Lixiong Shao

Title : Enhanced grain refinement, precipitates regulation, and improved mechanical properties of cast Al-Li alloy by Ti addition and heat treatment

Abstract:

The cast Al-Li alloy is a promising material due to its high strength-to-weight ratio and stiffness. This presentation systematically revealed the impact of minor Ti addition and heat treatment on grain structure, precipitates, precipitate-free zone (PFZ), and the performance of the Al-3Li-2Cu alloy. Minor Ti addition significantly refines the as-cast alloy, leading to a transition from columnar grains to equiaxed grains. This refinement is achieved through the formation of micro-sized Al3Ti primary phases and the introduction of Ti solutes. The primary Al₃Ti phases, with a low mismatch (0.99%) to the matrix, act as heterogeneous nucleation sites for α-Al, while the presence of Ti solutes restricts grain growth. As a result, the average grain size can be refined to 35.2 μm by adding 0.2wt.% Ti. Besides, minor Ti addition affects the corresponding precipitation behaviour of the alloy. Detailed TEM observations reveal that the addition of Ti inhibits the growth of δ'(Al3Li) precipitates and the accompanying δ'-PFZ, which should be related to the interaction between Ti solutes and vacancies. Ti solutes preferentially combine with vacancies to form Ti-vacancy clusters, thereby reducing the concentration of effective free vacancies in the matrix, further decreasing the diffusion rate of Li atoms and vacancies, ultimately inhibiting the growth of δ' precipitates and δ'-PFZ. Besides, reducing the aging temperature exerts a similar effect. Finally, the 0.2 wt.% Ti-modified alloy exhibits excellent mechanical properties after aging at 175°C for 8 h. It demonstrates a high yield strength (272 MPa), a high ultimate tensile strength (399MPa), and an acceptable elongation (5.9%), which represent an increase of 8.8%, 34.8%, and 293.3%, respectively, compared to the alloy without Ti addition. The simultaneous improvement in strength and ductility is attributed to the presence of fine and equiaxed grains, high-density and homogenous δ' precipitates, and a narrow δ'-PFZ. In-depth discussions are provided to elucidate the underlying mechanisms governing the microstructure and mechanical properties. These findings provide valuable insights for the advancement of the high-performance and lightweight cast Al-Li alloy.

Audience Take Away Notes:

• The refinement mechanism of cast Al-Li alloy through Ti addition will be systematically elucidated based on experimental and theoretical analyses, including comparisons with other refining agents. The intriguing findings are expected to offer valuable insights into the refinement of Al and its alloys
• The impact of Ti addition and heat treatment on the precipitation behavior will be uncovered, serving as reference for the compositional design of Al-Li alloy
• The distinctive characteristics of precipitates in Al-Li alloy will be clarified, accompanied by a theoretical analysis of their influence on mechanical properties
• This presentation aims to provide valuable insights and establish a theoretical foundation for the production and development of cast Al-Li alloys

Biography:

Mr. Shao graduated from Chongqing University (China) as BS in Metallurgical Engineering in 2017, and later as MS in the same field in 2020. In 2021, he embarked on a new academic journey by enrolling in Shanghai Jiao Tong University to pursue a Ph.D. degree. He focuses on the composition, structural design, and microstructure adjustment mechanisms of cast Al-Li alloy with high Li content. The impact of solute components, reinforcement particles, and heat treatment on the microstructure and mechanical properties have been investigated systematically. After years of dedicated research, he has published more than 10 research articles in SCI(E) journals.