Machinability study of high speed steel for focused ion beam (FIB) milling process – An experimental investigation at micron/nano scale

Nowadays the attention is focused on machining of non-silicon materials for miniaturized devices. High speed steel (HSS) is a non-silicon tool material, which is used in metal cutting applications as well as in micro-medical applications. Focused ion beam (FIB) milling process is highly suited for the fabrication of micro tools and other micro devices manufactured from HSS material. This study investigates the machinability aspects of HSS for FIB milling process. Beam current, extraction voltage, angle of incidence, dwell time and percentage overlap between beam diameters are the FIB process parameters, which have been analyzed experimentally to optimize FIB milling process for maximum material removal rate and minimum surface roughness. Beam current is found as the most significant parameter for controlling the material removal rate and surface roughness.     

Identification of the role of machinability and milling depth on machining time in controlled depth milling using abrasive water jet

Abrasive water jet machining (AWJM) is mainly used to through cut materials that are difficult to cut by conventional machining processes. This process may also be used for controlled depth milling (CDM) of materials. This work focuses on making blind pockets of controlled depths for a set of materials with AWJM. The materials used in the present work are AL 6061 alloy, AL 2024, brass 353, titanium (Ti6Al4V), AISI 304 (SS), and tool steel (M2 Rc 20). The effects of the milling depth and material characteristics on milling time are investigated. It is observed that machinability index and mechanical properties of the materials milled play important role in establishing milling time and surface roughness. It is found that traverse speed of AWJ process is lower for the materials with low machinability index and vice versa. Besides, the milling time increases non-linearly as the depth of milling increases due to loss of energy of jet and increase in standoff distance (SOD).     

Analytical modeling of work hardening of duplex steel alloys in the milling process

Severe plastic deformation in cutting operations such as milling might change mechanical properties (especially the strength and hardness) of the machined surface and its underlying layers. This phenomenon called work hardening and reduces machinability. This study presents an analytical solution to calculate the work hardening of the upper layers of the workpiece in the milling process of 2205 duplex stainless steel. In this regard, the stresses in the cutting regions are calculated to find the stress and temperature fields in the workpiece. Then the strain and strain rate values are calculated for each point of the surface and subsurface layers using the determined stress field. Finally, the Johnson-Cook material model is used to calculate flow stress and work hardening. Experimental results of the different machining conditions have been used to validate the proposed model. However, comparisons of subsurface microhardness and resultant cutting force obtained by an analytical model with experimental tests showed that the model properly predicts the amount of work hardening.

     

高速铣削时硬质合金刀具与NAK80模具钢化学性能匹配研究

高速铣削加工时温度较高,易引起刀具与工件之间产生化学反应,加剧刀具与工件磨损,缩短刀具寿命。实现刀具材料和工件材料的化学性能匹配,可有效保证模具型面加工质量和提高生产效率。化学性能匹配主要从刀具材料与工件材料的氧化以及扩散方面进行研究。通过理论计算得到刀具与工件材料的化学反应自由能;结合高温氧化试验,研究NAK80模具钢的高温氧化产物并探究其抗氧化性能;基于变系数浓度扩散模型,利用元素扩散试验获得了NAK80模具钢与刀具材料间的元素扩散强度。结果表明,NAK80模具钢的氧化程度随温度升高而加剧,温度高于1300K可发生完全氧化;NAK80模具钢中的Fe元素扩散深度随硬质合金刀具中Co元素含量的增加而增加。     

Tribological compatibility and improvement of machining productivity and surface integrity

Machining process productivity and machined part quality improvement is a considerable challenge for modern manufacturing. One way to accomplish this is through the application of PVD coatings on cutting tools. In this study the wear rate and wear behavior of end milling cutters with mono-layered TiAlCrN and nano-multilayered self-adaptive TiAlCrN/WN PVD coatings have been studied under high performance dry ball-nose end-milling conditions. The material being machined in this case is hardened H13 tool steel. The morphology of the worn surface of the cutting tool has been studied using SEM/EDX. The microstructure of the cross-section of the chips formed during cutting was analyzed as well. The surface integrity of the workpiece material was also evaluated. Surface roughness and microhardness distribution near the surface of the workpiece material was also investigated. The data presented shows that achieving a high degree of tribological compatibility within the cutting tool/workpiece system can have a big impact on tool life and surface integrity improvement during end milling of hardened tool steel.     

