Al-Cu Alloys Research Articles

Stress concentrator at the plate-like inclusion tip as an enhancement factor of diffusion flux

The work aims to evaluate stress-assisted effects in the diffusion growth of plate-like inclusions. We estimate the correction for diffusion flux onto plate-like inclusion tip accounting for the mechanical stress produced by phase transition. Also, the external stress effect on the stress-concentrator and upper-bound flux is evaluated. The stress factor results in the shift of solubility measured as the mean concentration in solution at precipitation. The impact of the stress concentrator was estimated for different thermodynamic systems: θ′-phase (Al2Cu) growth in Al-Cu alloy, zirconium hydrides growth, and perlitic transformation. The stress factor could play a sufficient role in nonstationary conditions and is negligible in steady-state approximation. Stress factor can also have substantial effects on the inclusion orientation if an external stress is present: rigid inclusion lengthening parallel to the external stress, while stacks of inclusions can behave differently depending on nucleation barriers competition for shear and hydrostatic stress components.

Long-term thermal ageing of the 2219-T851 and the 2050-T84 Al-Cu alloys

In this study, the influence of high temperature exposure on mechanical properties as well as on the microstructures of two Al-Cu alloys, has been investigated. Driven by an industrial will to use structural aluminium at higher temperature than usually stated, this work is focused on the characterization of the evolution of two commercial alloys, the 2219-T851 and the 2050-T84, induced by an ageing treatment of 1000 h at 200 °C. Careful TEM observations confirm the importance of the nanoscale precipitation and that the precipitate size and distribution strongly influence the mechanical properties of the studied alloys. Results unexpectedly show that the θ′−Al2Cu precipitates are more stable in time than the T1−Al2CuLi precipitates at this temperature. Regarding the mechanical properties related to ageing, it follows that the 2219-T851 alloy presents a reduced drop with time of its tensile test curves compared to the 2050-T84, thus a limited, and at least less pronounced, temperature dependence. For an industrial application on structural part, 2219-T851 alloy then appears to be a good candidate for long-term structural applications at intermediate temperatures.

Stepped-shoulder friction stir welding to alleviate weld thinning for dissimilar AA2195-T8/AA2219-T6 alloys

A stepped-shoulder friction stir welding tool containing a protruding inner shoulder and an outer shoulder was proposed to join AA2195-T8 Al-Cu-Li and AA2219-T6 Al-Cu alloys. The inner shoulder with a considerable plunge depth enhanced forging effect, and the outer shoulder with a low plunge depth prevented plasticized materials from overflowing out of welds, while the pin with milling facet promoted material flow. The sound joint with a weld thinning of 0.05 mm (about 0.8% of the workpiece) was obtained. The interface between the two materials was free of cavities and kissing bond. The joint fractured in thermo-mechanically affected zone on the side of AA2219-T6 alloy. The tensile strength reached 352 MPa, equal to 81.7% of base AA2219-T6 alloys.

Evaluating the stability and activity of dilute Cu-based alloys for electrochemical CO2 reduction.

Cu-based catalysts currently offer the most promising route to actively and selectively produce value-added chemicals via electrochemical reduction of CO2 (eCO2R); yet further improvements are required for their wide-scale deployment in carbon mitigation efforts. Here, we systematically investigate a family of dilute Cu-based alloys to explore their viability as active and selective catalysts for eCO2R through a combined theoretical-experimental approach. Using a quantum-classical modeling approach that accounts for dynamic solvation effects, we assess the stability and activity of model single-atom catalysts under eCO2R conditions. Our calculations identify that the presence of eCO2R intermediates, such as CO*, H*, and OH*, may dynamically influence the local catalyst surface composition. Additionally, we identify through binding energy descriptors of the CO*, CHO*, and OCCO* dimer intermediates that certain elements, such as group 13 elements (B, Al, and Ga), enhance the selectivity of C2+ species relative to pure Cu by facilitating CO dimerization. The theoretical work is corroborated by preliminary testing of eCO2R activity and selectivity of candidate dilute Cu-based alloy catalyst films prepared by electron beam evaporation in a zero-gap gas diffusion electrode-based reactor. Of all studied alloys, dilute CuAl was found to be the most active and selective toward C2+ products like ethylene, consistent with the theoretical predictions. We attribute the improved performance of dilute CuAl alloys to more favorable dimerization reaction energetics of bound CO species relative to that on pure Cu. In a broader context, the results presented here demonstrate the power of our simulation framework in terms of rational catalyst design.

