Rapid beam oscillation represents a promising technique for controlling process conditions and defects in laser welding of important alloy systems. Due to beam oscillation, laser energy distribution can be altered, leading to variations in weld dimensions, microstructure, and defect formation. Laser beam oscillation was initially used to improve gap tolerance in laser welding, while it has been investigated regarding many other aspects in laser processing, such as reducing penetration fluctuation, suppressing keyhole collapse porosity and solidification cracking, controlling surface roughness, and improving mechanical properties by impacting the microstructure. However, the effect of beam oscillation on the weld and defect formation during high power laser welding of nickel alloys is still largely undocumented, which requires careful investigation.
A series of autogenous laser welds were fabricated on 12.7 mm thick Inconel 740H plates. Individual welds approximately 50 mm in length were fabricated at laser powers ranging between 5 kW and 10 kW at a travel speed of 12.7 mm/s with a beam diameter in the sharp focus condition of 1.04 mm. Laser energy density distribution during beam oscillation was calculated with the consideration of laser power, welding speed, frequency, amplitude, and beam size.
Transverse cross-sections were extracted from different locations in each weld and prepared using standard metallographic techniques to a final polishing step with a 1 µm diamond slurry. To reveal the weld cross-section, electrolytic etching was used to reveal the fusion zone profile. Optical microscopy techniques were then used measure the weld depth and width. To capture the impact of beam oscillation on these defects, in-depth quantitative analysis was conducted using micro-computed x-ray tomography (µCT) techniques, which allow for a three-dimensional characterization of the defects.
To better understand the origination of the defects, a more in-depth characterization on the microstructure and elemental segregation in the fusion zone and HAZ is required. Samples representing each weld were characterized using a FEI Helios NanoLab 660 field emission gun (FEG) scanning electron microscopy (SEM) coupled with an Oxford Instruments X-Max X-ray energy dispersive spectroscopy (XEDS) and Nordlys Max electron backscatter diffraction (EBSD) detectors.
3. Results and discussion
The laser travel paths and the resultant spatial energy density distributions are determined by the complex interactions between the oscillation parameters, welding speed, and beam size. With higher oscillation amplitude, the laser energy is distributed to larger areas, resulting in lower energy density at the center while more energy is dispersed to the edges. As the frequency increases, the energy density distribution becomes more and more continuous, and no further change was observed when the frequency is higher than 50 Hz when other parameters remain the same.
The weld profiles and dimensions were not considerably impacted by the oscillation under amplitude of 0.4 mm with different frequencies. However, the weld profile and dimensions displayed significant differences when using oscillation amplitude of 1.6 mm. The weld depth was decreased while the width especially the region near the keyhole finger was increased compared with the corresponding linear welds, and these trends were consistently observed under different laser powers.
The application of oscillation amplitude of 0.4 mm has no benefit to mitigate or reduce the defects in the welds compared with the non-oscillation condition for all the laser power levels. On the other hand, no defect was detected in the welds made with oscillation amplitude of 1.6 mm with frequency of 300 Hz and 411 Hz under laser power of 5 kW. Considerably reduced porosity and solidification cracking was obtained with oscillation amplitude of 1.6 mm with frequency of 150 Hz and 300 Hz under laser power of 7.5 kW, indicating high amplitude relative to the beam size is beneficial to reduce the defects. On the other hand, relatively high oscillation frequency of 411 Hz resulted in considerably higher volume fraction of porosity compared with non-oscillation conditions, likely due to the high keyhole instability induced by the high oscillation frequency. Significant amount of defects exist with laser power of 10 kW under all the oscillation conditions, likely due to the deep penetrations.
The secondary dendrite arm spacing (SDAS )generally decreased along the weld for all the conditions, which was not significantly affected by the beam oscillation for different amplitudes and frequencies, indicating the overall cooling rate during beam oscillation laser welding is similar to the linear welding conditions. Similar grain structures were observed for all the different oscillation conditions, indicating that the beam oscillation under amplitude of 0.4 mm has minimal effect on the grain structure, likely due to the nearly the same spatial laser energy density distributions for these conditions. On the other hand, the grain structures with oscillation amplitude of 1.6 mm showed significant differences in both the grain size and orientation for all the frequencies.
4. Summary and conclusions
The impact of beam oscillation on weld dimensions, microstructure, defect formation, and mechanical property in high power laser welding of Inconel 740H was investigated. Due to the additional transverse and backwards motions with the laser beam oscillation, the laser energy density distribution was altered, leading to changes in weld dimensions. Although the weld depth was not obviously impacted by the oscillation frequencies used in this work, the weld dimensions were significantly affected by oscillation amplitudes, with higher amplitudes leading to wider while shallower weld pool. The microstructure in the fusion zone, specifically the secondary dendrite arm spacing and grain structure, was characterized, which show little dependence on beam oscillation. The characteristics of the defects produced during beam oscillation laser welding were captured through µCT, which allows for the optimization of beam oscillation conditions.