Experimental Study On Gradient Strengthened Cladding Layer Prepared By Laser Cladding On The Surface Of Crystallizer Copper Plate

Copper alloys are used to make continuous casting mold copper plates because of their excellent thermal conductivity.However,in service in harsh environments,the mold copper plates are prone to surface wear,high temperature oxidation,and hot cracking.In order to improve the working performance of the mold copper plate and ensure the safety of continuous casting production,the laser cladding technology was used to treat the surface of the mould copper sheet,and a metallurgical strengthening cladding layer was prepared,which is of great significance for improving the production efficiency of continuous casting,reducing the production cost of enterprises and saving metal resources.In this paper,Fe-based cladding layer and gradient cladding layer(Fe-based transition layer + Co-based cladding layer)were prepared on the surface of the copper plate by Nd:YAG solid-state laser and synchronous powder feeding laser cladding equipment.OM,SEM,EDS.XRD analysis methods and microhardness measurement,wear test,thermal shock test,softening temperature test were used to study the preparation process,microstructure and properties of the Fe-based cladding layer,the effect of alloy composition on microstructure and properties of Co-based cladding layer,the effect of VC addition on microstructure and properties of the gradient cladding layer.The test results show that the preparation parameters of the single-pass Fe-based cladding layer obtained by orthogonal test are:pulse width 2.6ms,pulse frequency 22Hz,current intensity 260A,scanning speed 350mm/min,powder feeding rate 7.5g/min,spot diameter 1.5mm.Under the above process conditions,the single-Fe-based cladding layer was well formed,and the average microhardness of the cross-section was 341.6 HV0.2.Multi-channel overlapping Fe-based cladding layer was prepared by 40%overlapping ratio.The cladding layer was metallurgically bonded with copper matrix,with smooth surface,no cracks and pore defects.From the interface between the cladding layer and the substrate to the surface of the cladding layer,the microstructures are plane crystal(white band),cellular crystal,cellular dendrite,fine and undirected dendrite and equiaxed crystal.The phases of Multichannel overlapping Fe-based cladding layer are composed of(Fe,Ni)solid solution,Fe-Cr,Fe9.7Mo0.3,MoC.Cr7C3 and Cr-Ni-Fe-C intermetallic compounds.The average cross-sectional microhardness of the overlapped Fe-based cladding layer is 322.4HV0.2.Four kinds of Co-based cladding layers with different composition were prepared on the Fe-based cladding layer,in which the CJ-4 Co-based cladding layer with the main element content of Co33%,Ni27.5%,Cr21.8%,Fe6.5%,W7.7%has good forming quality and forms a good combination with the Fe-based transition layer.The microstructure of the Co-based cladding layer includes cell crystals,cell dendrites,fine and non-directional dendrites,and equiaxed crystals,and no planar crystal formation is observed.The dendritic structure of the interface between the Fe-based transition layer and the Co-based cladding layer grows together,which ensures the overall strength of the gradient cladding layer.The phase composition of the Co-based cladding layer is:?-Co(face-centered cubic)solid solution,Cr4Ni15W,Fe0.64Ni0.36,CoCx and Cr23C6.The VC-enhanced gradient cladding layer(Fe-based transition layer +VC-enhanced CJ-4 Co-based cladding layer)was prepared on copper substrate,When the VC content increased from 0%to 15%,the surface of the Co-based cladding layer gradually became black and the effective thickness became thin.When the VC content was 20%,the forming quality of the Co-based cladding layer was poor and the thickness was thin.In the range of 0%-15%VC addition,with the increase of VC addition,the microstructure of Co-based cladding layer is refined gradually,and its microhardness and wear resistance are improved gradually;The phase composition of Co-based cladding layer with 15%VC content was as follows:y-Co(face-centered cube)solid solution,VC,Cr4Ni15W,Fe0.64Ni0.36,CoCx and Cr23C6.After thermal shock test of the gradient cladding layer with 15%VC and no VC was added,the cladding layer still bond closely with the copper matrix,forming a dense oxide film on the surface of the Co-based cladding layer,which shows better thermal shock resistance and high temperature oxidation resistance.The microhardness of Fe-based cladding layer,Co-based cladding layer without VC and Co-based cladding layer with 15%VC.addition were 89.73%,95.92%and 96.22%of the original microhardness respectively after holding at 800? for one hour and air cooling to room temperature,both greater than 80%,the cladding layer has a higher softening temperature.Considering the effect of VC content on the forming quality,structure and properties of Co-based cladding layer,the appropriate VC content is in the range of 10%-15%.