Microstructure and Mechanical Properties of Twin-Roll Strip Cast Mg Alloys

Abstract

Development of wrought Mg alloys, particularly in sheet form, is essential to support the growing interest for lightweight components in the automotive industry. However, development of Mg alloy sheets has been quite slow due to the complexity of sheet production originated from limited deformability of Mg. In this respect, twin-roll strip casting, a one-step processing of flat rolled products, can be an alternative for the production of Mg alloy sheets. In this study, AZ31 and experimental ZM series alloys are twin-roll strip cast into 2 mm thick sheets. The microstructure of the as-cast AZ31 alloy sheet consists of columnar zones near the roll side and equiaxed zones in the mid-thickness region. On the other hand, as-cast ZM series alloy sheets show equiaxed dendritic structure through the thickness of sheet. These alloys were subjected to various thermo-mechanical treatments and their tensile properties were evaluated. Twin-roll strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that twin-roll strip casting is a viable process for the fabrication of Mg alloy sheets. The experimental ZM series alloys have a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of twin-roll strip casting on microstructural refinement. It has been shown that the experimental ZM series alloys have superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by twin-roll strip casting.

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

“…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].…”

“…The AZ31B alloy (supplied by Timminco, now Applied Magnesium) was melted in a graphite crucible using a NORAX induction furnace under a gas mixture of sulfur hexafluoride (SF 6 ) and carbon dioxide (CO 2 ). A plate with a thickness of 4 mm was cast at 720 • C into a vertical copper die and at a cooling rate close to that experienced during the TRC process, i.e., ∼185 • C/s [21,23,27].…”

Critical Assessment 6: Magnesium sheet alloys: Viable alternatives to steels?

“…11 The tensile properties of TRC Mg alloy sheets are comparable with or better than those of Mg alloy sheets produced by conventional ingot casting and the subsequent complicated rolling process. 12,13 Although most TRC Mg alloys, either commercial or experimental, do not contain RE elements, the use of RE elements in conventionally produced Mg sheet alloys appears to be unavoidable at present when formability and isotropic mechanical properties are concerned due to the significant effect of RE elements on randomising and weakening the basal texture. [14][15][16][17] The amount of RE elements in most of these alloys ranges from 1 to 3 wt-% and therefore the increase in raw material cost by using RE elements would not be significant at present.…”

Rolling Twin Roll Cast Magnesium Strips with Varied Temperature and Degree of Deformation

“…There are worldwide a small number of industrial or laboratory scale twin roll casters installed at universities, companies and research facilities. Initial results from these activities on conventional wrought and cast alloys have shown promising sheet properties [2][3][4][5][6]. Former results indicate that the processing parameters of twin roll casting have a significant influence on the microstructure and texture of the strips [7].…”