Madalina Calamaz

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.

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.

Chip/Tool Interaction during Dry Machining of the TA6V Alloy

During chip formation, material is subjected to high deformations and high strain rates which generate high pressures and temperatures. Cutting fluids have an important role but produce many constraints: cleaning of parts, environment quality degradation, cost increase, diseases as identified by the European Agency for Safety and Health at Work. Dry machining is one of the future challenges although its implementation remains delicate, in particular for TiAl6V alloy. This paper aims at correlating the contact conditions at the tool/chip interface with the tool wear to understand the wear mechanisms of carbide tools in dry machining. Numerical simulations, experimentally validated, allow pointing out that the temperature distribution at the tool/chip interface depends on chip type (continuous, serrated). For continuous chips, the temperature is fairly uniform and stationary throughout the interface. For segmented chips, a cold zone between two temperature peaks is observed and moves along the tool rake face during the formation of a chip segment. The evolution of the normal stress at the interface is similar for both types of chips at the beginning of the localization phenomenon. These pressure and temperature fields allow the titanium to diffuse into the tungsten carbide and form the mixed carbide (Ti, W)C, which is very sensitive to oxidation above 500°C. This could explain the attrition of the tool, due to the brittleness of the oxycarbides. Contact conditions and tool wear are finally correlated.

3D modelling of the mechanical actions of cutting: application to milling

Along the cutting edge, the geometric and kinematic parameters vary greatly and the velocity vector at each point is very sensitive to the current position of the point considered on the cutting edge. The proposed study includes, for each of the three shear zones, the effect of velocity gradients on the strain fields and strain rates. These velocity gradients generate additional displacements of the chip, in three dimensions and, therefore, new force components and cutting moments. This study presents the overall approach for calculating cutting action starting with a detailed description of each feature area. The wrench of action is determined at the tip of the tool based on the elementary forces along the edge.

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).

Modeling and Adhesive Tool Wear in Dry Drilling of Aluminum Alloys

One of the challenges in aeronautic drilling operations is the elimination of cutting fluids while maintaining the quality of drilled parts. This paper therefore aims to increase the tool life and process quality by working on relationships existing between drilling parameters (cutting speed and feed rate), coatings and tool geometry. In dry drilling, the phenomenon of Built‐Up Layer is the predominant damage mechanism. A model fitting the axial force with the cutting parameters and the damage has been developed. The burr thickness and its dispersion decrease with the feed rate. The current diamond coatings which exhibit a strong adhesion to the carbide substrate can limit this adhesive layer phenomenon. A relatively smooth nano‐structured coating strongly limits the development of this layer.