Residual stresses after orthogonal machining of AlSl 304: numerical calculation of the thermal component and comparison
- PDF / 777,389 Bytes
- 8 Pages / 630 x 792 pts Page_size
- 24 Downloads / 181 Views
INTRODUCTION
M A N Y properties are dependent on the geometrical, mechanical, and structural surface and near-surface conditions of a component. The surface and the superficial surface layer can be characterized by their topographies, e . g . , Reference 1, their microstructure/hardness profiles, e.g., Reference 2 and their residual stresses. These characteristics are strongly influenced by machining operations and their parameters.t3] This article concentrates on the residual stress state caused by orthogonal cutting; the geometry and the forces and angles during orthogonal planing are shown in Figure 1. Residual stresses strongly influence many material properties and therefore have been of interest over the last 10 y e a r s . [4-7] The near-surface residual stresses are produced during machining primarily by mechanical and thermal impacts: (1) The mechanical impact is generally said to cause compressive residual stresses. This is true for near-surface plastic elongation which is constrained by the undeformed bulk material and causes compressive residual stresses. However, there are modifications of the surface layer caused by the mechanical impact of the tool which produce tensile residual stresses. (a) Strong work hardening and a considerable increase in microstructural defect density close to the surface during machining causes greater elastic relaxation upon unloading of a thin surface layer compared with the underlying workpiece. This relaxation is constrained by the bulk material, and tensile residual stresses are formed, tS] (b) Thermomechanical elastoplastic stress analyses of orthogonal machining using the shear plane as a boundary condition tg] have shown that there are two plastic deformation zones in the surface layer during cutting. A large compressive plastic
CHRISTOPH WIESNER, formerly Research Assistant with the Laboratoire de Mrtallurgie Mrcanique, Ecole Polytechnique Frdrrale de Lausanne, MX-D Ecublens, 1015 Lausanne, Switzerland, is Senior Research Engineer with TWI (The Welding Institute), Engineering Department, Abington Hall, Abington, Cambridge CB 1 6AL, England. Manuscript submitted April 5, 1991. METALLURGICAL TRANSACTIONS A
deformation zone is formed ahead of the advancing tool and a tensile plastic deformation zone behind it. This is illustrated in Figure 2 from Reference 9. The material in the surface layer is thus twice plastically deformed (with changing signs) during machining. Cutting conditions used for the calculations in Reference 9 cause greater effective plastic conpression ahead of the tool than tensile plastic deformation behind the tool, as can be seen in Figure 2. Therefore, a wake of plastically compressed material is left in the surface layer. The plastic deformations are constrained by the undeformed bulk of the workpiece which produces tensile residual stresses. (2) Strong thermal gradients in the workpiece during cutting produce thermal expansion of the near-surface region which is constrained by the cool bulk regions. This can lead to compressive plastic deformat
Data Loading...
