Simultaneous high-speed keyhole depth and absorptance measurements in laser spot welding of dissimilar metals

The integration of aluminum parts for purposes of lightweighting requires joining aluminum to steel. This process is infamously difficult, due to the poor laser weldability of aluminum and the brittle intermetallic phases that form when the two metals mix. Precise control of the process is enabled by deep understanding of the underlying physics. To aid in this effort, we collect simultaneous time-resolved keyhole depth measured by inline coherent imaging and absolute absorptance measured by integrating sphere radiometry, during spot welds on 6061 aluminum, 316 stainless steel, and on both materials in lap weld configuration. We find that laser welding through steel into aluminum carried the benefits of steel keyhole welding (laser absorptance near or above 0.9, predictable keyhole formation), while also leading to more stable keyholes (standard deviation of keyhole depth was reduced by 50% compared to steel welding alone). In comparison, laser welding through aluminum into steel faced similar problems as aluminum welding alone, such as unpredictable keyhole formation and large fluctuations in depth and absorptance (more than twice as large compared to steel alone).