AZ31 Billets Research Articles

Improved Corrosion Behavior of AZ31 Alloy through ECAP Processing

This study aims to establish the effects of equal channel angular pressing (ECAP) processing on the corrosion behavior and hardness values of the AZ31 Mg alloy. The AZ31 billets were processed through ECAP successfully at 250 °C and their microstructural evolution was studied using optical and field emission scanning electron microscopy. The corrosion resistance of the AZ31 alloy was studied before and after processing through ECAP. The homogeneity of the hardness distribution was studied using both sections cut parallel and perpendicular to the extrusion direction. ECAP processing resulted in highly deformed central regions with elongated grains aligned parallel to the extrusion direction, whereas the peripheral regions showed an ultra-fine-grain recrystallized structure. After processing, small ultra-fine secondary particles were found to be homogeneously dispersed alongside the grain boundaries of the α-Mg matrix. Regarding the corrosion properties, measurements showed that ECAP processing through 1-P and 2-Bc resulted in decreasing their corrosion rate to 67.7% and 78.3%, respectively, of their as-annealed counterpart’s. The corrosion resistance of the ECAPed Mg alloy increased with the number of processing passes. This was due to the refinement of the grain size of the α-Mg matrix and secondary phases till ultra-fine size, caused by the accumulation of strain during processing. On the other hand, ECAP processing through 2-Bc resulted in increasing the Vickers hardness values by 132% and 71.8% at the peripheral and central areas, respectively, compared to the as-annealed counterpart.

The simulation researches on hot extrusion of super-fined tube made of magnesium alloys

To research the deformation mechanisms of extrusion for artificial blood vessel. A series of experiments were done to obtain the simulation parameters: stress-strain curves, friction factors and heat transfer coefficient, etc. Three-dimensional, coupled thermo-mechanical finite element simulations of extruding a wrought magnesium alloy were performed. Computed parameters including workpiece material characteristics and process conditions (billet temperature, reduction ratio, and ram speed) were taken into consideration. The temperature distribution of the transient-state extrusion is different from the steady-state extrusion. There exists heat flow in two reverse directions. The maximum temperature is found to be at the center of small tube along the longitudinal direction, while the minimum temperature is found at the edge of the billet in contact with both the container and the die face. The material at the center of the profile has a lower strain rate and therefore it flows through the die with less severe deformation. It is clear that the metal flow velocity field along the negative Y axis remains steady during tube extrusion, so artificial blood vessel with homogeneous microstructures would be obtained. The maximum damage occurs at the billet surface in the exit region of the die. This is because critical damage occurs at the point of maximum tensile stress in the tube. The extrusion simulation was the reliable predictions of strain rate, effective strains, effective stresses and metal flow velocity in an AZ31 billet during direct extrusion.

Preparation of Non-Dendritic Microstructure of AZ31 Alloy Using Isothermal Heat Treatment of the Refined Billet

Semisolid metal forming (SSF) has been regarded as a potential process to shape the wrough Mg alloy base on the thixotropy and low deformation resistance of non-dendritic semisolid slurry. Therefore, it is necessary to develop the slurry production method of wrought Mg alloy as most current researches are focused on the cast alloy. In this paper, the AZ31 billet was refined by the self-inoculation method, and the partialy remelt method was employed to produce the non-dendritic microstructure. Results indicate that both the morphology and size of the primary Mg phase in AZ31 were refined by the grain refining treatment. When the billet were reheated under 595°C and 610°C with different holding times, the morphology of primary Mg phase undergo an evolution pattern from rosette or tiny dendritic to spherical, and with the prolonged holding time, the size of primary Mg phase increased and the shape turn to irregular. Moreover, the result show that the high holding temperature shorten the evolution time from irregular to spherical and the deteriorative grains obtained earlier compared to the low holding temperature.

Investigation on Grain Refinement of AZ31 Alloy by Self-Inoculation Method

The grain refining technology of AZ31 alloy by using self-inoculation method (SIM) has been developed. Results indicated that SIM significantly decreased the grain size of as-cast AZ31 billet from 287 to 58 μm, changed the morphologies and size of the β-Mg17Al12 phase and decreased the solute elements segregation along grain boundaries. The average grain size is sensitive to the diameter of a cast rod; the microstructures of the large diameter rod are relatively inhomogeneous. The grain refinement was mainly attributed to the grain multiplication which derives from heterogeneous nucleation induced by self-inoculants and free crystals and dendrite fragments enhanced by fluid director, combined with a high solidification rate of alloy melt in the metal mold.

