3.1 Weld appearance
Figure 3 shows the weld appearance of laser-MIG hybrid welded joints under different pressures of amplitude transformer (PAT). It could be found that the external ultrasound had an important effect on the weld formation. A smooth appearance with few spatters was observed, and the fish-scale pattern was not obvious when the ultrasound was not applied, as shown in Fig. 3 (a). With the pressure of amplitude transformer increasing to 62 N and 132 N, the fish-scale pattern was more obvious and the spatters were significantly reduced, as seen in Figs. 3 (b) and 3 (c). It suggested that the weld formation could be improved by the external ultrasonic with the amplitude transformer pressure of 62 N ~ 132 N. However, when the pressure of amplitude transformer was further increased to 168 N, as presented in Fig. 3 (d), the weld formation was extremely poor and plenty of spatters was obtained around the weld bead, representing the poor stability of hybrid welding process.
Figure 4 displays the cross-sectional morphology of laser-MIG hybrid welded joints under different pressures of amplitude transformer. The typical cross-sectional morphology of aluminum alloy laser-MIG hybrid welded joint, composed of arc zone and laser zone, was observed without the ultrasound. In addition, its weld depth and width were 3.6 mm and 7.4 mm, respectively, as shown in Fig. 4 (a). However, the obvious difference in cross-sectional morphology under varying pressures of amplitude transformer was observed. The weld depth was gradually increased to 3.7 mm and further to 4.2 mm, respectively, with the pressure of amplitude transformer increasing to 62 N and 132 N. Nevertheless, it was reduced to 2.9 mm when the large amplitude transformer pressure of 168 N was employed. The above phenomenon suggested that the external ultrasound could change the penetration ability of laser-MIG hybrid heat sources to aluminum alloy plates.
In the laser-MIG hybrid welding, the penetration ability of hybrid heat sources was mainly related to the shielding effect of the hybrid welding plasmas and the stability of keyhole [13]. The hybrid welding plasmas were composed of arc plasma and laser-induced plasma in laser-MIG hybrid welding. A strong shielding effect of hybrid plasmas on the laser beam energy would be generated owing to the refraction, absorption and scattering of plasmas when the hybrid plasmas were located at the path of laser beam [14]. To investigate the effect of external ultrasonic on the hybrid welding plasmas, the hybrid welding plasma morphologies with and without the ultrasound are suggested in Fig. 5. When no ultrasound was applied, the larger volume of arc was observed and more arc plasma existed at the path of the laser beam, suggesting a stronger shielding effect, as indicated in Fig. 5 (a). However, when the ultrasound with the amplitude transformer pressure of 132N was used, arc volume was more compressed, and the arc plasma at the channel of laser beam was significantly reduced, as suggested in Fig. 5 (b). As stated by Sarabia et al. [15] and Gallego-Juarez et al. [16], the ultrasonic wave could improve the particle collision frequency in the arc plasma, leading to an increase in plasma heat dissipation rate. According to the principle of minimum voltage, more heat would be generated when the greater heat of the arc was dissipated. In this case, the volume of arc plasma tended to be compressed, which was consistent with the study of Chen et al. [6]. As a result, the shielding effect of hybrid plasmas on laser energy was weakened with the addition of the external ultrasonic, leading to improvement of weld depth at the amplitude transformer pressure of 132 N.
Generally, with the amplitude transformer pressure further increasing to 168 N, the more weakened shielding effect of hybrid welding plasmas on laser beam energy could be obtained. However, the lowest weld depth (2.9 mm) was found when the amplitude transformer pressure was 168 N. It could be attributed to the effect of ultrasonic on the keyhole stability in the laser-MIG hybrid welding.
Figure 6 displays the longitudinal cross-sectional morphology of the weld under different pressures of amplitude transformer. The difference value between the maximum and the minimum weld depth is defined as extremum difference, which is shown in Fig. 7. The greater extremum difference represented the poorer stability of keyhole. When no ultrasound was applied, the keyhole extremum difference was 1.1 mm. However, as the pressure of amplitude transformer increased to 168 N, the values were increased to 1.74 mm, indicating the stability of keyhole was reduced. The decrease in the keyhole stability might be related to the disturbance of the molten pool by overly strong ultrasonic vibrations. In addition, Üstündağ et al. [21] had suggested that more laser energy was lost to maintaining the stability of the keyhole in the laser welding, which could also reduce the penetration ability of laser heat source to welded plate. Therefore, the weld depth was significantly reduced when the greater amplitude transformer pressure of 168 N was used.
