Assessing Thermomechanical Fatigue of a Cast Aluminum Alloy Cylinder Head of an Internal Combustion Engine

Cast aluminum alloys are used for cylinder heads in internal combustion engines to meet low weight and high strength (lightweight) design requirements. In the combustion chamber, the alloy experiences harsh operating conditions; i.e., temperature variation, constrained thermal expansion, chemical reaction, corrosion, oxidation, and chemical deposition. Under these conditions, thermomechanical fatigue (TMF) damage arises in the form of mechanical damage, environmental (oxidation) damage, and creep damage.

In the present work, several important properties that influence the TMF life of the cylinder head have been identified through TMF and finite element analysis (FEA). The results show that improving the strength at high temperatures helps improve TMF life on the exhaust side of the head. On the other hand, improving strength and ductility extend TMF life at low temperature on the intake side. Based on Neu-Sehitoglu TMF damage model, environmental (oxidation) damage is responsible for more than 80% of the total damage on the exhaust side. On the intake side, environmental factors are again a significant contributor to overall damage, but produce a smaller portion of total damage as compared to the exhaust side. Damage due to creep, relative to other categories of damage, is minimal and can therefore be neglected. Environmental (oxidation) damage is strongly influenced by the activation energy for oxidation. A small increase of the activation energy significantly increases the TMF life. The activation energy for oxidation is closely related to the chemical composition of the alloy and the kinetics of oxidation of each element in the alloy. FEA and TMF analysis facilitate optimized selection of target alloys that meet TMF life expectations as engine output increases.