Effect of Cu addition on the mechanical properties and corrosion behaviours of Al–9.2Mg–0.8Mn alloy

Li G., Pan X., Jiang J., Li J., Xie L., Liu H., Zhang M.

The ultra-fine grain structure of Al–Zn–Mg–Cu alloy was prepared by equal channel angular pressing (ECAP) and post cold rolling (CR) combined deformation process. The effect of different rolling deformation on the microstructure, microhardness and corrosion behavior of the ECAP alloy were studied. The results show that with the increase of rolling deformation, the dislocation density increased, the grain and second phases of the deformed alloy were obviously refined. Grain refinement was mainly due to the gradual transformation of some subgrain boundaries and low angle grain boundaries (LAGBs) of deformed alloys into high angle grain boundaries (HAGBs), which eventually form fine grains. The microhardness improvement was attributed to dislocation strengthening, fine grain strengthening and second phase strengthening. Simultaneously, the corrosion behavior of the alloy was tested by immersion corrosion and intergranular corrosion, which revealed that high-density lattice defects, refined grain and second-phase particles can improve the corrosion resistance of the deformed alloys.

Baek M., Shah A.W., Kim Y., Kim S., Kim B., Lee K.

This study investigated the microstructures, mechanical properties, tensile deformations, and strengthening mechanisms of the Al-7-mass%-Mg (7Mg) and Al-9-mass%-Mg (9Mg) alloys developed from the Mg+Al2Ca master alloy. The microstructures of both as-extruded alloys comprised an Al matrix and eutectic Mg2Al3, Al2Ca (C15), and Al6(Mn,Fe) phases. The 7Mg alloy mainly consisted of elongated grains surrounded by small equiaxed grains, whereas the 9Mg alloy contained only fine equiaxed grains. Higher Mg content resulted in higher fractions of evenly distributed C15 phases. Tensile tests revealed yield strengths and maximum-tensile strengths of 234 and 429 MPa, respectively, for the 7Mg alloy, and these values increased to 276 and 479 MPa, respectively, for 9% Mg content. The characteristic serrated flow in the 7Mg alloy started at approximately 10% of the engineering strain. In contrast, in the 9Mg alloy, the development of the serrated flow from the beginning of tensile deformation, immediately after the yield point, is considered to be facilitated by the fine equiaxed grains and higher Mg content and number densities of obstacles (owing to the formation of secondary phases). Solid-solution and grain-boundary strengthening mainly contributed to the overall yield strength of the aluminum-magnesium (Al-Mg)-based alloys. Three microstructural factors, higher Mg solid solubility, smaller grain size, and a higher fraction of secondary phases, were responsible for the higher yield strength of the alloy with higher Mg content. Theoretical calculations based on conventional strength-prediction models predicted yield-strength values almost identical to those obtained experimentally, demonstrating the potential of the conventional models to accurately predict the yield strengths of Al-Mg alloys with high Mg content.

Feng H., Chen Y., Yang H., Yang P., Zhang J., Shu B.

In this paper, the effects of as-quenched, single aging, and double aging on the mechanical properties, microstructure, corrosion resistance, and electrochemical behavior of an Al-Cu-Mg-Li alloy were investigated. Most changes only improved the mechanical properties or corrosion resistance of aluminum alloy alone, and very few of them improved the mechanical properties or corrosion resistance at the same time. The results showed that double aging treatment improved the strength, hardness, and corrosion resistance of the alloy but decreased its elongation. During the aging treatment, the corrosion morphology of the alloy gradually transformed from pitting corrosion (as-quenched) to a combination of pitting corrosion and corrosion cracks (single aging) and finally to pitting corrosion (double aging). The fracture morphology gradually changed from an uniform distribution of a mixture of large and small dimples (as-quenched) to an intergranular fracture (double aging) as aging progressed. Transmission electron microscopy results showed that the T1 phase within grains and at grain boundaries (GBs) was different between the as-quenched and double aged states. In addition, a precipitate-free zone (PFZ) formed in the double aged state. The optimal process of Al-Li alloy was determined to be pre-aging at 100 ℃ for 4 h and final aging at 160 ℃ for 24 h.

