Primary Al3 Research Articles

Influence of direct aging treatment on microstructure and mechanical properties of Al-4Cu-2Mg-0.3Sc-0.7Zr alloy processed by selective laser melting

This work investigates the microstructure and mechanical properties of select laser melting (SLM) fabricated Al-4Cu-2Mg-0.3Sc-0.7Zr alloys after direct aging treatment. In the as-fabricated alloy, typical duplex grain microstructure, primary Al3(Sc, Zr) precipitates and CuMg co-segregation are observed. After aging treatment at 203 °C for 2.5 h, it was found that two types of S (Al2CuMg) precipitates with {100}Al and {120}Al growth direction. The superlattice lattice confirmed the presence of the secondary Al3(Sc, Zr) precipitates with L12 structure after aging at 225 °C to 496 °C. The disappearance of CuMg segregation and the coarser scale Al3(Sc, Zr) particles lead to the poor Zener-pinning effect and abnormal growth of fine grains at 496 °C aged. The fine secondary Al3(Sc, Zr) precipitates enhanced mechanical properties greatly, the yield strength achieved 435.5 MPa (increased about 18 %) with the elongation of 10.5 % after aging at 440 °C.

Sc/Zr microalloying on strength-corrosion performance synergy of wire-arc directed energy deposited Al-Mg

ABSTRACT WADED Al-Mg and Al-Mg-Sc-Zr aluminium alloy thin-wall components were produced. Heat treatment (350°C aging for 4 h) was further conducted. The effects of Sc/Zr addition on porosity, microstructure, mechanical properties and corrosion behaviour were investigated. With Sc/Zr adding, pore area fraction decreased from 0.208 to 0.005% and grain size decreased from 84.34 to 19.54 μm. In WADED Al-Mg-Sc-Zr component, microstructure showed refined equiaxed grains. Mechanical properties were improved with 360 MPa ultimate tensile strength (UTS), 185 MPa yield strength, and 23.31% elongation. After heat treatment, UTS increased to 388 MPa with good ductility (22.53%) maintained. The grain refinement and Al3(Sc, Zr) particles were the key factors for mechanical property improvement. The corrosion behaviour was improved with introduced Sc/Zr and further optimised after heat treatment. The corrosion exhibited layered characteristics due to the distribution of primary Al3(Sc, Zr) while minor-size secondary Al3(Sc, Zr) along grain boundaries improved the corrosion behaviour.

A comparative study on Al-Mg-Sc-Zr alloy fabricated by wire arc additive manufacturing with controlling interlayer temperature and continuous printing: Porosity, microstructure, and mechanical properties

Wire arc additive manufacturing (WAAM) technique is a promising approach to producing large-scale metal components due to high deposition efficiency and low production cost. However, fundamental research about WAAM-processed Al-Mg-Sc-Zr alloy was still fewer. In this study, Al-6.54Mg-0.36Sc-0.11Zr (wt%) components were successfully manufactured by WAAM with an interlayer temperature at 100 °C (named IW) and continuous printing (named CP), and the corresponding porosity, microstructure, and mechanical properties of components were studied in detail. The porosity of components as-deposited was relatively low, about 0.385% and 0.116%, respectively. The microstructures of the two components exhibited the same distribution characteristics in XZ and YZ planes: fine equiaxed grains (FEG) at remelted zone + FEG and coarse equiaxed grain (CEG) alternative distribution at middle zone + FEG at the top zone of the molten pool. The average grain size of component IW was about 10.51 ± 6.01 μm, and that of component CP significantly increased, to about 11.85 ± 5.86 μm. The short-circuit transition mode of cold metal transfer technology and the heterogeneous nucleation effect of primary Al3(Sc, Zr) and Al3(Sc, Zr, Ti) phases together promoted the formation of equiaxed grains and refined the microstructures. After heat treatment at 325 °C and 6 h, nano-Al3Sc precipitated with a size of about 15–50 nm. The yield strength (YS) of components IW and CP increased from 171 ± 3 to 261 ± 1 MPa and 168 ± 7 to 240 ± 17 MPa, respectively. Component IW had the highest ultimate tensile strength, about 400 ± 1 MPa. For WAAM-processed Al-Mg-Sc-Zr alloys, the contribution of the strengthening mechanism to YS was solid solution strengthening > precipitation strengthening > fine grain strengthening > dislocation strengthening.

