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Vol. 8. Issue 3.
Pages 3044-3053 (May - June 2019)
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Vol. 8. Issue 3.
Pages 3044-3053 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.024
Open Access
Laser welding of AZ31B magnesium alloy with beam oscillation
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363
Kangda Haoa, Hekang Wangb, Ming Gaob,
Corresponding author
mgao@mail.hust.edu.cn

Corresponding author.
, Run Wua, Xiaoyan Zengb
a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
b Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology, Wuhan 430074, China
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Tables (2)
Table 1. Chemical compositions of base metal (wt. %).
Table 2. Welding parameters, where, r is the oscillating radius, f is the oscillating frequency, the laser power is 2kW, the welding speed is 2m/min.
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Abstract

Circular beam oscillation was introduced to laser welding of 2mm-thick AZ31 magnesium alloy. The appearance, microstructure and tensile properties of the welds were investigated. It was found that the low frequency about 50Hz and the small radius about 0.5mm are more beneficial to improve the weld appearance, under the given parameters of laser power 2kW, welding speed 2m/min and beam oscillating diameter 0.35mm. The undercut defect and even the totally collapse of the weld appear when employing the frequency higher than 75Hz or the radius larger than 1.5mm. The microstructure evolution mainly reflects in the variation of the proportions of the equiaxed zone (PEZ) and the average grain size (SG). The PEZ decreases from 85% to 42% with the beam oscillating frequency increasing from 25Hz to 100Hz, while the SG reaches the maximum of 37.5μm at 75Hz. The PEZ decreases and the SG increases with the increase of beam oscillating radius. The results showed that the tensile strength and elongation are closely related to the PEZ, the SG and the twins. According to the experimental results, the relationship of oscillating parameters, microstructure, and tensile properties was established.

Keywords:
Magnesium alloy
Laser oscillating welding
Microstructure
Tensile properties
Full Text
1Introduction

As the lightest structural metal, magnesium (Mg) alloy has advantages of high specific strength, specific stiffness and shock absorption, which is beneficial to achieve weight reduction and energy conservation for the fields of electronics, automobiles and aerospace [1]. However, its weld-ability is poor, defects like coarse-grain brittleness, pores and evaporation loss of the elements are generally occurred, and the solidification crack is easily induced by eutectic with low melting point in Mg weld, because of the high thermal conductivity, low solid solubility of hydrogen, easy oxidation and evaporation of magnesium and zinc. How to achieve high-quality weld of Mg alloy is then one of the most concerned issues.

Due to the high energy density and low heat input, laser welding would be potential for the welding of Mg alloy. During laser welding of 2mm-thick AZ31 Mg alloy, the weld with narrow heat-affected zone (HAZ) could be obtained under laser power of 2.2kW and welding speed of 5.5m/min, the coarse grain neighboring the fusion line was eliminated, and the hardness distribution was more homogeneous across the HAZ and the fusion zone [2]. The strength of the laser weld of Mg alloy could approach 90% of the base metal [3]. However, critical clamping accuracy is indispensable for laser welding because of the small spot of laser beam. In laser welding of die-casted AZ91D Mg alloy, the weld strength decreased by 10% once the gap width reached 5% of the sheet thickness [4]. Besides, the pores pre-existed in the base metal were easily gathered and grew up, because the liquid magnesium solidified quickly owing to the fast cooling rates of welding. The larger the heat input, the higher the porosity within the Mg weld [5].

Relatively, laser-arc hybrid welding can obtain better gap tolerance and weld quality by filling the welding wire. In laser-TIG hybrid welding of 1.7mm-thick AZ31 Mg alloy, it was found that the hybrid welding not only had advantages of low heat input and narrow HAZ, it could also stabilize the arc and improve the weld performance [6]. In laser-MIG hybrid welding of 5mm-thick AZ31 Mg alloy, it was found that the arc was compressed and stabilized by the laser beam, which promoted the droplet transferring from the wire tip to the weld pool smoothly, and the occurrence of the droplet explosion caused by low boiling point and high steam pressure of Mg was prevented to a certain extent [7]. However, the laser-TIG hybrid welding has problem of electrode burning, while the droplet explosion during laser-MIG hybrid welding hasn’t been solved fundamentally for the Mg alloy yet.

