Analysis of residual stresses induced by dry turning of difficult-to-machine materials

Critical issues in machining of difficult-to-cut materials are often associated with short tool-life and poor surface integrity, where the resulting tensile residual stresses on the machined surface significantly affect the component's fatigue life. This study presents the influence of cutting process parameters on machining performance and surface integrity generated during dry turning of Inconel 718 and austenitic stainless steel AISI 316L with coated and uncoated carbide tools. A three-dimensional Finite Element Model was also developed and the predicted results were compared with those measured.     

Laser-assisted high-speed finish turning of superalloy Inconel 718 under dry conditions

Inconel 718 (IN718) is used in aerospace applications due to its superior mechanical properties. This study investigates the high-speed machinability of this material under laser-assisted machining (LAM) and dry conditions. Finish turning tests were performed for cutting speeds up 500 m/min and feeds up to 0.5 mm/rev, using focused Nd:YAG laser beam and ceramic tool (SiAlON). At optimum machining conditions, nearly eight-fold increase in material removal rate and significant improvement in the tool life and surface finish were achieved, compared to conventional machining. The mechanisms of tool failure were identified. SEM analysis and microstructure examination of machined surfaces revealed the improvement in the surface integrity under LAM conditions.     

Surface integrity when machining age hardened Inconel 718 with coated carbide cutting tools

Considerable attention has been given to the use of ceramic cutting tools for improving productivity in the machining of heat resistant super alloys (HRSA). However, because of their negative influence on the surface integrity, ceramic tools are generally avoided particularly for finishing applications. As a result the main high end manufacturers are more or less dependent on carbide cutting tools for finishing operations. Still the improper use of carbide cutting tools can also result in poor surface integrity. The objective of this investigation is to develop a set of guidelines, which will assist the selection of the appropriate cutting tools and conditions for generating favorable compressive residual stresses. This paper specifically deals with residual stresses and surface finish components of surface integrity when machining (facing) age hardened Inconel 718 using two grades of coated carbide cutting tools specifically developed for machining HRSAs. The cutting conditions were obtained from investigations based on optimum tool performance. The effect of insert shape, cutting edge preparation, type and nose radius on both residual stresses and surface finish was studied at this optimum cutting condition. This investigation, suggested that coated carbide cutting tool inserts of round shape, chamfered cutting edge preparation, negative type and small nose radius (0.8 mm) and coolant will generate primarily compressive residual stresses.     

Effect of machining parameters and cutting edge geometry on surface integrity of high-speed turned Inconel 718

Stringent control on the quality of machined surface and sub-surface during high-speed machining of Inconel 718 is necessary so as to achieve components with greater reliability and longevity. This paper extends the present trend prevailing in the literature on surface integrity analysis of superalloys by performing a comprehensive investigation to analyze the nature of deformation beneath the machined surface and arrive at the thickness of machining affected zone (MAZ). The residual stress analysis, microhardness measurements and degree of work hardening in the machined sub-surfaces were used as criteria to obtain the optimum machining conditions that give machined surfaces with high integrity. It is observed that the highest cutting speed, the lowest feedrate, and the moderate depth of cut coupled with the use of honed cutting edge can ensure induction of compressive residual stresses in the machined surfaces, which in turn were found to be free of smeared areas and adhered chip particles.     

Residual stress and surface roughness when facing age hardened Inconel 718 with CBN and ceramic cutting tools

The demand for increasing productivity when machining heat resistant super alloys has resulted in the use of advanced cutting tools such as ceramics and cubic boron nitride (CBN). However, the effects of these tools on the surface integrity, especially the residual stresses created, in the high speed facing operation of Inconel 718 has not been dealt with. In this paper, the residual stresses and the surface roughness when facing age hardened Inconel 718 using CBN and mixed ceramic cutting tools at their respective optimum performance based on productivity has been investigated. The residual stress and surface finish generated during facing with CBN cutting tools have been investigated as a function of speed, depth of cut, coolant, tool geometry and nature of the tool coating. In addition, mixed ceramic cutting tools have been investigated for comparison. The results show that mixed ceramic cutting tools induce tensile residual stresses with a much higher magnitude than CBN cutting tools. The residual stresses and the surface roughness generated by CBN cutting tools are more sensitive to cutting speeds than depth of cut. The use of coolant results in either compressive residual stresses or lowers the magnitude of the tensile residual stresses, whereas dry cutting always resulted in tensile residual stresses. From this investigation, it is suggested that round CBN cutting tools should be used at slow cutting speeds (150 m/min) and small depths of cut (0.05 mm) and with the use of coolant to achieve compressive or minimal tensile residual stresses and good surface finish.     

