composed of many grains (crystallites) randomly oriented. If the grain size is big, fewer grains will be irradiated and therefore there will be fewer suitably oriented lattice planes fulfilling Bragg’s Law. In this case, the local crystal defect such as dislocations, vacancies, stacking faults will lead to a local fluctuation of the lattice spacing, resulting in a peak broadening [19]. This will reduce the accuracy of the X-ray diffraction methods. X-ray diffraction residual stress measurements were performed using a two-angle sine-squared-psi technique, in accordance with SAE HS-784, employing the diffraction of chromium K-alpha radiation from the (311) planes of the FCC structure of the A2618-T6 aluminum. X-ray diffraction residual stress measurements were made at the surface as shown in Fig. 14.7 and results are tabulated in Table 14.1. Measurements below the surface were attempted but were not successful because no suitable diffraction peak could be detected in the subsurface material. This is likely due to large grain size. Measurements were made in the hoop, 45 , and axial directions at 1, 2, and 3 as shown in Fig. 14.7 [22]. 14.4 Residual Stress Comparison and Discussion The residual stresses are predicted by finite element modeling and are shown in Figs. 14.8 and 14.9. It is shown that although considerable residual stresses are generated in the billet after water quenching, but majority of them are relieved after the machining process because the turbo wheel fan thickness is so small. Figures 14.8 and 14.9 show that principal residual stresses at most of the wheel locations are in the range from 15 MPa to 10 MPa, which is agreeable to strain 1 2 3 X-Ray Diffraction Fig. 14.7 X-ray measurement on the wheel Table 14.1 Residual stress measurement results by X-ray diffraction method Location Maximun (MPa) Minimum (MPa) PH ( ) 1 43 137 13.1 2 116 161 49.8 3 80 103 85.7 Fig. 14.8 Max. principal residual stress on the turbo wheel by numerical prediction 110 B. Xiao et al.
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