Laser-based directed energy deposition of novel Sc/Zr-modified Al-Mg alloys: columnar-to-equiaxed transition and aging hardening behavior

The control of grain morphology is important in laser additive manufacturing(LAM), as grain morphology further affects the hot cracking resistance, anisotropy, and strength–ductility synergy of materials. To develop a solidification-control solution and achieve columnar-to-equiaxed transition(CET) in Al-based alloys during LAM, Sc-and-Zr-modified Al-Mg alloys were processed via directed energy deposition(DED).CET was achieved by introducing high potent primary Al_3(Sc,Zr) nucleation sites ahead of the solidification interface. Furthermore, the relationship between the solidification control parameters and precipitation behavior of primary Al_3(Sc,Zr) nucleation sites was established using the time-dependent nucleation theory. Then, the CET was studied according to the Hunt criterion. The results indicated that coarse columnar grain structure was still obtained at the inner region of the molten pool at low Sc/Zr contents owing to the effective suppression of the precipitation of the primary Al_3(Sc,Zr) nucleation sites via rapid solidification during DED. In addition, the relatively low melt temperature at the fusion boundary unavoidably promoted the precipitation of primary Al_3(Sc,Zr) nucleation sites, which resulted in a fine equiaxed grains band at the edge of the molten pool. As the Sc/Zr content increased, the solidification cooling rate was not sufficient to suppress the precipitation of the primary Al_3(Sc,Zr) nucleation sites, and a fully equiaxed grain structure was obtained. Furthermore, the effect of the layer-by-layer manufacturing process on the subsequent precipitation strengthening of secondary Al_3(Sc,Zr) precipitates was discussed.Both the remelting and subsequent aging during thermal cycling should be considered to achieve greater precipitation strengthening.     

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase in Al alloys

Impurity effects on the nucleation and growth of primary Al₃(Sc,Zr) phase have been investigated in high purity Al alloys and commercial purity Al alloys, respectively. In the case of high purity Al alloys, primary Al₃(Sc,Zr) phases were found to be pushed to grain boundaries ahead of the solidification front. Such type of primary Al₃(Sc,Zr) phase did not contribute to the heterogeneous nucleation, and thereby the grain refinement of Al alloys. In the case of commercial purity Al alloys, the presence of Fe, Si, Cu, Mg, Ti, and other impurities significantly enhanced the heterogeneous nucleation of primary Al₃(Sc,Zr) phase. Most primary Al₃(Sc,Zr) phases were found to be located within the α-Al matrix, and kept an identical orientation relationship with the α-Al matrix. Furthermore, the presence of the impurities also changed the growth mode on the primary Al₃(Sc,Zr) phase. In the case of commercial purity Al alloys, a peritectic to eutectic reaction was induced due to the presence of the impurities. A layered growth was observed leading to a narrow particle size distribution. In contrast, in the case of high purity Al alloys, a featureless structure was observed. This investigation demonstrates that impurities and their concentrations are important factors affecting the nucleation and growth of primary Al₃(Sc,Zr) phases, and thereby for the successful grain refinement in Al-based alloys.     

Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al₃(Sc,Zr)?+?α-Al?+?Al₃(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.     

High Cycle Fatigue Property of Al–Mg–Sc–Zr Alloy Fabricated by Laser Powder Bed Fusion

Laser powder bed fusion (LPBF) of high-strength Al–Mg–Sc–Zr alloy possesses great potential application in the aerospace industry. However, fatigue performance has become a very important issue in the safety and durability design of engineering structures. Herein, the high cycle fatigue property of LPBF-fabricated Al–Mg–Sc–Zr alloy and its correlation with defects, microstructure, and precipitated phases are studied. The LPBF-manufactured Al–Mg–Sc–Zr alloy appears heterogeneous structure composed of equiaxed grains at the molten pool boundary and columnar grains at the inner of the molten pool. After aging treatment (325 °C/4 h), the nanosized Al₃Sc intragranular particles and Mn-rich intergranular particles are precipitated, leading to more difficult movement of dislocations that favors the fatigue strength. The ultimate tensile strength of the samples after aging treatment is 507.05 MPa and corresponding 10⁷ cycle fatigue strength (R = −1) is 106 MPa. The fracture morphology of the fatigue specimens shows that the fatigue cracks start from the surface defects with strong stress concentration, especially the lack of fusions, and then expand through the surplus part.     

