Mechanical and thermo-physical properties of rapidly solidified Al−50Si−Cu(Mg) alloys for thermal management application

Al?high Si alloys were designed by the addition of Cu or Mg alloying elements to improve the mechanical properties. It is found that the addition of 1 wt.% Cu or 1 wt.% Mg as strengthening elements significantly improves the tensile strength by 27.2% and 24.5%, respectively. This phenomenon is attributed to the formation of uniformly dispersed fine particles (Al₂Cu and Mg₂Si secondary phases) in the Al matrix during hot press sintering of the rapidly solidified (gas atomization) powder. The thermal conductivity of the Al?50Si alloys is reduced with the addition of Cu or Mg, by only 7.3% and 6.8%, respectively. Therefore, the strength of the Al?50Si alloys is enhanced while maintaining their excellent thermo-physical properties by adding 1% Cu(Mg).     

On the precipitation of magnesium silicide in irradiated aluminium–magnesium alloys

Thermal neutron irradiation of aluminium or its alloys causes the production of silicon by transmutation. In aluminium–magnesium alloys, the transmutation-produced silicon reacts with magnesium and forms small precipitates. The precipitation in irradiated Al–Mg alloys is similar to the early stage of aging in thermally treated Al–Mg–Si alloys. This study evidences the simultaneous generation of two crystallographically different precipitate types. On the basis of electron diffraction patterns, unit cell parameters are derived and compared with structures found in thermally aged alloys. One of the two precipitate types has an Mg₂Si composition, while the other is an Al–Si–Mg intermetallic compound with high aluminium and silicon but low magnesium content. The formation of magnesium poor precipitates is important since it indicates that the threshold neutron fluence for grain boundary precipitation of silicon may be much higher than estimated in the past.     

Phase stability and structural features of matrix-embedded hardening precipitates in Al–Mg–Si alloys in the early stages of evolution

The strength of Al–Mg–Si aluminium alloys depends critically on nanometre-size Mg_xSi_yAl_z-type precipitates that have a face-centered cubic-based structure. In this work, a large number of early structures are investigated by means of first-principles calculations. Both platelet-type and needle-type precipitates are considered. Calculations show that for alloys with an Mg:Si ratio smaller than one, needle-type precipitates with Si pillars extending in the needle direction are energetically favoured. The formation of Si pillars and the low density cylinder is described. For alloys with an Mg:Si ratio larger than one, platelet-type precipitates consisting of stacked layers of Mg, Si and Al atoms are energetically favoured. Using both the information on the formation enthalpies and the calculated lattice mismatch with the Al matrix, it is discussed which structures are likely to be formed. The earliest, most favourable structures with high Al content are the needle-type initial-β″ Mg₂Si₃Al₆ structure and the platelet-type structures {MgSi}₂Al₁₀, {MgAl}₁Al₁₀, Mg₃Si₂Al₅ and Mg₂Si₁Al₃.     

Effects of Si on deformation behavior and cavitation of coarse-grained Al–4.5Mg alloys exhibiting large elongation

Deformation behavior and cavitation characteristics of Al–4.5Mg–0.09Si and Al–4.5Mg–0.2Si alloys containing Mg₂Si particles have been investigated at 613–693 K. Tensile elongations above 350% were obtained in both alloys. Determination of the strain rate sensitivity and activation energy showed that in both alloys viscous dislocation glide creep deformation mechanism was operative and was not influenced by Si content. However, the level of cavitation was increased by the large addition of Si because Mg₂Si particles promoted more extensive cavity nucleation. Cavitation reduced the post forming tensile properties of both alloys. Investigation of the spatial distribution of cavitation revealed that the intragranular particles were more likely to nucleate cavities than the intergranular particles.     

