Abstract
Metal spinning technologies are attractive for single-piece fabrication of axisymmetric-shaped components for large aerospace structures, such as launch vehicle fuel tanks and aircraft fuselages. Aluminum (Al) alloys are of primary interest for structural weight optimization, particularly alloys with high specific strength, damage tolerance, and property retention at cryogenic temperatures. However, the ambient temperature properties of candidate Al alloys are such that alloys with adequate spin formability have limited strength, whereas alloys capable of high strength have limited formability. Of the metal spinning processes, spin forming poses additional challenges as a result of the free-space forming configuration, where material instabilities and a dominant tensile stress state can compromise the shaping process. Consequently, this work focuses on spin forming of Al 6061, Al 5083, and Al 2139 plate at room temperature, starting with 10-mm-thick preforms in the fully-soft, O-temper condition.
The overarching objective of this research is to correlate spin forming performance with various formability metrics by examining the stress states, strain paths, deformation mechanics, and failure modes involved. The approach incorporated (1) lab-scale spin forming trials, (2) mechanistic analyses of finite element model outputs, and (3) coupon-scale formability testing.
The first part of this dissertation examines the spin forming process through forming trials on candidate alloys and in-depth characterization of finite element model outputs in empirical regions of fracture. Spin forming trials of the three Al alloys revealed notable differences in the performance of each material that can be related back to material properties. Al 6061 served as a formability baseline, since the moderate-strength alloy proved robust to alternate forming paths and parameters required by the more crack-sensitive alloys. In contrast, the higher-strength alloys, Al 2139 and Al 5083, were prone to a range of failure modes, particularly on free surfaces subjected to tension. These locations included premature fracture in a localized neck bordering the clamping zone, surface roughening and cracking on the inner mold line (IML), and radial cracking from sensitivity to edge preparation of the starting preform. Analysis of finite element model outputs revealed that both the clamp zone border and the IML of the workpiece transition through a state of plane strain tension. These regions are also subjected to plastic cycling, with strain increments of 0.005-0.015 mm/mm with each roller interaction during the baseline forming sequence. The results suggest that the combination of the plane strain tension stress state with plastic cycling at a high mean stress was the primary contributor to the spin forming failures in Al 2139 and Al 5083. Such key elements of spin forming deformation are absent from spin formability assessments in the literature, which typically focus on tensile area reduction during quasistatic uniaxial tension testing.
The second part of this dissertation interrogated the effects of (a) changing strain rate, (b) deformation in plane strain tension, (c) plastic cycling, and (d) bending on the formability of the candidate Al alloys through alternate coupon-scale tests. Formability was evaluated at elevated strain rates (up to 1 s-1) and during strain rate jumps (akin to the passage under a forming roller) to simulate the dynamic forming environment. The results indicate the prevalence of dynamic strain aging (DSA) in the fully-soft, O-temper Al alloys and reveal that alloys with the more negative strain rate sensitivity (e.g. Al 2139 and Al 5083) fail prematurely. Plane strain tension properties were also evaluated due to the prevalence of the stress state in spin forming, and excellent correlation of the plane strain ductility was found with the spin forming failures at both the border of the clamp zone and the IML. Low cycle fatigue tests with tension-only plastic strain increments of ∆ep = 0.02, 0.005, and 0.001 mm/mm were also conducted with the plane strain specimen geometry to further reproduce spin forming conditions. The results confirmed a consistently lower fatigue life of Al 2139 compared to Al 5083 and Al 6061. All materials showed fatigue life knockdowns compared to literature data, attributed to the combined effects of the fully-soft O-temper and cycling at a high mean stress. Finally, three-point bend testing showed promise by replicating the surface roughening and premature cracking of Al 2139 on the IML during spin forming.
In summary, this investigation provides a framework that can be adopted to characterize spin formability when considering new alloys, tempers, or forming temperatures. The four alternative formability tests examined in this work were selected to reproduce specific stress states, strain paths, and dynamic loading conditions observed during spin forming. The primary conclusion is that testing in plane strain tension better reveals formability limitations of the candidate alloys at crucial locations in the workpiece compared to the incumbent metric derived from the uniaxial tension test. The identification of the prevalent plane strain tension stress state during spin forming and the linkage of premature failures in Al 2139 and Al 5083 with plane strain tension ductility advance the state-of-the-art for spin forming of plate materials.
