PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 452–462 Valentin LOURY--MALHERBE/ Structural Integrity Procedia 00 (2019) 000–000

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peened. In general it is reasonable to say that laser peening generates plastic response to depths 5 times or more than conventional peening techniques [8]–[11]. This depth is the critically important parameter that enables laser peening to provide superior fatigue performance [12]–[17].

4. Laser peening and thermal exposures of wrought In718: As In718 is in the class of nickel based superalloys and used in hot section turbine components of jet engines, the focus of this phase of our work was to thermally expose the material and evaluate retention of residual stress. After laser peening a sample and generating residual stress as shown in Figure 1, the sample was exposed at 760oC in an air oven for 48 hours. The unfilled circles in Figure 2 show the resulting stress change measured by x-ray diffraction (XRD) and etching. The compressive stress to a depth of about 1 mm (0.04 inches) has relaxed by about 75% to 140 MPa (20 ksi). Although relaxed to a significant degree, this stress is still a potentially useful amount for fatigue enhancement and consistent with the diffusion of the annealing heating effect into the sample. Also very significant, the deeper portion of the original stress appears to have been retained again consistent with diffusion from the surface of the thermal relaxation. A next test was to see if adding more layers of peening to the process would be conducive to better retention of the residual stress. The filled squares in Figure 2 show the residual stress for 8 layers of peening followed by 40 hours of thermal exposure at 760oC. A small difference of about 70 MPa (10 ksi) is seen in the residual stress, that is the retained stress with 8 layers is on the order of 210 MPa (30 ksi). Again, it is useful to note that beyond the 1 mm (0.04 inches) depth the original stress appears to be fully retained.

Figure 2. Test samples of wrought In718 were laser peened or treated with LP+TME and then exposed to 760oC for 48 hours. As described in the text, the sample treated with LP+TME showed impressive retention of residual stress. 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.

The next step in our work was to evaluate residual compressive stress following successive iterations of the laser peening followed by annealing. We decided to call this process laser peening plus thermal microstructure engineering (LP + TME). In this case, Process A was defined as comprising laser peening a single layer, annealing in the air oven at 760oC for 8 hours, peening a second layer, annealing again and finally peening a third layer. The measured