Performance enhancement of cryogenic machining and its effect on tool wear during turning of Al-Ticp composites

The current research is to focus on developing a liquid nitrogen diffusion system to optimize the usage of liquid nitrogen and maximizing cooling and lubrication capability by effective penetration. An atomized liquid nitrogen spray system was developed to diffuse liquid nitrogen effectively at a low flow rate (10–12 L/h) and as a high velocity (8–50 m/s) droplet jet to the machining zone. Using coated carbide tool with varying tool geometry (rake angle, approach angle, and nose radius), an investigation was performed to study the role of atomized liquid nitrogen spray-assisted machining, on performance of tool and surface quality of the machined workpiece during turning of Al-TiCp composite. To analyze the performance of liquid nitrogen spray-assisted machining, various experiments were conducted. The results obtained from the experiments reveal that the effective use of atomized liquid nitrogen spray machining is a feasible alternative to dry, wet, and cryogenically chilled argon gas. This technique significantly reduces heat generation in machining zone. The study also emphasizes the influence of tool geometry on the machinability of Al-TiCp metal matrix composites.     

低温加工及其应用

从改善难加工材料的切削加工性入手 ,论述低温加工的优势、效益及分类 ,同时介绍低温加工刀具材料 ,切削用量的选择。     

Lubrication effect of liquid nitrogen in cryogenic machining friction on the tool-chip interface

The liquid nitrogen as an environmentally safe coolant has been widely recognized in cryogenic machining, its function as a lubricant is plausible due to its chemical inertness, physical volatility and low viscosity. Since a reduced friction is a direct witness of the lubrication effect from a tribological viewpoint, this paper presents an evaluation of the apparent friction coefficient on the tool-chip interface in cryogenic cutting operations to prove and characterize the lubricity of LN2 in cryogenic machining. The cryogenic cutting technology used in this study is based on a cooling approach and liquid nitrogen delivery system which are intended to apply liquid nitrogen in well-controlled fine jets to selectively localized cutting zones and to penetrate liquid nitrogen to the tool-chip interface. It has been found that the apparent friction coefficient can be significantly reduced in cryogenic machining, depending on the approach of liquid nitrogen delivery.     

Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment

In a cryogenic environment components with interacting surfaces in relative motion (tribosystems) like bearings, seals and valves often generate undesired heat and experience high wear. Because conventional lubricants like oils or greases cannot be used in this temperature range, solid lubricants or materials with good frictional properties in unlubricated operation must be used. To test the suitability of conventional and advanced materials for tribosystems in this extreme environment and to obtain reliable materials data, model friction tests at low temperatures are performed. In most of the tests the samples are in ball-on-disc-configuration but various methods for cooling, loading, and measurement are applied. As an example the test results for amorphous carbon coatings in the temperature range 10–77 K are presented. These coatings can be suitable for cryogenic tribosystems, but their behaviour strongly depends on the composition and deposition method.     

Creation of ultrafine-grained surfaces by large strain extrusion machining (LSEM)

Large strain extrusion machining (LSEM) is examined as a route for achieving controlled microstructure refinement at freshly generated surfaces in a single pass of the machining tool. It is shown that the extrusion ratio λ of LSEM, which is the ratio of the thickness of the chip to that of the preset depth of cut, controls the extent of the ultrafine-grained (UFG) zone. Microstructure analysis was performed using orientation imaging microscopy (OIM) and mechanical testing using nanoindentation was used to characterize the UFG microstructure beneath the freshly generated surfaces. The mechanics of deformation in LSEM were examined using infrared thermography and modeled. The present research demonstrates LSEM as a novel platform for tailoring surficial microstructures and controlling their spatial extents in fabricated components.     

Coatings on Ti6Al4V as palliatives for fretting fatigue in cryogenic environment

Fretting, also known as small oscillatory sliding motion, can lead to catastrophic failure in industrial applications. To reduce the damage caused by fretting increasing use has been made of surfaces treatments. These treatments result in multilayer solids (coating, diffusion layer…). The understandings of fretting fatigue have enabled us to evaluate the fretting resistance of homogeneous and coated materials. The experimental part is associated to a numerical one to obtain cracking threshold of the coated materials to obtain lifetime evaluation. This present study seeks to compare the behaviour of bare substrate and coated substrate submitted to fretting and evaluates the improvement of the fretting fatigue strength.     

