## 5 Conclusions  
This study investigated the use of SLM technology in the production of aviation hydraulic parts. The results of the static tensile tests indicated that parts produced by SLM achieved similar tensile strength to the maraging steel 18 Ni1400 produced in a conventional manner. Comparison of the tensile fatigue results with the literature data revealed that the samples made with the additive technologies were broken at a lower load. Based on the test results at 300,000 cycles, it can be concluded that the AM samples are approximately 16% less resistant to fatigue compared with the weakest maraging steels. A Goodman diagram was plotted to visualize the impact of the building orientation vs. the load character on the fatigue strength of the AM specimens. Directions of load, stress, and building orientation in the samples tested for tensile fatigue were 0 degrees with respect to the specimen's longitudinal axis, and these specimens achieved a lower number of cycles to failure in comparison with pressure fatigue specimens. In contrast to the tensile testing, the pressure acting on the internal wall of the sleeves generated circumferential hoop stresses at 90 degrees to the specimen's build direction. The analysis of the fatigue tests shows that the fatigue limit cannot be strictly determined and is highly dependent on the load vs. the built direction. These findings are consistent with other research and relevant literature [24,44]. The application of additive technologies in aviation may completely redefine the design and manufacture of advanced hydraulic aircraft control systems; however, the design process should be well integrated with the manufacturing process. The build orientation and character of load, as seen in the Goodman diagram in Figure 10, can be used as a tool for designing parts for fatigues. The authors prepared this diagram based on experimental data, and it is valid for steel 1.2709. SLM technology allows the manufacture of components with functional and complex internal structures, such as channels and flow-through bores, with weights decreased by up to 50%. All dual servo valve ports were printed and directly connected with the external hydraulic ports on the actuator housing by complex internal galleries, allowing full redundancy of the system and complete mechanical separation of the flow paths [42]. Changing the cross-sectional area of the internal ports enabled the packing of two hydraulic servo valves and anti-jam control sleeves into a center gland housing with an unprecedented level of integration. This approach to designing hydraulic parts could revolutionize fluid  
Figure 17: The actuator response to a triangle-shaped input. power systems and significantly increase their potential in the aerospace industry. The use of 3D printing allows for significant reductions in the number of parts required for larger assemblies. The manufacture of parts by means of the technology in question decreases the usage of the target material, and thus the waste material (the turnings) is reduced to a minimum. Conceptualization, K.W., J.S., and P.B. ; methodology, K.W., J.S., and P.B. ; software, K.W., J.S., and P.B. ; validation, K.W., J.S., and P.B. ; formal analysis, K.W., J.S., P.B., and P.R. ; investigation, K.W., J.S., and P.B. ; resources, K.W., J.S., and P.B. ; data curation, K.W., J.S., and P.B. ; writing--original draft preparation, K.W. and P.R. ; writing--review and editing, P.R. and T.R. ; visualization, K.W., J.S., P.B., and P.R. ; supervision, P.B., P.R., and T.R. ; project administration, P.B. ; funding acquisition, P.B. All authors have read and agreed to the published version of the manuscript. The article has been realized within the framework of the project New generation of the hydraulic cylinder FBW made with application of Additive Manufacturing and machining technology at low temperatures (cryogenics)--co-financed by the INNOLOT Program 1/1.2/2015/POIR; measure 1.2: sectoral R&D programs 2014-2020, project No. POIR.01.02.00-00-0010/15. Not applicable.