## 1 Introduction
### Prior Work on Sinter-Joining Processes for Powder Metallurgical Manufacturing  
To date, there has been no published research on the sinter joining of binder-jetting components. In MIM, on the other hand, in which components are molded by injecting a mixture of metal powder and a thermoplastic binder into a tool, the limits of part sizes and part complexities that can be produced using tools were reached several years ago. Thus, sinter-joining methods have already been developed in MIM and are being used in economically relevant applications [10; 11]. After the debinding step, MIM components have a density of approximately 60% [12], which should make them comparable to binder jetting green parts during sintering. In addition to joining processes that are MIM-specific and not transferable to other processes, such as simultaneous injection of different feedstocks into a mold [13; 14; 15] or overmodling of steel inserts [16], there are processes that could in principle also work in binder jetting. One possibility of MIM parent joining was demonstrated by [PERSON] et al. For this purpose, MIM green parts were joined using a powder interlayer consisting of a mixture of 10 wt% cooking oil and iron powder. The oil was added for its adhesive effect, since loose metal powder cannot be applied to complex structures with a homogeneous distribution and uniform layer thickness, according to [PERSON] et al. However, sintering temperatures above 1250 degC lead to degradation of the hydrocarbon compounds of the oil, resulting in undesirable carburization of the base material [17]. This can even lead to local melting of the material by exceeding the eutectic temperature. In addition to the aforementioned processes, grazing alloys have been developed that are placed in the joining zone to create a material-to-material bond. In this case, conventional brazing alloys cannot be used because the brazing material is drawn into the open-pored green part after reaching melting temperature due to capillary effects [9]. Therefore, special solders have been developed that react and solidify with the steel of the green part after reaching the melting temperature, so that capillary action is interrupted [8]. Disadvantages of brazing are that the joint is significantly weaker than the base material [18] and the gap height between the components must be precisely adjusted [19; 20]. Further joining tests were carried out by [PERSON] using microcomponents that were to sinter together by mere contact. It was shown that the success of the joining process strongly depends on the initial contact of the joining partners and that the initial contact depends on the shape of the joining surface, the accuracy of the manufacturing process and the arrangement of the green parts before the joining process. The shape deviations of the components to be joined correlated negatively with the achievable joint strength [21]. This was also shown by [PERSON] et al. [22]. Roughness of surfaces to be joined is closely related to the initial contact of the surfaces. On a microscopic level, an increased roughness of the joining surfaces leadsonly to punctual contacting, resulting in an insufficient joint. The roughness thus directly influences the strength of the joint [23]. Therefore, it is absolutely necessary to investigate the surface finish and the resulting joining quality when developing a joining method for a new manufacturing process.