Abu Syed Humaun Kabir

Promotion of texture weakening in magnesium by alloying and thermomechanical processing: (I) alloying

The recrystallization and texture evolution of four Mg–Zn–Ce sheets with a warm-rolled microstructure obtained through two stages that can be characterised as rough rolling and finish rolling were investigated at different stages of post-rolling annealing. On annealing, the same regions of the microstructure, located by hardness indentations, were examined and tracked by electron backscatter diffraction (EBSD). Furthermore, intragranular misorientation axes (IGMA) analysis was used to investigate the associated deformation mechanisms in the as-deformed material. By combining these two methods, the development of the recrystallization microstructure was investigated and important aspects, such as preferential nucleation sites, correlation between activated deformation mechanism and initial orientation of the recrystallized grains, were studied. The results showed that the Mg–1Zn–1Ce alloy, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and the most homogenous distribution of shear banding/twinning. The IGMA analysis also showed that in Mg–1Zn–1Ce, other types of dislocations rather than basal 〈a〉 were activated; in particular, prismatic 〈a〉 type was activated during deformation. Therefore, the weakening of recrystallization texture during rolling resulting from the addition of RE elements was linked with a change in dynamic recrystallization (DRX) behaviour. Since the Mg–1Zn–1Ce alloy corresponds to the highest level of Ce in solid solution, the observed texture weakening was possibly due to decreasing grain boundary mobility as a result of solute partitioning of RE elements to dislocations and grain boundaries.

Effect of Postweld Heat Treatment on Microstructure, Hardness, and Tensile Properties of Laser-Welded Ti-6Al-4V

The effects of postweld heat treatment (PWHT) on 3.2-mm- and 5.1-mm-thick Ti-6Al-4V butt joints welded using a continuous wave (CW) 4-kW Nd:YAG laser welding machine were investigated in terms of microstructural transformations, welding defects, and hardness, as well as global and local tensile properties. Two postweld heat treatments, i.e., stress-relief annealing (SRA) and solution heat treatment followed by aging (STA), were performed and the weld qualities were compared with the as-welded condition. A digital image correlation technique was used to determine the global tensile behavior for the transverse welding samples. The local tensile properties including yield strength and maximum strain were determined, for the first time, for the laser-welded Ti-6Al-4V. The mechanical properties, including hardness and the global and local tensile properties, were correlated to the microstructure and defects in the as-welded, SRA, and STA conditions.

Influence of Static Precipitation on Microstructure and Texture of Annealed Cold-Rolled Mg-Al-Sn Alloys

The final mechanical properties of wrought magnesium alloys are mostly controlled by its microstructure and crystallographic orientation or texture. In the sheet form of common magnesium alloys, grain coarsening occurs during annealing, which only serves to strengthen the undesirable basal texture. One method to alleviate this problem is by stopping grain coarsening. Hence the aim of this work is to investigate the effect of static precipitation on microstructure and texture evolution during annealing at various temperatures after cold rolling. Mg-Al-Sn alloys were designed using thermodynamic modeling software, FactSageTM. It was found that static precipitates can only form extensively, after static recrystallization, at the recrystallized grain boundaries, and these retard grain growth during annealing. Presence of precipitates at the recrystallized grain boundaries also retard the strengthening of basal texture during annealing, related to grain coarsening.

Deformation Behavior and Dynamic Recrystallization of Micro-Alloyed Mg-Al-Ca Alloys During High Temperature Deformation

Two micro-alloyed magnesium alloys, Mg-0.3Al-0.2Ca (AX0302) and Mg-0.1Al-0.5Ca (AX0105), were designed based on the thermodynamics calculation in terms of precipitation temperature. Hot compression tests were conducted at temperatures of 300°C, 350°C and 400°C with strain rates of 0.1s−1, 0.01s−1 and 0.001s−1. Dynamic precipitation of Al2Ca could be found below 400°C in AX0302, while Mg2Ca dynamically formed in AX0105 during deformation at all three temperatures. At high temperature and low strain rate (400°C at 0.01s−1 and 0.001s−1 and 350°C at 0.001s-1), DRX mainly developed at the grain boundaries and formed necklace type microstructure in both alloys. However, at lower temperature and higher strain rate, DRX grains formed at both grain boundaries and twinning boundaries. The combination effect of twinning and dynamic precipitation on dynamic recrystallization was studied by comparing with two alloys.

Study of Dynamic Precipitation during Hot Deformation of Mg-Al-Sn Alloys

The objective of this work is to investigate the effect of dynamic precipitation on the hot deformation behaviour of magnesium. Mg-Al-Sn alloys have been designed using thermodynamic modeling software, FactSage, based on precipitation of Mg2Sn at hot deformation temperatures. Uniaxial compression has been introduced at the temperature range of 250-350 °C to enhance the formation of precipitates. The flow behaviour and microstructural evolvement were studied for a strain rate of 0.01 /sand deformation degree of 90 %. Dynamic recrystallization (DRX) occurs in this study and it is found that the volume percent of dynamic recrystallization and dynamically recrystallized grain size are related to the amount of precipitation formed during deformation. The formation of Mg2Sn precipitates during the dynamic recrystallization process may retard the dynamic recrystallization and slow down the grain growth by precipitation pinning effect at the DRX grain boundaries, resulting in a finer grain size.

