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

最近铝的发展主要集中在锻造合金上，通过使用成熟的铝挤压技术，诸如可热处理的6xxx系列合金越来越多地用于板材和其他要求相对较高的成形性和使用强度的应用中。Mg和Si是主要的合金元素，结合形成Mg2Si，即初硬化相；通常加入Cu以进一步提高强度。铁、锰和铬等金属以杂质或微量添加剂的形式存在。因此，除了α - al、Si和Mg2Si外，在合金的凝固、均匀化和热机械加工过程中还形成了广泛的含铁和含cu的金属间化合物，即Al-Fe、Al-Fe-Si、Al-Fe- mn -Si和Al-Mg-Si-Cu （Q相）。合金成分和铸造条件影响形成的金属间化合物的类型、形态和数量，所采用的热处理也是如此。脆性β - al5fesi薄片会对挤压性能产生不利影响，并在挤压材料中引起表面缺陷。铸态合金在530 ~ 600℃温度下均质化，β - alfesi相转变为更有利的α - al (FeMn)Si相，显著提高了合金的可挤压性和表面质量。本项目拟通过选择有利于每种金属间相形成的合金成分和条件，研究6xxx合金中发生的关键金属间相Mg2Si、β - fe、α - fe和含cu Q相的形成。该研究将重点关注Mn（0.02-0.9%）在稳定β -to- α转变中的作用，以及添加Zr和Sc等以部分取代昂贵的Mn的作用，从而为获得高合金成形性和提高拉伸性能提供经济有效的解决方案。硬度测量、拉伸性能以及广泛的TEM和EPMA分析将用于监测铸态、均匀化和冷轧阶段的显微组织演变，以及与合金成形性的关系。结果将有利于加拿大的汽车和建筑行业，这两个主要的应用领域感兴趣。

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.