Post-process surface finishing treatments for AlSi10Mg parts made by Selective Laser Melting technology

It is well established nowadays that Additive Manufacturing (AM) represents one of the most intriguing manufacturing processes family on the actual market. Given the high design flexibility, the more effective usage of materials and the possibility to tailor the physical and mechanical properties, AM could represent the starting point for a remarkable change in the traditional manufacturing contexts based mainly on material subtraction processes. Although AM processes belong to the near-net-shape processes family, a consistent series of pre and post-process operations are required to meet the desired product specifications. In this context, the post-process surface finishing step represents a fundamental operation that is needed to achieve a satisfactory surface quality, representing very often also a high cost operation when carried out manually and leading to a not reliable and repeatable output. All these considerations are particularly true when considering metal AM processes, among which the ones based on powder feedstock as the raw material are the most sensitive to the mentioned surface quality issues. Assumed that different surface treatments are currently object of study, aimed to reduce the surface roughness of metal AM parts, this Ph.D. thesis proposes an experimental and critical analysis of three different surface treatments, chosen according to their different interaction nature with the surface, i.e. mechanical, chemical and thermal. In order to investigate and understand the dynamics of the chosen treatments, the experimental activity reported in this work refers to flat geometry samples produced by means of the Selective Laser Melting (SLM) technology and employing one of the most widespread and studied alloys: the AlSi10Mg Aluminum alloy. In Chapter 1, a comprehensive overview of AM technologies and their related challenges and constraints is provided, according to the state of the art. In Chapter 2, the metal AM processes generalities are described as well as their main issues, with a specific focus on Powder Bed Fusion -based (PBF) technologies and the mechanisms that imply the poor surface quality of the final parts. With this premise, in Chapter 3 a general overview of the actual surface treatments for metal Additive Manufactured parts is reported, followed by a detailed description of the theoretical background of the investigated surface finishing treatments. On the basis of all the information reported and discussed, Chapter 4 describes in detail the aim and the scope of this Ph.D. thesis, considering all the provided theoretical elements. The description of the experimental work starts from Chapter 5, where the case study used for all the experiments is presented as well as the general characterisation methodology, used to analyse and quantify the effects of the investigated finishing treatments. Chapter 6 reports the results of the preliminary experiments carried out by means of the Fluidised Bed Machining treatment, that represents the mechanical interaction-based surface treatment considered in this work. To carry out the tests, a purposely made Plexiglas reactor was used to carry the experiments under different configurations. The process parameters considered in the reported preliminary experimental campaign are described, chosen according to the process background reported in Chapter 3. In Chapter 7, the preliminary experiments related to the Chemical Polishing treatment are described. According to the process theoretical basics reported in literature for the Aluminum alloys, a two-stage process is proposed that enables a consistent surface quality improvement in terms of smoothing effect as well as the enhancement of the surface symmetry and brilliance. As a matter of concern, the superficial chemical composition changes were investigated. Chapter 8 presents the results of the thermal interaction-based treatment considered: the Laser Surface Re-melting treatment (LSR). The premise of the experiments was to investigate the possibility to use a CO2 laser radiation to perform the desired smoothing process. Considering the Aluminum alloy examined, it is known that CO2 laser is not an effective solution due to the very low radiation absorption for its wavelength from traditionally machined Aluminum alloy surfaces, with lower roughness values compared to SLM. On the other hand, the high surface roughness of the considered SLM processed alloy allows to increase the absorption of the CO2 radiation, in accordance with the laser surface processing theory. Therefore, for the latter case, CO2 lasers could still represent a viable option for surface re-melting. Given the higher maturity level of LSR compared to the previous treatments about the identification and choice of the critical process parameters, as well as the more ease in changing the experimental process conditions, an experimental campaign based on the Design of Experiments (DoE) approach was used. Moreover, the influence of the treatment on the microstructure features of the considered alloy was investigated. According to the preliminary conclusions provided for each of the studied finishing treatments, more general conclusions are reported in Chapter 9, referring both to the main experimental outcomes observed as well as to the main limitations highlighted with respect to the experimental campaign and characterisation methodology. Furthermore, the aim of this Chapter is also to provide some insights for future work as well as a more detailed comparison of the investigated surface treatments, considering different aspects that deserves a critical analysis