Design and development of solid state additive manufacturing techniques

The aim of this research project is the investigation of how cold spray, a process used to create metal coatings, can be applied to 3D structuring, and the development of a manufacturing process for the creation of bulk, high fidelity surfaces.

Introduction

With increasing levels of interest and investment in metallic additive manufacturing technologies around the world, the cold spray process offers some distinctive capabilities that, if capable of being progressed to a freeform manufacturing method, will offer it a unique placement within the additive manufacturing landscape.

The practicality of this development is being investigated and the best concepts progressed, focusing specifically on:

1. Investigation of competitive technologies and opportunities for an additive cold spray system
2. Concept generation and experimentation to determine the net-shape deposition method
3. CFD simulation of the process area
4. Visualisation of the process in action

Investigation

Competitive techniques in both traditional and additive manufacturing sectors have been studied to analyse the relative strengths and weaknesses of each system when considering a cold spray additive manufacturing system. The proposed system would offer unique benefits due to the low processing temperatures, providing it with a niche advantage in some fields.

Opportunities for this potential system have been considered and classed into three categories; direct manufacturing, remanufacturing and support operations. Specific markets within these categories have been explored and quantifiable case studies are being generated.

Experimental work

The first practical challenge to be addressed in the development of a cold spray additive manufacturing system is the creation of net-shape walls. A test platform has been constructed allowing for the testing of various flow-altering techniques, to modify the deposition profile and allow net-shape creation. Methods being considered include; hard or sacrificial backstops to spray against, profile shaping using 2D masks or stencils, and flow separation techniques. Initial tests have been conducted with hard backstops, with following trials to investigate the effects of continuous flow on the backstop and optimisation of the deposited single-track wall height.

CFD simulation

A fluid model is in development, using the ANSYS Fluent package, to increase the understanding of the flow structure around the potential substrate and backstop combinations. The purpose of the model is to inform process parameters and highlight any issues that may arise from the proposed experimental trials. This will prove important when introducing objects into the flow path at the nozzle exhaust, as these will affect the generated shockwave and thus the powder deflection and velocity during deposition.

Visualisation

Investigations are ongoing for a method of observing both the gas and powder streams during deposition. This would allow verification of the CFD model and inform the choices of process parameters, such as standoff distance, gas pressure and temperature. One potential option is to utilise Schlieren photography of the gas jet only, allowing the visualisation of shockwaves and fluid flow at the nozzle exhaust. Particle image velocimetry would allow for the measurement of powder speeds in the exhaust, but would not provide data on the flow patterns or speed of the gas.