Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns. This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.

Machining induced surface integrity in titanium and nickel alloys: A review

Chatter is a self-excited vibration that can occur during machining operations and become a common limitation to productivity and part quality. For this reason, it has been a topic of industrial and academic interest in the manufacturing sector for many years. A great deal of research has been carried out since the late 1950s to solve the chatter problem. Researchers have studied how to detect, identify, avoid, prevent, reduce, control, or suppress chatter. This paper reviews the state of research on the chatter problem and classifies the existing methods developed to ensure stable cutting into those that use the lobbing effect, out-of-process or in-process, and those that, passively or actively, modify the system behaviour.

Chatter in machining processes: A review

Machining difficult-to-machine materials such as alloys used in aerospace, nuclear and medical industries are usually accompanied with low productivity, poor surface quality and short tool life. Despite the broad use of the term difficult-to-machine or hard-to-cut materials, the area of these types of materials and their properties are not clear yet. On the other hand, using cutting fluids is a common technique for improving machinability and has been acknowledged since early 20th. However, the environmental and health hazards associated with the use of conventional cutting fluids together with developing governmental regulations have resulted in increasing machining costs. The aim of this paper is to review and identify the materials known as difficult-to-machine and their properties. In addition, different cutting fluids are reviewed and major health and environmental concerns about their usage in material cutting industries are defined. Finally, advances in reducing and/or eliminating the use of conventional cutting fluids are reviewed and discussed.

/pdf/environmentally-conscious-machining-of-difficult-to-machine-32ov2j8z58.pdf

Environmentally conscious machining of difficult-to-machine materials with regard to cutting fluids

Recent advances in modelling of metal machining processes

Environmental friendly cutting fluids and cooling techniques in machining: a review

This paper presents a geometrical description of common end mills and of the engagement in the workpiece material for shoulder milling. The tool geometry is decomposed into elementary cutting edges in oblique cutting position and the elementary cutting forces are calculated from a thermomechanical approach and validated for a 42CrMo4 steel.

Parametric geometry for modelling of milling operations

In micro-milling, cutting forces are driven by many parameters, including feed per tooth, depth of cut, rake angle and machined material. Because of size effects, issues inherent in the micro-milling process, such as chatter, edge radius or tool deflection, influence the cutting forces making it difficult to understand the process. To overcome this problem, the best solution is to perform experiments independently on each parameter. With orthogonal and oblique micro-cutting experiments, chip formation is investigated with fewer parameters. This study aimed to model micro-cutting using the results obtained from orthogonal and oblique micro-cutting experiments, based on tube turning ignoring dynamic considerations. It was experimentally observed that a jump in cutting forces occurs when the uncut chip thickness decreases, described as a transient regime between ploughing and shearing dominant regimes. The ploughing regime for low uncut chip thicknesses is characterised by a normal force greater than the cutting force. At higher uncut chip thicknesses, the cutting force is dominant. A phenomenological model was developed to take into account the divergence in cutting forces at low uncut chip thicknesses. The model was divided into two terms that model, respectively, ploughing and shearing regimes with a continuous transient regime. The model reproduced the behaviour quite well and could be enriched with more parameters.

Phenomenological modelling of micro-cutting based on experimental results

Micro-milling can be defined as milling with end mills smaller than 1 mm of diameter. The top-down approach from milling to micro-milling is often used to define cutting conditions. Unfortunately geometries either for the active part or the overall shape are quite different from conventional tools, leading to inexistent problems at the macro-scale, such as a larger cutting edge radius to uncut chip thickness ratio leading to ploughing effect. Moreover, micro-milling can be used on particular material such as shape memory alloys in biomedical domain which are difficult to machine. This study focuses on burr formation during shoulder milling for two biocompatible NiTi alloys: a martensitic NiTi (shape memory effect) and an austenitic one (pseudo-elasticity effect). Design of experiment is used to highlight the influence of various parameters (cutting parameters and material phases) on the burr formation in micro-milling NiTi alloys. Burrs were observed and measured using confocal, optical and electronic microscopy and tend to be as large as shoulders dimensions. Material phase transformation was also examined. Analysis of variance emphasizes that the larger the feed per tooth and the smaller the width of cut are, the smaller the top burr is. Cutting strategy leads to different burr shape: up-milling burrs have a large curvature, whereas down-milling burrs are slightly bent. An affected layer of about 10 μm has been observed for the austenitic NiTi. The proposed experimental approach give the opportunity to study burr formation in micro-milling, the machinability of alloys or superelastic NiTi shape memory and a qualitative explanation of burr formation has been developed.

https://hal.archives-ouvertes.fr/hal-01207243/document

Study of burr formation and phase transformation during micro-milling of NiTi alloys

Inconel 718 Nickel-based superalloy has been applied widely in high temperature, high load and corrosion resistant environments due to its superior properties. They are known to be among the most difficult-to-cut materials. This paper reports some experimentals findings concerning the machinability at high cutting speed in milling process. To characterize the machining of Inconel 718 by tips tools , to identify the best cutting conditions, to define the modes of wear, to determine the tool life, and to classify the types of coatings. The study is conducted according to two phases. Phase one, determination of the cutting conditions for a comparative study. Machining over some lengths with selected tips in order to determine the strategy of milling. Phase two, Study of the types of wear. During machining, the modes of wears are observed and the powers of cut are raised and referred. High speed milling machine equipped by a power consumption recorder Wattpilote. The aim is to show the machinability of the superalloy, measuring the power and the tip wear by optical microscope and white light interferometer, using different strategies and cutting conditions. DOI: http://dx.doi.org/10.5755/j01.mech.19.2.4149

/pdf/dry-high-speed-milling-of-nickel-based-superalloy-290k8thtjc.pdf

Dry high speed milling of nickel-based superalloy

The forging work involves extreme impacts, forces, stresses and temperatures. Thus, mould dies must be extremely resistant. The aim of the project PROMETFOR, supported by the French National Agency (ANR), is to perform a rapid prototyping process answering to forging conditions integrating a Stellite 6 hardfacing deposed by a PTA process. This study talks about the dry machining of the hardfacing, using a two inserts machining tool and a high speed milling machine equipped by a power consumption recorder Wattpilote. The aim is to show the machinability of the hardfacing, measuring the power and the insert wear by optical microscope and white light interferometer, using different strategies and cutting conditions.DOI: http://dx.doi.org/10.5755/j01.mech.18.5.2707

/pdf/dry-high-speed-milling-of-cobalt-base-hard-facing-superalloy-mx3q9eypa9.pdf

D. Dudzinski

Papers

Parametric geometry for modelling of milling operations

Phenomenological modelling of micro-cutting based on experimental results

Study of burr formation and phase transformation during micro-milling of NiTi alloys

Dry high speed milling of nickel-based superalloy

Dry high speed milling of cobalt-base hard facing superalloy