Research On The Properties Of NiAl Coating Synthesized By HFIH/SHS

In this paper,NiAl coatings were prepared on the 5CrNiMo steel with Ni transition layer by high-frequency induction heating/self-propagating high-temperature synthesis?HFIH/SHS?to improve the service life of die-casting die.Standard Gibbs free energy change and adiabatic temperature of NiAl were calculated,and the preparation process of NiAl coating was optimized.The microstructure and phase composition of the coating were analyzed.The formation mechanism of the coating and joint interface were studied.The corrosion resistance of NiAl coating in molten aluminum alloy was studied by the immersion test.The structure of the diffusion-dissolution layer and the relationship between the thickness of the diffusion-dissolution layer and the time were ascertained.The formation mechanism of the diffusion-dissolution layer was analyzed.Finally,the corrosion resistance of NiAl coating and 5CrNiMo steel in molten aluminum alloy was compared theoretically.The main conclusions are as follows:?1?.Standard Gibbs free energy chang of NiAl is less than 0.At the preheating temperature of 298 K⁷00 K,the adiabatic temperature of NiAl is 1912 K,which reaches the melting point of NiAl.The coating synthesis process is non-full liquid phase synthesis.?2?.Dense and well-bonded NiAl coatings can be obtained at a connection pressure of 30 MPa,a preform pressure of 100 MPa,an induction heating simulation current of250 A,a holding time of 60 s,and a dwell time of 120 s.?3?.In addition to a few holes,the surface of the coating is smooth and flat,and the structure is compact.The composition is single phase NiAl.The microstructure of the coating cross-section interface is:NiAl/Ni₃Al/Ni/substrate metal,and the interfacial indentation is complete and crack-free,indicating that the coating is well bonded to the substrate.The formation mechanism of NiAl belongs to the dissolution-precipitation mechanism,and the formation of coating pores is mainly determined by the physical mechanism.?4?.The formation mechanism of the diffusion-dissolution layer of NiAl/Al corrosion interface conforms to the dissolution-precipitation mechanism.Ni₂Al₃ is the first formation phase.In a certain period of time,the diffusion-dissolution layer structure at the NiAl/Al corrosion interface is NiAl₃/Ni₂Al₃,As the test time increases,diffusion-dissolution layer structure is a single-phase NiAl₃ layer,and NiAl₃ is formed during the cooling process.The thickness of Ni₂Al₃ increases with time in accordance with the parabolic law.Theoretically,the corrosion resistance of NiAl coating is better than that of 5CrNiMo steel.