Twinning in weld HAZ of ZK21 commercial magnesium alloy

LIU Jun-wei(刘俊伟), CHEN Ding(陈 鼎), CHEN Zhen-hua(陈振华)

College of Materials Science and Engineering, Hunan University, Changsha 410082, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The microstructure and properties of Mg ZK21 laser beam weld without filler were researched using optical microscopy (OM), electron microscopy and mechanical test. The results show that the fracture strain of the joints after laser beam welding reduces by about 10.7% at room temperature. By means of laser beam welding, the fusion zones contain tensile RS, while the base material far away from the fusion line is under balancing compressive RS. The microstructures of the weld were characterized by a narrow heat affected zone and twins. Significant {} tension twins occur in the weld HAZ during laser welding processing. Due to the influence of temperature field and stress on morphologies, most of twins form twinning bands, which are nearly parallel to the welding direction.

Key words: magnesium alloy; twinning; laser beam welding; microstructure

1 Introduction

As an extremely light metal, magnesium alloys have excellent specific strength, excellent sound damping capabilities, good castability, hot formability, excellent machinability and good electromagnetic interference shielding and recryclability[1-3]. In specific structural applications, the utilization of magnesium and its alloys increases. In order to further widen the application field of Mg alloys, joining processes, such as tungsten inert gas welding (TIG), laser beam welding (LBW), friction stir welding (FSW) and electron beam welding, were applied to weld magnesium alloys [4-6].

However, low welding speeds, large heat affected zone (HAZ) and high residual stress of TIG and FSW caused attention to be drawn towards laser welding due to the low and precise heat input, small HAZ, deep and narrow fusion zone (FZ), low residual stress and high welding speed[7]. COELHO et al[8] carefully investigated the microstructures of the laser beam welds of AZ31B magnesium alloy. The results showed that the growth of the crystallites in the direction of the heat flow caused a slight tilt of the c-axis of the crystallites out of the sheet plane and no grain coarsening was observed adjacent to the fusion line. POORHAYDARI et al[9] found two types of transformation twins in the weld HAZ of a low-carbon high-strength micro alloyed steel, and suggested that twinning could occur in the ferritic fine-grained HAZ when the peak temperature and cooling rate were low. By examining the micro- structures of carbon steels, the dissolution of precipitates and local enrichment of the matrix could not be the main reason for twinning. BARNETT[10] studied the relationship between the orientations of favorable {} and {} twinnings and the applied loading directions with respect to the c-axis of hcp-Mg cell   (c/a≈1.624). However, the relationship between the orientation of twins in the weld HAZ and the applied loading directions was not mentioned before.

From above analysis, the importance of twinning and orientation perhaps was the effect on the mechanical properties. Therefore, the relationship between microstructures and mechanical properties of the laser beam welded Mg alloy ZK21 were investigated. Furthermore, the type of twinning was also observed in this study by optical microscopy (OM) and transmission electronic microscopy (TEM).

2 Experimental

2.1 Material

The chemical composition of ZK21 Mg alloy used is listed in Table 1. ZK21 Mg alloy plates in original

Table 1 Chemical compositions of hot-rolled AZ31 alloy sheet (mass fraction, %)

1) Maximum value.

dimensions of d 40 mm×80 mm were heated at 673 K for 1 800 s, and then rolled with a reduction of 10%-20% per one pass. The rolls were heated to 573 K prior to repeated rolling. The heating and rolling were repeated for 10 passes, and finally the blocks were rolled to a thickness of 1.1 μm.

2.2 Laser beam welding

HAZ samples were obtained by applying laser beam welding on the horizontal plates. The welding parameters chosen were laser power 2.2 kW, welding speed      5.5 m/min, focal point 0 mm and argon shielding gas, no post-welding heat treatment was performed.

2.3 Tests equipment

Tension tests were carried out using a computer servo controlled Gleeble 1500 machine. Tensile specimens with gauge section of 20 mm×4 mm were extracted by spark erosion cutting from the base material and the welds in transversal direction (TD) of the specimens (see Fig.1). Microhardness measurements were performed across the weld cross-section. In order to examine certain features of the samples in more detail, microstructure characterization was carried out by MM-6 optical microscopy (OM) and H-800 transmission electron microscopy (TEM).

Fig.1 Shape of AZ31B magnesium alloy plates joined by laser beam welding

3 Results and discussion

3.1 Mechanical properties

Microhardness measurements on the cross-section of ZK21 laser beam welds reveal an average microhardness of about HV 136.3 in the ZK21 sheet base material (see Fig.2). The HAZ and the fusion zone show a tendency towards slightly higher microhardness values (about HV 140.4) compared to the base material. In fact, for as-extruded alloys, little variation in hardness is observed between base material, HAZ and FZ. Though significant grain coarsening occurs in the HAZ of wrought AZ31 alloy, the hardness in the HAZ is still almost the same as that in base metal. The similar hardness is attributed to the complete compensation for the loss in work hardening by grain refinement. By this reason, the microhardness determined in different distances to the weld top surface is not significantly different from each other.

