磨削淬硬技术研究现状及其展望

磨削淬硬技术是一种绿色表面淬火技术,该技术视磨削热为积极因素,主动利用磨削热对工件表面进行热处理,使工件表层发生马氏体相变,达到与表面强化处理一样的性能.对磨削淬硬技术的研究现状进行总结,指出存在的问题.提出磨削淬硬技术的发展方向和展望.     

Fe modeling of the apparent spot technique in circular laser hardening

Circular laser hardening is the laser surface treatment used in the case of cylindrical workpieces. The single-track treatment is a particular case of circular laser hardening used when only one revolution of the workpiece is executed since the treatment of a narrow surface is required. As a result, an annular narrow hardening track is obtained. During the laser hardening, the initial and final parts of the workpiece are overlapped and treated twice. The main drawback of this treatment is the back-tempering effect focused on the overlapping zone. This phenomenon leads to a hardness decrease in the overlapping zone. To avoid this problem, a new technique called apparent spot (AS) was introduced by the authors. The aim of the AS technique is to increase in a fictitious way the dimensions of the laser spot. In the case of circular laser hardening, this technique results into a high-speed rotation (up to 1,000 rpm) of the cylindrical workpiece instead of the traditional low speed. So, a uniform hardening zone without overlapping and back tempering is obtained. However, despite these benefits, there is still a lack of knowledge about the physics of this treatment in particular referring to the thermal cycle that affects the workpiece. In order to enhance the knowledge of this technique in this work, the AS was modeled via the FE approach. DEFORM software was used to model the circular laser hardening process. The software was firstly validated by a comparison with experimental results. Once the software reliability was tested, a regression model was estimated to predict the surface temperature within the treatments. Good agreement was found between the prediction model and the numerical results.     

Grinding wheel effect in the grind-hardening process

The grind-hardening process is based on the utilization of the generated heat in the grinding zone for inducing a metallurgical transformation on the surface of the ground workpiece. The workpiece surface is locally heated above the austenitization temperature and subsequently is quenched to increase surface hardness. A theoretical model was developed for the prediction of the heat-generation rate as a function of the process parameters and the grinding wheel characteristics. The model combined with a database of relationships among the heat entering the workpiece, the process parameters, and the hardness penetration depth (HPD), which was presented by the authors in an earlier publication, allows the assessment of the grinding wheel’s effect characteristics on the hardening output of the process. The experimental results have verified the predictions of the theoretical model and served for its calibration.     

预应力淬硬磨削工件表面层质量试验研究

从抗疲劳制造与绿色制造的观念出发,融合预应力磨削与磨削淬硬技术原理,提出了将残余应力控制、表面淬火及磨削三者集成于一体的预应力淬硬磨削技术理论与方法。对45钢试件进行了预应力淬硬磨削加工试验,以工件淬硬层表面残余应力、硬度及粗糙度为研究对象,与相同条件下的磨削淬硬工艺试验结果进行了对比分析。结果表明：预应力淬硬磨削工艺可增大工件表面残余压应力,减小拉应力,其工件表面残余应力状态优于磨削淬硬工艺;预应力淬硬磨削工件表面硬度可以达到基体硬度的3倍左右,而工件表面粗糙度小于磨削淬硬工艺工件表面粗糙度。因此,在相同的加工条件下,预应力淬硬磨削工艺比磨削淬硬工艺具有更好的抗疲劳性、耐腐蚀性及表面完整性。     

40Cr调质钢外圆磨削淬硬试验研究

采用外圆磨削方式对40Cr调质钢进行磨削淬硬试验,通过测量试样的金相组织和显微硬度对外圆磨削淬硬的可行性、磨削参数对淬硬效果的影响规律等进行研究。结果表明:40Cr钢外圆磨削淬硬是可行的;试样的表层组织由完全淬硬层、过渡层和基体组成;表面硬度均在690 HV左右,淬硬层深度最高可达1.10 mm;磨削深度对表面硬度无显著影响,但对淬硬层深度具有显著影响,工件速度和砂轮型号对表面硬度及淬硬层深度均无显著影响。     

磨削强化技术对中碳钢磨削淬硬效果的研究

磨削强化技术是利用磨削加工过程中产生的大量磨削热对工件材料进行表面热处理的新工艺技术。它利用磨削热使退火或回火钢零件材料表层产生马氏体相变,达到表面强化的目的,实现了表面淬火工艺与机械加工过程的集成。本文选取45钢和60钢为研究对象,进行磨削强化试验研究。研究结果表明:在磨削温度场作用下缓进给磨削表面硬化层在深度和宽度方向上具有不同的组织变化趋势和显微硬度分布特征;表面硬化层始终存在由过渡区与未淬硬区或高温回火区组成的软化带。     

磨削淬火技术中温度场的理论研究

磨削淬火是利用磨削热对工件表面进行热处理,使工件表层发生马氏体相变,达到与表面强化处理一样的性能。本文采用倾斜移动热源模型对工件温度场进行了理论计算并与Jeager移动热源模型进行了比较;分析了各加工参数对工件表面最高温度的影响;利用实验研究验证了理论分析结果。研究结果表明,在磨削淬火技术中,采用倾斜移动热源模型,可提高磨削淬火技术温度场的预测精度。     

Experiment research on grind-hardening of AISI5140 steel based on thermal compensation

