Microstructure And Fatigue Properties Of Ni-Based Coatings Prepared By Plasma Cladding On Gray Cast Iron

A majority of weight bearing mechanical components,such as machine bed,brake drums,and gearbox cases,which have good damping and thermal properties,are made of gray cast iron.However,because of the service condition of overload and fatigue,above components'surface is heated and cooled alternately,noticeable cracks are easily initiated at the surface of cast iron equipment.The traditional surface technologies,such as arc welding,thermal spraying and gas shielded welding,have many shortcomings.For example,the quality of coating is poor,and there are many defects in the coating.Fabrication of surface coatings by plasma cladding on gray cast iron is an important research direction.Orthogonal experiments were carried out using L₉?3³?orthogonal table with three factors including working current,powder feeding rate and scanning speed to optimize the plasma cladding parameters for Ni60 coating.Micro-structure and mechanical properties of plasma cladding layer were analyzed by Optical Microscopy?OM?,X-ray Diffraction?XRD?,Scanning Electron Microscope?SEM?,the micro hardness tester,the micron indentation tester and the X-ray residual stress tester.The defects of cladding layer were further observed by metallographic microscope,and the causes were explored.The Ni60coatings with different Mo content were prepared on the surface of HT250 by using the best process parameters.The influence of Mo on the micro-structure,hardness and thermal fatigue property of cladding layers was explored.The influence mechanism of Mo elements on the crack propagation of cladding layer was further analyzed.The results show that:?1?The gradation of parameters decided by their effect on the synthesized points of the coating is:working current,scanning velocity,powder feeding,and when working current is 65 A,scanning velocity is 100 mm/min,powder feeding is 4g/min,the cladding layer will show the best comprehensive quality;?2??Fe,Ni???-Fe and?-Ni are shown in the coating,and the micro-structure of the coating matrix present distribution from plane crystal to columnar crystal,and then dendrite crystal.The micro-hardness of the cladding layer increases gradually from the matrix to the cladding layer.The maximum hardness appears in the middle of the layer.In addition,the modulus of elasticity changes smoothly from the matrix to the cladding layer;?3?There are cracks and holes in Ni60 cladding layer of gray cast iron,and there is shrinkage at the interface of the cladding layer;?4?With the increase of Mo content,the phenomenon of component segregation is obviously reduced,the dendrite changes from rod-shaped to dendrite,and the micro-structure of the cladding layer is obviously refined.When the content of Mo reaches6 wt.%,Mo₂C is precipitated in the cladding layer.The micro-hardness of the cladding layer increases with the rise of Mo content.And,it is found that the rate of crack growth decreases with the increase of content of Mo.