Changes in hardness of magnesium alloys due to precipitation hardening

Abstract

This paper deals with the evaluation of changes in hardness of magnesium alloys during precipitation hardening that are nowadays widely used in different fields of industry. It focuses exactly on AZ31, AZ61 and AZ91 alloys. Observing material hardness changes serves as an effective tool for determining precipitation hardening parameters, such as temperature and time. Brinell hardness measurement was chosen based on experimental needs. There was also necessary to make chemical composition analysis and to observe the microstructures of tested materials. The obtained results are presented and discussed in this paper.

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“…The curve also lacks the necessary underaged and overaged sections required for ageing response, as such showing that these alloys were not age hardenable. This was as expected due to the compositional similarity to the AZ31 range of alloys, which are typically used in wrought applications and are known for a lack ageing response[37].…”

“…This is observed in Figure 9 through a typical age hardening curve featuring underaged, peak and overaged sections. [37] Studies show that with Mg-Al alloys it is expected that Mg17Al12 precipitates must be reduced, since these are responsible for poor creep resistance properties. Other studies have shown that small amounts of cold working prior to ageing can sharply increase the rate of hardening, however this is beyond the scope of this project and any striking resulting in permanent tensile strain has been actively avoided [38].…”

“…48 hours was selected as a critical stage due to two primary reasons. Firstly, literature sources were found indicating a secondary hardness rise between 32 and 64 hours in an AZ31 alloy [37]. Due to this alloy not being hardenable, this rise cannot be labelled as a peak but it did provide an interesting point to analyse.…”

“…Despite the presence of hardening elements, prior studies suggest that the lack of ageing response was to be expected with the tested alloy compositions. Oršulová et al [37] found that ageing response was directly related to the aluminium addition when comparing the popular AZ91, AZ61 and AZ31 alloys. It was found that alloys with aluminium additions 6% and greater became susceptible to precipitation hardening illustrated in Figure 10.…”

“…An example of this can be found in the AZ31 alloy designation. Generally used in hot rolling applications with a H2x heat treatment, these alloys are not hardenable and as such show minor response to the T6 treatment as seen inFigure 10[37]. To see an age hardening response, it is usually required to alloy the magnesium with greater amounts of aluminium, such as in the AZ61 and AZ91 series.…”