## 5 Conclusions and Outlook  
Complex parts from the binder-jetting process cannot be fully depowdered due to the insufficient green-part strength. This study proposes a new approach to print such components in two parts and to join them through material diffusion in the sintering process by simply placing them on top of each other. Achieving the printability of such shapes would enable the possible economical usage of binder jetting in many areas. For this purpose, different influencing parameters on the joint quality were analyzed and a new method was developed to evaluate the contact ratio metallographically. In addition, a simulation model was developed to determine the contact between green parts. The main conclusions are summarized as follows:  
1. The air pressure during depowdering has a significant impact on the surface roughness of green parts. The lowest roughness of \(\mathrm{Sa}\approx 8.8\,\mathrm{\SIUnitSymbolMicro m}\) was measured at \(2\,\mathrm{bar}\). The roughness increases with increasing air pressure: \(3\,\mathrm{bar}\): \(\mathrm{Sa}\approx 9.9\,\mathrm{\SIUnitSymbolMicro m}\); \(4\,\mathrm{bar}\): \(\mathrm{Sa}\approx 12.2\,\mathrm{\SIUnitSymbolMicro m}\); \(5\,\mathrm{bar}\): \(\mathrm{Sa}\approx 14.6\,\mathrm{\SIUnitSymbolMicro m}\).
2. Depending on the print orientation, the roughness increases: upskin areas in the X-Y plane have better surface quality than the side surfaces with an average roughness of \(\mathrm{Sa}\approx 9.1\,\mathrm{\SIUnitSymbolMicro m}\). Downskin faces and side surfaces show the same, higher roughness of \(\mathrm{Sa}\approx 14.5\,\mathrm{\SIUnitSymbolMicro m}\).
3. Despite 30 measurements per orientation, the roughness measurements of the surfaces show a large coefficient of variation (top surface: 0.22; side surface: 0.17; bottom surface: 0.11), which suggests that surfaces are currently poorly reproducible in binder jetting.
4. The main influence on the contact ratio is roughness. Therefore, the smoothest part surfaces, the top sides, achieved the relatively highest (nevertheless low) contact ratios during sinter joining (average 13.5%). Even though the resulting connection holds the parts in place, it does not create a mechanically stable joint.
5. An initial contact pressure (\(100\,\mathrm{N}\)) on green parts improves the contact ratio to about 22%. This might be sufficient for a mechanically unstressed component, but certainly not enough to create a fluid-tight connection. However, this would be particularly necessary for components with internal channels.  
Even if the carried out studies show that it is possible to join binder-jetting components using the sintering process, the resulting joint achieves neither sufficient strength nor fluid tightness. To optimize the process, loose metal powder could be introduced into the joining zone to increase the contact ratio sufficiently. Solving the problem of depowderinginternal structures in binder jetting opens up a very large field of currently impossible-to-manufacture part shapes, which strengthens the economic use of the process.  
A.R.: Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Roles/Writing--original draft, Writing--review and editing; L.S.: Data curation, Investigation, Software, Validation, Visualization, Roles/Writing--original draft, Writing--review and editing; M.M.: Conceptualization, Funding acquisition, Resources, Supervision, Validation, Writing--review and editing; F.F.: Project administration, Resources, Writing--review and editing; All authors have read and agreed to the published version of the manuscript.  
We acknowledge support by the Open Access Publication Funds of Technische Universitat Braunschweig.  
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.  
The authors declare no conflicts of interest.