Al-Cu Alloys Research Articles

Hysteresis gap-shrinking and structural effects of minor Al and Ti modifications on binary CuAl-based high-temperature shape memory alloys

Cu-based shape memory alloys (SMAs), except for exhibiting shape recovery, superelasticity, and high damping, are desirable because these smart materials have higher electrical and thermal conductivity and much lower prices than NiTi SMAs. However, they also have some downsides in mechanical strength and brittleness (mostly stemming from their coarse grain structure) and thermal instability. Therefore, adding some grain refining elements to these SMAs to improve their shape memory effect (SME), and thermal, structural, and mechanical properties is a widespread and simple way that significantly affects their martensitic phase transitions, structure, and mechanical properties. One of these grain-refining elements is titanium. Its thermal conductivity is lower than those of Cu and Al elements and has a low solubility in Cu-matrix. Besides the effects of small Al variations, the use of minor amounts of titanium in binary CuAl-base alloys can show impressive effects on all characteristics of these shape memory alloys, such as shape memory effect properties, martensitic transformation kinetics parameters, and microstructural features. In this research work, CuAlTi ternary high-temperature shape memory alloys (HTSMAs) with new compositions were produced by the arc melting method without a complicating use of Mn or Ni components in usual ternary CuAlMn and CuAlNi shape memory alloys. Thermal analyses of the prepared samples of the alloys were investigated by using differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements. In contrast, x-ray diffraction (XRD) test results and optical micrographs were used for analyzing the structure of the alloy samples. The effect of different amounts of low soluble and grain refining Ti element on the binary CuAl alloy system was investigated.

In-situ microalloying of Al-Cu-Sc alloy manufactured by beam oscillating wire laser directed energy deposition: Grain refinement and properties improvement

Al-Cu alloys manufactured by wire laser directed energy deposition (WLDED) usually exist coarse columnar grains. The novelty of this study is adding Sc to Al-Cu alloy of beam oscillation wire laser directed energy deposition (O-WLDED), achieving grain refinement by in-situ microalloying of Sc. An Al-Cu alloy part and an Al-Cu-Sc alloy part are prepared by O-WLDED, respectively. Furthermore, how the Sc affects the microstructure and mechanical properties is explored and discussed. The in-situ microalloying of Sc provides more nucleation sites for grains and promotes the columnar to equiaxed transformation. The average grain size of the overlapping zone decreases from 21.64 to 14.32 μm, and that of the remelting zone decreases from 18.67 to 7.83 μm. The grain refinement is significant. The contents of θ-Al2Cu and θ'-Al2Cu at grain boundaries of the Al-Cu-Sc alloy reduce since Al3Sc particles precipitated from the Al matrix can hinder the segregation of Cu. The ultimate tensile strength and elongation of the Al-Cu-Sc alloy reach 433 MPa and 11.8 %, respectively, respectively 117 MPa and 2.6 % higher than those of the Al-Cu alloy.

Co-, Ni- and Fe-rich grain-boundary phases enhance creep resistance in θ′-strengthened Al-Cu alloys

Microstructural evolution and creep response were investigated in the cast Al-5.0Cu-0.3Mn-0.2Zr (wt.%) alloy with and without addition of slow-diffusing, intermetallic-forming elements Fe, Ni, or Co. Baseline Al-5.0Cu-0.3Mn-0.2Zr alloy exhibits high creep resistance at 300 °C, up to ∼75 MPa, which is attributed to high-aspect-ratio, intragranular θ′-Al2Cu precipitates that effectively suppress dislocation climb. However, θ′-Al2Cu precipitate-free zones form along grain boundaries upon heat treatment, whose extent is amplified during subsequent creep. Such weak regions experience dislocation creep, leading to strain localization and acceleration of grain-boundary sliding. Adding Ni and Co, individually or in combination, leads to the formation of grain-boundary precipitates (Al9Co2, Al3Ni2) which are resistant to coarsening, thus suppressing the formation of θ′-Al2Cu precipitate-free zones. This microstructure provides high creep resistance at stresses up to 75–80 MPa, with strain rates much lower than in unmodified Al-5.0Cu-0.3Mn-0.2Zr. Adding Fe, which results in extensive decoration of grain boundaries with coarsening-resistant Al7Cu2Fe, and then increasing the Cu content to compensate for the Cu loss to this new phase, leads to a new Al-7.4Cu-1.6Fe-0.3Mn-0.2Zr alloy with creep resistance at 300 °C that surpasses known cast aluminum alloys. Adding Fe to improve the creep resistance of Al-Cu alloys is both cost-effective and sustainable. Our findings offer guidelines applicable to various alloy systems on controlling the evolution of precipitate-free zones and its ensuing effects on creep deformation.

