Semi-solid Powder Rolling of AA7050 Alloy Strips: Densification and Deformation Behaviors
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trength aluminum alloys such as AA7050 with good hardness, excellent corrosion resistance, and high toughness were used extensively for aerospace applications as stated by Lang et al.[1] Generally speaking, flat or strip products are usually manufactured by conventional ingot metallurgy (I/M) process. Subsequently, the materials are further processed (e.g., heat treatment, hot rolling, cold rolling, etc.) to meet the required properties.[2] But the production run is long and energy wasting. Liu and Luo[3] proposed a novel strip manufacturing process termed ‘‘Semi-solid Powder Rolling (SSPR)’’, which combines powder rolling and semi-solid rolling with one step. This new process is mainly used to prepare strips with high quality, especially for alloys that have a wide freezing range and low melting point such as aluminum alloy (AA7050, 2124, etc.). Actually, SSPR has some similar features with semi-solid powder forming (SPF). Zu and Luo[4] studied semi-solid powder forming (SPF) and Wu discussed various processing routes of SPF for Al alloys.[5–8] Hamilton et al.[9] proposed a novel technique, a direct XIA LUO and ZHUOQIANG MO, Ph.D. Candidates, YUNZHONG LIU, Professor, and CAIXIN GU, Master Candidate, are with the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
powder semi-solid forming (DPSF) process, to prepare particle reinforced metal-matrix composites (PMMCs), which is also based on semi-solid for ming technique and powder metallurgy. In general, SPF and DPSF are mainly applied in the extrusion and compaction tests to prepare bulk materials, while SSPR is mainly used to prepare strips. Thereby, there must be some different features in the deformation and densification behaviors. Until now, literatures reporting powder processing under a semi-solid state have been very limited. Wu and Kim[10] discussed the compaction behavior of Al6061 powder in the semi-solid state and relative density distribution during processing. Shima–Oyane model was used to accurately predict the powder compaction behavior when the liquid fraction is lower than 20 pct. But the force condition in SPF is tri-axial compaction while in SSPR is bi-axial compaction. When the liquid fraction is higher than 20 pct, Shima–Oyane model is not suitable. German et al.[11] studied the densification behavior and mechanisms during liquid phase sintering (LPS) process in detail. However, the densification of SSPR is somehow different from that of LPS because there is no large deformation during LPS. Even though densification process and deformation during hot rolling of porous strip and semi-solid compaction when the liquid fraction is lower than 20 pct were theoretically investigated, the densification and deformation behaviors during rolling under semisolid state when the liquid fraction is higher than 20 pct are firstly discussed. In this work, t
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