Post-treatment of additive manufactured aluminium parts: Friction stir processing and hot isostatic pressing
(2018) ICEM 2018 - 18th international conference on experimental mechanics — Location: Brussels, Belgium (1.July.2018)
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- Metal additive manufacturing is a very promising production method. Its novelty aspect is somewhat a disadvantage compared to other more established manufacturing processes, but it also represents development opportunities. These improvements can be carried out in many of the various steps of the metal AM processing chain: new alloys and powder development, parameter optimisation or post-treatment. Post-treatment is a common way to improve performance. It is necessary in conventional manufacturing processes as well as in the additive manufacturing industry. When devising high performance applications, such as fatigue resistant Al-alloy based structural parts in the aerospace industry, the need for a combination of good mechanical properties and surface quality renders post-treatment indispensable. The laser heating source used in the most representative metal AM technique, selective laser melting (SLM), yields high thermal gradients, producing a very fine microstructure with good mechanical strength coupled with limited ductility. However, the resulting parts have inhomogeneities, porosity, residual stresses and surface roughness. In order to be able to propose a robust solution to the AM SLM parts that would enable high-end use, a good understanding of the effect of potentially beneficial post-treatments on SLM materials is needed. In this sense, a representative AM alloy, SLM AlSi10Mg, was selected as study subject, and friction stir processing (FSP), a method derived from friction stir welding that has been proven useful in cast material [1], was implemented. At the same time, taking into account FSP limitations, a complementary technique would be needed. While FSP can be easily implemented in the machining step (FSP tool can be used in a machining centre or milling machine) to locally improve mechanical behaviour, acting on specific features (structural or fatigue solicited areas, holes, union/attachment sites, etc.) of otherwise multifunctional complex parts, hot isostatic pressing could be the ideal solution to treat whole parts or lattice structures. Indeed, HIP yields very satisfactory results for Ti based AM alloys and is routinely applied in this industry, but no HIP based solution has been devised yet for Al-based AM alloys, although it is commonly used on their cast counterparts [2]. The typical 500°C/100MPa/2h cycle used for cast parts increases ductility, but it also ruins the SLM fine microstructure, depleting mechanical strength to an inadmissibly low level. A HIP cycle suitable for SLM Al parts should forcefully use a lower temperature [3]. To fulfil the objective of developing a solution to tailor the mechanical properties of metal AM parts, this research is focused on SLM AlSi10Mg. FSP was performed on 150x35x5 mm3 as built plates. Different process parameters were studied, both for the SLM and FSP processes. In another set of experiments, HIP was also performed on the same raw material. In order to characterize and understand the mechanical behaviour variation, various tests were performed on the processed material. Static mechanical (Vickers hardness, uniaxial tensile testing) and fatigue tests were carried out. Further characterization was done using microscopy (LOM, SEM) as well as laboratory X-ray computerized tomography and synchrotron radiation at ESRF ID11 beamline (the latter also allowed to perform in situ tensile and ex situ fatigue tests). These and other methods allow full determination and linkage of mechanical behaviour and microstructural features, including phase distribution, porosity and impurities (Fe-rich intermetallics). References [1] S. R. Sharma, Z. Y. Ma, R. S. Mishra. Effect of friction stir processing on fatigue behavior of A356 alloy, Scripta Materialia 51 237-241 (2004) [2] M. Garat, Casting made from aluminium alloy, having hot creep and fatigue resistance, EP 2329053 B1 (2017) [3] N. E. Uzan, R. Shneck, O. Yeheskel, N. Frage, Fatigue of AlSi10Mg specimens fabricated by additive manufacturing selective laser melting (AM-SLM), Materials Science & Engineering A 704 229-237 (2017)
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Santos Macias, J. G., Jacques, P., Bacroix, B., Van Hooreweder, B., Elangeswaran, C., Maire, E., Adrien, J., & Simar, A. (2018). Post-treatment of additive manufactured aluminium parts: Friction stir processing and hot isostatic pressing. Proceedings of the ICEM 2018 - 18th international conference on experimental mechanics. Published. ICEM 2018 - 18th international conference on experimental mechanics, Brussels, Belgium. https://hdl.handle.net/2078.5/220337