PSI - Issue 19

8

Valentin LOURY--MALHERBE/ Structural Integrity Procedia 00 (2019) 000–000

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 452–462

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Although evaluating the fatigue lifetimes of the AM In718 with and without forms of peening is of interest, the motivating application for this nickel superalloy is in high temperature applications. To that end fatigue tests were performed after thermally exposing the material to 600 o C for systematically increased durations. As seen in Figure 6 the fatigue benefit enabled by all forms of the peening, the shot, the dual shot and the laser peening, significantly dropped off after 50 hours air oven exposure at 600 o C. Going back to the residual stress measurements of laser peening stress of the wrought In718 (recognizing this was done at 760 o C and current work was motivated to be at 600 o C) we believe that thermal exposure in the range of 50 hours basically relieves most all of the residual stress for all standard forms of the peening. However the fatigue life for the Process A (LP + TME), remained unchanged at the impressively high cycle count. Again, referring back to the residual stress (wrought) measurements, this is due to only partially relieving of the protective compressive residual stress in the first millimeter of the samples and full retention of the deeper protective compressive stress.

Figure 6. Fatigue test results for AM In718 samples including results following 50 hours exposure in air oven to 600oC. Samples were tested at 683 MPa loading, 86% of yield stress. Process A(LP + TME) is able to hold full fatigue performance after the 50 hour thermal exposure. Process A is comprising laser peening a single layer, annealing in the air oven at 760°C for 8 hours, peening a second layer, annealing again and finally peening a third layer. Polished samples (the first three from the right) were hand polished to 3000 grit.

The obvious next step is to extend the thermal exposure time and of course explore further the idea of peening plus thermal exposure to even better retain residual stresses. Figure 7 summarizes the extent of our work to date where we recognized that Process A would probably begin to lose performance after longer exposures, so samples were treated