Stabilization Effect of Interfacial Solute Segregation on θ′ Precipitates in Al-Cu Alloys

The effects of Sc, Mg and Si elements in an Al-Cu alloy have been studied by means of hardness tests and transmission electron microscopy analysis. The experimental results show that additions of Sc, Mg and Si can improve the heat resistance of the Al-Cu alloy. The Sc/Mg/Si segregation-sandwiched structure is the most stable, when compared with Sc segregation or Si/Sc co-segregation at the interface of θ′/Al. The additions of Si and Mg promote the aging–hardening response of the Al-Cu alloy. Mg is a micro-alloying element with great potential in stabilizing the size of θ′ phases, which further promotes the number density greatly. Consequently, the Al-Cu alloy achieves a high strength, matched with excellent thermal stability, due to the microalloying of Sc/Mg/Si solutes.

The Effect of Precipitate Phases on Mechanical Properties of Al-Cu Alloys at Elevated Temperatures

The Effect of Precipitate Phases on Mechanical Properties of Al-Cu Alloys at Elevated Temperatures

Effects of external static electrical field on thermal and electrical conductivity in the Al-Cu, Al-Ni, and Al-Si eutectic alloys

This study aims to investigate the effects of external positive and negative static electric fields (E+ and E- respectively) on thermal conductivity (K) and electrical conductivity (σ) in Al-33 wt. % Cu, Al-6.4 wt. % Ni and Al-12 wt. % Si eutectic alloys. For this purpose, the solidifications of Al-Cu, Al-Ni, and Al-Si eutectic alloys were directionally done under E+ and E-. The directions of E were chosen to be parallel (E+) and antiparallel (E-) to the solid-liquid (S-L) growth direction and the magnitudes of E were approximately (+10) and (−10) kV cm−1 and (+16) and (-16) kV cm−1 for the Al-Cu, Al-Ni, and Al-Si eutectic alloys, respectively. The effects of E+ and E− on the K and σ were determined by the longitudinal heat flow and the four-point probe methods, respectively. While the K and σ values decreased with increasing temperature, the K and σ were increased and decreased with E+ and E−, respectively.

Revealing and predicting the mechanical and strain hardening behaviors of Cu-Al alloys based on microscopic mechanism

In order to reveal and predict the strain hardening behaviors of metallic materials, mechanical properties of Cu-Al alloys processed by rolling were investigated by tensile tests. The microstructure evolution of Cu-Al alloys processed by rolling were characterized. The true stress-strain curves were fitted by an exponential strain-hardening (ESH) model. Based on the microscopic mechanism of dislocation growth and annihilation, the true stress strain curves were predicted through macroscopic mechanical properties. The strengthening effects on Cu-Al alloys of different methods were analyzed quantitively by hardening exponent z and saturation strength σs which were derived from the ESH model. The fundamental assumptions and physical meaning of parameters have been validated by investigating strain hardening behaviors of Cu-Al alloys. The limited applicable materials and future research of ESH model are also proposed for the further development. This study could provide guidance to understand work-hardening behavior and predict tensile performance of Cu-Al alloys.

The Effects on Stability and Electronic Structure of Si-Segregated θ′/Al Interface Systems in Al-Cu Alloys

This study systematically investigates the energy and electronic properties of Si-segregated θ′(Al2Cu)/Al semi-coherent and coherent interface systems in Al-Cu alloys using ab initio calculations. By evaluating the bonding strength at the interface, it has been revealed that Si segregated at the A1 site (Al slab) of the semi-coherent interface systems exhibits the most negative segregation energy, resulting in a noticeable decrease in total energy and an increase in interface adhesion. The electronic structure analysis indicates the presence of Al-Cu and Al-Al bonds, with Si occupying the A1 site. The strong bond formation between Al-Cu and Al-Al is essential for improving interface bonding strength. The results of the calculating analyses are consistent with the results of the previous experiments, and Si can be used as a synergistic element to reduce the θ′/Al interface energy and further reduce the coarsening drive of the θ′ precipitated phase, which can provide new perspectives and computational ideas for the compositional design of heat-resistant Al-Cu alloys.

