Synergistic effect of gradient Zn content and multiscale particles on the mechanical properties of Al-Zn-Mg-Cu alloys with coupling distribution of coarse-fine grains

This study investigated the influence of graded Zn content on the evolution of precipitated and iron-rich phases and grain struc-ture of the alloys,designed and developed the Al–8.0Zn–1.5Mg–1.5Cu–0.2Fe(wt%)alloy with high strength and formability.With the increase of Zn content,forming the coupling distribution of multiscale precipitates and iron-rich phases with a reasonable matching ratio and dispersion distribution characteristics is easy.This phenomenon induces the formation of cell-like structures with alternate distribu-tion of coarse and fine grains,and the average plasticity–strain ratio(characterizing the formability)of the pre-aged alloy with a high strength is up to 0.708.Results reveal the evolution and influence mechanisms of multiscale second-phase particles and the corresponding high formability mechanism of the alloys.The developed coupling control process exhibits considerable potential,revealing remarkable improvements in the room temperature formability of high-strength Al–Zn–Mg–Cu alloys.

Effect of cold rolling deformation on microstructure evolution and mechanical properties of spray formed Al−Zn−Mg−Cu−Cr alloys

The impact of cold rolling deformation,which was introduced after solid solution and before aging treatment,on microstructure evolution and mechanical properties of the as-extruded spray formed Al−9.8Zn−2.3Mg−1.73Cu−0.13Cr(wt.%)alloy,was investigated.SEM,TEM,and EBSD were used to analyze the microstructures,and tensile tests were conducted to assess mechanical properties.The results indicate that the D1-T6 sample,subjected to 25%cold rolling deformation,exhibits finer grains(3.35μm)compared to the D0-T6 sample(grain size of 4.23μm)without cold rolling.Cold rolling refines the grains that grow in solution treatment.Due to the combined effects of finer and more dispersed precipitates,higher dislocation density and smaller grains,the yield strength and ultimate tensile strength of the D1-T6 sample can reach 663 and 737 MPa,respectively.In comparison to the as-extruded and D0-T6 samples,the yield strength of the D1-T6 sample increases by 415 and 92 MPa,respectively.

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al Zn Mg Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al Zn Mg Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al3(Sc,Zr) and Al3Zr dispersoids. The volume of Al3Zr particles is less than that of Al3(Sc,Zr) particles, but the distribution of Al3(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.

Quench sensitivity and microstructures of high-Zn-content Al−Zn−Mg−Cu alloys with different Cu contents and Sc addition

The Zn,Cu,and Sc contents of 7xxx Al alloys were adjusted according to the chemical composition of a 7085 Al alloy,and the effects of Zn and Cu contents and Sc addition on the microstructures,hardness,and quench sensitivity of the 7xxx Al alloys were studied.The alloys with high Zn content and Sc addition exhibited higher hardness than the 7085 alloy at the position 3 mm away from the quenching end.The density ofηand T phases increased with the increase in Zn and Cu contents,and the Sc addition led to the formation of the Y phase and moreηphases at the position 120 mm away from the quenching end.Compared with the 7085 alloy,the high Zn−high Cu and Sc-added alloys exhibited higher quench sensitivity,while the simultaneous increase in Zn content and decrease in Cu content could enhance the hardness and reduce the quench sensitivity of the 7085 alloy.

Theoretical design and distribution control of precipitates and solute elements in Al−Zn−Mg−Cu alloys with heterostructure

In order to simultaneously improve strength and formability,an analytical model for the concentration distribution of precipitates and solute elements is established and used to theoretically design and control the heterogeneous microstructure of Al−Zn−Mg−Cu alloys.The results show that the dissolution of precipitates is mainly affected by particle size and heat treatment temperature,the heterogeneous distribution level of solute elements diffused in the alloy matrix mainly depends on the grain size,while the heat treatment temperature only has an obvious effect on the concentration distribution in the larger grains,and the experimental results of Al−Zn−Mg−Cu alloy are in good agreement with the theoretical model predictions of precipitates and solute element concentration distribution.Controlling the concentration distribution of precipitates and solute elements in Al−Zn−Mg−Cu alloys is the premise of accurately constructing heterogeneous microstructure in micro-domains,which can be used to significantly improve the formability of Al−Zn−Mg−Cu alloys with a heterostructure.

