The quench sensitivities of an AlSi10Mg alloy in permanent mold (PM) and high-pressure vacuum die (HPVD) castings were investigated with time-temperature-transformation and time-temperature-property ...diagrams using an interrupted quench technique. The quench-sensitive temperature range of the HPVD casting sample is 275-450 °C, and its nose temperature is 375 °C. The quench-sensitive range of the PM casting sample is 255-430 °C, and the nose temperature is 350 °C. The mechanical strength versus the cooling rate in both casting samples were predicted via a quench factor analysis and verified experimentally. The critical cooling rate of the HPVD casting sample is 20 °C/s whereas it is 17 °C/s for the PM casting sample. With a shorter critical time, higher nose temperature, and higher critical cooling rate, the HPVD casting sample exhibits a higher quench sensitivity than the PM casting sample. The differences in the quench sensitivities of the AlSi10Mg alloy due to the different casting processes is explained via the different precipitation behavior. At the nose temperature, coarse β-Mg
Si precipitates mainly precipitate along the grain boundaries in the HPVD casting sample, whereas rod-like β-Mg
Si precipitates distribute in the aluminum matrix in the PM casting.
Thermomechanical fatigue loadings (TMF) applied on components in a certain temperature range with a variable state of stress (tensile and/or compression) produce a localized concentration of plastic ...strains that results in crack initiation and propagation. The time evolution of plastic strains must be known a priori to predict the lifetime of a part submitted to TMF loadings, which requires an extensive campaign of mechanical characterization conducted at different temperatures and aging conditions. Such a campaign was proposed for the aluminum alloy AlSi7Cu3.5Mg0.15 (Mn, Zr, V), which is recognized as being creep resistant. Combined isothermal low-cycle fatigue and isothermal creep tests were performed on this alloy to determine the constitutive parameters based on the Lemaître and Chaboche (LM&C) viscoplastic model. These laws were implemented within the finite element simulation software (Z-set) to model the response of the alloy to a thermomechanical fatigue test. The results of TMF Z-Set simulations, using the LM&C model adapted for two aging conditions, were then compared with results obtained from “Out of Phase” thermomechanical fatigue testings (OP-TMF) performed on a Gleeble 3800 machine. The modelling of the OP-TMF test revealed the complexity of the mechanical behavior of the material induced by the temperature gradient prevailing along with the cylindrical specimen. It was found that a better prediction of the evolution of plastic strains requires taking into account a larger range of plastic strain rates conditions for the determination of the constitutive law and eventually includes the role of the microstructure in the evolution of the material behavior, starting first with the yield stress.
Heat-treatable cast and wrought aluminum alloys are widely used for structural applications in the automobile and aerospace industries. To assess and diagnose the production and quality problems ...related to industrial heat treatments, differential scanning calorimetry (DSC) was used as a tool in the present work to determine the thermal histories of samples that had undergone different tempers of three commonly used aluminum alloys, namely a high-pressure die-cast AlSi10Mg0.3Mn alloy, permanent-mold cast Al-Si-Cu 319 alloy, and extruded Al-Mg-Si AA6082 alloy. Various peaks detected in the DSC curves were analyzed and characterized to identify the precipitation/dissolution reactions of metastable phases, aiming to establish a “fingerprint” of each temper of the three experimental alloys. Results showed that both the number and size of exothermic peaks varied with the temper owing to distinct precipitation behaviors, providing an effective means of fingerprinting the various tempers. Meanwhile, electrical conductivity and microhardness data provided the supplementary support for the fingerprinting. The thermal histories of three experimentally heat-treated alloys were well traced and distinguished by the combination of DSC characteristics and electrical conductivity and microhardness results, promoting the DSC application in the quality control and verification of industrial heat treatments. In addition, the microstructures after the various tempers were observed to confirm the evolution of the precipitation reactions revealed in the DSC curves.
Recent environmental restrictions constrained car manufacturers to promote cast aluminum alloys working at high temperatures (180 °C–300 °C). The development of new alloys permits the fabrication of ...higher-strength components in engine downsizing. Those technologies increase internal loadings and specific power and stretch current materials to their limits. Transition metals in aluminum alloys are good candidates to improve physical, mechanical, and thermodynamic properties with the aim of increasing service life of parts. This study is focused on the modified AlSi7Cu3.5Mg0.15 alloy where Mn, Zr, and V have been added as alloying elements for high-temperature applications. The characterization of the cast alloy in this study helps to evaluate and understand its performance according to their physical state: As-cast, as-quenched, or artificially aged. The precipitation kinetics of the AlSi7Cu3.5Mg0.15 (Mn, Zr, V) alloy has been characterized by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) observations, and micro-hardness testing. The Kissinger analysis was applied to extract activation energies from non-isothermal DSC runs conducted at different stationary heating rates. Finally, first-order evaluations of the interfacial mobility of precipitates were obtained.
In this paper, a novel analytical modeling of the growth and dissolution of precipitates in substitutional alloys is presented. This model uses an existing solution for the shape-preserved growth of ...ellipsoidal precipitates in the mixed-mode regime, which takes into account the interfacial mobility of the precipitate. The dissolution model is developed by neglecting the transient term in the mass conservation equation, keeping the convective term. It is shown that such an approach yields the so-called reversed-growth approximation. A time discretization procedure is proposed to take into account the evolution of the solute concentration in the matrix as the phase transformation progresses. The model is applied to calculate the evolution of the radius of spherical θ-Al2Cu precipitates in an Al rich matrix at two different temperatures, for which growth or dissolution occurs. A comparison of the model is made, with the results obtained using the numerical solver DICTRA. The very good agreement obtained for cases where the interfacial mobility is very high indicates that the time discretization procedure is accurate.
