Key Engineering Materials Vols. 611-612

Abstract: A first approach of tool-chip interface behaviour for high-speed machining modelling has been carried out by Al Brocail and 2010 and an empirical friction law has been first determined. This law has been established for high temperatures (initial sample temperature equal to 750 K) and low sliding velocities (less than 0.5 m.s^-1) and an extrapolation has been considered for higher velocities. This article intends to determine an empirical friction law for low temperatures (ambient) combined with high sliding velocities (up to 1.5m.s^-1) by means of a tribometer developed by Meresse and Al. A new experimental device is designed to carry out several tests and simulate the friction behaviour at the tool-chip interface. The experimental results are compared with a numerical model and an iterative method is used to minimize the error between experimental and numerical simulations on normal and tangential forces. This method allows to recover a Coulomb friction coefficient which is associated to local pressure, temperature and sliding velocity. The completion of several tests provides an empirical friction law for high sliding velocities and low temperatures.

Abstract: The composition of different materials and their specific properties like tensile strength and toughness is one way to achieve workpiece characteristics which are tailored to the later application. Another approach is the subsequent local heat treatment of workpieces made of homogeneous materials. However, both ways are costly and go along with several subsequent process steps. Therefore, mono-material workpieces which were manufactured by thermo-mechanical forming processes may provide such tailored properties in the form of functional gradations. Furthermore, the process chain is shortened by the combination of forming and heat treatment, but nevertheless machining processes are still needed for proper workpiece finish. This puts the challenge of varying process conditions due to hardness alterations within a single process step, e.g. turning. In addition to experimental investigations simulative analysis techniques are desired to evaluate mechanical as well as thermal loads on tool and workpiece. In the case of FE-based microscopic chip formation simulations proper material behaviour needs to be determined with respect to material hardness. This paper describes the approach of fitting Johnson-Cook material parameters as a function of workpiece material hardness. In order to achieve realistic stress states within the process zone, this approach considers the yield strength as a linear function of the hardness. It is shown how the hardness influences the cutting conditions and how the Johnson-Cook parameters are identified. Then these parameters are validated in three-dimensional simulations of exterior dry turning by comparison of simulated process forces and chip formation to experimentally achieved ones.

Abstract: Inconel 718 superalloy is one of the difficult-to-machine materials which is employed widely in aerospace industries because of its superior properties such as heat-resistance, high melting temperature, and maintenance of strength and hardness at high temperatures. Material behavior of the Inconel 718 is an important challenge during finite element simulation of the machining process because of the mentioned properties. In this regard, various constants for Johnson–Cook’s constitutive equation have been reported in the literature. Owing to the fact that simulation of machining process is very sensitive to the material model, in this study the effect of different flow stresses were investigated on outputs of the orthogonal cutting process of Inconel 718 alloy. For each model, the predicted results of cutting forces, chip geometry and temperature were compared with experimental results of the previous work at the different feed rates. After comparing the results of the different models, the most suitable Johnson–Cook’s material model was indentified. Obtained results showed that the selected material model can be used reliably for machining simulation of Inconel 718 superalloy.

Abstract: The evolution of carbon/epoxy composites use in aeronautics requires a better comprehension of the machining conditions influence on these materials. This study aim is to establish, based on the experimentation, the relationship between machining conditions and the behavior of drilled 2D and 3D carbon/epoxy composites. Two drill geometries, seam introducing and a range of cutting speed and feed have been tested. The effect of each parameter has been assessed in terms of thrust force, moment (recorded during machining) and defects (performed by macroscopic analyses and quantified using delamination factor F_d). Experimental results have shown significant influences of feed and drill geometry on delamination reduction. The use of a spur drill and a low feed generates minor defects and produces the best results. Furthermore, stitching helps reduce damage inside the hole.

Abstract: Machining is a process implying extremely high coupled thermo-mechanical stresses. The workpiece mechanical properties decrease with the temperature generated during the process and that temperature has a direct influence on wear intensity undergone by the tool. In the case of a drilling operation, the temperature generated by the cutting process can lead to metal burr formation and/or composite matrix degradation by burning. When these two materials are used in the form of a sandwich-type stacking, the temperature attained in the metallic part can cause new defects such as: i) a difference between the diameters measured in each material and ii) organic matrix damages due to heat diffusion from the metal towards the CFRP layer. Temperature reached at the tool/workpiece interface is difficult to measure during drilling operation, due to its enclosed configuration; numerical simulation is therefore a good alternative to access to this information. The purpose of this study is to develop and carry out numerical simulations in order to estimate the workpiece thermal field generated during drilling. The simulations are validated by comparing simulated and measured temperatures at 4 mm from the holes wall. This method is applied to evaluate thermal field generated during drilling (with chip removing cycles) of CFRP/Aluminum alloy stacks. The influence of the drilling kinematics on the workpiece thermal field is also investigated.

