Finding the edge: Net-shape surface sculpting during cold spray deposition
This thesis provides an overview of how a successful method for structuring cold spray deposition was developed, allowing the creation of defined surfaces from a previously uncontrolled deposition profile. When considering both additive and traditional manufacturing techniques, there remains a gap in the market for a high build rate, low-cost manufacturing system, capable of building net shape structures with good material properties from a range of difficult to work with engineering materials. A cold spray system had the potential to meet these requirements, and to provide additional benefits from being a solid-state fusion process, but the technique lacked the structural control capabilities required. The aim of this body of work was to develop methods for controlling the shape of depositing material, allowing the creation of three-dimensional structures, and determining an approach which would allow the creation of flexible manufacturing platform. A limited number of attempts had been previously made to control the deposition shape. These methods met with limited success, did not offer real control over the shape of the deposit during operation, and presented issues of accuracy, reliability, and repeatability. In this work, a series of concepts for shaping the deposition of the material were tested for the creation of flat vertical surfaces. Copper was used as the deposition powder as it readily deposits with cold spray under easily manageable conditions. The samples were investigated for shape conformity, surface roughness, porosity and build height, using optical microscopy and a white light interferometer. Successful shaping was delivered using masks, wide flow impeding backstops and thinner flow separating tools, provided the non-adhering powder had sufficient room to be cleared from the deposition zone. The thinner tool was further developed, as it allowed better positioning in smaller spaces for future systems. Computational fluid dynamics models were created to assist with the understanding of some inconsistencies in deposit quality. The results of these simulations showed minimal alteration to the particle trajectory was caused by the alteration of gas dynamics from the introduction of obstacles. The developed thin tool deposition concept was then successfully tested for robustness with deposition of further materials, and with the inclusion of laser irradiation of the substrate. It was demonstrated that the density, deposition efficiency and build heights are comparable with those expected from typical cold spray/supersonic laser deposition deposits. Following this, a range of building block structures were created, to further advance the shaping capabilities of the system, and demonstrate the freedom of deposition profile. Flat surfaces, thin walls, corners, curves, rings and overhangs were all shown to build efficiently without further complication. It is concluded that it is both possible to control the shape of the depositing material during cold spray, and possible to do so without adversely affecting the deposit characteristics that give cold spray manufacturing its specific advantages over other manufacturing methods. The next steps for this process are to create a more flexible system, automating the placement of the shaping tool and using a 5 or 6 axis bed and nozzle positioning setup. Further to that, precise control over the powder dosage, and the development of a known parameter space for select materials would progress the system to an additive capable platform.