Numerical modeling of dynamic heat flow behaviour during friction stir welding of Al-Cu alloy

Numerical modeling of dynamic heat flow behaviour during friction stir welding of Al-Cu alloy

Structures in grain-refined directionally solidified hypoeutectic Al-Cu alloys: Benchmark experiments under microgravity on-board the International Space Station

Benchmark solidification experiments were successfully performed under microgravity conditions on-board the International Space Station (ISS) within the ESAprogramme CETSOL (Columnar-to-Equiaxed Transition in SOLidification Processing). Cylindrical samples of grain-refined Al-4wt.%Cu, Al-10wt.%Cu and Al-20wt.%Cu alloys were directionally solidified in a gradient furnace to investigate columnar and equiaxed dendritic growth structures as well as the columnar to equiaxed transition under diffusive conditions. The determination of temperature gradients; interface velocities; and cooling rates at liquidus, solidus, and eutectic front positions provides well-defined thermal experimental characterization. The evaluation of the flight samples demonstrates that no significant macrosegregation along the sample axis occurred and no radial effects were observed. Therefore, purely diffusive solidification behaviour without any residual melt convection can be assumed for these microgravity experiments. The analyses of the microstructure in longitudinal cross-sections show dendritic structures without any pore formation and the averaged eutectic fraction is largely constant along the sample. The samples of refined Al-4wt.%Cu alloy show a sharp CET from columnar dendrites to a fine equiaxed steady-state grain structure whereas in the samples of refined Al-10wt.%Cu and Al-20wt.%Cu alloy, only equiaxed dendritic grain growth is observed. A quantitative analysis of the equiaxed grain morphology shows, that the shapes of the equiaxed dendrites depend on the applied temperature gradient, but the grain sizes in radial and longitudinal directions are identical. Therefore, a fully equiaxed dendritic growth structure without dendrite elongation was obtained. Compared to experiments in microgravity with non-refined Al-Cu alloys the average equiaxed grain size is about three times smaller.

A counter-gravity casting system for in situ high-speed synchrotron x-ray imaging characterization.

Counter-gravity casting (CGC) aims to eliminate turbulent melt flow and defect formation during filling and subsequent solidification by pushing high-temperature melt into the mold cavity against gravity with regulated pressure. However, limited by the opaqueness of molten metals and the complexity of the CGC apparatus, it is extremely difficult to directly quantify the high-velocity mold filling and pressurized solidification in real-time. Here, we report the design and characterization of a CGC system capable of in situ monitoring of mold filling and subsequent solidification processes in the synchrotron beamlines by deploying a high-energy, high-speed synchrotron x-ray imaging technique. The high-velocity melt flow and dendrite growth during pressurized solidification have been quantified for systematical process parameter analysis by investigating time-resolved x-ray images of an exemplary Al-Cu alloy. The high-speed imaging results demonstrate that the in situ CGC system provides a useful way to better understand the fundamentals of mold filling, pressurized solidification, and experimental inputs for high-fidelity modeling in scientific and industrial applications.

A secondary high-temperature precursor of the θ′-phase in Al-Cu-(Sc) alloys

The Al-Cu alloy is a historical model alloy system in the physical metallurgy of engineering aluminum alloys. Nevertheless, a few fundamental phenomena of phase transformation occurring in this simple alloy are still not adequately understood. Among all, for instance, the formation mechanisms of its key hardening θ′-phase remain mysterious. There is strong evidence that θ′-precipitates can form from a different high-temperature precipitation pathway, while their formation mechanism via the conventional pathway well-known since 1938 remains to be clarified. Using state-of-the-art electron microscopy, here we report a secondary high-temperature precipitation pathway of θ′-precipitates. It is demonstrated that led by a secondary high-temperature precursor, named θ′S-HTP, very fine θ′-precipitates can form in the undeformed bulk Al-Cu alloys at elevated temperatures (≥ 250 °C). Interestingly is that with Sc-microalloying the surviving rate of meta-stable θ′S-HTP precipitates increases drastically and the formed θ′-precipitates become much finer, significantly enhancing the alloys’ strength and thermal stability. It is also revealed that a θ′S-HTP precipitate can genetically evolve into a θ′-precipitate without having to change its morphology and orientation. Our study provides new insights into understanding the industry bulk alloys’ microstructures and properties.

