Reducing Macroscopic Defects at the Aluminum/Steel Interface in Bimetal Castings Made by the Cummins Process

Cast-on methods are the most cost-effective methods to reinforce aluminum parts with steel inserts but are prone to defect formation at the aluminum/steel interface. A Cummins process was developed for exploring ways to improve the bond quality. An initial trial was made to reveal macroscopic defects. Modeling was then performed to understand the thermal and fluid flow conditions in the casting. Based on modeling results, experiments were designed to reduce macroscopic defects at the interface. It was found that the bond quality was improved by modifying the shape of the inserts, reducing the size of insert and the number of insert per casting, and increasing the size of the riser. Mechanisms by which defects are formed using the Cummins process were examined. Mechanisms by which metallurgical bonding develop on the dual-layer-coated insert in a bimetal were proposed. It is suggested that high-quality bond can be produced in a bimetal casting when the surface temperatures of the insert are higher than the dendrite coherent temperature of the alloy for duration longer than a critical duration and when fluid flow is stronger enough to wash away bubbles and oxides that adhere on the surfaces of the insert.