Effect of Heat Treatment on the Mechanical Properties of Squeeze-Cast Al–5Si–3Cu Alloy for Automotive Applications
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Al–Si–Cu alloy (A319) castings are used in vehicle industry due to its better castability, high specific strength, corrosion resistance and low cost. Using this alloy, most of the automotive components are manufactured by gravity die casting or high-pressure die casting technique, but these castings contain higher porosity leading to rejections. Squeeze casting is an emerging casting technique in which solidification is done under high pressure which overcomes the drawbacks of the gravity and pressure die casting. It offers higher metal yield, minimum porosity and near-net-shaped products with enhanced mechanical properties. Typical Al–Si–Cu cast alloys are heat-treated for T6 condition for attaining tensile strength in the range of 250–300 MPa and elongation up to 3%. The mechanical properties of this alloy can be further improved by the addition of Mg and Sr modification and by subjecting to squeeze casting. The present study deals with the optimization of the heat treatment cycle of squeeze-cast alloy billets and compares squeeze-cast alloy with the gravity-cast alloy in terms of mechanical properties and wear behaviour.
KeywordsA319 alloy Sr modification Squeeze casting T6 heat treatment
Al–Si–Cu (A319) alloy is widely used in the automotive industries due to their good castability, low thermal expansion coefficient and excellent mechanical properties due to the formation of Al2Cu precipitate during heat treatment . The automotive components like suspension arms, engine blocks, etc., operate over a wide range of stress conditions which demands enhanced mechanical properties than the existing alloy. The literature indicates that the mechanical strength and wear properties of this alloy can be further enhanced by minor additions of Mg and Sr modification.
Squeeze casting (SC) is an advanced casting technique in which high pressure is employed during solidification of molten metal in the die cavity. Solidification under higher pressure eliminates gas and shrinkage porosities in the casting. Higher cooling rate caused by the improved thermal contact between the casting and the die results in the formation of fine grained microstructure. The squeeze casting technique is the most effective method of manufacturing complex near-net-shaped automobile components with high strength and desired elongation .
Mg addition to the existing A319 alloy improves the mechanical properties such as tensile strength and wear resistance due to the combined precipitation of CuAl2 and Mg2Si precipitates during heat treatment . The morphology of eutectic silicon in A319 alloy also has a considerable influence on the mechanical properties of the casting. The eutectic silicon morphology can be controlled by the addition of strontium to the melt which results in a fine and fibrous silicon structure during solidification thereby improving the ductility, fracture toughness and wear resistance [4, 5].
The chemical composition of Al–Si–Cu (Mg) alloy castings
After solidification, the castings were removed from the die and cut into the required dimensions and machined for the microstructural analysis and heat treatment studies (for the evaluation of hardness, wear and tensile behaviour). Hardness measurements were taken using Tinius Olsen hardness testing machine. Wear rate of the alloys was measured using dry sliding—pin-on-disc wear testing machine according to ASTM G99-05 [ASM, 1992] standard. Tensile specimens were prepared as per ASTM standards E8M-04. The tensile specimens were tested in a universal testing machine (Instron 5500R) at a constant cross head speed of 1 mm/min. For each condition, a total of at least three samples were tested and the average value was reported.
3 Heat Treatment
In the present study, T6 heat treatment was carried out and the samples were subjected to solution treatment at a temperature of 500 °C for varying time followed by quenching in water. Solution heat treatment was performed for a definite duration to obtain a homogeneous structure, followed by quenching to attain supersaturated solid solution at ambient temperature. During solution treatment, the eutectic Si underwent necking and separated into segments. There was a decrease in the average particle size due to this fragmentation, and the fragmented segments were spheroidized. The ageing temperature of 170 °C was adopted in all the cases, and the duration of ageing was varied from 1 to 16 h to find the time required for peak hardening.
4 Chemical Composition
The analysis of chemical composition using optical emission spectrometer confirms that the alloying elements are within the specified limits.
5 Results and Discussion
The combined precipitation of Al2Cu and Mg2Si precipitates during ageing of 319–0.5 Mg alloy improves the strength properties. Finer microstructure caused by squeeze casting process and ageing treatment together contributes to the improved hardness and mechanical properties of the alloy. The addition of strontium results in a fine and fibrous eutectic silicon structure which contributes to the improved ductility of the alloy [8, 9, 10].
6 Hardness and Wear Behaviour
7 Tensile Properties
The mechanical properties of the A319 alloy were enhanced by the addition of Mg, Sr modification and squeeze casting. A finer microstructure was observed for the squeeze-cast samples which contributed to a significant improvement in the hardness, tensile and elongation under peak-aged condition. The peak hardness of 110 BHN was attained for ageing for 7 h at 170 °C for squeeze-cast alloy samples compared to 12 h for the gravity-cast samples. A lower wear rate was observed for the peak-aged squeeze-cast alloy samples. The higher mechanical properties of the squeeze-cast alloy fulfiled the strength requirements of automotive components.
The authors would like to thank the Director, CSIR-NIIST, Trivandrum, for granting permission to publish this paper. The authors thank the members of Material Science and Technology Division, for their technical support for the present work.
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