Microwave processing in additive manufacturing

In this thesis the applications of microwave processing and measurement techniques for additive manufacture of metals by powder bed fusion (PBF) are investigated. This thesis presents an experimental setup developed for microwave heating and sintering of powder compacts using a cavity applicator. The setup uses directionally coupled power meters to achieve the functionality of a scalar network analyser, which allows the tracking of the resonant frequency during heating. Software is developed, which can be used to control the system and set the sample temperature using sensor measurements as feedback. Additionally, a cavity applicator is designed using a novel method for breaking frequency degeneracy of the excitation mode, TE011. This thesis shows that microwave sintering acts to reduce the microwave magnetic absorption of metal powder compacts. This is confirmed through experiment where various metal powders are sintered using microwave magnetic field. Measurements of the Q0 during the process are used to monitor the change in microwave loss of the powder compacts. This is important as previous results focus mainly on the postsintering properties, while there is very little data pertaining to in-process changes. This thesis presents a technique for measuring the surface resistance using a dielectric resonator. The novelty of the technique lays in its ability to determine the system losses by calibration in situ. This is achieved using a lift-off design, where the dielectric can be moved vertically within the resonator. Theoretical accuracy of this technique is shown to be good and simulation is used to investigate the effect on accuracy of different measurement conditions. Two prototype designs are produced and measurement results are presented for PBF manufactured metal parts, as well as metallised plastic surfaces produced by selective laser sintering (SLS) and stereo lithography (SLA)

Measurement technique for microwave surface resistance of additive manufactured metals

Additive manufactured (AM) metals are a subject of much interest for their performance in passive microwave applications. However, limitations could arise due to artifacts, such as surface texture and/or roughness resulting from the manufacturing process. We have, therefore, adopted a parallel plate microwave resonator for the accurate measurement of the surface resistance of flat metal plates, allowing for microwave current flow in two orthogonal directions by simply exciting a different resonant mode (at 5.3 and 6.4 GHz), without the need to remove and refix the sample. The systematic and random errors associated with the measurement of surface resistance are very small, less than 1% and 0.1%, respectively. The technique is demonstrated with measurements on a range of samples of the alloys, AlSi10Mg and Ti6Al4V, manufactured by laser powder bed fusion, in addition to traditionally machined samples of bulk metal alloys of aluminum and brass. For AM samples of AlSi10Mg, we have studied the effect on the surface resistance of directional roughness features, generated by the laser raster paths, in directions transverse or parallel to microwave current flow. Importantly for passive microwave device applications, we demonstrate that these samples exhibit no systematic anisotropy of surface resistance associated with such surface features

Liftoff dielectric resonator for the microwave surface resistance measurement of metal plates

A new method for accurate measurement of the microwave surface resistance of flat metal plates is proposed and verified experimentally, based on a sapphire dielectric resonator. System losses are accounted for by having continuous and controlled variation of the distance between the dielectric and the sample under test

Surface resistance of metallised 3D-printed plastic surfaces measured in the C-band using a dielectric resonator

In this paper the microwave surface resistance (Rs) of silver metallised plastic surfaces produced by selective laser sintering (SLS) and stereolithography (SLA) are measured. Measurements are performed using a specialised dielectric resonator that utilises a unique “Lift-Off” calibration step. Results for the Rs at 7.5 GHz are given for the samples in a variety of build orientations, encouragingly no bias towards a particular build direction is found. Differences in performance between the SLS and SLA are noted and commented on, with reference to their suitability for low-power, low-cost applications

Effect of build orientation and surface finish on surface resistance in microwave components produced by Selective Laser Melting

A number of metal surfaces, produced by Selective Laser Melting, have been compared from a microwave perspective. Test pieces were produced in Aluminium (Al10SiMg), Titanium (Ti6Al4V) and Cobalt Chrome in two orthogonal build orientations, half of which were grit blasted. Surface resistance measurements at 5.7 GHz were taken using a “lift-off” dielectric resonator method. The losses were compared with roughness measurements from drag profilometry. Microwave losses were found not to monotonically depend on root-mean-square surface roughness, which was attributed to the different size distributions of roughness features. Grit blasting a sample typically reduced its surface resistance but the effect was much more significant in vertically produced surfaces. These results have major implications for parts used in microwave applications

Monitoring changes in microwave asorption of Ti64 powder during microwave sintering

In order to properly understand the behavior of metal powders during microwave sintering, it is important to be able to observe material property changes in situ. One of the best ways to do this is by performing the sintering inside a microwave cavity resonator (MCR). MCRs spatially separate the magnetic and electric fields allowing the interaction with each to be studied in isolation. They also allow the determination of material properties by evaluating the unloaded Q. In this paper initial results are presented showing the changes in unloaded Q of a quasi-spherical cavity being used to heat samples of titanium (Ti64) powder. Partial sintering was confirmed in the sample by a significant and permanent reduction in absorption indicated by a sharp rise in the Q

Effects of post-processing treatments on the microwave performance of additively manufactured samples

Several flat metal samples, produced by selective laser melting (SLM), have under-gone a variety of post-processing treatments. Microwave surface resistance values at a nominal frequency of 7.5 GHz are used to form a comparison between treatments, whilst also performing a comparison with traditionally manufactured copper PCB and aluminium samples. Measured values are then used to create a 3D simulation of a C-Band waveguide filter, utilising measurements from both horizontally and vertically built samples. Overall performance (quantified via low measured surface resistance) was found to be best in the traditionally manufactured copper samples. However silver plating was found to reduce the surface resistance of some SLM samples close to the values of bulk silver and copper

A novel VHF heating system to aid selective laser melting

A novel approach is proposed for penetrative heating of metal powders for component manufacture using selective laser melting (SLM). Large volumes of metal powders have been shown to have unusual properties at higher RF frequencies, whereby magnetic fields can penetrate into them almost completely, at depths much greater than the expected skin depth. A resonant structure at VHF (100-500MHz) and a medium power VHF source (delivering up to 75 W) are described, analyzed and demonstrated, to efficiently heat tungsten carbide powdered samples of volumes of 40cm3 to above 7000C. Further development of this technique will help alleviate the thermal instability problems widely encountered in SLM which lead to large thermal stresses and mis-shaped component