Analysis of tool wear and residual stress of CVD diamond coated cemented carbide tools in the machining of aluminium silicon alloys

Aluminium alloys have found increasing applications in the automotive and aeronautical industries in recent years. Due to their extraordinary properties however, the machining of these alloys still poses difficulties, and requires the optimized combination of cutting tool material and geometry. The potential of CVD diamond coated carbide tools has been demonstrated in recent years, however tool wear and short tool life remain as issues to be resolved. Key to increasing the tool life of CVD diamond coated tools is the further development of the coating process to optimize the coating adhesion. An understanding of the substrate and coating residual stress profiles must be gained in order to achieve this. Compressive residual stresses in cutting tools can lead to a higher crack resistance, but also to early coating delamination and tool failure. To analyze the influence of residual stresses on the coating quality and tool life, the residual stress profiles of tungsten carbide substrates and CVD diamond coatings were measured using X-ray and synchrotron radiation. The influence of the tungsten carbide substrate type and the CVD diamond coating process on the residual stress profiles was thus determined. In order to analyze the performance of the coated tools and the influence of the residual stresses on the tool lifetime, machining tests were performed with two aluminium silicon alloys. The tool wear, tool lifetime and workpiece quality were examined. Finally, many of the commonly used wear tests used to analyze the wear resistance of tool coatings cannot be implemented for CVD diamond coatings due to their high hardness. An impact test was therefore constructed to allow the determination of the wear resistance of CVD diamond tools.     

CVD金刚石涂层刀具切削性能与磨损机理研究

针对普通刀具切削质量差、刀具耐用度低等问题,对CVD涂层刀具制备方法及切削性能进行研究。首先以硬质合金刀具为基体通过CVD方法制备金刚石涂层,分析涂层表面形貌。然后在不同条件下进行铝合金材料的干式切削试验,分析金刚石涂层对切削力、切削温度以及工件表面粗糙度的影响规律。最后,通过对刀具磨损机理的分析,讨论涂层对刀具使用寿命的影响。研究结果表明,所制备的涂层刀具能够降低切削力和切削温度,大大提高刀具的切削性能和工件的表面质量,并能有效提高刀具使用寿命。     

Using PCD for machining CGI with a CO2 coolant system

Polycrystalline diamond is widely used as a economic cutting material for machining non-ferrous materials such as aluminum. It is perceived that diamond cannot be used for cutting ferrous materials due to the high affinity of carbon to iron. Nevertheless, under certain conditions it is possible to use diamond materials for cutting ferrous metals. In order to avoid graphitization of the diamond matrix, it is necessary to keep the cutting temperature below the critical level of diamond graphitization. This paper presents the influence of a cryogenic CO₂ coolant strategy on the cutting process using PCD tools for cutting high strength compacted graphite iron (CGI). Investigations show, that tool wear behavior strongly correlates with the cutting speed, the cutting forces, cutting temperatures, and surface roughness of the workpiece. The test results show, that the tool life of PCD for cutting cast iron is dependent on the diamond grain size, the binder material, and the cutting parameters.     

Research Progress of Diamond Tool Machining Technology for Ferrous MetalsEI北大核心CSCD

金刚石刀具是超精密加工最理想的刀具之一,但在黑色金属超精密加工领域“石墨化”导致刀具快速磨损,其应用极大地受到了限制。首先,针对金刚石刀具的磨损机理进行介绍。然后,综述金刚石刀具切削黑色金属的几种常见方法,如刀具表面改性、工件表面改性、低温辅助切削、超声振动辅助切削等,通过研究实例来分析各方法的应用效果和存在问题,并从技术层面分析影响金刚石刀具在黑色金属加工领域发展的关键因素。最后,对金刚石刀具切削黑色金属未来的发展趋势进行探讨。总结金刚石刀具在黑色金属领域的加工方法,分析加工黑色金属时抑制金刚石刀具磨损的核心技术,对黑色金属的精密超精密加工具有重要的引领和推动作用。     