Thus, supplementary method was indispensable to achieve high-quality weld of the Mg alloy. By moving the welding head mechanically to achieve oscillating of the laser beam, it was found that the N2 porosity could be eliminated because of decreased aspect ratio, while the Ar porosity was also reduced due to the stirring effect of the oscillated laser, indicating the potential of beam oscillation in improving the weld quality of Mg alloy [8]. In arc oscillating welding of Mg alloy, it was confirmed that the oscillating behavior could promote the re-melting of the weld metal. On one hand, the re-melting preferentially occurred at the root of the secondary dendrite, which inhibited the growth of columnar grains and promoted the broken of dendrite. On the other hand, the temperature gradient was reduced and the composition super-cooling was easily obtained by the re-melting, which promoted the columnar grains transforming to equiaxed grains [9], indicating the oscillating behavior would be beneficial to improve the weld microstructure of the Mg alloy.

With the rapid development of high frequency galvanometer scanner recently, laser oscillating welding is considered one of the most promising welding technologies because of more precise modulation of oscillating parameters, which is significant to achieve sound weld of Mg alloy, and promote its industrial applications [10]. By laser oscillating welding of 6k21 aluminum alloy, the shear-tensile strength increased by 29% comparing with the conventional laser weld [11]. It was also demonstrated the spatter number was reduced either increasing the welding speed or decreasing the laser power [12]. By laser oscillating welding of AISI 304 austenite stainless steel, it was confirmed that the oscillating behavior could affect the weld appearance. With the increasing of oscillating frequency, the weld cross-section changed from slender nail to dumpy nail, V-shape and U-shape in sequence [13]. By laser oscillating welding of 6061 aluminum alloy, it was found that more equiaxed grains were obtained and the strain increased by 38% comparing with conventional laser weld [14].

The researches mentioned above indicate that laser oscillating welding is potential to improve the weld quality of Mg alloys due to the precise energy modulation. However, no attention has been addressed on laser oscillating welding of Mg alloys. Then, current study aims to reveal the effects of beam oscillating parameters on the characterization of Mg laser weld. The new phenomena in weld formation were studied, the difference of which with aluminum alloy and steel were discussed. Especially, the relationship of oscillating parameters, microstructure, and tensile properties was established, and the related mechanisms of microstructure evolutions were discussed, which is beneficial to deepen the understanding of laser oscillating welding of Mg alloys.

2Experiment

The base metal used was 2mm-thick AZ31 Mg alloy with chemical compositions shown in Table 1. The sheet size was 100mm in length and 50mm in width. Before welding, the sheets were steel brushed to remove the surface oxidation layer, cleaned by acetone and assembled in butt configuration.

Table 1.

Chemical compositions of base metal (wt. %).

Element  Al  Zn  Mn  Si  Fe  Cu  Ni  Mg 
AZ31  2.5–3.5  0.5–1.5  0.2–0.6  ≤0.1  ≤0.005  ≤0.05  ≤0.005  Bal. 

The laser oscillating welding system was composed of a Fanuc M-710 six-axis robot, an IPG YLS-6000 fiber laser and a SCANLAB hurrySCAN30 scanning system, as shown in Fig. 1. The maximum power of the laser source was 6kW with beam quality parameter of 6.9mm·mrad. The laser beam with wavelength of 1070nm was firstly transmitted by a fiber to a collimator, reflected by a copper mirror, and finally focused on the sheet surface with spot diamater about 0.35mm. The focal lengths of the collimator and the focus lens were 150mm and 350mm, respectively. During welding, the molten pool was observed using a Phantom V2012 high-speed camera with a Cavilux Hf light source. Besides, the top and root surfaces of the weld were shielded by Ar with flow rate of 20L/min and 8L/min, respectively.

Fig. 1.

Experimental set up for laser oscillating welding.

(0.38MB).

In this study, circular beam oscillation was employed. The welding parameters used were listed in Table 2. After welding, the welds were cut down to prepare the metallurgical, electron back-scattered diffraction (EBSD) and tensile samples. The metallurgical samples were grinded and polished, and etched by a solution of 3g picric acid, 20ml acetic acid, 20ml distilled water and 50ml alcohol. The microstructure was observed by optical microscope (OM) and FEI Quanta-200 environmental scanning electron microscope (SEM). The EBSD samples were electrolytically polished with the voltage of 15V for 120s by the solution containing 10ml perchloric acid and 90ml ethanol, and tested by EDAX-TSL OIM system equipped on FEI Sirion-200 field emission SEM. Besides, the tensile tests were carried out with the sample size as shown in Fig. 2. The tensile results were the average of three samples.

Table 2.

Welding parameters, where, r is the oscillating radius, f is the oscillating frequency, the laser power is 2kW, the welding speed is 2m/min.

