Residual stresses in aluminum castings

The majority of manufacturing processes induce residual stresses in the surface of the component produced. These residual stresses can be either beneficial or in some cases detrimental to the performance of the component in question. This investigation involves the use of a previously described new low-cost method to determine the superficial residual stresses in a casting of A356 aluminum alloy. The technique concerned involves the measurement of the change in spacing of previously applied pairs of micro-hardness indentations which occurs upon stress relief. In the case of a compressive residual stress, this spacing will increase after stress relief. In estimating the strain concerned it should be noted that the equivalent,original gage length' will be the relaxed spacing measurement. This strain multiplied by the modulus of elasticity will indicate the corresponding stress for a uniaxial stress state. The special 3 bar test frames that were employed had their residual stresses measured using both a cutting and a thermal stress relief technique. From the results it can be seen that the use of cutting is probably applicable to the more localized stresses that occur in the vicinity of the cutting zone. However, the stresses that remain in the frame redistribute themselves to maintain equilibrium, and are not measurable until the cut frame is stress relieved thermally. It was noted that the levels of strain measured were unusually high. This may be related to the occasional spontaneous failure of the castings on removal from their molds. The current paper describes the results of on-going research in this area. (C) 2007 Elsevier B.V. All rights reserved.