Krishnamurthy S.C., Arseenko M., Kashiwar A., Dufour P., Marchal Y., Delahaye J., Idrissi H., Pardoen T., Mertens A., Simar A.

The hot working of 5xxx series alloys with Mg ≥3.5 wt% is a concern due to the precipitation of β (Al3Mg2) phase at grain boundaries favoring Inter Granular Corrosion (IGC). The mechanical and corrosion properties of a new 5028-H116 Al-Mg-Sc alloy under various β precipitates distribution is analyzed by imposing different cooling rates from the hot forming temperature (i.e. 325 °C). The mechanical properties are maintained regardless of the heat treatment. However, the different nucleation sites and volume fractions of β precipitates for different cooling rates critically affect IGC. Controlled furnace cooling after the 325 °C heat treatment is ideal in 5028-H116 alloy to reduce susceptibility to IGC after sensitization.

Guo C., Chen Y., Zhang H., Ji H., Wu Z., Liu X., Nagaumi H.

To overcome the trade-off between strength and corrosion resistance of traditional 5xxx aluminum alloys, in this paper, we investigate the evolution of microstructure, mechanical properties and intergranular corrosion (IGC) resistance of the novel heat-treatable Al-Mg-Zn-Ag alloy processed by isothermal or non-isothermal ageing (NIA). Results indicate that NIA can not only accelerate age hardening response greatly, but increase the number density of precipitates. Meanwhile, fast precipitation behavior in NIA process causes discontinuous distribution of grain boundary precipitates with finer-scale, which is instrumental in improving the IGC resistance. Hence, the strength and corrosion resistance of Al-Mg-Zn-Ag alloy are improved synchronously. The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Shen P., Zhang B., Li Z., Pang X., Deng W.

Plastic flow machining (PFM), a novel severe plastic deformation machining, was proposed to prepare 7075 aluminum alloy sheets with a gradient structure. The forming performance and mechanism of PFM are investigated. Further, the corresponding microstructure was characterized by EBSD. During PFM, the 7075 aluminum alloy sheet was stratified into four obvious layers, whereas the microstructure presents layer-by-layer refinement from top to bottom. The above gradient structure can be controlled by adjusting the extrusion thickness ( t ch ) and extrusion angle ( θ ). As the increased t ch or the deceased θ , the refinement layer proportion could be increased. Meanwhile, the mechanical properties have been investigated by the tensile test and Vickers hardness test. With decreasing t ch or decreasing θ , there is an increase in strength and a decrease in the extension. It is possible to simultaneously elongate the fine and coarse grain layers for gradient structure, which effectively suppresses local strain and presents a different tensile process from homogeneous materials. Therefore, 7075 aluminum alloy sheets have excellent strength-ductility synergy, which higher strength can be obtained by sacrificing only a tiny amount of ductility. The overall hardness of the material was significantly improved. Meanwhile, the gradient structure sheets significantly improve corrosion resistance compared to the ST samples. The insights in this study exhibit that the 7075 aluminum alloy sheets processed by PFM have excellent properties and application prospects and may provide a feasible and efficient one-step machining for green machining lightweight materials with a gradient structure. • An aluminum alloy sheet with a gradient structure was prepared in one step • Explanation of grain refinement and gradient structure formation by the effects of strain and friction factors • The beneficial effects of grain structure gradient distribution on mechanical properties (i.e., strength-ductility relationship and hardness) were analyzed • The effect of grain refinement on the optimization of the corrosion properties of the sheet surface was analyzed

Zhang Z., Li Y., Li H., Zhang D., Zhang J.

