Inconel 718 (a nickel based superalloy) is widely used in the aerospace industry for the manufacture of aeroengine components that are subjected to relatively high temperature and stresses during operation, such as turbine disks, shafts, compressor blades and combustion chamber casings. Aeroengine manufacturers have recently begun to evaluate alternative production methods for these parts as opposed to the conventional route, which can involve arduous and environmentally hazardous chemical milling processes. High speed machining (HSM) involving the use of end / ball end mills is one such technology that has been earmarked as a potential substitute after successful trials on titanium components yielded significantly improved productivity and component quality. Metal cutting research has traditionally been a laboratory-based activity requiring massive investment in equipment, manpower, time and workpiece materials. Over the course of the last two decades however, computer based techniques such as finite element (FE) modelling have been shown to provide an acceptable and cost effective tool for metal cutting process simulation. Unfortunately, up until recently, the majority of published FE work on metal cutting has been confined to 2D orthogonal turning. Following an extensive literature review on analytical modelling techniques and FEM in metal cutting together with HSM technology, details are given of the development of a Lagrangian based, 3D finite element model to simulate the high speed ball nose end milling of Inconel 718 using the commercial FE package ABAQUS Explicit. Preliminary 2D, plane strain analysis of orthogonal turning was carried out utilising ABAQUS Standard followed by 3D turning using ABAQUS Explicit as a precursor to full scale milling research. ABAQUS Standard was deemed to be inappropriate for segmented / discontinuous chip and 3D formulations due to the nature of its code, which was not suited to highly non-linear and large deformation type problems. Outputs from FE models were validated against corresponding experimental data. Chip morphology studies were performed using an explosive quick stop device for orthogonal turning. This was used to investigate the effect of cutting speed (20,50 and 80m/min) and tool rake angle on chip formation, but also to obtain input 'parameters such as shear angle & deformed chip thickness for analytical modelling of cutting temperatures as a comparison with predicted FE results. Force measurements were carried out using Kistler piezoelectric dynamometers while chip surface temperatures were measured using infrared techniques. Flow stress data ofInconel 718 at elevated strain rates (up to lOOS-I) and temperatures (up to 850°) were determined through uniaxial compression tests on a Gleeble 3500 thermomechanical simulator. Based on this Gleeble data, material property constants for the overstress power law constitutive model were calculated. This was then applied in the finite element models in order for flow behaviour at high strain rates and temperatures to be reasonably defined. The 3D turning model in general provided reasonable predictions of tangential cutting forces and chip surface temperatures, with a difference of approximately 14% and 5% respectively to experimental results. The model however failed to describe the segmented chip morphology that was expected when turning at 50 & 80m/min. This was due to the lack of a user subroutine containing a suitable fracture criterion to define the adiabatic shear localisation and failure within the chip.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580322 |
| Date | January 2003 |
| Creators | Soo, Sein Leung |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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