Fundamentals of mechanical working

Abstract

Classification of mechanical forming processes and their main objectives. Hot working and cold working, and their comparison. Cold-work-anneal cycle and temperature limits for hot working. Warm working, its purpose and advantages. Temperature change during working of deforming metal, depending on its ideal plastic deformation, friction at its interface with tools or dies, and heat transfer between them. Effects of strain rate in working processes. Effect of varying pressure and strain rate on allowable hot working temperature range. Friction: Coulomb’s law of sliding friction and factors affecting Coulomb’s coefficient of sliding friction (COF). Shear friction factor and sticking friction. Maximum value of COF under sticking condition according to Von Mises’ and Tresca yielding criteria. Difference and advantages of shear friction factor model over Coulomb’s model of friction. Evaluations of friction factor and COF by ring-compression test. Adverse as well as beneficial effects of friction on mechanical working. Material pickup on tools. Functions and characteristics of a lubricant. Lubrication mechanism: hydrodynamic or full-fluid or thick-film lubrication, boundary lubrication, mixed-film lubrication, solid lubricants and melting solids. Mechanics of working process: slab method, uniform-deformation energy method, slip-line field theory, upper- and lower-bound solutions and finite element method. Slip-line field theory: slip lines, Hencky’s slip-line equations, stresses and slip lines at the boundaries of a plastic body, simple state of stress, Hencky’s first theorem, numerical method of solution, application of slip-line field to static system and steady motion. Upper-bound technique: derivation of its equation, its solutions for indentation of a semi-infinite slab and for compression. Deformation-zone geometry in terms of reduction in area of work-piece for different deformation processes. Anisotropy of mechanical properties: crystallographic texture and mechanical fibering. Problems and solutions.