Characterization of Static Performance and Failure of Resistance Spot Welds of High-Strength and Press-Hardened Steels
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JMEPEG https://doi.org/10.1007/s11665-019-03988-2
Characterization of Static Performance and Failure of Resistance Spot Welds of High-Strength and Press-Hardened Steels K. Paveebunvipak and V. Uthaisangsuk (Submitted August 15, 2018; in revised form March 5, 2019) The static performances and failure characteristics of resistance spot welds (RSW) of steel sheets were studied in this work. Both similar and dissimilar joints of press-hardened steel grade 22MnB5 and as-rolled high-strength steel grade 1000 were examined. From metallographic analyses and micro-hardness measurements, it was shown that the microstructures of base metal (BM), heat affected zone (HAZ) and fusion zone of steel joints were noticeably different. In particular, the HAZ close to BM or subcritical HAZ (SCHAZ) of 22MnB5 steel exhibited greatly decreased hardness due to occurring tempered martensite, in which carbide precipitation was clearly observed. Furthermore, tensile shear test and cross-tension test of the welded steel sheets were carried out. The determined maximum loads, energy absorbability and corresponding failure mechanisms were analyzed and compared. The RSW samples of similar steel grade 1000 provided the highest load-carrying capacity and absorbed energy in both tests. Due to the large hardness difference and soft zone in weldment of dissimilar steel specimen, failure loads and static absorbed energies were deteriorated up to 17 and 68%, respectively. The observed fracture occurrences of all investigated joints were pullout failure mode, in which BM and HAZ were the crack initiation sites of grades 1000 steel and 22MnB5 steel, respectively. The SCHAZ of welded boron steel considerably caused the failure onset in tension shear test, but seemed to have negligible effect on cross-tension test. Keywords
AHS steels, failure, microstructure, resistance spot welds, tension shear test
1. Introduction A variety of procedures for part assembly such as bolts, rivets, clinching, gas tungsten welding, resistance spot welding (RSW), laser welding, electron beam welding, friction stir welding as well as adhesive joints are available. The RSW is one of the most popular joining processes in the automotive industry, because it requires no highly skilled worker, low cost and shorter time. The RSW basically provides a joint with uniform load distribution. No precise preparation of sample edges is needed so that the welding can be done in a quick succession. To produce car body structures, advanced highstrength steel (AHSS) and ultra-high-strength steel (UHSS) sheets like dual-phase (DP) steels have been increasingly used because of their excellent mechanical and forming properties (Ref 1-3). The boron-alloyed steel grade 22MnB5 after hot stamping belongs to the family of UHSS. Hereby, the boron steel sheets are initially heated and subsequently cooled down in forming dies in order to achieve a fully martensitic transformation and thus the maximum yield and tensile strength
K. Paveebunvipak and V. Uthaisangsuk, Department of Mechanical Engineering,
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