Strong and ductile Al−Zn−Mg−Zr alloy obtained by equal angular pressing and subsequent aging

An ultrafine-grained Al−Zn−Mg−Zr alloy with superior mechanical performance was obtained by high passes of equal angular pressing(ECAP)and subsequent aging.After 8 ECAP passes and aging,the yield strength(YS)and ultimate tensile strength(UTS)of the solid-solutioned alloy are significantly improved from(98±10)and(226±7)MPa to(405±9)and(427±9)MPa,respectively.A large elongation is also maintained((17.4±2.5)%).The microstructure features including grain refinement,morphology of precipitates,and dislocation density,were revealed with multiscale characterizations,including transmission electron microscopy,electron backscattered diffraction,and X-ray diffraction.After 8 passes of ECAP,the original coarse elongated grains are refined to a unique bimodal grain structure consisting of ultrafine equiaxed and lath-like grains.Additionally,the effects of ECAP and subsequent aging on the strengthening contribution of a variety of strengthening mechanisms,such as dislocation strengthening and precipitation strengthening,were discussed in detail.

Effect of non-isothermal aging on microstructure and properties of Al−5.87Zn−2.07Mg−2.42Cu alloys

The evolution of microstructure and properties of Al−5.87Zn−2.07Mg−2.42Cu alloys during non-isothermal aging was studied.The mechanical properties of the alloy were tested by stretching at room temperature.The results show that in the non-isothermal aging process,when the alloy is cooled to 140℃,the ultimate tensile strength of the alloy reaches a maximum value of 582 MPa and the elongation is 11.9%.The microstructure was tested through a transmission electron microscope,and the experimental results show that the GP zones andη'phases are the main strengthening precipitates.At the cooling stage,when the temperature dropped to 180℃,the GP zones were precipitated again.Besides,the experimental results show that the main strengthening phase during non-isothermal aging isη'phases.

Tailoring phase fractions of T'and η' phases in dual-phase strengthened Al−Zn−Mg−Cu alloy via ageing treatment

Hardness tests and transmission electron microscopy were used to investigate the strategy of tailoring the phase fraction of precipitates in an Al-Zn-Mg-Cu alloy strengthened by T’ and η’ phases. Different phase fractions of T’ and η’ phases are presented in samples subjected to either single or two stages of ageing treatments at 120 and 150 ℃.For both types of ageing, the precipitation of η’ phase is found to be promoted by ageing at lower temperature and its phase fraction increases with prolonging ageing time at 120 ℃;whereas the phase fractions of T’ and η’ phases almost remain constant during ageing at 150 ℃. Besides, the strain fields produced by T’ and η’ phases were analyzed by using the geometric phase analysis technique, and on a macroscale the contributions of T’ and η’ phases to precipitation strengthening have been quantitatively predicted by combining the size, phase fraction and number density of precipitates.

Microstructure and mechanical properties of squeeze-cast Al−5.0Mg−3.0Zn−1.0Cu alloys in solution-treated and aged conditions

The microstructure and mechanical properties at different depths of squeeze-cast,solution-treated and aged Al−5.0Mg−3.0Zn−1.0Cu alloy were investigated.For squeeze-cast alloy,from casting surface to interior,the grain size ofα(Al)matrix and width of T-Mg32(AlZnCu)49 phase increase significantly,while the volume fraction of T phase decreases.The related mechanical properties including ultimate tensile strength(UTS)and elongation decrease from 243.7 MPa and 2.3%to 217.9 MPa and 1.4%,respectively.After solution treatment at 470℃ for 36 h,T phase is dissolved into matrix,and the grain size increases so that the UTS and elongation from surface to interior are respectively reduced from 387.8 MPa and 18.6%to 348.9 MPa and 13.9%.After further peak-aging at 120℃ for 24 h,numerous G.P.II zone andη′phase precipitate in matrix.Consequently,UTS values of the surface and interior increase to 449.5 and 421.4 MPa,while elongation values decrease to 12.5%and 8.1%,respectively.

