Effect of Cooling Rate on the Microstructure and Mechanical Properties of Al-5Cu-0.38Mg-0.32Mn-0.1Ag Alloy Cylinder Heads

For complex structural castings, the microstructure and mechanical properties in different parts of the castings are affected by the structure, casting process, and the temperature change during the solidification process. In this study, the microstructure, filling process, and mechanical properties of Al-5Cu-0.38Mg-0.32Mn-0.1Ag alloy cylinder heads prepared by the sand gravity casting method were systematically investigated by optical microscopy (OM), scanning electron microscopy (SEM), electronic universal testing, and MAGMASOFT5.4. The cooling curves at typical positions of the cylinder heads were measured. Although the cooling rate was different for the cylinder heads in different positions, equiaxed grains could be found. Fine equiaxed grains with a size of 62 μ m were found at the cylinder head fire surface, while coarse equiaxed grains with size of 116 μ m could be observed at the cylinder head intake airway side wall. The yield strength and tensile strength of the cylinder heads at different positions were 140–159 MPa, and 175–202 MPa, respectively, and their elongation percentage was 2–4%. The cylinder head fire surface has the maximum yield strength of 159 MPa. In the process of tensile fracture of the as-cast alloy, micro-cracks were first generated along the eutectic structure at the grain boundary, and then these microcracks grew, aggregated with each other, and propagated along the grain boundary, resulting in the final fracture failure of the material.