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Improving Fatigue Life of Metal AM parts

    High-Cycle Fatigue (HCF) is one of the most critical issues in mechanical engineering. From the first studies on railway axles to the infamous De Havilland Comet, this phenomenon led to some of the most tragic failures in the history of mechanics. Centuries of science and investigation led to a much better understanding of fatigue life & crack behaviour in conventionally manufactured parts, and today engineers are able to design and produce parts taking this phenomenon into account. When it comes to Metal Additive Manufacturing, two striking problems immediately emerge when talking about fatigue life: the lack of long-term and extensive research on the subject and the intrinsically inferior performance of as-built parts. While the former is being tackled more and more, with a recent surge in academic interest in the subject, the latter can be improved with specialised post-processing techniques such as our in-house Isotropic Super-Finishing process.

    Isotropic Super-Finishing (ISF) is a Chemically Accelerated Vibratory Finishing (CAVF) method developed by REM Chemical, to reduce the roughness of external surfaces of Metal AM parts and improve their fatigue behaviour. Being a tool-less process, it works by chemical conversion of a thin surface layer that is subsequently removed by contact with a vibrating non-abrasive media. The process comprises two steps: at first, the parts are placed within a circulatory vibrating bowl, where the part is submerged in an acidic compound, together with a high-density finishing media. Secondly, once the surfaces have been smoothened, a mildly alkaline chemistry is added to the mix, to neutralize and remove the previously altered surface layer. The process gives the treated components a uniform, non-directional surface finish with a significant reduction in Ra, dependant on the processing time: treated surfaces can achieve Ra values of 0.2÷0.8µm, compared to 5÷12µm Ra typical of as-built parts.

    Most importantly, Isotropic Super-Finishing allows for a significant increase in the fatigue life of Metal AM parts, showing an improvement in the endurance limit of 50÷100%. Tests on Ti6Al4V specimens are presented in the S/N plot of Figure 2: from this data is possible to calculate an increase in endurance limit from approximately 140 to 280 MPa, compared to 415 MPa for the wrought material. While traditional manufacturing retains a significant advantage in terms of endurance over Metal AM, ISF offers a credible improvement over the as-built material.

    Pict 1: S/N plot for Ti6Al4V samples, as-built, after ISF and wrought

    Yet ISF is not the only post-processing technique used in Metal AM that focuses on the reduction in surface roughness, and that should, therefore, bring relevant benefits to the fatigue life of treated parts. Both abrasive polishing and laser re-melting are two commonly used methods capable of a significant reduction in Ra: tested Inconel 625 samples achieved Ra values of 0.18 and 1.5 µm respectively after these treatments, however, neither proved capable of improving high cycle resistance. This is clear to see in figure 3, where post-processed and as-built specimens are all clustered along the same curve in the S/N plot. To understand why Isotropic Super-Finishing proved capable of improving the fatigue behaviour of Metal AM parts, whilst abrasive and laser polishing did not, we need to take a step back and understand why the as-built components have a limited fatigue resistance compared to wrought parts.

    Pict. 2: S/N plot of Inconel625, as-built, after laser polishing and after abrasive polishing

    Life-limiting features that influence HCF in as-built Metal AM parts include internal porosity, near-surface or surface-connected porosity and, most of all, surface roughness. Compared to machined parts of the same alloy, that can achieve sub-micrometric Ra values, those produced by Metal AM present a rougher surface finish (5÷12 µm Ra). This figure can even be higher if considering down-facing surfaces where there can be adhesion of the partially melted powder. As-built parts, therefore, present an HCF behaviour that is highly dependent on surface defects and roughness: these features increase the elastic stress concentration factor in areas where fatigue cracks are more prone to initiate, leading to a lower fatigue limit.

    All analysed post-processing techniques perform a smoothening of the surfaces, removing many of the most relevant surface defects and thus leading to a significant reduction in all surface roughness metrics. Yet we showed that these processes don’t improve the endurance of Metal AM parts. That’s because abrasive and laser polishing improve the surface finish by eroding the peaks of the roughness profile but are not able to affect the most critical features for fracture formation and development: the sharp, crack-like bottom of the valleys that offer a perfect spot for stress concentration.

    The significant advantage of ISF is in its ability to, at least partially, smoothen such features: thanks to the acidic chemical compound, the process is capable of reaching into the bottom of the smaller superficial cracks, not only improving the average roughness value but also expanding and rounding the tips of these critical features. While only the chemical action is possible in such locations, as the abrasive media cannot reach into the smaller cavities, this contributes to a decrease in the stress concentration factor and, therefore, to an increase in fatigue resistance. On the other hand, abrasive polishing and laser re-melting leave the roots of surface defects intact, giving fatigue cracks a perfect nucleation spot thus denying any advantage in terms of fatigue life.

    Picture 3: micrographic images of the as-built surface (left) and after ISF (right)

    While all considered post-processing methodologies provided significant advantages in terms of reduction of surface roughness, only Isotropic Super-Finishing proved to be a reliable way to improve the High Cycle Fatigue life of Metal Additive Manufacturing parts. Thanks to the active chemical compound, not only the peaks of surface roughness are affected by the process, but also its valleys are smoothened and rounded, thus reducing the likelihood of crack nucleation. This leads to a reduction in the elastic stress concentration factor and an improvement of 50-100% in the endurance limit. Moreover, ISF is capable of achieving a surface roughness that while dependent on the processing time, can reach sub-micrometrical values of Ra. For these reasons, ADDITIVA chose to offer this amazing technology in-house. From Aluminium to Stainless Steel, this system can process all of our available materials, to further improve the mechanical properties of our Metal Additive Manufacturing parts.

    Author: Fabio Baiocchi

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