Effect of post-heat treatment on the microstructure, mechanical, and corrosion properties of laser powder bed fused Al–Zn–Mg–Cu–Si–Zr–Er alloys

Three different post-heat treatments, including direct aging (DA), solution heat treatment + precipitate hardening (T6), and retrogression and re-ageing (RRA), were carried out on a Si–Zr–Er modified Al–Zn–Mg–Cu alloy, which is manufactured by laser powder bed fusion (LPBF). The results show that direct aging maintains the cellular structure of the as-printed (AP) alloy, while T6 and RRA promoted the damage and decomposition of the cellular structure. The size of the primary Al3(Er,Zr) phase increased after heat treatment, and secondary Al3(Er,Zr) phases were uniformly distributed in the matrix with a size of 5–20 nm. The Mg2Si phase in the DA alloy was a long strip pattern, but uniformly distributed in the T6 and RRA alloy. The ultimate tensile strength of the AP, DA, T6, and RRA alloys reached 468, 474, 389, and 354 MPa, respectively. The size, shape, and distribution of the Al3(Er,Zr) and Mg2Si phases have a significant impact on their corrosion resistance, with the latter having a stronger effect. To sum up, the strength of the DA alloy is increased, but its corrosion performance is decreased with heat treatment. On the other hand, the strength of the T6 and RRA alloys is somewhat sacrificed, but their plasticity and corrosion resistance are significantly enhanced.

Microstructure, mechanical properties, and damping capacity of high-Zn-content Al–Zn–Mg alloys with Sc and Zr additions

In this study, Al–15Zn–0.5 Mg–Sc–Zr alloys with different Sc and Zr contents were fabricated by casting, rolling, solution treatment, and aging at low and high temperatures. Primary and precipitated Al3(Sc,Zr) and Al3Zr phases could promote grain refinement in the high-Zn-content Al alloys more effectively than the Al3Sc phase, and the composition characteristics of high Zn and low Mg contents led to the Al–15Zn–0.5 Mg–Sc–Zr alloys exhibiting high thermal stability of η′ phases during aging at high temperature and long time. The Al3Sc and Al3(Sc,Zr) phases could promote the formation of fine η phases in the Al–15Zn–0.5 Mg–Sc–Zr alloys, but the Al3Zr phase could not. Finer grains and second phases led to the Al–15Zn–0.5 Mg–Sc–Zr alloys with Zr single addition and Sc, Zr collaborative additions exhibiting better mechanical properties than the alloy with Sc single addition. Meanwhile, the damping capacities of the Al–15Zn–0.5 Mg–Sc–Zr alloys with high-temperature aging were higher than those of the same alloys with aging at a low temperature due to their less numbers of dislocation pinning points and higher interface slipping ability.

Effect of Sc content on microstructure characteristics and evolution of W phase in Al–Cu–Li alloys under as-cast and homogenization conditions

In this study, the effect of Sc content on the microstructural characteristics and phase evolution of an Al–Cu–Li alloy under as-cast and homogenization conditions was investigated. The results show the grain refinement effect was distinct when the content of Sc was 0.16 wt%. However, further increased in Sc content resulted in a limited decrease in grain size. Besides, In the 0.16Sc alloy, the flocculent phase with primary Al3(Sc,Zr) phase coexisting with W (Al8-xCu4+xSc) phase appeared, and in the 0.32Sc alloy, the additional blocky primary Al3(Sc,Zr) phase was observed. With the increased of Sc content, the proportion of primary Al3(Sc,Zr) and W phases gradually increased, and the number and size of Al2Cu and Ag-containing Al2CuMg phases at grain boundaries gradually decreased. Two different morphologies of the primary W phase, flocculent and long strip shaped, were revealed within the Sc-containing alloy. The formation of the core-shell structure formed by Al3(Sc,Zr) particles and W phase was generated by the peritectic-eutectic reaction (Liquid + Al3(Sc,Zr)→α-Al + W) and the long strip of W phase was generated by the eutectic reaction (Liquid→α-Al + Al2Cu + W). During the homogenization process, the Al2Cu and Ag-containing Al2CuMg phases were completely re-dissolved after the two previous stages of homogenization, and only a small amount of the spheroidal residual W phase persisted. A subsequent low temperature treatment was employed to allow the Sc atoms that had diffused into the Al matrix to precipitate as Al3(Sc,Zr) dispersoids, resulting in a higher number density of Al3(Sc,Zr) particles for more efficiently utilizing the Sc elements.