Mechanisms of Solid-Solution Hardening of Single-Phase Cu-Al and Cu-Mn Alloys with a Mesh Dislocation Substructure

The dislocation structure and dislocation accumulation during deformation of polycrystalline FCC solid solutions of Cu-Al and Cu-Mn systems are studied by transmission diffraction electron microscopy. The Al content in Cu-Al alloys varies from 0.5 to 14 at.%. The Mn content in Cu-Mn alloys varies in the range of 0.4 ÷ 25 at.%. Alloys with a grain size in the range of 20 ÷ 240 µm are studied. The alloy samples are deformed by stretching at a rate of 2×10-2c-1 to failure at 293 K. The structure of samples deformed to various degrees of deformation is studied on foils using electron microscopes at an accelerating voltage of 125 kV. For each degree of deformation, the scalar dislocation density and its components are measured: statistically stored dislocations ρS and geometrically necessary dislocations ρG and some other parameters of the defective structure.
 The mechanisms and their contributions due to mesh and mesh-mesh dislocation substructures (DSS) are determined using the example of substructural and solid-solution hardening in polycrystalline Cu-Al and Cu-Mn alloys.
 The relative role of various mechanisms in the formation of the resistance to deformation of alloys at different grain sizes is determined. The role of the packaging defect energy on the value of solid-solution hardening for different grain sizes is revealed. The average scalar dislocation density is considered and determined along with its components: statistically stored dislocations ρS and geometrically necessary dislocations ρG. The dependences of the flow stress on the square root of the densities of geometrically necessary dislocations and the densities of statistically stored dislocations are found.

Effect of crystal orientations and precipitates on the corrosion behavior of the Al-Cu alloy using single crystals

Al-Cu single crystals with different typical orientations (i.e., (001), (101), and (111)) were prepared to investigate the corrosion behavior of the Al-Cu alloy with and without precipitates by scanning electron microscopy (SEM), transmission electron microscopy (TEM), immersion testing in EXCO solution, and electrochemical impedance spectroscopy (EIS). The results showed that the corrosion rates of the alloy with different orientations increased in the order of (111)< (001)< (101) in the solid solution state (without precipitates), while those of the alloy increased in the order of (101)< (001)< (111) in the aged state (with precipitates). The corrosion resistance of the alloy without precipitates was mainly determined by the atomic density of the crystal plane, and that of the alloy with precipitates was mainly provided by the corrosion product film. After aging, the precipitation of the θ' phase led to anodic dissolution of the nearby matrix and further accelerated the corrosion rates of the alloy. Furthermore, the corrosion pits of the alloy without precipitates showed geometric shapes corresponding to the crystal orientations. The corrosion of the alloy with precipitates propagated in the form of corroded bands, which were parallel to the {001}Al plane.

Insight on corrosion behavior of friction stir welded AA2219/AA2195 joints in astronautical engineering

Corrosion behavior of friction stir welded Al-Cu alloy and Al-Cu-Li alloy joints were revealed via immersion, intergranular, exfoliation and electrochemical corrosion. Precipitation-driven corrosion was discussed based on experiments and theoretical modeling. The corrosion potential of the entire joint was nobler than base materials, which was mainly attributed to solid solution and precipitation evolution of high equilibrium potential elements. The macro/micro galvanic effect induced by the redistribution of copper was studied with a numerical model to explain the relationship between precipitation evolution and corrosion resistance. Reducing heat input by increasing welding speed was an effective strategy to improve corrosion resistance.

