1Zn Magnesium Alloy Research Articles

Influences of grain size on mechanical properties and cold formability of Mg–3Al–1Zn alloy sheets with similar weak initial textures

Mg–3Al–1Zn magnesium alloy sheets processed by high temperature rolling were annealed at different temperatures ranging from 250°C to 500°C, which resulted in an increase in grain size from 7μm to 16μm but similarly retained weak basal textures. The annealed sheets were used for investigating the influences of grain size on mechanical properties and press formability at room temperature. Increasing the grain size results in a decrease in r-value and an increase in n-value, which are generally favored for the improvement of sheet formability. However, both stretch formability and deep drawability deteriorate with an increase in grain size. The sheet annealed at 250°C exhibits superior deep drawability with a high limited drawing ratio of 1.87. In contrast, the sheet annealed at 500°C fractures at the flange when drawn at a ratio of 1.72. For the former sheet, the restricted tensile twinning activity due to the smaller grain size leads to a delay of texture strengthening for the TD-texture forming at the flange during deformation, which is responsible for the enhanced deep drawability.

A comparative study between uniaxial compression and plane strain compression of Mg–3Al–1Zn alloy using experiments and simulations

Magnesium anisotropic behavior is enhanced by different deformations paths and modes. The mechanical anisotropy during plastic deformation of a Mg–3Al–1Zn magnesium alloy was examined by uniaxial compression (UC) and plane strain compression (or channel-die CD) at room temperature. The mechanical behavior and the observed microtextures were compared with the predictions obtained from different crystal plasticity models. A hot rolled AZ31 magnesium alloy plate with a strong basal texture was stressed along four different directions to initiate different deformation mechanisms. In both UC and CD, the compression along the transverse direction generated mainly {10−12} extension twins, while compression along the normal direction (Z=ND) was initially accommodated by pyramidal 〈c+a〉 slip followed by contraction twinning. During compression at 45° of the normal direction to the transverse direction, both slip and twinning took part in the deformation. The sample compressed in CD along the rolling direction and with the normal direction constrained was deformed by prismatic slip and tension twinning. It was found that basal slip was an important deformation mechanism in all the samples. Simulations were made with different models, and the simulated flow stresses were higher in CD than that in UC, but slightly underestimated in case of uniaxial compression. Modeling also corroborated higher {10−11} compression twinning activity in CD than that in UC, and that compression twinning resulted in a decrease in flow stress in CD. To fit with the experimental flow stresses, it was necessary to take into account an additional hardening caused by twin boundaries. In our model, the {10−12} twin boundaries were estimated to increase the critical resolved shear stress (CRSS) of prismatic slip by about 30%. However, twin induced hardening on pyramidal 〈c+a〉 and basal slip of about 10% is also necessary if contraction twinning do not soften the material.

Fabrication of ultrafine-grained Mg–3Al–1Zn magnesium alloy sheets using a continuous high-ratio differential speed rolling technique

Continuous production of ultrafine-grained magnesium alloy sheets could be realized through the development of a continuous high-ratio differential speed rolling technique. The obtained alloy sheets showed the enhanced room-temperature mechanical properties and superplasticity compared to the commercial magnesium sheets.

Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy

A semi-solid processed (thixomolded) Mg–9Al–1Zn magnesium alloy (AZ91D) was subjected to friction stir welding (FSW), aiming at evaluating the weldability and fatigue property of the FSW joint. Microstructure analysis showed that a recystallized fine-grained microstructure was generated in the nugget zone (NZ) after FSW. The yield strength, ultimate tensile strength, and elongation of the FSW joint were obtained to be 192MPa, 245MPa, and 7.6%, respectively. Low-cycle fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the BM, which could be well described by the Basquin and Coffin-Manson equations. Unlike the extruded magnesium alloys, the hysteresis loops of FSW joint of the thixomolded AZ91D alloy were basically symmetrical, while the non-linear or pseudoelastic behavior was still present. The FSW joint was observed to fail in the BM section rather than in the NZ. Fatigue crack initiated basically from the pores at or near the specimen surface, and crack propagation was mainly characterized by fatigue striations along with the presence of secondary cracks.

