Reduction of Hot Tears: Alloy and Casting Process Optimization Using Neutron Diffraction

摘要

The continued need of vehicle weight reduction provides impetus for research into the development of novel automotive casting alloys and their processing technologies. Where possible, ferrous components are being replaced by aluminum (Al) and magnesium (Mg) alloy counterparts. This transition, however, requires a systematic optimization of the alloys and their manufacturing processes to enable production of defect-free castings. In this context, prevention of hot tears remains a challenge for Al and Mg alloy thin-wall castings. Hot tears form in semi-solid alloy subjected to localized tensile stress. Classical methods of stress measurement present numerous experimental limitations. In this research, neutron diffraction (ND) was used as a novel tool to obtain stress maps of castings and to quantify the effect of two processes used to eliminate hot tears in permanent mold castings: 1) increasing of the mold temperature during casting of Mg alloys, and 2) grain refinement of Al alloys. In case of the AZ91D Mg alloy, the ND results indicate that increasing the mold temperature effectively decreased the level of tensile strain in a casting region with a geometric stress concentration, thereby preventing nucleation of hot tears. In the case of the B206 Al alloy, hot tearing was alleviated by the addition of 0.05 wt% Ti-B grain refiner and the ND mapping revealed a decrease in the stress level along with enhanced stress uniformity across the casting.