Magnesium and its alloys exhibit not only elastic and plastic behaviour but also anelastic behaviour upon loading. The presence of anelastic strain poses a challenge to the measurement of proof stress using conventional methods. As such, the proof stress measurement methods specified by American Society for Testing and Materials (ASTM) and International Standards Organization (ISO) are reviewed and applied to three common die-cast magnesium alloys including AE44, AM60 and AZ91. The methods in the standards give inconsistent results due largely to the inherent anelastic behaviour of these alloys. The widely used 0.2% offset strain method tends to underestimate proof stress whilst the 0.2% permanent plastic strain method requires repeated loading and unloading. In view of the fact that the non-equivalence between the offset strain and the residual plastic strain for magnesium alloys is a key obstacle to the accurate proof stress measurement, a conversion chart is constructed to enable the determination of the appropriate offset strain for a desired residual plastic strain for a range of magnesium alloys. It is shown that employing a higher offset strain than 0.2% has an advantage in reproducibility of proof stress measurement.
Self-piercing riveting (SPR) is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole. The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications. However, SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process. In this paper, SPR joints of advanced materials and their corresponding failure mechanisms are discussed, aiming to provide the foundation for future improvement of SPR joint quality. This paper is divided into three major sections: 1) joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions, 2) joint corrosion issues, and 3) joint optimisation via process parameters and advanced techniques.
The strain-rate effect in die-cast magnesium-aluminium based alloys under quasi-static strain rates ranging from 10 -6 to 10 -1 s -1 was investigated. The strain-rate sensitivity was shown to decrease with increasing aluminium solute level in the matrix. Microstructural examination by electron backscattered diffraction (EBSD) revealed that deformation twinning is more active in the alloys with lower strain-rate sensitivity. It is suggested that the decrease in strain-rate sensitivity with increasing aluminium solute level is likely due to dynamic strain ageing from the interaction between aluminium solute and dislocations. The correlation between strain-rate sensitivity and ductility in AE44 is briefly discussed.
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