Effects of initial microstructure on the microstructural evolution and stretch formability of warm rolled Mg–3Al–1Zn alloy sheets

Microstructural evolution and stretch formability of Mg–3Al–1Zn (AZ31) magnesium alloy sheets warm rolled at 225°C by a single differential speed rolling pass with two different initial microstructures were investigated. A cast ingot and that rolled by one pass at a high temperature of 550°C with average grain sizes of ~470μm and ~19μm, respectively, were used as starting materials. Both warm rolled sheets exhibit deformation microstructure in as-rolled condition and the occurrence of texture randomization during subsequent annealing as a result of static recrystallization. New recrystallized grains with a large orientation spread tend to persist in the original orientations of parent deformed grains and twin hosts. Regarding the nucleation sites at pre-existing grain boundaries and double twins, static recrystallization preferentially occurs at the sides of deformed grains and within double twins with their c-axes inclining toward the rolling direction (RD), resulting in a stable end annealing texture with a basal pole tilting toward the RD. For the sheet warm rolled from a cast ingot, incomplete recrystallization originating from less stored energy in the interiors of large deformed grains results in an insufficient texture weakening and an inhomogenous microstructure. Both sheets possess an improved stretch formability compared with conventional AZ31 alloy sheets due to the weakened textures. Benefiting from a weaker basal texture and a more homogenous microstructure, the sheet rolled at 550°C followed by warm rolling at 225°C exhibits a higher Erichsen value of 8.2 compared with that (6.1) of the sheet warm rolled directly from a cast ingot.

Multi-temperature equal channel angular pressing of Mg-3 wt%Al-1 wt%Zn alloy

Grain refinement of magnesium alloys at temperatures below 200 °C via deformation processes is a challenging task. Equal channel angular pressing is one of the severe plastic deformation techniques which can significantly refine grain size. Refined grains consequently improve mechanical properties of magnesium alloys. In this paper, Mg-3 wt%Al-1 wt%Zn magnesium alloy was processed by different multi-temperature equal channel angular pressing cycles without backpressure. The lowest finishing temperature of equal channel angular pressing was 150 °C leading to fine-grained microstructure with sporadically large elongated grains. Shear bands were observed after equal channel angular pressing finishing at temperatures of 175 and 150 °C. Electron backscattered diffraction revealed that grain boundary distribution is qualitatively the same for all equal channel angular pressing cycles. Crystallographic analyses of large elongated grains showed that {0 0 0 1} and {} planes in these grains tend to be aligned parallel to the shear plane. Microtexture measurements showed split of (0 0 0 2) poles. The shape of stress–strain curves and consequently yields tensile strength of Mg-3 wt%Al-1 wt%Zn alloy after multi-temperature equal channel angular pressing was influenced by final texture and twinning.

Effects of Zinc and Aluminum Concentration on the Microstructure and Properties of ZA Series Mg Alloys

The microstructures of a series of high Zn magnesium alloys Mg-(7-11) Zn-(2-6) Al alloys were clarified, and effects of Zn and Al concentration about the ZA system alloys on the microstructure and properties were also investigated. For Mg-(7-11)Zn-4Al alloys with the Zn content increase, improved creep resistance is obtained due to block τ-Mg32(Al, Zn)49phase, with severe reduction in ultimate strength and ductility, and the best creep property is obtained with 11wt.% Zn concentration. However, a bit increasing Al content of Mg-9Zn-(2-6)Al alloys contributes to the improvement in the ambient and elevated-temperature yield strength, but seriously deteriorates the creep properties for the formation of lamellar φ-Al2Mg5Zn2eutectics or Mg51Zn20phase.