Evolution and formation mechanism of residual stress in Al-Cu alloy rings during ring rolling and quenching

The large-scale ring components, as thin-walled structures, are prone to unpredictable machining deformation due to the initial residual stresses introduced by rolling and quenching, which seriously affect the dimensional accuracy and service life of the ring components. Therefore, the relief and homogenization of residual stress inside large-scale ring components is crucial. However, the existing studies rarely involve the essential causes and the evolution laws of residual stresses during the manufacturing of ring components, which are indispensable for understanding the failure mechanisms of components caused by residual stresses and specifying relevant protective measures. Therefore, the evolution and distribution laws of stress-microstructure-property non-uniformity during the hot forming (ring rolling) and heat treatment (quenching) of 2219 Al-Cu alloy rings were precisely and deeply explored using scaled experiments in this paper. The relationship between non-uniform deformation and residual stresses was analyzed, and the basic equation that residual stresses must satisfy was established. Furthermore, the concept of the “non-coordination coefficient” was proposed, revealing the physical mechanism of residual stresses induced by non-uniform deformation. In addition, the influencing factors and formation mechanisms of residual stresses in the rolled and quenched state of rings were studied, and the essential reasons for the differences in residual stresses between the rolling and quenching processes were obtained. The experimental results indicate that the circumferential and axial residual stresses increased from 1.48 and 0.51 MPa to −142.68 and −122.31 MPa, respectively. The standard deviation changed from 13.47 and 33.52 MPa to 63.42 and 61.61 MPa, respectively. The maximum range of tensile strength, yield strength and elongation rates of the rolling and quenched state is 12.82 and 20.16 MPa, 17.49 and 22.14 MPa, and 8.62 % and 7.56 %, respectively. All fractures are microporous aggregation ductile, including dimple transgranular fracture and intergranular fracture. The quenched state exhibits a significantly larger grain size compared to the rolled state. The appearance of residual stress is essentially to force the inconsistent deformation of various parts inside the object to maintain its integrity and continuity, which leads to a local mismatch of micro-elements inside the ring and further induces non-uniform strain and stress. The difference in the severity of “penetrability” induced by different temperature gradients determines the difference in residual stress between rolled and quenched rings, larger rings have higher and more non-uniform residual stress than smaller rings under the same process conditions. The above results deepen the theoretical understanding of residual stress formation mechanisms and lay a theoretical foundation for the comprehensive evaluation of performance degradation and service failure analysis of large ring components.

Effect of tool rotational speeds on material flow and strain rates during friction stir butt welding of AA2219-T87 plates

ABSTRACT Friction stir welding (FSW) process is widely used for joining Al-Cu alloy (AA2219-T87) plates because of its high joint efficiency and low residual stresses in contrast to fusion-based joining techniques. A two-way coupled, 3D thermal-material flow model has been developed in COMSOL software, and the effect of tool rotational speeds on thermal and material flow fields, strain rates, thermal histories, and size of weld zones are being studied. The non-uniform net heat generation during the FSW process is due to the friction between workpiece and tool materials and the deformation energy of plasticized material flow sheared by the rotating tool. An approximate range of temperatures for the weld zones has been proposed and found to be appropriate because the predicted sizes are in good agreement with the experiments. Furthermore, the authors suggested that a quality FSW joint must have the nugget zone smaller and the heat-affected zone larger than the tool shoulder diameter.

Effect of cooling rates and Fe contents on microstructure evolution of Al-Cu-Mn-Mg-Fe-Si alloys

During the solidification process, the recycled Al-Cu alloys form coarse Fe-rich phases, which seriously deteriorate the mechanical properties of the alloy. The study employed optical microscope (OM), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and synchrotron X-ray imaging to investigate the impact of cooling rate and Fe contents on the growth of Fe-rich phases, Al2Cu, and pores in recycled Al-Cu-Mn-Mg-Fe-Si alloys under synergistic effect ultrasonic melt processing (USMP) and Al-Ti-B. USMP can homogenize the distribution of TiB2 particles and provides more nucleation sites for the formation of Fe-rich phases. Under the synergistic effect of USMP and Al-Ti-B, increasing the cooling rate,the volume fractions of Fe-rich phases in 0.7FeUB (0.7 wt%Fe + USMP + Al-Ti-B) and 1.2FeUB (1.2 wt%Fe + USMP + Al-Ti-B) alloys decreased from 8.92% and 16.52% to 8.03% and 14.73%, respectively. The volume fraction of Al2Cu increased from 4.06% and 3.13% to 4.37% and 3.54%, respectively. Furthermore, the three-dimensional (3D) morphology of Fe-rich phases and Al2Cu became more compact and uniform. The volume fraction of pores in the 1.2FeUB alloy decreased from 0.412% to 0.072%. In-situ synchrotron X-ray radiography revealed that increasing cooling rates led to increased nucleation undercooling of primary α-Al and Fe-rich phases. These phenomena can be attributed to the combined effects of ultrasonic cavitation, ultrasonic streaming, and an increased cooling rate. These factors serve to homogenize the alloy composition, ensure uniform distribution of TiB2 particles, inhibit element diffusion, reduce solute segregation, and ultimately lead to the refinement of the grain size, α-Al, Fe-rich phases, and Al2Cu, while also inhibiting pore growth.

