Bonding Bare Steel Insert to A354 Sand Casting Using Ultrasound-Assisted Cast-on Methods

A conventional cast-on (CCO) method requires costly procedures for steel insert and molten metal processing in order to produce bimetal sand castings of acceptable bonding quality. We explored the feasibility of using ultrasound-assisted cast-on methods for producing bimetal sand castings using as-received bare steel inserts and an un-degassed molten A354 alloy. The ultrasound-assisted methods included a direct ultrasound-assisted cast-on (DUACO) method and an indirect ultrasound-assisted cast-on (IUACO) method. Mathematical calculations were carried out to understand the conditions under which metallurgical bonding could occur using CCO in a sand casting process. Results of this study showed that the CCO method was unable to metallurgically bond as-received bare steel inserts to A354 alloy under the conditions of this study. The IUACO method was capable of producing metallurgical bonding between as-received inserts and un-degassed A354 alloy with bonding quality identical to that obtainable using coated inserts following stringent procedures for insert and molten metal processing. The DUACO method was much more effective than the IUACO method in producing high-quality metallurgical bonding between bare steel insert and un-degassed A354 alloy. Microstructural analysis indicated that a continuous layer of intermetallic phases was formed at the steel/aluminum interface in castings produced by DUACO method. Push-out test revealed that the shear strength of the bond produced by the DUACO method approached around 80 MPa. Mechanisms under which defect-free metallurgical bonding was formed were discussed.