Microwave methods for additive layer manufacturing

• Main Author: Clark, Nicholas
• Published: Cardiff University 2017
• Subjects: 621.381
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720895

This thesis presents the novel application of microwave technology to the process of additive layer manufacturing (ALM). A particle size sensor, based on microwave cavity perturbation, is described and subsequently demonstrated by the measurement of the complex permeability of a series of Titanium powders. The results are compared with existing theory and finite element simulations of metallic powders. The ability to discern changing particle size distributions is important in ensuring the reliable operation of selective laser melting machines but, to remain industrially relevant, it is vital that the proposed system can be produced at low cost. By way of demonstration, the design and construction of an inexpensive scalar network analyser was completed. A systematic study of surface resistance of a number of metal surfaces, produced by Selective Laser Melting, was undertaken. Using a dielectric resonator with a “lift-off” calibration procedure, the losses of surfaces manufactured in orthogonal orientations and different surface finishes were compared. Surface roughness measurements showed that microwave losses were not monotonically dependent on root-mean-square surface roughness; this was attributed to differing roughness feature size distributions. For microwave characterization of materials over a wide temperature range, it is often desirable to perform cavity perturbation measurements at elevated temperatures. However, it is shown here that heat treatment can permanently modify the surface resistance of a metal surface and potentially lead to inaccurate perturbation results. X-Ray diffraction measurements confirm the source of modification is due to changes in surface stress and the appearance of solution precipitates. The sensitivity of microwave measurements to surface stress also demonstrates the potential for microwave assessment of surfaces produced by ALM. Finally, to stimulate further work in this area, the design of a single mode microwave heating system was discussed and a prototype developed.