Study On Microstructure,Crystal Structure And Property Of Laser Welded Joint Of WC-20Co/INVAR/45 Steel

The welded joints of WC-20Co/INVAR/45 steel was obtained with Fe₅₈Ni₄₂ invar as filler materials using IPG-YLS-5000W fiber laser.The 25L/min argon was chosen as shielding gas.The weld formation,microstructure,crystal structure and mechanical properties were investigated using optical micrography,scanning electron micrography,X-Ray Diffraction(XRD),Transmission Electron Microscopy(TEM)and four-point bend strength test.From the point of view of comprehensive mechanical properities,the well metallurgical bonding without defects was obtained if the welding heat input was controlled between 125J/mm¹66J/mm,and the laser power 2kW³kW.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 fusion zone.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 analysis of lengthwise section microstructure indicated that massive martensite can be observed at heat affected zone of steel side;?phase,Fe₃W₃C(M₆C),M₁₂C mixed carbides can be seen at lengthwise section fusion zone which may cause a stress concentration or micro crack which will increase the fragile products.At the interface of cemented carbide and welds,the grain growth was owed to the unmelting and aggregating of WC.The microstructure characterization research of as-welded Invar fusion zones indicated that after undergoing high temperature themal cycles,the microstructure of as-welded Invar fusion zones contains?-(Fe,Ni)solid–solution(nickel dissolving in?-Fe)with a face-centred cubic(fcc)crystal structure and mixed carbides(Eutectic colonies,mixed carbides between two adjacent grains).The mixed carbides exhibited larger,coarser eutectic microstructures with a decrease in welding speed and an increase in heat input.The structure of the initial Invar and as-welded Invar is face-centered cubic?-(Fe,Ni).As-welded Invar has a larger plastic deformation than initial Invar with an increase in local strain field and dislocation density.The analysis of TEM indicated that crystal defects of stacking fault,slip deformation and dislocation were observed in as-welded invar fusion zones.The bending test results and hardness test indicated that fratures were often occurred at Heat Affected Zone(HAZ)of cemented carbide side.Some cracks propogated from HAZ to fusion zone,others propogated from HAZ to base metal.The microhardness test indicated that the hardness value near the interface did not change sharply but smoothly from cemented carbide to fusion zone.