Tribochemical reactions of steel in cutting edge material during secondary wood products cutting

In this paper, evidence of tribochemical reactions of a steel cutting edge during cutting of Secondary Wood Products (SWP) is reported. Applying Quantum Chemical (QC) methods, with semiempirical results verified using Density Functional Theory (DFT), it was determined that some thermal degradation products of the principle components of SWP would stabilize iron, from steel and iron from the binder in cemented carbide tool materials, in an oxidized state via octahedral hydroxy organometallic complexes. Evidence for the very complex thermal degradation processes of SWP components interaction with Fe corrosion processes is also observed in Thermal Gravimetry Analysis (TGA).     

Surface layer microhardness changes with high-speed turning of hardened steels

In high-speed cutting of hardened steels, the surface layer is strongly affected by thermal effects and mechanical forces. Due to this, the surface layer of the machined material changes noticeably. Microhardness, one parameter of the surface integrity, is the most important. This paper deals with an investigation of microhardness. Measuring results are presented, and reasons for the sometimes significant changes in microhardness are analysed. It is proved on which part of the cutting edge the material removal will not take place but the cutting edge deforms the material plastically and how this part of the cutting edge can be reduced. With the measurement of the cutting force, the hypothesis is proved that the values of the cutting force components related to each other are different compared to the traditional turning. The passive force is 1.88?C2.25 times higher than the main cutting force. Hence, the force taking place on the flank face of the cutting tool is very high and the friction power significantly influences the cutting temperature. The friction taking place on the flank face of the cutting tool generates 2.8?C3.9 times higher heat than the cutting force. Due to the changes that occur in the surface layer, the hardness of this layer is higher with 100?C150?HV in depth of 0.1?mm than the original hardness or the hardness prescribed in the technical drawings. This phenomenon can be observed not only in internal hard turning but also machining of external and conical surfaces. Microhardness is compared after hard turning and after grinding. According to the measurements, the ground surface has not become a harder surface layer but softer. As an average result of many measurements, it has been proven that the original hardness (700?HV) after case hardening will increase to 800?C850?HV after hard turning and will decrease to 500?C550?HV after grinding. Microhardness changes are analysed considering the typical chip removal characteristics of hard turning. In this article, the focus is on how changes in microhardness influence the functional behaviour of the components and may affect their lifetimes. In this article, it has been proven that independently from the surface of the machined gear (bore, conical or face surface) the changes in the surface layer regarding microhardness do not differ.     

A state-of-the-art review of ductile cutting of silicon wafers for semiconductor and microelectronics industries

Silicon is a typical functional material for semiconductor and optical industry. Many hi-tech products like lenses in thermal imaging, solar cells, and some key products of semiconductor industry are made of single crystal silicon. Silicon wafers are used as substrate to build vast majority of semiconductor and microelectronic devices. To meet high surge in demand for microelectronics based products in recent years, the development of rapid and cost efficient processes is inevitable to produce silicon wafers with high-quality surface finish. The current industry uses a sequence of processes such as slicing, edge grinding, finishing, lapping, polishing, back thinning, and dicing. Most of these processes use grinding grains or abrasives for material removal. The mechanism of material removal in these processes is fracture based which imparts subsurface damage when abrasive particles penetrate into the substrate surface. Most of these traditional processes are extremely slow and inefficient for machining wafers in bulk quantity. Moreover, the depth of subsurface damage caused by these processes can be up to few microns and it is too costly and time consuming to remove this damage by heavy chemical–mechanical polishing process. Therefore, semiconductor industry requires some alternative process that is rapid and cost effective for machining silicon wafers. Ductile cutting of silicon wafer has the potential to replace the tradition wafer machining processes efficiently. If implemented effectively in industry, ductile cutting of silicon wafers should reduce the time and cost of wafer machining and consequently improve the productivity of the process. This paper reviews and discusses machining characteristics associated with ductile cutting of silicon wafers. The limitations of traditional wafer fabrication, the driving factors for switching to ductile cutting technology, basic mechanism of ductile cutting, cutting mechanics, cutting forces, surface topography, thermal aspects, and important factors affecting these machining characteristics have been discussed to give a systematic insight into the technology.     

