CDS: Controlled Diffusion Solidification - A novel casting approach

Wrought Al-based alloys such as the 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, and 7xxx are used extensively in the aerospace and automotive industries due to their high tensile strength and ductility compared to traditional Al-Si casting alloys. However, one of the biggest problems encountered during casting of these alloys is the formation of hot cracks or tears during solidification. Alloys with larger solidification ranges, higher eutectic liquid contents, and larger as-cast dendritic grain size are more prone to hot tearing than others. Controlled Diffusion Solidification (CDS) circumvents this problem and allows sound castings of wrought alloys with globular/cellular (non-dendritic) microstructures. In the CDS process, solidification of the alloy takes place by mixing two different liquid alloys with controlled mass and heat flow in order to achieve a predetermined alloy chemistry that solidifies with a non-dendritic microstructure without hot tears. CDS casting of 2014, 5056, and 7050 alloys is presented to illustrate the CDS process. The primary aluminum phase in all these castings is cellular and/or globular and the enhanced interconnectivity of the final solidifying liquid around the primary α-Al mitigates hot tearing. The CDS method can be extended to successfully rheocast both Al-based wrought alloys and casting alloys to attain the characteristic semi-solid microstructure.