Fabrication of Al alloys reinforced with AlN particles formed byin-situ reaction

The tensile properties and microstructures of Al alloys reinforced with AlN particles formed byin-situ reaction under a nitrogen atmosphere were analyzed. It was found that AlN particle layers formed on the surface of the Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the reaction, Mg₃N₂+2A1 → 2AlN+3Mg. The tensile strength and 0.2% offset yield strength in the control alloys were significantly greater than those in commercial alloys. This increase was due to the fine AlN particles formed by the abovein-situ reactions of the Mg₃N₂ formation and its decomposition into AlN. This article is based on a presentation made in the symposium “The l^st KIM-JIM Joint Symposium: High Strength Ratio Aluminum Alloys”, held at Inha University, Inchon, Korea, October 22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Japanese Institute of Metals.     

Influence of the microstructure on the machinability of heat-treated Al–10.8% Si cast alloys: Role of copper-rich intermetallics

This study was conducted with the intention of investigating a new experimental alloy, namely the 396 alloy which belongs to the Al–Si near-eutectic cast alloy group and contains about 10.8%Si. In the light of the above, the main purpose of the work is to report on the changes observed in the mechanical and machinability criteria resulting from the effects of the presence of two levels of Cu, namely 2.25% and 3.5%; and of the effects of two levels of Mg, namely 0.3% and 0.6%. In addition to the preceding, the effects of Mg-free alloys and Sr-modification on these same alloys were also investigated.The results demonstrate that the increase in the levels of Cu and/or Mg in the 396-T6 alloy has a detrimental effect on drill life. Such an effect may be attributed to the formation of large amounts of the coarse blocklike Al₂Cu phase, together with the formation of thick plates of the Al–Si–Cu–Mg phase. The Mg-free experimental alloy displays the lowest cutting force and moment in addition to producing the highest number of holes in the alloys studied. This observation may be explained by the cooperative precipitation of the Al₂Cu, Mg₂Si, Al₂CuMg, and Al₅Si₆Cu₂Mg₈ hardening phases in Mg-containing alloys which confer greater strength on the alloy than would be the case with the precipitation of only the Al₂Cu phase in the Mg-free alloy. A comparison of the non-modified alloy and the Sr-modified alloy (containing the same level of Mg and Cu additions) in terms of the number of holes drilled, reveals that the morphology of Si particles has a noticeable effect in governing the tool life of near-eutectic Al–Si alloys. The chip breakability of the alloys containing the Al₂Cu phase is superior to that of the alloys containing Mg₂Si. Thus, combined additions of Cu and Mg are expected to further refine the size of the chips produced.     

Effects of Al content on the corrosion properties of Mg–4Y–xAl alloys

The corrosion performances of Mg–4Y–xAl (x = 1, 2, 3, and 4 wt%) alloys in the 3.5% NaCl electrolyte solution are investigated by electrochemical tests, weight loss measurement and corrosion morphology observation. The results indicate that corrosion modes for the alloys are localized corrosion and the filiform type of attack. With Al concentration increasing from 1 to 4 wt%, the corrosion rate of Mg–4Y–xAl alloys decreases firstly and then increases, and WA42 alloy shows the best corrosion resistance. The addition of Al element to Mg–4Y alloys leads to the formation of Al₂Y and Al₁₁Y₃ intermetallic compounds and reduces the proportion of Mg₂₄Y₅ phase. Corrosion resistance of the Mg–4Y–xAl alloys mainly depends on the size and distribution of the second phases. Besides, the addition of excessive Al can greatly consumes the Y element in the matrix, thus leading to a less protective film on the alloys. The effect of the relative Volta potential changes between the second phases and α-Mg on corrosion resistance of Mg–4Y–xAl alloys is insignificant. The main corrosion products of the Mg–4Y–xAl alloys are Mg(OH)₂, Mg₃(OH)₅Cl·4H₂O, Mg_0.72Al_0.28(CO₃)_0.15(OH)_1.98(H₂O)_0.48, and Mg₄Al₂(OH)₁₂CO₃·3H₂O.     