Cutter path orientations when high-speed finish milling inclined hardened steel

The use of high-speed milling (HSM) for the production of moulds and dies is becoming more widespread. Critical aspects of the technology include cutting tools, machinability data, cutter path generation and technology. Much published information exists on cutting tools and related data (cutting speeds, feed rates, depths of cut, etc.). However, relatively little information has been published on the evaluation of cutter paths for this application. Most of the research focuses on cutter path generation with the main aim on reducing production time. Work concerning cutter path evaluation and optimisation on tool wear, tool life and relevant workpiece machinability characteristics are scant. This paper investigates and evaluates the different cutter path orientations when high-speed finish milling inclined hardened steel, at a workpiece inclination angle of 75°. The results demonstrate that employing a vertical downward orientation achieved the longest life. However, in terms of workpiece surface roughness, vertical upward orientation is generally preferred.     

Electromagnetic actuator for determining frequency response functions of dynamic modal testing on milling tool

Machine tools are the main driving forces of industrialization of a country. However, poor machinability because of chatter vibration results in poor surface quality, excessive noise, and reduced material removal rate. Modal testing is a useful method to investigate dynamic properties of a cutting tool system and improve material removal rate. However, at present, modal testing using impact hammer is limited by certain problems. This paper developed a non-contacting electromagnetic actuator (EMA) to determine frequency response functions (FRFs) under amplitude and speed dependencies of cutting milling tools. The geometry was designed using magnetic circuit analysis and generalized machined theory before finite element analysis was conducted using magnetostatic-ansys software. Next, EMA was used as a contacting and non-contacting exciter of a conventional milling machine to determine the FRFs and dynamic properties of milling tool with amplitude and speed dependencies including comparison with static FRFs. Subsequently, dynamic properties and FRFs are used to establish stability lobe diagram. Stability lobe diagram also shows an improvement of up to 5% of depth of cut at lower spindle speed. In conclusion, by generating force that applies to static and dynamic modal testing, an EMA can determine dynamic properties and stability lobe diagram for increasing material removal rate and production rate.     

Study of Residual Stress Distribution in the Machined Stainless Steel Components

The demand for high quality and fully automated production, coupled with advances in alloy development, focuses attention on the surface condition of products, especially the residual stresses on the machined surface because of its effect on component performance, longevity, and reliability. Although stainless steel is an important, material with wide application, it is not easy to obtain favorable surface condition, due to its sensitivity to thermal and mechanical operations. In order to obtain favorable surface conditions in a stainless steel component, it is necessary to have practical data which include information concerning tool wear, surface roughness and surface residual stress. In the research toward developing a machinability chart which can provide suitable cutting parameters for the high production rate and good quality surface, and can be used in computer controlled machining tools, surface residual stress distributions in the turning process for stainless steel were studied. Austenitic 304 stainless steel bars were selected as the workpieces and uncoated carbide tools were used in the tests. The obtained results will show residual stress conditions on the machined stainless steel components varying according to cutting conditions.     

AN EXPERIMENTAL AND FEM STUDY ON DRY TURNING OF ROTOR STEEL 26NiCrMoV145 USING MULTILAYER COATED CEMENTED CARBIDE TOOL

Rotor steel 26NiCrMoV145 has been widely employed in aerospace and power industry because of its unique properties. As to this new material that is used widely in important places, it is worth studying its machinability. An experiment and finite element (FE) simulation has been carried out to study the cutting forces, cutting temperature and tool wear mechanism during high speed dry turning of 26NiCrMoV145 using multilayer TiCN + Al₂O₃ coated carbide inserts. The orthogonal test was chosen and cutting forces were measured to validate the FE simulations. SEM analysis has been carried out on worn tools to determine the tool wear mechanism. Crater wear and coating peeling could be found at every cutting speed. Flank face wear appears smooth when cutting speed increases. Analyses of tool stress and temperature distribution during simulated cutting process were also taken out to help in understanding tool wear mechanism. At last, an optimization of the cutting conditions was suggested when considering material remove rate, cutting forces and tool wear.     

