Effect of ultrasonic vibration on Fe-containing intermetallic compounds of hypereutectic Al–Si alloys with high Fe content

The Fe-containing intermetallic compounds with high melting point in hypereutectic Al-Si alloys can improve the heat resistance and wear resistance at elevated temperatures. However, the long needle-like Fe-containing compounds in the alloys produced by conventional casting process are detrimental to the strength of matrix. The effect of ultrasonic vibration (USV) on the morphology change of Fe-containing intermetallic compounds in the hypereutectic Al-17Si-xFe (x=2, 3, 4, 5) alloys was systematically studied. The results show that, the Fe-containing intermetallic compounds are mainly composed of long needle-like β-Al₅FeSi phase with a small amount of plate-like δ-Al₄FeSi₂ phase in Al-17Si-2Fe alloy produced by conventional casting process. With the increase of Fe content from 2% to 5% in the alloys, the amount of plate-like or coarse needle-like δ-Al₄FeSi₂ phase increases while the amount of long needle-like β-Al₅FeSi phases decreases. In Al-17Si-5Fe alloy, the Fe-containing intermetallic compounds exist mainly as coarse needle-like δ-Al₄FeSi₂ phase. After USV treatment, the Fe-containing compounds in the Al-17Si-xFe alloys are refined and exist mainly as δ-Al₄FeSi₂ particles, with average grain size ranging from 26 μm to 37 μm, and only a small amount of β-Al₅FeSi phases remain. The mechanism of USV on the morphology of Fe-containing intermetallic compounds was also discussed.     

Microcladding of hypereutectic Al–Si alloys: technological aspects and structure features

Abstract

This study aims to investigate the technological possibility to generate high precision microrange structure from aluminium alloys using laser microcladding. Clad tracks of Al–Si alloys with different silicon contents with lateral resolution of ～200 μm were produced using co-axial laser microcladding. Particle size distribution with surface morphology and microstructure of the initial powders were investigated. Mechanisms of structure formation in Al–Si alloys during laser microcladding were studied. It was revealed that the chemical composition of the powder material influences the structure formation process and microhardness of hypereutectic Al–Si alloys produced by microcladding. In addition, the effect of processing parameters on the microstructure and geometry of the resulted tracks is discovered. The conclusions on technological possibility of manufacturing the single tracks with width of <500 μm from hypereutectic Al–Si alloys were made.     

Role of scandium additions in primary silicon refinement of hypereutectic Al–20Si alloys

This study investigates the effect of Sc on the formation of primary silicon during the solidification of hypereutectic Al–20Si alloys. The evolution of the microstructure was studied using thermal analysis. The results show that the addition of 0·2 and 0·4 wt-% Sc suppresses the nucleation of primary silicon due to the formation of ScP particles instead of AlP particles. For large Sc additions, at the centre of the samples, the primary silicon has a star-shaped morphology with thin branches. The addition of Sc decreases the P refinement ef?ciency in hypereutectic Al–20Si alloys, which may be a result of the formation of a ScP phase. The results suggest that the addition of Sc poisons the nucleant particles of the primary and eutectic silicon.     

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.     

Microstructure and hot cracking susceptibility of high conductivity Al–Fe–Si alloys

Aluminium–silicon based casting alloys have been extensively utilised in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable for high conductivity parts. In this research, Al–Fe–Si based high conductivity alloys containing limited silicon content were investigated. Al–0·5Fe–xSi alloys with silicon ranging from 0·5 to 2% showed significantly higher electrical conductivity than conventional Al–Si based alloys. The hot cracking susceptibility of Al–Fe–Si alloys became seriously high as the Si content increased up to 1·5%, then susceptibility rapidly reduced with the further increase in Si. The relationship between solidification characteristics and hot cracking susceptibility of Al–0·5Fe–xSi alloys was discussed based on the thermal and cooling curve analyses and microstructural observations.     

