NIAL-VC Laser Alloying On TC4 And Its Cavitation Resistance

In order to improve the fretting damage sensitivity and cavitation resistance of titanium alloy and its surface properties, the TC4 alloy used in long blade on large capacity steam turbine was selected as the substrate, and NiAl-VC was selected as adding alloying material, and the surface alloying processing was completed by laser alloying technology. The strengthened layer with no crack and pore, uniform and dense microstructure, a gradient decrease of microhardness and well wear property and cavitation resistance was obtained by optimizing several important parameters, Meanwhile, appropriate alloying processing parameters and the reason of excellent cavitation resistance were explored in the paper.The macroscopic morphology, microstructure, alloying element distribution, phase composition, the depth and width of alloying layer, hardness distribution, crack and blowhole condition with various laser power, laser scanning speed and alloy powder content were compared and analyzed respectively. The most appropriate alloying processing parameter and the mechanism of strengthening were discussed. Meanwhile, the wear property and cavitation resistance of strengthening layer were detected and the hardened mechanism was inferred.The experimental results show that in the condition of laser power from 1.8kW to 2.1kW, laser scanning speed from 180 mm/min to 300mm/min, the proporation of NiAl:VC of 50:50, the alloying layer with no pore, good surface condition, well depth, width and hardness could be obtained. However, with the increase of the weight proporation of NiAl, the hardness of the strengthened layer increases. And the amount of crack increases with the NiAl content over 70%. The strengthened phases in the alloying layer, such as TiC, Al2NiTi3C, NiTi, Ti2Ni and Ti3Al, precipitate from the liquid alloy during the cooling and solidification process. As the temperature gradient and the cooling speed varied, the top surface, the subsurface and the lower layer present significant different microstructures and phase distributions. The highest hardness and hardened depth come up to 995 HV0.1 and 770μm respectively in the alloying layer. The excellent wear property of the alloying layer due to the strengthened phases such as TiC, Al2NiTi3C and Ti3Al has greatly increased the surface hardness of the matrix material, which enhances the cavitation shockwave resistance to the surface. And its uniform distribution eases the stress of the strengthening phase effectively. The itermetallic compound NiTi with excellent resistance to cavitation performance, remarkably improves the material cavitation-resistant impact stress and deformation strain because of its good plasticity and fatigue strength.In addition, the refined grains with the result of the rapid heating and cooling during laser alloying increase the resistance ability of plastic deformation and hinder the crack growth effectively.