Effects of cutting conditions on microhardness and microstructure in high-speed milling of H13 tool steel

In the present study, high-speed side milling experiments of H13 tool steel with coated carbide inserts were conducted under different cutting parameters. The microhardness and microstructure changes of the machined surface and subsurface were investigated. A finite element model, taking into account the actual milling process, was established based on the commercial FE package ABAQUS/Explicit. Instantaneous temperature distributions beneath the machined surface were analyzed under different cutting speeds and feed per tooth based on the model. It was found that the microhardness on the machined surface is much higher than that in the subsurface, which indicates that the surface materials experienced severe strain hardening induced by plastic deformation during the milling process. Furthermore, the hardness of machined surface decreases with the increase of cutting speed and feed per tooth due to thermal softening effects. In addition, optical and scanning electron microscope (SEM) was used to characterize the microstructures of cross sections. Elongated grains due to material plastic deformation can be observed in the subsurface, and white and dark layers are not obvious under present milling conditions. The thickness of plastic deformation layer beneath the machined surface increases from 3 to 10 μm with the increase of cutting speed and feed per tooth. The corresponding results were found to be consistent and in good agreement with the depth of heat-affected zone in finite element analysis (FEA).     

EXPERIMENTAL INVESTIGATION ON SURFACE INTEGRITY OF END MILLING NICKEL-BASED ALLOY— INCONEL 718

Inconel 718 is a typical difficult-to-machine material, and its high speed end milling process has wide applications in manufacturing parts from aerospace and power industry. Surface integrity of these parts greatly influences the final characteristics. This paper presents an experimental investigation to evaluate surface integrity behaviors in high speed end milling of Inconel 718 with finishing cutting parameters in terms of surface topography, surface roughness Ra, residual stresses, subsurface microstructure, and microhardness. The results show that abraded marks can be observed on the machined surfaces, and high cutting speed is advisable to get better surface topography and roughness quality. Due to high cutting temperature, residual stress is mainly high tensile stress. After increasing the cutting speed beyond 80m/min, the cutting forces hardly increased and the chips take away more cutting heat, which leads to that the residual stress barely increases. Microstructures in subsurface layers have only slight deformations after high speed milling, and there was also no obvious difference when the cutting speed increased beyond 80m/min against the microhardness in subsurface increases together with the cutting speed.     

SURFACE INTEGRITY OF HIGH-SPEED FACE MILLED Ti-6Al-4V ALLOY WITH PCD TOOLS

This study is focused on the machined surface integrity of Ti-6Al-4V alloy using polycrystalline diamond (PCD) tools under wet milling condition. The surface integrity in terms of surface roughness, surface topography, microhardness, microstructure, and metallurgical alternations is investigated. The observations and conclusions are primarily focused on the effect of cutting speed (250–2,000 m/min) on the surface and subsurface of the machined Ti-6Al-4V. Experimental results show that machined surface integrity of Ti-6Al-4V alloy is sensitive to the variation of cutting speeds. Obvious machining (feed) marks can be found on the machined surfaces. Micro hardness examinations showed 5–20% hardening of the top machined surfaces than the bulk material. The analyses of microstructure and metallurgical alternations reveal that slight subsurface microstructure alteration such as plastic deformation on the subsurface and no phase transformation were observed. The evolution of crystallographic texture induced by the intense plastic deformation of the machined surface should be responsible for the modifications of the peak intensity radios in XRD patterns as well as higher peak broadening crystal structures.     

刀具几何参数对钛合金铣削力和表面完整性的影响

针对TC18钛合金铣削过程,采用正交试验研究了硬质合金刀具几何参数对铣削力和表面完整性的影响,建立了铣削力经验模型,并分析了铣削力对刀具前角、后角和螺旋角的绝对灵敏度和相对灵敏度;采用田口法分析了刀具几何参数对表面粗糙度和表面残余应力的影响。结果表明：大前角、小后角、大螺旋角的条件下铣削力较小,铣削力对刀具螺旋角的变化最敏感,对后角次之,对前角最不敏感;铣削表面均为残余压应力,刀具螺旋角对表面粗糙度的影响显著,刀具后角对表面残余应力的影响显著。     