Dynamic stress–strain properties of Ti–Al–V titanium alloys with various element contents

A series of Ti–Al–V titanium alloy bars with nominal composition Ti–7Al–5V ELI,Ti–5Al–3V ELI,commercial Ti–6Al–4V ELI and commercial Ti–6Al–4V were prepared.These alloys were then heat treated to obtain bimodal or equiaxed microstructures with various contents of primary a phase.Dynamic compression properties of the alloys above were studied by split Hopkinson pressure bar system at strain rates from 2,000 to 4,000 s-1.The results show that Ti–6Al–4V alloy with equiaxed primary a(ap)volume fraction of 45 vol%or 67 vol%exhibits good dynamic properties with high dynamic strength and absorbed energy,as well as an acceptable dynamic plasticity.However,all the Ti53ELI specimens and Ti64ELI specimens with ap of 65 vol%were not fractured at a strain rate of4,000 s-1.It appears that the undamaged specimens still have load-bearing capability.Dynamic strength of Ti–Al–V alloy can be improved as the contents of elements Al,V,Fe,and O increase,while dynamic strain is not sensitive to the composition in the appropriate range.The effects of primary alpha volume fraction on the dynamic properties are dependent on the compositions of Ti–Al–V alloys.     

Thermodynamic evaluation of hypereutectic Al–Si (A390) alloy with addition of Mg

This paper presents the thermodynamic evaluation of A390 hypereutectic Al–Si alloy (Al–17% Si–4.5% Cu–0.5% Mg) and alloys up to 10% Mg, using the Factsage® software. Two critical compositions were detected at 4.2% and 7.2% Mg where the temperatures of the liquidus, the start of the binary and of the ternary eutectic reaction are changed. These critical compositions show differences in the formation of Mg₂Si intermetallic particles during the solidification interval. For compositions up to 4.2% Mg, the Mg₂Si intermetallic phase first appears in the ternary eutectic zone. With Mg contents between 4.2% and 7.2%, Mg₂Si particle appears in both the binary and ternary eutectic reactions. Above 7.2% Mg, it solidifies as a primary phase and also during the binary and ternary reactions. The calculated liquid fraction vs. temperature curves also showed a decrease of the eutectic formation temperature (knee point temperature) with the addition of Mg content up to 4.2% Mg. This temperature becomes almost constant up to 10% Mg. The calculation of eutectic formation temperature shows a good agreement with differential scanning calorimetry (DSC) tests.     

Solidification characteristics of high Nb-containing γ-TiAl-based alloys with different aluminum contents

The effect of Al content on the microstructure and solidification characteristics of Ti–Al–Nb–V–Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction(XRD), scanning electron microscopy(SEM) equipped with energy dispersive spectroscope(EDS), and transmission electron microscopy(TEM). The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47Al7Nb2.5V1.0Cr(at%) alloy solidifying through the peritectic reaction. The b-solidifying Ti46Al7Nb2.5V1.0Cr(at%) alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48Al7Nb2.5V1.0Cr(at%) alloy is equivalent to that of Ti46Al7Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.     

Stirred casting Al–Pb monotectic alloys with high damping capacity

In this study, the stirred casting with various processing parameters, such as stirring temperature and stirring speeds, was carried out on the Al–Pb monotectic alloys in order to make Pb particles distribute much more uniformly. More importantly, their damping capacities were systematically studied. The results show tha mechanical stirring can not only make Pb in the aluminum matrix uniformly distribute but also dynamically influence the damping capacity of this alloy system. The Al–Pb alloy was prepared under a slow speed at solid–liquid temperature region, wherein high volume fraction of Pb in alloy could be obtained. The high volume fraction of Pb gives high overall damping capacity. The dislocation damping and interface damping theories are mainly dominated to the alloys.     

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.     

Magnetic and mechanical properties of FeSi alloys with high Si content

The chemical vapor（CVD） deposition-diffusion method was applied to prepare FeSi alloys with high silicon content up to 6.5%. In spite of various deposition and post-annealing, the sample remains α-Fe bcc structure. The cross section of the composition was analyzed to evaluate the Si content and distribution before and after annealing. The results show that the soft magnetic properties are improved by increasing the silicon content. For the samples containing about 6.5% Si, the coercivity decreases to 60 from 237.3 A/m of the original. It is also obtained that, in addition to the Si content, Si distribution has a large influence on the core loss due to the effect of resistivity. The micro-hardnesses were also evaluated along the cross-section after various annealings.     

