Factors influencing thermal conductivity and mechanical properties in 319 Al alloy cylinder heads

Aluminium alloys have gained popularity for automotive applications, due to their high strength-to-weight ratio and thermal conductivity. In particular, cylinder heads are commonly produced from Al-Si-Cu alloys, given their age-hardenability and excellent castability. However, operating temperatures can induce thermal stresses in the heads, which can cause failure if the mechanical and thermal properties are insufficient. This research is a pioneering attempt at presenting several relationships between microstructure, mechanical properties, and thermal conductivity via the characterization of commercial 319 Al alloy cylinder heads. Optical microscopy, SEM-EDX, XRD, porosity measurements, tensile testing, Rockwell hardness testing, and thermal conductivity measurements using the transient plane source method were conducted at several critical head locations after thermal sand reclamation (TSR) and T7 heat treatments. The higher cooling rate at the bottom of the castings led to a finer dendritic structure, smaller intermetallic compounds, lower aspect ratio eutectic Si particles, and less porosity. Also, the T7 treatment promoted the formation of fine Al2Cu precipitates distributed throughout the matrix and the coarsening and spheroidization of eutectic Si particles. Improvements in yield strength, tensile strength, and hardness were attributed to both grain boundary and precipitation strengthening, whereas improvements in thermal conductivity were attributed to solute depletion during aging.