An assessment of residual stresses and micro-structure during single point incremental forming of commercially pure titanium used in biomedical applications

Abstract

Single point incremental forming (SPIF) is a branch of incremental sheet forming where a very small portion of the sheet is deformed plastically at any moment. The highly localized point deformation is done by a simple hemispherical tool, whose path is numerically monitored by a Computer numerical control (CNC) machine, performs this progressive extremely localized deformation. Since no die is required during forming, highly customized and user-oriented sheet metal products can be manufactured employing the process. SPIF can be readily employed in the manufacturing of customized orthopaedic implants and braces, e.g., cranial implants, ankle implants, elbow and knee support braces. The forming of these sheets through SPIF would results in the generation of residual stresses in the sheet metal. With time and other physical factors, these residual stresses would be relieved resulting in dimensional inaccuracy. This inaccuracy is highly detrimental in the case of implants and highly undesirable for supporting braces. The objective of this work is to investigate, experimentally, the state and magnitude of residual stresses on commercially pure titanium grade 2 by SPIF for biomedical applications. The important process parameters: forming angle and incremental step depth are used for this investigation in the present study. The X-ray diffraction technique was used for the experimental measurements of the residual stresses. Microstructural behaviour of the final product at different incremental step depth and forming angles are also observed by EBSD (Electron backscattered diffraction) technique. The experimental findings showed the formation of increased tensile residual stresses with an increase in incremental step depth and steepness of forming angles.