The influence of Sc+Zr additions on the Portevin-Le Chatelier (PLC) effect and formability of Al-Mg alloys

Researcher: Folarin Bakare

Aim:

The aim of this project is to explore the impact of the presence, size and distribution of Al₃ (Sc, Zr) dispersoids on the inherent stretchers marks (PLC effect) and formability of 5xxx series Al-Mg alloys.

Background:

6xxx-series aluminium alloys are increasingly used in the automotive industry due to their excellent combination of strength, high corrosion resistance and surface quality after forming. The main drawback of these alloys are the need for a costly age-hardening treatment that does not coincide with modern paint baking cycles which then leads to increased manufacturing costs. A possible alternative is the utilization of 5xxx series Al-Mg alloys which do not require an age-hardening cycle for strengthening and can maintain high formability and good corrosion resistance. However, plastic instabilities usually observed as serrations on the stress-strain curve have been the reason for the limited use of 5xxx series Al-Mg alloys to the innermost parts of automotive. These serrations are generally referred to as “Portevin-Le Chatelier (PLC) effect”. They arise in an attempt to increase the strength and ductility by raising the Mg levels especially during subsequent forming of such alloys at room temperature. The detrimental effect of PLC are the fact that it causes loss of ductility and gives rise to unsightly surface markings during sheet metal forming processes. This makes it unsuitable for the visible outermost parts in automotive applications.

A copious amount of literature have reported that Sc can homogenise plasticity. Hence, it would be interesting to observe the influence on the formability of 5xxx series Al-Mg due to the formation of Al₃Sc nm-size dispersoids. This PhD project will explore the possibility of adding Sc+Zr to 5xxx alloys to homogenise plastic deformation and eventually prevent the PLC effect, ultimately leading to improved formability.

Methodology:

Alloys containing different Mg levels with and without Sc+Zr additions will be taken through different tensile testing, different homogenization treatment to obtain varied size and distribution of dispersoids. Hardness, tensile testing, microstructure analysis and GOM analysis will performed to investigate the effect of the presence of Al₃ (Sc, Zr) microstructure on the PLC effect, mechanical properties and formability of 5xxx series Al-Mg alloys.

Homogenization treatment will be used to obtain varied size and distribution of dispersoids. This will further show the effect of different size and distribution of Al₃ (Sc, Zr) dispersoids on  the PLC effect, associated mechanical properties and formability.

Key Findings to date:

The presence of Al₃ (Sc, Zr) dispersoids have shown that the inherent PLC effect can be suppressed even up to 5wt% Mg in 5xxx series Al-Mg alloys especially when plastic deformation is done at high strain rates of 0.01/s. The stress drop magnitude and PLC band type were evidence of the PLC suppression. Although, there is a weak dependence of Sc+Zr addition on critical strain. It is left to be seen if this can be further suppressed with a particular size and distribution of Al₃ (Sc, Zr) dispersoids,

Future Work:

• Homogenization treatment of 5xxx series Al-Mg alloys with and without Sc+Zr additions
• Investigating the mechanical and microstructure properties through hardness, tensile test, confocal microscopy and GOM analysis.

Contact:

Folarin Bakare

Institute for Frontier Materials (IFM)

Deakin University, Geelong

Email: [email protected]