Study On Cr₃C₂/Ni Composite Coating By Laser Cladding And Its Grinding Machinability

In this paper,the preparation process and grinding properties of Cr₃C₂/Ni cladding layer were studied.The Cr₃C₂/Ni composite coatings were prepared on 45 steel substrate by different laser cladding process parameters.In order to explore the best cladding process parameters,the effects of current,scanning speed and lap ratio on the forming quality,dilution rate,microstructure and hardness of the cladding layer were studied.On this basis,the effects of Cr3C2 with different particle sizes and content on microstructure,hardness,wear resistance and flexural strength of laser cladding Cr₃C₂/Ni composite coating were studied.The grinding experiments of cladding layer were carried out to study the grinding performance and the formation mechanism of the surface defects of laser clad Cr3C2/Ni composite coating with different Cr₃C₂ particle sizes and content.The optimal process parameters obtained in this experiment are as follow:laser current is 150A,scanning speed is 200mm/min,overlap rate is 60%,frequency is30Hz,pulse width is 30ms,spot diameter is 1.5mm.The surface of the cladding layer prepared by this process parameters has good quality,no cracks,pores and other defects,and it is well combined with the substrate.With the increase of Cr₃C₂ content in the cladding layer,the microstructure of the cladding layer is more dense.Moreover,when the content of Cr₃C₂ is high,there are more chromium carbides in the cladding layer.The hardness and wear resistance of the cladding layer increase significantly with the increase of Cr₃C₂ content.When the Cr3C2 content reaches 30wt.%,the hardness of the cladding layer fluctuates greatly,but the wear resistance of the cladding layer is much higher than that of the GCr15steel.The adhesive wear degree between the cladding layer and GCr15 steel decreases,and the wear degree of GCr15 steel increases.The degree of oxidation wear of GCr15 steel is aggravated.The flexural strength of the cladding layer gradually decreases with the increase of Cr₃C₂ content,and the brittle fracture characteristics of the bending fracture are obvious.Reducing the particle size of Cr3C2 in the cladding layer,the microstructure of the cladding layer is even smaller and well-distributed.The change of the particle size of Cr3C2 has little effect on the hardness of the cladding layer,but it will increase the wear resistance and bending strength of the cladding layer.Under the same grinding parameters,the normal grinding force and tangential grinding force increase with the increase of Cr₃C₂ content in the Cr₃C₂/Ni cladding layer,but the increase of the normal grinding force is greater than the tangential grinding force.The influence degree of cutting depth,grinding wheel speed,Cr3C2particle size and Cr3C2 content on grinding force decreased in turn.Under the same grinding parameters,the Ra value of the ground coating increases with the increase of the size of Cr₃C₂ particles and decreases with the increase of the percentage of Cr3C2.However,the Ra value increases slightly when the content of Cr₃C₂ exceeds 30wt.%.The influence of the cutting depth and the grinding wheel speed on the surface roughness of the coating is greater than the effect of the particle size and content of Cr₃C₂,but the influence of the cutting depth is greater.The defects on the ground surface of the laser cladding Cr₃C₂/Ni composite coating mainly include two categories:one is generated in the Ni based alloy which includes debris adherence,grinding traces,micro-crack and cracks;the other is the broken Cr₃C₂ particles and voids generated by the pullout of the broken Cr₃C₂.Debris adherence in the Ni based alloy is easier to form under the condition of smaller depth of cut and lower Cr3C2 content in the coating.As the cutting depth increases,deeper grinding marks and micro-crack appear.Cracks form in the Ni based alloy when the content of Cr3C2 in the coating is higher than 20wt.%.More voids are found on the ground surface when the percentage of Cr3C2 is 30wt.%.There are fewer defects on the ground surface when the size of Cr₃C₂ is smaller and the wheel speed is higher.