Developing a novel lightweight Al–Mg–Li alloy for laser powder bed fusion additive manufacturing: Parameter optimization, microstructure evolution, and mechanical performance

With the advancement of laser powder bed fusion (LPBF) for aluminum alloys, developing novel lightweight Al–Li alloy systems present enormous potential for aerospace applications. Conventional Al–Mg–Li alloys exhibit low densities but suffer from poor strength due to the lack of inherent strengthening phases. In the present work, a novel lightweight Al–Mg–Li–Ag-Sc-Zr alloy was developed for LPBF after optimizing the processing parameters. The as-built alloy featured a bimodal microstructure consisting of fine equiaxed grains and columnar grains. The submicron primary Al3(Sc, Zr) phases with a cubic L12 structure served as inoculants for grain refinement and exhibited good coherency with the α-Al matrix. Besides, submicron spherical quasicrystalline T-Mg32(Al, Ag)49 phases of and nanometer rod-shaped S1–Al2MgLi phases were scattered inside the grains. These results produced a yield strength (σy) of 286 ± 8 MPa, ultimate tensile strength (σUTS) of 406 ± 3 MPa, and elongation at fracture (εf) of 12.5 ± 0.3%. The direct aging at 325 °C for 12 h led to an increase in hardness to a peak value of 170 HV. The secondary phases at the grain boundaries were breached and a large amount of submicron T and S1 phases within the grains were prominently increased. Besides, spherical Al3(Sc, Zr, Li) and tiny δ′-Al3Li precipitates were observed in the α-Al matrix. A considerably enhanced σy of 375 ± 12 MPa and σUTS of 469 ± 4 MPa were achieved mainly due to the particle strengthening but decreased εf to 5.8 ± 0.2%. The alloys in both states exhibited a moderate work-hardening capability. The feasibility of LPBF for lightweight Al–Li alloy was successfully established but required further optimization of the alloy composition.

The effect of minor scandium addition on microstructure evolution and mechanical properties of spray formed Al-Zn-Mg-Cu alloy

In this investigation, both Scandium (Sc)-added and Sc-free Al-Zn-Mg-Cu aluminum alloys were prepared by spray forming. The as-deposited alloys were hot extruded and then subjected to solid solution and aging treatment at different temperatures. The effect of minor scandium addition on microstructure evolution and mechanical properties were studied. The results revealed that, with Sc added spray formed alloy (7055Sc alloy), Sc element was mainly observed in the primary Al3(Sc,Zr) phase, the secondary Al3(Sc,Zr) phase and the W(AlCuSc) phase in the alloy. Compared to the alloy without Sc addition (7055 alloy), the effects of fine grain strengthening, dislocation strengthening and precipitation strengthening of Al3(Sc,Zr) phase were enhanced. At low aging temperature, the precipitation of MgZn2 phase was inhibited and the size of this phase was found large in the 7055Sc alloy. Therefore, after 120 °C for 24 h aging, the yield strength of 7055Sc alloy was tested as 603.4 MPa, which was 46.9 MPa lower than that of 7055 alloy at the same aging temperature. After 160 °C for 24 h aging, in the 7055Sc alloy the quantity and density of MgZn2 phase increased and the size of the phase was smaller. As a result, the yield strength of the 7055Sc alloy was 576.9 MPa, which was 31.6 MPa higher than that of the 7055 alloy. The effects of Sc on the aging precipitation of spray formed 7055Sc alloy at different aging temperatures were explained by the vacancy mechanism of diffusion.