Material flow stress and failure in multiscale machining titanium alloy Ti-6Al-4V

Titanium Ti-6Al-4V alloy is a typical difficult-to-machine material due to its unique physical and mechanical properties. The material properties of Ti-6Al-4V play an important role in process design and optimization. However, the dynamic mechanical behavior is poorly understood and accurate predictive models have yet to be developed. This work focuses on the dynamic mechanical behavior of machining Ti-6Al-4V beyond the range of strains, strain rates, and temperatures in conventional materials testing. The flow stress characteristics of strain hardening and thermal softening can be predicted by the Johnson–Cook model coupled with the adiabatic condition. The predicted flow stresses at small strains agree very well with those from the split Hopkinson pressure bar (SHPB) tests, while the predicted flow stresses at large strains also agree with the calculated flow stresses based on the cutting tests with a suitable depth of cut. Heat fraction and temperature parameter control the range of thermal softening and the decrease rate of flow stress. The material may exhibit super plasticity at a small depth of cut with a large radius of the cutting edge in micromachining. Strain rate is one important factor for material fracture close to the cutting edge. The failure strain increases linearly with the increase of homologous temperature, while it only increases slightly with the strain rate.     

Thermal stress developed during the laser cutting process: consideration of different materials

The laser cutting of metallic substrates results in the development of thermal stresses around the cut edges. Depending on the cutting speed, laser power intensity, and material properties, stress levels reaching and exceeding the yielding limit of the substrate material can result. In the present study, the laser cutting situation is simulated and temperature as well as thermal stress fields are computed for steel, Inconel 625, and Ti-6Al-4V alloy. The cutting speed of the laser is considered to be constant and a constant temperature heat source with a focused spot diameter is assumed along the kerf surface at the cut edge, resembling the laser heat source. The equations for energy and thermal stresses are solved numerically using the finite element method (FEM). It is found that the temperature decays sharply in the vicinity of the cut edges and that the equivalent stress attains high values in this region. Inconel 625 results in the highest thermal stress levels in the vicinity of the cut edges and is then followed by steel and titanium alloy.     

Finite element modeling of residual stresses in machining induced by cutting using a tool with finite edge radius

A thermal elastic-viscoplastic finite element model is used to evaluate the residual stresses remaining in a machined component. An improvement in the accuracy of the predicted residual stresses is obtained by: (a) using a modified Johnson–Cook material model that is augmented by a linearly elastic component to describe the material behavior as non-Newtonian fluid; (b) using a remeshing scheme to simulate the material flow in the vicinity of the rounded cutting tool edge without the use of a separation criterion; (c) properly accounting for the unloading path, and (d) considering the thermomechanical coupling effect on deformation. Case studies are performed to study the influence of sequential cuts, cutting conditions, etc., on the residual stresses induced by orthogonal machining.     

加工高镍材料工件的刀具选用

高镍材料不会因为切削时产生高温而发生软化,因此在切削刃上会形成很高的压力。这样就可能引起刀刃因碎裂或变形而损坏。由于在切屑过程中,更强的材料产生更多的热,且导热性较低,因此形成高切削温度。大大增加了切削难度。     

Specific cutting force, tool wear and chip morphology characteristics during dry drilling of austempered ductile iron (ADI)

Dry machining is being recognized as ecological machining due to its less environmental impact and manufacturing cost. However, the choice of dry machining is mainly influenced by the workpiece material properties, machining operation and cutting conditions. The recent emergence of austempered ductile iron (ADI) can be considered a significant economic advantage to the increasing industrial demand for cost- and weight-efficient materials. However, due to its microstructure-induced inherent properties, ADI is considered hard-to-machine material. Thus, the dry drilling of ADI is investigated in this paper. The ADI material used in the present study is produced using an innovative process route for near net shape casting production. Drilling experiments are conducted on a DMU80P Deckel Maho five-axis machining centre using PVD-coated carbide tools under dry cutting environment. The dry drilling of ADI under different cutting conditions is evaluated in terms of specific cutting force and tool wear analysis. The influence of cutting conditions on chip morphology and surface roughness is also investigated. The experimental results revealed that the combination of the low feed rate and higher cutting speed leads to the higher mechanical and thermal loads on the tool's cutting edge, resulting in higher specific cutting force values. This behaviour is further supported by the chip morphology analysis, which revealed the formation of segmented chips at higher cutting speed with segment spacing increase with an increase in feed rate. Depending upon the cutting parameters, different modes of tool failures including crater wear, flank wear, chipping, breakage and built-up edge were observed. Surface roughness analysis revealed the influence of tool wear and chip morphology on the machined surface finish.     