Weld no.  #0  #1  #2  #3  #4  #5  #6  #7  #8  #9  #10 
r (mm)  –  0.5  1.0  1.5  2.0  2.5  1.0  1.0  1.0  1.0  1.0 
f (Hz)  –  50  50  50  50  50  10  25  75  100  150 
Fig. 2.

Schematic drawing of the tensile sample.

(0.15MB).
3Results and discussions3.1Weld appearance

As shown in Fig. 3, the undercut defect is general existed for the weld of Mg alloy whether employing beam oscillation or not, because the weight of the molten pool is hard to be held up due to the low surface tension coefficient of the liquid Mg. By modulating the longitudinal temperature gradient and the intensities of the eddying and convection effects within the molten pool via beam oscillation, the weld appearance can be improved [15], which will be subsequently discussed in detail according to the dynamic behavior of the molten pool.

Fig. 3.

Macro-profiles of the surface and cross-sectional welds, (a) without beam oscillation, (b) r=1.0mm and f=10Hz, (c) r=1.0mm and f=25Hz, (d) r=1.0mm and f=50Hz, (e) r=1.0mm and f=75Hz, (f) r=1.0mm and f=100Hz, (g) r=1.0mm and f=150Hz, (h) r=0.5mm and f=50Hz, (i) with r=1.5mm and f=50Hz, (j) r=2.0mm and f=50Hz, (k) r=2.5mm and f=50Hz.

(1.49MB).

Moreover, it can be found that the weld surface changes from wavy shape to continuous line with the frequency increasing to just 50Hz under the radius of 1.0mm. The undercut forms at 75Hz, and the collapse occurs at 150Hz. In laser oscillating welding of austenitic stainless steel, the frequency threshold achieving continuous weld surface was high up to 500Hz by vertical oscillating with amplitude of 1.0mm, and the undercut defect was negligible [13]. In laser oscillating welding of 6061 aluminum alloy, the weld appearance was also satisfied at frequency of 200Hz under radius of 1.0mm [14]. For the studied Mg alloy, the continuous weld surface is achieved at lower frequency, and complete collapse even occurs, because of lower boiling temperature, higher vapor pressure and heat sensitivity of the Mg alloy comparing with the above materials.

3.2Characteristics of molten pool

As shown in Fig. 4a and b, shrinkage and expanding of the keyhole appears during one cycle, indicating its fluctuation to the energy absorption. For the molten pool without beam oscillation, the melted metal flows from near the keyhole to the edge of the molten pool. The beam oscillation with radius of 1.0mm and frequency of 50Hz promotes the melted metal flowing along the oscillating direction, and is beneficial to expand the keyhole to 1.5mm2, 5 times larger than that without beam oscillation, which increases the eruption of the metallic vapor and reduces the instability of the molten pool.

Fig. 4.

Characteristics of the molten pools observed by high-speed camera, (a) without beam oscillation, (b) r=1.0mm and f=50Hz, (c) r=1.0mm and f=100Hz, (d) r=2.0mm and f=50Hz.

(1MB).

As shown in Fig. 4c, the heat is concentrated on the right side of the weld with the frequency increasing from 50 to 100Hz under radius of 1.0mm, because of the inflection point for the circular scanning path, and the enhanced stirring intensity of the oscillating laser to the molten pool makes the melt flow at the left side moves reversely. As shown in Fig. 4d, the molten pool is significantly enlarged with the oscillating radius increasing from 1.0 to 2.0mm at frequency of 50Hz. At time of 203.571ms, partly melted metal flows to the left side, causing excessive element burning and the formation of undercut.

3.3Microstructure characteristics

As shown in Fig. 5, the columnar zone neighboring the fusion line is continuous for the sample without beam oscillation, while that with beam oscillation is discontinuous. As shown in Fig. 6, the length of the columnar grain achieves 20μm under radius of 0.5mm and frequency of 50Hz, about 33% of the sample without beam oscillation. It then keeps about 35μm with the radius increasing and about 42μm with the frequency varying at radius of 1.0mm. It can be concluded that the length is more likely to be affected by the oscillating radius, but nearly irrelevant to the frequency.

Fig. 5.

SEM images of the weld microstructure neighboring the fusion line, (a) without beam oscillation, (b) r=0.5mm and f=50Hz, (c) r=1mm and f=50Hz, (d) r=1mm and f=75Hz, where the BM denoted base metal, the FZ denoted the fusion zone.

