Effect Of Interlayer Thickness On Microstructure And Properties Of Cemented Carbide/stainless Steel As-welded Joints By Laser Welding

The welded joints of WC-20Co/Invar/316L stainless steel on the thickness of 4mm and 6mm base metal,respectively,was obtained with thickness of 1mm,1.5mm and 2mm Ni₄₂Fe_50.9C_0.6Mn_3.5Nb₃ invar as filler materials by using IPG-YLS-5000W fiber laser.The weld macro-formation,microstructure,crystal structure was investigated by using X-ray non-destructive detector,Optical Microscopy?OM?,Scanning Electron Microscopy?SEM?with Energy Dispersive Spectrometer?EDS?,X-Ray Diffraction?XRD?,Transmission Electron Microscopy?TEM?,and mechanical properties were studied by hardness test and three-point bend strength test for analyzing effect of interlayer thickness on microstructure and properties of WC-20Co/Invar/316L joints.From the point of view of comprehensive reference parameters,the well metallurgical bonding with a few defects was obtained if the welding laser power was controlled in2.5k W,3.0k W,4.0k W for 4mm thickness of base metal,and 4.0k W for 6mm thickness of base metal respectively,laser scanning speed is 0.012m/s.The argon was chosen with the speed of 17L/min as shielding gas.The weldability analysis suggested that the main defects of joints were cracks and pores,which mostly gathered at the interface between cemented carbide and fusion zone,and furtherly propagate from Heat Affected Zone?HAZ?to WC-20Co side or to the weld fusion zone.It observed that the mutual movement of W,Fe or Co,C at the WC-20Co/Invar interface cause the formation of mixed carbides Fe3W3C and Co3W3C?denoted by M₆C,?phase?at the fusion zone,which are susceptible to cause a stress concentration or micro-crack and then lead to fracture.The width of fusion zone is increasing with the thickness from 4mm to 6mm of base weld,and the quality of weld becomes poor correspondly,which behave more pores and cracks due to the stress concentration and welding deformation.At the interface of cemented carbide and weld,the grain growth due to the incomplete melting and aggregating of WC,which will weak the mechanical properties of weld.The monophasic austenite grain and twinning crystal without impurity and drawbacks are observed in 316L weld side.Well fusion formatted at Invar/316L,and the crystal structure in Invar is facial cubic crystal-??-Fe,Ni?austenite solution.The movement of Co in Invar interlayer contributes to the toughness of weld as the buffer.The mechanical test results demonstrated that fractures are inclining to occurred at HAZ of cemented carbide side.The average bend strength improved with interlayer thickness increase from 1mm to 1.5mm due to the stress relax of the use of pre-placed Invar as a buffer,when thickness increase to 2mm,the pore density in the fusion zone increased rapidly and cracks occurred near the fusion boundary on the WC-Co side,which decline the bend strength.The microhardness test showed that the hardness values near WC-20Co/Invar interface with 1mm interlayer thickness are higher than that with 1.5mm and 2mm thickness interlayer.And the transition is smoother with 1.5mm interlayer at Invar/316L interface than that with other thickness.