Papers by Keyword: Differential Speed Rolling

Abstract: The present work investigates the influence of shear deformation on microstructural-texture and mechanical behavior of ZEW200 Mg alloy sheets. For the introduction of extra shear deformation during thermomechanical processing, the separate effect of differential speed rolling (DSR) and equal channel angular pressing (ECAP) were analyzed. The results were compared with the microstructure and mechanical behavior of equal speed rolled (ESR) ZEW200 sheets. No significant texture changes were observed after the utilization of DSR, while ECAP processing was effective in changing the character of the texture and reducing the texture intensity. The large yield stress asymmetry observed in the rolled sheet is strongly reduced in the ECAP processed sheets. Results showed the potential to use shear deformation to modify the crystallographic texture via the profuse activation of {10-12}<10-11> extension twins. The presence of a large twin fraction of the microstructure modified the work hardening behavior of the processed sheets due to the further activation of basal <a> slip. The application of extra shear deformation to tailor the texture during processing is, therefore, an alternative to optimize the deformation behavior of already formable Mg alloys.

Abstract: Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminum. However, the poor formability of magnesium semi-finished products (e.g. sheets) has hindered the massive application of this metal. This poor formability arises due to the formation of strong textures. Typically the basal planes of the HCP-structure align parallel to the sheet plane. Such preferred orientation of the basal planes limits the ability of basal <a> slip to accommodate plastic strain in the sheet plane and is thus unfavourable for ductility and formability of the sheets, especially at room temperature. A combination of new technologies could help to alleviate the strong textures formed in Mg sheets. In this regard the utilization of twin-roll casting (TRC) in combination with processing techniques which apply additional shear strain such as differential speed rolling (DSR) and/or equal channel angular pressing (ECAP) could refine the microstructure and modify the resulting deformation texture. In this study an AZ31 strip (3Al-1Zn-Mg Bal. wt.%) produced by TRC was hot rolled at 400 °C using a rolling speed of 10 m/min. A speed ratio of 10% between rolls was used. Rolled samples with dimensions of 200 x 200 x 1.8 mm receive a single pass of ECAP at 200 °C. After processing the microstructure of the samples were analyzed by means of optical microscopy, electron backscatter diffraction and X-ray diffraction. The results showed that the microstructure can be refined and the texture altered in comparison to conventionally rolled AZ31 sheets. The samples processed by ECAP show an increase in formability up to 50% higher than conventionally rolled sheets. This demonstrates the potential of using shear deformation for processing Mg sheets.

Abstract: AZ31 magnesium alloy sheets were prepared by multi-pass differential speed rolling (DSR) with small mismatch roll speed ratio. Effect of pass thickness reduction on microstructure, texture and formability at room temperature were investigated. The results showed that the low pass thickness reduction DSR is effective to improve the formability by refining the grain size and weakening the basal texture, which is characterized by the increasing of strain hardening exponent (n value) and elongation as soon as decreasing of yielding ratio and normal anisotropy ratio (r value). With the increasing of pass thickness reduction, more effective grain refinement is achieved due to the forming of deformation bands. While the ability of forming the title basal texture is weakened and the formability is reduced.

Abstract: The grain size and the distribution of crystal orientation have an important effect on the mechanical properties of wrought AZ31B magnesium alloy sheets. Because the AZ31B magnesium alloy sheets rolled by conventional rolling have a poor formability at room temperature, a new rolling technology of differential speed rolling is used to improve the mechanical properties of AZ31B magnesium alloy. The research shows that the number of twinning crystal decreases, the number of the core of dynamically recrystallized grain increases, and the grain size become fine and isotropy by differential speed rolling with the increase of the reduction and the improving of the rolling temperature to some extent. The differential speed rolling not only improves the isotropy of the basal texture and also improves the microstructure and mechanical properties.