Microstructure and mechanical properties of 600 MPa grade ultra-high strength aluminum alloy fabricated by wire-arc additive manufacturing

The utilization of wire-arc additive manufacturing (WAAM) technology for the preparation of Al-Zn-Mg-Cu aluminum alloy has made some progress in recent years. However, the challenge still remains to achieve ultra-high strength (600 MPa) in WAAM. In this study, the crack-free Al-Zn-Mg-Cu-Sc thin-wall component with ultra-high strength was successfully fabricated by the cold metal transfer (CMT) process using a self-prepared 7B55-Sc filler wire. The microstructures of both as-deposited and T6 heat-treated samples were all composed of fine equiaxed grains with an average size of about 6.0 μm. The primary Al3(Sc, Zr) particles acted as heterogeneous nuclei to promote the formation of equiaxed grains and refine the microstructures during the solidification process. A large amount of continuous eutectic structures rich in Al, Zn, Mg, and Cu elements formed along the grain boundaries under the as-deposited condition, and the precipitated second phases within the grains mainly included the equilibrium η phase, metastable η′ phase and large-sized T phase. After T6 heat treatment, the majority of the second phases originally distributed within grains and along grain boundaries were dissolved into the Al matrix, and a large amount of fine GP zones, η′ phase and secondary Al3(Sc,Zr) particles were precipitated within the grains during the aging process. The tensile strength reached a recorded level of 618 MPa in the horizontal direction after T6 heat treatment, which was considered a breakthrough for the manufacturing of 600 MPa grade aluminum alloy by WAAM.

Eutectic, precipitation-strengthened alloy via laser fusion of blends of Al-7Ce-10Mg (wt.%), Zr, and Sc powders

Laser powder-bed fusion was used to create an Al-7Ce-10Mg-0.71Zr-0.23Sc (wt.%) alloy, using hypoeutectic Al-7Ce-10Mg (wt.%) pre-alloyed powder blended with either elemental Zr and Sc powders (∼20 µm in size), or pre-alloyed, concentrated Al-10Zr and Al-10Sc (wt.%) powders (comprised of 1-5 µm Al3Zr or Al3Sc particles embedded in a <45 μm Al-rich matrix). The effects of process parameters and powder selection on the homogeneity of the as-fabricated microstructure were investigated via metallography and compared to predictions from a simple numerical model describing the dissolution of Zr powders in the melt pool during printing. Both experiments and model show that decreasing scan speed leads to greater dissolution of Zr and Sc, but remelting has no significant effect. The samples produced with Al-10Zr and Al-10Sc powder additions show greater amounts of Zr and Sc dissolution than those produced with elemental Zr and Sc additions, because of the smaller size (5 µm for prealloyed vs. 20 µm for elemental) and lower melting point (1580 and 1320°C for Al3Zr, Al3Sc vs. 1850 and 1541°C for Zr, Sc) of the dissolving species. The alloys fabricated with prealloyed powders display larger amounts of very fine grains (1-2 µm) nucleated by primary L12 Al3(Zr,Sc) precipitates, and a clear hardening response during aging, consistent with supersaturated Zr and Sc forming a high number density of secondary L12 Al3(Zr,Sc) nanoprecipitates.

High strength Al−Mg−Sc−Zr alloy with heterogeneous grain structure and intragranular precipitation produced by laser powder bed fusion

In this study, the intrinsic heterogeneous grain structure derived from the special temperature gradient in additive manufacturing and the intragranular precipitation were utilized for constructing a high-strength Al−Mg−Sc−Zr alloy. The alternative distribution of the coarse-grained (CG) layer in the middle of the molten pool and ultrafine-grained (UFG) layer at the boundary between the adjacent two molten pools were produced by using laser powder bed fusion (L-PBF) technology. High contents of Sc and Zr elements were introduced into the Al−Mg alloy, which contributed to the stabilization of UFG boundaries in the form of sub-micro primary Al3(Sc, Zr) particles and resulted in the formation of supersaturated Sc and Zr solid solutions in CG. The afterward aging treatment or well-controlled hot isostatic pressing (HIP) induced uniform precipitation of the high-density nano-scale coherent Al3(Sc, Zr) phase in CG. The HIP also promoted a significant decrease in porosity of the alloy and resulted in a combination of high ultimate tensile strength of 560 MPa and elongation of 12 % of the L-PBF Al−Mg−Sc−Zr alloy. The high strength of the alloy was attributed to the hierarchical formation of the Sc-rich phase, exerting a “dragging effect” on intergranular boundary migration as well as a “pinning effect” on intragranular dislocation sliding.