CBN-WC-10Co刀具材料的电火花线切割机理

采用电火花线切割方法对CBN-WC-10Co刀具材料进行了线切割加工实验,利用带能谱分析的扫描电子显微镜（SEM)观察了切割试件截面和表面的显微形貌并进行了成分分析。从微观角度分别研究了CBN颗粒和硬质合金基体的材料去除机理,分析了CBN-WC-10Co刀具材料的电火花线切割热损伤情况。研究结果表明:CBN-WC-10Co刀具材料的电火花线切割材料去除机理主要是通过放电通道产生的高温使材料局部区域熔化或者气化,在放电爆炸力作用下被抛出熔池,同时放电爆炸力使部分CBN颗粒直接从硬质合金基体脱落,形成凹坑;黏结剂金属钴因其导电性好、熔点和沸点低而容易被过量去除,导致WC颗粒因失去支撑结构而从表层脱落,且放电通道内产生的爆炸冲击波也会造成WC边缘局部碎裂。     

THE OXLEY MODELING APPROACH, ITS APPLICATIONS AND FUTURE DIRECTIONS

The Oxley machining theory which allows for the high strain-rate/high temperature flow stress and thermal properties of the work material is described. It is shown how the theory that was originally developed for the orthogonal process and later extended to oblique machining, can be used to predict cutting forces, temperatures and subsequently built-up edge formation conditions, tool life and cutting edge deformation conditions. It is also shown how the theory can be applied to obtain predictions in machining with restricted contact tools and in intermittent cutting processes, and to obtain work material properties using machining test results. Finally, some consideration is given to the future directions of machining research at UNSW. The Oxley Model can be used for predicting the performance parameters for different machining processes by taking into account the fundamentals of the chip formation process.     

Investigation on diffusion wear during high-speed machining Ti-6Al-4V alloy with straight tungsten carbide tools

During high-speed machining Ti-6Al-4V alloy, high-temperature at the tool–chip interface and the concentration gradient of chemical species between tool material and workpiece material support the activation of diffusion process, and therefore the crater wear forms on the rake surface of the cutting tool at a short distance from the cutting edge. In this paper, the diffusion analysis was theoretically proposed. The constituent diffusion at the contact interface between tool material and Ti-6Al-4V alloy at high-temperature environment, the crater wear on the rake surface of the tool, and the chips collected from high-speed milling Ti-6Al-4V alloy with straight tungsten carbide tools were analyzed by the scanning electron microscope with energy dispersive X-ray spectroscopy. The constituents inside the tool could diffuse into the workpiece and the diffusion layer was very thin and close to the interface. Compared with the diffusion of tungsten and carbon atoms, the pulling out and removing of the tungsten carbide (WC) particles due to cobalt diffusion dominated the crater wear mechanism on the rake surface of the cutting tool.     

MODELLING THE BASIC CUTTING ACTION AND MACHINING PERFORMANCE OF SINTERED METALLIC MATERIALS

Extensive ‘classical’ orthogonal cutting tests have been run to study and model the basic cutting action and machining performance of a popular sintered metallic material widely used in structural automotive components. These tests were also run to assess whether the ‘Unified-Generalised Mechanics of Cutting Approach’ to force and power prediction in practical machining operations such as turning and drilling is likely to be applicable to sintered materials. The orthogonal cutting tests have shown that the modified mechanics of cutting analysis, incorporating the edge force, was equally applicable to sintered metallic materials while comprehensive drilling and turning tests over a wide range of operation variables have shown good qualitative and quantitative correlation between predicted and measured forces and torques thus confirming the validity of the predictive force models for machining sintered materials. This investigation has provided further evidence of the generic nature of the predictive machining performance modelling approach developed at the University of Melbourne and its applicability to both conventionally produced and sintered metallic materials for structural components.

It has also been shown that the chip formation, cutting characteristics and mechanics of cutting analyses for the sintered metallic material for structural components, with its relatively high density and low porosity, are qualitatively similar to those for conventionally produced metallic materials. Quantitatively, however, the basic cutting quantities for use in the predictive force models are different and have to be established for each tool-workpiece material combination irrespective of whether the metallic material is a conventionally produced material or a sintered P/M material.     

Fabrication of polycrystalline diamond microtool using a Q-switched Nd:YAG laser

A Q-switched Nd:YAG laser (1,064 nm, 100 ns) was used to machine 2?×?1.5?×?0.5-mm rhombus-shaped tool inserts from a 60?×?0.5-mm circular disk of polycrystalline diamond. A systematic experimental study was undertaken to examine the effects of pulse repetition rate, feed rate, and number of laser passes on kerf, material removal rate, recast layer, surface morphology, and surface roughness. The optimal laser parameters for generating two-dimensional tool profiles were an average power of 3 W, a pulse repetition rate of 2 kHz, a feed rate of 1 mm/s, and a total of 45 laser passes. The beneficial results were a material removal rate of 0.02 mm³/min, kerf width of 27 μm, cutting edge radius of 6 μm, and surface roughness (Ra) of 0.625 μm. Recast layer formation, undulations, and striations were observed in the laser-cut regions. These features were attributed to the presence of a molten layer of cobalt binder, and amorphous carbon and graphite transitioned from diamond. An intriguing feature is the presence of fine particulate matter ranging in size from nanometers to a few micrometers in the laser-cut regions. It is believed that phase transition of diamond and cobalt during laser machining created thermal expansion mismatch stresses sufficient to fracture the solid into fine fragments.     

