Development and control of strength anisotropy and crystallographic texture during extrusion of aluminum 2195 and 7075

Abstract

The addition of lithium to high strength aluminum alloys significantly improves specific strength. Indeed, for aerospace applications, the third generation Al-Cu-Li alloy, Al 2195, is competitive with composite materials. However, unlike its non-lithium containing counterpart, Al 7075, it suffers from undesirable anisotropic mechanical properties in low aspect ratio extruded sections. To investigate the origins of this anisotropy, Al 2195 and Al 7075 were systematically extruded over a range of aspect ratios from 2-15 while maintaining a constant extrusion ratio. This study found that the interaction of high volume fractions of the Copper crystallographic texture with the strengthening precipitates in Al 2195 is responsible for the poor mechanical performance in low aspect ratio regions. Through a series of rolling studies, a higher initial billet temperature and a slower ram speed were hypothesized to minimize the Copper texture component in extruded Al 2195. As press trials are often cost prohibitive and lead to convoluted results due to imperfect press repeatability, the effects of extrusion press parameters on the final microstructure and properties would ideally be studied via simulations. However, it was found that the commercially available finite element software, HyperXtrude, was not able to predict the effects of press parameters on mechanical anisotropy. It was therefore recommended that the Barlat Method for prediction of anisotropic yield strengths be integrated into the HyperXtrude solver to allow for future computational parametric studies on the effects of extrusion variables on final strength anisotropy in extruded aluminum alloys.