Effects of Li content on microstructure evolution and mechanical properties of squeeze-cast Al-5Cu-xLi alloy assisted with ultrasonic treatment

Abstract

In this study, a new squeeze-cast Al-5Cu-xLi-0.5Mn-0.3Mg-0.15Ti (x = 0.3, 0.6, 0.9, 1.2 wt%) alloy (Al-5Cu-xLi alloy for short) was prepared with ultrasonic treatment (UT), and the effects of UT and Li content on the microstructure evolution and mechanical properties of Al-5Cu-xLi alloy were investigated. Unlike other aluminum alloys, UT applied to aluminum-lithium alloys can bring about significant degassing effects in addition to grain refinement and homogenization, which is beneficial for the high strength and toughness of Al-5Cu-xLi alloy. With the increase of Li content from 0.3 wt% to 1.2 wt%, the average grain size remains unchanged at first, and then increases significantly. Interestingly, the average size of α-Al grains is 30% smaller than that of Al-5Cu-0Li alloy (without Li element) after adding only 0.3 wt% of Li element, while the ultimate tensile strength (UTS), yield strength (YS) and elongation are increased by 11.3%, 15.6% and 26.9%, respectively. The optimal comprehensive mechanical properties are obtained in Al-5Cu-0.6Li alloy, which exhibits 300 MPa in UTS, 190 MPa in YS and 16.7% in elongation. However, by further increasing the Li content, the strength of the Al-5Cu-xLi alloy remains almost unchanged, while its elongation decreases significantly (but still more than 13.5%), due to grain coarsening and slight compositional segregation.