THE OXLEY MODELING APPROACH,ITS APPLICATIONS AND FUTURE DIRECTIONS

Abstract

The Oxley machining theory which allows for the high strain-rate/high temperature flow stress and thermal properties of the work material is described. It is shown how the theory that was originally developed for the orthogonal process and later extended to oblique machining, can be used to predict cutting forces, temperatures and subsequently built-up edge formation conditions, tool life and cutting edge deformation conditions. It is also shown how the theory can be applied to obtain predictions in machining with restricted contact tools and in intermittent cutting processes, and to obtain work material properties using machining test results. Finally, some consideration is given to the future directions of machining research at UNSW. The Oxley Model can be used for predicting the performance parameters for different machining processes by taking into account the fundamentals of the chip formation process.     

Study on tool wear mechanisms in milling laser sintered material

This paper investigates tool wear mechanisms of a ball end mill in cutting laser sintered material. Cutting edge temperature is measured by using a three-color pyrometer with an optical fiber. Bulk carbon steel JIS S55C is selected as the standard steel. Experimental results show that tool life in cutting sintered material is shorter than that in cutting JIS S55C. Observations by SEM show that adhesion of the work material and micro chipping are the main wear mechanisms in cutting sintered material. The corresponding cutting edge temperature shows a continuous increase as wear evolves with cutting time.     

基于修正滑移线场模型的倒棱刀具切削力预测

基于材料塑性滑移理论与刀具刃前材料流动状态分析,提出了一种考虑倒棱刀具负前角切削过程下的材料滞流区（死区）和预剪区的修正滑移线场模型,并给出了材料流动剪切应力和刃前切削几何参数的迭代求解方法,揭示了倒棱刃口几何形状与滑移线场几何参数之间的变化规律。将此模型应用于倒棱刀具切削过程,得到了适用于倒棱刀具正交切削力的预测方法。采用有限元仿真和切削试验相结合的方法对所提出的滑移线场模型和切削力预测方法分别进行了验证,模型预测结果与仿真结果和试验测量结果对比误差均在10%以内。研究结果为研究倒棱几何形状对工件材料流动特性和刀具切削性能的影响提供了参考。     

硬质合金圆盘刀分切硅钢片的磨损形态及机理研究

分析了圆盘剪分切加工过程中圆盘刀的受力,利用硬质合金圆盘刀进行了硅钢片的分切加工试验,研究了硬质合金圆盘刀的磨损,对比分析了圆盘刀刃口磨损前后的表面形貌、微观结构和钴元素含量的变化,探讨了其磨损形式及机理。结果表明,硬质合金圆盘刀磨损主要在刃口两侧形成磨损带,随着磨损的发生,圆盘刀切削刃过渡圆弧半径增大、刃口变钝,刀具的磨损形式主要表现为WC硬质颗粒裸露脱落及材料的黏结撕裂,磨损机理主要为硬质合金黏结相钴元素的流失、疲劳磨损和黏结磨损。     

Effects of cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and forces in finish turning of hardened AISI H13 steel

In this study, the effects of cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and resultant forces in the finish hard turning of AISI H13 steel were experimentally investigated. Cubic boron nitrite inserts with two distinct edge preparations and through-hardened AISI H13 steel bars were used. Four-factor (hardness, edge geometry, feed rate and cutting speed) two-level fractional experiments were conducted and statistical analysis of variance was performed. During hard turning experiments, three components of tool forces and roughness of the machined surface were measured. This study shows that the effects of workpiece hardness, cutting edge geometry, feed rate and cutting speed on surface roughness are statistically significant. The effects of two-factor interactions of the edge geometry and the workpiece hardness, the edge geometry and the feed rate, and the cutting speed and feed rate also appeared to be important. Especially honed edge geometry and lower workpiece surface hardness resulted in better surface roughness. Cutting-edge geometry, workpiece hardness and cutting speed are found to be affecting force components. The lower workpiece surface hardness and honed edge geometry resulted in lower tangential and radial forces.