Preparation Of NiAl/WC Composites By Thermal Explosion And Study On Their Properties

The NiAl intermetallic alloys have been recognized as the promising candidate for high temperature structural applications,due to its high melting point,low density,high thermal conductivity and excellent oxidation resistance at high temperature.However,the application of single-phase NiAl as a structural material is limited,because of its lack of ductility at room temperature and low strength at high temperature.WC ceramic is widely used in modern tools,corrosion,high-temperature materials because of its high hardness,high strength,and high wear resistance,low coefficient of thermal expansion,good oxidation resistance and corrosion resistance.Adding WC ceramic particles to NiAl alloy,the mechanical properties of NiAl alloys can be improved remarkably at room temperature and high temperature.In this paper,based on thermodynamic analysis of Ni-Al-WC system,NiAl/WC composites with different WC content were fabricated by thermal explosion and its mechanical properties and high temperature properties were investgated.In order to improve mechanical properties furtherly,Fe was added to NiA/WC composites and NiAl/WC-Fe composites were fabricated by thermal explosion.At last,NiAl/WC composite coating was produced on carbon steel by laser cladding and the microstructure of coating was investigated.The main research contents are as follows:The relation between adiabatic temperature(T_ad)and preheating temperature?To?,preheating temperature?T₀?and WC content?wt%?was analysized basing on thermodynamic principles.The results show that adiabatic temperature of the system decreases with the increase of WC content and the maximum of WC content in the SHS process is about 34%wt for preheating temperature of 973K.Adiabatic temperature rises with the increasing preheating temperature and properly increasing preheating temperature can ensure the reaction self-sustaining when WC content is excessive and adiabatic temperature is less than 1912K.In terms of the material preparation process,NiAl/WC composites with different content WC were fabricated by thermal explosion at a different heating rate,and the same composites were produced by thermal explosion with pressure of 30 MPa at 700?.The results show that there are still differences between two process.The appearance of NiAl/WC composites fabricated by thermal explosion without pressure has more holes,and the maximum WC content is about 20wt%.While NiAl/WC composites produced by thermal explosion at 700? has more dense microstructure,and WC content can reach 30wt%.The density of later NiAl/WC composites pressure is above 92%,so the fabrication process of NiAl based composite by thermal explosion assisted pressure offer a great potential technology.Moreover,bending strength and fracture toughness were investigated based on three-point bending principle.The results show that both bending strength and fracture toughness is improved significantly with addition of WC and Fe.SEM fractographic analysis of NiAl composites appear brittle fracture.Furthermore,microstructure and wear behavior of NiAl/WC composites at 800 ?were investigated in this paper.The results show that the friction mechanism of NiAl/WC composites is oxidation wear at 800 ?.With the increase of WC content,the friction coefficient of NiAl/WC composites tended to decrease after the rubbing period.With the increase of WC content,the wear rate of NiAl/WC composites decreased gradually.The amorphous WC ceramic particles obviously improved the wear resistance of NiAl/WC composites.In addition to,the high-temperature cyclic oxidation of NiAl/WC composites at 1000 ? was also investigated.The results show that the main components of the composites were ridge-like ?-Al₂O₃,a small amount of needle-like0-Al₂O₃,and NiO,and non-oxidized WC.The weight gain of composites in the initial stage of oxidation is obvious,and then the weight gain of the composites accords with the law of parabola in the course of time.The results also showed that the oxidation resistance of NiAl/WC composites decreased obviously with the increase of WC content,because the presence of WC destroyed the continuity of the Al₂O₃ film on the surface of the composites,resulting in cracks and holes,and the oxidation of grain boundaries occurred in the composites.Similarly,in the NiAl-WC-Fe material,with the increase of Fe content,the composites' high-temperature oxidation resistance also decreased.Laser claddding has a homogeneous composition which produced NiAl/WC coating;the coating is remelted and its microstructure is denser than the later due to the high energy laser beam.The NiAl/WC coating has the metallurgical bonding with the carbon steel.The results show that the particles in the coating with low WC content are transformed from original irregular shape to dendritic shape under the action of high-temperature laser energy,the particles in the coating with high WC content change little due to dilution.In addition to NiAl and WC,the FeAl was formed in the NiAl/WC coating with Fe added.The WC particles were distributed in FeAl after remelting.The NiAl/WC-Fe coating has no defects such as porosity and crack,and it has good metallurgical bonding with the carbon steel matrix.NiAl/WC composites with high density,good mechnicial properties and wear resistance in high temperature can be widely used in severe working environment.Because of simple process,high-density and low cost of product,the technology of thermal explosion assisted pressure for fabricating NiAl based composites has a potential application in industry.