预应力切削钛合金的有限元分析

为揭示预应力切削对钛合金Ti6Al4V加工表面残余应力的调整机理,探讨切削时锯齿形切屑的形成过程,基于预应力切削原理建立了钛合金的预应力切削有限元模型,模拟了0、280 MPa和560 MPa这3种预应力下的锯齿形切屑形成过程以及已加工表面的残余应力分布。结果表明:采用预应力切削方法可以调整钛合金已加工表面的残余应力状态;预应力对锯齿形切屑的形成过程和切屑特征无明显影响;在材料弹性极限内施加越大的预应力,表面层残余压应力效果越显著,次表层最大残余压应力值越高,残余压应力层分布也越深。     

Experimental Study on Cutting Forces and Surface Integrity in High-Speed Side Milling of Ti-6Al-4V Titanium Alloy

This article is concerned with the cutting forces and surface integrity in high-speed side milling of Ti-6Al-4V titanium alloy. The experiments were conducted with coated carbide cutting tools under dry cutting conditions. The effects of cutting parameters on the cutting forces, tool wear and surface integrity (including surface roughness, microhardness and microstructure beneath the machined surface) were investigated. The velocity effects are focused on in the present study. The experimental results show that the cutting forces in three directions increase with cutting speed, feed per tooth and depth of cut (DoC). The widths of flank wear VB increases rapidly with the increasing cutting speed. The surface roughness initially decreases and presents a minimum value at the cutting speed 200 m/min, and then increases with the cutting speed. The microstructure beneath the machined surfaces had minimal or no obvious plastic deformation under the present milling conditions. Work hardening leads to an increment in micro-hardness on the top surface. Furthermore, the hardness of machined surface decreases with the increase of cutting speed and feed per tooth due to thermal softening effects. The results indicated that the cutting speed 200 m/min could be considered as a critical value at which both relatively low cutting forces and improved surface quality can be obtained.     

MACHINING OF SODA LIME GLASS USING ABRASIVE HOT AIR JET: AN EXPERIMENTAL STUDY

Abrasive Jet Machining is becoming one of the most prominent machining techniques for glass and other brittle materials. In this article, an attempt has been made to combine abrasive and hot air to form an abrasive hot air jet. Abrasive hot air jet machining can be applied to various operations such as drilling, surface etching, grooving and micro finishing on the glass and its composites. The effect of air temperature on the material removal rate applied to the process of glass etching and grooving is discussed in this article. The roughness of machined surface is also analyzed. It is found that the Material Removal Rate (MRR) increases as the temperature of carrier media (air) is increased. The results have revealed that the roughness of machined surface is reduced by increasing temperature of carrier media. The mechanism of material removal rate has been discussed with aid of SEM micrographs.     

Optimization of cutting conditions for minimum residual stress,cutting force and surface roughness in end milling of S50C medium carbon steel

Residual stresses are usually imposed on a machined component due to thermal and mechanical loading. Tensile residual stresses are detrimental as it could shorten the fatigue life of the component; meanwhile, compressive residual stresses are beneficial as it could prolong the fatigue life. Thermal and mechanical loading significantly affect the behavior of residual stress. Therefore, this research focused on the effects of lubricant and milling mode during end milling of S50C medium carbon steel. Numerical factors, namely, spindle speed, feed rate and depth of cut and categorical factors, namely, lubrication and milling mode is optimized using D-optimal experimentation. Mathematical model is developed for the prediction of residual stress, cutting force and surface roughness based on response surface methodology (RSM). Results show that minimum residual stress and cutting force can be achieved during up milling, by adopting the MQL-SiO₂ nanolubrication system. Meanwhile, during down milling minimum residual stress and cutting force can be achieved with flood cutting. Moreover, minimum surface roughness can be attained during flood cutting in both up and down milling. The response surface plots indicate that the effect of spindle speed and feed rate is less significant at low depth of cut but this effect significantly increases the residual stress, cutting force and surface roughness as the depth of cut increases.     

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.

