium alloy structural parts. Up to now, several welding methods, such as laser beam welding (LBW), tungsten inert gas (TIG) arc welding, electron beam welding (EBW) and friction stir welding (FSW) have been applied to the welding of magnesium alloys [3 7]. Compared with other welding methods, TIG welding technology is the main welding method adopted for magnesium alloys because of its advantages of utility and economy [4,5]. Liu and Dong found that the difference of the grain size in the heat-affected zone (HAZ) between a TIG filler welded joint and a TIG welded joint without a filler resulted in a variety of the fracture location and the ultimate tensile strength (UTS) value of welded joints [8]. Zhu et al. found that a high rate of melting of α+β eutectic phases and a low rate of dissolution of the β-mg 17 Al 12 phase led to the formation of a partially melted zone (PMZ) [9,10]. Also, the formation mechanisms of pores [11] and liquation cracking [12] in the PMZ were investigated by Baseslack Corresponding author. Tel.: ; fax: address: shenjun2626@163.com (J. Shen) /$ see front matter 2009 Elsevier Inc. All rights reserved. doi: /j.matchar

2 1584 MATERIALS CHARACTERIZATION 60 (2009) Table 1 The TIG welding parameters in these experiments. Samples Current (A) Welding speed v (mm s 1 ) Heat input E (J mm 1 ) a b c d e et al. and Shen et al. Liu et al. found that the welded seam mainly consisted of dendritic crystals while the parts near the welded seam were composed of columnar crystals through a study of the microstructure of Mg/Al dissimilar materials TIG welded joints [13]. The above-mentioned investigations mainly focused on microstructural characteristics and mechanisms of the formation of defects in welded seams of TIG welded magnesium alloys. However, the influence of the welding parameters (in particular, heatinput)onthemicrostructure(suchasthegrainsizeandthe morphology of the eutectic phase) and the mechanical properties of TIG welded magnesium alloys is a valuable problem for an indepth study. Hence, in this paper, the effects of heat input on the microstructure (grain size and morphology of the β-mg 17 Al 12 phase) in both the HAZ and the fusion zone (FZ) of welded seams of TIG filler welded AZ61magnesium alloy and the mechanical properties (microhardness and UTS) were investigated. In addition, the morphologies of the tensile fractured surfaces and the fractured mechanisms of the welded joints of TIG filler welded AZ61magnesium alloy plates are discussed. 2. Experimental Procedures Hot-extruded AZ61magnesium alloy plates (provided by the Chongqing Magnesium Company, China) with a size of 30 mm 120 mm 3 mm were used for TIG filler welding tests. Prior to welding, in order to avoid the influence of impurities (such as surface contaminates and the oxide film) on the surface of the magnesium alloy plates and welding wires on the results of the welding tests, the surface of the samples were polished lightly with diamond powders and then cleaned with a solution (99 vol.% C 2 H 5 OH+1 vol.% HCl). The samples were butt-welded on top of a copper-backing strip containing a semi-circular groove with a dimension of 6 mm in width and 1.5 mm in depth. An AC penetrating square-wave welding procedure was adopted for the welding tests. The detailed welded parameters in these experiments are shown in Table 1. Here,thefluxof shielding gas (argon), welding voltage and welding speed are constant (the flux of shielding gas was10 l/min, welding voltage was 10 V and welding speed was 10 mm s 1 ). The variation of Fig. 1 A typical microstructural image of a TIG welded joint of AZ61 magnesium alloy at a heat input of 90 J mm 1, (a) whole welded joint, (b) BM of hot-extruded AZ61 magnesium alloy, (c) FZ and (d) HAZ.

3 MATERIALS CHARACTERIZATION 60 (2009) the heat input was achieved by adjusting the welding current. The relationship between the parameters is as follows: L = η UI v where L is the heat input during the TIG welding process, U is the welding voltage, v is the welding speed, I is the welding current and η is the efficiency of TIG welding (η=0.90 [14]). After the welding tests, the samples were sectioned and the cross-sections of the welded seams were prepared using standard metallographic procedures (grinding, polishing and etching with a solution of 4 vol.% HNO vol.% C 2 H 5 OH for ð1þ 20 s 40 s). In addition, tensile test specimens with a gauge length of 15 mm and a width of 4 mm were sectioned from the welded seam parts by a numerically controlled linear cutting machine. The tensile tests were carried out with a tensile test machine at room temperature and the tensile direction was perpendicular to the weld seams. Three tensile test results were collected in samples with the same heat input and the average values of them were adopted for discussion. The microhardness tests were performed with a Vickers hardness tester (V-1000) with a period of 20 s on, a load of 1000 g and a step size of 0.5 mm. The values of the microhardness of the FZ and HAZ were made from an average value of five data points. An optical microscope Fig. 2 Microstructural images of TIG welded joints with different heat inputs, (a) 63 J mm 1,(b)72Jmm 1,(c)77.4Jmm 1, (d) 81 J mm 1 and (e) 90 J mm 1.