Effect of Ti, Sc and Zr additions on microstructure and mechanical properties of rheo-diecasting Al-6Zn-2Mg-2Cu alloys

The microstructure and mechanical properties of rheo-diecasting Al−6Zn−2Mg−2Cu alloys microalloyed with Ti, Sc and Sc+Zr were studied by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness testing, and tensile testing. The swirled equilibrium enthalpy device (SEED) process was introduced to prepare the semisolid slurry. Results show that the addition of Ti, Sc, and Sc+Zr refines the grain size and improves the uniformity of the semisolid slurry and then suppresses the growth of the α-Al grain during solution heat treatment. The microstructure of the four alloys in as-cast state mainly consists of spherical α-Al and the Mg(Al, Cu, Zn)2 (η) eutectic phase. Moreover, primary Al3Sc, Al3(Sc, Zr) and Al3Zr are also found in the micro-alloying alloys. After solution and aging heat treatment, most of the Mg(Al, Cu, Zn)2 phases dissolve into the α-Al matrix, while part of Mg(Al, Cu, Zn)2 phases transform to Al2CuMg (S) phases. However, the coarse primary Al3Sc and Al3(Sc, Zr) still remain in the matrix, and promote crack initiation and propagation. With the tensile strength of 553 MPa, yield strength of 463 MPa and elongation of 13.4% at T6 state, trace Ti addition generates more attractive mechanical properties than the other three alloys.

Microstructures and Mechanical Properties of Al-Mg-Sc-Zr Alloy Additively Manufactured by Laser Direct Energy Deposition

An Al-Mg-Sc-Zr alloy was additively manufactured by laser direct energy deposition (DED) under different laser powers, and the microstructures and mechanical properties of the as-deposited samples were investigated. The samples showed a fully equiaxed grain structure with grain sizes of 2–30 μm. Most of the blocky primary Al3(Sc, Zr)-precipitated phases (<5 μm) were arranged along the grain boundaries. A small amount of fine granular secondary Al3(Sc, Zr) phases (<0.5 μm) were precipitated owing to the cyclic heat treatment during the DED forming process. According to the EBSD(Electron backscatter diffraction) results, the texture index and strength of the sample were only slightly greater than 1, indicating that the material structure exhibited a certain but not obvious anisotropy. The sample in the horizontal direction had better yield strength, tensile strength, and elogation properties (399.87 MPa, 220.96 MPa, 9.13%) than that in the building direction (385.40 MPa, 219.40 MPa, 8.24%), although the sample in the〈XOZ〉 plane had the finest equiaxed grains. The ductility of the 〈XOZ〉 sample deteriorated as the number of pores increased.

Microstructures and mechanical properties of Al–Zn–Mg–Cu alloy with the combined addition of Ti and Zr

The effects of combined Ti and Zr addition on microstructures and mechanical properties are systematically investigated in Al–Zn–Mg–Cu alloy by microstructure characterizations and a physical-based model. The results show that combined addition of Ti and Zr can promote the precipitation of nano L12 Al3(Ti,Zr) dispersoids, while primary D022 Al3Ti and Al3(Ti,Zr) phases are formed during solidification when Ti addition is over 0.2 wt%. As compared to the 0Ti alloy, the ductility and toughness is enhanced markedly by 0.1–0.2 wt%Ti addition since the volume fraction of nano L12 Al3(Ti,Zr) dispersoids is increased. Both the strength and ductility are significantly decreased when Ti addition is more than 0.35 wt%. The strain hardening behavior and fractured morphology analyses suggest that the deterioration of mechanical properties is mainly due to the localized recrystallization and cracks caused by coarse primary phases, which cause the higher dislocation dynamic recovery rate. Combined addition of minor Ti and Zr elements may provide a simple approach to improve toughness and simultaneously reduce cost in developing high-strength Al alloys.

