Key Engineering Materials Vols. 554-557

Abstract: The main purpose of composite materials drilling is the need to put together different parts of a structure, in aeronautics for example. The objective of this study is to experimentally analyze the influence of drilling on a carbon/epoxy composite. Three geometries of drill, a range of cutting speed and feed have been tested. Thrust forces and moments have been recorded during machining and macroscopic analyses have been conducted to examine outside and inside surfaces of the holes. Damages have been also quantified using delamination factor F_d. Experimental results have shown significant influences of feed and drill geometry on delamination and the best results have been obtained using a spur drill.

Abstract: In recent years titanium alloys demand has increased considerably. Although many progresses have been made during the last years, machining titanium alloys still is considered as quite challenging. This paper shows the results of the fundamental research carried out to understand the thermochemical loads when machining Ti-6Al-4V. A comparative study of empirical and modelling results is carried out on cutting forces and temperatures in orthogonal conditions. Empirical cutting forces are obtained with a piezoelectric sensor while the temperature fields are obtained with an infrared camera. The commercial software AdvantEdge (finite element modelling) is employed to obtain temperature and forces. In 3D, analytical modelling based on energy approach is used to estimate cutting forces while finite difference method is employed to obtain temperature fields. Results show a good correlation among all the approaches.

Abstract: During machining of hard materials, one approach to reduce tool wear is using a laser beam to preheat the material in front of the cutting zone. In this study, a new concept of laser-assisted milling with spindle and tool integrated laser beam guiding has been tested. The laser beam is located at the cutting edge and moving synchronously with the cutter. In experiment, a reduction in the resulting process cutting forces and tool wear has been observed in comparison to milling without laser. A three-dimensional finite element model in DEFORM 3D was developed to predict the cutting forces in the milling process with and without an additional laser heat source, based on a Johnson-Cook-type material constitutive model adapted for high strains and strain rates. Both in experiment and simulation, the deformation behavior of a Ti-6Al-4V workpiece has been investigated. The comparison of the resulting cutting forces showed very good agreement. Thus the new model has great potential to further optimize laser assisted machining processes.

Abstract: Besides developments in the area of dry machining and minimum quantity lubrication, the use of coolant lubricants is still essential when machining high alloyed steels or heat resistant materials like titanium and nickel based alloys. Experts agree that this fact will not change in the next decade. For this reason it is necessary to use coolant lubricants as effectively as possible to maximise their positive effect on productivity and process stability. High-performance cooling strategies like high-pressure cooling and cooling with cold gases (cryogenic cooling) have received increased attention in the last years. Through the targeted supply of coolant lubricants to the cutting site it is possible to decrease tool wear, increase cutting speeds, guarantee defined chip breakage and chip transport and – in terms of cryogenic cooling – waive part cleaning. This paper shows current research results in the above mentioned field. Since the performance of a high-pressure coolant lubricant supply in turning difficult to cut materials has been shown in many previous papers, this paper focuses on the quantification of the potential in turning different steels, namely quenched and tempered but also stainless steel in comparison to the conventional flood cooling. Since energy efficiency is very crucial, pressure and flow rate have to be adjusted carefully and in accordance with the cutting parameters to guarantee best results with less energy. Moreover the effects of cryogenic cooling will be evaluated in comparison to high-pressure cooling and conventional flood cooling. In latter field, cutting tests were carried out under variation of the flow rate in order to find the minimum required value for a certain machining task with the overall aim to prevent waste of the media used. Especially in cryogenic cooling technologies, many fundamental research regarding the working mechanisms but also further developments in cutting tool and machine tool technology are still necessary to make this technology ready for industrial use.

Abstract: A micro dimple machining is studied to control the surface functions with the micro-scale structures on the solid surfaces. The micro dimples are machined in milling with the inclined ball end mill. When the feed rate of the tool is high enough that the removal area of an edge does not overlap with that of the previous edge, periodical concave dimples are machined. A mechanistic model is presented to control the shape and the size of the dimples to be machined. Then, a machining error model is presented to control the deviation of the dimple shapes. Some machining examples are shown to verify the dimple model and the error model.

