Influence of Piston Lubricant on the Distribution of Defects in Cold Chamber High Pressure Die Casting

Abstract

In the cold chamber high pressure die casting process (CC-HPDC) for light alloys, the piston lubricants play a key role in protecting the piston tip from wearing and ensure adequate seal with the shot sleeve. However, during the production process, the pouring of overheated aluminum alloy melt into the shot sleeve would lead to evaporation and burning of the lubricants once in contact with the piston tip. The burning products, however, would form gas and non-metallic inclusions in the melt which would be transported and injected into the die area and finally trapped in the castings, all of which would affect the mechanical properties of the as-cast samples and deteriorate the product quality. To further investigate this issue, a pilot scale HPDC machine is used and the lubricant burning issue is studied based on material characterization and numerical modelling. The chemical composition, size, and morphology of the burned products are observed using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). In order to better explore the issue of lubricant combustion discovered in the experiment, a finite element model describing the entire HPDC process is established and the burning, motion, and trapping of the lubricant are calculated. The final distribution of the burned products such as gas and non-metallic inclusions are predicted and their influence on final solidification quality of the as-cast products under various process parameters are analyzed qualitatively. Finally, a slow shot velocity range of 0.4–0.6 m/s and an acceleration profile that ramps up to 0.3 m/s over 0–370 mm of the shot sleeve proved to be the most effective in reducing air entrainment and oxide inclusions to alleviate the burning of lubricant on final product quality.