Al-Mg-Zn-(Cu) crossover alloys, embracing the advantages of traditional 5xxx and 7xxx series aluminum alloys, are being designed to achieve a better trade-off between ductility and achievable strength. In this study, we achieve an excellent strength–ductility combination in a high Cu-concentration Al-4.0Mg-3.0Zn-1.5Cu (wt.%) crossover alloy. The effect of high Cu concentration on its mechanical property and microstructure evolution was investigated in detail. Its yield strength increases by 80.5% without loss of ductility as compared to the Cu-free alloy. This is because, on one hand, introducing 1.5 wt.% Cu accelerates the precipitation kinetics of the nano-sized precipitates and increases the solid solution strengthening contribution, thus enhancing the yield strength. On the other hand, this Cu alloying improves the strain hardening ability of this alloy and its ductility by reducing the width of the precipitation free zone (PFZ), enhancing the forest dislocation strengthening contribution, and introducing slowly coarsening precipitates. The study demonstrates a simple but efficient strategy to significantly accelerate the age-hardening response and improve the strength without obvious loss of ductility for Al-Mg-Zn-(Cu) crossover alloys.

Zhao S.B., Yan Y., Li X.W., Xue P., Ni D.R., Ma Z.Y., Tian Y.Z.

Low ductility has always been a drawback for nanostructured materials. In this study, we applied a low-temperature annealing process on a cold rolled Al-8.1Mg-0.15Zr alloy with supersaturated state. It is found that the uniform elongation of the rolled specimen was improved by 124% after a low-temperature annealing treatment at 333 K, while an enhanced tensile strength was obtained in the annealed specimen. The good combination of tensile strength and ductility was mainly attributed to the fully nanostructured microstructure and the formation of nanoscale precipitates and segregation of Mg element after annealing treatment.

Yang B., Gao M., Wang Y., Guan R.

Processing of Al–Mg alloys with high Mg content is a challenge owing to the presence of brittle β-Al 3 Mg 2 phases and coarse dendrites. In this work, the microstructural evolution and mechanical properties of rheo-extruded Al–Mg alloys with different Mg contents were investigated. The formation mechanism of equiaxed grains during continuous rheo-extrusion was revealed. The strengthening mechanisms of rheo-extruded Al–Mg alloys were discussed. The results showed that compared with the coarse grains with dendrites and secondary phases in the as-cast Al–Mg alloys, refined and equiaxed grains were formed in the rheo-extruded alloys, which were attributed to the high cooling rate and shear deformation. Furthermore, the continuous dynamic recrystallization resulting from the accumulative shear strain was responsible for the grain refinement during continuous rheo-extrusion. The Mg atoms with a high content that were dissolved in the matrix possessed a strong pinning effect on the dislocation motion and reduced the grain boundary mobility during deformation. With increasing Mg content from 5 to 7 wt%, the ultimate tensile strength and yield strength of rheo-extruded Al–Mg alloys were improved from 252.2 to 329.1 MPa and 97.2 to 139.9 MPa, respectively, accompanied by a slight decrease in elongation. Grain boundary strengthening, solid solution strengthening, and dislocation strengthening were the main strengthening mechanisms contributing to the yield strength enhancement for the rheo-extruded alloys.

Wang Y., Yang B., Gao M., Zhao E., Guan R.

• The mechanical property of Al–Mg alloys was tailored via compositional design. • An Al–Mg alloy with a good combination of strength and ductility was developed. • There was a combined effect of nano Al 6 Mn and Al 3 Sc phases on recrystallization. • The strengthening mechanisms of extruded alloys were established quantitatively. Based on the compositional design concept, an Al–6 Mg–0.8Mn (–0.2Sc) alloy with a good combination of strength and ductility was obtained by hot extrusion deformation with a large extrusion ratio. The microstructure evolution and mechanical property of an Al–6 Mg alloy with adding Mn and/or Sc elements were investigated. The results showed that the addition of Sc element resulted in the grain refinement and promoted the precipitation of nano-sized Al 6 Mn phases in the Al–6 Mg–0.8Mn–0.2Sc alloy. After hot extrusion, the number of nano-sized Al 6 Mn phases decreased with the morphology transforming into the rhomboidal/plate-like shape. The combined effect of nano-sized Al 6 Mn phases and Al 3 Sc dispersoids offered a strong pinning effect on both grain boundaries and dislocations, leading to a significant refinement for recrystallized grains. Strengthening mechanism analysis indicated that the grain boundary and dislocation strengthening play important roles in enhancing the yield strength of the alloy. In addition, the uniform distribution of solid solution Mg atoms and high recrystallized fraction contributed to the high ductility of the extruded alloy. The aim of this work is to provide a strategy to acquire Al–Mg alloys with excellent mechanical performance.

