Key Engineering Materials Vols. 554-557

Abstract: Titanium alloys, mainly because of their poor thermal conductivity, need to be cut at relatively low cutting speeds, with obvious negative consequences on the profitability of machining. An important amount of research activities has been done in order to increase productivity in titanium machining operations: high performance coatings and innovative technologies to improve inserts resistance to wear represent promising solutions. In this work, the cutting performance in Ti6Al4V rough turning of an innovative TiAlN coating obtained by Physical Vapor Deposition (PVD) magnetron sputtering and the effects of a Deep Cryogenic Treatment (DCT) have been experimentally investigated and a statistical analysis of the results has been performed. Typical commercially available inserts (TiAlN-AlCrO coated) have been used as a benchmark. Preliminary hardness and thermal conductivity measurements tests have been performed on the three types of tool to determine the differences caused by the different coatings and thermal treatment. The experiments have been conducted using a full factorial design in order to statistically evaluate, using ANOVA, the significance of the input factors on the process most interesting outputs. tool life and other variables of interest with different process parameters. The considered input factors are type of insert, cutting speed and feed rate. The analysed responses are average flank wear, surface roughness, cutting forces, coefficient of friction and chip morphology. The results show that even if friction coefficients are lower for standard tools, innovative inserts exhibit a higher resistance to wear. Taylor’s law parameters of PVD coated tools, with and without DCT have been determined, clearly showing that cryogenically treated tools present higher resistance at higher cutting speeds, mainly due to their superior hardness. In conclusion, it appears that relevant improvements of productivity or profitability of titanium turning can be obtained if an advanced PVD coating, deposited by magnetron sputtering is used on tungsten carbide inserts and if deep cryogenic of the inserts is performed afterwards.

Abstract: It is known that carbide inserts have better cutting performance. Coating structures will affect the inserts’ life and the cutting heat distribution on chip, workpiece and insert. In this manuscript, three kinds of turning inserts with different coating structures were used to test under the same cutting conditions and inserts geometry for cutting AISI1045 quenched and tempered steel. These coating structures are CVD-TiCN/Al₂O₃/TiN, PVD- TiAlN, CVD-TiCN/Al₂O₃. The chip temperature and the cutting force are measured by the infrared thermal image and cutting force instrument. Total heat includes the chip heat and the dissipated heat which is calculated using the formula deduced. The coefficient η of cutting heat flow of chip and the influence of coating structures on cutting heat distribution are investigated. The results show that the three kinds of turning insert with different coating structures have different effect on cutting heat distribution and the PVD- TiAlN coating performs better for taking away more heat on chip when turning AISI1045 quenched and tempered steel.

Abstract: Titanium alloys are known for their excellent mechanical properties, especially at high temperature. But this specificity of titanium alloys can cause high cutting forces as well as a significant release of heat that may entail a rapid wear of the cutting tool. To cope with these problems, research has been taken in several directions. One of these is the development of assistances for machining. In this study, we investigate the high pressure coolant assisted machining of titanium alloy Ti17. High pressure coolant consists of projecting a jet of water between the rake face of the tool and the chip. The efficiency of the process depends on the choice of the operating parameters of machining and the parameters of the water jet such as its pressure and its diameter. The use of this type of assistance improves chip breaking and increases tool life. Indeed, the machining of titanium alloys is generally accompanied by rapid wear of cutting tools, especially in rough machining. The work done focuses on the wear of uncoated tungsten carbide tools during machining of Ti17. Rough and finish machining in conventional and in high pressure coolant assistance conditions were tested. Different techniques were used in order to explain the mechanisms of wear. These tests are accompanied by measurement of cutting forces, surface roughness and tool wear. The Energy-dispersive X-ray spectroscopy (EDS) analysis technique made it possible to draw the distribution maps of alloying elements on the tool rake face. An area of material deposition on the rake face, characterized by a high concentration of titanium, was noticed. The width of this area and the concentration of titanium decreases in proportion with the increasing pressure of the coolant. The study showed that the wear mechanisms with and without high pressure coolant assistance are different. In fact, in the condition of conventional machining, temperature in the cutting zone becomes very high and, with lack of lubrication, the cutting edge deforms plastically and eventually collapses quickly. By contrast, in high pressure coolant assisted machining, this problem disappears and flank wear (VB) is stabilized at high pressure. The sudden rupture of the cutting edge observed under these conditions is due to the propagation of a notch and to the crater wear that appears at high pressure. Moreover, in rough condition, high pressure assistance made it possible to increase tool life by up to 400%.