Fig.2 Relationship between microhardness and distance from weld center line

Fig.3 shows the tensile properties of base material and ZK21 welded samples. From Fig.3, it is obvious that the tensile strength of the welded sample is smaller than that of the base material. The differences of the mechanical properties between the base material and the welded joint can be attributed to many reasons, such as precipitation hardening, grain size, twinning hardening and notch. The precipitates in the fusion zone are significantly coarser than that in the base material, therefore, a significant effect of precipitation hardening on the mechanical strength of the fusion zone is not expected. In the outer region of the fusion zone, the grain size is significantly smaller than that in the base material and in the centre of the fusion zone. According to Holl-Petch formula, the fine grain size in the fusion zone contributes to its strength[11]. During tension processing, twinning also can influence the deformation properties in a complicated way. Firstly, the twin boundaries formed act as barriers to dislocation motion, grain boundaries lead to an increase of the work hardening rate. In addition, the glissile dislocations transform into sessile dislocations within the twin interiors and hence contribute to strengthen the Basinski mechanism. Secondly, they accommodate strain along the c-axis, which gives rise to a decrease in the work hardening rate. Furthermore, the lattice rotation introduced by twinning can enhance or reduce work hardening depending on the type of twins formed[12]. Magnesium alloys usually have high notch sensitivity. The mechanical properties of welds are greatly influenced by geometric notches in weld seams. These notches are usually caused by root drop through metal burn-up. Thus, an important criterion for reliable joints is to avoid notches in weld joints. The strength of welded sample at RT is affected by complicated way, which is smaller than that of base material. So magnesium alloys are notch-sensitive and this greatly affects fatigue properties. HAFERKAMP et al[13] studied carefully the fatigue test data for laser welded magnesium joints. The results show that, due to lower notch sensitivity for the laser-welded specimens, the specimens can tolerate higher load amplitudes for a given number of cycles to failure compared to electron beam welded joints. From Fig.3, the fracture strain of the joints reduces by about 10.7% compared to that of the base material, and the fracture occurs in the weld metal.

Fig.3 Stress—strain curves at different temperatures and strain rates

3.2 Microstructure

The microstructure of base material is shown in Fig.4(a). The α-Mg grains are equiaxed, the average grain size is about 32.4 μm. After welding, in the HAZ of the material near the interface to the fusion zone, the grain size of the α-Mg is barely affected by the heat input during welding (see Fig.4(b)). The width of HAZ is about 180 μm. The results show that the significant twins occur in the weld HAZ during laser welding processing. Most of twins form a serial of cross-grain bands, which are nearly parallel to welding direction (Fig.4(b), region A). This can be credited to the influences of temperature field and residual stress. During the process of heating and rapid cooling, steep temperature gradients occur due to preferential heat flowing into the base material and the atmosphere. The inhomogeneous plastic deformation associated with shrinkage during cooling is the result during the formation of residual stresses. These welding residual stresses are higher for the highly stiff or constrained joint with lower distortion. So, in HAZ, if the critical resolved shear stress (CRSS) of twinning is surpassed, the significant twins can be activated during welding process. In fact, the residual stress in HAZ plays a rather completed character in the process of welding. Different temperature field and residual stress lead to different deformation mechanisms, in which the CRSS plays the crucial effect. In HAZ, the temperature is higher than 200 K. The relationship between CRSS of slip and twinning fulfill is as follows.

CRSS_{basal, slip}＜CRSS_{non-basal, slip}＜CRSS_twinning

A recent review of relevant literature reveals that the basal slip has the lowest CRSS ranging from 0.45-0.81MPa, the CRSS of twinning is 2-4 times larger, and the prismatic slip has an even 48-87 times larger CRSS compared to basal slip[14]. Therefore, significant

Fig.4 SEM images of samples: (a) Base material; (b), (c) ZK21 LBW welds

amount of basal slips are activated in the initial deformation stage. With the stress concentration, the CRSS of non-basal slip and this type of slip is activated. The probability of occurring twinning increases gradually with the increase of dislocation pile-up near grain boundaries. Then, twins begin to nucleate near the grain boundaries and further transverse to the grains. Because of the intersection of stress concentration near the grain boundaries between nearby grains, the nucleated twins form a twinning band (see Fig.4(c)).

Because the similar level of residual stress in the location parallels to welding direction in HAZ, the direction of twinning bands nearly parallels to the welding direction. Moreover, because the restrained thermal contraction of the weld pool during cooling in welding direction parallels to the fusion line, the fusion zone contains tensile RS while the base material far away from the fusion zone is under balancing compressive RS. BARNETT et al^[15] mentioned that compression perpendicular to the hcp c-axis favors the formation of twins on the {} plane and the flow stress remains low. While, the compression parallel to the hcp c-axis favors the formation of twins on the {} plane. The relationship between the favorable {} twinning and applied loading directions with respect to the c-axis of an hcp cell is shown in Fig.5. Because the HAZ is far away from the fusion zone and a strong basal plane texture of hot-rolled samples, balancing compressive residual stress in HAZ can induce the occurrence of {} twinning [15-16].

Fig.5 Schematic diagram of applied loading directions and types of twinning (Black arrows indicate applied loading directions of favorable {} tensile twinning; white arrows indicate applied loading directions of favorable {} contraction twinning)

Fig.6 shows TEM image of deformation twins. It can be seen from Fig.6 that the twins occur in the parallel bundles, and the average size in width is about 450 nm. The diffraction pattern of selected area of the twinning in

Fig.6 TEM image of HAZ and diffraction pattern of twinning

Fig.6(b) shows that the twin system is ()/<>. The twinning interface is determined to be ().

4 Conclusions

1) In transverse direction, lower yield strength of the welded joint than that of the base material is observed. This can be explained by the twinning occurrence, grain size evolution and notch. Moreover, the fracture strain of the joints after laser beam welding reduces about 10.7% at room temperature.

2) The residual stress distribution observed is typical for butt joints and thermally induced welding residual stresses. The fusion zones contain tensile RS while the base material far away from the fusion line is under balancing compressive RS.

3) Because of the influences of temperature field and residual stresses, many twinning bands nearly parallel to welding direction form. The type of these twins can be confirmed to be {} tension twins.

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(Edited by LI Yan-hong)

Corresponding author: CHEN Ding; Tel: +86-731-8820648; E-mail: liujw1981@yahoo.com.cn