Improving the formability of Mn-containing 6xxx Al alloys: role of additives and heat treatment

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Recent development efforts in aluminum have focused on wrought alloys where, through the use of the well-established technology of Al extrusions, such alloys as the heat-treatable 6xxx series alloys are being increasingly used in sheet and other applications where relatively high formability and in-service strength are major requirements. Mg and Si are the main alloying elements and combine to form Mg2Si, the primary hardening phase; Cu is commonly added, to further increase the strength. Metals such as Fe, Mn and Cr are present as impurities or trace additions. Thus, in addition to alpha-Al, Si and Mg2Si, a wide range of Fe and Cu-bearing intermetallics namely, Al-Fe, Al-Fe-Si, Al-Fe-Mn-Si and Al-Mg-Si-Cu (Q phase) form during solidification, homogenization and thermomechanical processing of the alloy. The alloy composition and casting conditions influence the type, morphology and amount of intermetallics formed, as do the thermal treatments applied. The brittle beta-Al5FeSi platelets adversely influence the extrusion properties and induce surface defects in the extruded material. The as-cast alloy is thus homogenized at temperatures of 530-600oC, during which the beta-AlFeSi phase transforms to the more favorable alpha-Al(FeMn)Si phase, which improves the alloy extrudability and surface quality considerably. This project proposes to study the formation of the key intermetallic phases, namely Mg2Si, beta-Fe, alpha-Fe, and the Cu-containing Q phase, occurring in 6xxx alloys by selecting alloy compositions and conditions favorizing the formation of each intermetallic type by itself. The study will focus on the role of Mn (0.02-0.9%) in stabilizing the beta-to-alpha transformation, and that of additions such as Zr and Sc to partially replace expensive Mn, and thus provide cost-effective solutions for attaining high alloy formability and increased tensile properties. Hardness measurements, tensile properties and extensive TEM and EPMA analyses will be used to monitor the microstructural evolution in the as-cast, homogenized, and cold rolled stages, in relation to alloy formability. The results would benefit the Canadian automotive and construction industries, two main application areas of interest.