“Free Complexity”: This well-known promise of additive manufacturing (AM) represents one of the biggest challenges for finishing AM components. This is all the more true as the surface treatment, just like the printing process itself, has a significant influence on the final quality of the finished part. Therefore, it is necessary to know the advantages and disadvantages of the many surface finishing methods when applied to AM components to take full advantage of these methods and take them into account in the design of the part. In this context, a decisive role is played not only by the achievable surface quality or economic efficiency, but in many cases also by the impact on mechanical properties such as fatigue resistance. Several studies by Fraunhofer IAPT therefore focus specifically on the post-processing of complex metallic AM components. and provide application-oriented decision aids to further accelerate the industrialization of additive manufacturing.
The challenges of post-processing additively manufactured metal components
The collective term “additive manufacturing” covers a large number of different processes for generating components layer by layer. The most popular variant in the metals sector is laser powder bed fusion (LB-PBF). Unlike conventional manufacturing, no significant constraints are generally placed on part design by undercuts, complex structures, or internal channels. On the other hand, fabrication is limited in terms of resolution as determined by the thickness of the layer (generally in the range 20-60 µm) and the width of the weld pool. The resulting surface quality can be strongly influenced by the resolution and the resulting stepping effect, as well as differences in heat balance when printing or adding support structures. An AM component therefore generally has a very heterogeneous surface with different levels of roughness in different segments of the part. In addition, the potential complexity of the parts is also often a huge problem for the surface finish in terms of accessibility for abrasives or other ablative media. The heterogeneity of the surface and the freedom of design of AM components therefore place the highest demands on performance and flexibility in post-processing methods.
Which post-processing method is suitable for my application?
Fraunhofer IAPT, Hamburg, set out to develop a comprehensive overview of the strengths and weaknesses of current market solutions for surface smoothing of AM components. Fraunhofer IAPT surface finish study thoroughly evaluated a wide range of eight different post-processing methods, representing the multitude of different technical solutions. Three geometry demonstrators have been developed with a variety of different shapes to meet various key characteristics and align them with real 3D printing applications. The demonstrators enabled an in-depth assessment of seven main criteria, namely surface roughness, hardness, ablation rate, edge rounding, penetration depth, readability and costs.
Three of the best-known alloys for printing LB-PBF AlSi10Mg (aluminum), 1.4404 (steel) and TiAl6V4 (titanium) were studied to determine the material specific differences. In total, over 100 test pieces were printed, 17,000 segment measurements were made, 700 working hours were spent on the measurements, and all results were summarized in a 120 page report. As shown in Figure 2, this report provides clear overviews of the performance of each method for the criteria studied for the three materials. In addition, it gives detailed data on the resulting surface qualities.
The following figure provides detailed information on the performance of each post-processing method, clearly revealing large variations in the roughness values obtained. The results are also very dependent on the observed surface. This excerpt is an example of the content of the study and indicates the clear differences between the internal and external surfaces, illustrating the accessibility for the respective smoothing media and therefore also the suitability of a finishing method for geometries and specific characteristics. The surfaces of titanium parts are also significantly improved by selecting an appropriate finishing process. Whereas conventional sandblasting already reduced the surface roughness by more than 50% to approx. 7 µm [mean Sa], other processes have achieved a surface roughness of up to 1 µm.
How does the surface quality influence the properties of my parts?
But since appearance is not everything, a second study (Additive Fatigue Study) also examined the influence of different surface finishing methods on the mechanical properties of FA components, with particular emphasis on fatigue performance. This second study observed that the fatigue behavior of the selected materials (TiAl6V4, IN718 [Inconel]) is not always directly correlated with the measured surface quality. The reasons include certain surface and process characteristics that specifically influenced fatigue resistance. For example, he showed that certain finishing methods could increase fatigue resistance by over 80%. On the other hand, the finishing performance obtained by other finishing methods was even worse than the surface as built. This underlines how important it is to choose the right finishing methods when the requirement is precise surface engineering and performance.
These discoveries and many others are available in the Fraunhofer IAPT studies, as well as process scalability and cost classification. Resulting from an independent and transparent development, they aim to provide easily understandable decision support for all users of additive manufacturing, designers, development and production engineers.
For more information, please contact M.Sc. Maximilian Kluge, Head of Materials and Finishes Fraunhofer IAPT, at [email protected]