Microstructure and formability performance of fiber laser welded 1.2 GPa grade hot-rolled TRIP steel joints

Abstract

Laser welding of 1.2 GPa hot-rolled transformation induced plasticity (TRIP) steels with thickness of 1.9 mm was carried out to study its microstructure, mechanical properties and deformation behavior. Defect-free joints with complete penetration were prepared under the applied welding parameters. It was found that the fusion zone (FZ) consisted of full martensite of high microhardness (489 HV). Compared with the base material (BM) (436 HV), the subcritical heat affected zone (SCHAZ) contained a narrow softening zone with 62 HV microhardness drop due to tempering of martensite. The laser welded joint achieved 96.6% joint efficiency comparing to the BM. The formability of welded specimen depended on the weld position. As the weld offset increased to 6 mm, the formability approached 95.7% of the BM. The deformation was small in FZ and was primarily concentrated in BM. The martensite in FZ effectively restrained transverse deformation in welded joint, such that main cracks occurred and propagated in the BM. Stretch-flange-formability in terms of hole-expansion ratio of weld specimens was degraded compared with the BM (33.3%). Premature failure appeared at joint with low punch force because complete martensite exhibited low ductility in the FZ.