# Monotonic and Fatigue Properties of Steel Material Manufactured by Wire Arc Additive Manufacturing
## 5 Conclusions and Outlook  
Monotonic and cyclic properties for steel material with an average tensile strength of \(\mathrm{R_{m}}=536\) MPa were determined in this study. These properties show a slight anisotropy but do not seem to be noticeably distributed between layer and layer boundaries. The monotonic properties of the investigated WAAM material differ significantly from the values expected from the same weld filler material in conventional welding use. The measured cyclic properties related to the tensile strength are in better agreement with cast steel than with rolled and forged steel. Optical strain measurements over several layers show that the plastic strain amplitudes are not distributed homogeneously. The recognizable areas with homogeneous behavior are larger than one single layer. This influences the local stresses and strains that occur in a WAAM component, which makes them challenging to predict. However, knowledge of the local stress and strain amplitude is one of the most important influences on a fatigue life calculation. This aspect needs to be examined more closely in the future. The test results for the investigated material and manufacturing process show that it is possible to increase the interlayer temperature from 150 to 300 \({}^{\circ}\)C without decisively reducing the performance in terms of monotonic and cyclic strength. The yield strength decreases slightly when the interlayer temperature is increased. Overall, a higher interlayer temperature can lead to a considerable reduction in manufacturing time (e.g., factor 2 for the cuboids manufactured for this study), more flexible planning of the process, and, thereby, improved economic efficiency of the WAAM process. It must be noted that the results presented here are only a first step towards the characterization of medium strength WAAM materials. The manufacturing process itself must be pushed to the limits of what is feasible in order to be as efficient as possible. For example, the question must be answered of at which interlayer temperatures--even higher than the ones investigated here--a component cannot be manufactured anymore, since the melt no longer solidifies with sufficient dimensional stability. In addition to the strength of the base material, other influencing variables such as surface texture and mean-stress sensitivity must be taken into account in order to be able to carry out a qualified fatigue assessment of WAAM components. The following are available online at [[http://www.mdpi.com/2076-3417/10/15/5238/s1](http://www.mdpi.com/2076-3417/10/15/5238/s1)]([http://www.mdpi.com/2076-3417/10/15/5238/s1](http://www.mdpi.com/2076-3417/10/15/5238/s1)), Table S1. Individual test results of strain-controlled tests, Figure S1. Test schemes for stress-controlled staircase tests according to DIN 50100, Drawing 1. Technical drawing of specimen for fatigue tests, measurements in mm, Drawing 2. Technical drawing of specimen for monotonic tensile tests at room temperature, measurements in mm, Drawing 3. Technical drawing of specimen for monotonic tensile tests at elevated temperatures, measurements  
Figure 15: Estimated cyclic stress–strain curves for steel (**a**) and cast steel (**b**) for \(\mathrm{R_{m}}=538\) MPa and individual test results from Figure 11.  
in mm, Drawing 4. Technical drawing of specimen for strain measurements with digital image correlation, measurements in mm. Conceptualization, M.W. and K.T. ; manufacturing of raw WAM material, K.T., S.K., and V.W. ; monotonic tensile tests at room temperature and cyclic tests, M.H., L.M., and A.E. ; monotonic test at elevated temperatures, M.L. and V.W. ; local strain measurements with DIC, C.L., S.H., and L.M. ; writing--original draft preparation, M.W. ; writing--review and editing, M.W., C.L., S.H., L.M., M.H., and K.T. ; supervision, A.E., V.W., and S.H. All authors have read and agreed to the published version of the manuscript. This research received no external funding.