Tool wear and tool life in end milling of 15–5 PH stainless steel under different cooling and lubrication conditions

Most of the machining operations on stainless steel alloys are carried out with cutting fluid due to the poor machinability of this kind of material. Tool wear mechanisms are directly influenced by the cooling and lubrication condition to which the tool is exposed, especially in interrupted cutting processes. This work investigates tool wear mechanisms for an end milling operation of a precipitation-hardened martensitic stainless steel under four different cooling and lubrication conditions. The results demonstrated that the cooling and lubrication condition strongly influences tool life and the tool wear mechanism, and furthermore, that tool lubrication rather than cooling should be the purpose for using cutting fluid in this kind of operation, in order to avoid damage caused by tool temperature variations.     

Experimental investigations from conventional to high speed milling on a 304-L stainless steel

Last years analytical or finite element models of milling become more efficient and focus on more physical aspects, nevertheless the milling process is still experimentally unknown on a wide range of use. This paper propose to analyse with accuracy milling operations by investigating the cutting forces values, shape of cutting forces curves obtained for different cutting speeds, and related phenomena as tool wear or tool run-out. These detailed experimental data in milling constitute a suitable experimental basis available to develop predictive machining modelling. All the tests have been conducted on the 304-L stainless steel in many cutting configurations and for different tool geometries. The machinability of the 304-L stainless steel with different tools geometries and configurations in shoulder milling is defined by three working zones: a conventional zone permitting stable cutting (low cutting speed; under 200–250 m.min^?1), a dead zone (unfavourable for cutting forces level and cutting stability; between 250 and 450 m.min^?1), and a high speed machining zone (high cutting speed; up to 450–500 m.min^?1). All the used criteria (cutting forces, chips, wear) confirm the existence of these different zones and a correlation is proposed with cutting perturbations as tool run-out, cutting instability, ploughing, and abrasive wear.     

Investigation of the performance of carbide cutting tools with hard coatings in hard milling based on the response surface methodology

Machining of hard materials has become a great challenge for several decades. One of the problems in this machining process is early tool wear, and this affects the machinability of hard materials. In order to increase machinability, cutting tools are widely coated with nanostructured physical vapor deposition hard coatings. The main characteristics of such advanced hard coatings are high microhardness and toughness as well as good adhesion to the substrate. In this paper, the influence of hard coatings (nanolayer AlTiN/TiN, multilayer nanocomposite TiAlSiN/TiSiN/TiAlN, and commercially available TiN/TiAlN) and cutting parameters (cutting speed, feed rate, and depth of cut) on cutting forces and surface roughness were investigated during face milling of AISI O2 cold work tool steel (～61 HRC). The experiments were conducted based on 3¹³ factorial design by response surface methodology, and response surface equations of cutting forces and surface roughness were obtained. In addition, the cutting forces obtained with the coated and uncoated tools were compared. The results showed that the interaction of coating type and depth of cut affects surface roughness. The hard coating type has no significant effect on cutting forces, while the cutting force F _z is approximately two times higher in the case of uncoated tool.     

Modeling dynamics and stability of variable pitch and helix milling tools for development of a design method to maximize chatter stability

Chatter is one of the major limitations in milling operations causing poor quality and reduced productivity. Stability diagrams can be used to identify deep stable pockets which usually occur at high spindle speeds. However, the required high cutting speeds may not be applied in some cases due to machinability or machine tool limitations. It is known that variable pitch and helix tools help suppressing chatter even at low cutting speeds. These tools may offer high productivity if they are properly designed. The literature on variable geometry milling tools is mainly limited to modelling and simulation whereas for industrial applications design guidelines are needed for selection of variation pattern and amount which is the focus of this paper. Dynamics and stability of variable pitch and helix tools are modelled and solved in frequency domain as well as using Semi-Discretization Method employing multiple delays. A practical but accurate design method is presented for selection of the best variation combination to maximize chatter free material removal rate without using time consuming computer simulations. Increased stability with the tools designed using the proposed method is demonstrated by several examples which are verified experimentally.     

