Abrasive Flow Machining, (AFM), is a non-traditional machining process that is achieved by extruding polyborosiloxane, (a viscoelastic polymer), containing abrasive grit additions, across surfaces, edges, and through component cavities. The AFM process is a complex one and its machining mechanism is still only partially understood since previous research into the process has mainly been limited to qualitative study. The present work undertook to investigate the relationship between the rheological characteristics of polyborosiloxane/grit mixtures and the associated machining parameters. A significant increase in the quantitative data available with respect to both the rheological and machining characteristics of these mixtures has been provided as a consequence of the investigations. Experiments were conducted using low viscosity, (LV), medium viscosity, (MV), and high viscosity, (HV), polyborosiloxane base media, in conjunction with silicon carbide abrasive grit of 60 and 100 Mesh size; the ratios of grit to base polymer utilised in the experiments were 0,1, and 2. The test pieces used in the experimental work were mild steel dies having a diameter of 15mm and a length of 1 5mm, and the equipment used to conduct the experiments was an Extrude Hone mark 7A machine. The investigations conducted have revealed that for all polymer/grit mixtures an increase in the number of extrusion cycles results in an increase in the stock removed, an improvement in the surface roughness, and an increase in the temperature of the mixture. Furthermore as the usage of the medium increases the grit particle wear increases so that there is a corresponding decrease in the machining parameters. For all mixtures there appears to be no correlation between the viscosities of the base media types and the machining parameters. However, a relationship is demonstrated between the machining parameters and variations in the viscosities of the grit/polymer mixtures based on a specific polymer base. The factors that appear to influence this relationship are the grit to polymer ratio, the grit size, and the temperature. The most important of these parameters are suggested to be the grit to polymer ratio and temperature since these variables appear to affect the viscosity behaviour and the associated machining parameters. In addition the investigations showed that the viscosities and associated rheological dependent parameters correspond to the qualitative viscosity nomenclature given to the different media types by the manufacturer. A shear history effect is also exhibited in each of the polymer types.

Abrasive flow machining (AFM) is a non-traditional manufacturing technology used to expose a substrate to pressurised multiphase slurry, comprised of superabrasive grit suspended in a viscous, typically polymeric carrier. Extended exposure to the slurry causes material removal, where the quantity of removal is subject to complex interactions within over 40 variables. Flow is contained within boundary walls, complex in form, causing physical phenomena to alter the behaviour of the media. In setting factors and levels prior to this research, engineers had two options; embark upon a wasteful, inefficient and poor-capability trial and error process or they could attempt to relate the findings they achieve in simple geometry to complex geometry through a series of transformations, providing information that could be applied over and over. By condensing process variables into appropriate study groups, it becomes possible to quantify output while manipulating only a handful of variables. Those that remain un-manipulated are integral to the factors identified. Through factorial and response surface methodology experiment designs, data is obtained and interrogated, before feeding into a simulated replica of a simple system. Correlation with physical phenomena is sought, to identify flow conditions that drive material removal location and magnitude. This correlation is then applied to complex geometry with relative success. It is found that prediction of viscosity through computational fluid dynamics can be used to estimate as much as 94% of the edge-rounding effect on final complex geometry. Surface finish prediction is lower (~75%), but provides significant relationship to warrant further investigation. Original contributions made in this doctoral thesis include; 1) A method of utilising computational fluid dynamics (CFD) to derive a suitable process model for the productive and reproducible control of the AFM process, including identification of core physical phenomena responsible for driving erosion, 2) Comprehensive understanding of effects of B4C-loaded polydimethylsiloxane variants used to process Ti6Al4V in the AFM process, including prediction equations containing numerically-verified second order interactions (factors for grit size, grain fraction and modifier concentration), 3) Equivalent understanding of machine factors providing energy input, studying velocity, temperature and quantity. Verified predictions are made from data collected in Ti6Al4V substrate material using response surface methodology, 4) Holistic method to translating process data in control-geometry to an arbitrary geometry for industrial gain, extending to a framework for collecting new data and integrating into current knowledge, and 5) Application of methodology using research-derived CFD, applied to complex geometry proven by measured process output. As a result of this project, four publications have been made to-date – two peer-reviewed journal papers and two peer-reviewed international conference papers. Further publications will be made from June 2014 onwards.

Additive Manufacturing, and specifically powder bed fusion processes, have advanced rapidly in recent years. Selective Laser Melting in particular has been adopted in a variety of industries from biomedical to aerospace because of its capability to produce complex components with numerous alloys, including stainless steels, nickel superalloys, and titanium alloys. Post-processing is required to treat or solve metallurgical issues such as porosity, residual stresses, and surface roughness. Because of the geometric complexity of SLM produced parts, the reduction of surface roughness with conventional processing has proven especially challenging. In this Thesis, two processes, abrasive flow machining and ultrasonic shot peening, are evaluated as surface finishing processes for selective laser melted 316L. Results of these experiments indicate that AFM can reliably polish as-built internal passages to 1 µm Ra or better but is unsuitable for passages with rapidly expanding or contracting cross-sections. AFM can also polish relatively small passages, but lattice components may prove too complex for effective processing. USP cannot achieve such low surface roughness, but it is a versatile process with multiple advantages. Exterior surfaces were consistently processed to 1.7 to 2.5 µm Ra. Interior surfaces experienced only partial processing and demonstrated high geometric dependence. USP significantly hardened the surface, but steel media hardened the surface better than ceramic media did. Both AFM and USP are recommended processes for the surface finishing of SLM manufactured parts. Good engineering judgement is necessary to determine when to use these processes and how to design for post-processing.

This contribution describes the investigation of flow channels which are designed to be directly integrated into an aerospike engine by means of additive manufacturing with laser powder bed fusion (LPBF). During the experimental testing of a previous aerospike engine in 2019, it was observed that high surface roughness of such additively manufactured integrated channels caused a significant reduction in the mass flow rates of the propellants ethanol and liquid oxygen as well as the coolant due to increased pressure drop. In an extensive study within the CFDmikroSAT project, various factors influencing this surface roughness are, therefore, being investigated, which include the geometry of the channels as well as selected manufacturing parameters of the LPBF process, such as layer thickness and component orientation. To further reduce the roughness after manufacturing, suitable post-processing methods are also being investigated for internal cavities, initially analysing the abrasive flow machining process. Within the paper, the overall investigation approach is presented, such as the overview of the considered specimens, and the initial results of a various studies with selected specimens are discussed. These studies consist of the examination of surface roughness reduction, shape accuracy and flow behaviour of post-processed cooling channel specimens. Finally, a brief overview of the already manufactured aerospike demonstrator is presented.