Zn Alloy Sheets Research Articles

High strength Mg-1.4Gd-1.2Y-0.4Zn sheet and its strengthening mechanisms

A high strength Mg-1.4Gd-1.2Y-0.4Zn (at%) alloy sheet was successfully prepared by the combination of extrusion and corresponding heat treatments. The microstructure, texture and mechanical properties were systematically investigated. The results show that the microstructure of the as-extruded alloy is composed of finer recrystallized grains with random orientation, elongated grains, and 14H-LPSO structures which are parallel to the extrusion direction, possessing a modified texture. For the as-aged alloy, the metastable β′ phases dispersively distribute within the grains. The as-extruded alloy already shows high tensile mechanical properties. After aging treatment, the tensile yield strength, ultimate tensile strength, and elongation to failure reach 395 MPa, 461 MPa, and 6.0%, respectively. The solution treatment and extrusion, which give rise to the dissolution of eutectic phases, the grain refinement and the accumulation of dislocations, contribute mainly to the strength of the as-extruded alloy. Aging treatment further strengthens the alloy by precipitating the metaphase β′ at the expense of the rare earth element dissolving into the matrix.

Formation of the elliptical texture and its effect on the mechanical properties and stretch formability of dilute Mg-Sn-Y sheet by Zn addition

All the time, it is difficult for magnesium alloys to equilibrate the relationship among strength, ductility and stretch formability at room temperature. Here, we added 0.6 wt% Zn into Mg-0.4Sn-0.7Y (TW00, wt%) alloy attempting to achieve a good combination among strength, ductility and stretch formability at room temperature. After extrusion, we found that Mg-0.4Sn-0.7Y-0.6Zn (TWZ000, wt%) alloy sheet exhibited a weak elliptical texture, accompanied with significant grain and second phase refinement. The formation of this weak elliptical texture was mainly originated from the joint contributions of multiple dynamic recrystallization (DRX) mechanisms consisted of twin-, compound- and dislocation slip-induced DRX behaviors. Grain boundary strengthening and second phase strengthening were mainly dedicated to the enhanced yield strength (~ 150.0 MPa) and ultimate tensile strength (~ 235.1 MPa), while non-basal slip, grain boundary cohesion, texture modification and twins commonly guaranteed the high ductility (~ 39.8%). Besides texture modification and non-basal slip, a large number of twins altered the initial microstructure along with the formation of new twinning textures during stretch forming. These new twinning texture facilitated the higher Schmid factor (SF) for basal slip beneficial to the enhancement in stretch formability (~ 7.0 mm). Therefore, TWZ000 alloy sheet provided an enormous potential to broaden magnesium alloys applications.

Microstructural evolution and high-temperature compressive properties of an extruded Mg–Dy–Zn alloy sheet

Abstract Microstructural evolution and compressive properties of an extruded Mg-2Dy-0.5Zn (at.%) alloy sheet at 350°C were investigated. As the compressive strain increased, the volume fraction of dynamic recrystallization increased, the fine lamellar 14H long period stacking ordered phase precipitated in the dynamic recrystallization grain, and the Mg12ZnDy phase with an 18R long period stacking ordered structure gradually bent. These secondary phases not only acted as nucleation sites to promote dynamic recrystallization but also restrained grain growth by inhibiting dislocation movement and grain boundary sliding. The compressive yield strength, ultimate compressive strength, and compressive strain of the alloy sheet were 161 MPa, 212 MPa, and 12.4% at 350°C, respectively. The high compressive strengths were mainly attributed to grain refinement, kink band strengthening of the 18R long period stacking ordered phase and precipitation strengthening of the fine lamellar 14H long period stacking ordered phase in the dynamic recrystallization grain.

