Metallurgical and mechanical properties of fusion zones of TRIP steels in laser welding

Abstract

Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and high plastic strain capacity that they offer. Si and Al are two important alternatives for alloying of TRIP steels in order to suppress carbide precipitation in the bainite holding temperature range during steel manufacture. Weldability of TRIP steel is one of the key factors governing its application in auto industry. In this paper, Al-alloyed TRIP steel was investigated with the diode laser welding process in terms of fusion zone metallurgical and mechanical properties, with Si-alloyed TRIP steel also included for comparison. It was found that the fusion zone of the Al-alloyed steel has a multiphase microstructure, containing skeletal ferrite, bainitic ferrite, martensite and retained austenite of two different morphologies. In contrast, the Si-alloyed steel fusion zone consists almost entirely of martensite. The high martensite content results in low fusion zone ductility in the Si-alloyed steel, only providing half the tensile elongation of the Al-alloyed steel. The Si-alloyed steel shows a greater decrease of the strength-ductility balance (ultimate tensile strength times elongation) due to welding, i.e., 62.9% compared to 45.2 % for the Al-alloyed steel in quasi-static tensile testing. High strain rate tensile testing with a Hopkinson Bar apparatus shows no significant effect of strain rate on the fusion zone ductility for either steel. The fusion zone of the Al-alloyed steel does not exhibit a detectable TRIP effect probably due to the low carbon content in the retained austenite. Al and Si are both relevant as agents to suppress cementite precipitation, but they are found to exert very different influences on steel weldability. © 2008 ISIJ.