The mechanism for eliminating hot crack in slender, complex thin-wall Al–Cu alloy casting by Backdown-Flowing and Integrated Casting-Forging (BFICF) process

This study develops an innovative Backdown-Flowing and Integrated Casting-Forging(BFICF) process to resolve severe hot cracking issues of Al-Cu Alloy under squeeze casting. The deformability and backdown space relieves stress, facilitates more effective feeding, and more important is fragmenting primary grains and make them flowing, leading to numerous heterogeneous nuclei. Furthermore, the strong forging forces significantly suppresses solute diffusion and undercooling in un-solidification regions, thereby refining the grain size and altering grain growth orientation. Compared to squeeze castings, the grain size and secondary dendrite arm spacing reduces from 102±7.5 μm and 21±7 μm to 42±2.5 μm and 0 respectively, with morphology transitioned from coarse dendrite to fine spherical grains structures. Also, increases area fraction of secondary phases from 4.22% to 7.36%, make it a broader continuous network structure that effectively compensated for shrinkage stresses and crack initiation zones during the final stage of solidification, consequently, eliminating the severe hot cracks. Due to serious defects under squeeze casting,the UTS is only 11±3.6 MPa. In contrast,the UTS,YS reach 402±12 MPa, 354±8 MPa after defects eliminating under BFICF, also improve 58.1% and 68.3% respectively compare to A356.0 alloy of 254.3±10.1 MPa, 210.3±5.8 MPa with EL on the same level under BFICF.