The quench sensitivity of an AlSi7MnMg alloy in high-pressure vacuum die (HPVD) casting was investigated by time-temperature-transformation and time-temperature-property diagrams with an interrupted ...quench technique. The quench sensitive temperature range of the alloy is from 260 to 430 °C and its nose temperature is 350 °C. The mechanical strength versus cooling rates of the HPVD casting was predicted using quench factor analysis method and verified by experimental results. The critical cooling rate is 6 °C/s to remain 95% of the maximal mechanical strength. The coefficients
k
2
-
k
5
, related to the nucleation and precipitation kinetics of TTP curves, and phase transformation diagrams were determined. The precipitation of Mg
2
Si phase in the castings was observed during isothermal treatment using transmission electron microscope. Moreover, the quench sensitivity and kinetics of the phase transformation of AlSi7MnMg alloy and AlSi10MnMg alloys were compared. It reveals that the quench sensitivity and phase transformation rate of the former are lower than that of the latter.
An exact analytical solution of the Fick’s second law was developed and applied to the mixed-mode growth of a multicomponent ellipsoidal precipitate growing with constant eccentricities in the ...quasi-stationary regime. The solution is exact if the nominal composition, equilibrium concentrations and material properties are assumed constant, and can be applied to compounds having no limitations in the number of components. The solution was compared to the solution calculated by a diffusion-controlled application software and it was found that the solute concentrations at the interface can be determined knowing only the nominal composition, the full equilibrium concentrations and the coefficients of diffusion. The thermodynamic calculations owing to find alternative tie-lines are proven to be useless in the mixed-mode model. From this, it appears that the search of alternative tie-lines is computationally counterproductive, even when the interface has a very high mobility. A more efficient computational scheme is possible by considering that a moving interface is not at equilibrium.
The effects of microalloying with Mg (0–0.23 wt%) on the microstructural evolution and mechanical properties of Al–Cu 224 cast alloys at ambient and elevated temperatures are investigated using ...transmission electron microscopy, differential scanning calorimetry, and tensile/compression testing. The results show that microalloying with Mg significantly enhances the precipitation of the θ′ phase during aging, producing fine, dense, and uniformly distributed θ′ precipitates. These precipitates are much more effective for alloy strengthening than are the θ″ precipitates in the alloy without Mg. During stabilization at 300 °C for 100 h, the dominant process becomes coarsening of the θ′ phase. The Mg-containing alloys have much finer and denser θ′ precipitates and thus considerably higher yield strengths at elevated temperature as compared to those of the alloy without Mg. The improvement is more pronounced at low Mg contents (0.09%–0.13%) than at high contents. The yield strength at 300 °C of the 0.13% Mg alloy is as high as 140 MPa, which is far superior to that of most cast aluminum alloys. Moreover, the enhanced yield strength of this alloy is well preserved during prolonged exposure at 300 °C for 1000 h, indicating that it is a promising lightweight material for high-temperature applications.
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•Effect of Mg on microstructure and room-/high-temperature strength of Al–Cu alloy.•Enhanced precipitation of the θ′ phase by microalloying with Mg.•Upon exposure at 300 °C for 100 h, the best characters of θ′ is obtained at 0.13% Mg addition.•Strength at 300 °C of Al–Cu alloy with 0.13% Mg highly exceeds that of most Al alloys.•Enhanced strength of alloy remains well preserved even after exposure at 300 °C for 1000 h.
Heat treatment for precipitation hardening is known to have a significant effect on the nano/microstructure of cast aluminum alloys, and thereby affecting its properties. In this study, precipitation ...kinetics after solutionizing and water quenching have been characterized by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and microhardness evaluations under different aging conditions. The Kissinger methodology was applied along with the Lee–Kim–Starink–Zahra kinetic equation to determine the kinetic parameters from the DSC runs at constant heating rates, assuming that the precipitates have an ellipsoidal shape. The TEM results showed evidence of semi-coherent
θ
′ precipitation in accordance with the microhardness evolution during isothermal aging at 190 °C and the kinetic analysis from the DSC data. The TEM bright field images of the specimens aged at two different times were used to record the size and number density of the precipitates. The activation energies for the precipitation kinetics of
θ
and
θ
′ were 330 and 114 kJ/mol, respectively. Finally, the values for the interfacial mobility were determined using the kinetic parameters derived from the DSC results and the TEM observations.
The influence of the additions of transition elements (Zr, V, and Mo) on microstructure, precipitation behaviour and tensile properties of Al-8%Si-0.3%Mg permanent mould castings were investigated. ...It reveals that the presence of the transition elements increased the solute concentration in aluminium matrix, refined eutectic Si particles, and reduced the length and aspect ratio of intermetallic phases in as-cast microstructure. Moreover, the transition element addition promoted the formation of solute cluster and increased the number density of β′ precipitates and dispersoids during T6 heat treatment. The additions of Zr, V, and Mo considerably improved the tensile strengths and elongation in both as-cast and T6 conditions. The effects of the combined addition of Zr, V, and Mo on the microstructure, precipitation behaviour, and tensile properties were stronger than that of the addition of Zr and V.
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