Temperature of internally-cooled diamond-coated tools for dry-cutting titanium

The role of cutting fluids is well known for the importance of removing heat from the cutting edge, lubricating the sliding chip contact and transporting the metal chips away from the cutting zone. Dry machining leads to increased cutting temperatures and higher wear rates resulting in shorter tool life; this is particularly evident in the cutting of high strength materials. Diamond coated cutting inserts are not usually considered for machining titanium due to rapid oxidation of the coating at the temperatures typical of titanium machining. This paper examines the formation of hot-spots on the rake face during dry and near-dry turning of titanium using conventional cemented carbide inserts. Machining performance is assessed by measurement of tool wear and tool life. Trials with an internally cooled tool with a specially designed, diamond coated insert have shown that the heat from the cutting operation can be rapidly diffused over the entire surface of the insert and thus successfully drawn away from the tool via closed loop recirculation of coolant through the tool holder. This enables wear to be inhibited by management of rake face temperature to keep it below the critical temperatures at which these prominent wear mechanisms operate. Measurements of change in coolant temperature before and after circulation are used to quantify the heat removed from the cutting process. The low friction coefficient and high thermal conductivity of diamond, assisted by the indirect cooling, results in longer tool life whilst maintaining high standards of surface finish.     

金刚石涂层刀具加工石墨的切削性能

为充分对比不同类型金刚石涂层刀具的切削性能,定制几种不同类型金刚石涂层刀具进行等静压石墨切削加工,并与WC硬质合金刀具和TiAlN涂层刀具的切削情况对比,分析不同类型金刚石涂层刀具的涂层形貌、切削寿命、加工后的表面质量以及切削力。结果表明:制备的金刚石涂层刀具的涂层形貌主要为纳米晶和微晶,其寿命是硬质合金和TiAlN涂层刀具的10倍以上,且几种不同类型的金刚石涂层刀具寿命差异较小;金刚石涂层表面的晶粒细化可以降低加工表面的粗糙度和切削力,涂层脱落是金刚石刀具的主要磨损形式。      

Machinability of ultrafine-grained copper using tungsten carbide and polycrystalline diamond tools

Equal channel angular extrusion (ECAE) is an effective process to produce bulk ultrafine-grained (UFG) materials from regular coarse-grained materials. Such ECAE-processed materials typically excel in strength, wear resistance, ductility, and high strain-rate superplasticity, with promising applications in lightweight transportation and medical industries. Precision machining work is generally indispensable for further applications after bulk materials are produced by ECAE. To effectively and efficiently machine such ECAE-processed materials for further broad applications, machining issues such as machinability and tool material selection should be considered. This study was undertaken to investigate the machinability of ECAE-processed pure copper using both tungsten carbide (WC) and polycrystalline diamond (PCD) cutting tools in order to facilitate broad applications of ECAE-processed UFG coppers. It is found that despite its higher cost, PCD is favored to machine UFG copper based on this study since it has better wear resistance, gives lower cutting forces, yields a better workpiece surface finish, and results in no smearing on the workpiece. In machining UFG copper, depth of cut notching was observed as the wear pattern and abrasion as the wear mechanism for the WC tool, while flank wear was observed as the wear pattern and diffusion as the wear mechanism for the PCD tool.     

CVD diamond coatings on geometrically complex cutting tools

The manufacturing of chemical vapour deposition (CVD) diamond coated shaft type cutting tools is demanding due to the complex design of the cutting edges and the cobalt content of the cemented carbide. The influencing parameters of substrate, pre-treatment and diamond film on the tool cutting performance are discussed. The optimised manufacturing route of CVD diamond coated thread milling drills is identified with the use of material and tribological tests. Following the optimised production of the tools, the thread milling drills are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.     

Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools

Hypereutectic aluminium silicon alloys, e.g. casted AlSi17Cu4Mg, are commonly used in the automotive and aeronautical industries. These alloys consist of hard, abrasive silicon particles in a soft aluminium matrix and thus place high mechanical loads on the tool during machining processes. Polycrystalline Diamond or CVD (chemical vapour deposition) diamond based cutting tools can be used for the high speed machining of these alloys due to their high hardness and wear resistance. Diamond thin film coatings of different film morphologies are commonly applied on cemented carbide tools using Hot Filament CVD. The distinguishing characteristic to other coatings is utmost hardness resulting in high resistance to abrasion, low tendency to adhesion and low friction coefficient. The manufacturing of CVD diamond coated shaft type cutting tools is challenging due to the complex design of the cutting edges and the demanding stress behaviour during tool application. The influencing parameters of substrate type, chemical and mechanical substrate pre-treatment as well as diamond film modification on the tool cutting performance are discussed. The manufacturing route of CVD diamond coated thread milling drills is analysed with the use of material and tribological tests. The complex thread manufacturing tools are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.     

Built-up edge effect on tool wear when turning steels at low cutting speed

In any machining process, it is very important to control the cutting variables used during the process because these will affect, for example, tool life and workpiece surface roughness. Since the built-up edge (BUE) increases the wear of the tool and affects the surface roughness of the workpiece, the study of this phenomenon is very important in predicting and minimizing the wear of a cutting tool. This research studies the influence of the BUE formation for coated carbide tools when turning medium- and high-strength steels. Different mathematical expressions were obtained to quantify this effect. Mathematical expressions for uncoated carbide tools were not possible to obtain, due to the fact that for these tools an increase in the wear and their premature fracture was observed.     

Cutting performance of multilayer diamond coated silicon nitride inserts in machining aluminum–silicon alloy

Aluminum–silicon (Al–Si) alloy is very difficult to machine and diamond tools are considered by far the best choice for the machining of these materials. Experimental results in the machining of the Al–Si alloy with diamond coated inserts are presented. Considering the fact that high adhesive strength and fine surface morphology play an importance role in the applications of chemical vapor deposition (CVD) diamond films, multilayer technique combining the hot filament CVD (HFCVD) method is proposed, by which multilayer diamond-coating on silicon nitride inserts is obtained, microcrystalline diamond (MCD)/ nanocrystalline diamond (NCD) film. Also, the conventional monolayer NCD and MCD coated inserts are produced for comparison. The as-deposited diamond films are characterized by field emission scanning electron microscopy (FE-SEM) and Raman spectrum. All the CVD diamond coated inserts and uncoated insert endure the aluminum-silicon alloy turning to estimate their cutting performances. Among all the tested inserts, the MCD/NCD coated insert exhibits the perfect behavior as tool wear due to its very low flank wear and no diamond peeling.     

A study of the diamond tool wear suppression mechanism in vibration-assisted machining of steel

Inability of machining steel strongly inhibits the application of diamond machining in manufacturing industry, especially in the fields of ultra-precision and micro machining. In recent years, vibration-assisted machining (VAM) has been proved to be capable of efficiently suppressing the diamond tool wear in cutting steel. Currently, the prevailing speculation claimed by most researchers for such suppression is that the tool–workpiece flash temperature was reduced in VAM, which would slow the chemical reaction between iron on steel and carbon on diamond. However, the correctness of this speculation has not been proved by any experimental or theoretical research. In this paper, in order to understand the true wear suppression mechanism of diamond tools in VAM of steel, a study is conducted by measuring the workpiece temperatures and modeling the cutting energy consumption in both VAM and conventional cutting (CC). Based on the comparison results, it is concluded that the cutting temperature and energy consumption in VAM are not smaller than in CC, and hence the reduced diamond tool wear in VAM should not be caused by the claimed reduced temperature, especially when the material removal rate is very small. Finally, based on the EDS analysis and the comparison of experimental results under different air pressure, two probable reasons are proposed for the significantly reduced diamond tool wear in VAM of steel: (i) increase of gas pressure at the tool–workpiece interface and (ii) generation of an oxide layer on the freshly machined surface.     

