PSI - Issue 31

David Liović et al. / Procedia Structural Integrity 31 (2021) 86– 91 David Liovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

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Based on midlife hysteresis loops, obtained from published experimental results of Zhang et al. (2019), at engineering strain levels of 1,2%, 1,4%, 1,6%, and 2%, engineering tensile cyclic stress – strain curve can be determined through points of maximum stress and strain at all strain levels (Fig. 3. a). Cyclic R – O material parameters (Fig. 3. b) can be found with the same procedure as monotonic R – O parameters, by using equation (2).

Fig. 3. (a) Hysteresis loops of annealed SLM Ti6Al4V tested at 2%, 1,6%, 1,4% and 1,2% strain amplitude and associated engineering tensile cyclic stress – strain curve (black); (b) Comparison of cyclic and monotonic stress – strain curves ( R – O monotonic (black) and true stress – strain curve (green), R – O cyclic (orange) and true stress – strain curve (blue)) 3. Results and discussion Despite the different orientation of the test specimens, and the different process parameters of the SLM-ed Ti6Al4V alloy monotonic mechanical properties, i.e., Young’s modulus ( E ), 0.2% offset yield strength ( R p0,2 ) and tensile strength ( R m ) do not differ significantly, as can be seen in Fig. 6. a, and Fig. 6. b. However, the results of monotonic tensile tests suggest that the orientation and process parameters have the greatest impact on reported elongation at break ( A t ) values, which are particularly low for SLM-ed Ti6Al4V alloy. According to ASTM F2924-14, the elongation of SLM-ed Ti6Al4V alloy must equal or higher than 10%, which often requires the application of heat treatment. The use of appropriate heat treatment shows the greatest impact on the elongation at break ( A t ) of the SLM ed Ti6Al4V alloy, but even in this case a minimum elongation of 10% is not achieved. Therefore, further research needs to be focused on more ductile titanium alloys, to achieve the necessary condition for their commercial application. According to Carroll, Palmer, and Beese (2015), Impact of specimen orientation on elongation at break ( A t ) may be associated with elongated columnar prior β grains along building direction, inside which the martensite phase ( α ’ ) is pronounced and grain boundary α phase which in turn provides a convenient location for damage accumulation, and consequently a preferential path for crack propagation. The influence of the orientation of test specimens and process parameters on low - cycle elastoplastic mechanical properties, as well as on low - cycle fatigue, has not yet been systematically researched. However, based on the presented values of the strength coefficient ( K ), and strain hardening exponent ( n) , it is noticeable that there is a significant impact of process parameters on the low – cycle elastoplastic behavior of the mentioned material. Cyclic softening, hardening or mixed behavior of the material under cyclic loading can be represented by displaying the monotonic and cyclic stress – strain curves on the same diagram (Fig. 4. a. and Fig. 4. b.). In all cases, the cyclic stress-strain curve is located below the monotonic stress-strain curve, which indicates that the SLM Ti6Al4V alloy has prominent cyclic softening behavior.