Faramarz Zarandi

Magnesium Sheet; Challenges and Opportunities

Where light-weight components are desired, e.g. in automotive and aerospace applications, magnesium can provide an important advantage over other structural materials such as aluminum and steel. The density of magnesium is two thirds that of aluminum and a quarter that of galvanized steel. The combination of low density and reasonable strength of magnesium leads to a specific strength, which is much higher than that of steel or aluminum. Stiffness is often as important as strength and, with respect to bending stiffness, magnesium offers major advantages over both steel and aluminum. Using more magnesium would significantly decrease the weight of automobiles, which is one of the important goals in automobile design. However, usage of magnesium lags far behind that of aluminum. To date, most magnesium applications in the automobile industry are in the form of die cast parts. Wrought magnesium, particularly in the form of sheet, represents a tremendous growth opportunity in the application of magnesium, e.g. inner door panel, engine bonnet, seat components, roof, and fenders. The use of magnesium sheet is, however, severely limited because of: (a) the high cost of magnesium sheet, (b) the poor room temperature ductility of magnesium, and (c) the relatively high propensity to corrosion. Sheet materials are required to display sufficient formability when subjected to bending stresses during fabrication into parts. The poor plastic flow characteristics of magnesium at room temperature have considerably hampered its sheet applications. Essentially, because magnesium has a hexagonal close packed (hcp) structure, active slip systems at low temperatures are mainly limited to those involving basal planes. This is because the critical resolved shear stress (CRSS) for basal plane slip in magnesium single crystal is 100 times lower than that for non-basal plane slip (prismatic or pyramidal planes) near room temperature. Thus, the distribution of basal planes (0001) in magnesium plays an important role in determining formability at low temperatures. As the temperature increases, the CRSS of the non-basal slip systems decreases and, therefore, there is a significant increase in formability. Consequently, sheet forming at elevated temperatures is one possibility that is being contemplated, and there is considerable ongoing research in this general area, including those concerned with the viability of superplastic forming. In this chapter, the challenges to the production of magnesium sheet and the route toward overcoming them are explained. Then, deformation characteristics of magnesium and associated mechanisms are discussed.

Effect of boron on the hot ductility of the Nb-microalloyed steel in austenite region

Two grades of the Nb-microalloyed steel, one modified with B, were subjected toin situ melting and the thermal schedules experienced by the billet surface in the continuous casting process. The hot ductility was evaluated at various temperatures at the straightening stage of the process. It was found that addition of B improves the hot ductility in the austenite region. Such improvement could be due to fast precipitation at grain boundaries as well as depletion of precipitants and strengthening elements in the matrix.

Effect of High Temperature Deformation on the Hot Ductility of Nb Microalloyed Steel

Low hot ductility of steel at the straightening stage of the continuous casting process is a problem found in steels containing microalloying and/or certain alloying additions. The thermal schedule undergone by the billet surface in the mill has a significant effect on the hot ductility. In this work, thermomechanical processing was employed to alleviate the problem of hot ductility in the Nb-microalloyed steel. Specimens were melted in situ and subjected to the billet surface thermal schedule in order to generate a microstructure similar to that present at the straightening stage of the continuous casting process. Some deformation schedules were incorporated with the thermal schedule at very high temperatures, specifically during solidification, within the d-ferrite region, and during the d®g transformation, and the hot ductility was subsequently evaluated at the end of the thermal schedule where the straightening operation is performed. After the thermal schedule alone, the steel exhibited a very low hot ductility at the straightening stage. It was found that deformation at very high temperatures prior to the straightening stage had a considerable effect on the hot ductility, either detrimental or beneficial, depending on the region in which the deformation has been executed. The mechanisms leading to loss and improvement of hot ductility are explained in this paper.

Application of Deformation to Improve Hot Ductility in the Peritectic Steel

The hot strip rolling of advanced microalloyed high strength steels still represents a new task to many mills due to the lack of data on the hot deformation resistance. With the aid of processing data from the Ispat-Inland hot strip mill, the “measured mean flow stresses” are calculated from the mill force using the Sims analysis and taking into account roll flattening, slip ratio and the redundant strain. A modification of the Misaka mean flow stress equation is proposed for C – Mn – Si – Al steels microalloyed with up to 0.02 % Nb. The effects of alloying and microalloying are then estimated. A new fitting parameter shows excellent agreement with the mean flow stress data from industrial processing of advanced high strength microalloyed steels. However, during the second half of the rolling schedule (lower temperature region), indications of austeniteto- ferrite transformation were found.

Effect of Annealing on Microstructure, Texture and Tensile Properties of Twin-Roll Cast AZ31B

Twin-roll cast (TRC) AZ31 alloy (Mg-3wt.%Al-1wt.%Zn) was subjected to heat treatment at 420 °C. As a result, the intensity of the original basal texture was reduced considerably. Crystallographic orientation analysis revealed that such a change in the texture is due to particle-stimulated nucleation of new grains with random orientations. The tensile test results indicate that annealing slightly increases ultimate tensile strength (UTS), however, dramatically improves the elongation.

As-Cast Microstructure and Texture of Twin-Roll Cast AZ31

In this study, the microstructure and texture of twin roll cast (TRC) AZ31 (Mg-3wt.%Al-1wt.%Zn) alloy has been investigated. The structure of the as-cast TRC AZ31 was dendritic containing both columnar and equiaxed grains. Second phase particles, Al-Mn and Mg-Al-Zn, were distributed evenly in matrix. The macro-texture analysis shows that as-received TRC AZ31 had a basal texture. The micro-texture analysis by electron-back-scattered diffraction (EBSD) revealed that the large grains were mostly basal, however the smaller grains tended to have varying orientations.