Ahmad Falahati

Precipitation in Al-Alloy 6016 – The Role of Excess Vacancies

6xxx Al alloys owe their superior mechanical properties to the precipitation of finely dispersed metastable β´´ precipitates. These particles are formed in the course of optimized heat treatments, where the desired microstructure is generated in a sequence of precipitation processes going from MgSi co-clusters and GP zones to β´´ and β´ precipitates and finally to the stable β and Si diamond phases. The entire precipitation sequence occurs at relatively low temperatures (RT to approx. 200 °C) and is mainly controlled by the excess amount of quenched-in vacancies, which drive the diffusional processes at these low temperatures. Very recently a novel model for the prediction of the excess vacancy evolution controlled by the annihilation and generation of vacancies at dislocation jogs, grain boundaries and Frank loops was developed and implemented in the thermo-kinetic software MatCalc. In the present work, we explore the basic features of this model in the simulation of the excess vacancy evolution during technological heat treatments. The focus of this article lies on the effect of vacancy supersaturation during different heat treatment steps, such as quenching, heating, natural and artificial aging.

Thermo-kinetic computer simulation of differential scanning calorimetry curves of AlMgSi alloys

Abstract The microstructure evolution in heat-treatable Al-alloys is characterized by a complex sequence of precipitation processes. These can be either endothermic or exothermic in nature and they can be investigated by thermal analysis. The individual peaks identified in a differential scanning calorimetry (DSC) analysis can be correlated to the nucleation, growth and dissolution of certain types of precipitates. Simultaneously, these data can also be obtained by thermo-kinetic simulation based on models implemented, for instance, in the software MatCalc. The simulations make use of information stored in thermodynamic databases, including stable and metastable phases. In the present work, a thermo-kinetic computational analysis of Al–Mg–Si DSC curves is carried out. The comparison with experimentally observed DSC signals for precipitation and dissolution of metastable GP-zones, β″, β′, as well as stable β-Mg2Si and Si precipitates provides a quantitative insight into the kinetics and sequence of precipitation during DSC probing. The combination of thermo-kinetic and experimental DSC analysis offers new possibilities in interpretation of DSC peaks with multiple metastable phases. In the present paper, we discuss the linking of the simulated precipitation sequence with the measured DSC signal. In addition, with the proposed methodology, a consistent set of parameters to describe the non-equilibrium kinetic parameters of a specific alloy system can be obtained, which can substantially aid in alloy and process development.

Assessment of parameters for precipitation simulation of heat treatable aluminum alloys using differential scanning calorimetry

Abstract Differential scanning calorimetry (DSC) has been used extensively to study different solid state reactions. The signals measured in DSC are associated with the growth and dissolution of different precipitates during a specific heat cycle. The time-temperature dependence of heat cycles and the corresponding heat flow evolution measured in the sample by DSC provide valuable experimental information about the phase evolution and the precipitation kinetics in the material. The thermo-kinetic computer simulation was used to predict the DSC signals of samples taken from 6xxx and 2xxx alloys. In the model, the evolution of different metastable and stable phases and the role and influence of excess quenched-in vacancies in the early stage of precipitation were taken into account. Transmission electron microscopy (TEM) and high-resolution TEM were used to verify the existence of precipitates, their size and number density at specific points of the DSC curves.

Thermo-Kinetic Simulation of the Yield Strength Evolution of AA7075 during Natural Aging

The yield strength evolution in aluminum alloy 7075 is investigated during natural aging. The thermo-kinetic simulation, capable of predicting nucleation, growth, coarsening and dissolution of metastable and stable hardening precipitates in Al-Zn-Mg-Cu during natural aging, is outlined briefly. A recent strengthening model for shearing and bypassing of precipitates by dislocations is utilized to calculate the evolution of the macroscopic yield strength at room temperature. The simulation accounts for vacancy-solute binding energies calculated with the help of first principles simulations that influence the diffusivity of the system due to the presence of excess quenched-in vacancies. These results provide predictions about the amount of excess vacancies trapped by solid solution alloying elements and how the lifetime of vacancies changes due to attractive or repelling binding forces between vacancies and different solid atoms in the aluminum matrix. In our approach, we calculate the strength evolution after quenching due to interaction between dislocations and changes in the microstructure by precipitation of different kinds of secondary phases. The predicted evolution of yield strength is finally verified on experimental measurements.

Simulation of Natural Aging in Al-Mg-Si Alloys

Natural aging during storage of Al-Mg-Si alloys at room temperature can significantly reduce the maximum strengthening potential (T6) during artificial aging and, therefore, is a key topic in aluminium research and industry. Many different strategies to understand and reduce the negative effect of natural aging have been investigated during the last decades, including analysis of different thermal pre-treatments and considering the effect of different microalloying elements. From these investigations, the vacancy evolution and the formation of clusters containing Mg and Si were found to be the governing aging mechanisms behind natural aging. In this work, we present a model to simulate and predict the behavior of these alloys when subjected to room temperature aging after solutionizing and demonstrate the effects of different thermal routes and chemical composition variations. In the implemented model, the evolution of excess quenched-in vacancies and the effect of solute vacancy traps are considered. Special emphasis is placed on co-cluster formation and its contribution to strengthening. The thermokinetic software MatCalc is used for the simulations and the results of the simulations are validated by experimental investigation.

Simulation of Yield Strength in Allvac® 718PlusTM

In the present study, we describe a comprehensive and consistent physical model for the yield strength change in Allvac® 718PlusTM caused by precipitation strengthening. The model incorporates the effect of different shearing and non-shearing mechanisms with respect to atomic continuity between the lattices of precipitates and matrix. We demonstrate that coherency and anti-phase boundary effects are the major strengthening mechanisms in this alloy. The final yield strength of Allvac® 718PlusTM during aging is investigated using the thermo-kinetic software MatCalc. The calculated final yield strength evolution is consistent with experimental results.