Qiu Y., Yang X., Li J., Xiang S., Shi J., Xu J., Sanders R.E.

Minor additions of Sc and Zr to a 5182 base alloy were used to study the effects of microstructure on the susceptibility to intergranular corrosion (IGC) of 5xxx sheet alloys. The results showed that minor Sc and Zr additions significantly decrease the IGC susceptibility as measured by the depth of IGC for the high-temperature annealed sheet. The improvement in IGC resistance is attributed to a high number density of nano-scaled coherent Al 3 (Sc,Zr) dispersoids which resist recrystallization, and produce a fine subgrain structure. The character of the subgrain structure is critical to prevent the occurrence of continuous intergranular Al 3 Mg 2 precipitates during sensitization. • Minor additions of Sc and Zr significantly declined the IGC susceptibility for high-temperature annealed AA5182 alloy sheet. • Coherent Al 3 (Sc,Zr) dispersoids effectively inhibited the recrystallization of AA5182 alloy sheet during annealing. • The intergranular Al 3 Mg 2 phase precipitated preferentially along high angle grain boundaries. • The subgrain structure can prevent the formation of continuous intergranular Al 3 Mg 2 phase.

Dun-Bo T., Hong Z., Cui-Hong H., Yu-Rong X., Zhen L., Ri-Qiang Z., Xiao C.

Abstract Electrochemical corrosion behavior of Sn-containing Al–Zn–Mg aluminum alloy has been studied in detail. The localized corrosion behaviors were studied by electrochemical impedance spectroscopy (EIS) analysis, and the potentiodynamic polarization measurements. The grain structure, grain-boundary microstructure, grain-boundary microchemistry, pitting and intergranular corrosion morphology were characterized and observed using SEM, EDS, TEM, SAED and HRTEM analyses. Based on these tests, the effects of grain-boundary on the corrosion resistance in our Sn-containing Al–Zn–Mg alloys before/after bake hardening were analyzed systematically. Finally, the relationship between chemical composition, microstructure evolution and corrosion behaviour was revealed. The results indicate that the bake hardening process improves the corrosion resistance compared to the pre-aging state. The grain size has little effect on the electrochemical corrosion bahavior.

LI X., XIA W., CHEN J., YAN H., LI Z., SU B., SONG M.

The Al–Mg alloy with high Mg addition (Al–9.2Mg–0.8Mn–0.2Zr-0.15Ti, in wt.%) was subjected to different passes (1, 2 and 4) of high strain rate rolling (HSRR), with the total thickness reduction of 72%, the rolling temperature of 400 °C and strain rate of 8.6 s −1 . The microstructure evolution was studied by optical microscope (OM), scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The alloy that undergoes 2 passes of HSRR exhibits an obvious bimodal grain structure, in which the average grain sizes of the fine dynamic recrystallization (DRX) grains and the coarse non-DRX regions are 6.4 and 47.7 μm, respectively. The high strength ((507±9) MPa) and the large ductility ((24.9±1.3)%) are obtained in the alloy containing the bimodal grain distribution. The discontinuous dynamic recrystallization (DDRX) mechanism is the prominent grain refinement mechanism in the alloy subjected to 2 passes of HSRR.

Zhao L., Yan H., Chen J., Xia W., Su B., Song M., Li Z., Li X., Liao Y.