3.2 Porosity
Figure 8 shows the porosity distribution and porosity rate in laser-MIG hybrid welds under different pressures of amplitude transformer. The weld porosity rate was calculated according to Eq. 1.
$${P}_{b}=\frac{\sum {S}_{b}}{{S}_{w}}$$
1
where the \({P}_{b}\) was porosity, \({S}_{b}\) was the area of the individual pore, \({S}_{w}\) was longitudinal section area of the weld. It could be seen that the porosity rate was 5.78% without the pressure of amplitude transformer, as shown in Fig. 8 (a). However, when the pressure of amplitude transformer was 62 N, the porosity rate in the weld was decreased to 1.48% and it was further reduced to 1.07% under the pressure of amplitude transformer of 132 N. Moreover, the porosity rate was 3.03% when the pressure of amplitude transformer increased to 168 N (in Fig. 8 (d)). It indicated that the porosity defects of aluminum alloy laser-MIG hybrid welded joints could be suppressed by the ultrasound.
In the laser-MIG hybrid welding of aluminum alloy, keyhole oscillation was usually found due to unbalance forces on the keyhole wall. The gas would be trapped by the metal pool when keyhole collapsed, which led to the formation of bubbles. When the bubbles could not escape from the molten pool before solidification, the pores would be formed in the weld [17]. The difference in porosity rate under different pressures of amplitude transformer could be attributed to the cavitation effect produced in the molten pool by the external ultrasonic. Nampoothiri et al. [18] pointed out that when the ultrasound was used to assist the welding, the cavitation effect could be generated in the molten pool.
During the laser-MIG hybrid welding of aluminum alloy, when keyhole collapse occurred, keyhole-induced bubbles were formed in the molten pool. In addition, the metal liquid would be pulled apart at the weak point when the acoustic pressure in the molten pool was greater than the melt cavitation threshold. In this case, a cavitation bubble would be formed [19]. Moreover, the ultrasonic cavitation could be divided into steady and transient cavitation. With the pressure of amplitude transformer increased, the transient cavitation intensity was gradually increased while the proportion of steady cavitation was reduced [9].
Figure 9 denotes the effect of steady cavitation and transient cavitation on bubble behavior. When ultrasonic cavitation occurred, during the negative acoustic pressure phase of the steady cavitation process, cavitation bubbles were pulled by the force generated by the pressure gradient as suggested in Fig. 9 (a) [20]. Thus, the cavitation bubble volume would be increased. The relation between escape velocity and the volume of bubbles could be obtained according to the following Stokes equation [12]:
$$v=\frac{2g{R}^{2}(\rho -{\rho }_{0})}{9\eta }$$
2
where the \(\rho\) was the melt density, \({\rho }_{0}\) stood the cavitation bubble gas density and \(\eta\) was the viscosity coefficient of melt. The escape velocity of the cavitation bubble was increased with the increase of the bubble radius. In this case, more cavitation bubbles could escape from the molten pool before its solidification, leading to the reduction of porosity rate of hybrid welded joint under different pressures of amplitude transformer.
As the pressure of amplitude transformer was increased to 168 N, the extremely strong transient cavitation would occur in the molten pool. In this case, the large cavitation bubble could be broke and additional microbubbles would be produced in the molten pool as shown in Fig. 9 (b) [21]. Meanwhile, the escape velocity of bubbles could not be accelerated due to the decrease of steady cavitation intensity. In addition, the more unstable keyhole was also obtained under the amplitude transformer pressure of 168 N, as indicated in Fig. 6 (d), which would result in the generation of more bubbles. As a result, the porosity rate in the welds under amplitude transformer pressure of 168 N was higher than that under the pressure amplitude transformer of 132 N.
3.3 Microstructure
Figure 10 shows the microstructure near the fusion line of 5052 Al welded joints with and without ultrasonic-assisted technology. It could be found that the external ultrasound had an important effect on the microstructure. When no ultrasound was applied, as shown in Fig. 10 (a), the width of columnar crystal near the fusion line was 705.4 µm. However, the width of columnar crystals near the fusion line was reduced with the addition of ultrasonic. The width of the columnar crystal zone was decreased to 600.4 µm and further to 137.5 µm, respectively, with the pressure of amplitude transformer increasing to 62 N and 132 N as illustrated in Figs. 10 (b) and (c). When the pressure of amplitude transformer increased to 168 N, the side of the weld was composed entirely of equiaxed crystals. It indicated that the ultrasonic vibration could promote the columnar-to-equiaxed transition of grain in the weld.