A Study on Compressive Anisotropy and Nonassociated Flow Plasticity of the AZ31 Magnesium Alloy in Hot Rolling

Effect of anisotropy in compression is studied on hot rolling of AZ31 magnesium alloy with a three-dimensional constitutive model based on the quadratic Hill48 yield criterion and nonassociated flow rule (non-AFR). The constitutive model is characterized by compressive tests of AZ31 billets since plastic deformations of materials are mostly caused by compression during rolling processes. The characterized plasticity model is implemented into ABAQUS/Explicit as a user-defined material subroutine (VUMAT) based on semi-implicit backward Euler's method. The subroutine is employed to simulate square-bar rolling processes. The simulation results are compared with rolled specimens and those predicted by the von Mises and the Hill48 yield function under AFR. Moreover, strip rolling is also simulated for AZ31 with the Hill48 yield function under non-AFR. The strip rolling simulation demonstrates that the lateral spread generated by the non-AFR model is in good agreement with experimental data. These comparisons between simulation and experiments validate that the proposed Hill48 yield function under non-AFR provides satisfactory description of plastic deformation behavior in hot rolling for AZ31 alloys in case that the anisotropic parameters in the Hill48 yield function and the non-associated flow rule are calibrated by the compressive experimental results.

Microstructure Formation and Mechanical Properties of AZ31 Magnesium Alloy Solidified with a Novel Mechanical Vibration Technique

A novel mechanical vibration for refining microstructure is reported where vibration energy was directly exerted into a molten alloy by a vibrating horn, and the vibrating horn was melted during vibration. Effects of vibration intensity and melt superheat on the microstructure and mechanical properties of AZ31 magnesium alloy were investigated. It is confirmed that the melting of the vibrating horn could effectively extract the superheat and latent heat from the interior of the molten alloy, leading to rapid cooling during the initial stage of solidification, and the cooling rate is strongly dependent on the vibration acceleration and melt superheat. This study showed that it was difficult to refine the solidified microstructure when the treated alloy was kept in the full liquid state within the entire vibrating duration. A significantly refined microstructure was obtained by applying mechanical vibration during the nucleation stage, and a globular microstructure could form in a few seconds after solidification. When the molten alloy was treated from 920 K to 903 K (647 °C to 630 °C), with increasing vibration acceleration from 2.5 to 19 m s−2, the coarse dendritic microstructure of the produced AZ31 billets transformed into a well-refined, reasonably uniform, and non-dendritic one, and mechanical properties were improved significantly. Moreover, the mechanisms of microstructure formation are discussed.

FEM Simulation on Extrusion of Magnesium Alloys

Extrusion of magnesium billets is associated with large deformations, high strain rates and high temperatures, which results in computationally challenging problems in process simulation. A simulation was carried out using the finite element software ABAQUS. The computed model was rotational symmetric and built up by meshing. Computed parameters including material characteristics and process conditions (billet temperature. reduction ratio, and ram speed) were taken into consideration. The distributions of temperature were different comparing the transient-state extrusion with the steady-state extrusion. The extrusion simulation was the reliable predictions of strain rate, effective strains, effective stresses and metal flow velocity in an AZ31 billet during direct extrusion.

Structure and mechanical properties of AZ31 magnesium alloy billets by different hot-top semi-continuous casting technology

AZ31 alloy billets of 200 mm in diameter were produced by three different processes of conventional direct chill (DC) casting, low-frequency electromagnetic casting (LFEC) and low-frequency electromagnetic vibration casting (LFEVC), respectively. The effect of LFEC and LFEVC on the microstructures, macrosegregation and mechanical properties of AZ31 alloy billets was investigated. In conventional DC casting, the AZ31 alloy billets exhibited coarse grains (about 370 μm) and severe segregation of Al and Zn. In the presence of a solo low-frequency alternating magnetic field or a low-frequency electromagnetic vibration field applied during DC casting of ϕ200 mm AZ31 billets, grains in the AZ31 alloy billets were effectively refined (about 210 μm) and the macrosegregation of Al and Zn in the billets was greatly decreased. Furthermore, the tensile strength, fracture elongation and hardness of the as-cast AZ31 alloy billets were improved by the processes of LFEC and LFEVC relative to that cast by the process of conventional DC casting.