An experimental and numerical study on the face milling of Ti-6Al-4V alloy: Tool performance and surface integrity

This paper is concerned with the experimental and numerical study of face milling of Ti-6Al-4 V titanium alloy. Machining is carried out by uncoated carbide cutters in the presence of an abundant supply of coolant. Experimental analysis is conducted by focusing on the measurement of specific cutting energy, surface integrity and tool performance. The experimental analysis is supplemented by simulations from a 3D finite element model (FEM) of face milling simulation where needed. A tool wear model parameterized from FEM predictions of the tool-chip interface temperature, contact stress and chip velocity is presented. Tool wear patterns are described in terms of various cutting conditions and the influence of tool wear on surface integrity is investigated. Tool wear predictions based on the 3D FEM simulation show good agreement with experimental tool wear measurements. The highest cutting speed realized for the cutting tool material is 182.9 m/min (600 sfpm). Good surface integrity in terms of favorable residual stress and surface finish is achieved under the machining conditions used with limited tool wear. Residual stresses imparted to the machined surface are shown to be compressive.     

New observations on tool wear mechanism in dry machining Inconel718

Tool wear is a problem in machining nickel-based alloy Inconel718, and it is thus of great importance to understand tool wear. Tool wear mechanism in dry machining Inconel718 with coated cemented carbide tools was analyzed in this paper. CCD and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS) were used to study tool wear mechanism. The results show that the main reason which causes cutting tool wear was that the tool materials fall off from the tool substrate in the form of wear debris. In addition,, element diffusion between tool and workpiece and oxidation reaction all accelerate the formation and the peeling of the wear debris. According to analysis of tool wear mechanism, tool flank wear model was established. The optimal temperature in machining Inconel718 with PVD-coated (TiAlN) tool was obtained through the established model. Excellent experimental agreement was achieved in optimal temperature calculated by the established model.     

Effect of Wire Material on Productivity and Surface Integrity of WEDM-Processed Inconel 706 for Aircraft Application

Inconel 706 is a recently developed superalloy for aircraft application, particularly in turbine disk which is among the most critical components in the gas turbine engines. Recently, wire electrical discharge machining (WEDM) attained success in machining of gas turbine components which require complex shape profiles with high precision. To achieve the feasibility in machining of these components, the research work has been conducted on Inconel 706 superalloy using WEDM process. And, the effect of different wire materials (i.e., hard brass wire, diffused wire, and zinc-coated wire) on WEDM performance characteristics such as cutting speed, surface topography, surface roughness, recast layer formation, residual stresses, and microstructural and metallurgical alterations have been investigated. Even though, zinc-coated wire exhibits improved productivity, hard brass wire was found to be beneficial in terms of improved surface quality of the machined parts. Additionally, lower tensile residual stresses were obtained with hard brass wire. However, diffused wire has a moderate effect on productivity and surface quality. Under high discharge energy, higher elemental changes were observed and also the white layer was detected.     

Investigations of White Layer Formation During Machining of Powder Metallurgical Ni-Based ME 16 Superalloy

Surface integrity of machined parts made from the advanced Ni-based superalloys is important for modern manufacturing in the aerospace industry. Metallographic observations of the ME 16 alloy microstructure were made using optical metallography and a high-resolution scanning electron microscope with energy dispersive x-ray spectrometer (HR SEM/EDS). Tool life of cemented carbide inserts with TiAlN coating during machining (finishing turning operation) of ME 16 superalloy has been studied and wear patterns of the cutting tools were identified. Surface integrity of the machined part after completion of the turning operation was investigated. The morphology of machined parts has been examined and cross-sections of the machined surfaces have been analyzed. The formation of white layer on the surface of the machined part was studied for varied machining conditions. It was found that a 2-4 μm thick white layer forms during turning of the ME 16 superalloy. This layer was investigated using EDS and XRD. The studies show that the white layer is an oxygen-containing layer with a high amount of aluminum, enriched by chromium and tungsten. Under specific cutting conditions, the structure of white layer transforms into a γ-alumina. Formation of this thermal barrier ceramic white layer on the surface of the machined part negatively affects its surface integrity and cutting tool life.     

Process design space for optimal surface integrity in finish hard milling of tool steel

Hard milling has the potential to replace finish grinding in manufacturing dies and molds. A significant impediment for wide-spread application of hard milling is the lack of understanding and control on the surface integrity of machined surface and subsurface. In this study, a Taguchi design-of-experiment based dry finish milling of AISI H13 tool steel (50?±?1?HRc) with (Ti, Al) N/TiN coated cutting tools was conducted to investigate the process-induced surface integrity. The mechanism of surface integrity in hard milling was investigated to understand the effects of mechanical/thermal loads on surface microstructure and properties. The microstructure, microhardness and residual stresses were characterized. Phase transformation was not observed under the process parameters, while the increased microhardness and high compressive residual stresses obtained are beneficial for improving fatigue properties and wear resistance of the machined components. Finally, the process design space for the desired surface properties has been established via the microhardness and residual stress maps.     

