High-strength aluminum alloys are susceptible to the formation of porosity and solidification cracking during laser welding. The porosity is generally attributed to the collapse of the keyhole driven by an imbalance between the vapor recoil pressure formed within the keyhole and surface tension forces driven by fluid flow in the surrounding liquid metal in the fusion zone. Several mitigation approaches using shielding gases that are soluble in the molten metal and reducing the ambient pressure have demonstrated some success in other material systems. Beam manipulation through the use of galvo-driven optics has become more widely available and allows the beam path and oscillation frequency to be tailored to alter the fluid flow patterns within the fusion zone. This technique has shown some initial promise for decreasing the rather high levels of porosity obtained during the laser welding of high-strength Al alloys. However, the role of the beam path and oscillation frequency on the morphology and distribution of the pores within the fusion zone has been largely unexplored.
A series of laser welds were fabricated on butt joints comprised of AA6061 and AA4047 aluminum alloy plates using a multi-mode Yb-fiber laser equipped with a D30 wobble head (IPG Photonics) that produced a beam diameter of 55 Ám. Circular scanning patterns with an amplitude of 0.4mm and 0.8mm and frequency ranging 150Hz and 1000Hz were used to fabricate the different welds at a laser power of 800 W and travel speeds between 22 mm/sec and 50 mm/sec. The alloy combination was selected to mitigate the potential for solidification cracking in 3.175 mm plate material. Weld dimensions were measured from cross sections obtained from each welding condition using Zeiss Smartzoom optical microscope. Pores present within the welds were characterized using X-ray Computed Tomography (XCT) techniques, a GE v|tome|x L300 system operated at an accelerating voltage of 210 kV and current of 70 μA with a 0.5 mm Cu filter to produce a voxel size of approximately 0.016mm. The size and morphology of the individual pores were measured from three-dimensional representations of the welds processed using Avizo 9.7.0 scans and analyzed using ImageJ, an open-source image-processing platform.
Results and Discussion:
The use of circular beam deflection during laser welding impacts both the energy density distribution as well as the resulting weld shape and dimensions. In general, the addition of circular deflection to the beam path by varying the diameter of the circular beam path and the frequency at which the beam traverses, the energy density distribution, and heat input are impacted even for the same laser power and travel speed. By changing the energy density distribution and beam path, corresponding changes in the weld profiles are observed. An increase in the diameter of the circular deflection path from 0.4 mm to 0.8 mm produce wider yet shallower welds with widths on the order of 1.6 to 1.8 mm and depths on the order of 1.4 mm. Changes in the frequency of the beam oscillation, however, had little impact at the smaller beam deflection but resulted in shallower pools when the frequency was increased at the larger beam amplitude.
While changes in weld dimensions are expected, the use of beam deflection has been mainly employed to attempt to mitigate the formation of keyhole collapse porosity, which is common in laser welding of high-strength Al alloys. By examining the welds in a three-dimensional fashion, the size, morphology, and distribution of pores for various beam deflection parameters were able to be quantified. While increasing the frequency up to a level of 300 Hz, lower porosity levels in the different welds were noticed, but the pores were not completely removed for keyhole mode welds and comprised a bimodal distribution of pore sizes. The smaller pores were largely spherical, with sphericity values above 0.95, and distributed at locations corresponding to 80% of the weld depth. On the other hand, the larger pores were elongated in shape and displayed sphericity values below 0.85.
Emerging beam deflection technologies are being pursued as a means for mitigating and eliminating the pores resulting from keyhole collapse during the laser welding of aluminum alloys. The application of circular beam deflections at different diameters or amplitudes and frequencies has been evaluated during the laser welding of AA6061 and AA4047 alloys, and the pore size, morphology, and distribution have been quantified. While different oscillation parameters have an effect on the pore distribution, number, size, and shape, the pores are not completely removed from the welds. Porosity reaches the lowest measured levels (0.49%) at the smallest amplitude (0.4 mm) and a frequency of 300Hz frequency, with the pores being rather small and spherical and located near the weld root region, corresponding to the bottom of the keyhole.