Developing an effective die cooling technique
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In permanent mold casting, die design for cast aluminium alloy and magnesium alloy products includes a number of high conductivity material cooling blocks (also called channels or cooling circuits) that are aimed to extract heat away from molten metal through direct conduction heat transfer and freeze the casting as quickly as possible in a directional manner. One of the biggest problems during this solidification process occurs when the molten metal naturally shrinks away from the mould as it solidifies. This makes it increasingly difficult to efficiently and effectively cool targeted areas in the casting through conduction, since the direct contact between the solidifying casting and the cooling block is significantly reduced or even lost. A typical cooling block (termed in this thesis as a “chill”) is a cooling circuit that is embedded in a permanent mold (or die) and positioned to enable high heat transfer (effective cooling) to a targeted large section in the casting. If a large volume section in a casting does not cool efficiently and in the correct sequence in the overall product (i.e. solidification first in the furthest part from the sprue inlet followed by successive and ordered solidification towards the sprue inlet, until finally the sprue inlet itself), then it will create a “hot spot” which will create macro-shrinkage in the casting. This can create millions of dollars of waste in terms of casting rejects, lost productivity, and reworks for a given manufacturing company. When the molten metal solidifies, it shrinks by about 6.6 % for aluminium alloys and 4.0 % for magnesium alloys. This creates an air gap at the casting and mold interface. This air gap causes inefficient, random and isolated pockets of heat transfer from the casting to a contacting chill, which in turn causes a significant variation in the temperature distribution in the casting and die during solidification. A die operating in an incorrect and unstable temperature band will very likely produce adverse secondary effects in the final product such as macro shrinkage, micro shrinkage, hot tearing, gas porosity, or even misruns. This aim of this study is to theoretically understand and experimentally develop a cooling technique that can offset or close up the growing air gap and maintain high heat transfer between the casting and contacting chill, by ensuring that the chill is pushed closer into the casting at specific times during the solidification (and shrinking) process. A movable copper chill was designed and built to push forward into an insulated mold. The experiments were carried out using commercially available A356 aluminium alloy. The chill was pushed into the casting as it solidified in the mold. Studies were carried out to understand the effect of a movable chill with different cooling conditions compared to a fixed chill. Numerical simulations were conducted using developed boundary conditions in a commercial casting solidification package(ProCASTTM). The boundary condition used to emulate the air gap is a temporally distributed interfacial heat transfer coefficient function between the casting and chill and this is manually calculated using inverse modelling in an in-house developed optimisation package (OPTCASTTM) to compare and validate with experimental data. The resulting sensitivities of the casting due to different chill conditions (i.e. fixed vs. moving) are described through physical phenomenon, metallographic analysis and computational modelling. Results show that the effective cooling can be increased by 39.2 % by using movable chill with cooling compared to fixed chill with cooling. The percentage of complete contact between the casting and chill has been increased from 10 % in case of fixed chill with cooling to 76% in case of movable chill with cooling. Apart from effectiveness of cooling, the quality of casting produced with new cooling technique has significantly improved. The secondary dendrite arm spacing (SDAS) of the casting produced under the movable chill have be reduced by 26 % compared to fixed chill.