Fine-grained Mg−1Mn−0.5Al−0.5Ca−0.5Zn alloy with high strength and good ductility fabricated by conventional extrusion

Mg−1Mn−0.5Al−0.5Ca−0.5Zn(wt.%)alloy was fabricated by conventional extrusion at 673 K with an extrusion ratio of 25:1,followed by aging at 473 K.The microstructure was characterized by scanning electron microscopy,electron back-scattered diffraction,and transmission electron microscopy.The mechanical properties were determined by the tensile test.The peak-aged sample shows fine recrystallized grains with an average grain size of 1.7μm.Area fraction of Al−Ca particles in the alloy increases significantly after peak aging.Meanwhile,botháañandác+añdislocations were observed to remain in the alloy after hot extrusion.Thus,the peak-aged sample exhibits simultaneously high strength and good ductility with the ultimate tensile stress,tensile yield stress,and tension fracture elongation of 320 MPa,314 MPa,and 19.0%,respectively.

Effect of compression ratio on microstructure evolution of Mg−10%Al−1%Zn−1%Si alloy prepared by SIMA process

A new Mg−10%Al−1%Zn−1%Si alloy with non-dendritic microstructure was prepared by strain induced melt activation(SIMA)process.The effect of compression ratio on the evolution of semisolid microstructure of the experimental alloy was investigated.The results indicate that the average size ofα-Mg grains decreases and spheroidizing tendency becomes more obvious with the compression ratios increasing from 0 to 40%.In addition,the eutectic Mg2Si phase in the Mg−10%Al−1%Zn−1%Si alloy transforms completely from the initial fishbone shape to globular shape by SIMA process.With the increasing of compression ratio,the morphology and average size of Mg2Si phases do not change obviously.The morphology modification mechanism of Mg2Si phase in Mg−10%Al−1%Zn−1%Si alloy by SIMA process was also studied.

Influence of Si Contents on the Microstructure Evolution and Mechanical Properties of Al-Mg-Si-Cu-Zn Alloys

The influence of different Si contents on the microstructure evolution and mechanical properties of Al⁃Mg⁃Si⁃Cu⁃Zn alloys was systematically studied using tensile testing,OM,SEM,EDS,and EBSD.The results indicate that the grain size of as⁃cast alloys was gradually reduced with the increase of the Si content,which mainly resulted from the formation of many iron⁃rich phases and precipitates during the casting process.During homogenization treatment,the plate⁃likeβ⁃AlFeSi phases in the alloy with a higher Si content easily transformed to the sphericalα⁃Al(FeMn)Si phases,which is helpful for improving the formability of alloys.The microstructure evolution of the alloys was also greatly dependent on the content of Si that the number density and homogeneous distribution level of precipitates in the final cold rolled alloys both increased with the increase of the Si content,which further provided a positive effect on the formation of fine recrystallization grains during the subsequent solution treatment.As a result,the yield strength,ultimate tensile strength,and elongation of the pre⁃aged alloys in the direction of 45°with respect to the rolling direction were all increased with increasing Si content.

Fatigue strength and microstructure evaluation of Al 7050 alloy wires recycled by spray forming,extrusion and rotary swaging

Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7 xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated(solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 ℃, imparting higher tensile(586 MPa) and fatigue(198 MPa) strengths.

