Research Status of High-Impact Aluminum Alloys Domestically and Internationally

The "Energy-saving and New Energy Vehicle Technology Roadmap" emphasizes the importance of reducing automobile carbon emissions, with lightweighting being a key method to achieve energy conservation and emission reduction. Mainstream lightweight solutions include structural optimization, material substitution, and process improvement, with aluminum alloys becoming the preferred material due to their comprehensive performance advantages. Traditional automotive cast aluminum parts primarily use high-strength aluminum alloys like ADC12 and A380, as well as heat-treated aluminum alloys such as AlSi10MnMg and A356 for producing key structural parts.

In recent years, Tesla has led the adoption of integrated die-casting technology, using large-tonnage die-casting machines to manufacture the Model Y rear floor, which not only reduces product weight but also lowers production costs. As a key component of integrated die-casting, heat-free aluminum alloy has become a focal point in industry research due to its high strength, toughness, and lack of need for heat treatment. It not only meets technical needs but also reduces production costs, demonstrating significant development potential.
 

Heat-free aluminum alloy is a unique material with high strength and toughness in the cast state, exhibiting an elongation of over 10%, and its mechanical properties can be further enhanced through heat treatment. Rheinfeld's Silafont-36 has established a presence in the automotive parts field with the EN AC-43500 grade (AlSi10MnMg). Although high-strength and tough aluminum alloys possess the mechanical properties of heat-free alloys, they are not specifically optimized for integrated die-casting processes. For example, Alcoa's C611, patented in 2004, primarily focuses on specific heat treatment processes. Therefore, when Tesla was promoting integrated large-scale die-casting technology, the company decided to independently develop new heat-free alloys. In recent years, studies on heat-free alloys in China have increased, involving OEMs, material manufacturers, casting companies, and research institutions, with a primary focus on developing new heat-free alloys by optimizing material composition. Many companies opt to independently develop rather than directly adopt existing materials domestically and internationally. The reasons are: ① Heat-free alloys require not only high strength and toughness in the cast state but also excellent casting and connection performance during the die-casting process, making material composition requirements extremely stringent. ② Independently developing advanced new materials allows companies to secure patent protection and avoid potential intellectual property risks. Currently, many domestic entities possess independent intellectual property rights for heat-free alloys, including Tsinghua University (in cooperation with FAW Foundry Co., Ltd.), Shanghai Jiaotong University (in cooperation with Human Horizons), Xiaomi Automobile, Guangdong Hongtu, Shanxi Ruige, Lizhong Group, Aluminum Corporation of China, Nantong Hongjin, Suzhou Huijin, and others. Additionally, companies like Shuai Yichi sell C611 aluminum ingots in China by obtaining authorization for material patents such as C611 from Alcoa.
 
High-strength and tough aluminum alloys are primarily divided into two series: Al-Si and Al-Mg. Currently, the integrated die-casting process primarily uses Al-Si alloys. Al-Si alloys are further divided into the common Al-Si-Mg series, such as Silafont-36, and the Al-Si series without Mg, such as Castasil-37. In the research and application of aluminum alloys, concepts such as non-heat-treated and non-heat treatment strengthening for high-strength and tough materials are usually equivalent to "heat-free" alloys in practical applications. In a strict sense, non-heat-treated materials specifically refer to alloys such as Castasil-37, which do not contain Mg and Cu and whose performance changes after heat treatment are negligible.
 
For integrated die-casting heat-treatment-free alloys, the chemical composition design is complex, typically including basic elements, trace strengthening elements, and reasonably allowed impurity elements. Among these, Si, as a basic alloying element, is crucial to the fluidity of the alloy, and its content is typically more than 6%. Mn is used to replace Fe to ensure proper demolding, and its content is typically 0.3% to 0.8%. Ti and Sr are used as refiners and modifiers, respectively. The appropriate addition of these elements can simultaneously improve the material's strength and toughness. Mg and Cu can be optionally added. Adding Mg will form the Mg2Si strengthening phase. Every 0.1% increase in Mg can raise yield strength by about 10MPa, but it also reduces elongation; its content is generally kept below 0.3%. When Mg content exceeds 0.5%, strength does not increase, and casting performance declines. Cu has a similar effect to Mg, but excessive addition may reduce the alloy's corrosion resistance, so its content must also be strictly controlled.
 
The differences in currently developed integrated die-casting heat-treatment-free aluminum alloys are reflected in the trace strengthening elements. These trace elements have three characteristics: ① The addition of a single trace element is generally no more than 0.2%. ② The trace elements are mainly concentrated in the transition elements of the 4th period, the IB-MIB elements of the 5th period, and the rare earth elements of the 6th period. Representative elements include La, Ce, V, Zr, Cr, Mo, and others. ③ They are usually added in combination. Xincheng Jiang and others found that the single addition of Sc did not significantly improve tensile strength and elongation, but the composite addition of Sc and 0.15% Ti and 0.2% Sc significantly refines the alloy structure.
 
The primary impurity element is Fe. Since Fe is inevitably introduced during the production of Al liquid and the die-casting process, considering the recyclability of aluminum alloys and smelting process requirements, the Fe content cannot be kept extremely low, necessitating an appropriate Fe tolerance. Currently, the Fe content of heat-treatment-free alloys used for integrated die-casting is typically not higher than 0.15%, but it is expected to be relaxed to 0.25%. Additionally, Zn and non-metallic elements such as C and B are detrimental to the plasticity of alloys, so their content must be strictly controlled.