Unusual texture formation in Mg–3Al–1Zn alloy sheets processed by slope extrusion

A novel extrusion process, namely, slope extrusion (SE), was proposed to produce Mg–3Al–1Zn alloy sheet, thereby tailoring the strong basal texture of conventional extruded (CE) sheet. The grain morphology and texture were examined, and the tensile tests were performed along 0°, 45°, and 90° with extrusion direction. Moreover, the finite element simulation was performed to have a better understanding on the extrusion processes. An elongated weak double-peak texture titled from extrusion direction to transverse direction about 48° was obtained for the SE sheet and meanwhile the two peaks titled away from normal direction by ~ 35°. Therefore, the SE sheet showed lower yield strength and higher ductility compared with CE sheet. Especially, in terms of the transverse direction specimen of the SE sheet, the ductility reached 27.3%, which was considered as an excellent value for the wrought AZ31 sheet.

Effects of rolling conditions on the microstructure and mechanical properties in a Mg–Al–Ca–Mn–Zn alloy sheet

Mg–1.2Al–0.3Ca–0.4Mn–0.3Zn (AXMZ1000) (wt%) alloy was rolled in three different conditions in order to understand the effects of the rolling conditions on the microstructure and mechanical properties. The sheet subjected to high temperature reheating at 500 °C for 5 min prior to each 100 °C rolling delivers the highest Index Erichsen (I.E.) value of 7.0 mm. The sheet rolled at 300 °C with 500 °C reheating for 5 min delivers the lowest I.E value of 5.4 mm, while the sheet continuously rolled at the same temperature shows a moderate I.E. value of 6.0 mm. Systematic observations of the microstructures of T4-treated and 3 mm stretched formed samples reveal that the large stretch formability of the sheet rolled at 100 °C with 500 °C reheating is associated the weak basal texture and fine-grained microstructure.

Microstructural refinement and improvement of mechanical properties of hot-rolled Mg–3Al–Zn alloy sheets subjected to pre-extrusion and Al-Si alloying

In this study, an appropriate alloying and rolling method was sought to improve the mechanical properties of Mg-3Al-Zn alloy sheets. The effects of a pre-extrusion process, combined with Al-Si eutectic addition, on the microstructure, texture, and mechanical properties of Mg-3Al-Zn alloys were investigated in this study. The results demonstrated that the hot-rolled Mg-3Al-Zn alloys subjected to pre-extrusion and Al-Si alloying addition exhibited a distinct bimodal-distribution characteristic and optimal grain refinement. The microstructural refinement could be attributed to the particle-stimulated nucleation (PSN) effect generated by the refined Mg2Si particles, as well as the dynamic recrystallization (DRX) that occurred in the twins, resulting in ultrafine grains. The as-rolled Mg-3(Al-Si)-Zn alloy sheets subjected to pre-extrusion and Al-Si alloying (ASZ-ER) exhibited superior mechanical properties. Their yield strength (YS), ultimate tensile strength (UTS), and total elongation values were 274 MPa, 355 MPa, and 12.9%, respectively. The enhanced mechanical properties of the ASZ-ER alloy could be ascribed to grain refinement and second-phase strengthening effects.

Influence of Continuous Bending Process on Texture Evolution and Mechanical Properties of AZ31 Magnesium Alloy

Mg–3Al–1Zn alloy sheets have been deformed by continuous bending (CB) to investigate the effects of processing parameters (bending angle and repetitive passes) on the texture and formability. The samples exhibited a bimodal microstructure with abnormal growth grains in the surface region and fine grains distributed in the center after CB process followed by annealing. The texture evolution measured by XRD indicated that the basal poles were rotated from ND toward RD, and the texture intensity decreased with the bending angle decreasing and repetitive passes increasing. Compared with the as-received sample, the yield strength of CBA-120-2 sample significantly decreased from 183 to 112 MPa, and a smaller r-value and a larger n-value were obtained. The formability of CB processed samples in annealing condition was significantly enhanced with the highest of Erichsen value of 5.4 mm, increased by about 135%. The improvement of formability was likely attributed to the weakened and RD-tilt basal texture and coarse grains in the surface part.