Revisit of the structure of Ω precipitate in Al-Cu-Mg-Ag alloys

Re-examination of the structure of Ω precipitate in Al-Cu-Mg-Ag alloys through first-principles density functional theory (DFT) calculation reveals that the widely-accepted orthorhombic structure of the ‘metastable’ Ω phase is actually tetragonal that is identical to that of the equilibrium θ phase in binary Al-Cu alloys. When the tetragonal θ (i.e. Ω) is embedded in α-Al matrix as thin plates, the elastic distortion imposed by the surrounding α-Al matrix can reduce the original 4-fold symmetry of the tetragonal structure to the 2-fold symmetry of the orthorhombic structure. The DFT calculation results also indicate that the presence of Mg and Ag at the θ/α-Al interface can dramatically facilitate the θ formation. The clarification of the Ω structure and the role of Mg and Ag in θ formation provides insights into how to facilitate precipitation of an intrinsically strong equilibrium phase as a key strengthening constituent in the microstructure.

On the interaction of solute atoms with vacancies in diluted Al-alloys: A paradigmatic experimental and ab-initio study on indium and tin

To study the vacancy-solute-atom interaction, diluted Al-In and Al-Sn alloys with 0.005 and 0.025 at.% indium/tin cast from very high purity elements were investigated by positron annihilation spectroscopy (PAS). Therefore, the alloys have been solution heat treated at temperatures ranging from 320 to 620 °C and then rapidly quenched into ice water freezing-in most thermal vacancies. Positron annihilation lifetime (PALS) and coincidence Doppler broadening spectroscopies (CDBS) were combined to unambiguously identify vacancy-solute-atom complexes. For enabling a direct comparison to experiment, we did employ ab-initio DFT calculations of vacancy-solute-atom complexes providing relaxed atomic coordinates, which are used to calculate PAS annihilation parameters. In the as-quenched state vacancy-solute-atom pairs as well as vacancy clusters were observed in both alloys for all concentrations. During isochronal annealing vacancy clusters, formed during quenching, dissolved at about 130 °C, leaving vacancy-solute-atoms complexes as the only remaining defects. Thus, we could unambiguously identify those by a combination of PALS and CDBS with ab-initio calculations. Employing isothermal annealing the binding energy EB of vacancies to In and Sn solute atoms was determined experimentally by PALS as well as by ab-initio calculations. We find from our experiments EB = (0.20 ± 0.03) and (0.32 ± 0.10) eV for In and Sn, respectively, which is in very good agreement with our ab-initio calculations giving 0.23 and 0.26 eV, respectively. Our results clearly identified vacancy-In/Sn complexes as responsible for the retardation of vacancy migration after quenching. The vacancies bound to In and Sn solute-atoms are released around 150 °C shifting, when added to AlCu alloys as trace elements, the transport of copper atoms to higher temperatures necessary for the formation of finely distributed ″precipitates″, which efficiently strengthen this kind of alloys.

Influence of Mould Thickness on Microstructure, Hardness and Wear of AL-Cu Cast Alloys

Aluminium – copper alloys have a wide range of industrial applications especially in military vehicles, rocket fins and aerospace. Solidification plays a vital role in controlling, the mechanical and tribological properties, and influencing the microstructure of metallic alloys in general and aluminium alloys in particular. Therefore, the researchers have made many efforts to figure out the solidification behaviour of Al-Cu alloys. Despite all these endeavors, however, the behavior is not yet fully understood. This research aims to investigate the effect of cooling rate on the microstructure, mechanical and tribological properties of aluminium-copper cast alloys (Al-Cu alloys) under dry sliding conditions. Four cooling rates were achieved by using four various steel moulds made of different thicknesses and one of them was surrounded with green sand, to get a lower cooling rate, with the same respective mould hole geometries. The microstructure results showed that the grain size increases with decreasing the cooling rate. While the hardness increased largely due to the refinement of the microstructure. Finally, it was concluded that the wear rate increases with decreasing the cooling rate, and this due to the reduction in hardness.

