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ASM International 1059-9495/$19.00

Effect of Al-Si Coating on Weld Microstructure and Properties of 22MnB5 Steel Joints for Hot Stamping Wenhu Lin, Fang Li, Dongsheng Wu, Xiaoguan Chen, Xueming Hua, and Hua Pan (Submitted June 23, 2017; in revised form January 11, 2018) 22MnB5 hot stamping steels are gradually being used in tailor-welded blank applications. In this experiment, 1-mm-thick Al-Si coated and de-coated 22MnB5 steels were laser-welded and then hot-stamped. The chemical compositions, solidification process, microstructure and mechanical properties were investigated to reveal the effect of Al-Si coating and heat treatment. In the welded condition, the coated joints had an Al content of approximately 2.5 wt.% in the fusion zone and the de-coated joints had 0.5 wt.% Al. The aluminum promoted the d-ferrite formation as the skeletal structure during solidification. In the highaluminum weld, the microstructure consisted of martensite and long and band-like d-ferrite. Meanwhile, the low-aluminum weld was full of lath martensite. After the hot stamping process, the d-ferrite fraction increased from 10 to 24% in the coated joints and the lath martensite became finer in the de-coated joints. The tensile strengths of the coated joints or de-coated joints were similar to that before hot stamping, but the strength of the coated joints was reduced heavily after hot stamping compared to the de-coated joints and base material. The effect of d-ferrite on the tensile properties became stronger when the fusion zone was soft and deformed first in the hot-stamped specimens. The coated weld showed a brittle fracture surface with many cleavage planes, and the de-coated weld showed a ductile fracture surface with many dimples in hot-stamped conditions. Keywords

d-ferrite, Al-Si coating, hot stamping, laser welding

1. Introduction Weight reduction is one of the important issues in vehicle manufacturing. One method is to use advanced high-strength steels (AHSS) and ultrahigh-strength steels (UHSS) (Ref 1). Among these steels, press-hardened steels (PHS), such as 22MnB5 steel, exhibit very high ultimate tensile strength of approximately 1500 MPa after hot stamping (Ref 2). To prevent the surface oxidation and decarburization during the hot stamping process, PHS sheets are usually coated with an Al-Si coating, which can offer good anticorrosion ability and high-temperature oxidation resistance (Ref 3). The other method is to use advanced design, such as tailor-welded blanks (TWBs) (Ref 4). The blanks are welded by a laser beam, which offers flexible delivery, high precision and less thermal distortion and a small HAZ (Ref 5). In comparison, hotstamped sheets have a softening problem in the heat-affected zone (HAZ) after welding (Ref 6).

Wenhu Lin, Dongsheng Wu, and Xiaoguan Chen, School of Materials Science and Engineering, Shanghai Key Laboratory of Laser Processing and Materials Modification, Shanghai Jiao Tong University, Shanghai 200240, China; Fang Li and Xueming Hua, School of Materials Science and Engineering, Shanghai Key Lab