Study on the effect of the surface treatment on the residual stress gradient in silicon carbide (SiC) reinforced aluminum metal matrix composites

Silicon carbide (SiC) whiskers and particle-reinforced aluminum alloys show promise as metal matrix composites (MMC) stiff and in high-strength lightweight applications. When a MMC is cooled down to room temperature from the fabrication or annealing temperature, residual stresses are induced in the composite as a result of the mismatch of the thermal expansion coefficient between the metal matrix and fibers. The magnitude of the residual stresses has a very important effect on the yield stress and fatigue strength of the MMC. Many techniques are available for residual stress measurement on mechanical parts. X-ray diffraction, bending deflection, and hole drilling are three techniques that are commonly used. This paper discusses a study on measuring residual stress on MMC components by the incremental hole-drilling and X-ray diffraction methods. The modified hole-drilling method is used for measuring the macroscopic residual stress gradient in depth, and the X-ray method is used for measuring surface residual stresses. New developments have shown that the strain measured on the surface during incremental drilling can be related to the residual stress gradient. Finite element software has been developed for the calculation of calibration coefficients for the hole-drilling method. Some special aspects of this technique, drilling system, and stress gradient will be presented. Different aluminum matrices (2024, 2124, and 6061) and SiC fiber proportions have been tested. The effects of heat treatment, machining, and shot-peening treatment on the residual stress distribution of these materials will be analyzed and compared. Comparison between measurement with the X-ray diffraction method and the incremental hole-drilling method will also be discussed. This study shows that it is possible to optimize the residual stress distribution of MMC materials with adequate posttreatment.