Abstract: The paper investigates the machinability characteristics of the CoCrMo alloy ASTM F1537, usually utilized for the production of joint replacements and fixation devices thanks to its high strength, good wear and corrosion resistance, and excellent biocompatibility. This research work intends to overcome the lack of literature data about this alloy machinability, even if its use in the biomedical sector is intensive and it is usually subjected to various machining operations for the implants production. Orthogonal cutting tests were carried out under both dry and lubricated conditions at different cutting speeds and feeds typical of finishing operations. The alloy machinability was analysed in terms of tool wear, cut surface integrity, and chip morphology. In particular, the cut surface integrity was evaluated through its mechanical properties, metallurgical state and topological parameters, highlighting the influence of the process parameters.

Abstract: Airspace industry components frequently need high added value part including some featuredifficult to manufacture. One of the best example is the thin walls of parts (airplanes frames orthe turbine blades) that have a very low stiffness. The finishing operations for high height to thicknessratio parts lead to chatter vibrations, unacceptable dimensional errors or poor surface finish. The optimalmachining strategy determination is often based on trial and error and may not be cost effective(acceptable conditions can be far from the optimum). Simulation of the milling process is a powerfulmean to accelerate the search for better cutting parameters. Cutting forces, vibrations, geometricerrors or roughness can be predicted before the production of the first parts. The classical mechanisticapproach is even though limited while machining flexible parts because the dynamic response ofthe workpiece changes with the position of the cutter. The objective of this paper is to demonstratethe adaptation of numerical simulation of milling operation for the machining of thin-walled plates.Three complementary approaches are developed: location-dependent stability lobes, quasi static approachand full dynamic simulation. Location dependent stability lobes extend the classical theoryto take into account the variation of dynamic response along the workpiece. Quasi static approach isintended to deal with form error during chatter-free machining operations. Full dynamic simulation isa more complex approach intended to simulate the behavior of the complete tool/machine/workpiecesystem. The numerical approach is compared to experimental tests performed on thin plate of titaniumalloys.

Abstract: This paper deals with the analysis of the cutting process in the BTA (Boring Trepanning Association) deep hole drilling. The process is a major technique of drilling when the machining with a conventional tool is not possible. Poor training and/or poor chips evacuation often cause a temperature rise and excessive wear detrimental to the tool life and the dimensional stability of machined parts. The process is relatively not explored enough, because it is difficult to instrument experimental tests (measurement of forces acting at each insert of the BTA drilling tool, temperature at each cutting edge…). Moreover, the thermomechanical phenomena related to the cut are localized at the end of the BTA drilling head and confined in a zone inaccessible to the observation. Hence, a study of this process based on a scientific approach has been proposed. The evaluation of the chips morphology has been performed. Indeed, it is a good indicator of the stability of the cutting process and it can therefore be a serious help in the selection of optimal cutting parameters. Adequate parameters are proposed to highlight the impact of cutting conditions on the cutting process. Macro and microscopic observations of generated chips under several cutting conditions are performed. Fragmentation and segmentation of chips are some examples of analysed phenomena. In this sense, experimental tests have been conducted. The chips have been sorted according to their morphology and identified according to their origin and then proposed physical parameters are assessed. The quantitative and qualitative analysis of chips allowed identifying the impact of the cutting speed and feed rate on the cutting process.