Abstract: Residual stress is one of the most important surface integrity parameter that can significantly affect the service performance of a mechanical component, such as: contact fatigue, corrosion resistance and part distortion. For this reason the mechanical state of both the machined surface and subsurface needs to be investigated. Residual stress induced by dry and cryogenic machining of hardened AISI 52100 steel was determined by using the X-ray diffraction technique. The objective was to evaluate the influence of the tool cutting edge geometry, workpiece hardness, cutting speed, microstructural changes and cooling conditions on the distribution of the residual stresses in the machined surface layers. The results are analysed in function of the thermal and mechanical phenomena generated during machining and their consequences on the white layer formation.

Abstract: Machining of advanced aerospace materials have grown in the recent years although the difficult-to-machine characteristics of alloys like titanium or nickel based alloys cause higher cutting forces, rapid tool wear, and more heat generation. Therefore, machining with the use of cooling lubricants is usually carried out. To reduce the production costs and to make the processes environmentally safe, the goal is to move toward dry cutting by eliminating cutting fluids. This objective can be achieved by using coated tool, by increasing cutting speed and by improving the product performance in term of surface integrity and product quality. The paper addresses the effects of cutting speed and feed on the surface integrity during dry machining of Waspaloy using coated tools. In particular, the influence of the cutting conditions on mechanical power consumption, the tool wear and some important indicator of the surface integrity (surface roughness, affected layer, microhardness, grain size and microstructural alteration) were investigated. Results show that cutting conditions have a significant effect on the parameters related to the surface integrity of the product affecting its overall performance.

Abstract: The main objective of this paper is to identify the thermo-mechanical behaviour of the spheroidal graphite (SG) iron EN-GJS-700. In the first instance, compression tests are carried out using Gleeble 3500 system enabling a precise control on testing temperature and strain rate. The effects of these testing parameters on the SG iron behaviour are studied. Through this, the occurrence of dynamic recrystallization phenomenon is highlighted. Specific rheological models based on metallurgy are introduced. Finally, shear tests on hat shaped specimens are performed to reach higher strain rates.

Abstract: An effect of micro-textured tool on a reduction in cutting force has reported in recent years. Some researchers have discussed that the microstructures contribute to the reduction in friction force at tool-chip interface by acting as cutting fluid reservoirs. In this study, on the other hand, a reduction in cutting force achieved by using the textured tools was confirmed even under dry condition. The results of orthogonal cutting tests employing AISI 1045 steel with non-textured and textured tools indicated that the cutting force and calculated friction coefficient at the tool-chip interface definitely reduce at relatively high cutting speed. For this friction reduction effect, effective texture patterns and optimal area ratio of concave portion to total surface were empirically suggested. Moreover, from results of the tests using tools with sectionally textured surface, it was revealed that the texture only around the position in which the chip flow separates from the tool rake face is effective to reduce the cutting force. Close observation of both tool edges and formed chips under various cutting tests indicated that changes in geometry and dimensions of dead metal formed around the cutting edge is essential for the reduction in cutting force. Finally, a mechanical cutting model having an agreement with the experimental results was discussed by employing the slip-line field method.

Abstract: Nickel super-alloys are characterized by: high temperatures resistance, high hardness and low thermal conductivity. For this reason they are widely used in critical operating conditions. However, due to their excellent properties, nickel super-alloys are hard to machine. Tool wear is a major problem in nickel super-alloy machining; the high temperature at the tool rake face is a principal wear factor. Flank wear is the most common type of tool wear; it offers predictable and stable tool life evaluation. In this work, the authors present a flank wear evaluation in Inconel 718 turning, in order to develop a predictive model for CAM optimization. An appropriate database has been developed thanks to an experimental activity (V_B as a function of: the cutting time T, cutting speed S and feed rate F). The objective of the optimization procedure is to maximize the Material Removal Rate (MRR) under the constraint represented by the flank wear limit. The developed procedure operates directly on the part program code, using the original one as starting point for the application of the knowledge about the wear behaviour. After the optimization phase the given output is represented by a new part program code obtained in accordance with: the maximum MRR within the respect of the wear limit. s and tables etc.