Evolution of micro-chemistry during solidification and homogenisation of AA 3xxx aluminium–manganese alloys

Abstract

An important aspect of the characterisation of non-heat-treatable Al alloys is the constitution of the material in terms of its alloying elements in solid solution and, in turn, volume, size, morphology and species of second-phase particles. These constitutional characteristics, conveniently summarised as micro-chemistry, have an impact on physical and mechanical materials properties of Al semi-finished products. In the present study, the evolution of micro-chemistry in the Al–Mn alloys of the AA 3xxx series during solidification and homogenisation is analysed. Especially, we focus on the impact of additional alloy elements, including Si, Mg and Cu, on the phase selection during solidification and the changes in solute level and volume, size and type of second-phase particles during subsequent homogenisation.

Formation of intermetallic particles during solidification and homogenisation of two Al–Mg–Si alloys

“…In AA 6016 the α-phase dispersoids contained roughly 6% Fe, 5% Mn and 3.5% Si, resulting in a slightly lower mean value of (Fe + Mn):Si of 3.6 (Figure 11(b)). Thus, the newly forming α-phase dispersoids revealed a larger Mn:Fe ratio than the α-Al(Fe,Mn)Si-phase constituent particles ([16]; see also [38]).

Figure 11 Chemical composition of the dispersoids after single-step homogenisation annealing (a) alloy AA 6005C and (b) alloy AA 6016 (SEM/EDS, in wt-%).

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“…In AA 6016 the α-phase dispersoids contained roughly 6% Fe, 5% Mn and 3.5% Si, resulting in a slightly lower mean value of (Fe + Mn):Si of 3.6 (Figure 11(b)). Thus, the newly forming α-phase dispersoids revealed a larger Mn:Fe ratio than the α-Al(Fe,Mn)Si-phase constituent particles ( [16]; see also [38]).…”

“…More in-depth information on size, density, spatial arrangement and, particularly, the chemical composition of the second-phase particles was gathered by means of area measurements in an SEM. To this end, we employed the field-emission SEM Zeiss Merlin equipped with the silicon drift detector SSD X-Max 80 and the software package AZtecFeature from Oxford Instruments [37,38]. SEM samples were prepared by argon ion beam milling using a Hitachi IM4000 cross-section polisher.…”

“…The evolution of microchemistry during solidification and one and two-step homogenisation of the two alloys was studied by means of optical and electron microscopy as well as measurements of the specific electrical resistivity. Most notably, we apply statistical area measurements in a scanning electron microscope (SEM) to probe the size distribution, area density and chemical composition of the second-phase particles [37,38]. The interpretation of the experimental findings was supported by an appraisal of the constituent phases forming during solidification with a microstructure model termed Alstruc which is built on standard solidification theory [39].…”

Intermetallic and dispersoid structures in AA3104 aluminium alloy during two-step homogenisation

“…Engler et al . 22 identify that most industrial processing of AA3XXX alloys incorporates a single homogenisation at 600 °C or has a multi-step homogenisation in the range of 600 °C. Therefore, the higher temperature of the first soak is in line with industry standards.…”