PROCESSING AND PERFORMANCE OF Al-Li-Cu-X EXTRUSIONS

Abstract
The extrusion characteristics of 2090 (Al-Cu-Li) and 8090 and 8091 (Al-Li-Cu-Mg) alloys from the standpoints of ease of extrusion and compatibility with plant operations were investigated. Experimentation was conducted to determine the effects of extrusion temperature and extrusion rate on extrudability and product performance. Structure studies were carried out to explain variations in mechanical properties. Experimental extrusion limit diagrams for Al-Li-Cu and Al-Li-Cu-Mg alloys showed that the addition of magnesium lowered the working range by increasing the flow stress and enhancing the onset of incipient melting and surface tearing, particularly at the higher alloying levels. Plant trials were conducted with 2090 alloy. The extrudability of this alloy was more similar to 6061 alloy from the standpoints of press effort and run out speed than to the conventional 2XXX and 7XXX aircraft alloys. The mechanical properties of the extrusions were a function of microstructure and test direction. High longitudinal ultimate tensile and yield strength (> 600 MPa) and good toughness were obtained in the peak strength condition when the extrusions had fibrous, elongated grains. Transverse strengths and toughness were substantially lower. Material with a more recrystallized, equiaxed grain structure showed less variation with test direction, but significantly lower strengths were obtained. It was concluded that 2090, 8090 and 8091 alloys can be extruded using conventional equipment and methods. Alloy 2090 is easier to extrude than the Mg-bearing compositions. The mechanical properties of these alloys are very structure sensitive, which can be controlled to some extent by the extrusion parameters.