(1.16MB).
Fig. 6.

The average length of the columnar grains, (a) effect of beam oscillating radius at frequency of 50Hz, (b) effect of beam oscillating frequency at radius of 1.0mm.

(0.19MB).

As shown in Fig. 7, the weld microstructure was in-homogeneously mixed with columnar and equiaxed grains for the sample without beam oscillating. At radius of 1.0mm, the weld center is composed of homogeneous equiaxed grains at frequency of 25Hz, mixed structure with coarser columnar and equiaxed grains at 75Hz, and almost total refined columnar grains at 100Hz. Under frequency of 50Hz, the weld center is mostly composed of fine and homogeneous equiaxed grains at radius of 0.5mm, and coarse columnar grains at radius of 2.0mm.

Fig. 7.

Inverse pole figures, (a) without beam oscillation, (b) r=1.0mm and f=25Hz, (c) r=1.0mm and f=75Hz, (d) r=1.0mm and f=100Hz, (e) r=0.5mm and f=50Hz, (f) r=2.0mm and f=50Hz.

(2.32MB).

According to the statistical data of the microstructure characteristics, as shown in Fig. 8, the proportion of the equiaxed zone (PEZ) within the weld decreases from 85% to 42% with the frequency increasing at radius of 1.0mm, while the average grain size (SG) reaches the maximum of 37.5μm at 75Hz. On the other hand, the PEZ decreases from 92% to 14%, and the SG increases from 21μm to 44μm with the radius increasing at frequency of 50Hz.

Fig. 8.

Statistical data of the microstructure characteristics, (a) effect of beam oscillating frequency at radius of 1.0mm, (b) effect of beam oscillating radius at frequency of 50Hz.

(0.24MB).

The results above show that the beam oscillation can not only break up the columnar grains near the fusion line and reduce the grain size, but also promote the formation of equiaxed grains. However, the equiaxed grains at the weld center may be replaced by coarse columnar grains when employing excessive frequency or radius. Thus, the effects of the oscillating behavior on the weld microstructure can be explained as following.

As shown in Fig. 9a, the temperature gradient (G) is not homogeneous for the molten pool without beam oscillation, because the heat is transferred from the keyhole to the edge of the molten pool. The columnar grains are formed at the edge due to the high enough G. However, partly columnar grains start to be broken up because of the critical composition super-cooling caused by the decrease of the G. Then, the broken grains move within the molten pool for re-nucleation with the convection provided by the surface tension and the thermal buoyancy. In this case, only a small amount of equiaxed grains form since partly nucleation sites are melted under the high enough temperature.

Fig. 9.

Schematic drawing of the microstructure formation, (a) without laser oscillation, (b) oscillation with appropriate parameters, (c) oscillation with frequency higher than 75Hz, (d) oscillation with radius larger than 1.5mm.

(0.9MB).

As shown in Fig. 9b, the keyhole moves along the oscillating path of the laser, and the heat is transferred from the keyhole to the surrounded areas by Marangoni flow. According to the simulation results of Wang (2016) and Gao et al. (2017) [15,16], the G within the molten pool tends to be homogeneous, and the composition super-cooling is easier to be achieved under appropriate oscillating parameters. This phenomenon was also observed in arc oscillating welding of AZ31 Mg alloy [9]. The eddying and convection effects are thus enhanced by the Marangoni melt flow. A large amount of fine turbulent flows from the tail of the molten pool to the mushy zone, and acts on the root of the secondary dendrite. Then, the columnar grains are broken up in addition with the re-melting effect. On the other hand, the broken grains are immediately rolled from the edge to the center by the melt flow, the number of the nucleation sites is increased and the maximum atomic group is enlarged, which promotes the occurrence of nucleation. Due to the reasons above, the equiaxed grains are more likely to form at the weld center under appropriate oscillating parameters.

At frequency higher than 75Hz, as shown in Fig. 9c, the G at the weld center is more homogeneous due to more concentrated distribution of the heat, which promotes the formation of the equiaxed grains. However, the merge and coarsening of the sub-grains may be nurtured, and the actual nucleation sites are reduced, because of the enlarged molten pool and the prolonged holding time at high temperature according to the results of high-speed photography, which promotes the formation of partly columnar grains at the weld center.

