Evaluation of Precipitation Hardening Characteristics of Rheology-Forged Al 7075 Aluminum Alloy Using Nano- or Microinde
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INTRODUCTION
SEMISOLID forming (SSF) processes, including the new rheology (rheo)-casting process and rheo-forging, have been established for the production of near-netshaped parts in various industrial fields.[1–3] The SSF processes provide good microstructures and smaller defects because the laminar filling of the semisolid melt suppresses air entrapment, resulting in decreased macrosegregation, grain size refinement, and fine Si morphology.[4,5] In the rheo-forming process, material is heated to a liquid state. Molten alloy is then directly cooled from a liquid state to a semisolid state and is formed by forging and casting processes. Recently, rheoforming has been used to mold aluminum alloys into a semisolid state by producing globular microstructures, which are crushed by dendrite structures through electromagnetic stirring.[6,7] The SSF processes are suitable for the production of parts requiring high strength, whereas heat treatment enhances mechanical properties, especially the ductility, of aluminum alloys. Wrought aluminum alloys such as Al 7075 and Al 6061 are used for various applications after T6 heat treatment. In T6 heat treatment, forged parts are artificially aged directly after fabrication with a solid solution heat treatment. Incorrect heat treatment such as overaging can occasionally lead to poor mechanical properties. Usually, the age-hardening response is significantly influenced by time factors. Therefore, it is important to find the optimum time factors for T6 heat treatment. In this study, the nano- and microstructure, the aging response from T6 heat treatment, and the mechanical and tribological properties of rheo-forged Al 7075 H.H. KIM, Postdoctoral Candidate, Graduate School of Mechanical and Precision Engineering, and C.G. KANG, Professor, School of Mechanical Engineering, are with Pusan National University, Pusan 609-735, Korea. Contact e-mail: [email protected] Manuscript submitted December 4, 2008. Article published online January 23, 2010 696—VOLUME 41A, MARCH 2010
wrought alloy parts were investigated through nano- or microindentation and mechanical scratching incorporated with optical microscopy and AFM. Microstructures are usually investigated using scanning electron microscopy (SEM). However, SEM does not provide quantitative information on the height distribution of the structure, because the on-line image analysis system, which discovers the contents of elements, is unsupported. Atomic force microscopy can be used as an additional and reliable technique for investigating micro- or nanostructures on the surface of materials.[8] The combination of nanoindentation with AFM enables mechanical, tribological, and morphological properties to be measured at different regions, such as the eutectic and primary a-Al phase, down to the nanometer scale.[9,10]
II.
EXPERIMENTAL PROCEDURE AND EQUIPMENT
We used Al 7075 wrought aluminum alloy in this study. Table I shows its chemical composition. The liquidus and solidus lines are 644 C and 477 C, respectively. SSF of Al 7075 alloy was carried
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