Effects of the heat input in the mechanical integrity of the welding joints welded by GMAW and LBW process in Transformation Induced Plasticity steel (TRIP) used in the automotive industry

Abstract

In this work an Advanced High Strength Steel (AHSS) sheet of the Transformation Induced Plasticity (TRIP) type currently employed in the automotive sector was welded using a Gas Metal Arc Welding (GMAW) and a CO2 Laser Beam Welding (LBW) processes. The mechanical properties of welded tensile specimens including microhardness were determined and the results were related to the exhibited microstructures. It was found that LBW lead to relatively high hardness in the fusion zone (FZ) indicating that the resultant microstructure was predominantly martensite. In the Heat Affected Zone (HAZ), a mixture of phases consisting of bainite and ferrite was present. Similar phase mixtures were found in the HAZ and Fusion Zone (FZ) of the GMAW samples. The exhibited microstructure did not result in mechanical degradation when the GMAW specimens were tested in tension as all the fractures occurred in the BM. In contrast, the region adjacent to the HAZ of most tensile specimens welded using LBW failed by brittle cleavage. Apparently, in this region tempering effects due to heat dissipation in the LBW process promoted carbide growth and a relatively coarse microstructure. No embrittlement was found that could be associated with the development of martensite.