Development of a high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy using Laser Powder Bed Fusion: Design of a heterogeneous microstructure incorporating synergistic multiple strengthening mechanisms

Laser Powder Bed Fusion (L-PBF) provides great advantages in creating supersaturated solid solutions due to its intrinsic ultrafast cooling and high solidification rate, which is particularly desired for enhanced solid solution strengthening and precipitation strengthening. In the present work, a novel high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy with a good L-PBF processability is investigated. The as-built alloy exhibits a heterogeneous microstructure featuring a bi-modal grain structure with coarse columnar and fine equiaxed grains and heterogeneously distributed nanometer and submicrometer-sized cuboid-shaped primary Al3(Sc,Zr,Hf) particles. The Al3(Sc,Zr,Hf) phase, exhibiting a homogeneous elemental distribution of Zr, Sc, and Hf, adopts a cubic L12 structure with a lattice parameter of 0.4044 ± 0.009 nm, showing a good coherency with α-Al. Additionally, the rapid L-PBF solidification enables the formation of submicrometer-sized elongated and globular primary Al6Mn and a supersaturated Al matrix with a high dislocation density. This results in a good combination of strength (yield strength of 438 ± 3 MPa and ultimate tensile strength of 504 ± 2 MPa) and ductility (elongation at fracture of 10.9 ± 1.4 %), with a moderate work hardening strength of 113 ± 12 MPa. Direct-ageing at 325 °C for 10 h promotes the formation of a large amount of rod-shaped Al6Mn precipitates and a few spherical Al3(Sc,Zr,Hf) nanoprecipitates. The heat treatment increases the hardness from 166 ± 2 HV in as-built condition to 173 ± 4 HV and enhances the tensile strength (yield strength of 487 ± 2 MPa and ultimate tensile strength of 542 ± 3 MPa) but slightly reduces the ductility to 7.4 ± 0.7 %. The high strength was achieved by the synergistic effect of grain boundary strengthening, solid solution strengthening, and Orowan strengthening mechanisms.

Effects of solute atoms re-dissolution on precipitation behavior and mechanical properties of selective laser melted Al–Mg-Sc-Zr alloys

Selective laser melted (SLM) Al–Mg-Sc-Zr alloys still needs be post-treated to exhibit outstanding combination of strength and ductility. However, the influences of the different post heat treatments on the SLM Al–Mg-Sc-Zr alloys have not been systematically investigated. Hence, the SLM Al–Mg-Sc-Zr alloys were post-treated by a series of different heat treatments to investigate their influences on the SLM Al–Mg-Sc-Zr alloy, including the phase transitions, microstructural evolutions, and mechanical properties. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to study the emergence and effects of Al3(Sc,Zr) precipitates. Strengthening mechanisms of the different heat-treated samples were recognized and calculated in details based on the observations of cubic primary Al3(Sc,Zr) particles and spherical secondary Al3(Sc,Zr) precipitates behavior. Benefiting from the grain boundary strengthening and precipitation dispersion strengthening mechanisms, ultimate tensile strength of the aged samples increased from 353.7 MPa to 528.0 MPa.

Investigation and characterization of an Al-Mg-Zr-Sc alloy with reduced Sc content for laser powder bed fusion

Only few high strength Al alloys are processable by Laser Powder Bed Fusion due to the occurrence of hot cracks during solidification. In recent years, the addition of Zr and Sc in Al-Mg alloys revealed an effective solutions to suppress solidification cracking and improve strength. Nevertheless, since Sc is classified as a critical raw material by European Commission due to its high cost and supply risk, its content should be desiderably reduced. It is therefore necessary to focus on novel Al alloys featuring both enhanced processability and low amount of Sc. In this study, we investigated the microstructure and mechanical behavior of an Al-5.2Mg-0.8Zr-0.3Sc alloy, commercially available as m4p™ StrengthAl, produced by Laser Powder Bed Fusion. Simulations of equilibrium phase diagrams and Scheil solidification curves showed the precipitation of primary Al3Zr and Al3(Sc,Zr) in the liquid phase on cooling. These particles revealed able to act as nuclei for heterogeneous nucleation of grains, giving rise to a fine equiaxed structure which is able to suppress hot cracking and increase the processability of the Al-Mg-Zr-Sc alloy. Despite the reduced Sc content, the formation of secondary Al3(Sc,Zr) nano-phases during the annealing treatment led to a sharp increase of micro-hardness values, whereas a stress relief effect was monitored by residual stress measurements during aging. Both as-built and aged alloys show a bimodal grain size distribution and a similar crystallographic texture. Yield strength and ultimate tensile strength of 460 MPa and 485 MPa, respectively, were recorded in samples aged at 350 °C for 24 h and at 375 °C for 8 h.

