Silent, creeping strain and stress change and distort the casting, and may intensify to the The casting also suffers, being stretched as on a rack. The mould suffers, and may crush and crack. The solidified casting strives to contract while being resisted by the mould. The subsequent cooling to room temperature is no less dramatic. The surface sinks are halted, but the internal defects now start. This will continue until the casting is solid, or unless the increasing stress is suddenly dispelled by an explosive expansion of a gas or vapour giving birth to a shrinkage cavity. In those regions cut off from the flow, continuing contraction causes the pressure in the residual liquid to fall, possibly becoming negative (as a tensile stress in the liquid) and sucking in the solid surface of the casting. During freezing, liquid surges through the dendrite forest to feed the volume contraction on solidification, washing off branches, cutting flow paths, and polluting regions with excess solute, forming segregates. ![]() The mould shocks with the vicious blast of heat, buckling and distending, fizzing with the volcanic release of vapours that flood through the liquid metal by diffusion, or reach pressures to burst the liquid surface as bubbles. The surging and tumbling flow of the melt through the running system can introduce clouds of bubbles and Sargasso seas of oxide film. The liquid metal attacks and is attacked by its environment, exchanging alloys, impurities, and gas. But sense the excitement of this new arrival: The first moments of creation of the new casting are an explosion of interacting events the release of quantities of thermal and chemical energy trigger a sequence of cataclysms.
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