Effect of keyhole mode on weld shape and mechanical properties of thin sheets of Al-Si coated 22MnB5 steel

The great change of plasma or plumes and weld pool flows existed between “blind” keyhole and open keyhole, especially for thin sheets. The keyhole mode could be changed by increasing the power density for deeper keyhole penetration. In this study, weld shape, aluminum content, δ ferrite fraction, and tensile strength were investigated in laser welding Al-Si coated steel. “Blind” keyhole led to “Y” weld shape, lower aluminum content, less δ ferrite, and higher strength at the optimized parameter. On the contrary, open keyhole led to “X” weld shape, higher aluminum content, more δ ferrites, and lower strength. The microstructure was more homogeneous in low speed “Y” weld. From the high speed images, the open keyhole was accompanied with fierce molten flow on the bottom side. The bottom coating was also important in the aluminum increase. In the suitable combination of welding parameters for “blind” keyhole, their strength could reach the level of decoated sample with no need for decoating preparation.