Abstract
Abstract The mechanical properties of age hardenable Al alloys depend strongly on the precipitate microstructure. This work has investigated the relationship between properties such as strength and ductility and the distribution of precipitates, using three Al-Mg-Si(-Cu) alloys (Cu ≲ 0.1 at.%). A range of ageing conditions was examined in order to understand the effect of an evolving precipitate microstructure, and the results were used as input for strengthening models. The mechanical properties were obtained by tensile tests and microstructure characterisation was attained by transmission electron microscopy. The results showed that minor changes to the Si, Mg, and Cu additions – the total addition (at.%) kept approximately equal – had a significant impact on material properties, with corresponding changes in the precipitate microstructure. On the peak strength plateaus differences as large as 35 MPa in yield strength were measured between the strongest and the weakest alloy, obtained as 410 MPa and 375 MPa, respectively. Higher material yield strength correlated well with a refined precipitate microstructure comprising higher number densities of smaller precipitates. Differences with respect to material ductility first appeared after moderate overageing of the alloys, showing negative correlation with material strength. At significantly overaged conditions the differences in strength exceeded 100 MPa, demonstrating large differences with respect to the thermal stability of these materials, which has important consequences for alloys exposed to elevated temperatures under in-service conditions. The highly comprehensive body of data presented here should serve as a valuable reference in the development of precipitation and strengthening models for the Al-Mg-Si-Cu system and will hopefully incite further investigations on the topics covered.
Highlights
The AA6xxx series Al-Mg-Si(-Cu) alloys contain important age hardenable alloys for extruded products in automotive, marine, and building construction applications
All alloys showed a similar rate of hardness increase, all reaching close to peak hardness after roughly 2 h ageing, with values ranging from 122 HV for alloy S to 128 HV for alloy M
It was found that relatively small changes in alloy compositions caused significant differences in the precipitate microstructures, with corresponding significant differences in the measured material proper ties, including yield strength and fracture strain
Summary
The AA6xxx series Al-Mg-Si(-Cu) alloys contain important age hardenable alloys for extruded products in automotive, marine, and building construction applications They are widely used due to their low cost, high machinability, and to a combination of attractive material properties, including high strength, low weight, nice surface finish, and good corrosion resistance. For automotive components that will be exposed to elevated tempera tures under in-service conditions the thermal stability is important, and the change of σy with prolonged ageing is highly relevant in this regard The strength of these alloys is caused by the formation and growth of a high density of nanosized rod- and/or lath-shaped precipitate phases with long axes parallel to 〈100〉Al, acting as obstacles to dislocation motion. Changes in composition and TMP will affect the distribution and size of precipitates, which will have a direct influence on material properties
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