Investigating the Effect of Heat Treatment on the Microstructure and Hardness of Aluminum-Lithium Alloys

In this study, the effects of heat treatment on the microstructure and strength (micro-hardness) of an aluminum-lithium (Al-Li) base alloy containing copper (Cu) and scandium (Sc) were investigated, with a view to enhancing the alloy performance for aerospace applications. The heat treatment conditions were investigated to understand the precipitation behavior and the mechanisms involved in strengthening. Aging was carried out at temperatures of 130 degrees C and 150 degrees C for aging times of 1 h, 2.5 h, 5 h, 10 h, 15 h, 25 h, 35 h, and 45 h at each temperature for Al-Li alloy and at 160 degrees C, 180 degrees C, and 200 degrees C for aging times of 5 h, 10 h, 15 h, 20 h, 25 h, and 30 h at each temperature for Al-Li-Cu and Al-Li-Cu-Sc alloys. The investigation revealed that both solution heat treatment and artificial aging had a notable impact on strengthening the hardness of the alloy. This effect was attributed to the characteristics of the precipitates, including their type, size, number density, and distribution. The addition of copper (Cu) and scandium (Sc) was observed to have an impact on grain size refinement, while Cu addition specifically affected the precipitation behavior of the alloy. It led to remarkable changes in the number density, size, and distribution of T1 (Al2CuLi) and theta' (Al2Cu) phases. As a result, the hardness of the alloy was significantly improved after the addition of Cu and Sc, in comparison with the base Al-Li alloy. The best heat treatment process was determined as: 580 degrees C/1 h solution treatment +150 degrees C/45 h artificial aging for Al-Li alloy and 505 degrees C/5 h solution treatment +180 degrees C/20 h artificial aging for Al-Li-Cu and Al-Li-Cu-Sc alloys.