Microstructure and Hot Deformation Behaviour of Twin-Roll Cast AZ31 Magnesium Wire

Abstract

Due to their low density and high specific strength, magnesium alloys offer great potential as a design material for lightweight construction. An economical and energy-efficient method for the production of magnesium wire is the technology of twin-roll casting. In this work, the deformation behaviour of twin-roll cast and heat-treated AZ31 wire pre-profile is investigated for the first time during the compression test at different temperatures (250–400 °C) and forming speeds (0.01–10 s−1). To obtain optimal parameters, a processing map is created, and the microstructural changes during the hot forming processes are determined by accompanying microstructure characterization through an optical microscope and scanning electron microscope. The heat treatment causes a reduction in segregation and a homogeneous microstructure. The average activating energy for plastic deformation of twin-roll cast and heat-treated magnesium alloy AZ31 is 159.008 kJ·mol−1. The instability region of the process map starts at a forming temperature of 250 °C and extends into the range of high forming speeds (1–10 s−1). In this area, cracks in the microstructure can be detected during hot forming. At high temperatures (300–350 °C), dynamic recrystallization at the grain boundaries is observed as the main forming mechanism. Based on these results and observations, existing models for describing the hot forming behaviour of magnesium alloys can be extended and validated.

Twin-Roll Casting as a Grain Refinement Method and Its Influence on the Microstructure and Deformation Behavior of an AZ31 Magnesium Alloy Wire

“…In contrast, the sample's middle exhibits equiaxed grains with segregations and precipitation zones between the grain boundaries. Similar results were found in previous studies [43] and for the AZ31 strips after twin-roll As the segregation region is enriched in probably Al or Zn, these regions may contribute to the formation of non-uniform grain sizes. It can be observed that at the edge of the sample, elongated, coarse grains are formed.…”

“…In contrast, the sample's middle exhibits equiaxed grains with segregations and precipitation zones between the grain boundaries. Similar results were found in previous studies [43] and for the AZ31 strips after twin-roll casting [25]. With the help of EDX analysis (measured in the sample center), it could be detected that the Mg 17 (Al, Zn) 12 precipitations and Al 8 Mn 5 particles are probably present (see Figure 3c).…”

“…Some newly formed grains are aligned with the old grain boundaries, forming a necklace structure. This was already investigated when deforming a TRC and heat-treated AZ31 sample at lower temperatures and speeds (300 • C and 0.01 s −1 ) [43]. In the case of deforming a TRC and heat-treated AZ31 sample at 350 • C and 0.01 s −1 , CDRX was also present.…”

“…Due to the formation of the segregations, a more non-uniform microstructure is present in the TRC state. This can be avoided with the help of a heat treatment [43]. After the short heat treatment, more uniform grains are present, and the particles are more evenly distributed.…”

Influence of Aluminum Content on the Microstructure, Mechanical Properties, and Hot Deformation Behavior of Mg-Al-Zn Alloys

“…Regarding the hardening parameter n, in the literature, it is said that there is a creep of dislocation climb present n > 5 [48,49]. Therefore, it is concluded that the dislocation climb creep is the dominant mechanism for both alloys since AZ31 and AZ91 exhibit higher values than 5 (7.965, resp., 7.128).…”