Creep aging behavior of retrogression and re-aged 7150 aluminum alloy

Creep aging behavior of retrogression and re-aged(RRAed)7150 aluminum alloy(AA7150)was systematically investigated using the creep aging experiments,mechanical properties tests,electrical conductivity tests and transmission electron microscope(TEM)observations.Creep aging results show that the steady-state creep mechanism of RRAed alloys is mainly dislocation climb(stress exponent≈5.8),which is insensitive to the grain interior and boundary precipitates.However,the total creep deformation increases over the re-aging time.In addition,the yield strength and tensile strength of the four RRAed samples are essentially the same after creep aging at 140℃ for 16 h,but the elongation decreases slightly with the re-aging time.What’s more,the retrogression and re-aging treatment are beneficial to increase the hardness and electrical conductivity of the creep-aged 7150 aluminum alloy.It can be concluded that the retrogression and re-aging treatment before creep aging forming process can improve the microstructure within grain and at grain boundary,forming efficiency and comprehensive performance of mechanical properties and electrical conductivity of 7150 aluminum alloy.

Fundamental research on recovering metals from hot-dip Zn-Al-Mg dross by supergravity separation

The dross generated in the hot-dip Zn–Al–Mg coating process is a valuable co-product,since it contains high quantities of recyclable alloy.A new method to recover Zn–Al–Mg alloy from the industrial Zn–Al–Mg dross was proposed using supergravity separation.The separation efficiency was analyzed as a function of gravity coefficient(G),separation time,and separation temperature.The separation of Zn–Al–Mg alloy from the dross can be achieved at G>100.The alloy content in the dross decreased gradually with an increase in the gravity coefficient,the separation time,and the separation temperature.The alloy ratio in the enriched dross decreased almost linearly as the gravity coefficient increased,and the recovery of Zn–Al–Mg alloy from the dross exceeded 78%;these results were consistent with the results of the FactSage software calculation.The purified alloy can be in-situ used in the hot-dip Zn–Al–Mg bath for production.The feasibility of supergravity separation as a promising process for efficiently recovering Zn–Al–Mg alloy from Zn–Al–Mg dross was thus demonstrated.

Solidification behavior and elimination of undissolved Al_(2)CuMg phase during homogenization in Ce-modified Al–Zn–Mg–Cu alloy

The solidification behavior and intermetallic phase evolution during homogenization annealing of an Al-Zn-Mg-Cu alloy with 0.12 wt%Ce addition were examined.The residual Al_(2)CuMg phase is completely dissolved after homogenization and is replaced by a large number of dispersed micro/nanoscaled AlCuCe enrichment phases within Al matrix.This change occurs because of the formation of a large number of finer lamellar eutectic network structures which are more easily dissolved into Al matrix during the homogenization process.In addition,the trapping of Cu atoms in the stable AlCuCe phase also prevents the formation of Al_(2)CuMg phase,leading to the complete dissolution of Al_(2)CuMg phase in the Al-Zn-MgCu alloy.The grain refinement behavior in Al alloy with Ce addition is similar to that in alloys with the addition of Sc,because of the formation of primary Ce-enriched Al_(11)Ce_(3)phase as the nucleation agent ofα(Al)during solidification.

Distribution uniformity of added elements in twin-roll cast Al–Zn–Mg–Cu alloy by multi-electromagnetic fields

The micromorphology and the concentration o massive precipitates produced by twin-roll casting(TRC processes without and with multi-electromagnetic fields fo Al–Zn–Mg–Cu alloy at 670 °C were investigated in detai by means of optical microscopy(OM) and electron probe micro analyzer(EPMA). The results clearly show tha under a 0.2 T static magnetic field, the macro-segregation bands are remarkably alleviated according to the order o uniform static magnetic field, half-wave oscillating elec tromagnetic field as well as alternating oscillating elec tromagnetic field, as compared with the non-field TRC process(B = 0). Moreover, under the alternating oscillat ing electromagnetic TRC process, almost all segregation bands disappear. Additionally, through the observation on a smaller scale, the netlike precipitates elongated and broken by electromagnetic force, and replaced by numerous bulk depositions. EPMA analysis shows that the added atoms are diffused from deposition to a(Al) matrix, resulting in that the solute concentration in and around the precipitates is tending toward uniformity and stability.

