Microstructure Of Work Material Research Articles

Fundamental study of exit burr formation mechanisms during orthogonal cutting of AlSi aluminium alloy

Burr formation during machining is an important issue in industry. It causes an additional deburring operation, which is time consuming and has a negative economic impact.This study aims to analyse burr formation mechanisms and its accumulation in successive passes during orthogonal cutting of a cast aluminium alloy. A customized experimental setup was developed, which includes a high speed imaging system and a laser profilometer. A design of experiments using the setup mentioned previously is carried out and a methodology for geometric burr characterization is developed and applied. Furthermore, statistical representation of the obtained results is performed, which allows the understanding of the geometric heterogeneity influence associated to burr formation mechanisms and to work material microstructure.Based on the exit burr analysis, new geometrical criteria are proposed for the characterisation and the definition of two main burr formation mechanisms.The influence of cutting parameters on burr morphology along the workpiece exit edge is investigated in depth. The results show that two types of burrs can be produced simultaneously along the workpiece exit edge due to the work material microstructure heterogeneity. The results present as well the influence of low uncut chip thickness that leads to a higher proportion of burrs without chamfer. This type of burr is higher and more propice to burr accumulation. After performing several cutting passes, these burrs may be eliminated and replaced by a burr with chamfer.

Prediction of the Cutting Forces and Chip Morphology When Machining the Ti6Al4V Alloy Using a Microstructural Coupled Model

Titanium and its alloys are often used in aerospace, power and biomedical applications due to their low density, high tensile strength, resistance to corrosion and high temperatures. However, these materials are well known to be difficult-to-cut materials and require to follow some special techniques to improve their machinability. It should be also noticed that some phenomena like segmentation and recrystallization can occur during the chip formation process. Therefore, fine grains are observed in the adiabatic shear bands located in the chip segments. The machined surface also presents fine grains because of the recrystallization of the microstructure. In the present work, a 2D finite element model based on Lagrangian formulation was developed in Abaqus/Explicit to simulate the orthogonal cutting process of the Ti6Al4V alloy. To take into account the recrystallization phenomenon, a new material constitutive model denoted ‘Multi-Branch Model’ (MB) was developed. The MB model is based on the Johnson-Cook (JC) flow stress model and its modified formulation, known as the tangent hyperbolic model (TANH), to introduce the softening effect due to the recrystallization process. This new model is coupled to a microstructural criterion in order to simulate the work-material microstructure evolution during the machining process. The recrystallized grains size field, cutting forces and chips morphology are compared to those obtained with the TANH model. Based on these results, a relationship between recrystallization and chip segmentation has been found and deeply discussed.

Combined microstructure-based flow stress and grain size evolution models for multi-physics modelling of metal machining

Intense thermomechanical loading in metal machining induces a microstructure change at different zones in the workpiece (machined surface, chip and tool tip). This paper deals with a multi-physics modelling of the cutting process, where the thermo-viscoplastic behaviour of the workmaterial is described by a physical model, denoted Mechanical Threshold Stress (MTS) model, and by the classical Johnson–Cook (JC) flow stress law for comparison purpose. The workmaterial microstructure change during cutting (grain size evolution) is described by a physical-based Dislocation Density (DD) model, which is combined with the MTS model in the framework of an ALE-FE approach. The model is developed for a 2D orthogonal cutting simulation. Combined MTS–DD material models were implemented in Abaqus/Explicit FE code. The first step of the multi-physics model is analysed by comparison of predicted cutting forces and chip thickness with experimental ones and those predicted by the JC model. In the second step, the grain refinement during cutting is predicted, revealing zones where the microstructure is highly affected, particularly through the depth of the machined surface, where the thickness of the affected subsurface is estimated.

Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V

Titanium alloys are known to produce shear-localized chips during machining, resulting in cyclic variations in cutting forces which in turn could cause severe problems with vibrations. However, at low cutting speeds and feed rates, continuous chips are formed, with an increase in both parameters favoring the transition to shear-localized chips. This transition is affected by work material microstructure, where a coarse microstructure gives anisotropic effects, e.g., when the size of alpha colonies is on the same order of magnitude as the primary cutting zone. The change in chip morphology with an increase in cutting parameters will then be dependent on the orientation of alpha colonies within the cutting zone. The microstructure of work material can show large variations depending on product form, e.g., cast, wrought, or sheet material, thus affecting whether the chip formation is isotropic or anisotropic. Other sources of variations also exist that can be found within the same component, such as segregation of alloying elements and differences in thermo-mechanical history during processing due to geometry. In this study, the interaction between work material microstructure, process parameters, and the machining system’s structural characteristics is studied. The aim is to further increase the knowledge about vibrations during machining of titanium and the role of microstructure and machining system properties. Different microstructures were produced by adding boron to cast Ti-6Al-4V material, where the resulting colony sizes gave both isotropic and anisotropic chip formation within the chosen cutting data range. The machining systems dynamic properties were varied by using different tool overhangs, thereby simulating different configurations of natural frequencies and stiffness. The results show the influence of both microstructure and machining system’s structural characteristics on the dynamic response of the system for different process parameters. This information can be used to increase robustness of machining operations taking into consideration this three-way relationship.

