Authors: Forhad Hossain
Source: Repository@Nottingham (University of Nottingham) (article, 2025-12-09)
Field: Chemistry — Organic Chemistry

Design of microwave systems for metal additive manufacturing Metal Additive Manufacturing (metal AM) is a fascinating technology which is already revolutionizing the metals manufacturing industry with much future potential in view. Its unique advantages have led to the development of numerous techniques aimed at fully utilizing its benefits. While laser and electron beam technologies are widely used as heating sources in metal AM, their high energy consumption and expensive operational costs limit their accessibility for mass adoption. Microwave energy, with its demonstrated capabilities in sintering, coating, and cladding, offers promising alternatives. Its unique ability to selectively couple with materials and efficiently transfer energy creates opportunities for a cost-effective, scalable, and environmentally sustainable solution for metal AM. This thesis explores the feasibility and applicability of microwave energy for metal AM through two complementary approaches: (1) developing a microwave cavity applicator as a proof-of-concept for melting and generating metal droplets as a crucial step toward Drop-on-Demand (DoD) metal jetting and (2) developing a novel microwave coaxial line applicator for metal powder bed fusion. These efforts seek to evaluate the performance of microwave-based systems, determine the key influencing operational parameters, and establish a foundation for microwave-driven metal AM techniques. A comprehensive methodology was employed, combining numerical simulations, experimental investigations, and advanced characterisation techniques. Numerical models were developed using COMSOL Multiphysics® to simulate the distribution of electromagnetic fields, their interactions with metal powders, and to predict heating patterns. These simulations guided the design and development of experimental configurations tailored for the DoD and coaxial studies. A single-mode rectangular waveguide cavity was developed to study the effect of electromagnetic field distribution on heating efficiency and sintering behaviour. A cylindrical microwave cavity was developed as a proof-of-concept for generating plasma to melt metal powders, serving as a precursor for producing molten droplets in a Drop-on-Demand jetting process. The coaxial line applicator was designed to deliver highly focussed microwave energy incident to a metal powder bed thus facilitating localized heating and fusion of metal powders. A real-time monitoring system was implemented to measure temperature and power absorption during experiments ensuring precise process control. Advanced characterisation techniques, including optical microscopy, SEM, XRD and Vickers hardness testing were used to analyse the processed metal parts. Experimental results revealed that microwave heating of conductive materials is strongly influenced by particle size and the positioning of the sample within regions of strong electric (E) or magnetic (H) fields. Smaller particles with higher surface- area-to-volume ratios exhibited enhanced microwave absorption and improved densification. Positioning the sample at the maximum H-field location achieved uniform heating and optimal sintering and near-theoretical densities and refined grain structures. Among the tested materials stainless steel (SS410) demonstrated the highest energy absorption due to its magnetic properties and hysteresis losses that resulted in superior densification compared to non-magnetic metal. This thesis presents the design, development, and validation of a microwave cavity for high-temperature melting and molten droplet formation of metal powders, targeting Drop-on-Demand additive manufacturing applications. A cylindrical cavity operating at 2.45 GHz in the TM010 mode was successfully developed to ignite and sustain a macroscale microwave plasma, that achieved efficient heating and metal melting. The study addresses a key limitation in metal additive manufacturing particularly for high- melting-point materials, where conventional heating methods often suffer from thermal inefficiencies and restricted material compatibility. Experimental validation demonstrated successful melting of aluminium, bronze, and two different stainless steels. Controlled droplet ejection for bronze was optimized by adjusting aperture size, deposition height, and ejection speed. The findings indicate that microwave-driven melting provides a material-flexible, energy-efficient alternative to conventional techniques, with potential implications for precision metal jetting in additive manufacturing. However, some challenges remain, particularly in crucible durability under extreme thermal conditions. Alumina and stabilized zirconia crucibles failed due to thermal shock and cracking, disrupting plasma stability and resulted in incomplete melting. Quartz crucibles, while more resilient, exhibited compatibility limitations, especially with aluminium, where chemical interactions led to degradation and contamination. Achieving consistent droplet formation required precise control over iv process parameters to mitigate issues such as coalescence, premature solidification, and nozzle blockages. This study establishes microwave-assisted metal melting and jetting as a viable alternative for next-generation AM applications, particularly in processing irregular or recycled metal feedstocks, opening new possibilities for cost- effective and sustainable additive manufacturing technologies. This thesis presents the design and development of a novel microwave coaxial applicator for powder bed fusion metal additive manufacturing. The applicator operates in the transverse electromagnetic (TEM) mode and was numerically optimized to ensure efficient energy transfer and targeted heating. Simulations of a melt envelope indicated that microwave-driven localized heating could effectively sinter and melt metal powders when positioned within 1 mm of the applicator tip. Experimental validation was conducted using bronze and stainless steel 316L powders, demonstrating that a minimum energy density of 150 J/mm3 was required to produce cohesive tracks, while an optimal range of 170-255 J/mm3 resulted in well- structured, stable geometries confirmed by optical microscope and scanning electron microscope analysis. Preheating the powder bed to 950C significantly improved track quality by reducing balling effects and heat conduction losses, yet challenges in layer adhesion and track consolidation remained. Furthermore, the results highlight the microwave coaxial applicator’s potential as an alternative energy delivery system for powder bed fusion, particularly in processing high-reflectivity and refractory metals that are challenging for laser-based AM. Unlike traditional techniques, this system offers the possibility of processing irregular, lower- cost feedstocks, contributing to more sustainable and cost-efficient metal AM solutions. Compared to laser and electron beam-based additive manufacturing microwave-based processing can offer significantly lower capital investment, as solid- state microwave generators are generally more affordable than high-power lasers or electron beam systems. Again, microwave systems can be tailored for processing irregular and lower-cost feedstocks, potentially reducing operational expenses and minimizing maintenance requirements. However, key challenges remain, including ensuring strong interlayer bonding, improving energy efficiency, and mitigating oxidation effects. Further work should focus on refining the applicator’s geometry for higher power delivery, incorporating advanced substrate preheating strategies, and v exploring vacuum or controlled atmospheres to reduce oxidation and enhance part quality. Integration of machine learning-driven process optimisation has the potential to improve precision, repeatability, and scalability, advancing the applicability of microwave-based powder bed fusion for industrial additive manufacturing. Microwave ; Energy consumption ; Coaxial ; Microwave heating ; Metal powder ; Waveguide ; Electromagnetic environment ; Reliability (semiconductor) ; Key (lock) Microwave-Assisted Synthesis and Applications ; Food Drying and Modeling ; Solidification and crystal growth phenomena