The Microstructure and Processing in Twin Roll Casting of Magnesium Alloy Strip

Abstract

Magnesium has a hexagonal close-packed crystal structure which makes it more difficult to deform than aluminum. Conventional strip production usually requires several process steps to reach the final strip thickness. Strip casting can reduce some of the process steps and make strip processing simpler and easier, especially for alloys with poor deformability. The twin-roll casting process can directly obtain strips with thickness less than one or two millimeters. In this paper, the metallurgical characteristics of the twin-roll strip casting process were analyzed and discussed. A laboratory scale vertical twin roll caster in prepared AZ31 magnesium alloy strips, with 1.0 to 2.0 mm thick and 150mm wide. Process stability in the thin strip casting process of the alloy has been studied, the casting temperature (superheat) was proven to be a key factor influencing process stability and casting strip quality. The as-cast microstructure of the alloy was analyzed and evaluated by optical microscopy, which showed that the as-cast microstructure was composed of developed dendrites when the superheat was high, and of a rosebush-like structure when the superheat was low. When the casting temperature was close to the liquid phase temperature，the as-cast microstructure became global or grainy, and the grain size of the cast strip was very small. This remarkably improved the deformability of the as-cast strip.

The twin-roll casting of magnesium alloys

“…35 However, in the alloys exhibiting wider freezing ranges than AZ31 alloy, such as AZ61 (∆T o =85 K), AZ91 (∆T o =130 K), and several Mg-Zn based alloys (∆T o 115 K) that have larger alloying content than AZ31 alloy, a fully equiaxed dendritic structure was observed through the thickness of the as-cast strip. [14][15][16]23,26,[58][59][60] Another important characteristic of the Mg sheet fabricated by TRC is the less pronounced basal texture than the one by ingot casting.…”

“…On the other hand, fabrication of Mg alloy strips by vertical TRC has also been reported. [35][36][37] Ding et al conducted a comparative study between vertical and horizontal TRC for Mg alloys. They concluded that horizontal TRC is suitable for producing wide sheets and thus favorable for commercialization due to the relatively easy control of uniform melt distribution while vertical TRC is advantageous with fast casting speed.…”

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

“…However, the cost can be reduced via near-net shape casting processes such as thin slab casting, single-belt casting and twin-roll casting (TRC) [8]. The TRC produces the thinnest as-cast sheet (with thickness less than 6 mm) [9] and is the most economical process, with investment costs that are 1/3 to 1/4 of the conventional Mg sheet production process [10][11][12].…”

Research and Development of Processing Technologies for Wrought Magnesium Alloys

“…They investigated the optimum operating parameters such as melting temperature, casting speed and roll gap. Yang et al [44] studied the processing stability of the twin-roll thin strip casting of the AZ31 alloy and the influence of process factors on the strip structure.…”