Influence of processing window on laser welding-brazing of Al to press-hardened 22MnB5 steel

Laser welding-brazing of AA6061 alloys and press-hardened 22MnB5 steel in lap configuration was carried out with Zn-Al15 filler metal. The processing window in different defocused distances was investigated and its influence on microstructure and mechanical properties of Al/steel dissimilar joints was explored. The results revealed that good appearances could be acquired in different processing windows with variation of defocused distances. The processing window was enlarged with increase of laser spot diameters, from laser power of 2.1-2.3 kW in defocused distance of +20 mm (laser spot diameter of 1.4 mm) to 2.8-3.6 kW in defocused distance of +40 mm (laser spot diameter of 2.4 mm). The thickness of interfacial intermetallic layer increased gradually from 0.47 to 1.86 mu m for defocused distance of +20 mm to 3.76-11.03 mu m of +40 mm. The newly formed phase at the interface at defocused distance of +20 mm was identified as Fe-2(Al, Zn)(5) and those were identified as the mixture of Fe-2(Al, Zn)(5) and FeZn10 when the defocused distances were +30 mm and +40 mm. The tensile-shear test of laser welded-brazed joints produced in various processing windows showed two types of fracture modes occurred. The interfacial failure appeared with the defocused distance less than +30 mm. The maximum fracture load reached 2793 N, representing 84.6% joint efficiency in regard to aluminum alloys. However, when the defocused distance increased to +40 mm, the maximum tensile-shear force decreased to 2242 N and failure occurred in the softening region of HAZ in aluminum alloy caused by effect of large laser irradiation. At larger defocused distance, the weak position of the dissimilar joints would transfer from the interface to the HAZ of the aluminum alloy, resulting in a bottleneck to improve mechanical property of joints.