Estimation and Control of Friction in Bulk Plastic Deformation Process
Solomon, Christopher S
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Friction plays a significant role in bulk plastic deformation processes in controlling the tool life, formability of the work piece material and the quality of the finished product such as, surface finish, microstructure and mechanical properties. Friction causes in-homogenous deformations, leading to defects in the finished products.Excessive friction leads to heat generation, wear, pick-up and galling of the tool surface, resulting in premature failure of the tools. Computer simulations based on Finite Element Methods are being extensively used for process planning and tool designing metal working industry for bulk plastic deformation process.Material and friction models used in simulation packages are very important to have accurate process simulations. Hence it is essential to estimate the friction and understand its role on deformation of the work piece to do realistic process simulations. Friction in bulk plastic deformation processes is influenced by many factors such as velocity, temperature, contact pressure and tribological conditions such as surface roughness, lubrication etc. Among the above factors, Surface Roughness and Surface Topography (ST) of the die material are the important parameters that influence the friction between the dies and the work-piece. The abbreviation “ST” is used for surface topography. Transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and ploughing, are controlled by the ST. Ploughing component is mainly the frictional resistance caused by asperities of hard surface ploughing through soft material. The force required for plastic flow of softer material represents the ploughing friction component. Adhesion component of friction is due to the cold welding/adhesive bond occurring in the real contact area of asperities. The force required to shear the adhesion junctions formed at the interface represents the adhesion component. Though surface characteristics such as roughness were dealt with by many researchers, the influence of surface topography on friction and microstructure evolution in bulk plastic deformation is still to be understood well.Though the work done by Menezes et al. has shown that friction is influenced by the surface roughness, ST and transfer layer, but does not link this ST to the micro structural evolution of the material.Friction influences the strain and strain rates imparted to the deforming material. The strain and strain rate (apart from the temperature) imposed on the deforming material would in turn influence the microstructural evolution of the work-piece. Thus, for application to industrial scale it is important that the influence of friction on the bulk deformation and microstructural evolution, if any, be understood.Further, the techniques used by Menezes et al. for generating the STs are very difficult to be adopted in industrial scale. The present thesis addresses the following three issues on the possible influence of friction in metal forming. • Use of surface generation techniques that can be easily adapted at the industrial scale. • Role of ST on friction during room temperature and high temperature deformation. • Role of this friction on the microstructural evolution during bulk plastic deformation of Aluminium alloys.