Effect of Mg on the Oxide Growth of Al–Cu Alloys at Elevated Temperature

In this study, we investigated the effect of Mg on oxide growth of Al-Cu alloys at elevated temperatures. Results show that Al2Cu as eutectic phase was distributed in the as-cast microstructure of Mg-free Al-5 mass%Cu alloy, whereas Al2CuMg_S phase was formed by eutectic reactions with the addition of Mg. Scanning electron microscopy and thermodynamic calculations revealed that after oxidation at 500 °C for 30 h, Al-5 mass%Cu alloy had only slightly oxidized surfaces, whereas significantly grown oxide clusters formed as the Mg content increased. MgO and spinel were the main oxides in the Mg-containing alloys, whereas no Cu-based oxide was involved in the oxidation of the alloys. Only Al2CuMg_S phase contributed to the oxidation of the base metal and the formation of Mg-based oxides. The Al-5 mass%Cu alloy showed no inter-products with Mg-based oxides, including MgO and spinel.

Effect of powder processing and alloying additions (Al/ZrB2) on the microstructure, mechanical and electrical properties of Cu

The present work elucidates the effect of powder processing conditions (milling/mixing) and conductive alloying element (Al: aluminium) and ceramic (ZrB2: zirconium diboride) reinforcement addition on the densification, microstructure and electrical conductivity of copper (Cu) processed via hot pressing route. Disregard of alloying element/reinforcement/content or powders preparation method, the density of Cu materials varied between 92.16 and 99.76% ρth (theoretical density) after hot pressing at a low temperature of 500 °C. In case of Cu-Al alloys, the powder processing method significantly influenced its microstructure and conductivity. Particularly the Cu-Al alloys processed using mixed powders consisted of various phases Cu, α-Cu, γ1 (Cu9Al4), δ (Cu3Al2), ζ1 (Cu4Al3), η2 (CuAl) and θ (CuAl2) and the Cu alloys prepared using milled powders composed of either only α-Cu or α-Cu and γ1 (Cu9Al4) phases (depending on the Al content). Whereas, only Cu and ZrB2 phases were observed with the Cu-ZrB2 composites processed using either milled or mixed powers. In case of Cu-Al alloys, the hardness (0.88–3.41 GPa) and strength (540.30–1120.18 MPa) of Cu increased with the addition of Al. Interestingly, the hardness (0.88–2.55 GPa) and strength (508.50–970.60 MPa) of Cu increased upto 5 wt% ZrB2 and then they lowered with further addition of ZrB2. In particular, the hardness and strength of Cu-ZrB2 composites are lower than Cu-Al alloys reflecting the effectiveness of solid solution strengthening in the Cu alloys as compared to dispersion strengthening mechanism in Cu composite. The pure Cu prepared using milled powders exhibited low conductivity (75.70% IACS) than Cu processed using as-received/un-milled powders (97.00% IACS). Also, the Cu-ZrB2 composites measured with better electrical conductivity than Cu-Al alloys. Depending on the milling conditions and alloying/reinforcement amount, the conductivity of Cu-ZrB2 composites varied between 44.10 and 88.70% IACS.

Valorization of Aluminum Dross with Copper via High Temperature Melting to Produce Al-Cu Alloys.

The valorization of aluminum dross for Al recovery was performed via its mixing with metallic copper to produce Al-Cu alloys. This approach was with the intention of establishing a new smelting process to treat the dross with Cu scrap use. To evaluate the high temperature interaction of the materials, the wettability of a Cu-containing aluminum alloy with the non-metallic components of the dross was studied by the sessile drop method. It was found that the wetting was weak via temperature changes at 973–1373 K, and consequently no proper metal separation occurred. To better separate the metallic and non-metallic phases with larger density differences, a higher Cu portion was considered to obtain a significantly denser metallic phase, and it was found that partial separation of the Al in an Al-Cu alloy is possible. The complete separation of the metallic components of the dross was, however, experienced by the dross and copper melting with the addition of pre-melted calcium aluminate slags at elevated temperatures. It was found that Al-Cu alloys were produced and separated from the adjacent slags, and the aluminum oxide of the dross ended up in the slag phase. Moreover, the characteristics of the produced slags depend on the process charge.

