Fatigue Cracks in Aluminum Cylinder Heads for Diesel Engines

During the development of diesel engine cylinder heads a series of parts were produced using different casting conditions in order to evaluate the process capability and to establish the limits of the process parameters. The cylinder heads were gravity cast in a water-cooled permanent mold with sand cores using A357 alloy in the refined and modified condition. The parts were heat treated to T7 condition. Cylinder heads with high contents of microporosity or with new oxide film inclusions failed during thermo-cycle durability test. The article describes the failure analysis of some of these failed cylinder heads that developed fatigue cracks. Clear evidence was found of fatigue crack nucleation at microporosity and new oxide film inclusions. A detailed analysis identified the causes of the failures and allowed the selection of process parameters for the production of the cylinder heads. New parts, produced with low hydrogen content and with redesigned gating systems, complied with the specified test.