Key Takeaways & Executive Findings
- •• Combining Al-Ti-C-B master alloy addition with deep cryogenic treatment (DCT) reduces average grain size of Al-12Si-4Cu-2Ni-1Mg alloy from 0.92 mm to 0.50 mm, achieving significant grain refinement. • DCT for 24 hours followed by aging spheroidizes and refines eutectic Si and Al7Cu4Ni precipitates, enhancing microstructural uniformity. • The refined microstructure leads to a 22.5% increase in elongation and a 6.8% improvement in tensile strength, while also significantly extending fatigue life. • This novel strategy offers an environmentally friendly and effective method to overcome Si poisoning in grain refinement of near-eutectic Al-Si alloys, with potential for automotive lightweighting applications.
Abstract
Near-eutectic Al-Si alloys are widely used in automotive manufacturing due to their superior wear resistance and high temperature performance. Because of high Si content, the grain refinement of near-eutectic Al-Si alloy has been a problem for many years. In this study, the effect of deep cryogenic treatment (DCT) on the microstructure and mechanical properties of Al-12Si-4Cu-2Ni-Mg alloy with addition of Al-Ti-C-B master alloy was fully investigated. Results show that the average grain size of the alloy is greatly reduced from 0.92 mm to 0.50 mm, and the eutectic Si and Al7Cu4Ni precipitates are spheroidized and refined in Al-12Si-4Cu-2Ni-Mg after DCT for 24 h and aging treatment. Thereby these changes of microstructures result in a significant increment of about 22.5% in elongation and a slight enhancement of about 6.8% in tensile strength. Moreover, the refinement of microstructure also significantly improves the fatigue life of the alloy.
1. Introduction
Near-eutectic Al-Si alloys exhibit superior wear resistance, corrosion resistance, thermal expansion properties, and high temperature strength [1-3], which are widely used in automotive industry, such as the engine hook and cylinder [4]. Furthermore, high strength Al alloy is needed as a substitute for steel with high density in vehicles to reduce carbon emission. However, the mechanical properties of aluminum alloy are much lower than those of high-performance steels. Solution strengthening, precipitation strengthening, work-hardening are usually used to enhance the mechanical properties of Al alloys. Among these strengthening strategies, grain boundary strengthening has proved to be one of the most effective strengthening methods for casting Al-Si alloys in previous studies [5-7].
According to the solidification theory, the grain size is determined by the nucleation rate and growth rate. The increase of nucleation rate can contribute to the refinement of grains during the solidification. The master alloys, providing heterogeneous nucleation sites or inhibiting the growth of crystal nuclei during solidification, have been introduced into Al alloy to increase the recrystallisation nucleation rate so as to refine the grain structure of the alloy [7-9]. Nowadays, intermediate alloy-type refiner has been successfully used for grain refinement of Al alloys, improving the castability and mechanical properties. However, the grain-refining effect of Al-Ti-C and Al-Ti-B will be diminished by high amounts of Si in Al-Si alloys or some amount of Zr in Al alloys [10]. Si elements adhere to the surface of nucleating substrate Al-B2, Ti-B2, etc., forming silicates or worsening the interfacial bond between the nucleating substrate surface and the α-Al matrix [11, 12]. To solve this problem, the researchers started to optimize the composition and structure of the master alloy. Recently, the novel Al-Ti-C-B master alloy exhibits good anti Si/Zr poisoning due to its unique diphase composite structure [13], which provides a better choice for the refinement of Al-Si alloy.
In addition to chemical methods, deep cryogenic treatment (DCT), as an environmentally friendly method, can significantly eliminate microstructural defects such as shrinkage, greatly affecting the microstructure and properties of Al alloys [14]. For instance, some studies found that heat treatment, including solution and artificial aging, combined with DCT, effectively promotes the precipitation and refines grains in Al-Cu alloys [14, 15]. The lattice shrinkage at low temperatures leads to the generation and movement of dislocations. As the DCT time extends, these dislocations gradually accumulate at the grain boundaries, which is conducive to the nucleation of recrystallization [16]. DCT is also used to eliminate residual austenite and refine carbon compounds, while improving the strength and hardness of steel [17]. These studies provide good references for us to further improve the mechanical properties of near-eutectic Al-Si alloy by means of DCT.
Therefore, a novel strategy was proposed, in which DCT and master alloy were combined to tailor the microstructure of near-eutectic Al-Si alloy with the aim of enhancing the mechanical properties. Firstly, the microstructure and mechanical properties of Al-12Si-4Cu-2Ni-1Mg alloy with different...
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Lin-fei Xia, Wen-bo Li, Zuo-shan Wei, Yu-ying Wu, Xiang-fa Liu (2025). Achieving further refinement of grain structure and improvement of mechanical properties in Al-12Si-4Cu-2Ni-1Mg alloy by Al-Ti-C-B master alloy addition and deep cryogenic treatment. China Foundry. https://doi.org/10.1007/s41230-024-4124-4
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Frequently Asked Questions
What is the effect of deep cryogenic treatment on Al-12Si-4Cu-2Ni-1Mg alloy?
Deep cryogenic treatment (DCT) for 24 hours followed by aging significantly refines the grain structure, reducing average grain size from 0.92 mm to 0.50 mm, and spheroidizes eutectic Si and Al7Cu4Ni precipitates, leading to improved elongation (22.5% increase) and tensile strength (6.8% increase), as well as enhanced fatigue life.
How does Al-Ti-C-B master alloy help in grain refinement of Al-Si alloys?
Al-Ti-C-B master alloy provides effective heterogeneous nucleation sites and exhibits anti-Si/Zr poisoning due to its unique diphase composite structure, overcoming the issue of Si poisoning that diminishes the refining effect of conventional Al-Ti-C and Al-Ti-B master alloys.
What are the mechanical property improvements after combined treatment?
The combined treatment of Al-Ti-C-B master alloy addition and deep cryogenic treatment results in a 22.5% increase in elongation and a 6.8% enhancement in tensile strength, along with significant improvement in fatigue life, making the alloy more suitable for automotive applications.
Why is grain refinement important for near-eutectic Al-Si alloys?
Grain refinement is crucial because it improves mechanical properties such as strength, ductility, and fatigue resistance. In near-eutectic Al-Si alloys, fine grains enhance the alloy's performance, making it a viable lightweight alternative to steel in automotive components, thereby reducing carbon emissions.
Is deep cryogenic treatment an environmentally friendly method?
Yes, deep cryogenic treatment is considered environmentally friendly as it does not involve harmful chemicals and is a physical process that reduces microstructural defects, improving material properties without adverse environmental impact.
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