**4. Conclusions**

The static and dynamic mechanical behaviour of PBF-LB/M- and hybrid-PBF-LB/Mbuilt lattice structures of maraging tool steel, with the hybrid approach consisting of standard PBF-LB/M combined with in situ high-speed milling, was studied. In addition, the effect of different heat-treatment processes, namely solution treatment combined with an aging treatment process as well as hot isostatic pressing, have been examined, comparing the PBF-LB/M-built and machined specimens. BCC unit cells with a relative density of about 13% were chosen as lattice structures to evaluate the particular influence of the superior surface quality of the in situ milled lattices. For the evaluation of the mechanical load behaviour, static testing has been performed, showing a typical ductile load behaviour for the as-built specimens, while the heat-treated specimens perform with a strong but brittle behaviour, developed due to the post processing.

However, the difference between the PBF-LB/M-built and the hybrid-built specimens persists whether the components are post processes or in the as-built state. As a result of the subsequent machining, the maximum load of the milled specimens exceeds that those generated solely with the PBF-LB/M. The in situ milling can eliminate surface defects and superficial cracks, increasing the mechanical behaviour within the static testing. Further, the machined specimens exceed the PBF-LB/M-built at dynamic testing, as an offset of cycle-numbers is persisting nearly continuously over the different sections of fatigue testing. Partially, milled specimens can be assigned to the VHCF, while the PBF-LB/M-built components fall into the range of HCF at the same load. Overall, this study outlines a comprehensive evaluation of the mechanical properties of hybrid-built lattice structures for static and dynamic testing, highlighting the superior load behaviour for the hybrid approach, combining Laser powder bed fusion and in situ high-speed milling as an innovative manufacturing technology.

**Author Contributions:** Conceptualization, D.S. and R.H.; methodology, D.S.; validation, D.S. and R.H.; formal analysis, D.S.; investigation, D.S.; resources, R.H.; data curation, D.S.; writing—original draft preparation, D.S.; writing—review and editing, D.S., C.E. and R.H.; visualization, D.S.; supervision, C.E. and R.H.; project administration, R.H.; funding acquisition, R.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
