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Texture and mechanical properties of Strip Cast and hot rolled magnesium AZ31
Abstract
Strips of AZ31 were produced by Direct Strip Casting and subsequent hot rolling at 400 degrees C. The texture and microstructure of the strips in as cast and hot rolled condition were investigated. The cast strip exhibited a random distribution of the grain orientations while a strong basal-type texture was observed in the hot rolled sheets. The flow curves of the Direct Strip Cast material were obtained by hot compression experiments and dynamic recrystallization was found to occur at all studied temperatures and strain rates. The tensile properties of strip cast and hot rolled magnesium were examined by tests at room temperature. Grain refinement and the development of the basal texture during hot rolling resulted in high strength of the hot rolled magnesium sheet.
... In this regard, it is helpful that the grains, which are typically oriented randomly after casting (Zi et al., 2011;Asgari et al., 2014), deform regularly on preferred slip planes during manufacturing processes (e.g., in rolling, extrusion, forging). Due to the crystal and sample symmetries, the resulting texture also shows symmetrical relationships (Bunge et al., 1981), often resulting in typical preferred orientations for process and material combinations. ...
Microstructure analysis via electron backscatter diffraction has become an indispensable tool in materials science and engineering. In order to interpret or predict the anisotropy in crystalline materials, the texture is assessed, e.g. via pole figure diagrams. To ensure a correct characterization, it is crucial to align the measured sample axes as closely as possible with the manufacturing process directions. However, deviations are inevitable due to sample preparation and manual measurement setup. Postprocessing is mostly done manually, which is tedious and operator-dependent. In this work, it is shown that the deviation can be calculated using the contour of the crystal orientations. This can also be utilized to define the axis symmetry of pole figure diagrams through an objective function, allowing for symmetric alignment by minimization. Experimental textures of extruded profiles and synthetically generated textures were used to demonstrate the general applicability of the method. It has proven to work excellently for deviations of up to 5∘, which are typical for careful manual sample preparation and mounting. While the performance of the algorithm is reduced with increasing misalignment, good results have also been obtained for deviations up to 15∘.
... For discussion of the measured texture, orientation density functions (ODF) were calculated from the discrete datapoints and plotted in pole figures (PF) as depicted in Figure 8. While the texture of the cast stock can be assumed to be random [33,34], the extruded material shows a typical extrusion texture, further discussed in Section 1 of the Supplementary. The EBSD measurements have also been used to gain an overview of the grain sizes and the respective degree of recrystallization, as listed in Table 2 and Figure 9, confirming the grain structures described in Section 4.2.1. ...
The intensifying search for light weighting possibilities in transportation have repeatedly brought attention on wrought Mg alloys. While various alloys have been investigated a trend in development towards heat treatable Mg alloys with low alloying content has been noticeable. Investigations on wrought alloys are done predominantly by extrusion or rolling, but not by forging. As forgings are an indispensable part in structural components used today, it is important to gain an in depth understanding of the interaction between material, forming process and heat treatments of forged parts.
In this study, the forging process of a piston rod using an age-hardenable lean Mg alloy AXMZ1000 is investigated on a semi-industrial scale, comparing two different stock materials: cast and homogenized versus extruded forging material. The microstructural evolution and mechanical properties during the production process are analyzed and assessed. Comparable microstructures are obtained with both starting materials. The mechanical properties achieved are slightly better with extruded feedstock than with the cast counterpart, but are at a satisfactory level comparable to extruded or rolled components made of similar alloys.
A consortium formed by research institutes, steel producers and a plant builder is developing the DSC-strip production process. It is based on a casting process where liquid steel is fed on a moving conveyer belt cooled from below by water. After solidification a strip is obtained which is directly in-line hot rolled. The developmental state achieved at the pilot plants, main steps in the process development and obtained product properties are given. Based on the state reached at the pilot plants, it can be concluded that the DSC-process is suitable for production of high quality strip. Such a production process would provide a very high productivity which is a basic requirement for a strip casting process to fit into an integrated plant. The investment and production costs would be comparatively low. By avoiding reheating processes to a far extent and by optimising the amount of hot reduction a significant energy saving can be achieved. A far reaching perspective of the new near-net-shape casting processes, and especially of the DSC-process, is the production of strip from new steel materials having properties which are significantly superior to conventional steel grades. The next big step in order to establish the DSC-process is the design and erection of a production-scale demonstration plant.
The deformation behavior of a coarse grained AZ31 alloy during hot rolling was investigated. The basal texture originates from the {10–12} twins at the initial stage of rolling. During subsequent deformation, the grains with the c-axis parallel to compression stress are insensitive to dynamic recrystallization. This is considered to be the main reason for the progressive strengthening of the basal texture.
For a number of metallic polycrystalline aggregates, it is shown experimentally that [sgrave]f, the flow stress at constant strain, is related to the grain diameter l by where [sgrave]0 and k are constants. This has the same form as the relationship between the lower yield point, when this occurs, and grain size. An explanation is given by an extension of Taylor's theory to allow for the resistance at the grain boundary to the formation of a slip band. Dislocation-locking and a small number of slip systems are shown to favour a strong grain-size dependence of [sgrave]f and this explains the variation of this dependence amongst the common metals and alloys.
The effect of strain path change on the rolling behavior of twin roll cast (TRC) magnesium alloy AZ31 was investigated by subjecting the TRC material to hot unidirectional and cross rolling, respectively. In general, unidirectional rolling exhibited stronger texture intensities compared to cross rolling. Increasing the thickness reduction per rolling pass, i.e. reducing the total number of rolling passes, gave rise to weaker textures. For cross rolling, the texture intensity was independent of the strain amount per pass.
Equal channel angular pressing (ECAP) and large-strain hot rolling (LSHR) are widely used methods for refining the grain size in magnesium alloys. The hardening capability of the processed materials confers the resistance to develop tensile mechanical instabilities, therefore controlling ductility. In this work various magnesium alloys were processed using ECAP, LSHR and annealing treatments in order to control the texture and the grain size. Emphasis was laid upon the influence of these factors on work hardening behavior and dynamic recovery. In addition to the direct effect of texture through the change in the orientation factor for basal and prismatic slip, effects were found on dynamic recovery and the appearance of stage II of work hardening. The grain size refinement causes a strong decrease in the hardening rate. A contribution of grain boundary sliding to deformation gives a plausible explanation of these results.