## 1 Introduction  
Metal binder jetting (MBI) is a powder bed-based additive manufacturing (AM) process in which metal powder is deposited layer by layer onto a build platform. An organic binder is selectively injected into this powder bed within a predetermined geometry. This is followed by thermal curing of the binder, depowrdering, debinding, and sintering. During sintering, the MBI green bodies are consolidated, which is accompanied by a linear shrinkage of about 15-20% [1; 2; 3]. Sometimes, anisotropic shrinkage behavior is reported [4; 5], which could be attributed to the layered structure and a not-optimally adjusted binder saturation [1; 6]. MBI components can already be found in many applications in aerospace, energy, and automotive sectors [7; 8; 9; 10]. Compared to beam-based AM technologies, such as powder bed fusion with laser beam (PBF-LB) or electron beam (PBF-EB), the MBJ process offers several advantages: low residual stresses since no high temperature gradients occur in the process, a significantly reduced need for support structures, and easy up-scaling of the process for larger production quantities due to high build-up rates of more than 2000 cm\({}^{3}\)/h. However, achieving almost complete densification during sintering is still a challenge, and MBJ components often have lower mechanical properties than conventionally manufactured parts due to their residual porosity [11]. This is known from other sinter-based technologies such as metal injection molding (MIM) or powder compaction. While these drawbacks limit the range of application, they can be overcome to a certain extent by post-treatment with hot isostatic pressing (HIP) [12; 13]. HIP is a widely established manufacturing process in powder metallurgy, in which powder materials enclosed in a metallic capsule are consolidated to absolute density by the simultaneous application of high pressure and high temperatures [14]. In addition, HIP is very commonly used to post-densify components that may exhibit process-induced pores and defects, as is often the case with cast components. Highly loaded safety-relevant components are HIP post-densified by standard. One example are turbine blades for aircraft engines. Additionally, for materials produced by AM processes such as laser powder bed fusion (PBF-LB), HIP post-densification can provide remarkable improvement in reliability and mechanical properties [15; 16; 17; 18; 19]. To achieve densification without encapsulating the materials in the HIP process, no open porosity is permitted in the component, as the applied gas pressure would also act within the pores and these could not be closed. Only inner pores and thus a closed porosity can be densified. At a so called critical density, pore closure appears during sintering, which means that pores are no longer interconnected. According to theoretical models and experimental verifications, the critical density ranges from 89 to 94% [20]. The residual porosity of components produced by MBJ is exactly in this range between open and closed porosity and known to scatter locally [21]. Therefore, a high sinter density and a residual porosity not connected to the surface are highly important for achieving successful HIP densification of MBJ parts. Even though additive manufacturing has come a long way in recent years, the materials available are still limited. For conventional manufacturing using subtractive processes, there are well over 2000 different types of steel; in additive manufacturing, the range of commercially available steels for AM processes is much smaller. Within the portfolio of the common AM machine manufacturers, the average number of qualified materials offered for the PBF-LB and MBJ processes is around 30, of which, between 12 and 14 are steel grades. One reason could be that many materials are not directly suitable for the AM process because, for example, they tend to crack in laser-based additive manufacturing or have poor sintering properties in sinter-based additive manufacturing.