Ce对Al-Zn-Mg-Cu合金亚快速凝固铸造组织的影响

通过XRD,DSC,SEM,EDS等现代分析方法,研究了稀土元素Ce在不同凝固冷却速率下对Al-Zn-Mg-Cu合金显微组织、凝固温度的影响,分析讨论了Ce对合金晶粒细化和熔体净化作用的原理。结果表明,合金的主要析出相为α-Al和MgZn_2型共晶相,MgZn_2固溶了Al,Cu,Mg等元素并形成了Mg(Zn,Cu,Al)_2相,在晶界上溶质元素浓度较高,与α-Al基体共晶形成层片状共晶组织。添加Ce能使合金枝晶间距减小,并减小共晶层片间距和细化共晶组织,显著细化晶粒,并抑制铝合金中的杂质相Al7Cu2Fe的出现。Ce还将合金α-Al基体和共晶相的析出温度分别降低了6.4℃和5.6℃。     

Optimizing microstructure,shrinkage defects and mechanical performance of ZL205A alloys via coupling travelling magnetic fields with unidirectional solidification

ZL205A alloys tend to form disordered and defective microstructure due to the large solidification intervals and multi-phase.Accordingly,finding ways to effectively optimize the microstructure and mechanical performance is of great significance.In this regard,the coupling of travelling magnetic fields (TMF) with unidirectional solidification was used to continuously regulate the mushy zones of ZL205A alloys.Additionally,experiments are combined with simulations to systematically reveal the mechanisms on the optimizations at each stage of solidification process.Current findings demonstrate that different directional strong melt flows generated by TMF are responsible for these optimizations.Additionally,the effects of TMF on microstructure are different at each stage of solidification process.Specifically,downward TMF coupled with unidirectional solidification can refine and uniform the microstructure,decrease the formation of precipitation,promote the growth consistency of matrix phase α-Al growing along the <001 > crystal orientation,reduce the secondary dendrites and overlaps between dendrites,eliminate the shrinkage defects,and increase the ultimate tensile strength,yield strength,elongation and hardness from 198.3 MPa,102.2 MPa,7.5 ％ and 82.3 kg mm-2 without TMF to 225.5 MPa,116.1 MPa,13.6 ％ and 105.2 kg mm-2.Contrastively,although upward TMF can reduce Al3Ti and refine α-Al,it increases the formation of Al6Mn,Al2Cu,secondary dendrites,overlaps between dendrites,and shrinkage defects;then it deflects and disorders the growth of α-Al,further to decrease the overall performance of alloys.     

Investigation of primary Al3(Sc,Zr) particles in Al–Sc–Zr alloys

Abstract

This paper studied the primary Al₃(Sc,Zr) particles formed during solidification in three Al–Sc–Zr alloys with various Zr contents. It has been shown that the primary Al₃(Sc,Zr) particles formed during solidification since the Sc and Zr concentrations are above the solid solubility limit in Al matrix. Scanning electron microscopy line scanning results indicated that the distributions of Sc, Zr and Al atoms in the primary Al₃(Sc,Zr) particles differ a lot from those in the secondary Al₃(Sc,Zr) particles formed during annealing. In the primary Al₃(Sc,Zr) particles, both Sc and Zr contents are found from rim to the centre of the particles, with an increasing trend from the rim to the centre, while the Al content drops sharply on the rim of the particle and slightly from the rim to the centre.     

