Investigation On Microstructure And Wear Properties Of WC-Co-Cr Coatings Based On Laser Linear Energy Density Regulation

Titanium alloys are widely used in aerospace,petrochemical and marine engineering fields due to their high specific strength and heat resistance property.However,the low hardness and poor wear resistance limit their further promotion.As a surface treatment technology,laser cladding has the advantages of high productivity,high controllability and a wide range of material options,which is often used to strengthen the specific properties on the surface of structural materials.In this study,the microstructure of WC-Co-Cr composite coating is regulated and the wear resistance is optimized by the variation of linear energy density（LED）on Ti-6Al-4V substrate.The effect of LED on the metallurgical qualities,phase content and microstructure characteristics of the composite coatings were systematically investigated.The evolution behavior and growth mechanism of WC in the laser cladding process were revealed.Meanwhile,the microhardness and wear resistance of the composite coatings prepared by different LED were studied,the wear mechanism of the coatings with different LED was clarified,and the relationship between LED-microstructure-wear resistance was established.It is expected to provide a theoretical basis and experimental solution for the preparation of wear-resistant coating on titanium alloy surface.The results show that all the WC-Co-Cr composite coatings have metallurgical bonding with the Ti-6Al-4V substrate and no defects can be observed on the surface.The coatings consist of WC,W₂C,Cr₇C₃,Ti C andη-Co phases after laser cladding.The WC among them exhibits morphological transformation and grain growth compared with pre-processing and shows a small angular shift in the XRD diffraction pattern.In addition,the main phase WC has an evolutionary behavior including two stages of dissolution and growth during laser cladding.The WC is dissolved by the forms of releasing W and C,and grows into specific sharp-edged angular morphologies along three habit planes of（10-10）,（0-110）and（-1100）according to the Lifshitz-Slyosov-Wagner model,while the degree of dissolution and growth scale are both positively correlated with LED.Therefore,as the LED increases,the content of W₂C increases,the index of carbide retention decreases（91.7-69.8）and the average size of WC grains increases up to three times（1.89μm-5.81μm）.The microhardness of the WC-Co-Cr composite coatings significantly increases compared with the Ti-6Al-4V substrate and is positively correlated with LED,the maximum increase among them is nearly 6 times.All coatings occur abrasive wear due to WC fragmentation under high loads during sliding wear.The adhesive wear occurs in low LED coating due to the direct contact of the exposed soft phase with the friction pair,WC peeling off in high LED coatings during wear resulting in the high content of the hard and brittle phase W₂C,the above additional wear behaviors lead to higher wear mass loss of the coatings.The coating prepared by 166.7J/mm among them has the best sliding wear performance with 0.2 mg wear mass loss.The abrasive wear is the main wear mechanism of the coatings during dynamic load abrasive wear and is supplemented by the cutting of the coating by quartz sand abrasives.The coating prepared by low LED and high LED exhibits additional mass loss in the form of severe cutting due to low hardness and partial coating shedding due to the high brittleness,respectively.The best dynamic abrasive wear performance among them is the coating prepared by 112.5 J/mm（Wear mass loss:0.7378 g）.