Rolled AZ31 Sheet Research Articles

Microstructural basis for improved corrosion resistance of laser surface processed AZ31 Mg alloy

Despite their excellent strength-to-weight ratio, wider use of magnesium (Mg) alloys for light-weight applications is limited by their poor corrosion resistance, especially in chloride-containing environments. The present study shows improved corrosion resistance imparted by laser surface processing (LSP) of a commercial AZ31 (Mg-3Al-1Zn) alloy. Nanosecond laser processing at three different power settings was carried out on the surface of a 1-mm-thick rolled AZ31 sheet. Electrochemical studies and salt spray testing (ASTM B117) indicate substantial enhancement of corrosion resistance in LSP-treated AZ31. The underlying reasons behind improved corrosion resistance of the LSP-AZ31 surface have been studied through detailed microstructural characterization and chemical analysis by SEM, TEM, and XPS. Formation of a ∼0.5 μm thick mixed metal (Mg, Al) oxide surface film, together with refinement in the size and number density of Al-Mn intermetallic particles, are shown to play a major role toward improved corrosion resistance after LSP treatment of the AZ31 alloy.

Mechanical Anisotropy Induced by Strain Path Change for AZ31 Mg Alloy Sheet

The variation of strain paths induces anisotropy during practical sheet forming processes, which is very important for the subsequent processing technology of anisotropic Mg alloys. In this study, two-step loading tests (tension-tension) were performed to clarify the effect of strain path changes on the evolution of anisotropy on rolled AZ31 sheet. Specimens were preloaded with tension along the rolling direction (RD) with 9% of prestrain. Then, second tension was conducted along 0°, 30°, 45°, 60° and 90° from the RD. It was found that yield strength during the second loading increased along the same direction compared to uniaxial tension without prestraining. For the second loading, the yield strength and flow stress decreased with the increase of the angle from the RD. It was found that the strain path change resulted in stronger anisotropy than that induced by texture. Moreover, it was found that the main deformation modes were basal and prismatic slips during the second loading based on visco-plastic self-consistent (VPSC) modeling. The relative activities of basal and prismatic slips were affected by the second loading direction due to texture evolution. The mechanical anisotropy induced by strain path changes was ascribed to the coupling of the heterogeneous distribution of dislocations and texture evolution induced by prestraining.

Effect of Pulsed Magnetic Field on the Residual Stress of Rolled Magnium Alloy AZ31 Sheet

A novel method of pulsed magnetic field (PMF) treatment was developed to eliminate the residual stress of rolled magnesium alloy AZ31 sheet in this study. The effect of PMF on residual stress of rolled AZ31 sheet was investigated and its mechanism was analyzed. The experimental results revealed that the pulse frequency had a significant impact on residual stress. After 10.0 Hz PMF treatment, the average and maximum reduction rates of residual stress along the rolled direction were 26.6% and 30.3%, respectively. It was found that the dislocation density and parallel dislocation in grains of the rolled sheet increased after it was treated by the pulsed magnetic field. The simulation results showed that the Lorentz force generated by the pulsed magnetic field can lead to basal slip, thereby resulting in local plastic deformation. Besides, the Joule heat produced during the PMF treatment was conducive to the elimination of residual stress.

Influence of Wavy Bending on Microstructure and Mechanical Properties of a Rolled AZ31 Sheet

In the present work, cross-wavy bending at room temperature was carried out to tailor the microstructure and stretch formability of rolled AZ31 sheets. Wavy bending processing generates profuse {10–12} twins and a tilt basal texture. Subsequent recrystallization annealing causes grain coarsening and enhances the intensity of twin-orientation. The combined use of wavy bending and annealing can maintain high tensile ductility and remarkably enhances the stretch formability of rolled AZ31 sheet. It can be mainly attributed to the non-basal texture in the wavy bent sheet which increases the thinning capability during in-plane tension.

Improving Strength and Formability of Rolled AZ31 Sheet by Two-Step Twinning Deformation

The present work aims to improve strength and stretch formability of rolled AZ31 sheet by two-step twinning deformation, i.e., compression (8%) along the rolling direction (RD) and subsequent tension (5%) along RD. A recrystallization annealing was carried out between two twinning deformations to avoid detwinning. It shows that two-step twinning deformation can form strong c-axis//RD texture and generate profuse crossed twin lamellae in each grain. The former can generate a refinement hardening effect to enhance the strength and the latter can contribute to a high stretch formability at room temperature.

Investigation of microstructure and texture during continuous bending of rolled AZ31 sheet by experiment and FEM

Various continuous bending experiment with three loading rates (100, 300 and 500 mm/min) at room temperature was adopted to investigate the diversity of microstructure, springback and texture of sheet after straightening. Due to grain size at the central layer of the bent plate decreased with the augment of bending rate, and the grain size was finest and most homogeneous at the bending rate of 500 mm/min. After each bending process, due to the springback and the shift of the neutral layer, the load of each time was gradually degressive. The deviations of springback between bending passes declined by degrees under 300 and 500 mm/min rates. However, the springback difference reached a minimum on the ninth pass under 100 mm/min bending rate. In addition, the intensity of {0002} basal texture, which is close-packed plane got to a minimum on the 100 mm/min bending rate. Meanwhile, the intensity of {10 1¯ 1} and {10 1¯ 2} textures, which shifted from TD-split to concentrating on the side of the RD, declined with incresing the bending rate. The FE software was used and the experimental process was simulated accurately.

