Effects of rare earth composite refiner on the as-cast microstructure and mechanical properties of A356 aluminum alloy under different cooling rates

To investigate the coupling effects of Al-5Ti-1B-xCe (x = 0, 1, 5, and 10 wt.%) refiners and cooling rate on A356 aluminum alloy, wedge-shaped castings with different thicknesses were prepared to obtain various cooling rates. The effects of refiner composition and cooling rate on microstructural evolution and mechanical properties were systematically studied. Compared with conventional Al-5Ti-1B, rare-earth composite refiners more effectively improved the microstructure and mechanical properties of A356 alloy. The Ce content strongly influenced the microstructure of the refiner and its subsequent refinement efficiency. With increasing Ce content, the refinement effect was gradually enhanced, and the finest secondary dendrite arm spacing (SDAS) was obtained using the Al-5Ti-1B-10Ce refiner. Increasing cooling rate directly refined the alloy microstructure and also acted synergistically with the composite refiner, further improving SDAS refinement. As a result, the mechanical properties were significantly enhanced. The alloy refined by Al-5Ti-1B-10Ce at a cooling rate of 22.1 °C/s exhibited the best overall properties, with an ultimate tensile strength of 229.6 MPa, yield strength of 98.4 MPa, and elongation of 12.0%. In-situ tensile observations revealed that microcracks preferentially initiated at second-phase particles, such as Si and Mg 2 Si phases, phase boundaries, and grain boundaries. Crack propagation mainly occurred along second phases and grain boundaries, with limited transgranular propagation, eventually leading to fracture. Plastic deformation near the crack tip was mainly governed by single-slip and double-slip mechanisms.