High ductility combining with relative high strength is of great significance for the application of Al–Mg alloys. In this study, excellent ductility and moderated strength are obtained in the as-hot-rolled Al-9.2Mg-0.5Zn sheet, with ultimate tensile strength ( σ b ), yield tensile strength ( σ 0.2 ) and elongation ( δ ) are respectively 434 ± 5 MPa, 228 ± 2 MPa and 48 ± 2%. High ductility of the as-hot-rolled Al-9.2 Mg-xZn (0-1.5 wt%) sheets can be ascribed to their strong work-hardening abilities, which increase firstly and then decline with higher Zn content. The highest work-hardening component ( n ), work-hardening capacity ( H c ) and the initial work-hardening rate ( θ 0 ) in the Al-9.2Mg-0.5Zn sheet are 0.428, 1.793 and 3496 MPa, respectively. The microstructures of the Al–Mg–Zn sheets are characterized by electron backscattered diffraction (EBSD), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning electron microscope (SEM). Low density of pre-existing dislocations (PDs), random grain orientations and reduced stacking fault energy (SFE) contribute to the strong work-hardening capacity and lead to desirable ductility in the as-hot-rolled Al–Mg–Zn sheets. However, numerous precipitates with an uneven distribution are harmful for work-hardening and thus lead to the reduced ductility in the Al–Mg sheet with high Zn content. • Effects of Zn content on the work-hardening behaviors of Al-9.2 Mg alloy are studied. • Hot rolled Al–Mg–Zn sheets show high ductility and strong work-hardening effect. • The Al-9.2Mg-0.5Zn alloy exhibits high δ (48 ± 2%) and large n (0.428). • The 0.5%Zn addition reduces the PDs density and enhances the work-hardening behavior. • Numerous Al 3 Mg 2 and Mg 32 (Al,Zn) 49 precipitates are developed in the Al-9.2Mg-1.5Zn sheet and accelerate the alloy failure.

Pan Y., Zhang D., Liu H., Zhuang L., Zhang J.

High-performance aluminum alloys are desired for applications that require lightweight materials, but it is challenging to overcome the tradeoff between their strength and intergranular corrosion resistance. To overcome this tradeoff, we have prepared novel Al–Mg–Zn (-Cu) alloys with highly Zn content combined with Cu addition. The alloys with a (Zn + Cu)/Mg ratio below 1.5 are different from traditional 2000, 5000 and 7000 series aluminium alloys. The results show that the number density and lattice spacing of intragranular precipitates increase, while the precipitate sizes decreases with the increase of (Zn + Cu)/Mg ratio, which is the reason for the increase of the alloy strength. Meanwhile, the intergranular corrosion resistance of the alloys is also enhanced by decreasing the potential between grain boundary precipitates and the Al matrix, especially in alloys with high (Zn + Cu)/Mg ratios. These results provide a new method for obtaining high-strength and corrosion-resistant aluminum alloys. • The novel Al-Mg-Zn(-Cu) alloys with (Zn + Cu)/Mg ratio below 1.5 are different from traditional 2000, 5000 and 7000 alloys. • The precipitation hardening of T-Mg 32 (AlX) 49 phase is the main reason to improve the alloy strength. • The IGC resistance of the higher (Zn + Cu)/Mg ratio alloy are mainly depended on the continuity of GBPs.

Hao K., Xia W., Li Q., Yan H., Chen J., Su B.

Microalloying is believed to improve the performance of Al-Mg alloys, and the effect of Sn addition (0-1.2 wt.%) on the microstructure, mechanical properties and intergranular corrosion behavior of Al-9.2Mg-0.8Mn-0.2Zr alloys has been investigated to extend the application of Al-Mg alloys. The Sn addition forms the Mg2Sn phase, which refines the grain size and improves the strength, where the yield strength of the alloy improves by 27 MPa with the addition of 0.6 wt.% Sn. Moreover, the addition of Sn improves the intergranular corrosion resistance of the alloys, where the intergranular corrosion mass loss of as-sensitized alloy with the addition of 0.3 wt.% Sn decreases by 35.2% compared to the Sn-free alloy. This is attributed to the formation of the Mg2Sn phase which reduces the driving force for the precipitation of the β-Al3Mg2 phase.