Modeling subsurface deformation induced by machining of Inconel 718

Traditionally, the development and optimization of the machining process with regards to the subsurface deformation are done through experimental method which is often expensive and time consuming. This article presents the development of a finite element model based on an updated Lagrangian formulation. The numerical model is able to predict the depth of subsurface deformation induced in the high- speed machining of Inconel 718 by use of a whisker-reinforced ceramic tool. The effect that the different cutting parameters and tool microgeometries has on subsurface deformation will be investigated both numerically and experimentally. This research article also addresses the temperature distribution in the workpiece and the connection it could have on the wear of the cutting tool. The correlation of the numerical and experimental investigations for the subsurface deformation has been measured by the use of the coefficient of determination, R². This confirms that the finite element model developed here is able to simulate this type of machining process with sufficient accuracy.     

Comparative investigation on using cryogenic machining in CNC milling of Ti-6Al-4V titanium alloy

Ti-6Al-4V titanium alloy is one of the most important materials in industry, 80% of which is used in aerospace industry. Titanium alloys are also notoriously difficult-to-machine materials owing to their unique material properties imposing a major bottleneck in manufacturing systems. Cryogenic cooling has been acknowledged as an alternative technique in machining to improve the machinability of different materials. Although milling is considered to be the major machining operation for the manufacture of titanium components in aerospace industries, studies in cryogenic machining of titanium alloys are predominantly concentrated on turning operations. To address this gap, this article provides an investigation on the viability of cryogenic cooling in CNC end-milling of aerospace-grade Ti-6Al-4V alloy using liquid nitrogen in comparison with traditional machining environments. A series of machining experiments were conducted and surface roughness, tool life, power consumption, and specific machining energy were investigated for cryogenic milling as opposed to conventional dry and flood cooling. Analysis revealed that cryogenic machining using liquid nitrogen has the potential to significantly improve the machinability of Ti-6Al-4V alloy in CNC end-milling using solid carbide cutting tools and result in a paradigm shift in machining of titanium products. The analysis demonstrated that cryogenic cooling has resulted in almost three times increased tool life and the surface roughness was reduced by 40% in comparison with flood cooling.     

阀门低温试验装置的节约型设计

阀门低温试验装置兼有深冷处理和低温试验的双重功能,是低温阀门生产过程中的重要设备,其冷媒和试验介质等消耗巨大且价格昂贵,对低温阀门的整机成本具有重要影响。提出了一种低温阀门试验装置的节约型设计,从组成装置的低温系统、压力试验系统和测控系统等方面着手,寻找减少消耗的途径,并重点讨论了多种高效保温技术,通过液氮喷淋实现自动调温的方式,贵重试验介质回收重复利用技术,数据采集和智能化操作模式等,提出了应从多渠道实现装置的节约型设计。     

Size effects in finish machining of porous powdered metal for engineered surface quality

Porous tungsten is conventionally machined with the aid of a plastic infiltrant to achieve acceptable surface finish. For dispenser cathode application, both high surface porosity and low surface roughness are necessary. Cryogenic machining has already been demonstrated to be capable of eliminating the need for plastic infiltration by greatly reducing smearing of pores. In order to address the problem of undesirable brittle fracture during cryogenic machining, the importance of uncut chip geometry is investigated. The value of critical chip thickness, beyond which brittle fracture occurs, is found to be closely linked to the microstructure of the workpiece material. While machining with very low uncut chip thickness leads to ploughing and spalling of the workpiece surface, ductile mode machining of porous tungsten with cryogenic cooling is found to yield excellent surface quality. When the maximum uncut chip thickness is approximately equal to the average ligament diameter of 80% density porous tungsten (d ≈ 8–9 μm), ductile mode machining is possible under both dry and cryogenic conditions. Changes in shock compaction behavior of the workpiece material, leading to altered physical properties, is hypothesized to be the underlying mechanism of ductile mode machining of porous tungsten.     

虚拟加工过程薄壁工件铣削变形模型研究

在薄壁工件的切削加工中,工件的受力变形是影响加工过程的一个主要因素,文中针对薄壁工件侧面的立铣加工,建立了不同复杂度的切削过程模型,以仿真工件薄壁在加工中受到切削力发生的变形,虚拟实现受到工件变形影响下的切削过程。通过仿真创成的工件表面形貌与实际切成的工件表面的比较,对文中各模型进行了验证。     

