The heat treatment of Al–Si–Cu–Mg casting alloys

The mechanical properties of aluminium–silicon casting alloys containing Cu and Mg are known to be improved by heat treatment. Over 60 papers are reviewed here in order to clarify the sequences of microstructural changes which occur during heat treatment, and their influence on the mechanical properties. It is shown that the changes occurring during solution treatment are relatively well understood, and that the equilibrium phase diagram can be used to predict the stability of phases at the solution treatment temperature. The influence of quench rate and natural ageing on subsequent artificial ageing needs to be studied further, but some conclusions can be drawn. These include: (1) An increase in quench rate above 4 °C/s gives a small increase in yield strength after ageing, while the concomitant influence on elongation is more complicated and depends on the alloy. (2) Natural ageing is shown to have a large influence on subsequent artificial ageing response of Al–Si–Mg alloys, while there is a significant lack of knowledge for Cu-containing alloys. Artificial ageing of Al–Si–Mg alloys in the temperature range 170–210 °C gives peak yield strengths of the same level, while Cu-containing alloys show a decrease in peak yield strength with increasing ageing temperature. The precipitation sequences in Al–Si–Mg and Al–Si–Cu alloys are relatively well known. In Al–Si–Cu–Mg alloys several precipitation sequences are possible, which need further investigation.     

Microstructure and material properties of an Al–Cu alloy provided by the Ohno continuous casting technique

The specific objective in this work is an exploration of the material properties produced by a recently developed continuous casting technique. The study focuses on an Al–Cu alloy provided by the Ohno continuous casting process. The experimental approaches give rise to unusual microstructural characteristic in the cast samples, namely a fine lamellar eutectic structure with unidirectional growth along its axial direction together with a regularly oriented lattice structure. Such microstructural characteristics significantly increase the tensile and fatigue properties.     

Effects of homogenisation conditions on semisolid microstructures of Al–Mg–Si–Mn alloys produced by D-SSF process

Abstract

The effects of different heating rates to a homogenisation temperature on the semisolid microstructure of Al–Mg–Si–Mn alloys are investigated. It is found that the size, morphology and distribution of the α-Al₁₂Mn₃Si₂ intermetallic compound (Mn containing dispersoid) depend on the heating rate in the homogenisation process. Fine spherical and homogeneously distributed Mn containing dispersoid particles are found in the slow heated samples (0˙7°C min^?1), while inhomogeneously distributed coarser particles with a rod-like shape are found in the rapid heated samples (110°C min^?1). The homogenised sample is deformed by 60% cold rolling. It is found that the recrystallised and semisolid grain sizes of the rapid heated sample are smaller than those of the slow heated sample in all conditions. Compared with the M4 alloy (0˙4 mass-%Mn), the M7 alloy (0˙72 mass-%Mn) has much finer semisolid grain size and smaller values of the shape factor close to 1. The Mn containing dispersoid greatly affects the semisolid grain size of the alloys. The results in this work show that the rapid heating in the homogenisation process is useful to produce high quality semisolid products of the Al–Mg–Si–Mn alloys.     

Mechanical properties of intermetallic phases in multi-component Al–Si alloys using nanoindentation

The presence of additional elements in a multi-component Al–Si alloy system allows many complex intermetallic phases to form. The mechanical properties of different intermetallic phases have been investigated using nanoindentation. In particular, the hardness and modulus of a number of phases have been established for a range of alloy compositions. The results show that both hardness and reduced modulus increase as the Ni ratio of the Al–Cu–Ni phases increases. The elastic modulus can be correlated with the formation temperature of the intermetallic phases. The intermetallic phases with a high heat of formation have a strong binding between atoms and therefore, their elastic modulus is also higher.     

Change in microstructure of Al–Si–Cu casting alloys during high temperature solution treatment

Abstract

The solution treatment in Al–Si system casting alloys is usually performed to obtain supersaturated solid solution and spheroidising Si particles. It can be inferred that a high temperature solution treatment enhances mechanical properties without any special apparatuses or techniques. However, it is well known that the solution treatment close to an eutectic temperature causes local melting. In this study, the change in microstructure of Al–Si–Cu casting alloys, which have been solution treated at temperatures ranging from 773 to 824 K, have been investigated from a viewpoint of Cu concentration and the distributions of micropores and locally melt regions due to eutectic reaction. Tensile and hardness tests were carried out to discuss the relationship between mechanical properties and microstructures. In addition to a surface observation, an internal microstructural observation was carried out using the high resolution X-ray computed tomography. The burnt regions during the high temperature solution treatment were identified to be Cu rich. Porosity increased with increasing the solution treatment temperature. The porosity in the sample solution treated above a binary eutectic temperature was confirmed to be >0˙2 vol.-%. The Cu concentration in the α-phase increased below the binary eutectic temperature.     