Cutting Performance and Wear Behavior of AlTiN-and TiAlSiN-Coated Carbide Tools During Dry Milling of Ti-6Al-4V

Machining, especially dry machining of titanium alloys, has been one of the most significant challenges for carbide cutting tools. In this study, aluminum-rich AlTiN coating, as well as TiAlSiN nanocomposite coating, were successfully employed for dry milling of Ti-6Al-4V alloy with high efficiency and long tool life. At the cutting speeds of 150 m/min and 200 m/min, the tool life of the TiAlSiN-coated tool exceeds that of AlTiN-coated tool by 32 and 66%, respectively. The wear modes for both coated tools include the uniform flank wear, smooth wear, chipping, coating and substrate flaking, crater and notch wear, and the wear mechanisms include adhesion, diffusion, oxidation and crack. Among them, the wear mechanism is dominated by the adhesion and oxidation wear. As compared with AlTiN coating, TiAlSiN coating exhibits better mechanical properties and oxidation resistance, which contribute to a better cutting performance, fewer thermal cracks and smaller and uniform workpiece chips during the dry milling of Ti-6Al-4V alloy.     

Thermo-Chemical Wear Mechanism of Diamond Tool in Machining of Ferrous Metals

To understand the wear mechanism of diamond tool in machining of ferrous metals, an erosion test simulating wear process and ab-initio molecular orbital calculation are carried out. The results of the tests and analyses show that the essential wear mechanism at the temperature higher than 1000K is the dissociation of carbon atoms on diamond surface due to the interaction with iron surface. The wear rate is controlled by the removal rate of dissociated carbon atoms from the tool-work interface such as diffusion into workpiece. At the temperature lower than 900K, the mechanism involves the removal of carbon atoms due to oxidization of diamond accompanied with deoxidization of iron oxide.     

Study on wear mechanisms and grain effects of PCD tool in machining laminated flooring

Polycrystalline diamond (PCD) tools have gained increasing application in woodworking industry for the phenomenal tool life and cutting finish compared with carbide tools. In the paper, machining experiments with PCD tools were conducted to mill laminated flooring with Al₂O₃ overlay. Four kinds of PCD products with different original diamond grain sizes were used to fabricate the cutters. Wear volume was measured by optical microscopy and wear morphology was examined by SEM and optical microscopy.The experimental results show that the wear mechanisms of PCD tools, in the machining process, involve inter-granular wear and partial cleavage fracture. The microcracks in PCD tools are a key reason for the wear of tools. By comparing the flank wear, the experiments reveal that PCD tools with middle original diamond grain size have long tool life. The influences of original diamond grain size on cutting edge and wear properties have also been discussed in detail.     

Tool wear control in single-crystal diamond cutting of steel by using the ultra-intermittent cutting method

Excessive tool wear is a major drawback to the ultraprecision cutting of steel with geometrically defined single-crystal diamond tools. This paper presents a new approach to reduce this wear. In general, the wear of the diamond tool is due to chemical reactions such as diffusion into the steel, oxidation, graphitization, and carbide formation under cutting conditions of high temperature and high pressure. To suppress these types of chemical reactions, the contact time between the diamond tool and the steel in the cutting process was controlled by intermittent cutting method such as fly-cutting or milling. A series of intermittent cutting experiments were carried out to control the tool–workpiece contact time in one cutting cycle by changing the cutting speed and cutting length in each cutting cycle. The experimental results showed that the diamond tool wear was highly dependent on the tool–workpiece contact time, regardless of the cutting speed, and that the wear was greatly reduced by decreasing the contact time to less than 0.3 ms under these cutting conditions. It is expected that steel can be successfully cut with a single-crystal diamond tool by controlling the tool–workpiece contact time.     