Thermodynamic Modeling and Experimental Measurement of Precipitation Formation during Dynamic Recrystallization for Magnesium Alloys

Magnesium alloys have low formability at room temperature associated with its hexagonal closed pack structure. Formation of precipitates during deformation may pin the grain boundary and reduce the final grain size, which literally means the improvement of formability. The aim of this study is to design magnesium alloys that are capable of forming precipitate during hot deformation. Thermodynamic modeling software, FactSage, has been used to design Mg-Al-Sn alloys based on forming target Mg2Sn precipitate at hot deformation temperatures between 250 and 350oC. Uniaxial compression at elevated temperatures has been performed to simulate the hot deformation behaviour as well as to enhance the formation of precipitates. Strain rates used in this study were in the range of 1.0 to 0.001s-1 for a constant deformation degree of 90%. It was found that the formation of precipitate depends on deformation temperature and strain rate. Measured amounts of precipitate were compared with the calculated equilibrium results from FactSage.

Effect of Precipitation on Texture Evolution during Dynamic Recrystallization in Mg-Al-Sn Alloys

In common magnesium alloys, recrystallization is usually not accompanied with a noticeable change of deformation texture resulting in, strong mechanical anisotropy. The aim of this work is to investigate the effect of dynamic or strain induced precipitates on texture evolution during hot deformation of Mg-Al-Sn alloys. Mg-Al-Sn alloys have been designed using thermodynamic modeling software, FactSage, based on forming precipitates at hot deformation temperatures. Two alloys have been chosen in a way that one forms precipitates during the hot deformation process, another does not at a certain temperature. Uniaxial compression has been introduced at different strain rates at this elevated temperature to simulate the hot deformation behaviour. Two alloys have been compared in terms of the intensity of basal texture. It was found that the formation of dynamic precipitates during deformation weakens the texture compared to the case where no precipitates were formed.

Precipitation behaviour of micro-alloyed Mg-AL-CA alloys during heat treatment and hot compression

Based on thermodynamic calculations, two micro-alloyed Mg-Al-Ca alloys, Mg-0.3Al-0.2Ca and Mg-0.1Al-0.5Ca, were selected in terms of the equilibrium precipitation temperatures of Al2Ca and Mg2Ca, respectively. The basic idea is to form precipitates during hot compression to examine their effect on hot deformation. Both alloys, cast by copper mould, were solution treated at 500°C for 8 hours to dissolve eutectic precipitates which formed in the as-cast microstructure, and then isothermally heat treated at 350°C for different times. SEM analysis of the heat treated alloys generally agreed with thermodynamic calculations. Hot compression tests were also conducted on solution treated alloys at 350°C with a strain rate of 0.01s-1 and different strains (10%, 30%, 60%, and 90%). The precipitation behaviour and microstructural evolution which was characterized by optical microscopy and SEM with BSE and EDS detectors during isothermal heat treatment and hot compression were compared.

Effect of Dynamic Recrystallization on Microstructure Evolution and Texture Weakening During Annealing of High Speed Rolled AZ31 Magnesium Alloy Sheets

Magnesium AZ31 (Mg-3 wt. % Al-1 wt. % Zn) alloy sheets were rolled at a high speed of 1000 m/min at 100 °C to reductions of 30% and 49%. Annealing was then conducted on the as-rolled specimens at temperatures of 200 °C and 350 °C for different times. The microstructure was characterized by optical microscopy and the macrotexture was analyzed by X-ray diffraction. The as-rolled microstructure of the specimen subjected to the reduction of 30% was heavily twinned and shear banded, while a partially dynamically recrystallized and twinned microstructure was observed at the reduction of 49%. Effect of the initial microstructure on static recrystallization behavior and texture evolution during annealing was studied at different temperatures. Texture weakening was found during annealing at both reductions. However, the weaker texture can be achieved in the specimen subjected to the reduction of 30% than that of 49%.

Non-Basal Texture Evolution During Annealing of Cold-Deformed Magnesium Alloy

In this study, effects of statically and dynamically formed precipitates on the microstructure and texture evolutions were investigated after two step deformations and subsequent post deformation annealing of Mg-3Al-3Sn (wt.%) alloy. At first, the alloy was compressed at low temperature and at a relatively high strain rate to simulate the rolling behavior. Annealing at moderate temperature recrystallizes the microstructure and forms static precipitation. Further deformation was performed on the recrystallized microstructures with the presence of precipitation followed by a subsequent annealing at the deformation temperature. Microstructures were characterized by optical and scanning electron microscopes and macro- and micro-texture were measured by X-ray diffraction (XRD) and Electron Back-Scattered Diffraction (EBSD) techniques, respectively. Fine recrystallized grains were obtained after annealing due to the presence of large volume fraction of precipitates. Also, the texture was weakened significantly after the final annealing step probably due a mechanism that involves the precipitates.