In-situ rod-shaped nanoparticles in Mg–Zn magnesium alloy: Towards high strength and ductility

Abstract Zinc ingot was added to molten ZK60A magnesium alloy to improve tensile and compressive properties after hot extrusion. In tension, Zn addition increased strength (by up to +71%) and ductility (by +109%). The addition of Zn resulted in significant grain refinement by about 1 order of magnitude, and increased alignment of the basal plane along the extrusion direction (or force axis). More importantly, the addition of Zn enabled the significant formation of rod-shaped intermetallic nanoparticles (β nanorods) responsible for significant strengthening during tensile deformation. The β nanorods were preferentially formed from hetrogenous precipitation of dissolved Zn on finer α nanorods. The observation of non-basal slip in the high strain zone (HSZ) adjacent to the β nanorod (after room temperature tensile deformation) indicated the sufficiently robust nature of the interface between the β nanorod and the alloy matrix. In compression, Zn addition increased strength (by up to +74%) but at the expense of ductility (decreased by −34%). There was buckling and fracture of the β nanorod (loss of strength) during compression but this was offset by the significant grain refinement and increased alignment of the basal plane along the force axis (gain in strength). Also, there was HSZ formation from the tip of one β nanorod to another, but the strain hardening rate was overall higher due to Zn addition, resulting in ductility loss.

Investigation on the impression creep properties of a cast Mg–6Al–1Zn magnesium alloy

In the present study, the creep behavior of a cast Mg–6Al–1Zn (AZ61) magnesium alloy was investigated using impression creep technique at the temperature range of 423–495K under shear modulus normalized stress (σimp/G) between 0.02 and 0.0425. Based on the obtained results in the applied stress range, the creep behavior of the alloy was divided into two low and high stress regimes. Stress exponent varied in the range of 4–6 and 11–12 at the low and high stress regimes, respectively. Also, creep activation energy (Q) varied in the range of 113–199kJ/mol and 170–188kJ/mol at the low and high stress regimes, respectively. The activation energy values obtained at the low stress regime were between activation energy for magnesium lattice self diffusion (135kJ/mol) and dislocation pipe diffusion (92kJ/mol). Considering the obtained stress exponents and creep activation energies, it can be stated that the climb controlled dislocation creep and power law breakdown are the dominant creep mechanisms at the low and high stress regimes, respectively.

Influence of initial texture on cold deep drawability of Mg–3Al–1Zn alloy sheets

Mg–3Al–1Zn magnesium alloy sheets rolled at 350°C, 450°C and 525°C, which exhibit approximately the same grain size but a significant decrease in basal texture intensity with increasing rolling temperature, were used for investigating the influence of initial texture on deep drawing behaviour at room temperature. The sheets rolled at 350°C and 450°C fracture at drawing ratios of 1.5 and 1.7, respectively, while the sheet rolled at 525°C can be successfully drawn at a large drawing ratio of 1.8. The fracture site changes from the shoulder of drawn cup to the edge of the flange with the weakening of initial basal texture. At the flange, the basal texture dramatically changes to the TD-texture with basal planes perpendicular to the sheet plane accompanied with sheet thickening. Both tensile twinning and basal slip contribute to this texture change due to compressive stress along the circumferential direction. In the case of weak initial texture, relatively restricted tensile twinning and more activated basal slip delay the development of the TD-texture, which is favoured for maintaining deformation capability at the flange and in turn enhances deep drawability. The deformation behaviour is discussed in terms of the change in Schmid factor for basal slip during deep drawing.

Effects of grain size and heat treatment on the tensile properties of Mg–3Nd–0.2Zn (wt%) magnesium alloys

This paper reports the tensile properties and deformation behavior of a newly cast magnesium alloy Mg–3Nd–0.2Zn–xZr (NZ30Kx, x=0, 0.1, 0.3, 0.5, 1.0, 2.0wt%) with different grain sizes and heat treatment conditions. Zirconium (Zr), as an effective grain-refiner in NZ30K alloys, is found to significantly improve the yield strength (YS), ultimate tensile strength (UTS) and elongation of these alloys. The NZ30K alloys show a significant response to heat treatment, achieving 60 and 47% increases in YS and UTS, respectively, in the peak-aged condition (14h at 200°C), compared with those of the as-cast alloy. Both grain refinement and age hardening significantly improve work-hardening at strains below 1%, leading to higher yield strength. At large strains, however, only grain refinement continues to show a slight strain hardening. The strain-hardening exponents of both NZ30Kx–T4 and NZ30Kx–T6 alloys decrease with reducing grain size at large strains. The Hall–Petch relationships of true flow stresses of NZ30Kx–T6 alloys at low strains (0.001–0.04) show a tendency of two-slope feature as plastic strains. All tensile failures occur prior to global instability, indicating the existence of localized damage.