Abstract: In Mg-Al-Zn alloy sheets, the grains were refined from 30 mm to 2 mm during the process of differential speed rolling at 473 K, having approximately the same size dominate at a high thickness reduction rate of 60 %. In addition to these microstructural changes, the rolled sheets showed a weaker texture characterized by pole than those of commercial sheets. The DSR sheets have equal value of elongation at 0, 45, and 90 degrees from the rolling direction. With the mechanical properties, the results of Erichsen test indicate that DSR process remarkably enhanced ductility of the present Mg alloy sheets at room temperature.

Abstract: The AZ61 magnesium alloy was subjected to the differential speed rolling at different rolling conditions including reduction per pass, rotation speed ratio and rolling temperature, and the influences of the rolling conditions on microstructure, texture, mechanical properties and formability of the as-rolled sheets were investigated. Increasing the reduction per pass results in a more homogeneous microstructure and a relatively weaker basal texture. With increasing the rotation speed ratio, the inclination angle of basal pole toward the rolling direction increases and the stretch formability enhances correspondingly. Increasing the rolling temperature has an effect on weakening the basal texture intensity and leads to the enhancement of stretch formability.

Abstract: It is known that well developed <111>//ND texture increases Lankford value (r-value) of not only bcc metals but also fcc metals and alloys. However, <111>//ND texture cannot be formed in fcc metals by conventional rolling and annealing processes. The <111>//ND orientation is one of the major components of shear texture. Accordingly, this orientation develops in aluminum sheet when shear deformation is introduced. Al-Mg-Si alloy 6016 sheet was processed by two-pass differential speed rolling at room temperature under a high friction conditions. The rolling direction of the second pass was so selected that the direction of shear deformation introduced in the second pass was either similar (unidirectional shear rolling) to or opposite (reverse shear rolling) to that in the first pass. The roll speed ratio was 2.0. Large shear strain was successfully introduced through the thickness uniformly by the differential speed rolling. The shear texture with major components of {001}<110> and {111}<110> were developed throughout the thickness. Though large reduction in thickness of 75% was applied to the sheets by the rolling, conventional rolling texture such as {112}<111> or {123}<643> orientation was not detected in any part of the thickness. By solution treatment after the rolling, intensity of shear texture weakened and grain size decreased. It has been found that r-value is improved by the differential speed rolling subsequently followed by solution treatment.

Abstract: For the Magnesium alloy AZ31, hot rolling is usually carried out in the temperature range between 250 and 400°C but the processed sheets usually exhibit high anisotropy in mechanical properties. In the current study, DSR process was found to be effective in improving anisotropy of mechanical properties and ductility at room temperature. Full recrystallization takes place from 200°C and above. A large drop of UTS occurs above 200°C where full recrystallization starts. Tensile elongation increases with annealing temperature but anisotropy degrades from 200°C onwards. Texture change during recrystallization is believed to be responsible for this result.

Abstract: The aims of this study ares to investigate the microstructure evolution of AZ31 Mg alloys with normal rolling and different speeds rolling during hot rolling affects microstructure, texture and mechanical properties of AZ31 Mg alloy. In the microstructures of as-rolled both samples, twins are clearly apparent, small and recrystallized grains are visible along some grain boundary and twinned regions. The tensile strength and yield strength of DSR sample were slightly higher than that of NR sample. Also, in the case of the NR sample, tensile strength indicated different values to the rolling directions. From this result, NR sample compared to DSR sample strongly indicated to the plastic anisotropy tendency. Therefore, it is noted that DSR sample could be presented to the good formability, comparing to the NR sample. DSR samples deformed at 473K and 523K could be perfectly formed, indicating the potential application of the DSR process to improve formability of the Mg alloys at warm temperatures.

Abstract: An AZ31 (Mg-3Al-1Zn-0.35Mn in mass%) alloy sheet exhibiting the inclination of the basal plane from the rolling plane at about 15º, was fabricated by a DSR processing with a roll speed ratio of 1.167. Compared with the normal rolled sheets, the DSR processed sheets showed a lower 0.2% proof stress, a larger elongation, a smaller r-value and a larger n-value. The Erichsen value at room temperature significantly increased from 2.2 to 3.1, and the deep drawability also improved.