Figure 11 displays the evolution mechanism of columnar-to-equiaxed transition of grain in the weld. In the ultrasonic-assisted laser-MIG hybrid welding, the ultrasound was transmitted into the base metal when the ultrasonic tool head was in close contact with the upper surface of the base metal. In addition, the input energy of the ultrasonic to the molten pool in the welding was positively relevant to the pressure of amplitude transformer [11]. Preview literature [22] had suggested that the cavitation effect and acoustic streaming effect would be generated in the molten pool when the external ultrasonic was used. First, the cavitation effect was formed in the molten pool, owing to the impact of the burst of a cavitation bubble on the fluid metal, as stated by Wang et al. [23]. When ultrasonic waves were introduced into the molten pool, cavitation usually occurred in the molten pool and resulted in cavitation bubbles. According to the study of Chen et al. [6], the cavitation bubbles would expand, plug and burst rapidly in the molten pool, resulting in an instantaneous local high-pressure pulse. The impact effect would be formed by the high-pressure pulse. Thus, the generated columnar crystals were broken under the action of instantaneous high-pressure pulses as shown in Fig. 11 (c), leading to the refinement of the grain in the welded joint.
Additionally, during the brief growth of the cavitation bubble, the bubble would absorb ambient heat, resulting in subcooling, leading to the formation of many nuclei at the periphery of the cavitation bubble. As declared by Yuan et al [24], the generated nucleus would act as new grain growth sites, which would promote grain refinement. Second, Jian et al. [25] reported that the ultrasonic energy attenuated during the propagation, which led to the generation of the sonic-press gradient. In this case, the fluid flow in the molten pool would be promoted, and the more spiral vortices could be formed by the sonic-press gradient, showing an acoustic streaming effect [24]. As proposed by Xu et al. [26], the vibration and stirring effect on the molten pool would also be created by the external ultrasonic because of the acoustic streaming effects. Thus, the temperature gradient of the molten pool and the cooling rate of the molten metal was reduced, which was also beneficial for suppressing the growth of columnar grains. Meanwhile, cavitation-induced nuclei and dendrite fragments were uniformly distributed in the molten pool due to acoustic streaming effect as indicated in Fig. 11 (b). Therefore, the width of columnar grains in the weld was reduced and grain size was finer with the increasing pressure of amplitude transformer, owing to the cavitation effect and acoustic streaming effect.
3.4 Mechanical properties
3.4.1 Microhardness
Figure 12 shows the microhardness distribution in the hybrid welded joint with and without the ultrasound. It could be found that the average microhardness value was about 55 HV in the weld under no ultrasound. However, when the ultrasonic was applied, the average microhardness of the weld was increased. Specifically, the average microhardness of weld increased to 65 HV when the pressure of amplitude transformer was 132 N, as shown in Fig. 12 (b). The increase of the weld microhardness under ultrasound could be attributed to the refinement of grain size (in Fig. 10), which corresponded to the study of Chen et al [27].
3.4.2 Tensile properties
Figure 13 shows the tensile properties of hybrid welded joints under different pressures of amplitude transformer. When no ultrasound was applied, the average tensile strength was 184.8 MPa. However, the average tensile strength of the specimens produced with the pressure of amplitude transformer was improved. In detail, with the pressures of amplitude transformer increased to 62 N and 132 N, the average tensile strength was increased to 226.8 MPa and 240.9 MPa, which was improved by about 22.70% and 30.33%, respectively. When the pressure of amplitude transformer was 168 N, the average tensile strength was 233.6 MPa, which was improved by approximately 26.1%. Additionally, the elongation of the hybrid welded joint was improved by about 20.3%, 44.3% and 25.3% under three different pressures of amplitude transformer, compared to that without the ultrasonic.
Figure 14 displays the fracture surface morphology of hybrid welded joints under different pressures of amplitude transformer. As shown in Figs. 14 (a)(b), more quasi-cleavage surfaces and fewer dimples without ultrasound were observed, representing a mixed fracture mode of brittleness and ductility. However, with the assistance of different pressures of amplitude transformer, similar morphology features with more dimples and fewer pores were observed in fracture surfaces as shown in Figs. 14 (c)-(h), indicating the mode of ductile fracture.
The increase in tensile strength of hybrid welded joints with the increasing pressures of amplitude transformer was due to the lower porosity rate and finer grain size. When the external tensile stress was used, the pores in the weld would reduce the area of bearing stress and cause stress concentration, resulting in the formation of cracks [28]. When the cracks propagated in the welds, the fracture would be formed. As shown in Fig. 8, the porosity rates were decreased from 5.78–1.48%, 1.07% and 3.03% with the increasing pressures of amplitude transformer. It suggested that the stress-bearing area was increased and the number of crack sources was reduced with the assistance of ultrasound. In addition, the volume of grain boundaries usually was increased with the refinement of grain size, which could enhance its ability to absorb the energy of crack propagation [29]. Therefore, the tensile properties of welded joints assisted by ultrasound were greatly improved.