Improvement of Superplasticity in High-Mg Aluminum Alloys by Sacrifice of Some Room Temperature Formability
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UMINUM alloys with high-Mg content, such as AA5083, are widely used as structural materials in ship building, automotive, aircraft, and other manufacturing industries. These alloys were initially designed to supply as fully annealed (e.g., O temper) or near-fully annealed (e.g., H32 temper) sheet products for cold forming, so the chemical compositions were optimized for balanced strength and formability at room temperature. They usually contain 4.0 to 4.9 pct Mg, 0.4 to 1.0 pct Mn, 0.2 to 0.3 pct Fe, and very low Si (all in wt pct hereinafter). The 4.0 to 4.9 pct Mg gives nearly the maximum solid solution hardening effect without causing serious technical issues, such as hot tearing, edge cracking, and reduction in corrosion resistance.[1–4] Manganese is added for grain size control, providing grain boundary hardening, and preventing orange peeling upon forming, but it is usually kept below 1 pct to avoid significant reduction of room temperature formability.[5] The Fe
H. JIN and B. SHALCHI AMIRKHIZ are with the CanmetMATERIALS, Natural Resources Canada, Hamilton, ON L8P 0A5, Canada. Contact e-mail: [email protected] D.J. LLOYD is with Aluminum Materials Consultants, Bath, ON K0H 1G0 Canada. Manuscript submitted July 31, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and Si are the most common impurity elements, coming from Bauxite, recycled aluminum scraps, and steel tools used in the smelter and cast house. The Fe is usually controlled around 0.2 to 0.3 pct, while higher level damages the formability and lower one increases the material cost. Since Si forms Mg2Si phase and thus reduces the Mg in solid solution, it is tightly controlled at 0.1 pct or below. High-Mg aluminum alloys are also supplied in heavily cold-rolled condition (e.g., H18 temper) for superplastic forming (SPF).[6–11] In SPF of these alloys the work piece is heated up in minutes to the desired forming temperature at 425 C to 525 C, while a uniform fine grain structure is generated. The fine grain structure enables grain boundary sliding and thus leads to extended plasticity. The extended plasticity in high-Mg aluminum alloys is often not prominent, because the grain size is not fine enough to fully realize the potential of high temperature formability. For example, for commercial AA5083 H18 sheet products under typical SPF condition, the mean grain size is 15 to 20 lm and the maximum tensile elongation is around 300 to 350 pct. Meanwhile, the famous Al-6 pct Cu-0.5 pct Zr Supral 100 alloy, which was specifically designed for SPF for aerospace applications, is able to maintain a mean grain size below 5 lm and to reach more than 1000 pct tensile elongation.[12–14]
Nevertheless, the aluminum products having true superplasticity are usually very expensive, because they often contain expensive alloying elements, e.g., Sc and Zr,[15,16] and/or are fabricated through complex processing routes with additional overaging.[17, 18] Moreover, many products specifically developed for SPF are metal matrix composites,[19,20] eutectic alloys,[21,22] or regular
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