Study on Laser Welding of Advanced High Strength Martensite Steel

Abstract

The growing demand for lightweight yet crashworthy automotive structures has promoted the use of 1.5 GPa grade martensitic steels (MS1500 1). However, laser welding of such ultra high strength materials is challenged by heterogeneous microstructures and severe hydrogen induced delayed cracking (HIDC) risks. This study systematically investigates the laser welding behavior and hydrogen embrittlement susceptibility of MS1500 1, focusing on three aspects: (1) the effect of surface oil contamination (oil free, 1.5 g/m² as received, and 3.0 g/m² excessive lubricant) on microstructural evolution and mechanical performance of laser lap welded (L bend) and butt welded joints; (2) the hydrogen permeation and diffusion behavior in the base metal using electrochemical permeation tests; and (3) the correlation between microstructural features of distinct heat affected subzones, local hardness distribution, and overall joint integrity. Comprehensive microstructural characterization (optical microscopy, scanning electron microscopy, X ray diffraction), mechanical evaluation, and hydrogen permeation measurements reveal that surface oil significantly alters the weld thermal cycle and hydrogen uptake, leading to pronounced softening and increased HIDC susceptibility. The results establish a fundamental understanding of hydrogen trapping characteristics and mechanical reliability of laser welded MS1500 1 joints, providing a scientific basis for mitigating delayed cracking and developing robust welding protocols for ultra high strength steel components in automotive manufacturing.