4 1586 MATERIALS CHARACTERIZATION 60 (2009) (MDJ200) and a scanning electron microscope (TESCAN, Inc. VegaIILMU SEM) were used for microstructural observations. Energy dispersive X-ray spectroscopy (OXFORD, Inc. ISIS300) was used to determine the phases formed in the welded seam. 3. Results and Discussion 3.1. The Typical Microstructure of TIG Welded AZ61 Magnesium Alloy Plates Fig. 1 shows a typical microstructure of a TIG welded joint with a heat input of 90 J mm 1. It was found that the welded joint was composed of a FZ, a wide HAZ and a base material (BM) area (see Fig. 1 (a)). The BM is characterized by a fine and uniform equiaxed structure (the average grain size is about 39 μm), as shown in Fig. 1 (b). The presence of this structure is due to the recrystallization of grains during hot rolling. The FZ mainly consisted of the α-mg phase with a surrounding eutectic β- Mg 17 Al 12 phase (see Fig. 1 (c)). Compared with the grains both in the FZ and BM, the grains had coarsened markedly in the HAZ (see Fig. 1 (d)). This is because during the welding process, the temperature in the HAZ may be up to 527 K [15], whichismore than the recrystallization temperature of the AZ series magnesium alloys (about 478 K [16]). Therefore, recrystallization and grain coarsening may easily occur in the HAZ Effects of Heat Input on the Microstructures of HAZ Fig. 2 illustrates the evolution of microstructure in the HAZ with an increase of the heat input. In agreement with the literature [3], the results of EDX analysis indicated that the HAZ mainly consisted of a white α-mg phase (marked with an arrow A in Fig. 2 (a)) and a black β-mg 17 Al 12 phase (marked with arrow B in Fig. 2 (a)). The average grain size of α-mg in the HAZ and the width of HAZ were quantitatively analyzed by an image analyzer (UTHSCSA Image Tool 3.0) and the results are shown in Fig. 3. It can be seen that the width of the HAZ and the grain size of α-mg in the HAZ increased with an increase of the heat input. When the heat input was low (63 J mm 1 ), the average grain size of α-mg (about 34.9 μm) in the HAZ was close to that of the BM. This indicated that when the heat input was relatively low (63 J mm 1 ), although recrystallization occurred in the HAZ, the coarsening of grains in the HAZ was not obvious. When the heat input increased to the maximum value (90 J mm 1 ), the grains in the HAZ grew up to an average grain size of 72.9 μm, which was about double the size of that in the BM. The increase of heat input led to a coarsening of the grains in the HAZ because the increase of heat input provided more driving force for grain boundary migration which then speeded the growth of grains. Fig. 3 also shows that with an increase of heat input, the width of the HAZ also increased due to the extra heat input to the BM. Moreover, it is worth noticing that when the heat input was 63 J mm 1, a continuous β-mg 17 Al 12 phase formed in the HAZ (see Fig. 2 (a)). However, with an increase of heat input, the amount of the continuous β-mg 17 Al 12 phase decreased while the amount of the granular β-mg 17 Al 12 phase increased. When the heat input was 90 J mm 1, only a small area of the continuous β-mg 17 Al 12 phase can be observed in the HAZ (see Fig. 2 (e)). This is because the increase of temperature led to more of the continuous β- Mg 17 Al 12 phase dissolving into the α-mg matrix and the Fig. 3 The effect of heat input on the width of the HAZ and the average grain size of α-mg in the HAZ. precipitation of the solute Al in the interior of α-mg matrix to form granular β-mg 17 Al 12 [17] Effects of Heat Input on the Microstructures of FZ The effect of heat input on the microstructures of the FZ is shown in Fig. 4. It can be seen that α-mg in the FZ is characterized by fine equiaxed dendrites with a surrounding β-mg 17 Al 12 phase. The results for the quantitative analysis of the effect of heat input on the average grain size of α-mg are shown 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 about 17.5 μm, which is smaller than that in the BM. The formation of a fine grain structure in the FZ is due to the relatively high cooling rate. When the heat input was 90 J mm 1, the average grain 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 heat input. By careful observation (Fig. 4 (a) (e)), it can be seen that more continuous β-mg 17 Al 12 phase formed in the FZ at the lowest heat input (63 J mm 1 ). However, with the increase of heat input, the amount of continuous β-mg 17 Al 12 phase decreased while the amount of discontinuous/granular β- Mg 17 Al 12 phase increased. This is because a high cooling rate due to a low heat input in the FZ restrained the growth of the α-mg grain Effect of Heat Input on Mechanical Properties of Welded Joints Tensile Strength The effect of heat input on the UTS of the TIG welded joint is depicted in Fig. 6. In this study, at the lowest heat input (63 J mm 1 ), a partial penetration and pores were observed in the welded seam. During the welding process, the shielding gas usually protected the surface of the molten pool, while the back of the sample failed to be protected. Hence, air may intrude into the molten pool easily through the gap between two plates and result in the formation of pores in the welded seam (see Fig. 6). In this condition, the UTS of the welded seam is 135 MPa, which