Modelling Yield Strength in an A6061 Aluminium Alloy

The yield strength of an A6061 Aluminium alloy for different artificial ageing times, strain rates and temperatures is modelled taking into account precipitation, solid solution and dislocation forest strengthening. Precipitation kinetics during artificial aging and the individual strength contributions are simulated with the thermokinetic software package MatCalc. In the present contribution, we introduce the model for the temperature and strain rate dependence of the yield-strength based on thermal activation theory. The experimental work presented here is performed on a Gleeble 1500 thermo-mechanical simulator, where the solution annealed and quenched samples are heat treated to produce materials in various microstructural conditions. We demonstrate that yield strength simulation is a powerful tool to reduce experimental effort and to cut down costs in the process of alloy engineering. This approach consistently represents the yielding behaviour of alloys in a variety of microstructural conditions with respect to the production history of the alloy and the testing conditions, i.e. temperature and strain rate.

Modelling Microstructure Evolution in Polycrystalline Aluminium – Comparison between One- and Multi-Parameter Models with Experiment

The plastic response of an aluminium alloy type A6061 is modelled using different state parameter‐based approaches. Several of these models (one‐ and two‐parameter models) have recently been implemented into the thermo‐kinetic software package MatCalc. In the present work, a model based on the Kocks-Mecking-law is used to investigate the capabilities of one and two parameter approaches in order to model experimental data. The experimental work presented here is performed on a Gleeble 1500 thermo‐mechanical simulator for different natural ageing times. We demonstrate that one‐parameter models offer a ready‐to‐use and easy‐to‐calibrate solution for a rough correlation between flow‐curve data and microstructure. Such models describe the evolution of the average dislocation density in time. In many practical cases, a single state parameter is insufficient and multi‐parameter models must be utilized, for instance, with consideration of separate populations of dislocations in walls and subgrain interior. These approaches can consistently represent the deformation behaviour of alloys in a variety of conditions with respect to temperature and strain rates.

Prediction of Yield Strength at Room Temperature for Squeeze Cast A226

Predicting yield strength of the cast is difficult, mainly due to inherent chemical inhomogeneity of the microstructure and metal matrix composite nature of the cast. In our approach to predict the yield strength of as cast material AlSi9Cu3(Fe), Scheil-Gulliver model has been used to calculate the phase fraction and chemical composition of each phase during solidification and at each temperature step. Inhomogeneity of the microstructure has been taken into account by considering the evolution of pre-eutectic and eutectic fractions separately. The solidification time-temperature data and cooling to room temperature are recorded using thermocouples and serve as input for the thermo-kinetic software “MatCalc”, that has been used for Scheil simulation and takes into account the evolution of microstructure after solidification and during any arbitrary cooling rate. The strengthening model takes into account the contributions of the intrinsic yield strength of the aluminum matrix, solid solution strengthening, precipitation hardening, effect of eutectic silicon portion and dendrite arm spacing size effect. The phases taken in to consideration include α-Al, Intermetallics, Si and Cu-rich precipitates. The predicted yield strength values are validated by comparing with the experimental values. This approach is extendable to calculate yield strength of the as-cast and heat-treated aluminum alloys.

The Effect of Si on the Precipitation Behaviour in Al-Mg-Si Alloys Studied by Thermo-kinetic Simulation and DSC Experiments

The precipitation sequences of T4 (solutionized and quenched) treated Al-Mg-Si alloys that occur during continuous heating—investigated by Differential Scanning Calorimetry (DSC)—are modelled with the help of computational thermo-kinetic methods. Simulations, including coupled nucleation growth and coarsening of fine dispersed precipitates, are presented to study the influence of the Mg/Si ratio on the formation of the precipitates, especially on the metastable ß” phase. Excess silicon above the content required to form stoichiometric Mg2Si is not believed to alter the precipitation sequence but to increase the effective amount of the intermediate metastable and stable Mg-Si phases. The applied framework is able to predict number densities and sizes of the main hardening precipitates. DSC thermographs of different Mg/Si ratios in 6xxx alloys are simulated, discussed, and compared to experimental data.ZusammenfassungDie Ausscheidungssequenzen einer T4 (lösungsgeglüht und abgeschreckt) behandelten Al-Mg-Si Legierung werden mittels thermo-kinetischer Modellierung simuliert. Die gekoppelten Modelle bezüglich Keimbildung, Wachstum und Vergröberung ermöglichen es, Einflüsse des Mg/Si Verhältnisses vorherzusagen, insbesondere deren Wirkung auf die in der Matrix fein verteilten Ausscheidungen. Besonderes Interesse gilt hierbei der ß” Ausscheidung. Dieser Phase wird die höchste Festigkeitssteigerung des Materials während einer Warmauslagerung zugeschrieben. Generell finden Al-Mg-Si Legierungen mit Überschuss an Si, mehr als dies zur reinen Bildung der Phase Mg2Si notwendig wäre, Verwendung. Dieser Überschuss an Si ändert nicht die generelle Ausscheidungssequenz, trägt aber wesentlich zur speziellen Teilchendichte und Verteilung von metastabilen festigkeitssteigernden Phasen bei. Mit den verwendeten Modellierungsansätzen sind Prognosen zu Teilchenradien und Teilchendichten möglich. Experimentelle Daten einer dynamischen Differenzkalorimetriemessung werden zur Verifizierung der Modellierung den simulierten Werten gegenübergestellt und diskutiert.