An algorithm for automatic identification of double twin variants in Mg alloys

101¯1−101¯2 double twins (DTs) are profuse in Mg alloys after rolling at a large reduction (>10%). In many cases, the secondary twins completely occupy the primary ones, causing difficulty in variant analysis. Here, we propose an algorithm for automatic identification of twin variants. This is applicable for the 101¯1−101¯2 DTs without knowing the primary twins. In addition, two kinds of stereographic projections are constructed to assist such variant selection. By this method, the variants of 67 DTs observed in a rolled AZ31 sheet are determined. Based on 14 sets of DTs with known primary twin orientations, the high efficiency and reliability of this proposed algorithm is verified. The reason for unreliable variant identification in two special cases is also discussed.

Enhancing stretch formability of rolled Mg sheets by pre-inducing contraction twins and recrystallization annealing

It is commonly accepted that weakening or tilting basal texture is an effective way to improve stretch formability of Mg alloy sheets. In this study, contraction twins (including double twins) were introduced in rolled AZ31 sheet by pre-cold rolling to a thickness reduction of 10%. It was noted that the stretch formability of rolled AZ31 sheet can be greatly enhanced by pre-cold rolling and subsequent recrystallization annealing. The Erichsen value of rolled AZ31 sheet was increased up to 66% by pre-cold rolling and annealing at 400°C. The improvement in stretch formability was mainly attributed to the formation and growth of recrystallization grains with weak texture nucleated at contraction twins during annealing.

The influence of the recrystallization mechanisms and grain growth on the texture of a hot rolled AZ31 sheet during subsequent isochronal annealing

The texture development during isochronal annealing at 250, 300 and 350°C for 30min of a hot rolled Mg AZ31 sheet produced by twin-roll casting was studied in this work. It was found that the rolling texture shows some features that resemble the textures that develop in rolled Mg alloys with rare earth additions. During further heat treatment, special attention was given to the deformation and recrystallization mechanisms, and grain growth that control the texture development. It was found that at 250 and 300°C extended recovery and discontinuous recrystallization influence the texture by generating strain free grains with off-basal orientation. Conversely, during annealing at 350°C growth of grains with their c-axis close to the (0001) leads to the formation of the well-known basal type texture.

On the Controversy about the Presence of Grain Boundary Sliding in Mg AZ31

Highly-textured, rolled AZ31 sheet material shows a significant drop in the plastic anisotropy (r-value; r=ew/et) in tension between 25°C and 200°C. This behavior was initially explained as a result of the increased activity of non-basal slip with increased temperature. Other authors suggested, however, that the mechanism responsible for this phenomenon was the activation of grain boundary sliding (GBS). Here, in-situ tensile tests have been carried out in an SEM at various temperatures in order to obtain further evidence of the role of GBS during moderate to high temperature deformation of Mg alloys, which remains highly controversial.

Twinning, grain orientation and texture variation of AZ31 Mg alloy during compression by EBSD tracing

In order to investigate the micro-mechanism of warm forming of Mg alloys, three specimens cut from a rolled AZ31 sheet were chosen to be compressed along the Rolling Direction (RD) at 100 °C, 170 °C and 230 °C, separately. During compression, an in situ measurement of grain orientation in the plane of RD × TD (Transverse Direction) was carried out with EBSD method. Experimental and analytical results show that temperature has remarkable impact on activation of twinning and variation of texture. As the temperature was raised from 100 °C to 230 °C, the number of grains with twins activated decreased substantially during deformation, and rolling texture varied from quick vanishing at 100 °C to always existing at 230 °C. Tracing for orientation of individual grains during deformation shows that there are obvious different orientation changes between grains with twins activated and those without twins activated. Twinning plays a significant effect on texture variation during compression. The extension twin variant really activated during deformation is the one with maximal Schmid factor.

Research on the Rolling of AZ31 Strip Prepared by Twin Roll Casting

The AZ31 thin sheet (minimum thickness less than 1mm) was obtained by rolling the cast strip which was prepared by vertical twin roll casting in this paper. Since the absolute deformation during rolling was smaller,due to the small thickness of the strip of 3mm, the rolling of cast strip was different from the conventional rolling process. It was found that homogenizing time at 400°C for the cast strip was 4h and reduction per pass should be 8-10% for producing thinner (1-1.5mm) rolled AZ31 sheet at 350°C. Mechanical properties of the sheet were equivalent to conventional rolling sheet’s.

Properties of AZ31 and AZ91 Sheets Made by Twin Roll Casting

A feasibility study has been made to produce the Mg alloy sheet of high tensile strength and formability by strip casting and an optimized rolling process. 4mm thickness AZ31 and AZ91 strips were cast successfully. While the microstructure of the cast AZ31 strip consisted of columnar zones, the cast AZ91 strip consisted of fine, equiaxed dendrite and fine precipitations. All samples were rolled to 0.5 mm thickness using an optimized finish-rolling schedule, and their grain size was around 4μm. The ultimate tensile strength (UTS), yield strength (YS) (0.2% offset) and elongation (EL) of the rolled AZ31 sheet were higher than those of commercial AZ31B sheet, and the UTS and YS of the rolled AZ91 sheet were 30% higher than commercial AZ31B sheet. The press formability of the rolled AZ31 and AZ91 sheet was equal or superior to commercial AZ31B sheet.