Solidification Microstructure and Mechanical Properties of Hot Rolled and Annealed Mg Sheet Produced through Twin Roll Casting Route

Abstract

The use of wrought magnesium for automobile structural components is an important component of the mass reduction strategy for automobiles to improve their fuel efficiency. Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31 magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions. Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and mechanical properties were comparable with commercial AZ31 sheet.

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

“…A heat treatment reduces the amount of segregation in the twin-rolled cast magnesium wire pre-profile (460 • C/15 min). Similarly, within the investigations by Aljarrah et al (2011) [22], heat treatment (400 • C/1 h) significantly reduced the amount of centerline segregation detected in a twin-roll cast AZ31 strip. Analogous results were obtained by Kawalla et al (2011) [23].…”

Thermal fluid mathematical modelling of twin roll casting (TRC) process for AZ31 magnesium alloy

“…Owing to different cooling conditions and the presence of the solute rich liquid material an equiaxed zone is formed at the strip centre. 23 Figure 8 shows more details of the microstructure of the AZ31 twin roll cast strip above the centreline of the strip. The black arrows on the microstructure illustrate the direction of the columnar dendrites growth.…”

Twin roll casting of magnesium

“…The real solute profi le measurement shows that the Al content from surface to ~45% of the thickness of the strip is almost constant, at about 2.7~2.8 wt% (0.2~0.3 wt% lower than the original AZ31 composition), while the Al content in the central region (only ~20% of strip thickness) reaches about 6 wt% (Aljarrah et al , 2011). Although the calculated solute profi le across the thickness does not exactly correspond to the real TRC strip profi le, the results are qualitatively similar.…”

“…The typical overall compositions of the columnar dendrite and center line segregation region in central equiaxed zones are roughly Mg-2.7%Al-0.9%Zn-0.3%Mn and Mg-8%Al-4%Zn-0.2%Mn, respectively (Aljarrah, 2011). Figure 7.11 shows the phase distribution in the columnar and central zones, as calculated using the Scheil cooling scheme.…”