Gravity and Centrifugal Casting of Light Metal Alloys using rapidly produced sand moulds
Traditional sand casting is a well understood method to produce metal shapes and has been used for many years in industry. It is a relatively simple method but has a significant drawback, with the requirement of a pattern to form the internal cavity. Patterns are produced at high cost through Computer Numerical Controlled machining or wood pattern making with significantly high lead times. Rapid Prototyping is seen as a solution to this problem, with the ability to produce sand moulds directly from Computer Aided Design platforms and thus eliminate the requirement of a pattern. Through layered manufacturing, the sand mould can be produced with complex internal geometry directly, minimising labour costs and involving short waiting times. While initial research was mainly concerned with the use of Selective Laser Sintering, with the advent of 3D printing, pattern-less sand moulds can be produced more easily and cheaply. With the process gaining more and more popularity, there was a need to scientifically assess the suitability of the process for sand casting as well as, establish influences of typical process parameters on significant responses. Critical mould properties, such as permeability and compressive strength, were investigated with respect to varying time and temperature of baking. To this end, mathematical models of permeability and compressive strength were developed. Also, the influence of mould material, mould coating, alloy type and pouring temperatures were investigated in static sand casting of light metals. Further work utilised the centrifugal casting process using these 3D printed moulds to establish links between process factors, such as rotational speed and cast strength using light metals. Compressive strength results for the rapidly produced materials were acceptable compared to traditional values. Permeability was however lower than commonly used foundry sand. Results showed, nevertheless, that permeability and compressive strength were both improved by baking times and temperatures. Significant model effects were established for ZP131 and ZCast501 with respect to increased compressive strength and mould permeability. Multi-factorial experiments involving simultaneous variation of factors such as mould materials, surface coatings, alloys and pouring temperatures were conducted and static casting results in general show good as-cast mechanical properties with the factors having significant effects on surface roughness, percent elongation and hardness. Centrifugal casting of aluminium alloys initially produced below average tensile properties, due to the large presence of hydrogen porosity. However, upon degassing, much improved tensile strengths were obtained, being superior to both static casting and traditionally sand cast aluminium. Also a Magnesium alloy was successfully trialled with the centrifugal process using 3D printed moulds in spite of numerous practical difficulties. Substantial data relating to the process factors for mould materials and casting processes was produced. Analysis of factor influences facilitated optimum process configurations for the production of moulds and castings. These combinations of factors at optimum levels comprehensively showed that light metals such as aluminium and magnesium alloys could be successfully processed by rapidly produced moulds, both statically and centrifugally.