Abstract
The effect of solution treatment on mechanical properties (UTS, elongation, Brinell hardness) and microstructure (Si-morphology and Si-size) of an aluminium alloy (A356) used for casting cylinder heads was studied. The tests were carried out with specimens machined from the bulkheads of V8 engine blocks cast by the low pressure process. The samples were tested in as-cast and T6 heat treating conditions (solution heat treatment at 530°C with different time - 2, 3, 4, 5, 6, 7 hours, quenching in water at 20°C and precipitation hardened for 4 hour at 160°C). The results show that used heat treatment improves mechanical properties of the cylinder head casts. Tensile strength and hardness of specimens increase with solution treatment time. The hardness is a reflection of solution strengthening and silicon particle distribution in matrix. Solution temperature 530°C and 5 hours solution time is appropriate to obtain better morphology and distribution of Si particles in microstructure. Prolonged solution treatment (more than 5 hours) leads to a coarsening of the Si particles, while the numerical Si density decreases. As the particle density decreases, a fewer number of sites are available for crack nucleation, and hence, the fracture properties are improved. The data obtained from this study will be used to improve process control, and to help the selection of heat treatment of the casting for future products.
Highlights
The cylinder head is an integral component of internal combustion engines
High strength at elevated temperatures is crucial to ensure that the engine blockcylinder head assembly can withstand the combustion forces and the forces resulting from thermal expansion and contraction during service cycles without losing tightness of the cylinder head gasket
This modification treatment improves the mechanical properties of the Al-Si alloy by reducing the notching effect of needle shaped natural eutectic silicon [6, 9, 18]
Summary
The cylinder head is an integral component of internal combustion engines. It conveys air and gasoline to the combustion chamber and serves as a cover for the cylinders. Aluminium cast alloys used in the production of such complex cast parts as engine blocks or cylinder heads must meet a combination of requirements which include low cost, excellent castability, low hot tearing tendency, good machinability, moderate strength at elevated temperatures and resistance to fatigue, i.e. high fatigue strength at elevated temperatures. High strength at elevated temperatures (up to 250°C) is crucial to ensure that the engine blockcylinder head assembly can withstand the combustion forces and the forces resulting from thermal expansion and contraction during service cycles without losing tightness of the cylinder head gasket. The present study is part of a larger research project which was conducted to evaluate the heat treatment of an AlSi7Mg0.3 cast alloy used in the manufacture of engine blocks and cylinder heads. The manufacturer required selection of optimal solution heat treatment, respectively shorter dissolution annealing times
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