Abstract: Nickel alloy is widely applied to aero-engines, marine structures, chemical plants, etc. This alloy has superior mechanical and chemical properties, but is one of difficult-to-machine materials because of its superior properties. For this reason, various machining methods, such as cryogenic machining, hot machining, machining using high pressure coolant and rotary machining have been studied for increasing the cutting speed and cutting efficiency of this alloy. Recently, a new lubrication method called air jet assisted (AJA) machining was developed to extend tool life by 20-30% in finish-turning of Inconel 718 and Ti-6Al-4V. In this machining method air jet as well as coolant was applied to the cutting area. In this study, AJA machining was applied to high speed machining of Inconel 718 with a SiC whisker reinforced ceramic tool. The air jet was applied from an air nozzle at the tool flank face. Because the ceramic tool is likely to suffer from severe notch wear, the influence of AJA machining on flank and notch wear of the ceramic tool was investigated experimentally. As a result, it was found that as compared to conventional wet machining, AJA machining increased flank wear but reduced notch wear, which is usually critical to the tool life of the reinforced ceramic tool. The tool wear characteristics peculiar to AJA machining were effective for extending tool life in high speed machining of this alloy.

Abstract: This contribution first aims at providing a more detailed analysis of the cutting mechanisms of a normalized carbon steel from a microscopic and metallurgical point of view. SEM and EBSD are used to highlight, in the main intensive deformation zones of the chip, a drastic grain refinement induced by a dynamic recrystallization process. The second part of this study intends to emphasize the consequences of the latter on the workmaterial behaviour. A rheological study based on dynamic compression tests is briefly presented and enables to understand their occurrence as well as their influence on the flow stress of the material. A "metallurgy based" constitutive equation is identified to reach a better description of the flow stress. Finally, this one is implemented in a Finite Element code (Abaqus/Explicit) to assess the potential of this method. It is shown that microstructural evolutions could be relatively well predicted especially when considering the level of recrystallization.

Abstract: Machining of advancedaerospace materials have grown in the recent years although the hard-to-machinecharacteristics of alloys like titanium or nickel based alloys cause highercutting forces, rapid tool wear, and more heat generation. This paper presentsan experimental evaluation of machining of Inconel718 alloy under dry conditions at varying of cutting speeds and feed rates.The influence of the cutting conditions on surface integrity was studied interms of surface roughness, affected layer, grain size variations and phasechanges/modification. Also, the machining process performance was evaluatedthrough the power consumption and tool-wear.

Abstract: A hybrid experimental-numerical methodology is presented for the identification of the model parameters regarding a mixed hardening anisotropic finite plasticity fully coupled with isotropic ductile damage in which the micro-crack closure effect is given account for, for steel sheets made of DP1000. The experimental tests involve tensile tests with smooth and pre-notched specimens and shear tests with specimen morphologies recently proposed by D.R. Shouler, J.M. Allwood (Design and use of a novel sample design for formability testing in pure shear, Journal of Materials Processing Technology, Volume 210, Issue 10, 1 July 2010, Pages 1304-1313). These tests cover stress triaxiality ratios lying between 0 (pure shear) and 1/√3 (plane strain). To neutralize machine stiffness effects displacements of the chosen material surface pixels are kept track of using the digital image correlation system ARAMIS, where recorded inputs are synchronized with force measurements. On the numerical part, developed constitutive model is implemented as user defined material subroutine, VUMAT, for ABAQUS/Explicit. FE models for the test cases are built using 3D brick elements (rather than thin shells) and devising developed VUMAT for the constitutive model, model parameters are identified using an inverse parameter identification procedure where the objective function relies on the difference of experimentally observed-numerically predicted forces for the selected pixel displacements. The validity of the material model and transferability of its parameters are tested using tests involving complex strain paths.

Abstract: Bron and Besson yield criterion has been used to investigate the plastic anisotropic behavior of an aluminum alloy AA5086. The parameters of this anisotropic yield model have been identified by two different methods: a classical one, considering several homogeneous conventional experiments and an exploratory one, with only biaxial test. In this paper, the parameter identification with conventional experiments has been carried out with uniaxial tensile tests and simple shear tests in different orientations to the rolling direction and with a hydraulic bulge test. For comparison’s sake, Hill’s 48 yield function has also been calibrated analytically from uniaxial tensile tests. Numerical simulation for the cross biaxial test has been carried out with the anisotropic parameters identified from the conventional tests. From this simulation, the principle strains along a specified path in the gauge area of the cruciform specimen have been evaluated. A good agreement is observed between experimental and numerical values of principal strains for a large range of strain paths.