Study On Grain Growth Of WC And Crostructure Performance Of Joint In Laser Welding

The welding of cemented carbide to 45# carbon steel was carried out using YLS-5000 ytterbium doped multimode fiber laser manufactured by Apache Corp(IPG, USA). The specimens were prepared using 3mm base materials without groove and 4mm base materials with the INVAR as filler metal under argon atmosphere at the rate of 20-25L/min. Optical microscopy, scanning electronic microscopy, X-rays diffraction, and four point bend strength were used to investigate the weld formation, microstructure distribution, and mechnical properties. And then analyzing on the harmful compound carbide(η phase) and the formation mechanism and influencing factors on abnormal growth of tungsten carbide particles.The results indicate that the welded joint of WC-20 Co to 45# steel can be obtained at optical parameters. The well welded joint without filler material can be reached with the parameters at-8mm defocusing, laser power 3.5kW, laser scanning speed 0.012m/s and the laser power is 4kW, the laser scanning speed is 0.016m/s. The welded joint formation using INVAR alloys as interlayer are as a whole better. But from the point of view of comprehensive mechanical properties, the well welded joint without defects can be reached if the welding energy input were controlled between 125J/mm~170J/mm, and the laser power were controlled among 2kW~3kW.The defects analysis of joint indicated that the main defects of joints were cracks and porosity. The cracks were mainly concentrated at the interface between cemented carbide and welds. Two growth mechanisms on cracks propogation were observed in the experiments: some cracks propogated from the interface of cemented carbide and welds to the cemented carbide base metal; others grew from the welds near cemented carbide to the cemented carbide base metal. There are two kinds of craking: cold cracks and hot cracks. There are hot cracks in the high welding energy input condition, and cold cracks are more often in the lower welding energy input condition. The porosity was often formed at the upper of the welds. In addition, the appearance of brittle eta phase Co2W4 C, Fe3W3 C, M12 C composite carbides tends to cause a stress concentration or micro crack which will increase the fragile products.The corrosion results showed that for the welded joints of WC-20 Co cemented carbide to 45# carbon steel in HNO3, H2O2 and Haruki reagent, the corrosion rate is not the same. The corrosion rate of 45# carbon steel in 5% HNO3 was the fastest; the composite carbide MxC and WC are the main etched in 5% H2O2. In the 20%Haruki reagent, the corrosion rate is the slowest, and MxC is the main corroded.At the cemented carbide/welds interface, the grain growth was owed to the unmelting and aggregating of WC. The aggregated WC particles led to the WC normal grain growth(NGG) with large concentration of stresses and cracks. The NGG behavior can use LSW theory to explain. Under the action of laser, small particles of tungsten carbide dissolves in INVAR alloy molten pool(liquid) or in steel, and then precipitate in the large particles surface. Under the effect of laser, the tiny or large particles of tungsten carbide are gathered by liquid migration and growing again. Tungsten carbide particles grow up along a crystal plane orientation and small particles attached to large particles in the processing of growing. The phenomenon of abnormal grain growth(AGG) behavior appeared in this melting transition process.The bending test results and EDS point analysis indicated that the fractures were often occurred at the interface between cemented carbide and welds. The intergranular fracture and transgranular fracture are the two forms of crack that in microstructure.