Investigation On Preparation Of Directional Microstructure In Ni-based Alloy Coating And Its Properties

Supersonic atmospheric plasma spraying(SAPS) technology which had many advantages was widely used in preparation of coating of infusibility spraying material such as wear and corrosion resistance coating based on metal, alloy and thermal barrier ceramic coating.The coating still exists the problems of insufficient bond strength, low cohesion and defects as a result of constrained processes and parameters.These problems had not only limited applications but also shortened service life of coating. In recent years,the researchers have tried to solve the above problems by using different preparation techniques or adding different sizes of hard phases, rare earth and self lubrication phase. In the process of alloy research, the oriented crystal has improved the longitudinal mechanical and physical properties of the material, so it has been focused by researchers and producers.Therefore,under the condition of invariable original composition, directional microstructure was prepared by SAPS,high frequency induction remelting and directional forced cooling method in the coating of metallurgical bonding so that it could deeply improve performance such as friction and wear properties.The Ni60 alloy coating was prefabricated by supersonic plasma spraying technology on 45#steel substrate, and the coating was remelted by the induction heating and forced cooling by using directional cooling device under the conditions of cooling water of different flux.The effect of cooling water of different flux on the microstructures, phases, and elements development, or properties of the induction remelting coating were investigated by OM, SEM(EDS),XRD,EPMA,the macro and microhardness tester,friction and wear testing machine. Phase types and porosity of coating were analyzed by Origin 7.5,Jade 6.5,Image-pro plus 6.0 software.The research results of microstructures,phases,elements development of coating showed that great changes have occurred in the microstructure of the coating and a metallurgical bond were formed between the coating and substrate with the increase of the flux of cooling water.When the flux was 20,80 and 120L/h, the directional crystal was formed in the coating.When the flow rate was 80L/h, the structure of columnar dendrites was relatively dense,fine and the central symmetry of columnar dendrites of cross shape of this coating surface was the best. Besides, its deviation angles were minimum along the [001] direction and the range of deviation angles were between0°-8°. At the same time, it had a highly orientation along the(200)crystallographic plane.Spraying coating was mainly composed of γ-Ni, Ni2.9Cr0.7Fe0.36,Fe Ni3, Cr23C6, Cr2 B, Fe3 B. After the induction remelting, new γ-(Fe,Ni) appeared,the rest were basically unchanged and Fe Ni3 was not detected in the coating. Induction remelting and forced cooling coating was mainly composed of γ-Ni,γ-(Fe,Ni),Cr C,Cr B,Ni2.9Cr0.7Fe0.36.The distribution of elements showed that Ni and Fe element mainly enriched in initial columnar crystal, hard phase elements which enriched in between the grains enhanced columnar crystal and contributed to improvement of the wear resistance.The research results of properties of coating showed that section microhardness of spraying coating increased at first and then decreased along the direction of the substrate to coating, but section microhardness of induction remelting and directional solidification coating slightly increased. Additionally, the surface macro and section microhardness of the latter two coating were less than spraying coating.The porosity of prefabricated Ni60 coating after induction remelting was changed from 2.45% to0.59%, and the porosity of the directional solidification coating was the minimum(0.23%). Under room temperature and dry sliding condition, the wear volume and rate of directional solidification coating were the lowest with the increase of load, which showed the wear resistance of directional solidification coating was the best in the process of coatings of three different microstructure against1Cr18Ni9 Ti stainless steel. Compared with spraying coating, the friction coefficient of directional solidification coating increased a bit, which displayed good friction drag characteristic. In addition, the main wear mechanism of spraying coating attributed to the fatigue spalling and adhesive daub, induction remelting coating attributed to adhesive daub and slight desquamation, directional solidification coating attributed to adhesive daub.