Consolidation and Forging Methods for a Cryomilled Al Alloy
- PDF / 1,564,826 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 12 Downloads / 243 Views
BALL milling of a metal powder leads to the development of a nanocrystalline microstructure via the severe plastic deformation that occurs upon repeated crushing between the milling balls.[1,2] The small size of the grains imparts a high level of strength compared to material processed by standard ingot metallurgy routes.[3] For alloys containing Al, the incorporation of N during milling in liquid N2 is believed to impart excellent thermal stability, as well as extra strength, to this structure due to the formation of stable nanoscale N-containing dispersoids that prevent excessive grain growth upon exposure to elevated temperatures.[3] In common with other Al alloy powders, the cryomilled powder must be consolidated to take advantage of the increased strengths in a bulk form. The compact must be deformed by shear stresses to break up the Al oxide/hydroxide layers of the prior particle boundary (PPB) structure and obtain sufficient tensile ductility.[4] For example, the tensile ductility of a cryomilled Al 5083 billet consolidated only by hot isostatic pressing (HIP) is A.P. NEWBERY, formerly Researcher, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, is unaffiliated. Contact e-mail: [email protected] B. AHN, Doctoral Student, and S.R. NUTT, M.C. Gill Professor, are with the Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089. E.J. LAVERNIA, Distinguished Professor, is with the Department of Chemical Engineering and Materials Science, University of California. R.W. HAYES, President/CEO, is with Metals Technology, Inc., Northridge, CA 91324. P.S. PAO, Materials Research Engineer, is with the Naval Research Laboratory, Washington, DC 20375. Manuscript submitted August 18, 2007. Article published online June 6, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
generally very low, despite the achievement of nearly full density.[5] A combination of HIP and extrusion has often been used in the past to produce cryomilled Al alloy rod or bar with high strength.[5] However, the transverse ductility of extrusions can still be low.[6] In addition, if the required product is plate, an alternative method of deformation processing, such as forging, is more suitable to produce a preform for rolling. For convenience, open-die forging of the ‘‘HIPped’’ compact is attractive. Alternatively, quasi-isostatic (QI) forging (formerly known as Ceracon forging), where the HIPped compact is forged within a die filled with a particulate ‘‘pressure transfer medium’’ (PTM) has produced plate from HIPped cryomilled Al 5083 that has very good ductility and fracture toughness.[7] The QI forging can also be used to carry out the initial consolidation of the canned powder, so that the preform for rolling can be made by a two-step forging process without the need for HIP. Note that Al 5083 is a work-hardened Mg-containing alloy with an excellent combination of strength, weldability, and corrosion resistance often used for marine structures
Data Loading...
