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

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.