Manas Paliwal

Modelling temperature and concentration dependent solid/liquid interfacial energies

ABSTRACT Models for the prediction of the solid/liquid interfacial energy in pure substances and binary alloys, respectively, are reviewed and extended regarding the temperature and concentration dependence of the required thermodynamic entities. A CALPHAD-type thermodynamic database is used to introduce temperature and concentration dependent melting enthalpies and entropies for multicomponent alloys in the temperature range between liquidus and solidus. Several suitable models are extended and employed to calculate the temperature and concentration dependent interfacial energy for Al–FCC with their respective liquids and compared with experimental data.

Multiphase Diffusion Study for Mg-Al Binary Alloy System

Multiphase diffusion simulation and annealing experiments have been performed for Mg-Al binary alloys at various temperatures. Annealing experiments of Mg-3wt% Al and Mg-6wt% Al alloys were carried out at 330 and 400 °C for various times and the change of concentration profiles of Al in grains were measured by Electron Probe Micro Analyzer (EPMA). In order to simulate this annealing process and understand the diffusion of Mg-Al alloys, diffusion model was developed by using Finite Difference Method (FDM) coded in FORTRAN. In the diffusion simulations, composition-independent inter-diffusion coefficients were used and the intermetallic phases were assumed to have equilibrium compositions.

Evolution of as-Cast Microstructure of Mg-Al Alloys with Solute Content and Cooling Rate

Although numerous solidification experiments have been conducted for Al, Cu and Si alloys to investigate microstructural features like primary and secondary dendrite arm spacing, solute distribution with in secondary arms and second phase fraction, no systematic solidification study on Mg alloys has been performed to understand the evolution of microstructural features as a function of cooling rate and solute content. The present study focuses on the experimental microstructural evolution of Mg-3, 6 and 9 wt. % Al alloys in the cooling rate range of 1 K/sec to 1000 K/sec. The results suggest that secondary dendrite arm spacing and amount of second phase formation are strongly dependent on both solute content and cooling rate.

Thermodynamic and Kinetic Calculations for TRC (Twin Roll Casting) Mg Alloy Design

Twin roll casting (TRC) process is most cost competitive casting process to produce wrought Mg alloys. At the moment, this process is successfully applied to the production of AZ series Mg alloys with low Al content. From microstructure viewpoint, one of the most concerns in the process is the control of inverse segregation which is inherent problem in TRC process. The inverse segregation is known to be formed when the remaining highly solute enriched liquid between solidified columnar zone is squeezed out through the weak columnar layer due to the compressed force during the solidification in TRC process. The alloys with a large amount of remaining liquid at low temperature and a long solidification range can be more prone to produce severer inverse segregation in TRC process. The other constraint of TRC alloys is the secondary phase amount during rolling process. Too much secondary phase can increase the roll force and induce defects during warm rolling. However, the precipitation of secondary phase at low temperature during the final heat treatment will be beneficial for increasing strength. In the design of new TRC Mg alloys, all these aspects should be considered.

Microstructural Modeling of Mg Alloys and Experimental Validation

A systematic investigation was carried out in the present study to study the impact of solidification variables like thermal gradient (G), solidification velocity (V) and cooling rate on the second phase fraction and microsegregration in Mg-Al (3, 6 , 9 wt.%) binary alloys. Solidification experiments such as directional solidification and gravity castings were conducted to obtain a cooling rate in the range of 0.05-700 K/s. The experimental results were tested against the microstructural modeling data and a reasonable agreement was obtained between them.

Solidification Studies of Mg–Al Binary Alloys

The knowledge of as cast microstructure of Mg alloys is of prime importance to design the downstream thermo mechanical processes like rolling and annealing which then follows with commercial applications of Mg alloys in various industrial sectors [1]. Specifically, the emerging technologies like Twin roll casting that require minimal thermo mechanical processing; it is the solidified microstructure at the casting stage which determines the mechanical properties of the final product. Hence, the as cast microstructural information of Mg alloys becomes imperative but there has been little or no work on the same. In this regard, the present study focuses on the influence of the solidification parameters like thermal gradient (G), solidification velocity (V), cooling rate and solute content on the microstructural features of Mg-Al alloys. Directional solidification experiments and gravity experiments have been performed to explore the relationship between secondary dendrite arm spacing (SDAS) of Mg-Al alloys and the solidification parameters. The transition between cellular-columnar growth modes for Mg-Al alloys has also been investigated in context to the solidification parameters.