Novel Approach to Thermal Processing Development for Precision Sand Casting Process (PSCP) of Aluminum Engine Blocks

The latest generation of high-performance engine blocks are produced with the precision sand casting process (PSCP). Their enhanced mechanical properties are provided by an integrated cast iron chill in the bulkhead section and thus result in large variations in secondary dendrite arm spacing (λ2) throughout the whole casting, providing unique challenges in both heat treat (T7 temper) development and compliance to the CQI-9 protocol. This research examines the influence of chemistry, controlled solution/quench, residence time between quench and the start of artificial age and artificial age temperature, on the mechanical properties and cryogenic testing observations on PSCP engine blocks. The results are obtained from large population engine block castings (n = 204), heat treated within an industrial plant environment with strict control over furnace temperature gradients. The results indicated that age temperature differences of as little as 4 °C (within CQI-9 for furnace temperature specification of ± 5 °C) can alter yield strength and elongation significantly, which in turn changes crack susceptibility (warranty issues). In addition to the above analysis, it was found that extending the time between the solution/quench operation and the start of artificial age results in a continual drop in yield strength due to the influence of natural aging.