A Noval Concept of Combined Cold Expansion and Surface Processing of Fastener Holes Using Rotating Tool

Panaskar, N J (2013) A Noval Concept of Combined Cold Expansion and Surface Processing of Fastener Holes Using Rotating Tool. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

Bolts, rivets and location pins are commonly used in various engineering applications for assembly of components and structures. The holes produced for the assembly is susceptible to cracks due to fatigue caused by tensile loads applied. These cracks can be prevented or its propagation can be prolonged by inducing compressive residual stress in the material around the hole. Cold expansion hole is one of the techniques used to induce compressive stress zone, by inserting a tapered mandrel/pin or ball into an undersized hole. A novel application of the cold expansion and surface modification of walls of the cylindrical hole is presented. Predrilled holes of different diameters in commercially available aluminum and Al- 2014-T6 aluminum alloy plates are cold expanded using a specially fabricated tapered tool that rotates and simultaneously traverses through the hole. The tapered tool expands the hole to the desired diameter and also stirs the internal wall of the hole. In order to understand the role of frictional heating and stirring in the cold expansion of holes three mediums, namely, dry (without cooling and lubricating), metal working fluid (cooling and lubricating) and Al2O3 nanoparticles (lubricating and frictional heating) are used. The residual stress, surface roughness and hardness of the wall of the holes are measured and SEM images are captured. Significant residual stresses are induced in the material around the hole region. Significant hardness improvement with reasonable surface finish is obtained. Moreover the cold expanded holes showed considerable improvement in the fatigue life of the holes. The fatigue life is found to be directly proportional to residual stresses induced around the hole and inversely proportional to surface roughness. Hardness has a negative impact on fatigue life. The rotating tool cold expansion process is compared with conventional cold expansion process and good efficiency of cold expansion is achieved in case of Al-2014-T6 aluminum alloy material using nanoparticles, in terms of fatigue life achieved. It is observed that frictional heating and the plastic deformation have combined effect on the performance of the cold expansion of cylindrical holes.

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