Effects of segmented thermal-vibration stress relief process on residual stresses, mechanical properties and microstructures of large 2219 Al alloy rings

Large 2219 Al-Cu alloy transition rings are extensively employed in propellant tanks of heavy launch vehicles. These rings have a diameter exceeding 5 m or even reaching 10 m, with a less than 2% thickness-to-radius ratio, and low stiffness which can cause machining deformation due to the residual stress. Thus, residual stress relief of these rings is necessary in their extreme manufacture. A novel effective method is thermal-vibration stress relief (TVSR), which integrates the conventional thermal stress relief (TSR) and vibratory stress relief (VSR); however, the existing TVSR equipment cannot meet the requirements of large rings, and the underlying mechanisms of TVSR remain unclear and a quantitative interpretation is still lacking. Therefore, a segmented TVSR (STVSR) process suitable for large rings was proposed and the corresponding experiment was carried out with a self-made STVSR experiment platform. Then this study investigated the evolution and distribution laws of the residual stresses, tensile properties, Vickers hardness, dislocations, precipitated phases and metallography during STVSR. Based on the experimental results, multi-scale mechanics theory and strengthening mechanisms were applied to quantitatively reveal the underlying mechanisms of residual stress relief by STVSR. The results showed that the circumferential and axial residual stress relief rates can reach 44.43% and 45.14% after STVSR, respectively. The residual stress relief after STVSR is attributed to the dynamic evolution of dislocations and precipitated phases in the material. The improvement of mechanical properties mainly depends on the precipitated phases and is also affected by the residual stress. The findings confirm the significant effects of STVSR on metal plasticity and provide valuable insight into the underlying mechanisms of TVSR.

Contribution of inter- and intraband transitions into electron\u2013phonon coupling in metals

We recently developed an approach for calculation of the electron–phonon (electron–ion in a more general case) coupling in materials based on tight-binding molecular dynamics simulations. In the present work, we utilize this approach to study partial contributions of inter- and intraband electron scattering events into total electron–phonon coupling in Al, Au, and Cu elemental metals and in AlCu alloy. We demonstrate that the interband scattering plays an important role in the electron–ion energy exchange process in Al and AlCu, whereas intraband d–d transitions are dominant in Au and Cu. Moreover, inter- and intraband transitions exhibit qualitatively different dependencies on the electron temperature. Our findings should be taken into account for the interpretation of experimental results on the electron–phonon coupling parameter.Graphic abstract

Influence of Silicon and Magnesium on the Mechanical Properties of Additive Manufactured Cu-Al Alloy.

By using cold metal transfer technique, Cu-Al alloy with addition of silicon (Si) and magnesium (Mg), viz (1) Cu-6.5% Al and (2) Cu-6.5% Al-1.2% Si-0.5% Mg were manufactured additively. Four samples were deposited: Cu-6.5% Al alloy as samples 1 and 2, and Cu-6.5% Al-1.2% Si-0.5% Mg alloy as samples 3 and 4. The alloys were homogenized by heat treatments: (1) 800°C (2 h) for sample 2 and (2) sample 4, to improve their mechanical properties. Detailed microstructural investigation conducted using scanning and transmission electron microscopies showed formation of various intermetallic phases. Results revealed that (1) the addition of Si and Mg increases the strength properties and ductility and (2) heat treatments improved strength properties but reduce the ductility of the alloys. The article discusses the correlation of the identified microstructure and the evaluated mechanical properties of the additively manufactured alloys.

A new mechanism of surface phase formation on precipitation-hardening alloy under ion irradiation

Understanding surface modification of materials by Ga-ion milling has attracted growing research interests. This research reports that ion beam milling can induce rapid formation of θ-phase on the surface of a precipitation-hardening Al-Cu alloy. Atom probe tomography analysis reveals that an Al-Cu alloy tip develops a surface Cu-rich layer after electro-polishing. Cu-rich solute clusters in the 1-week naturally aged alloy sample are effective to assist the nucleation for rapid formation of surface θ-phase under the ion milling. The growth of the surface θ-phase is via a combination reaction of the surface Cu-rich layer with the Al matrix, rather than a simple decomposition of the Al-Cu alloy under the Ga ion irradiation. Partition and segregation behaviours of Ga with respect to the surface θ-phase are addressed by this investigation.

