In recent years there has been a growing demand to produce lightweight high pressure die cast (HPDC) parts for structural applications to decrease vehicle mass and to reduce manufacturing costs. Due to the coupled rapid heat flow and complex flow/deformation that occur in the process, the formation of microstructure and defects in HPDC are still not fully understood. Developing a better understanding of microstructure formation is essential to enable advances in die design and process optimisation, as well as alloy development, to improve the quality and productivity of HPDC components. Therefore, this thesis aims to enhance this understanding by conducting detailed microstructural analysis of samples produced in controlled HPDC experiments. In the first series of experiments, various microstructure characterisation techniques were used to study salient HPDC microstructural features. The microstructures of castings were characterised at different length scales, from the scale of the casting to the scale of the eutectic interlamellar spacing. The results show that the salient as-cast microstructural features, e.g. externally solidified crystals (ESCs), defect bands, surface layer, grain size distribution, porosity and hot tears were similar for both two HPDC-specific Al alloys used, AlSi4MgMn and AlMg5Si2Mn. The formation of these features has been explained by considering the influence of flow and solidification during each stage of the HPDC process. The formation of defect bands is further studied by investigating the ratio between band thickness ( ) and average grain size in the band ( ). Suitable methods for measuring w and dsb in HPDC have been developed. The w/dsb relationship of defect bands has been investigated in HPDC specimens from a range of alloys, casting geometries and band locations within castings. The bands were measured to be 7-18 mean grains wide. This is substantial evidence that defect bands form due to strain localisation in partially solidified alloys during cold-chamber and hot-chamber HPDC. At the end of solidification, dilatant shear bands contain a higher eutectic volume fraction and/or porosity content than adjacent material. In the cross-section of the AM50 Mg alloy, the centrally-located band contains a much higher volume fraction of concentrated porosity than the second-outermost band and insignificant porosity was found in the outermost band. The level of porosity in bands was attributed to the relative difficulty of feeding shrinkage for each band location. As the feeding of material during the intensification stage is important for the reduction of porosity, the influence of intensification pressure (IP) and gate thickness on the transport of material through the gate during the latter stages of HPDC were investigated. Microstructural characterisation of the gate region indicated a marked change in feeding mechanism with increasing IP and gate size. Castings produced with a high IP and/or thick gate contained a relatively low fraction of total porosity and shear band-like features existed through the gate, suggesting that semi-solid strain localisation in the gate is involved in feeding during the pressure intensification stage. When a low IP is combined with a thin gate, no shear band was observed in the gate and feeding was less effective, resulting in a higher level of porosity in the HPDC component. As equiaxed primary crystals are subjected to intense shear during HPDC, their agglomeration and bending behaviour were investigated in the last series of experiment. Samples produced by near-static cooling, HPDC and Thixomoulding®, where the solidifying crystals experience different levels of mechanical stresses, were characterised. The electron backscatter diffraction (EBSD) technique was used to acquire grain misorientation data which is linked to the crystal agglomeration and bending behaviour during solidification. The number fraction of low-energy grain boundaries in HPDC and Thixomoulded samples was substantially higher than in ‘statically cooled’ samples. This is attributed to the much higher shear stresses and pressure applied on the solidifying alloy in HPDC and Thixomoulding, which promote crystal collisions and agglomeration. In-grain misorientations were found to be significant only in branched dendritic crystals which were subjected to significant shear stresses. This is related to the increased bending moment acting on long protruding dendrite arms compared to more compact crystal morphologies.
| Identifer | oai:union.ndltd.org:ADTP/279207 |
| Creators | Somboon Otarawanna |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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