Abstract: The present work analyzes the influence of an orthogonal machining process on the generation of nanocrystalline surface layers. Thereby, AISI 4140 is used as work piece material. Metallic parts with a severe nanocrystalline grain refinement in the near-surface area show many beneficial properties. Such surface layers considerably influence the friction and wear characteristics of the work piece in a subsequent usage as design elements working under tribological loads. The focus of this paper is an experimental analysis of a finishing orthogonal cutting operation, carried out with a broaching machine, to generate nanocrystalline surface layers. The influence of process and geometry parameters on the generation of nanocrystalline surfaces is investigated with the aim to massively decrease the grain size in the work piece surface layer. Parameters that are studied and taken into account in the manufacturing process are cutting edge radius rβ, depth of cut h and cutting velocity vc. The cutting edge radius rβ is modified by a drag finishing process. The generation of nanocrystalline surface layers is especially influenced by the design of the uncoated carbide cutting tools. Additionally, cutting force Fc and passive force Fp are determined by a 3-component dynamometer to calculate the relationship between specific cutting force kc and specific passive force kp. The temperature beneath the clearance face is detected by a fiber optic pyrometer. These measurement methods and devices are applied to detect the impact of the most relevant measurement values occurring during machining and causing a drastic reduction of grain size in the surface layer. The evaluation of the manufacturing process is carried out by detailed analyses of the microstructural conditions in the surface layer after processing using a Focused Ion Beam (FIB) system. These material characterizations provide information about the surface engineering concerning the microstructural changes in the surface layer of the work piece due to finishing orthogonal cutting processes.

Abstract: High-speed machining is submitted to economical and ecological constraints. Optimization of cutting processes must increase productivity, reduce tool wear and control residual stresses in the workpiece. Developments of numerical approaches to simulate accurately high-speed machining process are therefore necessary since in situ optimization is long and costly. To get this purpose, rheological behaviour of both antagonists and representative friction models at tool-chip interface has to be studied as encountered during high-speed machining process. The study is led with AISI 1045 steel and an uncoated carbide tool. An experimental device has been first designed to simulate the friction behaviour at the tool-chip interface only in the zone near the cutting edge. Several tests are then performed to provide experimental data and these data are used to define the friction coefficient versus to the contact pressure, the sliding velocity and the interfacial temperature by a new formulation frictional law given by Brocail et al (2010). A two-dimensional finite element model of orthogonal cutting is developed with Abaqus/explicit software. An Arbitrary Lagrangian-Eulerian (ALE) formulation is used to predict chip formation, temperature, chip-tool contact length, chip thickness, and cutting forces. This numerical model of orthogonal cutting is then validated by comparing these process variables to experimental and numerical results obtained by Filice et al. (2006). This model makes possible qualitative analysis of input parameters related to cutting process and frictional models. A sensitivity analysis has been performed on the main input parameters (coefficients of the Johnson-Cook law, contact and thermal parameters) with the finite element model. The interfacial law determined by Brocail et al (2010) is implemented on this finite element model of machining and leads to improve numerical approaches of machining. Nevertheless, even if this law enables to reduce results between experimental and numerical approaches, some differences are still substantial. The advanced friction law must be complemented for higher sliding velocities and this work using a new specific experimental device will be presented in the second part of this paper.

Abstract: The studies of biomaterials machinability applied in the medical field are extensive, however many of these studies use models of regular geometry and use elementary machining operations. In this work, a femoral prosthesis with a complex geometric shape was experimental milled using two different commercial Computer Aided Manufacturing (CAM) applications. The toolpaths defined in both CAM applications were similar and carefully selected according with the femoral prosthesis geometry. Roughing, semi-finishing and finishing passes were applied in this work. The influence of toolpath strategy was studied and predicted results from software’s simulation were compared with milled part.