Spray forming of thin walled net-shaped components of hard materials by high velocity oxy-fuel thermal spraying process
Maksud Helali, Md (1994) Spray forming of thin walled net-shaped components of hard materials by high velocity oxy-fuel thermal spraying process. PhD thesis, Dublin City University.
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Spray forming is a near-net shape fabrication process in which a spray of finely divided molten particles of metallic material is deposited onto a suitably shaped substrate to form a coherent solid. This technology offers unique opportunity for simplifying material processing by elimination of a number of unit operations and can be an alternative to conventional metal working technology for the production of certain type of components. The components thus formed may have some properties viz. hardness and wear resistance, surpassing those of their cast and wrought counter parts.
Cemented carbide belongs to a class of hard wear resistant refractory materials which are very difficult to process. This material can be used to make thin-walled inserts which can be utilised to improve the surface property of the engineering components.
A high velocity oxy-fuel thermal spraying system has been installed. This coating process has been employed to spray form thin walled near net shaped components. Materials used to fabricate components were tungsten carbide/cobalt, nickel chromium alloy and stainless steel. Special attention was given to determine the processing parameters of tungsten carbide/cobalt components.
Forming cores of different sizes, shapes were made from different materials. Materials in the form of powder were deposited by spraying with the HVOF thermal spray gun on the forming core surface after applying a releasing layer on the forming core. The releasing layer was so chosen that it facilitated deposition without fracture and after deposition it can be debonded easily from the forming core such that the deposited layer can be separated without fracture. Heating and cooling of the forming core-deposit assembly at different stages of spray forming process have profound effect on the success in obtaining the component without fracturing. Optimum ranges of values of the processing variables for different types of material were determined.
The effects of the processing parameters on the properties of the components were also investigated. The density of the spray formed components vary between 96- 99.5% of the theoretical values. The other properties of the components such as hardness, roughness and composition were also measured and compared with the standard values.
The components thus formed were found to be brittle. To improve the toughness of tungsten carbide/cobalt components, multi-layer components were fabricated. The toughness of the multi-layer component was found to be higher than the single layer tungsten carbide/cobalt, component. Components of materials with gradual change in composition were also fabricated to improve the toughness of tungsten carbide/cobalt components.
Depending upon the processing variables, the spray formed components contain residual stress. For tungsten carbide/cobalt component the amount of residual stress was measured and the effect of processing variables on the level of residual stress was investigated. An optimum condition was established to obtain stress free tungsten carbide/cobalt component.
These spray formed components were found to be suitable as replaceable insert in a nozzle or cylinder for low stress application. These replaceable inserts can be fitted with adhesives or can be shrink fit with then* counter parts. The cost analysis of fabricating tungsten carbide/cobalt component shows that spray forming might be a viable alternative route for the production of carbide components.
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