Measurements of electrical and thermal properties with growth rate,alloying elements and temperature in the Al–Si–X alloys

In this work, effect of alloying elements (X = Cu, Co, Ni, Sb and Bi) and growth rates on the microstructure, physical properties (electrical resistivity, enthalpy and specific heat) of the directionally solidified Al–Si eutectic alloy have been investigated. Al–12.6Si–2X (wt. %) samples were prepared using metals of 99.99% high purity in the vacuum atmosphere. These alloys were directionally solidified under constant temperature gradient, G (7.80 K/mm) and different growth rates, V (8.3–166.0 μm/s). Flake spacing (λ) and electrical resistivity (ρ) were measured from the solidified samples. The variation of electrical resistivity with temperature in the range of 300–500 K for alloying elements in the Al–Si eutectic cast alloy was also measured. The enthalpy of fusion (ΔH) and specific heat (C_p) for the same alloy were determined by a differential scanning calorimeter from the heating curve during the transformation from solid to liquid.     

Modelling of the age hardening behaviour of Al–Mg–Si alloys

In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.     

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.     

A comparative study on corrosion of Mg–Al–Si alloys

Corrosion behavior of various Mg–Al–Si alloys (AS11, AS21, AS41, AS61 and AS91 series), cast under the same cooling conditions and controlled alloying composition, was investigated systematically. Optical microscopy and scanning electron microscopy were used for microstructural examinations. The corrosion behavior was evaluated by immersion tests and potentiodynamic polarization measurements in 3.5% NaCl solution. The results from both immersion tests and the potentiodynamic polarization measurements showed that marginal improvement in corrosion resistance was observed with 2.0% Al (mass fraction) containing alloy (AS21) whereas Al addition above 2.0% (AS41, AS61 and AS91) deteriorated the corrosion resistance which was attributed to β phase, acting as cathode, and the interruption of continuity of the oxide film on the surface of the alloys owing to coarsened β and Mg₂Si phases.     

Effects of boron on eutectic modification of hypoeutectic Al–Si alloys

The effects of boron on the eutectic modification and solidification mode of hypoeutectic Al–Si alloys have been studied adding different boride phases. The results show that boron does not cause modification of the eutectic silicon. Boron-containing samples display eutectic nucleation and growth characteristics similar to that of unmodified alloys.     

Phases and microstructures of high Zn-containing Al–Zn–Mg–Cu alloys

Phases and microstructures of three high Zncontaining Al–Zn–Mg–Cu alloys were investigated by means of thermodynamic calculation method, optica microscopy(OM), scanning electron microscopy(SEM)energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) analysis. The results indicate that similar dendritic network morphologies are found in these three Al–Zn–Mg–Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg(Cu, Zn, Al)_2 eutectic phases, and fine intermetallic compounds g(MgZn_2). Both of as-cast 7095 and 7136 aluminum alloys involve a(Al)eutectic Al/Mg(Cu, Zn, Al)_2, intermetallic g(MgZn_2), and h(Al_2Cu). During homogenization at 450 °C, fine g(MgZn_2) can dissolve into matrix absolutely. After homogenization at 450 °C for 24 h, Mg(Cu, Zn, Al)_2 phase in 7136 alloy transforms into S(Al_2Cu Mg) while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zncontaining Al–Zn–Mg–Cu alloys.     

Grain refinement of primary Si in continuously cast hypereutectic Al–Si alloy by electromagnetic vibration

Abstract

Refinement of Si crystals in hypereutectic Al–20 wt-%Si alloys was carried out using low frequency electromagnetic vibration direct chill casting of billets with 100 mm diameter. Low frequency electromagnetic vibration was produced in the liquid metal by the simultaneous application of a stationary magnetic field and a variable magnetic field. The stationary magnetic field was generated by supplying an induction coil with direct current. The variable magnetic field was created by another induction coil below the top of the mould, which is supplied with an alternating current. The effects of low frequency and addition of chromium and/or iron were tested. The frequency <37 Hz promoted inhomogeneous distribution of primary Si due to heavy stirring. The diameter of primary Si applied with 27 Hz was ～19 μm. Cr addition, even on the strong magnetic stirring of 15 Hz, shows refinement and homogeneous distribution of primary Si.