Effect of effective strain gradient on texture and mechanical properties of Mg–3Al–1Zn alloy sheets produced by asymmetric extrusion

In the present study, Mg–3Al–1Zn (AZ31) alloy sheets were fabricated by conventional extrusion (CE) and asymmetric extrusion (AE) processes. The microstructures, textures and mechanical properties of the AE and CE AZ31 sheets were examined and compared. The results showed that a tilted weak basal texture and microstructure refinement were obtained in the AE AZ31 sheet compared with the CE AZ31 sheet. The reason for the texture weakening and microstructure refinement was ascribed to an introduction of large effective strain gradient throughout the normal direction during the AE process. In addition, the AE AZ31 sheet exhibited lower yield strength and r-value, but larger ductility and n-value. Improving mechanical properties for extruded AZ31 sheet were achieved by the AE process.

Microstructure evolution and enhanced mechanical properties of hot rolled Mg–3Al–Zn alloy with the addition of Al and Si as a eutectic alloy

Abstract

Simultaneously enhanced strength and corrosion resistance of Mg–3Al–1Zn alloy sheets with nano-grained surface layer produced by sliding friction treatment

Simultaneous improvement of the two disparate properties such as mechanical strength and corrosion resistance of Mg alloys is the object researchers have been pursuing all the time. In this study, by means of sliding friction treatment (SFT), a nanocrystalline surface layer (with an average grain size of 70 nm on the topmost surface) was successfully generated on coarse-grained (CG, mean grain size: 25 μm) Mg–3Al–1Zn (AZ31) alloy sheet. The SFTed AZ31 exhibited simultaneously enhanced mechanical strength and corrosion resistance as compared to the CG counterpart. Specifically, the yield strength (YS) of SFTed sample is 236 MPa, which is almost twice that of CG sample (123 MPa). Meanwhile, the hydrogen evolution rate during 80 h immersion in 3.5% NaCl is reduced from 0.44 ml/cm2/h to 0.17 ml/cm2/h after SFT treatment. Effective grain refinement was responsible for the notable strength increment. The improvement in corrosion resistance might be correlated with SFT-induced grain refinement, good surface quality, second-phase particles fragmentation and strong basal texture. These combined capabilities of SFT render it a commercially viable manufacturing process for comprehensive properties modification of Mg products.

Improving Formability of Mg–Ca–Zr Sheet Alloy by Microalloying of Zn

Addition of 0.1 at% Zn to the Mg–0.5Ca–0.1Zr alloy is found to significantly improve the stretch formability and ductility. The remarkable improvement in the stretch formability and ductility is attributable to the enhanced grain boundary cohesion and weakened texture. The as‐annealed Mg–0.5Ca–0.1Zr–0.1Zn alloy sheet shows a 0.2% proof strength of ≈134 MPa along the rolling direction, which is lower than that of as‐annealed AZ31 alloy sheet (≈147 MPa). However, by post‐forming ageing, the 0.2% proof strength of the Mg–0.5Ca–0.1Zr–0.1Zn alloy sheet is considerably increased to ≈201 MPa, exceeding that for the annealed AZ31 alloy sheet.

Microstructure and Mechanical Properties of Severely Deformed Mg-4.5Al-1.0Zn Alloy Processed by Asymmetric Rolling on Ingot and Twin Roll Cast Strip

The twin roll cast Mg-4.5Al-1.0Zn alloy sheets, processed by asymmetric rolling and subsequent annealing were investigated. The asymmetric rolling on Mg-4.5Al-1.0Zn ingot was also studied for comparison in terms of microstructure, texture and mechanical properties of the sheets. The results can be summarized as follows. The rolled sheets show intensively deformed band structures and fine grains due to dynamic recrystallization. After annealing at 375 °C for 1 hr, both the twin roll casted and ingot processed sheets show the fine equiaxed grains with around 9 μm in mean diameter. The twin roll casted magnesium alloy sheet shows superior tensile strength and low elongation compared to the ingot cast one. The results of this work suggest that asymmetric rolling can improve the formability of magnesium alloy and the technique could be extended to enhance the sheet qualities of different alloys in jewelry industry.

