Multiphysics Simulation and Experimental Study of Microwave - based Melting of AA6061 Alloy

Abstract

The increasing focus on energy-saving requirements has encouraged various alternative approaches to efficiently process metals compared to conventional methods. Microwave-based processing of metal materials has emerged as one of the promising methods due to its cost effectiveness, environmental friendliness, and rapidity compared to traditional melting processes. The research effort is established to improve the process's capabilities, aiming for it to be accepted as a commercial production method. In this thesis, first, the progress and recent development in the field of microwave material processing of metals have been reviewed, and the current status of the process has been recognized. Based on the literature review, the research gap has been identified, and the research objectives have been framed. The first part of the thesis focuses on setup development for bulk metal processing using microwaves. Microwave melting and casting of bulk metals require a microwave applicator and special tooling assembly that provides easy processing for microwave-reflective metal materials. This research addresses the specific tooling assembly design for microwave-based melting and casting of AA6061 in domestic microwave applicator. A low-cost set-up has been developed for microwave-based melting and casting of bulk metal using a commercial microwave source. The tooling assembly consist of a casket, susceptor, and mold with a specific arrangement for microwave hybrid heating. AA6061 is a highly utilized aluminium alloy grade manufactured in various forms to meet diverse engineering needs. To achieve the required composition with different manufacturing methods, AA6061 has been tailored to accommodate various industrial uses. Among various casting methods, microwave-based metal casting offers considerable precision in regulating heating and solidification stages. The research investigates the complex relationship between temperature fields and material properties during the melting process, aiming to explain the fundamental mechanism for microwave hybrid heating and melting of bulk metals. The second part of the thesis focuses on multiphyiscs simulations and experimental validations, which give critical insights regarding heating dynamics. In this research work, the effect of casket, susceptor, mold, and microwave power has been investigated by performing various simulations with experimental validation. Moreover, specific materials for tooling design for efficient melting have been identified for improving uniform heating. The time-temperature characteristics obtained during simulation and experimental studies lies within 5% error. The findings contribute to advancing the understanding of microwave-assisted metal casting techniques, offering valuable insights for enhancing process efficiency and product quality in industrial applications. The third part of the thesis focuses on melting and solidification effects on structural characterizations and correlation with mechanical properties for the developed microwave cast specimens. The microstructural characteristics have been evaluated by phase analysis, elemental analysis, and grain structure formation. The results demonstrated that in-situ casting of the AA6061 alloy exhibits superior mechanical properties and enhanced grain structure compared to the ex-situ casting. In-situ casting of this alloy at high temperature pouring inside the mold yields fine dendrites and reduced dendritic arm spacing, resulting in grain refinement. The finer dendrites and reduced dendritic arm spacing have been observed due to enhanced cooling rates during solidification. The uniform temperature breaks the initial dendrites growing during the solidification process. Results demonstrated that in-situ casting, the AA6061 alloy exhibits an increase in hardness, low residual stress, low porosity, and fine grain texture compared to the ex-situ casting. The fourth part of the thesis focuses on multiphysics simulation and analysis of two charge geometries with edges and smooth surfaces (Cube and Cylinder). In the present study, a numerical simulation has been carried out to analyse the effect of shape on electric and temperature fields. The microwave absorption capacity of cube shape is higher with nonuniform heating, whereas cylinder have a lower absorption capacity with uniform heating. It is also possible to improve the melting efficiency by adjusting the size and shape of the specimen and appropriate tooling components. In summary, this work has contributed to the knowledge of microwave melting and casting of AA6061. By integrating simulation and experimental approaches, this study aims to explain the fundamental mechanism for microwave-based melting and casting of bulk metals. Insights gained from this study may pave the way for the optimization of microwave processing parameters and the development of more efficient and sustainable methods for aluminum alloy melting in industrial context.