Effect Of B₄C On Microstructure And Properties Of The Laser-clad Coatings

In this paper, to solve the problems such as the low microhardness and poor wear resistance on Ti alloy, the effects of different contents of B₄C（0 wt.%, 5 wt.%, 15 wt.%, 25 wt.%） on the evolution of the microstructure and properties of laser-clad coating was studied, and even the suitable content of B₄C in the cladding materials was investigated. Based on this, in order to understand the effects of different heat-treatment time on microstructure and properties of laser-clad coating, composite coatings with a suitable content of B₄C（10 wt.%） were produced by laser clad, and then they were taken a heat-treatment at 600 oC with different heat-treatment time（0 h, 2 h, 4 h and 6 h）.The results of the experiments with different contents of B₄C showed that the coatings were mainly composed of TiNi/Ti₂Ni and TiC/TiB₂ as the matrix and reinforcement particles, respectively. The phase constituents of the coatings were not influenced by addition of different contents of B₄C. The microstructure of the reinforcements in the coatings presented the following evolution: hypereutectic consisting of blocky（TiC+TiB₂）e eutectic and primary Ti Cp cellular dendrites（0 wt.% B₄C）, mixture of hypereutectic and willow-shaped（TiB₂+TiC）p pseudoeutectic（5 wt.% B₄C）, and pseudoeutectic（15 and 25 wt.% B₄C）. With increasing B₄C content, the volume fraction and size of the pseudoeutectic structures as well as the average microhardness of the coatings（850, 889, 969, and 1002 HV0.2） were increased. By contrast, the average fracture toughness of the coatings was gradually decreased（4.47, 4.21, 4.06, and 3.85 Mpa×m1/2） along with their wear volumes（0 wt.%, 5 wt.%, and 15 wt.% B₄C）. The increase in B₄C content to 25 wt.% did not further reduce wear loss. The wear mechanism transformed from micro-cutting（0 wt.% B₄C） into a combination of micro-cutting and brittle debonding（5 wt.% B₄C） and finally led to brittle debonding（15 wt.% and 25 wt.% B₄C）. Coatings with suitable contents of B₄C（less than 15 wt.%） showed excellent comprehensive mechanical properties.The results of the experiments with different heat-treatment time illustrated that the coating without the heat treatment was mainly composed of Ti Ni and Ti₂Ni as the matrix, and TiB₂ and TiC as the reinforcements. TiB₂ and TiC belonged to the eutectic microstructure in which a large number of fine columnar/equaxied TiC particles were uniformly distributed within the coarse willow-shaped TiB₂ particles. The eutectoid structure consisting Ti₂Ni and TiNi₃ were precipitated within the initial TiNi grains by the eutectoid reaction Ti Ni?Ti₂Ni+TiNi₃. With the extension in heat treatment time, more eutectoid structure was formed. With the increase in heat treatment time, the average microhardness was decreased: 941.3 HV0.2（0 h） 933.2 HV0.2（2 h）, 903.8 HV0.2（4 h）, 890.4 HV0.2（6 h）, the average fracture toughness presented an opposite trend: 4.18 Mpa×m1/2（0 h）, 4.26 Mpa×m1/2（2 h）, 4.37 Mpa×m1/2（4 h）, 4.9 Mpa×m1/2（6 h）.