Removal of oxides and brittle coating constituents at the surface of coated hot-forming 22MnB5 steel for a laser welding process with aluminum alloys

The surface of a press-hardened steel 22MnB5 coated with Al-base (AlSi10Fe3) and Zn-base (ZnNi10) was conditioned by a pulse laser and by sandblasting to remove undesirable oxides and brittle phases. Oxides formed on coating surfaces counteract the wettability of welding filler during a welding or brazing process. Furthermore, welding and brazing joints of 22MnB5 coated with aluminum alloys failed along the brittle intermetallic phases in the coating under a low mechanical load. Treated 22MnB5 surfaces were analyzed microscopically, and the phase compositions were investigated by synchrotron diffraction measurements. It was found that brittle phases could be locally removed by laser ablation; however, high laser energies led to remelting and oxidation of the coating surface. In contrast, sandblasting homogenously removed oxides and brittle intermetallic phases. Surface-treated 22MnB5 steel sheets were joined to AA6016 aluminum sheets by laser welding, and the strength of the weldment was determined by tensile tests. The measured mechanical strength of the aluminum/steel joints was 210-230 MPa. Failure of the weldments under tensile loading occurred within the aluminum sheet, away from the steel surface/welding filler interface if brittle coating components and oxides were removed homogenously.