Deformation mechanics and microstructure evolution during microforming of metals

摘要

Deformation mechanics including dynamic strain, strain-rate and rotation of material elements and its spatio-temporal scaling behavior was studied using in situ characterization of prototypical microforming operations- Equal Channel Angular Pressing (ECAP), Indirect Extrusion (IE) and Deep Drawing (DD) across length scales (sub-millimeter and micron). Microforming devices including ECAP, IE and DD dies in plane strain condition were designed and fabricated to manifest process outcomes/anomalies in small length-scale deformations for a range of imposed strains: severe (ECAP), moderate (IE) and low (DD). This was captured by conducting in situ experiments on commercially pure metals: Ni 200, Oxygen Free High Conductivity (OFHC) Cu, Al 1100 and Pb.;Set up microforming stages capable of in-situ observation in various length scales were implemented to employ Digital Image Correlation (DIC) technique in order to quantify the mechanics of deformation particularly in deformation zone where the temperature filed captured by in situ Infra-Red (IR) thermography completed the detailed understanding of thermomechanical phenomena prevail in microforming operations. To do this, ECAP and IE devices were designed with a transparent viewing window made of Sapphire block enables the imaging of the material flow during deformation using high-speed (CCD) and IR cameras. While DD of metallic sheets was performed in a microforming setup that sits inside the chamber of Scanning Electron Microscope (SEM) enables in situ characterization of material flow behavior using SEM based DIC. Pre and post--Mortem Microstructure analysis was carried out by performing Orientation Imaging Microscopy (OIM) across the microformed machine elements aiming to correlate spatially evolved microstructures/textures across the deformation zone with the mechanics of deformation obtained by in situ observations for the given materials system. In microforming, variables such as initial microstructure, process configuration and tooling design along with the deformation process parameters are known as crucial factors that determine the deformation behavior of material and therefore the process consequences including failure characteristics and quality of microparts surface finish. In the present dissertation, the effect of process parameters and scaling was studied and the role of characteristics of prior microstructures such as grain size and its distribution, grain morphology, twin size and density, pre-existing textures, etc. and their contribution in improving or disproving the formability was delineated for different deformation geometries and material systems.;These studies revealed the strong dependence of the morphology and characteristics of plastic deformation zone (PDZ) to the process outcomes e.g. microstructure evolution, surface roughening, sudden failure, etc. which are the results of the mechanical/microscopical responses of material to the geometric confinements and strain gradients. The systematic studies of the effect of microscopic/macroscopic boundary conditions allows to determine the presence of any spatial confinement switchover in the mechanism of microscopic material response that will be eventually appeared in the quality of micro-machined components.