Study On The Modification Of Laser Clad Cobalt-based Composite Coating On Titanium Alloy Surface

As the excellent aerospace material, titanium alloy is always limited in its application by low hardness and poor wear resistance. But laser cladding served as one of the research hotspots in the field of surface modification recently can provide a very promising approach to solving this problem. Meanwhile, cobalt-based alloy with good high temperature strength is the excellent material in aviation industry. Although it is often used in laser cladding, its application in the cladding modification of titanium alloy surface is less now. Therefore, the cobalt-based composite coatings were fabricated on TA15heat resistant titanium alloy surface in order to obtain high hardness and good wear resistance in this paper.The cladding materials chosen in this paper are KF-Co50powder with a little Ti powder, cobalt-based powder containing a high proportion of Cr and Stellite6alloy powder with a little S13N4powder, respectively. The single track and primary multiple tracks overlapping laser cladding experiments were conducted on TA15titanium alloy surface by transverse flow CO2laser, then the cobalt-based composite coatings reinforced with multiple particles could be fabricated. Metallurgical microscope, XRD, EPMA, SEM with EDS, were used to study the microstructures of the cladding layers; A DHV-1000type digital Vickers hardness tester was used to measure the microhardness values of the cladding layers; An UMT-2type multifunctional wear testing machine was used to process the wear tests of the cladding layers and substrate.The results of laser cladding ingredient-optimized KF-Co50composite coating show that: The cladding layer has an excellent metallurgical bond with the substrate, and it is in finer microstructures; The cladding layer is mainly composed of γ-Co, a little α-Ti solid solution and dispersed in-situ synthesized TiB2, Cr5Si3, TiC, WB and Co3T1reinforced phases, and so on; The microscopic morphologies in the cladding layer are multiple reinforced phases particles dispersively distributed between the small dendrites; Compared with the substrate, the microhardness and wear resistance of the cladding layer both have improved a lot. Its average microhardness value is about three times higher than that of the substrate, and the wear rate is about1/12of the substrate. The wear mechanism of the cladding layer and the substrate are both a mixture type of adhesion and abrasion. The results of laser cladding in-situ cobalt-based composite coating reinforced with boron-chromium compounds show that:The in-situ synthesized CrB, CrB2, TiC, TiB2, Cr7C and WB reinforced phases of the cladding layer are mainly distributed in γ-Co with a little a-Ti solid solution; The acicular-streak structures precipitated from the lower part of cladding layer are CrB and CrB2phases. And the acicular-streak structures become more dense and thick with the increase of Cr content; At the same time, the microhardness and wear resistance of the cladding layer have been improved obviously. Its average microhardness value is about three times higher than that of titanium alloy, and the highest wear resistance is about thirty-six times more than that of the substrate; The Cr content of the cladding material should be appropriate.The results of laser cladding cobalt-based composite coating added with a little Si3N4powder show that:The main microstructures of the cladding layer are y-Co with a little a-Ti solid solution, and the reaction phases TiC, Ti(C,N), Co3Ti, NiC, Cr5Si3, β-Si3N4which disperse on the solid solution; Average microhardness value of the cladding layer is about three times higher than that of the substrate, and the highest wear resistance is about thirty-eight times more than that of the substrate; The Si3N4content of the cladding material should be appropriate, if adds too much, the cracking tendency of the cladding layer will be increased. And the multiple tracks overlapping laser cladding experiments were also studyed.