Papers by Author: Ramin Ghelichi

Abstract: The effect of slight surface REM finishing, that is a chemically accelerated polishing process, in order to reduce surface roughness after shot peening has been studied on different series of specimens shot peened by varied peening parameters. Room temperature rotating bending fatigue tests have been performed on the shot peened series and the series subjected to shot peening and successive REM finishing process. The results represent how REM finishing affects the final fatigue behavior of differently treated specimens in two ways: decreasing surface roughness and removing a thin layer of material with compressive residual stresses.

Abstract: Coating deposition processes such as cold spraying are commonly employed to increase wear and fatigue resistance and consequently to enhance longevity of engineering components. Such processes typically introduce residual stresses into the coated surface, which in turn affect efficiency of coatings and play an important role in coating durability. Present study describes alteration of residual stress state and surface work-hardening of two types of aluminum coatings that are cold sprayed onto aluminum substrate, and subsequently treated by air blast shot peening (ABSP). Residual stress measurements have been made by means of X-ray diffractometer (XRD) on coated samples both before and after shot peening process. The results indicate that that the effect of shot peening is more marked in terms of surface work-hardening than of residual stresses and allow some consideration to correctly orient the choice of peening parameters in practical application.

Abstract: Surface grain refinement is a relatively new process aimed to enhance mechanical material properties. In this study Al7075-T6 bars have been shot peened with parameters much different from those of conventional shot peening. Microstructure gradient has been observed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and nano indentation tester. A fine grained layer was found on top surface of the specimens. Measurements indicate of notable improvements in cases of hardness and elastic modulus in comparison with untreated material. XRD results also show significant depth affected both in terms of residual stress and FWHM. The results imply that using optimized shot peening parameters, it would be possible to create a fine grained layer on surface of component and consequently increase fatigue life and any other property affected by grain size.

Abstract: Effects of surface defect size and the method used to obtain the defect itself on bending fatigue behaviour of nitrided and shot peened steel was studied. Two different series of 15 sandglass specimens with a microhole in the minimum section generated after nitriding and shot peening treatments were considered. In one series the microholes were obtained by controlled electro-erosion. In the other series microholes were obtained by indentation, much more invasive that electro-erosion, simulating a damage due to a mechanical impact or scratch on the surface material, common on real components during the life span. In both cases the microholes act as a pre-existent crack. The results show that defects obtained by indentation lead to a minor fatigue limit with respect of larger defects obtained by electro-erosion. This was interpreted as the modification of the residual stress field due to the mechanical indentation. On the basis of the results some consideration is formulated about generating micro-holes with one of the discussed ways.