Modeling of thermomechanical shear instability in machining

The modeling of thermomechanical shear instability in the machining of some difficult-to-machine materials leading to shear localization is presented. Shear instability was observed experimentally in high-speed machining (HSM) of some of the difficult-to-machine materials, such as hardened alloy steels (e.g. AISI 4340 steel), titanium alloys (e.g. Ti-6A1-4V), and nickel-base superalloys (e.g. Inconel 718) yielding cyclic chips. Based on an analysis of cyclic chip formation in machining, possible sources of heat (including preheating effects) contributing toward the temperature rise in the shear band are identified. They include the four primary heat sources, the four preheating effects of the primary heat sources, the image heat source due to the primary shear band heat source, and the preheating effect of this heat source. The temperature rise in the shear band due to each of the heat sources is calculated using Jaeger's classical model for stationary and moving heat sources. Based on this temperature, Recht's classical model of catastrophic shear instability in metals under dynamic plastic conditions developed in 1964 is extended by predicting analytically the conditions for the onset of shear localization. This is done by comparing the strength of material in the shear band, σ′ under the combined effects of thermal softening and strain hardening with that of the material in the vicinity of the shear band, σ where the material has undergone small strains (i.e. up to yield point) and at the temperature caused by the preheating heat sources. Thus, σ is nearly the original strength of the work material. If σ′ < σ, thermal softening predominates at the shear band and shear localization will be imminent. The cutting speed for the onset of shear localization can be predicted based on thermomechanical shear instability model presented here. High-speed machining results reported in the literature for an AISI 4340 steel agree reasonably well with the analytical values developed in this investigation.     

为模具加工选择加工中心

本文针对模具数控加工的特点，提出了选用加工中心时应考虑的若干方面，以保证加工质量和提高生产效率。     

机械加工和电火花加工的组合技术在叶轮制造中的应用

介绍了机械加工和电火花加工的组合技术在窄流道闭式叶轮制造中的应用,通过对比试验可以充分征明,这种组合加工技术是电火花加工效率低、机械加工有干涉等问题的有效解决方法。     

超精密加工技术研究现状及发展趋势

超精密加工是多种技术综合的一种加工技术,是获得高形状精度、表面精度和表面完整性的必要手段。根据当前国内外超精密加工技术的发展状况,对超精密切削、磨削、研磨以及超精密特种加工及复合加工技术进行综述,简单地对超精密加工的发展趋势进行预测。     

伞形齿轮加工工装开发

在没有专业伞形齿轮加工设备的条件下,通过对伞形齿轮加工原理分析,结合数控车床刀架台和卡盘的位置特征,设计了一套数控车床加工伞形齿轮的工装装置。利用工装夹具的分度功能和数控车床刀架台的进给功能,解决了伞形齿轮轮齿的分度与铣削,实现了通用数控车床加工伞形齿轮。     

Chemical machining as a precision material removal process

The adaptation of a photochemical machining process to replace a precision machining operation is presented. An ultrasonically activated photoresist was used to establish the cutting boundary, and an etchant was used as a cutting tool to remove material from the workpiece. This precision material removal process proved to be highly effective when compared to more conventional precision machining techniques.     

Investigations aiming to increase electrochemical machining accuracy

Under conditions of electrochemical sinking, the standard deviation of inter-electrode gap thickness decreases with the decrease of machined area, while the machining accuracy increases. Advantage may be taken of this fact when using an electrode with a working area smaller than its machined area and when using different electrode kinematics than for sinking. For instance, the working electrode should move above the machined area at a velocity v_p. Theoretical and expeirmental research shows, that electrochemical machining (ECM) processes can show a significant increase of machining accuracy when compared to classical electrochemical sinking. This results mainly from the fact that parameters of the process can be chosen in such a way that the uneven distribution of physical properties in the machining space does not radically affect changes of inter-electrode gap thickness. Because the productivity of the discussed case is lower than for classical electrochemical sinking, it should be applied for finishing operations in which a small material excess is removed.     

A five-axis rough machining approach for a centrifugal impeller

A clear trend shows that most products or mechanical components, especially those regarding aerospace applications, are designed to fit the requirements of free form surface features. When a 3-axis computer numerical controlled (CNC) machining centre is used to produce a typical centrifugal impeller, great difficulties, i.e., collisions between the cutting tool and impeller, need to be overcome. In this case, sophisticated five-axis machines have to be utilised. Presently, most commercial computer-aided manufacturing (CAM) systems for five-axis control are lacking generality, and functions for the rough tool-path generation are far less than required. The rough machining is recognised as the most important procedure influencing the machining efficiency and is critical for the success of the following finishing process. However, great difficulties are expected to arise in performing five-axis rough machining. The main objective of the present study is to overcome this problem by combining related machining technology. As a result, CL data based on the geometry model of blade and hub of the impeller are generated. Finally, the CL data is confirmed through software simulation. The results of verification prove the machining methodology and procedure to be successful.     

虚拟加工系统研制与加工误差分析

面向数控车削加工过程研制了一个虚拟数控加工系统。该系统以Windows2000为开发平台，以Visual C 6．0为开发工具，基于OpenGL技术实现了数控车削加工过程3维仿真。该系统能够有效地仿真数控车削过程，具有更接近实际车削加工过程的特点。同时在仿真研究中，分析了加工过程中由切削力导致的加工误差，实现了对虚拟车削工件加工误差的预测。