Papers by Author: C. Pilé

Abstract: The aim of this work is to use ultrasonic shot peening, a mechanical surface treatment derived from conventional shot peening, in order to increase the fatigue life of TiAl alloys. The goal of this treatment is to generate compressive superficial residual stresses which are aimed to enhance fatigue crack initiation and growth resistance. For this purpose, different ultrasonic shot peening tests have been carried out on Ti-48Al-2Cr-2Nb samples in order to optimise treatment conditions. The first results reveal that it is possible to generate very high stress levels (»1000 MPa) beneath the surface, far much higher than the tensile yield stress of the material which is in the range 350-600 MPa. Such a phenomenon was also observed in ultrasonic shot peened iron or stainless steel and seems to be associated to the creation of a new homogeneous and nanometric structure below the surface of the alloy [1, 2]. In the light of these encouraging results, the shot peening treatment was optimised in terms of residual stresses profile and surface quality. The influence of different parameters of shot peening like the treatment time, the shot diameter as well as the specimen-sonotrode distance were studied. S-N curves were realised on polished specimens as well as on shot peened samples in order to study the effect of the treatment on the fatigue life of this intermetallic alloy.

Abstract: The aim of this work is to reach a better understanding of the ultrasonic shot-peening process and, in particular, the evolution of the shot speed distribution. A simple 1D modelling of the interaction between the shots and the sonotrode is carried out. The impact is considered as inelastic with an energy absorption that depends on the speed of the shot. It is found that after about 10 interactions (» 1s) the speed distribution in the chamber follows a Maxwell-Boltzmann distribution, which is the distribution found in a perfect gas at equilibrium. The influence of various process parameters such as the sonotrode amplitude, the vibration frequency on the average speed and on the Almen intensity is studied.