The first report on formation of Al-W-Cu grain boundary phase and its influence on mechanical behavior of 2D-WS2 reinforced Al-4Cu alloy matrix composites

Nanostructured two-dimensional (2D) WS2 reinforced Al-4Cu alloy matrix composites were prepared via spark plasma sintering (SPS) at 550 and 570 °C. The microstructure and interface characteristics of the composites were investigated. In situ formation of Al-W-Cu ternary phase at the grain boundaries of the Al-rich matrix phase was observed on sintering. High-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) with quantitative mapping confirmed that the structure of ternary Al-W-Cu (Al12W0.6Cu0.4) belongs to bcc phase (space group: Im3̅ (204), similar to that of binary bcc-Al12W phase. Such bcc intermetallic phase was absent in pure Al-matrix under identical sintering conditions. The ternary intermetallic phase along with the retained 2D-WS2 contributed to about 41% increase in the hardness of the alloy composite. Nanoindentation hardness and Young’s modulus of the Al-W-Cu ternary intermetallic were found to be around 4.6 GPa and 104 GPa, respectively. The formation mechanism of Al-W-Cu intermetallic phase from 2D-WS2 nanosheets and Al-4Cu matrix was elucidated with Calphad (Calculation of Phase Diagrams)-based Gibbs energy analysis.

Effects of stacking fault energy and reduction ratio on mechanical properties geometry size effect of cold-rolled pure Cu and CuAl alloy

With the trend of increasing product miniaturisation, it has become necessary to consider the service performances of microparts. If the material exhibits the ‘smaller is stronger’ geometry size effect (GSE), the mechanical properties data of conventional dimension specimens can be used to design the microparts, obtaining improved service safety of the microparts. The surface layer model shows that when the exterior mechanical properties of material are higher than these of the interior, the ‘smaller is stronger’ GSE can be obtained. Stacking fault energy (SFE) and reduction ratio (RR) have been reported to affect the distribution of mechanical properties along the thickness direction of a rolled material, so that they are expected to affect the GSE of rolled sheets. Therefore, the tensile mechanical properties of different SFE Cu-Al alloys cold rolled at different RR were examined in this work. The average grain size of the surface layer of the rolled specimens was smaller than that of the interior, whereas the average strain and microhardness of the surface layer were larger than those of the interior. The difference in the strain between the exterior and interior of the specimen was found to decrease with the specimen thickness t, and increases with RR and SFE. The ‘smaller is stronger’ GSE trend was found in pure Cu and CuAl alloy rolled at 30 –50% RR, while GSE changed to ‘smaller is weaker’ at RR = 70%. The magnitude of the GSE of the rolled specimens increased with increasing RR and SFE, resulting from the effects of RR and SFE on the microstructure, microhardness and strain distribution. New models for the relationship between the strengths and t and SFE were established for three RRs. The results predicted by the models were in good agreement with the experimental values.

The nature of the maximum microhardness and thickness of the gradient layer in surface-strengthened Cu-Al alloys

The presence of a gradient layer with an increased maximum microhardness and extended thickness is extremely attractive because it can enhance the service performances for surface-strengthened metallic materials. In this research, Cu-Al alloys with different Al contents and microstructures were processed by surface spinning strengthening (3S) and studies were conducted to examine the gradient microstructure and microhardness distributions of the 3S Cu-Al alloys. The experimental results show that each group of the 3S Cu-Al alloys having the same Al content has approximately the same maximum microhardness at the topmost surface layer and the maximum microhardness increases with an increase in the Al content. In addition, the thickness of the gradient layer in the 3S Cu-Al alloys increases with a decrease of yield strength and an increase in the work-hardening exponent, respectively. The relationship between the maximum microhardness and chemical composition which determines the Young's modulus and plastic deformation mode, and the relationship between the thickness of the gradient layer and the microstructure which governs the strength and work-hardening capacity, were both investigated. It is found that the maximum microhardness of the gradient layer depends mainly on the chemical composition; whereas the thickness of the gradient layer depends primarily on both the strength and work-hardening capacity which are closely related to the microstructure. By combining the compositional design and microstructure optimization of the materials, and improving the surface strengthening intensity, a gradient layer of the surface-strengthened materials with an enhanced maximum microhardness and an extended thickness may be achieved.