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A 780 MPa galvanised dual phase (DP) steel was two-, three- and four-layer resistance spot welded under different welding parameters and joint configurations. The splash situation, nugget sizes and tensile shear properties of the spot welds were evaluated systematically. The results showed that the splash occurring current increased in the order of two-, three- and four-layer spot welds owing to the increasing metal volume for melting. The joint configuration had a great influence on the tensile shear properties of the spot welds. The three- and four-layer spot welds with proper joint configurations had better weld quality than the two-layer spot weld. Therefore, well designed multilayer spot welds could be used to replace the commonly used two-layer spot welds in actual production. [ABSTRACT FROM PUBLISHER]

A 1000 MPa galvanised dual phase steel was fibre laser lap welded by different welding parameters. The surface appearance, cross-section macrostructure and tensile shear properties of laser lap welds were evaluated. The different methods for improving the laser lap weldability of galvanised steel were analysed. The results showed that the increasing laser power resulted in the formation of the keyhole, which could be used as the escape route for the vapourised zinc formed at the faying surface. Therefore, the weld defects, such as spatter, cavity and blow hole, were eliminated, and the full penetrated laser lap welds with smooth surface appearances were acquired. A proper joint gap also worked as the escape route for the zinc vapour and led to the formation of sound welds. Adding proper preheating procedures improved the mechanical properties of the welds to some degree, but weld defects still existed owing to the residual zinc. [ABSTRACT FROM AUTHOR]

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel, a 980 MPa transformation induced plasticity (TRIP) steel and a 980 MPa twinning induced plasticity (TWIP) steel were resistance spot welded under different welding parameters. The macrostructure, splash situation, nugget size and mechanical properties of the spot welds were evaluated systematically. The results showed that the different heat conductivities, melting points and surface electric resistances of the DP, TRIP and TWIP steels resulted in the asymmetric weld nuggets of the dissimilar spot welds and the increase in the splash occurring current in order of DP/DP/DP, TRIP/DP/TWIP, TWIP/TRIP/DP, TRIP/TWIP/DP (TRIP/TRIP/TRIP), TWIP/TRIP/DP and TWIP/TWIP/TWIP. For the dissimilar spot welding of galvanised DP1000, uncoated TRIP980 and uncoated TWIP980, the TRIP/TWIP/DP combination can obtain better weld quality than the TRIP/DP/TWIP and TWIP/TRIP/DP combinations. [ABSTRACT FROM AUTHOR]