Franck Girot

NUMERICAL SIMULATION OF TITANIUM ALLOY DRY MACHINING WITH A STRAIN SOFTENING CONSTITUTIVE LAW

In this study, the commercial finite element software FORGE2005®, able to solve complex thermo-mechanical problems is used to model titanium alloy dry machining. One of the main machining characteristics of titanium alloys is to produce a special chip morphology named “saw-tooth chip” or serrated chip for a wide range of cutting speeds and feeds. The mechanism of saw-tooth chip formation is still not completely understood. Among the two theories about its formation, this study assumes that chip segmentation is only induced by adiabatic shear band formation and thus no material failure occurs in the primary shear zone. Based on the assumption of material strain softening, a new material law was developed. The aim of this study is to analyze the newly developed models capacity to correctly simulate the machining process. The model validation is based on the comparison of experimental and simulated results, such as chip formation, global chip morphology, cutting forces and geometrical chip characteristics. A good correlation was found between the experimental and numerical results, especially for cutting speeds generating low tool wear.

Wear of Form Taps in Threading of Steel Cold Forged Parts

Tapping of blind or through holes is a common operation in several applications in the automotive, windmill towers, general machinery, and other industrial sectors. On the other hand, tapping is usually performed at the final manufacturing stage of components, so operation errors can lead to very expensive scraps. This article focuses on the performance of different forming taps since there are no standardized procedures for evaluating form tapping, beyond the mere visual assessment or go/no-go tests. Tool behavior was studied according to four aspects: (a) wear analysis of tap forming lobes, (b) measurement of thrust force and torque during tapping, (c) metallographic study of threads, and finally (d) evaluation of coating and tap geometry. In the work presented here, tap wear and thread quality are correlated with processes monitoring parameters. A useful conclusion for tap manufacturers is that taps with hexagonal section and 5 pitches in the chamfer zone led to lower thrust forces and torques than pentagonal section ones, along with a lower wear. Hexagonal/5-pitch taps led to better results due to a more gradual deformation process for producing a thread. A larger number of pitches before reaching the nominal diameter resulted in a reduction of strain between successive lobes, generating lower force and torque values.

Damage modes of the aeronautic multidirectional carbon/epoxy composite T300/914 in machining

Damage mechanisms of tipped tungsten carbide tools have been investigated when dry machining of multi-directional carbon fibre composites. The objective of this study is to determine the effects of the cutting parameters on the tool?workpiece interface and to identify the different wear modes. Different rake angles (0?, 15? and 30?), and feed rates (0.05, 0.1 and 0.2 mm/rev) were tested for a constant cutting speed (60 m/min). The cutting loads were determined, and the tool surface topography was analysed using complementary techniques. All aspects of material removal were found to be primarily dependent on the fibre orientation.

Numerical Simulation of Workpiece Thermal Field in Drilling CFRP/Aluminum Alloy

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.

Strain Field Measurement in Orthogonal Machining of a Titanium Alloy

Improving the cutting processes by optimizing operating parameters necessarily involves understanding the thermo-mechanical mechanisms generated during chip formation. For this, numerical simulations are used to obtain the strain, stress and thermal fields near the tool tip. Nowadays, the validation of numerical simulation models of cutting is based on macroscopic results such as chip geometry and cutting forces generated by the machining process. However, it is not appropriate to validate local fields by macroscopic results. So, it is important to validate numerical cutting simulations on the bases of measured local strain fields. This article aims to study the feasibility of strain field measurement in orthogonal machining of the titanium alloy Ti64. A high-speed camera was used to provide data for segmented chip formation analysis. A microscope was related to the camera to observe an area of about 0.7x0.7mm² around the tool tip. An optimum adjustment of camera settings, lighting, workpiece surface preparation and cutting conditions allowed to obtain an acceptable image quality for analyzing with Correli [1] software. At low cutting speed, Correli qualitatively identify the position of the primary shear band and its evolution over the time.

Analysis of coating performances in machining titanium alloys for aerospace applications

The current study emphasises the role of coating materials in enhancing the wear resistance of the cutting tool and improving the tool-chip contact. The wear mechanisms have been investigated through a series of cutting experiments performed on an instrumented planer machine. Machining tests were conducted on the usual Ti-6Al-4V alloy (workpiece) and cemented carbide tools. Four new coatings were especially designed for the study: 1diamond (thin layer, about 2 to 3 μm) 2diamond+TiB2+CrN/DLC (diamond like carbon, about 3, 5 μm) 3diamond (thick layer, 6 μm) 4TiB2+CrN/DLC (3 μm). The performance of each coating material was analysed and compared in one hand to the uncoated carbide tools and on the other hand to the CBN reinforced carbide tools in terms of cutting forces and tool wear mechanisms.

Using Image Analysis Techniques for Single Evaluation of the Chip Shrinkage Factor in Orthogonal Cutting Process

The forces involved in a cutting process are related, for example, with the power consumption, with the final quality of the workpiece and with the chip geometry obtained, since these forces determine the compression experimented by the chip and therefore its final geometry. The orthogonal cutting process assisted with a High Speed Filmation (HSF) permit obtains a digital filmation of the process with high magnification. This filmation permits to obtain a measurement of the longitudinal changes produced in the chip. This deforms are related with the Shrinkage Factor, ζ. And in this case the Stabler hypothesis is enabled, by that using the shear angle and the rake angle is possible obtain a value of the Shrinkage Factor in a different conditions.

Cutting Forces Prediction in the Dry Slotting of Aluminium Stacks

Cutting forces are one of the inherent phenomena and a very significant indicator of the metal cutting process. The work presented in this paper is an investigation of the prediction of these parameters in slotting processes of UNS A92024-T3 (Al-Cu) stacks. So, cutting speed (V) and feed per tooth (fz) based parametric models, for experimental components of cutting force, F(fz,V) have been proposed. These models have been developed from the individual models extracted from the marginal adjustment of the cutting force components to each one of the input variables: F(fz) and F(V).

Experimental study and synthesis of the dynamic behaviour in high speed drilling

This paper concerns an experimental approach of vibration phenomena in High Speed Drilling (HSD) of aluminium alloys. In order to study these phenomena, different results have been chosen to illustrate that these vibrations are generated by different deformations of the drill and the machining system. The relevant parameters at the origin of these vibrations in HSD are pointed out and their influence on the quality of drilling is analysed. The effect of the drill geometry on surface quality is reported and in particular the role of the chisel edge, the helix angle which controls the sharpness of tool corner, the thickness and the number of margins which allow the drill to be held in the hole and the rigidity of the drill materialised by its overhang. Depending on its length, the drill can deform differently. The influence of the drilling conditions on surface quality has also been evaluated: the feed rate per tooth and the cutting speed as well as the rotation frequency and the feed speed. The effect of lubrication and machine rigidity (in our case an industrial robot) has been analysed. Finally, we pointed out the apparition of vibration when two aluminium plates are drilled together. The behaviour is very variable and different steps of the drilling have to be considered: the drill entry in the aluminium part, the drilling, the drill exit from the plate and the drill backward movement. This allows for a particular value of drill diameter and drill overhang to determine the optimised feed rate and rotation frequency in order to achieve the best quality of the hole surface.