Effects of ESMT on Microstructure and Mechanical Properties of Al-8Zn-2Mg-1.5Cu-0.15Sc-0.15Zr Cast Alloy in Squeeze Casting Process

Al-8Zn-2Mg-1.5Cu-0.15Sc-0.15Zr alloy with high-strength performance as well as good castability has been developed. In this study, effects of electromagnetic stirring melt treatment (ESMT) on microstructure and mechanical properties of the alloy in the squeeze casting process were investigated. The results show that solidification structure and mechanical properties are significantly improved by ESMT; compared with the conventional squeeze casting, the average grain size decreases from 112 μm without ESMT to 53 μm with ESMT. Meanwhile coarse primary Al3(Sc, Zr) particles unavoidably occurred in cases without ESMT disappear, and segregation degree of the main elements of Zn, Mg, Cu are greatly alleviated; the tensile strength increases from 590 MPa to 610 MPa, and the elongation increases from 9% to 11%. The structure refinement and homogenization should owe to uniform temperature and composition distribution by ESMT under squeeze casting with rapid solidification.

A Study on the Interrelationship between the Microstructural Features and the Elevated Temperature Strength of Multicomponent Al-Si-Cu-Ni Casting Alloys

The elevated temperature strength of multicomponent Al-Si alloys is greatly affected by the volume fraction and the interconnectivity of hard phases formed upon solidification. In the present investigation, such influences were examined for two Al-Si-Cu-Ni alloys with different total volume fractions of hard phases. To control the microstructural features related to the size of the phase, the specimens were prepared with and without ultrasonic melt treatment (UST) at different cooling rates. The microstructures of the alloys were composed of primary Si, eutectic Si, (Al,Si)3(Zr,Ni,Fe), Al9FeNi and Al3(Cu,Ni)2 phases. The microstructural features, such as the size and aspect ratio of each phase, changed with UST and cooling rate, and accordingly, the elevated temperature strength at 350 oC was changed. The alloy with a high volume fraction of about 30 vol.% exhibited increased elevated temperature strength at 350 oC when ultrasonic melt treated, and the alloy having a volume fraction as low as about 18 vol.% exhibited the opposite results. Considering the microstructural features of the multi-component Al-Si alloy, a hexagonal shear-lag model was suggested, based on the well-known shearlag model proposed by Nardone and Prewo (Scr. Metall. 20;1986:43-48). Using the 2-D microstructural factors such as the size, aspect ratio of the phase and secondary dendrite arm spacing, the elevated temperature strength was calculated and compared with the measured value. Based on the hexagonal shear-lag model, the influence of microstructural factors on the elevated temperature strength was discussed for multi-component Al-Si-Cu-Ni alloys.

Effect of minor Sc addition on high temperature tensile behavior of Al–7Zn–2Mg–2Cu–0.2Zr alloy

Alloys of Al–7Zn–2Mg–2Cu–0.2Zr and Al–7Zn–2Mg–2Cu–0.2Zr–0.2Sc were prepared by extrusion casting. Elongation–to–failure tests and strain–rate–change tensile tests were conducted at 300, 350, 400 and 450 °C, and deformation mechanisms were investigated based on microstructure, stress exponent and deformation activation energy. The results show that the 0.2 wt% Sc addition improves both tensile ductility and strength by refining grains and forming second phase particles. Primary Al3(Sc, Zr) particles precipitate during solidification and refine the as–cast grains, and secondary Al3(Sc, Zr) particles precipitate and interact with moving dislocations during high temperature deformation. The Sc addition has no significant effect on high temperature deformation mechanism. The deformation of both alloys is dominated by dislocation creep strengthened by second phase particles at 300 and 350 °C. Such strengthening effect weakens with the increase in temperature, and the deformation transfers to solute–drag dislocation creep at 450 °C.