Optimisation of the HVOF thermal spray process for coating, forming and repair of components

The High Velocity Oxy-Fuel (HVOF) Thermal Spraying technique has been widely adopted in many industries due to its flexibility, and cost effectiveness in producing superior quality of coating. The demand of high-technology industries and the availability of new advanced materials have generated major advances in this field. The HVOF thermal spray process has been utilised in many industries to apply coatings on components to protect against wear, heat and corrosion, and also to build up worn components. This spraying technology is not limited to coating substrates but also encompasses the manufacture of net shaped component from materials which are sometimes difficult to form by conventional methods. A knowledge of coating properties, testing and evaluation methods is essential in order to apply coating technology to a specific application. While spraying parameters and substrate surface preparations directly impact the coating properties, it is equally important to know the spraying technique required to deposit coating having these properties and the processing parameters which have to be applied. The thesis reports the development and optimisation of the HVOF thermal spray process for coating, forming and repair of components. A die was designed to manufacture free standing WC-Co inserts, and a similar technique was then followed to fabricate free standing annular rings and solid discs. The effects of spraying parameters on the components properties such as residual stresses and hardness were investigated and limitations identified. Experiments to assess the coatings properties involved the combinations of three spraying powders, (1) Austenitic stainless steel (2) WC-Co and (3) Tool steel match powder on stainless steel 316L andD2 tool steel substrates. Investigations were carried out on the effect of spraying distance, sprayed coating thickness and pre-spray heat treatment on coating properties including hardness, bond strength and residual stress. Results reveal that there are strong correlations between the bonding strength, coating thickness and residual stress in coatings. The tensile residual stresses coupled with increasing coating thickness cause the degradation of bond strength with increasing coating thickness. Optimisation of the repair of damaged components using the HVOF technique involved the use of similar combinations of powder and substrate materials. Tests were carried out to identify the adhesion strength of the repaired material sprayed under various conditions which were varied, including (1) repair thickness (2) prerepair and post-repair heat treatment (3) repair wall angle and (4) substrate surface preparation. In addition, the finish machining possibility of these repaired components was evaluated