Optimization Of Laser Cladding Co-9Al-12W Alloy Process Variables

Co-Al-W alloy is strengthened by a ternary compound?'-Co₃?Al,W?phase with the precipitation strengthening on?-Co matrix novel Co-based superalloys.It still has high strength and excellent comprehensive performance when used at temperatures above 1000°C.Alloying elements can further improve the high temperatu re properties of the superalloys.The cobalt-based Co-Al-W superalloys is used as the cladding material to coating the surface of the material and workpiece,which can improve the working temperature of combined cycle system of gas turbine and steam turbine in thermal power plant,meet"extreme"environment requirements,reduce the cost of alloy,improve the utilization efficiency of coal gasification and coal-fired power plants,expand the application scope of local resource products and promote local economic development.Based on the preliminary work of the research group,the LDM2000 fiber laser processing system and the preset powder laser forming process were used to coat the Co-9Al-12W alloy on the surface of 304 stainless steel by single-track and multi-track.Under the nonequilibrium condition of laser cladding,the influence of process parameters on the macroscopic morphology and dilution rate of Co-9Al-12W alloy cladding layer was studied,the phase composition and microhardness of the cladding layer were determined,the microstructure,element diffusion,distribution and cracking behavior of cladding layer were analyzed.The main research results are as follows:?1?In the single-track laser cladding experiment,Co-9Al-12W alloy cladding layer with good bonding with the base metallurgy was successfully prepared.The cladding alloy was mainly composed of?-Co matrix and its intermetallic compound Co _XAl and carbide,such as Cr₂₃C₆,Co₆W₆C,etc.The microstructure of the cladding layer can be roughly divided into the following regions:interface bonding zone,cellular crystal region,dendrite region and equiaxed crystal region.The bottom of the cladding layer was a columnar dendrite beam which partially perpendicular to the interface growth.The middle part of the cladding layer was mainly composed of cross-dendrites with a certain orientation and smaller equiaxed crystals.The top of the cladding layer was almost entirely fine and dense equiaxed crystal,according to the results of X-ray diffraction,the top of the cladding layer consists of?-Co dendrites and eutectic such as Co6W6C and Cr23C6,but the eutectic on the top of the cladding layer was more prominent than that in the middle of the cladding layer.Under the experimental conditions,P=1.6KW and V=360mm/min were selected as the optimal process parameters for single-track laser cladding.?2?In the multi-track laser cladding experiment,the microstructure of cladding layer generally followed the morphology of the directional solidification growth,but the multi-track-joined cladding layer does not appear plane crystals at the interface.The multi-track-joined cladding layer was mainly composed of dendrites which were basically perpendicular to the bonding zone,and the dendrites grow in many directions.In the overlapping zone,the microstructure of cladding layer appeared obviously coarsening phenomenon.The multi-track-joined cladding alloy was mainly composed of?-Co matrix and its intermetallic compound CoxAl and carbide,such as Cr₂₃C₆,CoCx,Co₆W₆C,etc.When the laser power was constant,the microstructure of the cladding layer becomes smaller with the increase of scanning speed.When the scanning speed is constant,with the increase of laser power,the microstructure o f cladding layer becomes larger.The Co,Al,Fe and Ni elements are mainly distributed in the dendrites,and the C,Cr and W elements are mainly distributed in the interdendritic regions.A high-hardness cladding layer is obtained by multiple lap joints,and the hardness of the cladding layer generally decreases from the surface to the substrate,and the highest hardness can reach 732.9HV _1.0.Under the experimental conditions,P=1.6KW,V=360mm/min,f=40%were selected as the optimum process parameters for multi-pass laser cladding.?3?The existence of large residual stress in the multi-track cladding layer is the cause of cracks.This internal stress is the result of the combination of thermal stre ss,tissue stress and restraint stress during laser cladding.The position generated in the middle can be divided into crack surface layer crack,interface matrix crack and overlap zone crack.The surface layer crack of the cladding layer is mostly located in the middle part of the cladding layer,and cracks are not easily generated at both ends of the cladding layer.In the lap-bonded laser cladding layer,the cracks usually exhibit three different macroscopic features:?a?a certain angle with the scanning direction;?b?fish scales;?c?almost perpendicular to the scanning direction.The lap joint cladding layer is more susceptible to cracks than the single pass cladding layer,where in the crack depth at the surface crack of the cladding layer is 64.4?m and the average depth is about 60?m.