Surface integrity of dry machined titanium alloys

The study is focused on the machined surface integrity of titanium alloy under the dry milling process. Roughness, lay, defects, microhardness and microstructure alterations are investigated. The result of surface roughness shows that the CVD-coated carbide tool fails to produce better Ra value compared to the uncoated tool. Lay is found to be dependent on cutting speed and feed speed directions. Microhardness is altered down to 350 μm beneath the machined surface. The first 50 μm is the soft sub-surface caused by thermal softening in the ageing process. Down to 200 μm is the hard sub-surface caused by the cyclic internal work hardening and then it gradually decreased to the bulk material hardness. It was concluded that for titanium alloys, dry machining can be carried out with uncoated carbide tools as far as cutting condition is limited to finish and/or semi-finish operations.     

切削Inconel718时新型陶瓷刀具JX-2-Ⅰ的切削性能研究

JX－2－Ⅰ是最新研制的碳化硅晶须（SiCw）增韧和碳化硅颗粒（SiCp）弥散增强氧化铝（Al2O3）新型陶瓷刀具。本文详细研究了该刀具加工Inconel718时的切削性能，结果表明，在低速干切时的刀具抗磨损能力为YG8＞JX－2－Ⅰ＞JX－1＞JX－2－Ⅱ；在105m／min的高速湿式切削时，JX－2－Ⅰ的切削性能与JX－1差不多，但是在42m／min的速度时JX－2－Ⅰ的切削性能好于JX－1（Al2O3＋SiCw）。同时发现在用JX－2－Ⅰ中高速切削Inconel718时必须使用冷却液。由于切削温度对工件材料加工硬化的影响，以及对工件材料高温强度屈服拐点的影响而存在一个切削速度的最佳选取范围。SEM分析表明，刀具磨损的主要形式是后刀面磨损、边界磨损、切深沟槽磨损和前刀面月牙洼磨损；刀具磨损的主要机理是粘结磨损、磨粒磨损和塑性变形磨损。     

Surface characteristics of machined Inconel-718 nickel-base superalloy using natural and controlled contact length tools

The surface characteristics of machined Inconel-718 nickel-base superalloy were investigated using natural and controlled contact length tools, at various cutting speeds, and under both dry and lubricated conditions. Surface damage in the form of cavities and holes, fractured areas, and long and short straight grooves parallel to the direction of cutting was produced when machining was carried out at low cutting speeds and with tools having natural or controlled contact length, and at high cutting speeds with tools having natural contact lengths. The intensity of surface damage and total area affected decreased as the cutting speed was increased. The quality of the machined surface was drastically improved at high cutting speeds, when tools having controlled contact lengths were used. The application of lubricants was less effective for improving the surface quality. The results were interpreted in terms of the mechanics of chip formation, the interaction between the tool-nose region and freshly machined surface, and the tool forces resulting from the machining process.     

镍基高温合金表面完整性研究现状分析

表面完整性是评价零件表面加工质量的重要标准之一。在切削镍基高温合金GH4169的过程中,加工硬化、变质层的形成等是重要的加工难题,因此研究GH4169零件的表面完整性对其高速切削加工过程有重要的意义。从表面粗糙度、表面加工硬化、白层以及表面残余应力等方面对切削镍基高温合金的表面完整性研究进展进行总结,从切削用量、工艺参数和刀具的选用等方面为提升加工零件表面质量提供了重要依据。     

An experimental study of the effect of high-pressure water jet assisted turning (HPWJAT) on the surface integrity

This study deals with the effect of High-Pressure Water Jet Assisted Turning (HPWJAT) of austenitic stainless steels on chip shape and residual stresses. The machining of the austenitic stainless steels represents several difficulties. Recently, research has shown that the introduction of a high-pressure water jet into the gap between the tool and the chip interface is a very satisfactory method for machining applications. In this article, the effect of a high-pressure water jet, directed into the tool-chip interface, on chip shapes breakage and surface integrity in face turning operations of AISI 316L steel has been investigated. Tests have been carried out with a standard cutting tool. The cutting speeds used were 80 and 150 m/min, with a constant feed rate of 0.1 mm/rev and a constant cutting depth of 1 mm. Three jet pressures were used: 20, 50 and 80 MPa. Residual stress profiles have been analysed using the X-ray diffraction method in both longitudinal and transversal directions. The results show that jet pressure and cutting parameters influence the residual stresses and the chip shapes. Using a high-pressure jet, it is possible to create a well fragmented chip in contrast to the continuous chip formed using dry turning. It is also possible to control the chip shape and increase tool life. When the jet pressure is increased the residual stress at the surface decreases; however it is increased by an increase in cutting speed. It can be concluded that surface residual stresses can be reduced by the introduction of a high-pressure water jet.     