Compressive anisotropy of extruded Mg–Dy–Zn alloy sheet

Compressive anisotropy of extruded Mg–2Dy–0.5Zn (mole fraction, %) alloy sheet was investigated. The alloy sheet was mainly composed of α-Mg, (Mg, Zn)xDy phase and a large number of long period stacking ordered (LPSO) phases distributed along the extrusion direction. The compressive experimental results show that the alloy sheet exhibits an obvious compressive anisotropy. The compressive strength of the specimen in the extrusion direction (ED) is higher than those of the specimens in the transverse direction (TD) and 45° inclined to the extrusion direction. The compressive yield strength (CYS), ultimate compressive strength (UCS) and compressive strain of the specimen in the ED are 274.65 MPa, 518.94 MPa and 12.93%, respectively. The compressive anisotropy is mainly attributed to the distribution of LPSO phase and formation of fiber texture in the deformed grains.

Dynamic recrystallization mechanisms during high speed rolling of Mg–3Al–1Zn alloy sheets

A high reduction of more than 70% was achieved at a rolling speed of 100m/min in magnesium AZ31 alloy (Mg–3wt.%Al–1wt.%Zn) sheets at 100°C. Dynamic recrystallization (DRX) was found at the reductions of 37% and higher. The dominant mechanism was twins/shear bands related dynamic recrystallization, which led to a unique partially recrystallized and twinned microstructure. In the untwinned/less twinned grains, continuous and discontinuous recrystallization mechanisms were also involved at higher reductions as the temperature increased.

A comparison of microstructure and mechanical properties of Mg–9Al–1Zn sheets rolled from as-cast, cast-rolling and as-extruded alloys

The effects of different initial microstructures of Mg–9Al–1Zn alloy on microstructure and mechanical properties after rolled were investigated in this study. Three original Mg–9Al–1Zn alloys involving as-cast, cast-rolling and as-extruded conditions present different initial grain sizes and initial textures. After a rolling process, all the corresponding rolled sheets show a fine-grained recrystallized microstructure (2.7–3.6μm) and a strong basal (0002) texture. The tensile tests of the three rolled sheets show that the as-extruded alloy after rolled exhibits the highest yield strength (~248MPa) and the highest tensile strength (~382MPa) at room temperature, and has the greatest ultimate elongation (~697%) at 300°C. Therefore, the Mg–9Al–1Zn alloy sheet via a combination of extrusion and rolling possesses the best integrated performance in service at room temperature and presents the most excellent superplastic capacity at elevated temperature among all the three sheets. The results in this study would be of high industrial or scientific interest in industrial applications and superplastic preprocessing on magnesium alloys.

Pinless Friction Stir Spot Welding of Mg‒3Al‒1Zn Alloy with Zn Interlayer

With the addition of a thin Zn interlayer, 2.4 mm thick Mg‒3Al‒1Zn alloy sheets were friction stir spot welded (FSSW) using a pinless tool with flat, convex and concave shoulder shapes. The results showed that an alloying reaction took place between the Mg substrate and Zn interlayer during FSSW, forming a discontinuous intermetallics layer composed of dispersive (α-Mg + MgZn) eutectic structure underneath the shoulder and a Mg‒Zn intermetallics bonding zone at the outside of the joints. This alloying reaction increased the bonded area and eliminated the hook defects, thereby producing sound FSSW joints with a shallow keyhole without hook defects. The increase of plunge depth was beneficial to the Mg‒Zn diffusion, thereby increasing the tensile-shear load of the joints. However, excessive plunge depths resulted in a decrease of the effective sheet thickness, reducing the strength of the joints. At a small plunge depth, the convex and concave shoulders were more beneficial to the interface reaction than the flat shoulder. The maximum joint load of 6.6 kN was achieved by using the concave shoulder at a plunge depth of 1.0 mm. A post-welding heat treatment promoted the dissolution of the discontinuous reaction layer in the joints; however, it led to the occurrence of void defects, influencing the bonding strength.

