不等厚ULC-BH钢激光拼焊接头硬化机制

Hardening mechanism of tailored bland laser welding different thickness ULC-BH steels

采用两种激光光斑加热位置,对不等厚超低碳烘烤硬化钢板进行激光拼焊,分别测定拼焊接头的显微硬度,对比分析其组织性能差异,探讨热影响区硬化软化机制.结果表明,光斑偏向厚板时,拼焊焊缝两侧组织硬度曲线较为对称;在焊接热循环作用下,热影响区基体中饱和科特雷尔(Cottrell)气团析出细小弥散的NbC等第二相颗粒,使得热影响区强硬化;光斑置中时,容易使薄板热输入过渡,析出的第二相颗粒聚集长大,热影响区重新软化,容易导致冲压时开裂.

Ultra low carbon bake hardening (ULC-BH) steel sheets with different thickness were welded by using high power laser beam with different offsets of laser beam heating position. The microhardness of cross-section of laser tailor-welded blank was determined, and its microstructure was observed with optical microscopy and scanning electron microscopy. The hardening and softening mechanism of heat-affected zone (HAZ) were analyzed. The results show that with appropriate offset, the microhardness curve of the cross-section is symmetrical. Because of weld heat input, some dispersive and slight NbC particles are precipitated from the saturated Cottrell atmospheres of bake hardening steel, which help to harden the HAZ. However, without offset more laser power is required to melt the thick sheet, but thin sheet also receives more power. Surplus heat input results in the obvious precipitate growth and the softening HAZ of the thin side sheet. Stamping cracking always occurs in the HAZ of the thin side sheet.