Improvements engineered in UTS and elongation of aluminum alloy high pressure die castings through the alteration of runner geometry and plunger velocity

摘要

Improvements in ultimate tensile strength (UTS) and elongation were engineered in as-cast tensile specimens made using cold chamber high pressure die casting (HPDC) by increasing the melt flow velocity in the runner. Such improvements were achieved irrespective of casting size, runner geometry and alloy condition (virgin vs. recycled) as long as melt flowed faster. Round tensile specimens were cast using Australian alloy CA313 (an A380 equivalent). Two sets of geometries were used for the tensile castings-with the first satisfying dimensions stipulated by the ASTM standard for HPDC test specimens and the other the ASTM standard for permanent mold casting (PMC) test specimens. In the HPDC set of casting trials, melt velocities were increased by using a higher plunger velocity. By contrast, in the PMC set of castings, the melt was accelerated by introducing a constriction within the runner, upstream of the gates. The reasons for the higher properties are proposed to be a result of the refinement and more homogeneous dispersion of externally solidified crystals and defect-forming suspensions such as cold flakes, oxides and gas bubbles that flow along with the molten alloy and their more homogeneous dispersion. It is suggested that the fragmentation of the suspensions is caused both by the greater rates of shear achieved in the melt, particularly when it flowed through a constriction, and by the viscous dissipation of turbulent energy. The increased dispersion of these suspensions is proposed to be caused by the higher levels of turbulence. The above suggestions are supported with calculations performed using computational fluid dynamics (CFD) simulations which reconstructed some of the physical experiments involving the PMC set. Importantly, CFD modeling indicates that the tensile piece castings were well vented due to the fill patterns achieved, indicating the results may be generalized for such situations where sufficient venting is available including when vacuum assisted venting is employed. The 0.2% proof strength of the alloy was the least affected by the engineered change introduced here, as this property is more dependent on the bulk microstructure including the α-Al grain size which is determined predominantly by cooling rates.