Is there room for any more

Al-Mg-Si alloys (i.e. 6xxx alloys) form the basis of many heat-treatable wrought alloys and are used in a range of products due to their outstanding physical and chemical characteristics [2]. Formability and paint bake respone are hugely important for 6xxx sheets [3]. However, the low cold work hardening of Al-Mg-Si alloys, which is considerably poorer than that of non-heat-treatable Al-Mg alloys (5xxx), reduces its stretch formability [4,5].A high Fe content is typical for 8xxx foil alloys, which are primarily made from Al with Fe as the alloying element [7]. An important characteristic of Fe in 8xxx alloys is grain refinement, which is essential in foils in order to retain the material’s formability at low foil thicknesses [7-9]. A crossover approach aims to combine the positive characteristics of 6xxx alloys with the microstructure control provided by primary intermetallic phases (IMPs), which are important in foil materials. Given that the microstructure does not reflect the sum of the material’s mechanical properties, these structural optimizations should produce higher strength values as well as improved ductility despite the high proportion of iron-rich intermetallic phases. At low solidification rates, however, coarse and brittle IMPs form, which reports suggest can reduce ductility and negatively impact the formability of wrought alloys [2,10-12]. These coarse IMPs, and particularly those with an elongated morphology, are sites where damage typically occurs due to the formation of microcracks in particles or the loss of cohesion at the matrix - IMP interface [13,14].The variable composition of aluminium scrap and the introduction of impurities complicates recycling of aluminium. Fe presents a particular challenge as it is difficult to remove with metallurgical methods and gradually accumulates in wrought alloys when secondary aluminium is recycled [11,15].A number of strategies are used to control the morphology of iron-containing IMPs. Alloying elements such as manganese, which acts as an “Fe corrector”, can be used to suppress the formation of the elongated, needle-shaped β-AlFeSi-phase and promote the development of fine-branched α-AlFeSi [11,16,17]. Rapid solidification (> 10 K/s) also promotes the beneficial formation of a refined α-phase instead of β-AlFeSi, with a higher solidification rate required for a higher Fe content. [11,18]. Fe-rich IMPs can also be partially dissolved [19], fragmented [20,21] and converted into rounder shapes using heat treatments [7].

This study shows that Fe-rich IMPs can be effectively deployed in thermomechanical processes to positively influence grain size and material properties. Even coarse IMPs that form during slow solidification can be sufficiently fragmented through conventional rolling and used to control microstructure development in Al-Mg-Si-Fe crossover alloys. This significantly improves the alloys’ capacity for cold work hardening while maintaining good hardening potential [1].

Successful particle fragmentation and microstructure modification throughout the examined range of compositions

As mentioned in the introduction to this report, age-hardening is particularly important during paint bake of 6xxx series alloys. For this reason, the alloys (slow solidification) were also subjected to a paint-bake simulation: pre-aging for 5 hours at 100°C → 11 days of natural aging → application of 2% plastic elongation → 3 days of natural curing → artificial aging for 20 minutes at 185°C. Table 2 summerizes the results ΔRp0.2 stands for the curing reaction of the paint-bake treatment compared to the condition after 14 days of natural aging (T4). These test alloys have pronounced artificial aging potential and mechanical properties that are very comparable with those of the 6016 reference alloy with a low Fe content [1]. The size and morphology of Fe-rich primary intermetallic phases can be effectively modified by casting conditions and, more importantly, through thermomechanical treatments. Furthermore, IMPs are used to achieve a finer-grained structure than the 6016 reference alloy. Both fine-branched IMPs (which form at fast solidifcation rates) and significantly coarser intermetallic AlFeSi phases (which form at slow hardening rates, comparable with those of industrial continuous casting) can be fragmented into fine particles. The age hardening potential of Al-Mg-Si alloys and the microstructure-controlling effect of primary intermetallic phases on processing in the context of foil materials were successfully combined with a crossover approach. This offers considerable potential to expand the properties of Al-Mg-Si alloys to include higher cold work hardening capacity and improvements in recyclability [1]. We have completed the first laboratory scale development phase and have now commenced industrialization. The first industrial-scale casting and rolling tests have been successful. We are now examining a wide spectrum of material characteristics of CrossAlloy®.68 to provide comprehensive data for interested customers.