     

TC4铣削中超临界CO2混合油膜附水滴的冷却润滑性能

在干切削、超临界CO2（scCO2）以及scCO2与油膜附水滴(OoW)混合三种绿色切削方式下对钛合金进行了铣削试验。通过单因素试验分析了铣削参数和冷却润滑方式对切削力、切削温度、表面粗糙度的影响规律，研究了scCO2与OoW混合冷却方式在钛合金铣削中的冷却润滑性能。结果表明，三种冷却润滑方式下，随着切削速度、每齿进给量和径向切宽的增大，切削力和切削温度均呈现增大趋势；当切削速度进一步增大时，依据高速切削加工理论，切削力和温度有增长变缓和下降的趋势；不同加工参数下，相比干切削和scCO2，scCO2与微量油膜附水滴混合冷却方式能有效减小切削力和降低切削温度，并获得良好的加工表面，具有良好的冷却润滑性能。     

高速铣削时钛合金刀具的磨损及对工件表面粗糙度的影响

观察高速铣削钛合金刀具时后刀面及被加工表面的形貌,通过对刀具后刀面磨损量和被加工表面粗糙度值的测量研究了刀具磨损对被加工表面粗糙度的影响,揭示了钛合金铣削加工时提高表面质量的规律。     

AlTiN涂层刀具精铣TC4钛合金工艺参数的影响与优化

采用AlTiN涂层4刃?10 mm硬质合金立铣刀,在VMC850立式加工中心上对TC4钛合金进行铣削精加工试验。利用高精密数字化检测设备,对加工成形的TC4钛合金试件表面粗糙度、平面度、平行度、表面形貌、残余应力及显微硬度测量。分析AlTiN涂层刀具在设定不同工艺参数条件下TC4钛合金的整体加工质量和表面形貌变化规律。结果表明:在主轴转速n=8000 r/min、每齿进给量f z=0.04 mm/z、切削深度Δd=0.5 mm的最优精铣工艺参数下,TC4钛合金工件的加工质量和表面形貌好,刀具寿命长,其平面度为0.39μm,平行度为0.33μm,表面粗糙度为0.70μm,表面残余应力为-175 MPa,表面显微硬度为269 HV 0.2,实现了TC4钛合金的高质量高效率的精铣加工。     

高速铣削铝合金时切削力和表面质量影响因素的试验研究

对高速铣削典型铝合金框架结构工件时的切削力和加工表面质量进行了试验研究。在高速进给铣削时 ,当进给方向发生改变 ,机床的加减速特性将导致在拐角处进给量减小、铣刀切入角增大 ,从而引起切削力增大和加工振动。在恒切削效率条件下高速铣削铝合金的试验结果表明 ,高速铣削时宜采用较小的轴向切深和较大的径向切深 ,以减小铣削力、提高加工表面质量 ;刀具动平衡偏心量是高速铣削时引起轴向振纹的主要原因     

An experimental approach to study the capability of end-milling for microcutting of glass

Glass is a hard and brittle material. It is finding mounting quantum of applications in semiconductor, opto-electronics, and mold manufacturing sectors. However, glass is not amenable to machining because of its low fracture toughness. If machined with conventional approach, the mechanism of material removal in machining of glass is fracture based that results into poor quality of the machined surface and imparts subsurface damage. In order to achieve superior surface finish, glass must be machined in ductile mode. Ductile-mode machining is now a well-established technique but most of the work has been performed with single-point cutting processes. To assess the capability of ductile-mode machining with multipoint cutting process, fundamental studies are highly desired. This paper reports the results of an experimental investigation into ductile-mode machining of glass by milling process. Side-milling tests have been performed on the glass workpiece to identify the key parameters governing the ductile-brittle transition mechanism. Experimental results demonstrate that fracture-free surface can be machined on glass by milling process. Cutting forces were analyzed to comprehend the dynamic behavior of the cutting process in ductile mode.     

高速铣削近α钛合金的切削温度研究

切削温度不仅直接影响刀具的磨损和耐用度,而且也影响工件的加工精度和已加工表面质量。由于钛合金导热性差和化学亲和性强等原因,通常在其切削加工时切削温度高、刀具磨损严重,致使切削速度难以进一步提高。本文重点对钛合金高速铣削时的切削温度进行试验研究,阐明夹丝半人工热电偶法测温原理和所测热电势信号的物理意义。试验选用了3种不同类型的硬质合金刀具,系统地研究了切削用量、冷却条件及刀具磨损等因素对近α钛合金高速铣削时切削温度的影响。     

Measurement of equivalent residual stresses generated by milling and corresponding deformation prediction

We introduce a method to measure the equivalent residual stresses and depths of the affected layers, which are generated in the workpiece during the milling process. These layers lie immediately under the milled surface. After machining, two material layers on the opposite side from the machined surface were chemically removed, and strain changes on the machined surface were measured after each layer removal. Based on the strain changes and the thickness of each removed layer, the equivalent residual stresses and depths of the affected layers in different directions were obtained. Based on the measured results, the corresponding deformations caused by the milling induced residual stresses can be predicted in workpieces with different rigidities. The predicted deformations were validated by the experimental results. We found that our measuring method can be successfully used in practice to evaluate the machined surface properties and predict the deformations caused by milling induced surface residual stresses accurately.     