Microstructure,tensile properties and thermal stability of AlMgSiScZr alloy printed by laser powder bed fusion

The new generation of Sc and Zr modified Al alloys has been attracted wide concerns in aerospace industry,owing to the excellent mechanical performances and superior thermal stability than other normal Al alloys. By microalloying with Sc and Zr, the Al3(Sc, Zr) particle forms as the grain refiner during the solidification, which is extremely beneficial for the laser powder bed fusion(LPBF) processed Al alloys. In this study, a new type Al-14.1 Mg-0.47 Si-0.31 Sc-0.17 Zr alloy was additively manufactured by LPBF, and the microstructure, tensile properties and thermal stability were studied in detail. By using a single melt-67°scanning strategy, the LPBF-processed specimen with a relative density of 99.4 % and tensile strength of 487.7 MPa was obtained at 160 W-200 mm/s. And this AlMgSiScZr alloy can still exhibit an excellent tensile strength of 393.9 MPa at a moderate temperature of 473 K. After the aging treatment, the tensile properties further increased due to the precipitate hardening of Mg2 Si and Al3(Sc, Zr), and the maximum value(580 MPa) was reached at an aging time of 10 h. The average crystal size was almost unchanged after aging treated at 325°C and 24 h, indicating this AlMgSiScZr alloy has an improved thermal stability.The AlMgSiScZr alloy is recommended to substitute some particular titanium alloys in aerospace field afterwards.     

Effect of Sc and Zr alloying on microstructure and precipitation evolution of as cast Al–B4C metal matrix composites

Abstract

Additions of Sc and Zr were introduced into Al–15 vol.-%B₄C composites, and eight experimental composites with different Sc and Zr levels were prepared via a conventional cast process. Optical microscopy, SEM and TEM were applied for observing the as cast microstructures, including the interfaces between the Al matrix and the B₄C as well as the evolution of the precipitates. It was found that Sc involved the interfacial reactions with B₄C that partially consumed the Sc. On the other hand, no major Zr reaction products were found in the interfaces, and the major part of Zr remained in the matrix for precipitation strengthening. The Sc addition yielded considerable precipitation strengthening in the as cast and peak aged conditions. The combination of Sc and Zr significantly enhanced the precipitation strengthening. Nanoscale precipitates Al₃Sc and Al₃(Sc,Zr) were found in the as cast microstructure and contributed to the significant increase of matrix hardness.     

Effect of Sc and Zr additions on microstructures and corrosion behavior of Al-Cu-Mg-Sc-Zr alloys

The effects of adding the alloy element Sc to Al alloys on strengthening, recrystallization and modification of the grain microstructure have been investigated. The combination of Sc and Zr alloying not only produces a remarkable synergistic effect of inhibition of recrystallization and refinement of grain size but also substantially reduce the amount of high-cost additional Sc. In this work, the microstructures and corrosion behavior of a new type of Al-Cu-Mg-Sc-Zr alloy with Sc/Zr ratio of 1/2 were investigated.The experimental results showed that the Sc and Zr additions to Al-Cu-Mg alloy could strongly inhibit recrystallization, refine grain size, impede the segregation of Cu element along the grain boundary and increase the spacing of grain boundary precipitates. In addition, adding Sc and Zr to Al-Cu-Mg alloy effectively restricts the corrosion mechanism conversion associated with Al2 Cu Mg particles, which resulted in the change of the cross-section morphology of inter-granular corrosion from an undercutting to an elliptical shape. The susceptibility to inter-granular corrosion was significantly decreased with increasing Sc and Zr additions to the Al-Cu-Mg alloy. The relationships between microstructures evolution and inter-granular corrosion mechanism of Al-Cu-Mg-Sc-Zr alloys were also discussed.     

锆微合金化增强铝合金的研究进展EI北大核心CSCD

锆(Zr)元素是铝合金中研究较为深入、实际应用较为广泛的微合金元素之一。由于Zr在铝中具有低的固态扩散速率且可形成低密度、高熔点、低界面错配度的Al_(3)Zr弥散相,因此合金展现出高温下服役的潜力。然而,Al_(3)Zr粒子的弥散强化效果主要受到粒子低数量密度或体积分数的制约;此外,多元合金体系凝固、变形、热处理过程中多组元间交互作用复杂,Al_(3)Zr弥散强化与各体系中本征相强化作用往往难以兼得,上述问题均对合金的力学强度造成了不利的影响。本文综合近年来的相关报道,对含Zr铝合金中Zr的存在形式、析出和粗化行为以及强化机制进行了概述;简要介绍了复合微合金化促进Al_(3)Zr析出机理与最新研究结果;对某些体系铝合金中Zr微合金化的应用进行了归纳与总结,结合当前新型耐热铝基合金发展的新趋势,指出铝合金内Zr的微量添加对调控微结构、提升室温和高温强度的重要意义。     