Different annealing behaviours of warm rolled Mg–3Al–1Zn alloy sheets with dynamic recrystallized microstructure and deformation microstructure

The annealing behaviours of Mg–3Al–1Zn magnesium alloy sheets warm rolled at 225°C and 150°C using differential speed rolling, which are mainly characterized by dynamic recrystallized microstructure and deformation microstructure, respectively, were investigated at different annealing stages. Static recrystallization occurs for the sheet rolled at 150°C while only grain growth proceeds for the sheet rolled at 225°C. For the sheet rolled at 225°C, the grain growth of dynamic recrystallized grains eliminates the basal pole inclination in the rolling direction. In contrast, for the sheet rolled at 150°C, the grain growth after static recrystallization retains the basal pole inclination, which is favoured for sheet forming. This difference in texture evolution may be attributed to the dynamic or static recrystallization prior to grain growth, which results in residual dislocation densities for the former and thus leads to the preferential growth of harder oriented basal grains at the centre of (0001) pole figure with lower dislocation densities during annealing.

Effect of grain size on prismatic slip in Mg–3Al–1Zn alloy

The grain size of Mg–3Al–1Zn magnesium alloy was systematically varied from 33 μm down to 350 nm, while keeping the crystallographic texture nearly constant, via multi-temperature equal-channel angular processing. Nearly the same texture allowed the activation of mostly prismatic slip in tension and thus evaluation of the effect of grain size on prismatic slip. The maximum tensile yield strength of 385 MPa and ultimate tensile strength of 455 MPa were achieved in materials with an average grain size of 350 nm, accompanied with 13% ductility.

Influence of initial texture on rolling and annealing textures of Mg–3Al–1Zn alloy sheets processed by high temperature rolling

The Mg–3Al–1Zn magnesium alloy sheets rolled at 573 and 798K characterized by strong and weak basal textures, respectively, were further subjected by a single high temperature rolling pass at 798K. The sheet rolled only at 798K throughout rolling exhibits a double peak texture in both of as-rolled and annealed conditions. By contrast, for the sheet rolled at 573 and 798K, the reduction rate of basal texture intensity is larger and the single peak of the rolling texture changes to the double peak with basal poles largely tilting in the RD at ∼20° after annealing, which is rarely observed in wrought magnesium alloys. This phenomenon is resulted from static recrystallization and may be related to enhanced pyramidal 〈c+a〉 slip during high temperature rolling due to the initial strong basal texture.

Effect of aging treatment on low-cycle fatigue behavior of extruded Mg–8Al–0.5Zn alloys

The low-cycle fatigue properties of Mg–8Al–0.5Zn (AZ80) magnesium alloy have been studied as a function of precipitation state. It has been shown that the presence of precipitates significantly reduces tension–compression yield asymmetry, compared with solution treated material. This decreased asymmetry significantly reduces tensile mean stress during low-cycle fatigue process. As the cyclic deformation progressed, an abrupt increase in the plastic strain amplitude prior to failure is observed, representing the onset of fatigue crack initiation. This increase disappears in the aged sample, which leads to the significantly decreased area of crack propagation zone, and shorter lifetime. Due to the enlarged reverse plastic zone size, the aged sample showed microscopically rough faceted fracture surfaces in the fatigue crack propagation zone.

Flow softening behavior during dynamic recrystallization in Mg–3Al–1Zn magnesium alloy

The flow softening behavior of Mg–3Al–1Zn magnesium alloy in the temperature range 325–500 °C is quantified using plane strain compression. The flow softening ratio, defined as the steady-state stress at 0.6 strain to the peak stress, shows a linear relationship with the Zener–Hollomon (Z) parameter and varies from 45 to 70%. Higher levels of flow softening at high Z are associated with fully recrystallized structures, whereas lower flow softening levels at low Z exhibit a mix of large recrystallized and fine non-recrystallized grains.