An experimental study of tool wear and cutting force variation in the end milling of Inconel 718 with coated carbide inserts

Inconel 718 is a difficult-to-cut nickel-based superalloy commonly used in aerospace industry. This paper presents an experimental study of the tool wear propagation and cutting force variations in the end milling of Inconel 718 with coated carbide inserts. The experimental results showed that significant flank wear was the predominant failure mode affecting the tool life. The tool flank wear propagation in the up milling operations was more rapid than that in the down milling operations. The cutting force variation along with the tool wear propagation was also analysed. While the thermal effects could be a significant cause for the peak force variation within a single cutting pass, the tool wear propagation was believed to be responsible for the gradual increase of the mean peak force in successive cutting passes.     

58SiMn高强度钢车削表面完整性的试验研究

目的研究58Si Mn高强度钢表面完整性评价指标受切削参数影响的变化规律。方法分别设计单因素和正交试验,采用涂层硬质合金刀具对58Si Mn高强度钢进行车削加工试验,通过采集相关数据,分别讨论了切削深度、进给速度和切削速度变化对表面粗糙度、残余应力、显微硬度和表层微观组织变化等方面的影响。结果进给速度对表面粗糙度的影响最显著,切削速度次之,切削深度的变化对表面粗糙度无直接影响。已加工表面的残余应力随切削速度和进给量的增大而增大。显微硬度随切削深度的增大而减小,随进给量的增大而增大,层深上的显微硬度则呈现先减小后增大的趋势。表层微观组织受切削速度影响不大,未出现明显的相变和晶粒歪曲。结论降低进给速度是减小工件表面粗糙度最直接有效的方法,提高切削速度并不能使表面粗糙度明显减小。工件表面的轴向和切向残余应力均为拉应力,为提高零件使用性能,应采取相应的措施使之转化为压应力。     

Laser-assisted machining of Inconel 718 with an economic analysis

Superalloys have high strengths at elevated temperatures, which make them attractive toward various applications and also make these materials difficult to machine at room temperature due to excessive tool wear and poor surface finish. Laser-assisted machining (LAM) offers the ability to machine superalloys more efficiently and economically by providing the local heating of the workpiece prior to material removal by a single point cutting tool.An existing transient, three-dimensional heat transfer model is modified for modeling LAM of Inconel 718. Suitable coating conditions are determined for increasing the laser absorptivity in metals and an approximate absorptivity value is determined. The thermal model is validated in axial and circumferential directions by temperature measurement using an infrared camera.The machinability of Inconel 718 under varying conditions is evaluated by examining tool wear, forces, surface roughness, and specific cutting energy. With increasing material removal temperature from room temperature to 620 °C, the benefit of LAM is demonstrated by a 25% decrease in specific cutting energy, a 2–3-fold improvement in surface roughness and a 200–300% increase in ceramic tool life over conventional machining. Moreover, an economic analysis shows significant benefits of LAM of Inconel 718 over conventional machining with carbide and ceramic inserts.     

Influence of cryogenic heat-treatment soaking period and temperature on performance of sintered carbide cutting tools in milling of Inconel 718

Cryogenic heat treatment is an innovative heat treatment applied to improve the mechanical properties of sintered carbide cutting tools. In this study, the effect of cryogenic heat treatment used at different temperatures and soaking periods on the mechanical properties of carbide cutting tools was investigated. Cryogenic heat treatment was applied at two different temperatures, −145 °C and − 196 °C, for 24 and 36 h soaking periods. The microstructure of the cryogenic heat-treated cutting tool was investigated as microhardness and grain size. As a result of microstructure analysis, heat treatment soaking period was found to be more effective than heat treatment temperature. Milling tests of the Inconel 718 superalloy were performed under dry conditions using cryogenic heat-treated cutting tools and non-heat treated tools. The effect of heat treatment on cutting tool performance was studied in terms of the amount of tool wear, cutting force, surface roughness values. According to the results obtained, the cryogenic heat treatment applied to the cutting tools has a positive effect on cutting performance. In addition, in the examinations carried out on worn cutting tools, effective wear mechanisms were found to be oxidation, adhesion and abrasion.     

Investigation of machining performance in high-pressure jet assisted turning of Inconel 718: An experimental study

This paper investigates the application of the high-pressure jet assistance (HPJA) in rough turning of Inconel 718 with coated carbide tools. The region of operability, which sets the boundaries of the process parameters, has been experimentally determined using the tool–material pair (TMP) methodology. The assessment of HPJA machining performance is based on response surface methodology (RSM), which integrates a design of experiment (DOE) and a regression modelling technique for fitting a model to experimental. This leads to a new insight into the influence of the cutting process on chip breakability, cutting forces, contact length, surface finish and tool temperature, which are the key machining performance measures.