Effects of Zr,Ti and Sc additions on the microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe alloys

The evolution of microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe (wt.%) alloys, micro-alloyed with Zr, Ti and Sc, were investigated. The addition of 0.2%Zr to base alloy accelerates the precipitation of Si-rich nano-phase in α-Al matrix, which plays an important role in improving the mechanical properties of an alloy. The tensile strength increases from 102 MPa for the base alloy to 113 MPa for the Zr-modified alloy. Adding 0.2%Zr + 0.2%Ti to base alloy effectively refines α-Al grain size and accelerates the precipitation of Si and Cu elements, leading to heavy segregation at grain boundary. By further adding 0.2%Sc to Zr + Ti modified alloy, the segregation of Si and Cu elements is suppressed and more Si and Cu precipitates appeared in α-Al matrix. Accompanied with the formation of coherent Al₃Sc phase, the tensile strength increases from 108 MPa for the Zr + Ti modified alloy to 152 MPa for the Sc-modified alloy. Due to excellent thermal stability of Al₃Sc phase, the Sc-modified alloy exhibits obvious precipitation hardening behavior at 350 °C, and the tensile strength increases to 203 MPa after holding at 350 °C for 200 h.     

Microstructural characterization and grain refinement of AA6082 gas tungsten arc welds by scandium modified fillers

The refinement in weld metal grain size and shape results in both improved mechanical properties (ductility and toughness) as well as a significant improvement in weldability. In the present study, the influence of scandium (Sc) additions to the fillers on the structure and mechanical properties of AA6082 gas tungsten arc (GTA) weldments were investigated. Controlled amounts of scandium as grain refiner were introduced into the molten pool of AA6082 by pre-deposited cast inserts (AA4043 and AA5356) by GTA welding. Full penetration GTA welds were prepared using alternating current (AC). It was observed that grain size decreased with increasing amounts of scandium. The grain refinement is mainly caused by the Al₃Sc particles, which act as heterogeneous nucleation of α-Al grains. It has been shown that welds prepared with AA5356 cast insert exhibited high strength and ductility when compared with other welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, strength and ductility. Post-weld aging treatment resulted in improved tensile strength and hardness of the weldments and this aging response could be attributed to the weld dilution from the base metal. The slow diffusion of Sc in Al matrix and stability of Al₃Sc precipitates at elevated temperatures were suggested to be responsible for the improved high temperature yield strength of welds made from Sc modified fillers.     

An additively manufactured Al-14.1Mg-0.47Si-0.31Sc-0.17Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance

A novel Al-14.1 Mg-0.47 Si-0.31 Sc-0.17 Zr alloy was applied in the printing process of selective laser melting(SLM),and the corresponding microstructural feature,phase identification,tensile properties and corrosion behavior of the Al Mg Si Sc Zr alloy were studied in detail.As fabricated at 160 W and 200 mm/s,the Mg content of bulk sample decreased to 11.7 wt%due to the element vaporization at high energy density,and the density of this additively manufactured Al Mg Si Sc Zr alloy was 2.538 g/cm³,which is4.2%8.5%lighter than that of other SLM-processed Al alloys.After heat-treated(HT)at 325℃and 6 h,the microstructure was almost unchanged with an alternate distribution of fine equiaxed crystals and coarse columnar crystals.Nano-sized Al3(Sc,Zr)and Mg₂Si phases precipitated dispersedly in the Al matrix,and the tensile strength increased from 487.6 MPa to 578.4 MPa for precipitation strengthening and fine grain strengthening.With a fine grain size of 2.53μm,an excellent corrosion resistance was obtained for the as-printed(AP)Al Mg Si Sc Zr alloy.While the corrosion resistance of HT sample decreased slightly for the formation of non-dense oxide layer and pitting corrosion induced by diffuse precipitation distribution.This SLM-printed Al Mg Si Sc Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance has broad prospects for the aerospace and automotive applications.     

