Experimental Study On Laser Cladding Properties Of Ni-based Powders And Co-based Powders On The Surface Of H13 Steel

H13(4Cr5MoSiV1), the mostly widely used hot work die steel, which has high thermal strength, wear resistance, impact-toughness and excellent thermal fatigue resistance, is widely used to manufacture various forging dies, hot extrusion dies and casting dies of magnesium and aluminium alloys. Because hot work dies suffer from various physical and chemical actions, such as wear, thermal fatigue, erosion, stress corrosion and surface thermal wielding in service, their service life are not long. It is well known that failure of mould starts from the surface. To improve the surface microstructure and mechanical properties is an effective way to extend its service life. Method of surface laser cladding is used to improve surface properties of H13 steel, eapecially its surface hardness, wear resistance, corrosion resistance and heat resistance by making use of high radiation, high brightness, sound directivity and monochromaticity of laser beam, and so its service life is extend. In practical, large area cladding should be done, but the diameter of the incident laser spot is usually too small to meet the requirement. Over lapping between contiguous laser scanning zones is the simplest solution.In this paper, a 5kW power carbon dioxide flow transverse laser system was adopted for preparation of Ni-based, co-based alloy cladding layer on the surface of H13 hot die steel by means of overlapping large area cladding. The microstructure and phase composition of cladding layer as well as its microstructure and morphology after high temperature oxidation are analyzed using optical microscope (OM) and X-ray diffraction (XRD). The hardness of cladding layer was tested using HVS-1000A microhardness tester and its wear resistance was analyzed by wear testing machine.Microstructure analysis showed that the laser cladding Ni-based and co-based alloy coatings were divided into three areas from the aspect of organizational structure: the cladding, the combining area and the heat affected zone. At the cladding layer bottom of Co-based alloy were mostly planar and cellular grains growing normal to the interface. In the middle were dendritic crystals growing multi-directionally, and at the top zone were small dendritic crystals growing parallelly to the laser scanning direction. Good metallurgical bonding between the coating and the substrate and good comprehensive performances were obtained. The phases in Ni60A cladding layer mainly consisted of Cr₂₃C₆ and Ni2Si reinforced phase and theγ(Fe-Ni) supersaturated solid solution; the phases in Co50 cladding layer mainly consisted of (Cr-Fe)₇C₃ reinforced phase andγ-Co supersaturated solid solution; and the phases in Co42 mainly consisted of reinforced phases of carbides such as M₂₃C₆ and a littleγ-Co supersaturated solid solution.The laser cladding coating had a high surface hardness because of its fine structure. In this paper, the multi-track laser cladding coating was adopted, and the wear test results showed that Ni60A microhardness ranged from 800HV0.2 to 900HV0.2, Co50 microhardness was around 500HV0.2, Co42 microhardness was approximately 475HV0.2, and that hardness of the H13 steel matrix was around 450HV0.2. In other words, descending order of the hardness was as follows: Ni60A→Co50→Co42→H13.It's discovered that the hardness of Co-based alloy keep stable and it is higher than that of Ni-based alloy after long-time high-temperature oxidation test, which showed that the Co-based alloy enjoyed a good thermal stability even at 600℃after long time holding. The wear amount curve of H13 steel matrix and cladding coating of Ni60A, Co50 and Co42 under different loads showedΔM Co50 >ΔM Co42 >ΔM H13 >ΔM Ni60A, which meant that wear resistance of the Ni60A cladding coating was the best.The failure mold of die cast mold in service is thermal fatigue, erosion, light metal corrosion, high temperature corrosion and surface thermal welding. So it is not necessary for die cast mold to possess extortionate hardness. Therefore, heat-resistant Co-based alloy is the best choice in the surface cladding process of H13 die cast mold.