PSI - Issue 34

Raffaele Sepe et al. / Procedia Structural Integrity 34 (2021) 172–177 R. Sepe / Structural Integrity Procedia 00 (2021) 000–000

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Nine specimens were tested under fatigue conditions at a fixed value of R = -1 and under different load amplitudes ranging between 8 and 12 kN so as to derive the Wohler diagram shown in Figure 4. For the investigated joints following characteristic behaviors were observed:  The knee point of the Woehler-curves, i.e. the transition from medium to high-cycle fatigue, is around 1·10 6 cycles.  An endurance limit of σ a, k = 268.5 MPa ( N k = 1·10 6 ) about 55% of the static yield strength was measured.  The steep slope of k = ( Δ log N )/( Δ log σ a ) = 11.88 for N < 1·10 6 cycles becomes after N > 1·10 6 cycles extremely shallow, k = 45, which means a drop of the stress amplitude of 5% from one decade to the next one.  The results are covered by the uniform scatter of the stress amplitudes T σ = 1/[ σ ( P s = 10%) / σ ( P s = 90%)] = 1:1.10 between the probability of survivals P s = 10 and 90%.  All specimens reported the failure within the HAZ.

Fig. 4. Wohler curve for steel butt joints manufactured by laser welded of AM 17-4 PH plates.

4. Conclusions The currently existing AM processes for metals allows to produce relatively small parts due to the small sizes of the building chambers and/or to the severe deformations occurring during building. The investigation reported in this document was aimed at evaluating the potential usage of LBW to join AM parts made of 17-4 PH steel and manufactured by LPBF. A SLM machine was used to manufacture 3 mm thick plates that were welded by LBW process and consequently machined to produce specimens to test under static and fatigue loading. Static tests demonstrated a null degradation of the mechanical performances due to the LBW process when comparing welded and as-built (unwelded) specimens. Fatigue tests reported an endurance limit of nearly 270 MPa. The possibility to join AM parts via laser welding to manufacture larger parts seems to be very promising and further developments along this line of research are currently in progress, e.g. to have a comparison on the fatigue resistance of welded with as-built specimens.