High-Speed X-Ray Investigation of Pore Formation during Full Penetration Laser Beam Welding of AA6016 Aluminum Sheets Contaminated with Lubricants

The presence of lubricants on the surface of sheets favors the formation of pores in laser welded seams. This formation process was investigated by means of high-speed X-ray imaging of the full penetration laser beam welding process of two AA6016 aluminum sheets in overlap configuration. The measurement of the growth velocity of the bubbles indicated their sudden growth once they started to form. Further analysis of the X-ray images identified the point of origin of the pores: a few millimeters behind the capillary between the two aluminum sheets. The study shows that the lubricant does not affect the stability of the capillary, which evidences that the formation of these pores is not caused by the fluctuations of the capillary. These results explain for the first time why pore formation cannot be avoided by process strategies, which stabilize the capillary, when welding uncleaned sheets.