Aiden Beer

Effect of Grain Size on the Deformation and Dynamic Recrystallization of Mg-3Al-1Zn

The influence of grain size on the deformation of extruded Mg-3Al-1Zn tested in tension at temperatures between room temperature and 300°C is investigated. The results enable estimation of the deformation conditions for the transition from slip to twinning dominated flow and for the initiation and completion of dynamic recrystallization. A map illustrating these critical parameters is constructed and it is shown that the operating conditions of the common wrought processes straddle key transitions in microstructure behaviour.

The Application of Magnesium Alloys to the Lightweighting of Automotive Structures

This paper investigates some of the opportunities for magnesium alloys based on magnesium’s attributes, such as excellent die castability and high strength to weight ratio. Barriers to the uptake of magnesium alloys are considered and recent technical advancements outlined including the development of creep resistant alloys, high extrudibility alloys, joining technologies and corrosion performance. Finally issues to do with the life-cycle performance of magnesium alloys are discussed.

Grain Size in Mg Alloys: Recrystallization and Mechanical Consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying addition on both of these reactions is briefly considered.

Compositional Effects on the Restoration Behaviour in Mg-Zn-RE Alloys

Additions of rare earth elements to magnesium alloys are qualitatively reported in the literature to retard recrystallisation. However, their effect in the presence of other (non-rare earth) alloy additions has not been systematically shown nor has the effect been quantified. The microstructural restoration following the hot deformation of Mg-xZn-yRE (x = 2.5 and 5 wt.%, y = 0 and 1 wt.%, and RE = Gd and Y) alloys has been studied using double hit compression testing and microscopy. It was found that, in the absence of rare earth additions, increases in zinc level had a negligible influence on the kinetics of restoration and the microstructure developed both during extrusion and throughout double hit testing. Adding rare earth elements to Mg-Zn alloys was found to retard restoration of the microstructure and maintain finer recrystallised grains. However, in the Mg-Zn-RE alloys, increasing the zinc concentration from 2.5 wt.% to 5 wt.% accelerated the restoration process, most likely due to a depletion of rare earth elements from solid solution and modification of the particles present in the matrix.

An Initial Assessment of the Effects of Increased Ni and V Content in A356 and AA6063 Alloys

Changes in calcined coke composition associated with different crude oil sources have caused nickel (Ni) and vanadium (V) levels in aluminum to rise. To ensure cast product quality is not compromised an understanding of the effects of these changes is needed. An initial investigation has been conducted for two commonly used alloys, A356 and AA6060/6063. Castings were produced with low typical levels of Ni-V and with high Ni-V levels approaching the maximum P1020 specification of 300ppm each. Micro structural changes were assessed using optical and scanning electron microscopy and tensile properties and corrosion resistance were measured. For as-cast A356 alloy, there was no significant difference in corrosion performance, but adding Ni and V had a small effect on tensile properties. For AA6060/6063 alloy there was no significant difference in the tensile properties of extrusions with low and high Ni-V levels but a small drop in corrosion performance was measured at high Ni-V levels.

Nucleation of Recrystallization in Mg Alloys during and after Hot Working

Recrystallization nucleates at heterogeneities. The impact of this on local texture and stress-strain response in hot worked magnesium is considered in the present paper. Two aspects of bulge nucleation during dynamic recrystallization are considered.

The Influence of Process Parameters and Themomechanical History on Streaking Defects in AA6060 Extrusions

Streaking is a common surface defect on architectural extrusions. The current study investigates the effect of processing parameters and thermomechanical history on the surface finish of a streaked AA6060 extrusion. Streaked profiles were intentionally produced via extrusion using a die with internal geometry deliberately designed to produce thermomechanical variation in specific regions of the profile geometry. Extrusions were conducted at various ram speeds and the profiles were given an anodization pre-treatment process to reveal streaking in the designated regions. Thermomechanical variation was analyzed using the “ALE” code HyperXtrude®. The surface profile and roughness of the treated and mill finish extrudates were analyzed using optical profilometry. The presence of the streaks were associated with strain rate variability as determined by the die geometry, independent of the ram speed. The streaked regions of the profile were found to be more recessed compared to the bulk regions.

Effect of grain size on the performance of extruded magnesium alloy tubes in three-point bending

The performance of extruded AZ31, AZ61 and AM-EX1 tubes was examined in three-point bending. Different extrusion temperatures were used to investigate the effect of grain size on the load-carrying capacity, energy absorption and fracture propensity of the tubes. Results showed that while the peak load increased with a smaller average recrystallised grain size, the retention of large elongated un-recrystallised grains in the microstructure reduced the load. The presence of the large elongated grains also appeared detrimental to the ability of the tube to deform before fracture.