Effect of Different Proportion of Coarse and Fine Grain Microstructure on Superplastic Forming Characteristics

Abstract

The study was carried out to understand the effect of inhomogeneous microstructure on thickness variation in superplastically formed bulge. Friction stir processing was performed at rotational and traverse speeds of 720rpm and 155mm/min respectively on a 6mm sheet maintaining 50% overlap on the retreating side. Different probe dimensions were selected to obtain different proportions of fine grained stir zone in thickness direction. The proportions of the fine grained stir zone were 25%, 50%, 72% and, 100%. The sheets containing inhomogeneous microstructure were subjected to superplastic bulge forming under constant gas pressure up to a bulge height of 23.5mm. The sheet which was processed with 72% fine grains showed lower thickness variation from edge to apex and the bulge shape in this condition was close to the ideal spherical profile.

Deformation Behavior of Inhomogeneous Layered Microstructure

“…Higher elongation to failure in 78% fine grain material can be attributed to concomitant microstructural evolution and increase in m value with strain. Comparison with previous work on Al-Li 8090 alloy [1][2][3] and Al 5086 alloy [4] with present work brings out the fact that to observe better superplasticity in a material with inhomogeneous microstructure the proportion of fine grain microstructure should be equal to more than 50%.…”

“…Microstructural prerequisites are; uniform equiaxed fine grain microstructure (grain size <10 µm) having predominantly high angle grain boundaries. However in recent years, work on Al-Li 8090 alloy [1] and, FSPed material having inhomogeneous layered microstructure containing coarse and fine grain microstructures have shown significant higher ductility then uniform fine grain microstructure [2]. In Al-Li 8090 alloy better superplastic properties were reported by Fan et al [1] for sheet having 67% fine grain microstructure whereas, Pancholi and Kashyap [3] showed better superplastic forming properties in 50% fine grain microstructure.…”