Experimental study in high speed milling of titanium alloy TC21

In this paper, a number of experiments were carried out to study the machinability of a new damage-tolerant titanium alloy (TC21). Firstly, the effects of milling parameters, tool material, and tool wear on cutting forces were investigated. Secondly, the effects of milling parameters and tool wear on cutting temperature were studied. Finally, the effect of tool material on tool life was explored. Results showed that cutting force, cutting temperature, and tool life were greatly influenced by milling parameters, tool material, and tool wear. Thus, cutting tool matching with proper milling parameters should be carefully chosen to satisfying the tool life in actual production.     

Study of Ti-1023 milling with toroidal tool

The Ti-1023 beta alloy titanium is used in aeronautics for the production of structure parts such as landing gear and as most of titanium alloys its machinability is poor explained by its physical and mechanical properties. This work presents the machinability results carried out for Ti-1023 milling in using toroidal tool. The aim of this research is the understanding of tool wear mechanisms and to establish the relation between cutting conditions, milling tool geometry, and tool life. A section is also devoted to the chip analysis. The wear modes are also analyzed and defined for different cutting tool geometries where several steps occur. This study is focus on material removal rate, tool life, and volume.     

Micrometer-scale machining: tool fabrication and initial results

Conventional milling techniques scaled to ultrasmall dimensions have been used to machine polymethyl methacrylate (PMMA) with micrometer-sized milling tools. The object of this work is to achieve machining of a common material over dimensions exceeding 1 mm while holding submicrometer tolerances and micrometer size features. Fabricating the milling tools themselves was also an object of the study. A tool geometry for nominal 25 micrometer diameter cutting tools was found that cuts PMMA with submicrometer tolerances over trench lengths of 2 mm. The tool shape is a simple planar facet cut by focused ion beam milling on ground and polished 25 micrometer diameter steel tool blanks. Pairs of trenches 24 micrometers wide, 26 micrometers deep, 2.3 mm long, with a 14 micrometer separation were milled under various machining conditions. The results indicate that the limits of the machining process in terms of speed, pattern complexity, and tolerances have not been approached. This is the first demonstration of a generic method for microtool making by focused ion beam machining combined with ultraprecision numerically controlled milling. The method is shown to be capable of producing structures and geometries that are considered inaccessible by conventional materials removal techniques, and generally regarded as applications for deep X-ray lithography.     

Machining of sintered steel under different lubrication conditions

Cutting experiments of sintered steel Fe–2Cu–0.8C under different lubrication/cooling conditions and with different tool materials were performed in order to clarify the reasons for the low machinability of the sintered steel, and investigate the applicability of minimum quantity of lubricant (MQL) to the machining of sintered steel. As a theoretical approach to the short tool life in machining sintered steel, the impacts to the cutting edge caused by micro-voids were theoretically modeled, and it was revealed that the cutting edge suffered from impacts at a very short interval causing high cycle fatigue. Extremely short tool life in dry machining of sintered steel already used for wet machining proved that coolant definitely increased the stress amplitude in high cycle fatigue. Application of the air jet to a tool showed the positive effect in reducing tool wear. This indicated the possibility that micro-cracks nucleated by high cycle fatigue were healed by oxidation. MQL cutting with small amount of oil and air jet is proved to be acceptable for machining sintered steel from the results of cutting experiments using uncoated and coated carbide and cermet tools. Finally, it is confirmed that thick coating of a carbide tool is effective for dramatically improving the machinability.     

Prediction of specific force coefficients from a FEM cutting model

This paper presents a method to obtain the specific cutting coefficients needed to predict the milling forces using a mechanistic model of the process. The specific coefficients depend on the tool–material couple and the geometry of the tool, usually being calculated from a series of experimental tests. In this case, the experimental work is substituted for virtual experiments, carried out using a finite element method model of the cutting process. Through this approach, the main drawbacks of both types of models are solved; it is possible to simulate end milling operations with complex tool geometries using fast mechanistic models and replacing the experimental work by virtual machining, a more general and cheap way to do it. This methodology has been validated for end milling operations in AISI 4340 steel.