Defect Formation Mitigation During Laser Welding of High-Strength Aluminum Alloys Using Beam Oscillation
Open Access
- Author:
- Saha, Abhirup
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 02, 2024
- Committee Members:
- Laura Cabrera, Program Head/Chair
Judith Todd Copley, Major Field Member
Todd Palmer, Chair & Dissertation Advisor
Reginald Hamilton, Major Field Member
Tarasankar Debroy, Outside Unit & Field Member - Keywords:
- Laser Welding
Beam Oscillation
Aluminum Welding
X-ray Computed Tomography
Keyhole Porosity
Machine Learning
Heat Transfer and Fluid Flow - Abstract:
- High-strength aluminum alloys, extensively used in automotive and aerospace industries, often suffer from processing defects such as solidification cracking and keyhole collapse porosity during laser welding. These defects, particularly keyhole collapse porosity arise from imbalanced vapor recoil pressure, surface tension, and hydrodynamic forces. Laser beam oscillation, which allows for manipulation of the beam path in various patterns, has shown promising impacts in mitigating these defects. In order to determine the efficacy of different beam oscillation parameters, accurate measurements of the porosity levels are needed, but they have been hindered by the use of two-dimensional optical microscopy-based techniques that capture single longitudinal or transverse cross sections. With the emergence of X-ray computed tomography tools, internal porosity can be characterized in three dimensions across the entire weld volume, vastly increasing the accuracy of porosity measurements and allowing the location and morphology of individual defects to be obtained. When comparing porosity measurements obtained using these tools, two-dimensional microscopy tools underestimated the porosity by 50 to 80%. These conventional two-dimensional methods are also unable to accurately characterize the morphology of irregularly shaped pores. Circular beam deflection has proven effective in reducing defects in various alloy systems, though the impact of specific welding parameters remains unclear. Experiments conducted on AA6061 and AA4047 alloy plates indicated that travel speed significantly influences porosity levels, with slower welding speeds leading to lower porosity. Further evaluation of the beam oscillation conditions and the corresponding defect structures observed in the welds highlighted the important role that the ratio between the laser beam diameter and oscillation amplitude played. Increases in beam overlapping during oscillation were driven by a combination of the beam diameter and oscillation amplitude and frequency, with higher beam diameters and slower travel speeds appearing to be more impactful on defect formation. Despite these findings, when operating in the keyhole mode, no combination of beam deflection parameters completely mitigated the formation of these keyhole collapse defects, but trends were observed in the number, size, and shape of defects with changes in beam oscillation parameters. Across all conditions, small spherical pores were primarily located near the bottom of the welds. Keyhole mode laser oscillation welding has the capacity to bridge narrow gaps, enhance mechanical properties by improving microstructural features, and mitigate processing defects of welds. However, the impact of similar laser beam oscillation welding parameters on different alloy systems remains inadequately understood. In this research, experimental and computational approaches have been employed to investigate the effect of oscillation welding parameters on metallurgical factors for four commonly used structural alloys with different thermophysical properties – 304L stainless steel, AA6061, Inconel 740H, and Ti-6Al-4V. Results showed that Al6061 displayed larger fusion zones than 304L stainless steel, Inconel 740H, and Ti-6Al-4V due to its lower density and boiling point. Al6061 exhibited the highest thermal gradient (G), cooling rate (GR), and solidification mode (G/R), where the solidification rate is R, with a decreasing cooling rate as weld depth increased. Conversely, Ti-6Al-4V showed the lowest thermal gradient, cooling rate, and G/R. Process maps were generated to understand the impact of welding parameters on achieving the desired fusion zone geometry, providing crucial insights for optimizing welding processes and selecting appropriate parameters to achieve high-quality welds across diverse materials.