Effect of aluminum alloy fluidity on formation of localized surface stains induced by die lubricant residue during HPDC

This study aims to reveal the formation mechanism of a newly discovered surface defect, namely localized surface stains induced by die lubricant residue, during high-pressure die casting (HPDC). Three commercial aluminum alloys (ADC12, A360, and AlSi10MnMg) were subjected to HPDC experiments, and a die lubricant thermal shock test was designed to simulate the thermal decomposition behavior of die lubricant during HPDC. The results revealed that among the three alloys, only the ADC12 alloy castings exhibited distinct localized surface stains after sand blasting and passivation treatment. These surface stain defects penetrated to a depth of 60-120μm beneath the casting surface and were identified as a reticular mechanical mixture composed of SiO 2 particles derived from the thermal decomposition of polysiloxane-based die lubricant and the alloy matrix. Thermodynamic calculations demonstrated that the ADC12 alloy possessed the greatest superheat and the narrowest solidification interval, exhibiting the highest fluidity compared to A360 and AlSi10MnMg alloys. During the HPDC process, the molten ADC12 alloy with better fluidity more readily infiltrated the interstices of the coral-like aggregates formed by SiO 2 particles generated from the thermal decomposition of die lubricant, thereby forming this surface stain defect. The results indicate that alloy fluidity is a critical factor influencing the formation of this surface stain defect induced by die lubricant residue, and reducing fluidity can effectively suppress the formation of such defects.