Junjie He

Deformation behaviour of hot extruded Mg alloy AZ31 during compressive deformation

Abstract Deformation behaviour of a hot extruded Mg–3Al–1Zn alloy subjected to uniaxial compression at room temperature was investigated. Microstructure evolution showed that the amount of lenticular shaped extension twins increased with the increase in true strain when it was lower than ∼4% but decreased when it was higher than ∼4%. Previous studies indicated that detwinning occurred. However, we have demonstrated in the present paper that no detwinning occurred, but a process of twin growth and coalescence, leading to an analogous detwinning phenomenon observed in the optical micrographs because most of the matrixes were consumed. A strong texture evolution occurred in this process, and the reorientation produced by strong twinning was mainly responsible for strain hardening when compressed along the extrusion direction.

Improved ductility of magnesium alloy sheets by pre-hardening and annealing

Abstract The mechanical behaviour of AZ31 magnesium alloy sheets were evaluated at different directions tilting 0, 45 and 90° to rolling direction. A suitable critical point of pre-stretching has been investigated through the work hardening curve of the magnesium alloy sheets. The microstructure and mechanical properties were examined and compared. The results indicated that the average yield strength of the 0, 45 and 90° pre-hardened 4% specimens followed by annealing decreased 30 MPa and the average uniform elongation increased up to 38% compared with the as received sheet. Pre-hardening process is suggested as a potential technique to improve the ductility and anisotropy of magnesium alloy sheets.

Reducing tension–compression yield asymmetry by free end torsion in extruded Mg alloy

Free end torsion was carried out on extruded Mg alloy AZ31. Subsequently, tension or compression was implemented on pretorsion specimens. The microstructures of samples with 100 and 200° of pretorsion reveal that no obvious change has occurred; however, samples with 300 and 400° of pretorsion show that there are extensive twins. Compressive yield stress of pretorsion samples increases with increasing pretorsion angle. However, their tensile yield stress exhibits practically the same value with extruded samples. Consequently, yield asymmetry ratio increases from 0.55 of extruded sample to 0.84 of sample with 400° of pretorsion. Both textures rotated around the radial axis by free end torsion, and different deformation mechanisms of compression and tension deformation result in decrease in yield asymmetry.

Enhancing strength and ductility of AZ31 magnesium alloy sheets by the trapezoid extrusion

Abstract The microstructure evolutions and mechanical properties of AZ31 magnesium alloy sheets processed by the conventional extrusion and the trapezoid extrusion (TE) were investigated. The textural evolutions of TE sheets at near surface are different throughout the sheet thickness direction due to an additional shear deformation. This enforces the basal plane to tilt toward the direction of shear deformation. Moreover, it weakens the basal texture intensity. Trapezoid extrusion process exhibits a considerable grain refinement. Improved strength and ductility of magnesium alloys sheets were achieved by TE approach due to the tilted weak basal texture and the grain refinement.

Effects of prior compression on ductility and yielding behaviour in extruded magnesium alloy AZ31

Abstract In the present paper, the effects of precompression along extrusion direction (ED) on subsequent compression perpendicular to ED were investigated in an extruded magnesium alloy AZ31. The results showed that the yield stress under compression perpendicular to ED increased if there was precompression along ED. The evolution of deformation mechanism was responsible for increase in yield stress because plastic deformation was dominated by both basal slip and {10–12} twinning under compression perpendicular to ED in samples without prestrain, but basal slip was difficult to be activated and {10–12} twinning dominated deformation in samples with precompression. However, because basal slip had no obvious contribution to plastic deformation, the ductility decreased if there was precompression along ED.

Continuation of twins across grain boundary in extruded Mg–3Al–1Zn alloy

Abstract Paired twins in an extruded Mg–3Al–1Zn alloy are investigated by using electron backscattered diffraction in the current paper. The results show that these paired twins are discovered at low misorientation grain boundaries. The twin variant (1–102)[−1101] is operated in the paired twins. Additionally, a macroscopic angle exists in the paired twins and is determined by the c axis misorientation of the grains.

Roles of detwinning in strain hardening and ultimate elongation in extruded Mg–3Al–1Zn alloy

Abstract For extruded magnesium alloy, prior compression along extrusion direction has great influences in the flow stress during subsequent tension. Detwinning plays an important role for these influences. In the present study, the effects of different prestrains on strain hardening behaviour during subsequent tension were examined in an extruded magnesium alloy AZ31. The results showed that the existence of detwinning decreased the tensile yield stress. Samples with different prestrains exhibited different strain hardening behaviour during subsequent tension. The reorientation due to detwinning had a great effect on strain hardening during tension. In addition, the effect of detwinning on ultimate elongation was investigated. The results showed that the sample with higher prestrain always has higher ultimate elongation due to the contribution of detwinning on macroscopic strain.

Microstructural study of forged magnesium pipes at room temperature

Abstract We report the impact of room temperature forging on microstructure evolution and mechanical properties of hot extruded Mg pipes with a special focus on twinning behaviour and texture. The twinning is identified as an important deformation mechanism. The amount of twins are first increased with the increase in forging strain and then reduced remarkably, followed by a sudden increase until the saturation state. Different types of twins are identified, and the role they play in the deformation process is discussed. The basal planes are found to be rotated by a right angle from the extrusion direction during the room temperature forging. The Vickers hardness is increased drastically once imposing the room temperature forging, thereby demonstrating that this technique is effective in modifying microstructures and mechanical properties of metals.

Effect of SiO2 nanoparticles as lubricating oil additives on the cold-rolling of AZ31 magnesium alloy sheet

Abstract Magnesium alloy sheets were processed by cold rolling without lubrication and with SiO2 nanolubricant so as to investigate cold rolling performance by measuring rolling force as well as the surface roughness of the sheets. A series of ball on disc tests without lubrication and with SiO2 nanolubricant conditions were carried out to simulate the contact and sliding during the cold rolling process. The results indicate that the SiO2 nanolubricant effectively reduced the rolling force and improved the surface quality of sheets by SiO2 nanolubricant during cold rolling process. The excellent cold-rolling performances of AZ31 Mg alloy with SiO2 nanolubricant were attributed to the friction-reducing and the anti-friction mechanism was based on the deposition of nanoparticles on the magnesium alloy surfaces.