Rotating Tool Cold Expansion

Motali, B K and Sharma, Abhay (2013) Rotating Tool Cold Expansion. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

Many engineering parts in industries are assembled by rivets, bolts, pins etc. These joints are more vulnerable to fatigue loadings and a crack is emanated. As structure is in use crack starts propagating over a period of time. Crack propagation can be obstructed by inducing a compressive residual stress zone around the vulnerable parts. Cold expansion hole (CEH) is one of the techniques used to induce compressive stress zone, by inserting a tapered pin or a large diameter ball into an undersized hole. As the oversized object is inserted into the hole, the surrounding material is elastically deformed and when the object comes out from the other side, the expanded material springs back to form tangential compressive residual stress around the hole. In most of the cases the crack origins on the surface, a good surface finish is also an important parameter for increasing the strength at the surface and stop crack emanation. However, good surface finish is not achieved by CEH. A novel technique called rotating tool cold expansion (RTCE) technique is developed where a tapered mandrel is inserted into an undersized hole besides rotating it simultaneously. The tool rotation breaks down the coarse grains into fine grain structure achieving good surface finish. In this method, RTCE is performed with three undersized diameters, namely, 9.2, 9.5 and 9.9 mm diameters to a maximum of 10 mm diameter. The tool material is high speed steel and the work piece material is commercial aluminium. The induced residual stresses are measured using X-ray diffraction method and compared for three different Cold expansion holes. Due to the rotation of the tool, heat is generated because of both friction at the interface and plastic deformation. A 3-D thermo-mechanical analysis is performed in ANSYS FEA for different values of coefficient of friction and plastic deformation rate and validated with experimental results. Compared to the degree of cold expansion through conventional techniques (upto 4%) the proposed technique is capable to produce good results up to 8.69% of cold expansion. Role of heat generated through friction and plastic deformation is critical to attain the best possible result, equal contributions of both are important.

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