Tool wear characteristics in machining of nickel-based superalloys

Nickel-based superalloy is widely employed in aircraft engines and the hot end components of various types of gas turbines with its high strength, strong corrosion resistance and excellent thermal fatigue properties and thermal stability. However, nickel-based superalloy is one of the extremely difficult-to-cut materials. During the machining process, the interaction between the tool and the workpiece causes the severe plastic deformation in the local area of workpiece, and the intense friction at the tool–workpiece interface. The resulting cutting heat coupled with the serious work hardening leads to a series of flaws, such as excessive tool wear, frequent tool change, short tool life, low productivity, and large amount of power consumption etc., in which the excessive tool wear has become one of the main bottlenecks that constraints the machinability of nickel-based superalloys and its wide range of applications.In this article, attention is mainly focused on the tool wear characteristics in the machining of nickel-based superalloys, and the state of the art in the fields of failure mechanism, monitoring and prediction, and control of tool wear are reviewed. The survey of existing works has revealed several gaps in the aspects of tool self-organizing process based on the non-equilibrium thermodynamics, tool wear considering the tool nose radius, thermal diffusion layer in coated tools, tool life prediction based on the thermal–mechanical coupling, and industrial application of tool wear online monitoring devices. The review aims at providing an insight into the tool wear characteristics in the machining of nickel-based superalloys and shows the great potential for further investigations and innovation in the field of tool wear.     

Experimental investigation on the effect of the material microstructure on tool wear when machining hard titanium alloys: Ti–6Al–4V and Ti-555

An experimental investigation was conducted in this work to analyze the effect of the workpiece microstructure on tool wear behavior and stability of the cutting process during marching difficult to cut titanium alloys: Ti–6Al–4V and Ti-555. The analysis of tool–chip interface parameters such as friction, temperature rise, tool wear and workpiece microstructure evolution under different cutting conditions have been investigated. As the cutting speed increases, mean cutting forces and temperature show different progressions depending on the considered microstructure. Results show that wear modes of cutting tools used for machining the Ti-555 alloy exhibit contrast from those obtained for machining the Ti–6Al–4V alloy. Because of the fine-sized microstructure of the near-β titanium Ti-555, abrasion mode was often found to be the dominate wear mode for cemented cutting tools. However, adhesion and diffusion modes followed by coating delamination process were found as the main wear modes when machining the usual Ti–6Al–4V alloy by the same cutting tools. Moreover, a deformed layer was detected using SEM–EDS analysis from the sub-surface of the chip with β-grains orientation along the chip flow direction. The analysis of the microstructure confirms the intense deformation of the machined surface and shows a texture modification.     

CVD金刚石薄膜涂层刀具切削性能研究

本文采用不同涂层工艺的ＣＶＤ金刚石薄膜刀具切削高硅铝合金,观测比较刀具的磨损过程、磨损与破损形貌及工件表面粗糙度,分析ＣＶＤ金刚石薄膜刀具切削主崖裂口合金的磨损机理和失效原因。研究结果可为涂层工艺的提供了理论依据。     

High Speed Face Milling of a Aluminium Silicon Alloy Casting

High speed machining of aluminium silicon alloy castings has gained significant interest from automotive industry involved in the development of the new generation of lightweight vehicles. This paper investigates the influence of workpiece microstructure, namely the secondary dendritic arm spacing (SDAS), tool material and geometry on tool wear mechanisms, cutting forces and surface integrity when face milling at cutting speeds of 5,000 m min⁻¹. It was found that the SDAS is the parameter with the main influence on tool wear rate; higher SDAS values require polycrystalline diamond (PCD) tooling due to the lower wear rates when compared with carbide tools. Finite Element Analysis (FEA) was employed to study the influence of tool wear on temperature and shear stress distribution in the workpiece material.