Fatigue properties of Al–Si casting alloy with cold sprayed Al/SiC coating

Abstract

The fatigue properties of Al–Si alloy cold sprayed Al and Al–SiC composite coatings have been studied. The specimens coated with composites reinforced with a large volume (25%) of fine SiC particles exhibited improved adhesion strength at the interface due to crater formation, and cyclic fatigue lives at room temperature more than three times those of uncoated specimens. In high temperature low cycle fatigue tests at 250°C, the pure Al coatings showed longer fatigue lives than the Al–SiC composite coatings, which is attributed to an increment in ductility at the surface retarding fatigue crack initiation.     

Rheologic behaviors of A356 aluminum alloy billet produced by semisolid continuous casting process

The experiments for rheologic behaviors of semisolid continuous casting billets of A356 alloy in semisolid state had been carried out with a multifunctional rheometer. The results show that the deformation rate increases with loading time, the maximum strain reaches to 120％ (which is one time larger than that of traditional mold casting billet) and the strain can be rapidly eliminated to 10％ after unloading. Moreover, there is a critic stress for billet deformation even in semisolid state, which is named as critic shear stress. This stress increases with the decreasing of heating time. The theologic behaviors can be expressed by five elements mechanical model (H_2- [N_1|H_2]-[N_2|S]) and can be modified with the increasing of heating time.     

Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg₅Si₆, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.     

Microstructure and high temperature tensile properties of Al–Si–Cu–Mn–Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast, squeeze cast, and semi-solid thixoformed Al–Si–Cu–Mn–Fe alloys after thermal exposure at 300 °C were discussed. The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys, especially after thermal exposure for 100 h. The ultimate tensile strength (UTS) of semi-solid thixoformed alloys after thermal exposure at 300 °C for 0.5, 10 and 100 h were 181, 122 and 110 MPa, respectively. The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications. The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.     

Determining hot cracking index of Al–Si–Cu–Mg casting alloys calculated using effective solidification range

The possibility of determining the hot cracking index using the calculated value of the effective solidification range is investigated for multicomponent cast aluminium alloys based on the system Al–Si–Cu–Mg with Mn, Ni, Fe and Zn additives. The upper limit of the effective solidification range was calculated as the temperature of formation of 65 wt-% solid phase using Sheil model. The linear relationship of the hot cracking index and the effective solidification range in the industrial and experimental multicomponent alloys based on the Al–Si–Cu–Mg system is demonstrated.     

Fabrication and new electrochemical properties of nanoporous Cu by dealloying amorphous Cu–Hf–Al alloys

To produce monolithic nanoporous Cu (NPC) ribbons with good mechanical integrity, we investigate chemical dealloying behavior of an amorphous Cu_52.5Hf₄₀Al_7.5 alloy with good mechanical properties by immersion in 0.5 M HF solution for different times. With the design concept of adding ductile amorphous supporter to NPC ribbons, an amorphous alloy-containing interlayer with a certain thickness is successfully fabricated in the NPC ribbon by controlling dealloying time to ensure the good bendability of the dealloyed product. New electrochemical properties of the obtained NPC ribbon are further developed. The current signal and the specific capacitance of the NPC-supported MnO₂ composite electrode are remarkably enhanced as compared to those of pure MnO₂ powders. NPC substrate with large specific surface areas and excellent electrical conductivity can effectively promote the morphological change of MnO₂ from globular particles to nanoflakes for larger specific surface area and improve the utilization of MnO₂ surface active sites, which reveal new electrochemical properties of NPC in a potential application for supercapacitor composite electrodes.     

Structure and properties of cast and splat-quenched high-entropy Al–Cu–Fe–Ni–Si alloys

The effect of the composition and cooling rate of the melt on the microhardness, phase composition, and fine-structure parameters of as-cast and splat-quenched (SQ) high-entropy (HE) Al–Cu–Fe–Ni–Si alloys was studied. The quenching was performed by conventional splat-cooling technique. The cooling rate was estimated to be ~10⁶ K/s. Components of the studied HE alloys were selected taking into account both criteria for designing and estimating their phase composition, which are available in the literature and based on the calculations of the entropy and enthalpy of mixing, and the difference between atomic radii of components as well. According to X-ray diffraction data, the majority of studied Al–Cu–Fe–Ni–Si compositions are two-phase HE alloys, the structure of which consists of disordered solid solutions with bcc and fcc structures. At the same time, the Al_0.5CuFeNi alloy is single-phase in terms of X-ray diffraction and has an fcc structure. The studied alloys in the as-cast state have a dendritic structure, whereas, after splat quenching, the uniform small-grained structure is formed. It was found that, as the volume fraction of bcc solid solution in the studied HE alloys increases, the microhardness increases; the as-cast HE Al–Cu–Fe–Ni–Si alloys are characterized by higher microhardness compared to that of splat-quenched alloys. This is likely due to the more equilibrium multiphase state of as-cast alloys.     