Fracture localisation in retreating side of friction stir welded magnesium alloy

Friction stir welding was successfully applied to joint Mg–3Al–1Zn alloy sheets. Serious strain localisation was observed during transverse tensile tests of the welds. Fracture was initiated from the interface of thermal–mechanical affected zone and stir zone in retreating side. The reasons for such localised fracture behaviour were explained by analysing the texture, Schmid factor, grain size and strain localisation characteristics in retreating side and advancing side. The results suggest that the coarser grain size and higher Schmid factor resulted in the easier activation of basal slip in retreating side than advancing side, which caused more serious strain localisation and stress concentration, and hence promoted fracture initiation from the interface of thermal–mechanical affected zone and stir zone in retreating side.

Effects of high-density pulse current on mechanical properties and microstructure in a rolled Mg–9.3Li–1.79Al–1.61Zn alloy

The ultimate tensile strength of 209MPa and the elongation to failure of 24% were demonstrated in as-rolled Mg–9.3Li–1.79Al–1.61Zn (designated as LAZ922) alloy sheets. Variation in true stress with true strain showed that stress decreases and elongation to failure increases under high density pulsed current. The stresses at the current density of 1.44×103 and 2.11×103A/mm2 decreased by 57% and 82%, respectively, relative to the stress without a current. The elongations to failure at above current density were 33.2% and 78%. These were somewhat larger than the elongation to failure of 24% without a current. The imposed current promoted the appearance of some equiaxed grains within the elongated grains in the gauge section and induced partial dynamic recrystallization. The formation of needle grains near the fracture site at the current density of 2.11×103A/mm2 can be attributed to the occurrence of new grain boundaries caused by the lattice expansion due to an increase in temperature. Dislocation studies revealed that, as the current density increases, the structure evolution such as the formation of tangled dislocations, the formation of dislocation walls at the subgrain boundary, and the disappearance of dislocations within the subgrain takes place. New models of dislocation density and number of dislocations inside the grain in the presence of current were established. The changing trend of the model prediction was consistent with experimental results.

Influencing mechanism of Zn interlayer addition on hook defects of friction stir spot welded Mg–Al–Zn alloy joints

Abstract 2.4 mm thick Mg–Al–Zn alloy sheets were friction stir spot welded (FSSW) without and with the addition of 0.1 mm thick Zn interlayer. The influence of interlayer addition on the microstructural features and mechanical properties of FSSW joints was investigated by optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffraction and tensile testing. The results show that the addition of Zn interlayer resulted in complex alloying reactions between Mg substrate and Zn interlayer, forming a bonded zone composed of α-Mg, (α-Mg + MgZn) eutectoid structure and a mixture of Mg 4 Zn 7 and unreacted Zn, thereby increasing the area of bonded zone and reducing the hook defects. This results in a significant increase in tensile–shear load from 2.4 kN to about 4 kN.

Enhanced mechanical properties in Cu–Zn alloys with a gradient structure by surface mechanical attrition treatment at cryogenic temperature

This paper describes the effects of gradient structure on mechanical properties of Cu–Zn alloys processed by surface mechanical attrition treatment (SMAT) at liquid nitrogen temperature (77K). This method leads to the formation of a gradient structure with surface fine-grained regions and a coarse-grained interior. In this study, a fine-grained surface layer with a thickness of 10μm is formed on a coarse-grained Cu–Zn alloy sheet after the cryogenic SMAT process. Microstructural observations and microhardness measurements demonstrate that a significant microstructure refinement in grain size and a gradient increase in hardness from the coarse-grained core to the top surface. Tensile tests at room temperature showed superior strength–ductility synergy for the SMAT sample.