Optimization of Aluminum Alloy AA5083 for Superplastic and Quick Plastic Forming
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INTRODUCTION
THE aluminum-magnesium-manganese alloy AA5083 has been widely used in superplastic forming (SPF) and quick plastic forming (QPF).[1–9] It contains 4.0-4.9 pct Mg, 0.4-1.0 pct Mn, ~ 0.25 pct Fe and less than 0.1 pct Si (all in wt pct hereinafter), and is usually supplied in heavily cold-rolled H18 temper. Besides the major alloying element Mg, which gives high strain rate sensitivity at elevated temperatures and solution hardening at room temperature, the other two critical elements are Mn and Fe.[10–13] The Mn is an alloying element for grain size control, while the Fe is an impurity element. In Al-Mg alloys with very low Si, the Mn forms the Al6Mn intermetallic phase, but the Fe could form various phases, such as Al3Fe and AlmFe, depending on the cooling rate in casting.[14–19] However, in commercial direct chill (DC) casting of AA5083, where the Mn level is high and the cooling rate is relatively low, the Fe preferentially forms the Al6(Mn, Fe) with Mn.[14,15] The Al6(Mn, Fe) phase is either the coarse constituents formed in the early stage of casting, or the small dispersoids formed in the late stage of casting, homogenization and early stage of hot rolling.[20,21]
H. JIN is with the CanmetMATERIALS, Natural Resources Canada, Hamilton, ON, L8P 0A5, Canada. Contact e-mail: [email protected] Manuscript submitted March 7, 2019. Article published online June 5, 2019 3868—VOLUME 50A, AUGUST 2019
Commercial AA5083 H18 sheet products are fabricated through DC casting, scalping, homogenization, hot rolling and cold rolling.[22,23] An industry-size DC cast AA5083 ingot is normally 4–6 m in length, 1.6–1.9 m in width and 0.635 m in thickness. It has a typical dendritic structure with extensive micro-segregation of Mg, and the intermetallic phases are mainly the Al6(Mn, Fe) and Mg2Si. After scalping off the shell zone from rolling surfaces, the ingot is homogenized at ~ 813.15 K (540 C) for several hours, while the micro-segregation is minimized, the Mg2Si phase is dissolved and the sub-micrometer Al6(Mn, Fe) dispersoids are precipitated out of the super-saturated solid solution. The hot rolling is done from ~ 600 to ~ 8 mm, with the entrance temperature slightly below 813.15 K (540 C) and the exit temperature around 573.15 K (300 C). During the hot rolling, the dendrites and many coarse Al6(Mn, Fe) constituents are turned into smaller particles by the break-down effect, and the smaller ones are re-distributed along the rolling direction (RD). In the early stage of hot rolling while the temperature is high, small Al6(Mn, Fe) dispersoids may further precipitate out. In practice, the thickness reduction in the subsequent cold rolling is ~ 75 pct. In typical SPF and QPF processes of AA5083, e.g., air blow forming, the work piece in H18 temper is heated and formed concurrently by blowing hot air.[3,5–7] The heating rate is very high, a few minutes from room temperature to the forming temperature, while a uniform fine grain structure is generated. Conventionally, the desired forming temperature is 748.15 K to 79
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