微量Sc和Zr对Al—Az—Mg合金组织与性能的影响

采用铸锭冶金法制备了Al-6.2Zn-2.0Mg-0.25Zr和Al-6.2Zn-2.0Mg合金,测试不同处理态的拉伸力学性能。利用金相显微镜和透射电子显微镜研究其不同处理态的显微组织,结果表明：添加微量Sc和Zr可明显细化合金的铸态晶粒,并显著提高Al-Zn-Mg合金的力学性能,其作用机理主要为Al3(Sc,Zr)造成的细晶强化,亚结构强化和弥散强化。     

Formation of metastable phases and nanocomposite structures in rapidly solidified Al-Fe alloys

In the present work the structure and morphology of the phases of nanocomposites formed in rapidly solidified Al-Fe alloys were investigated in details using analytical transmission electron microscopy and X-ray diffraction. Nanoquasicrystalline phases, amorphous phase and intermetallics like Al₅Fe₂, Al₁₃F₄ coexisted with α-Al in nanocomposites of the melt spun alloys. It was seen that the Fe supersaturation in α-Al diminished with the increase in Fe content and wheel speed indicating the dominant role of the thermodynamic driving force in the precipitation of Fe-rich phases. Nanoquasicrystalline phases were observed for the first time in the dilute Al alloys like Al-2.5Fe and Al-5Fe as confirmed by high resolution TEM. High hardness (3.57 GPa) was measured in nanocomposite of Al-10Fe alloy, which was attributed to synergistic effect of solid solution strengthening due to high solute content (9.17 at.% Fe), dispersion strengthening by high volume fraction of nanoquasicrystalline phase; and Hall-Petch strengthening from finer cell size (20-30 nm) of α-Al matrix.     

Optimization of hardening of Al–Zr–Sc cast alloys

The effects of composition, cooling rate after the end of solidification, and annealing regime on the structure and hardening of binary and ternary alloys of the Al–Sc–Zr system are studied. The liquidus in Al–Sc–Zr alloys is experimentally assessed in order to facilitate the correct choice of casting temperatures. The precipitation during slow cooling after the end of solidification causes hardening in the as-cast state and decreases the hardening effect during annealing. It is shown that the full hardening ability of precipitates can be achieved only upon their homogeneous distribution in the matrix. The optimum total concentration of Sc and Zr in aluminium alloys should be about 0.3 wt% at the ratio Zr:Sc ≥ 2. That allows conventional casting temperatures and considerable hardening during annealing.     

Effect of additional doping with scandium and scandium with zirconium on strength properties of the alloys of Al–Mg<Subscript>2</Subscript>Si system

The effect of Sc and Sc + Zr on the structure and properties of Al–Mg–Si alloys is studied using the methods of hardness measurement, resistivity, mechanical properties, and light and electron microscopy. A considerable strengthening of Al–Mg₂Si alloys doped with transition metals (Sc or Sc + Zr) during natural aging and its almost complete absence during artificial aging are established. The observed effects are explained using electron microscopy and local X-ray analysis.     

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al₃Sc/Al₃(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al₃Sc/Al₃(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.     

微量钪在Al-Cu-Li-Zr合金中的存在形式和作用

通过显微组织观察和室温拉伸实验,研究了微量Sc在Al-Cu-Li-Zr合金中的存在形式和对合金微观组织和拉伸性能的影响.结果表明:微量Sc在Al-Cu-Li-Zr合金中主要以初生Al3(Sc,Zr)和次生Al3(Sc,Zr)两种形式存在.初生的Al3(Sc,Zr)是合金凝固过程中形成的,可成为有效的非均质形核中心,显著细化铸态晶粒组织,具有细晶强化和增塑作用;次生Al3(Sc,Zr)是合金在热加工过程中析出的,对位错和亚晶界起钉扎作用,稳定亚结构并有效抑制合金再结晶,具有亚结构强化和直接析出强化作用.因此,加入微量Sc的Al-Cu-Li-Zr合金的强度和塑性大大提高.