钛合金型腔圆角高速铣削特征研究

钛合金型腔圆角加工容易发生切削负载增大和颤振等现象,导致圆角表面质量较差,难以实现钛合金高效加工。通过钛合金型腔圆角铣削试验,基于铣削力和圆角表面质量检测,分析内圆角铣削特征和原理,并优化铣削参数。试验表明:采用小切削宽度的高速加工,可实现钛合金内圆角的高效加工;在Vc=90m/min,ft=0.06mm/t,ap=20mm,ae=1mm切削参数组合下,切削力相对较小,加工效率高,切削表面质量高。     

Influence of cutting speed on surface integrity for powder metallurgy nickel-based superalloy FGH95

Powder metallurgy (PM) nickel-based superalloy FGH95 has been widely used for components, which requires the greatest service performance. The surface integrity is becoming more and more important in order to satisfy the increasing service demands. However, the machined surface of FGH95 is easily damaged due to its poor machinability. The purpose of this paper is to investigate the effects of dry milling process parameters on the surface integrity of FGH95. Experiments were conducted on a CNC machining center under different cutting speeds. The machined surface is evaluated in terms of surface roughness, microhardness and white layer. Experiments results show milled surface integrity of FGH95 is sensitivity to the cutting speeds. The machined surface roughness decreases with increase of the cutting speed, but with further increase of cutting speed between 80?m/min to 100?m/min an increase in surface roughness appears. For microhardness, it can be seen that the machined workpiece surface hardens seriously. It can also draw the conclusion that cutting speed has the marginal effect on the white layer thickness generated in the machined subsurface.     

Influence of cutting edge radius on surface integrity and burr formation in milling titanium

The influence of the cutting edge micro geometry on cutting process and on tool performance is subject to several research projects. Recently, published papers mainly focus on the cutting edge rounding and its influence on tool life and cutting forces. For applications even more important, however, is the influence of the cutting edge radius on the integrity of the machined part. Especially for titanium, which is used in environments requiring high mechanical integrity, the information about the dependency of surface integrity on cutting edge geometry is important. This paper therefore studies the influence of the cutting edge radius on surface integrity in terms of residual stress, micro hardness, surface roughness and optical characterisation of the surface and near surface area in up and down milling of the titanium alloy Ti–6Al–4V. Moreover, the influence of the cutting edge radius on burr formation is analysed. The experiments show that residual stresses increase with the cutting edge radius especially in up milling, whereas the influence in down milling is less pronounced. The influence of the cutting edge radius on surface roughness is non-uniform. The formation of burr increases with increasing cutting edge radius, and is thus in agreement with the residual stress tests.     

Fundamental investigation on partially overlapped nano-cutting of monocrystalline germanium

Molecular dynamics simulations on partially overlapped nano-cutting of monocrystalline germanium with different feeds are carried out to investigate the surface topography, cutting force and subsurface deformation. The results indicate that the side-flow material piling up on the edges of tool marks is a decisive factor for the surface topography with the machining parameters in this study, ignoring the tool wear and machining vibration. In the partially overlapped nano-cutting, the lateral force is more affected by nominal depth of cut than by pitch feed with the value of feed being in some range. The atomic sights on the subsurface deformation show that the thickness of deformed layer after machining is much thinner than that after single cut with the same nominal depth of cut. Laser micro-Raman spectroscopy and cross-sectional transmission electron microscopy are used to detect the subsurface deformation of monocrystalline germanium after eccentric turning. The crystalline structure with defects rather than the amorphous germanium is observed in many areas of machined surface. Both molecular dynamics simulation and experimental results indicate that the amorphous-damage-less and even amorphous-damage-free machined surface can be achieved by partially overlapped nano-cutting.