Study on microstructure and refining effect of deformed Al-4.5Er-1Zr-1.5Ti master alloy

The microstructure and grain refinement of Al-4.5Er-1Zr-1.5Ti master alloy were analyzed by the refinement experiment, OM, SEM and XRD. The results show that the grain size of pure aluminum is reduced from 14,000μm to 202μm by Al-4.5Er-1Zr-1.5Ti master alloy, which is mainly due to the nucleation promoted by Ti2Al20Er, Al3Er and Al3Ti. Plastic deformation further improves the refining effect of the material by improving the primary phase size, and the Al-4.5Er-1Zr-1.5Ti −2ARB can refine the pure aluminum from 202 mμm to 150μm, and the refinement was increased by 25.7 %. The master alloy showed a better refinement effect in Al-5Cu alloy than pure aluminum, with a grain size of 92μm and a refinement improvement of 97.8 %.

Effect of Si Addition on the Weibull Distribution of Tensile Properties of Fe-bearing Al-Cu Alloys

Effect of Si Addition on the Weibull Distribution of Tensile Properties of Fe-bearing Al-Cu Alloys

Atomic-scale investigation on the evolution of T1 precipitates in an aged Al-Cu-Li-Mg-Ag alloy

The T1 (Al2CuLi) phase is one of the most effective strengthening nanoscale-precipitate in Al-Cu alloys with Li. However, its formation and evolution still need to be further clarified during aging due to the complex precipitation sequences. Here, a detailed investigation has been carried out on the atomic structural evolution of T1 precipitate in an aged Al-Cu-Li-Mg-Ag alloy using state-of-the-art Cs-corrected high-angle annular dark field (HAADF)- coupled with integrated differential phase contrast (iDPC)-scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDXS) techniques. An intermediate T1’ phase between T1p and T1 phase, with a crystal structure and orientation relationship consistent with T1, but exhibiting different atomic occupancy and chemical composition was found. We observed the atomic structural transformation from T1p to T1’ phase (fcc→hcp), involving only 1/12<112>Al shear component. DFT calculation results validated our proposed structural models and the precipitation sequence. Besides, the distributions of minor solute elements (Ag, Mg, and Zn) in the precipitates exhibited significant differences. These findings may contribute to a further understanding of the nucleation mechanism of T1 precipitate.

Anode-Free Aqueous Aluminum Ion Batteries.

Aqueous aluminum ion batteries (AAIBs) possess the advantages of high safety, cost-effectiveness, eco-friendliness and high theoretical capacity. However, the Al2O3 film on the Al anode surface, a natural physical barrier to the plating of hydrated aluminum ions, is a key factor in the decomposition of the aqueous electrolyte and the severe hydrogen precipitation reaction. To circumvent the obnoxious Al anode, a proof-of-concept of an anode-free AAIB is first proposed, in which Al2TiO5, as a cathode pre-aluminum additive (Al source), can replenish Al loss by over cycling. The Al-Cu alloy layer, formed by plating Al on the Cu foil surface during the charge process, possesses a reversible electrochemical property and is paired with a polyaniline (cathode) to stimulate the battery to exhibit high initial discharge capacity (175 mAh g-1), high power density (≈410Wh L-1) and ultra-long cycle life (4000 cycles) with the capacity retention of ≈60% after 1000 cycles. This work will act as a primer to ignite the enormous prospective researches on the anode-free aqueous Al ion batteries.

Theoretical Insights into Enhancing Catalytic Performance of Al-Cu Alloy for CO2 Electroreduction toward Ethene Production.

The understanding of the reaction mechanism of CO2 electroreduction (CO2RR) is essential for the precise design of catalysts for specific products with high selectivity. In this work, combined with the computational hydrogen electrode model and kinetic energy barrier calculations, CO2RR pathways on Cu(100) and Al1Cu3(100) are intensively investigated. The free energy barrier of the rate-determining step of ethylene formation is reduced from 1.08 eV for *CCOH formation on Cu(100) to 0.51 eV for *CH2OCHOH formation on Al1Cu3(100) and enhances the catalytic activity. The reaction free energy of *CO-*CO coupling is remarkably reduced from 0.86 eV on Cu(100) to -0.43 eV on Al1Cu3(100) and the coupling barrier is reduced from 0.97 to 0.37 eV, suppressing the production of gas phase CO and enhancing the production of C2 products. Furthermore, the selectivity toward C-O breaking of *CH2CHOH on Cu(100) and *CH2CH2OH on Al1Cu3(100) ensures high selectivity toward ethene rather than ethanol.