Consolidation and Forging Methods for a Cryomilled Al Alloy
The method used to consolidate a cryogenically ball-milled powder is critical to the retention of superior strength along with acceptable tensile ductility in the bulk product. In this study, gas-atomized Al 5083 powder was cryomilled, hot vacuum degassed, and consolidated by hot isostatic pressing (HIP) or by quasi-isostatic (QI) forging to produce low-porosity billets. The billets were then forged, either at high strain rate (without a die) or quasi-isostatically, and subsequently hot rolled to produce three 6.5-mm-thick plates. Despite extended periods at elevated temperatures and differences between the consolidation/deformation methods, a similar predominantly ultrafine grain microstructure was obtained in all three plates. The plates possessed similar ultimate tensile strengths, about 50 pct greater than standard work-hardened Al 5083. However, in terms of fracture toughness, there were significant differences between the plates. Debonding at prior cryomilled powder particle surfaces was an important fracture mechanism for “HIPped” material, leading to low toughness for crack surfaces in the plane of the plate. This effect was minimized by the implementation of double QI forging, producing plate with good isotropic fracture toughness. The type of particle boundary deformation during forging and the influence of impurities appeared to be more important in determining fracture toughness than the presence of ∼10 vol pct coarser micron-sized grains.
KeywordsFracture Toughness Ultimate Tensile Strength Rolled Plate Cryomilled Powder Prior Particle Boundary
Financial support for this work was gratefully received from the Office of Naval Research (Contract No. N00014-03-C-0163). The authors also thank Feng Tang, previously of UC Davis, for the scanning electron micrograph shown in Figure 3, and Kevin Doherty and Ernie Chin of the Army Research Laboratory for the donation of the standard Al 5083-H131 plate.
- 5.D. Witkin, B.Q. Han, and E.J. Lavernia: Metall. Mater. Trans. A, 2006, vol. 37A, pp. 185–94Google Scholar
- 6.A.P. Newbery, T. Topping, B. Ahn, P. Pao, S.R. Nutt, and E.J. Lavernia: J. Mater. Proc. Technol., 2008, vol. 203, pp. 37–45Google Scholar
- 8.Aluminum: Properties and Physical Metallurgy, J.E. Hatch, ed., ASM INTERNATIONAL, Materials Park, OH, 1984Google Scholar
- 10.Test Method for Plane-Strain Fracture Toughness of Metallic Materials, ASTM Standard, ASTM INTERNATIONAL, West Conshohocken, PA, 2005Google Scholar
- 11.A.P. Newbery: SSM Program Report (Dec.)—Novel Multifunctional Lightweight Systems using Multi-Scale Materials: From the Nanoscale to the Mesoscale, University of California, Davis, CA, 2006Google Scholar
- 12.D. Witkin, E.J. Lavernia: in Processing and Properties of Structural Nanomaterials, L.L. Shaw, C. Suryanarayana, R.S. Mishra, eds., TMS, Warrendale, PA, 2003, pp. 117–24Google Scholar
- 15.P.S. Pao, H.N. Jones, C.R. Feng, D.B. Witkin, and E.J. Lavernia: in Ultrafine Grained Materials IV, San Antonio, TX, 2006, Y.T. Zhu, ed., TMS, Warrendale, PA, 1996, pp. 331–36Google Scholar