Study On In-situ Synthesis NiAl/Al Composite Coating By Laser Cladding

NiAl intermetallic compound had a low density, good thermal conductivity, high meltingpoint and good oxidation resistance at high temperature, etc. It was used to be compositematerial reinforcement. Therefore, it was widely used as high temperature structural materialand composite coating material in aerospace field.In this paper, the NiAl/Al in situ composite coating was prepared in the surface ofsubstrate material Q235by the means of laser cladding technology. Aording to DSC and XRDresults and combining with theory calculation, thermodynamics and kinetics of Ni-Al systemin situ reaction were studied. With the help of optical microsopy and XRD method,microstructure and phase composition of composite coating were analyzed, and the wear ofcomposite coating was characterized by the multi-functional tester and friction and weartesting machine. Conclusions were as follows.(1) According to analysising the thermodynamics and kinetics, the results showed thatNi-Al system can form the intermetallic compound, such as NiAl, Ni3Al, NiAl3and Ni2Al3.Basing on the calculation results of Gibbs free energyΔG θT, their thermodynamics stability,from low to high respectively, was NiAlâ†'Ni3Alâ†'NiAl3â†'Ni2Al3. When the value oftemperature was893K, in situ reaction generated NiAl3phase, with temperature increasing,NiAl3would spread to nickel-rich region and final product was NiAl and Ni2Al3when thevalue of temperature was1138K. The kinetics mechanism to in situ reaction of Ni-Al systemwas melting; diffusion; reaction and crystallization. Reactant composition would change thedirection of in situ reaction and reaction rate; reducing Al powder particle could improve insitu reaction rate; because of elevating temperature, atoms of Brownian motion strengthened,as a results, what increased possibility of in situ reaction.(2) From the fusion zone to the surface of the composite coating, composite coatingmicrostructure emerged gradually thinning phenomenon. Microstructure that was close tosubstrate showed cellular dendrite. With distance that was from the surface of compositecoating to substrate material increasing. The microstructure rapidly grew into equiaxedstructure. Growth patterns of microstructure were cellular dendrites-dendrites-exquiaxed.(3) With the increase of laser power and scanning speed, wear rate of composite coatinghad a minimum value that was1.2‰（power:500W, sanning rate:8mm/s, spot diameter:1mm）. Wear mechanism of composite coating surface was mainly adhesive wear and abrasivewear. In fusion zone of composite coating, where resulted in a crack, propagation direction ofcrack perpendicular to the direction of laser scanning speed and continuously extened. The reason why was that thermal expansion rate and contraction rate of composite coating andmatrix material were inconsistent, then residual stress generated. When the ultimate strengthof the composite coating was less than the residual stress, it would have a crack.Microhardness of composite coating distributed unevenly. Microhardness of compositecoating where surface was along the direction of fusion zone firstly increased and thendecreased.