Influence of the Processing of Magnesium Alloys AZ31 and ZE10 on the Sheet Formability at Elevated Temperature

Abstract

The substitution of conventional materials such as aluminium alloys and steels with the lightest structural metal magnesium and its alloys can yield significant weight saving in the transportation industry and hence, reduce vehicle weight and greenhouse gas emissions. Producing magnesium sheets by conventional hot rolling is expensive due to the large number of rolling passes to final gauge and annealing steps at elevated temperatures between the rolling passes. Twin roll casting is a well established processing route for aluminium sheets which can reduce the necessary rolling passes to a bare minimum to reduce the production costs. This process is receiving increasing attention for the production of magnesium sheets. This study reveals first hand results of sheet metal forming experiments on magnesium sheets rolled from twin roll cast strip as well as conventional DC cast slabs. Two different alloys, AZ31 (Mg-3Al-1Zn-Mn) and rare earth element containing ZE10 (Mg-1Zn-RE) were investigated. It is known that these alloys show significant differences in the microstructure development during conventional rolling as a result of recrystallisation. For hot rolled AZ31, distinct textures are formed with the majority of basal planes oriented in the sheet plane and hence, unfavourably for basal slip. Conventionally rolled ZE10 commonly shows a much weaker texture. Forming limit diagrams are presented and discussed with respect to the initial texture of the sheets. Strain response to various strain paths and plastic anisotropy are evaluated. Results of twin roll cast sheets are compared with conventionally hot rolled sheet of the same alloys. Competitive formability can be achieved at 200°C for all tested sheets. While conventionally rolled sheets show a generally higher formability than their twin roll cast counterparts, ZE10 outperforms AZ31 for both processing routes.

Processing Effects on the Formability of Magnesium Alloy Sheets

“…The final thickness of the sheets was 1.5 mm. The developed microstructure and texture as well as the mechanical properties of these sheets are comparable to other works in the literature [3,7,10,17]. The extruded sheets were produced from the cast billets of the respective alloys.…”

“…The results for the rolled AZ31 and ZE10 sheets were not repeated in this work, but rather taken from an earlier work [3]. Specimen geometries according to Hasek [20] were used to represent different strain paths.…”

“…In this regard, limits have been seen for a long time, which were based in the limited deformation behavior of magnesium alloys, and often associated with the hexagonal close-packed lattice structure of this metal and its alloys. Many works on the microstructure-property relationship [3][4][5][6] have concluded that the formation of strong textures, especially with a preferential alignment of basal planes in the sheet plane, is an important reason for limited formability. This limitation is primarily due to the limited strain accommodation, especially by basal slip [7], e.g., such textures are formed during the rolling process.…”

“…However, rare earth-containing alloys have been found to enable significant texture changes during the rolling of magnesium sheets, resulting in a weaker alignment of basal planes in the sheet plane [7][8][9][10] or in textures with tilt components of basal planes. Especially, the latter type of texture has been correlated to higher strain accommodation and higher ductility and formability based on a potential balance of active deformation mechanisms [3,11].…”

Substitution of Rare Earths in Magnesium Alloys

“…The poor plastic deformability of common magnesium sheet materials is caused by its pronounced rolling texture and is the reason why until now there are only a few applications of magnesium sheets especially in the automotive industry. The reason for this is the limited number of deformation systems that could be activated in magnesium crystals for plastic deformation at low temperatures [2,3]. Rare earth elements containing magnesium alloys show the demanded randomized texture and fine-grained microstructure [4][5][6].…”