Modeling the Stress-Strain Behavior and Hot Tearing during Direct Chill Casting of an AZ31 Magnesium Billet

Quantitative knowledge of the thermal mechanical history experienced during direct chill (DC) casting aids the scientific understanding of the process especially in terms of defect formation such as hot tearing. In this work, a thermomechanical finite element (FE) model has been developed to simulate the DC casting of magnesium alloy AZ31 billets. The mathematical model simulates the evolution of temperature, stress, and strain within the billet during an industrial DC casting process. These quantities were then used to calculate the evolution in pressure, and hence hot tearing tendency, within the semisolid regime via the Rappaz–Drezet–Gremaud (RDG) criterion. The temperature predictions were validated against experimental thermocouple data measured during a plant trial at an industrial magnesium DC casting facility. In addition, the residual elastic strains predicted by the model were compared to residual strain measurements made at the Canadian Neutron Beam Centre (CNBC) using a magnesium billet produced during the industrial casting trial. The validated model was then used to quantitatively assess the impact of casting speed on the hot tearing tendency in AZ31 billets.

A three-dimensional finite element simulation of the warm extrusion of AZ31 alloy billets

Extrusion of magnesium billets is associated with large deformations, high strain rates and high temperatures, which results in computationally challenging problems in process simulation. A series of experiments were done to obtain the simulation parameters: stress–strain curves, friction factors and heat transfer coefficient etc. Three-dimensional, thermo-mechanically coupled finite element simulations of extruding a wrought magnesium alloy AZ31 into a small bar at certain ram speeds were performed. The computed model was rotational symmetric and built up by meshing. Computed parameters including workpiece material characteristics and process conditions (billet temperature, reduction ratio, and ram speed) were taken into consideration. The distributions of temperature were different comparing the transient-state extrusion with steady-state extrusion. The extrusion simulation was the reliable predictions of strain rate, effective strains, effective stresses and metal flow velocity in an AZ31 billet during direct extrusion.

Production of Magnesium Alloy Billets by Semisolid Continuous Casting

In order to produce AZ91 and AZ31 magnesium alloy billets semisolid continuous casting by using an inclined cooling plate was carried out. The molten magnesium alloy in an electric furnace was flowed on the inclined cooling plate and then poured in a tundish. The slurry prepared using this process was solidified by drawing a dummy bar from a water-cooled copper mold, consequently producing a billet. As a result, we could produce the magnesium alloy billets by semisolid continuous casting. Magnesium alloy billets with refined and globular primary α crystals were successfully and continuously cast by optimizing the solidification conditions. In this case, the grain size of AZ91 billets was about 55μm and that of AZ31 billets was 50μm. The AZ91 billets produced by semisolid continuous casting were employed in thixocasting experiments. The experiments revealed that thixocasting could produce the mobile phone casings with spherical primary α crystals at 858K. Moreover, we proved that the difference of shot cycle did not influence the microstructures for up to 251 shots.

Microstructures and mechanical properties of Mg–Al–Zn–Ca alloys fabricated by high frequency electromagnetic casting method

The microstructures and mechanical properties of AZ31 and 1 wt%Ca-containing AZ31 billets fabricated using EMC (Electromagnetic Casting) and EMS (Electromagnetic Stirring) were examined. The results show a great potential of producing high-quality surface magnesium billets with fine-grained microstructure at a relatively high casting speed. Application of EMC + EMS for production of the 1 wt%Ca-AZ31 alloy billet with a diameter of 150 mm produced a reasonably homogeneous microstructure composed of fine grains with an average size of 45 μm. Attainment of the fine-grained and homogeneous microstructure by EMC + EMS was attributed to reduction of temperature gradient and fragmentation of dendrite structure under electromagnetic force. Strength of the EMC and EMC + EMS 1 wt%Ca-AZ31 billets was higher than that of the EMC AZ31 billet due to the grain size and particle strengthening effects.

Microstructure Evolution of AZ31 Magnesium Alloy during Warm Compression Forming

Warm compression tests of AZ31 Mg alloy were carried out at five temperatures in 30°C intervals from 210°C to 330°C. The samples of different thickness which were machined from as-cast and pre-strained AZ31 billets were compressed into thickness 1mm and then cooled in the air to room temperature. The microstructural evolution of AZ31 Mg alloy was investigated during warm compression forming. The results show that all the samples have undergone a microstructure changes to different scales in the range investigated. The twinning is the predominant deformation mechanism for magnesium alloys at moderate temperatures and its occurrence is dependent on temperature and strain. Microstructural evaluation indicates that the mean size of the recrystallised grains decreases with increasing effective strain and temperature because of sufficient dynamic recrystallization. The original grain has significant influence on microstructural evolution during warm forming.