Combustion synthesis of Ni, Al and NiAl based composites by induction heating
Intermetallic NiAl has the potential to be used for elevated temperature applications. Self-propagating high-temperature synthesis (SHS) has been developed as a relatively simple route to obtain intermetallics. To date different ignition techniques have attempted to synthesize NiAI and produce coatings. Induction heating has been used to produce coatings and differs from conventional heating techniques in which the material is heated from the inside. This paper considers the use of induction heating to preheat and ignite the synthesis directly and investigates the effect of induction parameters on the phase transformation, microstructures and properties of Ni/ Al compacts synthesized by SHS. During synthesis the temperature profiles were measured with infrared thermometers and a high resolution thermal image camera to monitor the reaction process. Scanning electron microscopy (SEM), Energy Dispersive X-ray test (EDX) and X-ray Diffraction (XRD) were used to characterize products. The mechanical properties of the products were evaluated by measuring hardness. The results show that single phase NiAI can be produced by induction heating whilst processing parameters such as heating rates and green densities have a significant effect on the properties and structures of the sintered products. To further improve the mechanical properties and control the deformation of NiAI during combustion reaction caused by the formation of liquid, Al[sub]2O[sub]3 was used as an additive and dilution agent. The results show that single phase NiAI can be produced by this process regardless of the addition of Al[sub]2O[sub]3. However, the addition of Al[sub]2O[sub]3 is found to have a significant effect on heating rates, combustion behaviour and properties of the synthesized products. Additionally, there is a critical concentration for Al[sub]2O[sub]3 above which the compacts cannot be ignited by induction heating. Tests showed that the addition of Al[sub]2O[sub]3 can significantly improve the mechanical properties of NiAl. The synthesis of TiC and NiAl/TiC composites using induction heating via SHS process was also studied in this project. High density NiAl/TiC composites and two-layer TiC-NiAI structures were successfully produced using this process. The results show that the reaction was complete and that stoichiometric products of NiAI and TiC were produced. The properties of the NiAl/TiC composites were found to be functions of composition and processing parameters. The reaction mechanism was analyzed using temperature monitoring, thermodynamic analysis and microstructure investigation. A computer simulation using ANSYS was carried out to investigate the effects of processing parameters on the temperature distribution in induction heating. Experimental work has shown that the simulation results had a good agreement with experimental tests and the simulation can be applied to explain the heating behaviour during induction heating. The simulation was also used to investigate the solidification process to understand the cooling process during SHS.