5 MATERIALS CHARACTERIZATION 60 (2009) Fig. 4 SEM images of the microstructure in the FZ with different heat inputs, (a) 63 J mm 1,(b)72Jmm 1,(c)77.4Jmm 1,(d)81Jmm 1 and (e) 90 J mm 1. only accounted for 47% of that of BM (285 MPa). This result illuminates that during the TIG welding of magnesium alloy AZ61, too low a heat input easily resulted in the presence of welding defects, which seriously decreased the tensile strength of the welded joint. With an increase of the heat input, the UTS of the welded joints increased. The highest UTS of a welded joint, with 90% of that of the BM, was obtained at a heat input of 81 J mm 1 due to less welding defects (such as partial penetration and pores) in the welded seam (see Fig. 6). However, when the heat input was increased to 90 J mm 1, the UTS of the welded joint decreased slightly due to the different volatility of elements in the AZ61 magnesium alloy, such as magnesium and

6 1588 MATERIALS CHARACTERIZATION 60 (2009) Fig. 5 The effect of heat input on the average grain size of α- Mg in FZ. zinc. During a welding process, magnesium and zinc evaporated easily at a high temperature due to the lower melting points/ boiling points and the higher vapor pressures of these elements compared with that of aluminum (the melting points/boiling points of aluminum, magnesium and zinc are: 2333 K/933 K, 1380 K/923 K and 1023 K/693 K [18]). In general, the strengthening effect of zinc on Mg Al Zn alloys is both by solution strengthening and by increasing the solubility of aluminum in the magnesium alloy [19,20]. So, the vaporization of zinc due to an over high heat input weakens the effect of solution strengthening by aluminum and zinc and this decreased the UTS of the welded joint. This result is in agreement with the result reported by Quan et al., in which dissimilar magnesium alloys (AZ31, AM60 and ZK60) were welded by a CO 2 laser [21]. SEM images of typical tensile fracture surfaces of AZ61 magnesium alloy (BM hot extruded) and the TIG welded joint (90 J mm 1 )areshowninfig. 7(a) and (b), respectively. The fracture surface of the BM (hot extruded) mainly exhibited the features of a ductile fracture, which was characterized by more tearing fibers and ridges (see Fig. 7 (a)). Cleavage surfaces (marked with an arrow A in Fig. 7 (b)), secondary cracks (marked Fig. 7 SEM images show typical tensile fracture surfaces of the magnesium alloy, (a) BM of hot-extruded AZ61 magnesium alloy and (b) TIG welded joint (90 J mm 1 ). Fig. 6 The effect of heat input on the UTS of the TIG welded joints. 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 as typical brittle fracture features, could be seen on the fracture surface of the welded joint. The features of the fracture of the welded joint indicated that the TIG welded joint underwent both ductile deformation and brittle deformation during the tensile test. It should be pointed out that the fracture of the TIG welded joint occurred in the HAZ because this is the weakest zone in the TIG welded joint of the magnesium alloy. This is because the grain coarsening in the HAZ due to the effect of the thermal cycling resulted in a decrease of the tensile strength of the welded joint. In addition, the secondary phase particles (β- Mg 17 Al 12 ) along the grain boundaries in the HAZ produced local stress concentrations [4]. Hence, the cracks may initiate at the brittle β-mg 17 Al 12 phase along the grain boundaries.

