Properties of AZ31 Magnesium Alloy Sheet Produced by Twin Roll Casting

Abstract

Twin roll casting has been used to produce AZ31 magnesium alloy strip. The size of the cast strip is 5mm thick and 250mm wide. The microstructure of the as-cast AZ31 alloy strip consists of columnar zones from the surface to mid-thickness region and equiaxed zones in the mid-thickness region. The secondary dendrite arm spacing varies from 5.0 to 6.5 μm through the thickness and width of the strip and is much smaller than 34μm of an AZ31 alloy DC ingot used for comparison. In the temperature ranging from room temperature to 400°C, the tensile strength and elongation of the as-cast strip are higher and smaller than those of DC ingot, respectively. Elongation is evidently improved by homogenization treatment and becomes more than that of the DC ingot. Such an improvement of elongation is though to be caused by homogenization of segregation of Al and Zn atoms in the dendrite cell boundaries. The non-homogenized and homogenized strips were subsequently hot rolled. Their hot rolled sheets have much higher value of elongation with equivalent strength, compared to the hot rolled sheet of flat-bar extruded from DC ingot in the temperature ranging from 200°C to 300°C.

Microstructure and texture studies on twin-roll cast AZ31 (Mg–3wt.%Al–1wt.%Zn) alloy and the effect of thermomechanical processing

“…Due to the high cooling rate and directional heat transfer, large columnar grains have grown against the heat extraction direction where the solidifying material has been in contact with the cold casting rolls. In contrast to the results found by Nakaura and Ohori [22], no bleeding was observed below the sheet surface. Equiaxed grains (Fig.…”

“…A reduction in the centerline segregation has been also reported as a result of annealing [5]. Fine equiaxed grains with an average size of 6 m are produced via TRC followed by warm rolling and annealing [5,[21][22]24,27]. Recently, the present authors have observed texture alteration upon postannealing of the TRC sheet [25].…”

“…Cooling rates of 556-130 • C/s have been determined for these SDAS values using an equation developed for the AZ91 (Mg-9 wt.%Al-1 wt.%Zn) alloy [21,23,27]. These SDAS values are significantly smaller and the cooling rates are much higher than those commonly seen in DC casting (SDAS of 34 m and cooling rate of 10 • C/s [21,22,27]). …”

“…The TRC that combines casting and in situ warm rolling produces a unique combination of cast and deformed structure. The structure of the TRC AZ31 sheet consists of a chilled-zone close to the sheet surface, followed by columnar-grains formed as a result of directional solidification [5,[21][22][23][24][25]. Equiaxed grains observed in the central region of the TRC sheet can be attributed to constitutional undercooling.…”

Magnesium Sheet; Challenges and Opportunities

“…The microstructure of a twin-roll cast AZ31 alloy, with about 5 mm thickness, consists of a columnar-grain zone from the surface to the mid-thickness region and an equiaxed-grain zone in the mid-thickness region. The secondary-dendrite arm spacing is within 5-6.5 μm range through the thickness and the width of the strip, which is much smaller than ~34 μm of a DC ingot AZ31 alloy (Nakaura & Ohori, 2005). The small dendrite arm spacing in a twin-roll cast AZ91 alloy improved the workability at elevated temperatures.…”