On the Influence of Work Material Microstructure on Chip Formation, Cutting Forces and Acoustic Emission When Machining Ti-6Al-4V

The influence of heat treatment of work material on chip formation, when machining Ti-6Al-4V, was studied through microstructural investigation of chips, as well as response on cutting forces and acoustic emission. Three different microstructures were investigated; equiaxed, bimodal and Widmanstätten. It is well known that machining of titanium produces shear localized chips at all industrially practical cutting speeds and feed rates, however there is also a transition from aperiodic to periodic saw- tooth chip formation. The feed rate was varied at constant cutting speed to study this transition from aperiodic to periodic saw-tooth chips in the three microstructures. Face turning cutting tests were used when sensor signals were collected. The results from this investigation stress the importance to consider work material microstructure when studying the chip formation process, and its impact on cutting forces and acoustic emission, when machining Ti-6Al-4V.

S134033 カムシヤフト加工における切削メカニズムに関する研究

Generally, camshafts used in automobile engines are firstly made by forging or casting processes and then turning process is applied to remove a rough layer on the work materials as cast or forged. This process contains relatively harder conditions for cutting tool because of the unstable intermittent cutting derived from the eccentricity in shape and/or lack of uniformity of the work material microstructure. Thus the productivity in camshaft machining is limited in production time, tool life and surface quality of machined surface. In this study, cutting force analysis in rough turning of camshaft was performed. High-speed movie was successfully captured the transition from unstable interrupted cutting into stable continuous cutting, and the tool-work rubbing phenomena was observed after stop the feed. The amplitude of vibration in cutting and thrust forces were about 300N and 250N respectively, and rubbing forces were 200N and 80N respectively after stopping the tool feed. This rubbing may affect the tool life greatly, NC program was suggested to be modified at the machine shop in automobile factory.

Ultra-precision machining of aluminium alloy surfaces for optical applications

Fabrication of optical quality freeform surfaces using ultra-precision machining technologies enables to achieve sub-micrometric form accuracy and nanometric surface finish. In this paper, ultra-precision machining of large aluminium alloy optical components has been conducted to investigate the machining performance and machined surface integrity. An ellipsoidal mirror surface and a lenticular lens array have been machined out using slow slide servo (SSS) turning, grooving and milling techniques with a single crystalline diamond tool on an ultra-precision machine centre. It was found that there are some striations on one segment of the machined ellipsoidal mirror surface. Through data density optimisation of angular increment dθ and arc length increment dc in C -axis positioning, the striation problem has been successfully solved, and surface roughness Ra of the machined ellipsoidal mirror surface has been achieved less than 10 nm. Work material microstructure and thermal stability of the machining centre significantly affected the machined lenticular lens surface quality. Compared with grooving process, milling process could produce a better surface integrity and form accuracy of large aluminium lenticular lens array.

An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning

White layers formed in machining of hardened alloys are known to be very hard and resistant to standard etchants used in metallographic studies. Many studies have been performed on this subject, but only with little progress showing definite results concerning the actual effectiveness of white layer formation. Hence, the basic question that remains unanswered is: are the white layers a tribological advantage for the manufacturing industry producing parts/components from hard alloys? The focus of this study is to investigate the evolution of white layers produced during progressive tool flank wear in dry hard turning with CBN (cubic boron nitride) tools, and to correlate this with the surface integrity of the machined surface. The following four materials were machined: X160CrMoV12 cold work steel (AISI D2), X38CrMoV5 hot work steel (AISI H11), 35NiCrMo16 high toughness steel and 100Cr6 bearing steel (AISI 52100). Samples of chips were metallographically processed and observed under an electronic microscope to determine whether white layers are present or not. More specifically, chip-forms/shapes were studied to determine how they developed during machining with potential appearance of white layers, with a view to correlating the chip-forms/shapes with the white layer formation. Finally, by using scanning electron microscopy and EDS techniques on these chip samples, properties and microstructures of white layers were deduced in order to verify some of the prevalent theories.

On Chip Morphology, Tool Wear and Cutting Mechanics in Finish Hard Turning

Topography of surfaces produced in finish hard turning using cubic boron nitride (CBN) tools is affected by a large number of factors including tool wear and the mechanics of the chip formation process. This paper shows first that tool wear rates are affected by interactions between the work material and the binder phase of the CBN tool. For finish hard turning, low CBN content, ceramic binder tools give longer lives and better finish than high CBN content metallic binder tools. For low CBN tools, wear rate is directly related to the microstructure of the work material and to the CBN grain size. SEM studies suggest that chip morphology is independent of work material microstructure, but varies with tool wear. Orthogonal cutting tests show that, above a critical speed, segmented chips are formed by catastrophic localized shear and that chip segmentation spacing may be reflected in a modulation of the machined surface. Segment spacing is a function of depth of cut, rake angle, and surface speed, approaching a limiting value with speed. Specific cutting energies decrease with speed, also approaching an asymptote. A simple mechanical model gives reasonable predictions of segment spacing along the original surface, although a full thermo-plastic model will be required to account for other aspects of the chip formation process.