Abstract
With the aim to investigate the effect of parameters and the quenching process on the joint microstructure and mechanical properties of hot stamping steel by laser welding, BR1500HS boron steel was welded by wire-filling laser welding with ER70-G welding wire under different parameters. The welded specimens were heated to 900 °C and held for 5 min before water quenching. A universal material test machine, optical microscope, Vickers hardness tester, scanning electron microscope, and electron backscatter diffraction (EBSD) were used to characterize. The results show that the heat input should be greater than 1040 J/cm and the optimal wire-feeding speed is between 160 cm/min and 180 cm/min. The tensile strength of the quenched joint can reach greater than 1601.9 MPa at compatible parameters. More retained austenite distributes in the fusion zone (FZ) and fine grain zone (FGZ) than the coarse grain zone (CGZ) before quenching. However, the retained austenite in FZ and heat-affected zone (HAZ) decreases clearly and distributes uniformly after quenching. The grain diameter in FZ before quenching is not uniform and there are some coarse grains with the diameter greater than 40 μm. After quenching, the grains are refined and grain diameter is more uniform in the joint. With the increase in heat input, the microhardness of FZ and HAZ before quenching decreases from 500 HV to 450 HV. However, if the wire-feeding speed increases, the microhardness of FZ and HAZ before quenching increases from 450 HV to 500 HV. After quenching, the joint microhardness of all samples is between 450 HV and 550 HV. The fracture morphology of the joint before quenching consists of a large number of dimples and little river patterns. After quenching, the fracture morphology consists of a large amount of river patterns and cleavage facets due to the generation of martensite.
Highlights
The tendency of current lightweight automotive parts is the application of more and more high strength steel to reduce weight [1]
Automobile parts inevitably need to be connected by welding technology and the heat affected zone is softened due to the effect of thermal circulation in the welding process for high-strength steel, which dramatically decreases the strength of the joint
Whether conducted with filler wire material or not, laser welding can be divided into autogenous laser welding and wire-filling laser welding [5]
Summary
The tendency of current lightweight automotive parts is the application of more and more high strength steel to reduce weight [1]. In order to meet the welding demand of high-strength steel and reduce the defects in the welding process, laser welding technology has recently been applied more widely [2]. With many advantages such as high heat input, concentrated energy, fast cooling rate, narrow heat affected zone, and excellent joint properties, etc, laser welding is applied more frequently in the connection of high strength steel [3,4]. Wire-filling laser welding can allow a small gap in the joint, solving the problem of autogenous laser welding requiring a high-precision assembly of the joint [7]. It is meaningful to carry out an experiment to investigate the process parameters, microstructure, and mechanical properties of wire-filling laser welding for high-strength steel
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