Effect of erbium addition on the corrosion behaviour of Mg–Al alloys

The corrosion behaviour of two Mg–Al–Er alloys with increasing content of erbium (Mg₉₅Al₃Er₂ and Mg₉₅Al₂Er₃) has been evaluated in borate buffer solution and compared with that of the commercial AM60 alloy (Mg–6.0Al–0.13Mn, at%) employed in the automotive industry. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples, prior and after the electrochemical tests. The experiments were carried out in 0.05 M H₃BO₃+0.075 M Na₂B₄O₇ solution with pH=8.4. Anodic polarization curves were recorded, potentiostatic current decay transients were obtained at anodic potentials and the polarization resistance was determined. The alloys show similar microstructures characterized by a Mg-based solid solution, surrounded by a very small amount of a secondary phase. The alloys investigated exhibit enhanced corrosion resistance with respect to AM60 alloy. Electrochemical investigations reveal that the surface layers formed on the erbium-containing alloys provide a better protective effectiveness than the magnesium hydroxide or aluminium hydroxide layer formed on AM60 in borate buffer solution. Based on preliminary analysis, the incorporation of erbium in the Mg(OH)₂ lattice is believed to be responsible for the improved corrosion behaviour of the Mg–Al–Er 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.     

铝含量对AS系列铸造镁合金机械加工性能的影响(英文)

研究铝含量对AS系列铸造镁合金机械加工性能的影响。通过测量切削力和表面粗糙度对镁合金的机械加工性能进行评估。研究合金的微观结构和拉伸性能。结果表明,切削力随着铝含量的增加而增大;AS91镁合金的表面粗糙度和力学性能最高;对力学性能有影响的主要机制是存在金属间相Mg2Si和Mg17Al12。在机械加工镁合金中,切削力随着切割速度的增大而增大。所测得的数据与机械加工合金的力学性能一致。     

The Microstructure and Tensile Properties of a Newly Developed Mg–Al/Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> In Situ Composite in As-Cast and Extruded Conditions

The microstructures and tensile properties of Mg–x wt%Al–y wt%Sb alloys have been studied where x/y ratio was 1 and Sb(Al) contents were 5, 10, 15 and 20 wt%, respectively. The results indicated that by increasing Sb(Al) content, not only the crystals of primary Mg₃Sb₂ alter from small flake-like particles to polygonal or needle-like morphology, but also the eutectic structure changes from semi-continuous network in Mg–5Al–5Sb to continuous network in Mg–20Sb–20Al alloy. The results obtained from thermal analysis revealed different peaks related to the formation of Mg₃Sb₂ as primary phase and eutectic structure containing Mg₁₇Al₁₂?+?Al₃Mg₂ intermetallic phases. Further results also revealed that Sb(Al) additions change the solidification performance of the material by depressing the Mg₃Sb₂ nucleation temperature, reducing solidification range and widening eutectic area. Tensile testing results showed that with the increase in Sb (Al) content, ultimate tensile strength (UTS) and elongation values of the alloys are decreased in as-cast condition. But, significant improvement in the UTS and elongation values of the extruded specimens was attributed to the severe fragmentation of intermetallic phases and well distributed fine particles in the matrix which provided proper obstacles for dislocation motion. It was interesting to note that the fracture behavior of intermetallic particles was found to be different, while Mg₃Sb₂ was ductile, intermetallic compounds in eutectic regions were brittle.     

Microstructure characteristics in non-modified and Sr modified Al–Si–Cu–Mg 319 type alloys

Abstract

This study was carried out on 319 alloys containing low and high levels of Mg, in the non-modified and Sr modified conditions (150 ppm Sr addition). Single step, two step and triple step heat treatments were applied to identify the optimum solution heat treatment to minimise incipient melting of the copper phases Al₂Cu and Al₅Mg₈Cu₂Si₆ in these alloys in relation to the alloy properties. In Mg free alloys, no incipient melting of Al₂ Cu was observed even in samples heat treated at 520°C. Addition of Sr leads to modification of Si particles but also to an increase in area per cent porosity and pore length, especially when the solution temperature reaches 520°C. Addition of Mg results in a decrease in the Si particle aspect ratio but an increase in particle size. Magnesium was also found to increase the possibility of incipient melting resulting from the formation of the insoluble Al₅Mg₈Cu₂Si₆ phase. To some degree, Sr decreases the effect that Mg has in increasing the area per cent porosity and pore length, while Mg impairs the effects that Sr has on modifying Si particles, even though the lowest Al–Si eutectic temperature is obtained for the 319 alloy containing both Mg and Sr.     

Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

The solute clusters and the metastable precipitates in aged Al–Mg–Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg–Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e. pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serve as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al–Mg₂Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al–Mg–Si alloys is discussed.     

Hardness and microstructural developments of spray formed aluminium alloys with high Mg2Si content during hot extrusion and heat treatment

Abstract

Spray forming offers the possibility of producing alloys with very fine, homogeneous microstructures. Even materials with high contents of intermetallic precipitates, which cannot be produced by casting because of the high solidification rates required, can be distributed homogeneously. Alloying aluminium with high contents of Mg and Si (>20 wt-%Mg₂Si) gives an increase in stiffness plus a significant reduction in density, but a very fine distribution of the Mg₂Si particles in the aluminium matrix is required. Therefore, such alloys are commonly produced by spray forming. Post-spraying processes such as forming and heat treatment are generally carried out to optimise properties. To examine the microstructure and hardness as a result of subsequent processing, aluminium alloys with high Mg₂Si content (22–30 wt-%) have been produced under a variety of spray forming conditions. The duration and temperature of heating before extrusion were varied. In addition, some specimens were preheated without extrusion. The influence of subsequent heat treatment was investigated by varying the age hardening parameters. Hardness measurements were conducted and the distribution and size of the precipitates were evaluated by light microscopy. Image analysis was used to study the coarsening behaviour of primary Mg₂Si. The results indicate that the subsequent processing conditions have a strong influence on the microstructure and hardness of the material. Further, a significant dependence of coarsening rate during subsequent processing on the initial state of the material after spray forming was observed. Knowledge of correlations between process parameters and microstructural development offers the possibility of optimising the hot extrusion and heat treatment parameters for high Mg₂Si containing aluminium alloys.     

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.     

Atomic size and chemical effects of alloying elements Cu,Mg and Si on the structure and dynamics of molten 8090-based AlLi alloy

The structural and dynamical properties of liquid Al₉₁Li₉ and Al₉₁Li₉M₃ (M = Cu, Mg, Si) alloys are investigated by means of ab initio molecular dynamic simulation. Pair distribution function analysis suggests that the atomic distances of Li–Li and Al–Li decrease after addition of alloying elements. The additive Cu and Si are manifested surrounded by Al and Li and hardly meet each other owing to the effects of atom size and their negative mixing enthalpy with Li. The topological environments of Al and Li in Al₉₁Li₉ are changed significantly by adding minor alloying elements. The diffusion of Al and Li is hindered by alloying elements, among which Cu and Si play more significant role. Furthermore, the metalloid additive Si illustrates different effects from the metallic additive Cu and Mg on the diffusivity of Al₉₁Li₉ liquid alloys.     

Morphology and formation of Fe–Al intermetallic layers on iron hot-dipped in Al–Mg–Si alloy melt

We have examined the morphology and the growth of Fe–Al intermetallic layers of η-Fe₂Al₅ and θ-FeAl₃ phases formed on pure Fe sheets dipped in an Al-8.2Mg-4.8Si (wt.%) alloy melt and pure Al melt at 750 °C. The η phase layer grows one order of magnitude slower in the Al–Mg–Si alloy melt than in the pure Al melt. The change in thickness of Fe sheets with dipping time is less pronounced in the Al–Mg–Si alloy melt than in a pure Al melt. Microstructure observations suggest that the retarded interfacial reaction between solid Fe and liquid Al–Mg–Si alloy is associated with a continuous θ phase layer formed in the Al–Mg–Si alloy melt, which acts as the diffusion barrier.