7 MATERIALS CHARACTERIZATION 60 (2009) Microhardness Fig. 8 shows the Vickers microhardness profiles measured along the mid-thickness line of the cross-section of a TIG welded joint of the AZ 61 magnesium alloy at heat input of 81 J mm 1.The relationships among the microhardnesses of the BM, the HAZ and the FZ of welded joints were as follows: FZ>BM>HAZ. During a process of TIG welding in the magnesium alloys, the FZ consisted of fine equiaxed grains due to the high cooling rate while grain coarsening occurred in the HAZ because of the effect of the thermal cycling. Therefore, the grain sizes of the three zones were FZ<BM<HAZ. According to the Hall Petch equation, the smaller the grain size is, the higher is the microhardness. Hence, the change of the microhardness should be inversely proportional to the square root of the grain size. The effects of heat input on the microhardness of the FZ and the HAZ are shown in Fig. 9. It can be seen that the lower the heat input was, the higher was the microhardness of the FZ and the HAZ. In general, the increase of the hardness can be attributed to the grain refinement and the strengthening effect of the brittle and hard β-mg 17 Al 12 phase [22]. During a welding process, grain coarsening occurred in both the HAZ and the FZ with an increase of the heat input. Hence, a higher microhardness value was achieved with a lower heat input. In addition, it can be seen that with a further increase of heat input, the microhardness of the FZ and HAZ changed slightly. This is because with a relatively high heat input, more granular β-mg 17 Al 12 phase was formed in the HAZ and in FZ, which partially offsets the effect of the grain coarsening on the decrease of the microhardness of the HAZ and FZ of the welded joint. 4. Conclusions In this study, the effect of heat input on the microstructure and mechanical properties of TIG butt-welded AZ61 magnesium alloy plates was investigated by microstructural observations, tensile tests and microhardness tests. The main conclusions may be summarized as follows: 1) An increase of heat input resulted in an increase of the width of HAZ and the grain coarsening of α-mg in both the Fig. 9 The effect of heat input on the average microhardness of both the HAZ and FZ. HAZ and FZ. Moreover, the continuous β-mg 17 Al 12 phase deceased while the granular β-mg 17 Al 12 phase increased in both the HAZ and the FZ with an increase of the heat input. 2) In general, the UTS of welded joints increased with an increase of the heat input because too low a heat input led to the presence of partial penetration and pores. However, too high a heat input decreased the UTS of welded joints slightly due to the evaporation of the zinc from the AZ61 magnesium alloy. The tensile fracture of the welded joints usually occurred in the HAZ and the fracture surfaces of the welded joints were characterized by brittle and ductile components. 3) The microhardness of the HAZ was lower than that of the BM and FZ due to the grain coarsening of α-mg in the HAZ. With an increase of heat input, the microhardness of both the HAZ and FZ decreased sharply at first and then decreased slightly due to the formation of the granular β- Mg 17 Al 12 phase when a relatively high heat input was used. Acknowledgements This research was financially supported by a Research Fund for the Doctoral Program of Higher Education of China (Project No ) and Key Scientific and Technological Project of Chongqing (Project No. CSTC, 2009AC4046). REFERENCES Fig. 8 A typical microhardness profile across the TIG welded joint at a heat input of 81 J mm 1. [1] Westengen H. Magnesium die-casting: from ingots to automotive parts. Light Metal Age 2000;58: [2] Mordike BL, Ebert T. Magnesium: properties applications potential. Mater Sci Eng A 2000;302: [3] Coelho RS, Kostka A, Pinto H, Riekehr S, Kocak M, Pyzalla AR. Microstructure and mechanical properties of magnesium alloy AZ31B laser beam welds. Mater Sci Eng A 2008;485: [4] Munitz A, Cotler C, Stern A, Kohn G. Mechanical properties and microstructure of gas tungsten arc welded magnesium AZ91D plates. Mater Sci Eng A 2001;302: [5] Cao X, Jahazi M, Immarigeon JP, Wallace W. A review of laser welding techniques for magnesium alloys. J Mater Process Technol 2006;171:

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