Factors affecting the properties of Friction Stir Welds between aluminum and magnesium alloys

Abstract Extruded Al–0.5Mg–0.3Si (6063) aluminum and rolled Mg–3Al–1Zn (AZ31B) magnesium alloy sheets were joined by Friction Stir Welding. The dissimilar metal weld joints exhibit tortuous interfaces. The nugget grain size on both the Al and Mg sides monotonically increase as the tool rotational speed increases. The midplane microhardness traverses show fluctuating hardness peaks due to the presence of different microstructural phases in the nugget zone. The maximum tensile strength of the dissimilar weld joint is 68% of the 6063-T5 base metal with a maximum elongation of 1%. The low ductility is attributed to the formation of brittle intermetallic phases at the Al–Mg interface in the weld joint. Transverse tensile test results are correlated with several interface features: (1) actual interface length, (2) extent of interpenetration between the aluminum and magnesium base metals, (3) maximum intermetallic layer thickness, and (4) area fraction of micro-void coalescence on the tensile fracture surfaces. Results indicate that maximizing the extent of interpenetration and hence promoting mechanical interlocking between the metallic phases, is the key to attaining reasonable transverse strength in Al–Mg dissimilar metal welds. The welding process control variables that promote higher mechanical interlocking of the weld joints are discussed. The process response variables (welding torque, power, x-axis force and the nugget grain size) are presented for a range of welding parameters.

Semisolid Die-Casting Process of Mg-20Al-0.8Zn Magnesium Alloy

Based on microstructure evolution of Mg-20Al-0.8Zn magnesium alloys realized by semisolid isothermal heat-treatment (SSIT), we obtained the non-dendrite or spherical grains microstructure under the suitable technological parameters that isothermal temperature is 495 °C and holding time is 120 min. With the help of special experimental equipment, the semisolid die-casting process has been studied and the specimens have been analyzed. The effects of different parameters as injection speed and pressure on tensile strength, elongation rate, hardness, etc have been investigated. The results indicate that tensile strength was improved along with increasing injection speed and pressure. However, excessive speed will involve gas, which formed defects and reduced the mechanical properties. When the injection pressure is 40MPa and injection speed is 4m/s, the tensile strength and elongation rate can reach maximal 220MPa and 5.63% respectively. Its fracture mechanism was intercrystalline cracking.

Improvement of stretch formability of Mg–3Al–1Zn alloy sheet by high temperature rolling at finishing pass

The effects of increasing rolling temperature from 723 K to 828 K at the last rolling pass on microstructure, texture, mechanical properties and stretch formability of a Mg–3Al–1Zn magnesium alloy previously rolled at 723 K were investigated. In the as-rolled condition, the basal texture strengthens slightly with increasing the rolling temperature whereas it weakens more remarkably after static recrystallization during annealing for the sheets rolled at higher temperatures. Only by increasing the rolling temperature from 723 K to 798 K, the Erichsen value is significantly increased from 4.5 to 8.6 due to the weakened texture for the annealed sheets. Further increasing the last rolling temperature does not appear to further improve the stretch formability.

Low-frequency damping properties of as-extruded Mg–11.2Li–0.95Al–0.43Zn magnesium alloy

Low-frequency damping capacities of as-extruded Mg–11.2Li–0.95Al–0.43Zn (LAZ1110) alloy are studied by dynamic mechanical analysis tests. Experimental results reveal that it is β-phase matrix with α precipitates in major. The heating internal friction Q −1( T) curves show P 1, P 2 and P 3 peaks appearing at 30 °C, 107 °C and 215 °C, respectively, and P 1 and P 2 peaks have been reported, but P 3 peak has not yet. P 1 and P 2 peaks are frequency-dependent and their activation energies are calculated as 0.85 eV and 1.02 eV, respectively. The heating Q −1( T) curves also show a significant high-temperature damping background (HTDB) at temperature above 275 °C and its activation energy is calculated as 0.75 eV which is relatively low due to the abundant α/ β phase boundaries. XRD tests indicate that the textures are developed during recrystallization and grain growth of the heated specimens and the texture development is suggested to be closely related to the occurrence of P 3 peak.