At radius larger than 1.5mm, as shown in Fig. 9d, the molten pool is obviously enlarged. The previously melted metal starts to be solidified when the base metal heating by the laser is just melted. Then, the heat is dispersed, and the weld center is mixed with solid and liquid states. Besides, the melted metal at the tail of the molten pool flows along the welding direction to transfer the heat to the surrounded areas, especially the weld edge. Thus, the G at the weld edge is reduced, and the columnar grains are broken up. Moreover, the holding time at high temperature of the weld center is further prolonged because of the enlarged molten pool and decreased solidifying rate, the nucleation sites are then rapidly reduced, and the weld center is mainly composed of columnar grains with small portion of equiaxed grains.

3.4Tensile properties and plastic deformation mechanism

All the welds crack along the fusion zone during tensile tests. It should be noted that the weld tensile properties cannot be improved by the beam oscillation, but are obviously affected, as shown in Fig. 10. The ultimate tensile strength (UTS) decreases from 240 to 208MPa with the oscillating radius increasing from 0.5 to 2.0mm at frequency of 50Hz, while the elongation rate (EL) decreases from 9.6% to 7.5%. With the frequency increasing at radius of 1.0mm, the EL achieves the maximum of 9.2% at 75Hz, while the UTS keeps about 232MPa when the frequency is lower than 75Hz, and decreases to 222MPa when it increases to 100Hz.

Fig. 10.

Tensile properties of the welds, (a) effect of beam oscillating frequency at radius of 1mm, (b) effect of beam oscillating radius at frequency of 50Hz.

(0.26MB).

According to the above results, it is found that the weld tensile properties are closely related to the PEZ and the SG. The PEZ decreases while the SG increases with the increase of the oscillating radius at the frequency of 50Hz, which is corresponding with the decrease of the UTS and the EL. With the increase of the frequency at radius of 1.0mm, the PEZ also decreases and the SG reaches the maximum at 75Hz, but the higher UTS and EL are reversely obtained. By further analysis, as shown in Fig. 11, it is related to the formation of tension twins found within the fracture surface of this sample. The slip system of Mg alloy is few due to its close-packed hexagonal structure, which couldn’t coordinate any deformation. Other than the slipping deformation activated under high temperature, the twining deformation is induced by stress, which can compensate for the lack of the slip systems and coordinate the plastic deformation, and then improve the weld ductility under room temperature. The related researches [17–20] showed that the dislocation slip path was short for the fine grains of Mg alloy, the stress then was relieved by cross-slip, non-basal slip or dynamic recovery. Thereby the twin nucleation was suppressed. However, the dislocation slip path was long, and the local stress neighboring the grain boundaries was high for the coarse grains, the twin nucleation is then activated easier, which improves the weld ductility.

Fig. 11.

Crystallographic characteristics of the fractured weld, (a) IPF of the fractured weld with f=75Hz and r=1.0mm, (b) IPF of the fractured weld with f=25Hz and r=1.0mm, (c) Orientation analysis of the black rectangle area.

(0.63MB).
4Conclusion

  • (1)

    The continuous weld surface is achieved at frequency of 50Hz, and the undercut defect appears at frequency higher than 75Hz or radius larger than 1.5mm, which are obviously lower than the thresholds of the aluminum alloy or steel. Under the given parameters of laser power 2kW, welding speed 2m/min and beam oscillating diameter 0.35mm, the beam oscillation with small radius about 0.5mm and low frequency about 50Hz is more beneficial to achieve better weld appearance.

  • (2)

    The columnar grains neighboring the fusion line are broken up by the beam oscillation, and are rolled to the center acting as nucleation sites. The number of the nucleation sites is reduced increasing whether the oscillating frequency or the radius, resulting in the melt of the nucleation sites and the formation of coarse columnar grains.

  • (3)

    The proportion of the equiaxed zone (PEZ) within the weld decreases from 85% to 42% with the frequency increasing at radius of 1.0mm, while the average grain size (SG) reaches the maximum of 37.5μm at 75Hz. On the other hand, the PEZ decreases from 92% to 14%, and the SG increases from 21μm to 44μm with the radius increasing at frequency of 50Hz.

  • (4)

    The weld tensile properties cannot be improved by the beam oscillation, but are obviously affected, which are closely related to the PEZ, the SG and the twins. The decrease of the PEZ and the increase of the SG decrease the UTS and the EL with the increase of the oscillating radius. On the other hand, the PEZ also decreases with the increase of the oscillating frequency and the SG reaches the maximum at 75Hz, but the formation of tension twins during tensile deformation promotes the improvement of the tensile properties.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgments

This research is financially supported by the National Natural Science Foundation of China (grant nos. 51775206 and 51475183).

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Journal of Materials Research and Technology

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