| Boron Steel not only can increase the safety of automobile body, but also can de-crease the weight of automobile body, because of its ultra high strength. Laser weld-ing technology is an advanced welding technology, because it possesses good welding quality, high efficiency and high degree of automation. Now laser welding technology has been widely used in automobile industry. Therefore, it is very important to re-search laser welding technology of boron steel for the application of this kind of steel in the automobile industry. The B1500HS Boron steel, which is produced by Bao steel company, was selected as welding base metal. Finite element analysis method was used to optimize welding parameters at first. Then microstructures and mechanical properties of welding join at different welding parameters were calculated by software. Laser butt welding experiment and laser lap welding experiment were conducted to research the influence of different welding parameters on microstructures and me-chanical properties of welding joints.The result of welding parameters optimization showed that high power and high speed welding could increase the cooling rate of heat affected zone(HAZ) and de-crease tempering time of soften region.Simulation results of microstructures and mechanical properties of the welding joint showed that microstructure of weld and completely quenched region was mar-tensite. Martensite in soften region decomposed and M3C carbide formed in soften zone. Welding speed had less effect on microstructures and mechanical properties of weld and completely quenched region. Hardness and strength of weld and completely quenched region were almost same with base metal. However, hardness and strength of soften region decreased with, the increase of tempering time and tempering temper- ature.Laser butt welding experiment showed that microstructure of weld and com-pletely quenched region was martensite. Microstructures of incompletely quenched region were martensite and ferrite. However, tempered martensite and ferrite were found at the soften region in HAZ. Hardness distribution of welding joint was uneven. Soften region had the lowest hardness. According to the result of tensile test, the sam-ples were fractured at the soften region. Tensile strength of welding joints increased with the increase of welding speed. Three-point bending test showed that maximum bending angle decreased with the increase of welding speed. The welding joint had good mechanical properties when the welding power was 1000W and welding speed was between 10mm/s and 15mm/s, because both its tensile strength and bending strength were more than 1100MPa and the bending angle could be 180°.The change rule of microstructure and hardness of lap welding joint was similar to butt welding joint. Martensite decomposed more completely and hardness was lower at the soften region. Pore appeared in fusion zone for the laser lap welding ex-periment. Tensile test specimens fractured in interface. The shear force of lap welding joints decreased with the increase of welding speed.Frequency had little impact on the strength of laser butt welding joint, but it had significant effect on the porosity and shear force of lap welding joint. Oxide coating on the surface of the sample after quenching could increase laser absorption efficiency and increase the size of weld. Oxide coating decreased the strength of laser butt welding joint and increased the shear force of lap welding joint. |
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