Investigation of the microstructure and properties of Al–Si–Mg/SiC composite materials produced by solidification under pressure

Composite materials based on alloys of the Al–Si–Mg system have been obtained via the introduction of 5, 10, and 15 wt % of SiC particles into the alloy melt and the solidification under a pressure. As a result of solidification under pressure, the porosity of the composite materials decreased substantially. An increase in the content of SiC particles in the composites enabled a smaller size of dendritic cells to be obtained. It has been shown by the X-ray diffraction method that, in the process of solidification under pressure, an interaction occurred between the matrix and reinforcing SiC particles. The presence of SiC particles in the structure of composites led to the acceleration of the aging process and to an increase in the peak hardness in comparison with the matrix alloy.     

Refinement of primary Si in Cu–50Si alloys with novel Al–Zr–P master alloy

In this article, a novel Al-6Zr-2P master alloy with ZrP particles was successfully synthesized, and the refining performance of this novel master alloy for the primary Si in Cu-50Si alloys was also investigated. By means of the fracture plane observation, it is found that the ZrP phase would precipitate first in the solidification process, and then, the ZrAl 3 phase grows around them. Furthermore, it is observed that the refining effect can be remarkably improved by changing the addition sequence of the raw materials. After the melting of commercial Cu, the 2.0 wt% Al-6Zr-2P master alloy and crystalline Si were added in sequence, and the mean size of the primary Si in Cu-50Si alloy can be significantly refined from 255.7 to 75.3 lm. Meanwhile, the refining mechanism was discussed in detail.     

Effect of thermal exposure on microstructure and mechanical properties of Al−Si−Cu−Ni−Mg alloy produced by different casting technologies

The effect of thermal exposure at 350 °C for 200 h on microstructure and mechanical properties was investigated for Al−Si−Cu−Ni−Mg alloy, which was produced by permanent mold casting (PMC) and high pressure die casting (HPDC). The SEM and IPP software were used to characterize the morphology of Si phase in the studied alloys. The results show that the thermal exposure provokes spheroidization and coarsening of eutectic Si particles. The ultimate tensile strength of the HPDC alloy after thermal exposure is higher than that of the PMC alloy at room temperature. However, the TEPMC and TEHPDC alloys have similar tensile strength around 67 MPa at 350 °C. Due to the coarsening of eutectic Si, the TEPMC alloy exhibits better creep resistance than the TEHPDC alloy under studied creep conditions. Therefore, the alloys with small size of eutectic Si are not suitably used at 350 °C.     

The quasicrystalline phase formation in Al–Cu–Cr alloys produced by mechanical alloying

Almost single-phase decagonal quasicrystal with periodicity of 1.26 nm along 10-fold axis was produced in Al₆₉Cu₂₁Cr₁₀ and Al_72.5Cu_16.5Cr₁₁ alloys using combination of mechanical alloying (MA) and subsequent annealing. Phase transformations of as-milled powders depending on annealing temperature in the range of 200–800 °C are examined. Since the transformations can be explained based on kinetic and thermodynamic reasons it seems that applied technique (short preliminary MA followed by the annealing) permits to produce the equilibrium phases rather than metastable ones.     

Effect of microalloying on the structure and properties of alloys in the Al−Si−Cu system

Conclusions  

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Controlled semisolid microstructures and mechanical properties of Al–Mg–Si–Mn wrought alloy produced by D-SSF

Abstract

The semisolid microstructures and the mechanical properties of Al–1˙35Mg–1˙04Si–0˙67Mn alloy produced by deformation semisolid forming (D-SSF) process were studied. Fine α-Al₁₅Mn₃Si₂ compounds precipitate homogeneously during the homogenisation treatment. These compounds effectively inhibit the coarsening of recrystallised grains during heating to the semisolid temperature. When the liquid fraction is controlled to be ～23%, the complete die filling is not achieved. Therefore, in order to achieve good fluidity, it is necessary to control the liquid fraction to be more than 30%. The average grain size and the liquid fraction at the semisolid temperature influence directly mechanical properties. Therefore, the relationship among the average grain size, the liquid fraction at the semisolid temperature and mechanical properties was evaluated. Furthermore, the optimum semisolid microstructure was determined and the condition for the D-SSF process was established.