Phases and microstructures of high Zn-containing Al–Zn–Mg–Cu alloys

Phases and microstructures of three high Zncontaining Al–Zn–Mg–Cu alloys were investigated by means of thermodynamic calculation method, optica microscopy（OM）, scanning electron microscopy（SEM）energy dispersive spectroscopy（EDS）, X-ray diffraction（XRD）, and differential scanning calorimetry（DSC） analysis. The results indicate that similar dendritic network morphologies are found in these three Al–Zn–Mg–Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg（Cu, Zn, Al）2 eutectic phases, and fine intermetallic compounds g（MgZn2）. Both of as-cast 7095 and 7136 aluminum alloys involve a（Al）eutectic Al/Mg（Cu, Zn, Al）2, intermetallic g（MgZn2）, and h（Al2Cu）. During homogenization at 450 ℃, fine g（MgZn2） can dissolve into matrix absolutely. After homogenization at 450 ℃ for 24 h, Mg（Cu, Zn, Al）2 phase in 7136 alloy transforms into S（Al2Cu Mg） while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zncontaining Al–Zn–Mg–Cu alloys.

Effects of Solution Treatment on the Microstructure,Tensile Properties,and Impact Toughness of an Al–5.0Mg–3.0Zn–1.0Cu Cast Alloy

This study investigates the eff ect of solution treatment(at 470°C for 0–48 h)on the microstructural evolution,tensile properties,and impact properties of an Al–5.0Mg–3.0Zn–1.0Cu(wt%)alloy prepared by permanent gravity casting.The results show that the as-cast microstructure consists ofα-Al dendrites and a network-like pattern of T-Mg32(AlZnCu)49 phases.Most of the T-phases were dissolved within 24 h at 470℃;and a further prolonging of solution time resulted in a rapid growth ofα-Al grains.No transformation from the T-phase to the S-Al2CuMg phase was discovered in this alloy.Both the tensile properties and impact toughness increased quickly,reached a maximum peak value,and decreased gradually as the solution treatment proceeded.The impact toughness is more closely related to the elongation,and the relationship between impact toughness and elongation appears to obey an equation:IT=8.43 EL-3.46.After optimal solution treatment at 470℃for 24 h,this alloy exhibits excellent mechanical properties with the ultimate tensile strength,yield strength,elongation and impact toughness being 431.6 MPa,270.1 MPa,19.4%and 154.7 kJ/m^(2),which are comparable to that of a wrought Al–6.0 Mg–0.7 Mn alloy(5E06,a 5 xxx aluminum alloy).Due to its excellent comprehensive combination of mechanical properties,this cast alloy has high potential for use in components which require medium strength,high ductility and high toughness.

Improving mechanical properties of age-hardenable Mg–6Zn–4Al–1Sn alloy processed by double-aging treatment

Microstructure and mechanical properties of a new high-strength Mg–6 Zn–4 Al–1 Sn alloy were investigated. Microstructure of the as-cast Mg alloy exhibited partially divorced characteristics. The dendritic structure of the Mg–6 Zn–4 Al alloy was significantly refined with the addition of 1%（in weight） Sn, but Mg2 Sn phases were not formed. In addition, an icosahedral quasi-crystal phase was formed in the as-cast Mg–6 Zn–4 Al–1 Sn alloy. It was found that after the double-aging treatment through two different heat treatments on the Mg–6 Zn–4 Al–1 Sn alloy, the precipitates were finer and far more densely dispersed in the matrix compared with single-aged counterpart, resulting in a significant improvement in tensile strength with yield strength, ultimate tensile strength and elongation of 175 MPa, 335 MPa and 11%,respectively.