The grind-hardening process utilizes the heat generated to induce martensitic phase transformation. However, the maximum achievable harden layer depth is limited due to high grinding forces, and the tensile residual stress appears on the ground surface in the grind-hardening process. This paper proposes a new grind-hardening technology using thermal compensation. The workpiece of AISI5140 steel is preheated by electric resistance heating, and ground under the condition of the workpiece temperature 25°C, 120°C, 180°C and 240°C. The grinding force, harden layer depth and surface quality including residual stress on ground surface, surface roughness and micro-hardness are investigated. The experimental results show that a deep harden layer with a fine grain martensite can be obtained with the thermal compensation. The ground workpiece surface produces a certain compressive residual stress, and the residual compressive stress value increases with preheating temperature. As the preheating temperature increases, grinding force slightly decreases, while there is slightly increment of surface roughness. Compared with the conventional grind-hardening process, both the harden layer depth and residual stress distribution are significantly improved.

     

Characterization of microplastic deformation produced in 6061-T6 by using laser shock processing

High dislocation densities are formed in the irradiated region of the workpiece during the laser shock processing; in which case, surface hardening is resulted. The process involves with recoil pressure loading at the workpiece surface with the minimum heating effects in the irradiated region. This favors the process to be a good candidate for the surface treatment of metallic materials. Therefore, in the present study, laser shock processing of 6061-T6 aluminum alloy is carried out and the influence of a number of laser pulses and irradiated spot diameter on the treated layer characteristics, including morphology and hardness, are investigated. It is found that the number of laser pulses has significant influence on the resulting surface characteristics such as surface roughness, crystallite size, micro-strain, and microhardness of the alloy. In this case, surface roughness is deteriorated by increasing number of laser pulses and pulse intensity. In addition, fine crystallite structure takes place in the laser-treated region.     

基于分段变磨削力的磨削强化层仿真

分析了磨削强化工艺过程中实际磨削时间以及磨削过程中磨削力的变化规律,提出了分段变磨削力磨削温度仿真方法来预测磨削强化层深度分布。首先对磨削力进行离散,计算相应的热流密度;然后将热流密度按砂轮与工件实际接触长度依次施加到工件的磨削表面,对工件磨削过程中的温度场进行仿真分析,得到了磨削强化层的分布;最后将所提出仿真方法与实验和传统仿真方法进行了比较分析。结果表明,基于分段变磨削力仿真可以更准确地预测工件沿磨削方向的磨削强化层分布。     

40Cr钢磨削强化的试验与数值仿真

利用磨削过程中产生大量的磨削热使40Cr钢表层的温度升高,超过奥氏体化的温度,然后快速冷却,使40Cr钢表层发生马氏体相变,达到强化40Cr钢表层的目的。在磨床上对40Cr钢进行磨削试验,观测并分析工件横断面相变层的组织变化、厚度值和硬度变化,以及加工后工件表面的粗糙度。借助有限元分析方法,对工件的温度场进行仿真,得出工件各处的温度变化历程,由马氏体相变条件来获得表层马氏体相变层的厚度值。试验结果表明,40Cr钢的表层发生了马氏体相变,表层的硬度值得到极大提高,表面粗糙度也满足常规磨削的要求。由有限元仿真得出的相变层厚度值和试验结果相吻合。因此利用磨削强化技术替代40Cr钢高频淬火强化工艺是可行的,并且这项技术可以对其他合金钢进行强化。借助有限元方法对工件表层的温度场进行仿真,可以预测相变是否发生和相变层的厚度,优化磨削参数,减少试验研究的次数和成本。     

Effect of High-Pressure Coolant on Machining Performance

Machining of hardened steel and other difficult-to-cut materials requires instant heat transfer from the cutting edge of the tool to improve tool life. Supply of high-volume and high-pressure coolant often provides the best answer. This paper deals with an experimental investigation on the effect of high-pressure coolant on workpiece hardness, comparing it with dry cut and conventional coolant. The effectiveness of high-pressure coolant is evaluated in terms of improvement of surface finish, reduction in tool wear and cutting forces, and control of chip shape. It is found that the cutting force is reduced, surface finish improved, and chip width is reduced with the use of high-pressure coolant.     

磨削淬火技术的温度场分析和材料相变研究

磨削淬火技术是利用磨削热对工件表面进行热处理 ,使工件表层发生马氏体相变 ,达到与表面强化处理一样的性能。通过对温度场的分析和材料相变的研究 ,发现利用计算机仿真磨削淬火温度场 ,不仅可以节省大量分析时间和试验费用 ,而且可以利用磨削温度场仿真技术 ,通过选择不同的工艺参数来预测和控制相变层厚度     

基于BP神经网络的外圆磨削淬硬试验研究

磨削淬硬是一种将磨削加工与表面淬火相集成的绿色加工工艺,具有显著的经济和社会效益。在外圆磨床上对40Cr钢进行磨削淬硬试验,观测并分析其磨削温度、表面硬度以及淬硬层深度等,最后利用试验数据建立其BP神经网络模型对其淬硬效果进行预测,通过预测结果与试验结果的对比表明该模型是有效可行的。     

Experimental study on operating temperature in laser-assisted milling of silicon nitride ceramics

Operating temperature plays a significant role in laser-assisted milling (LAMill) of silicon nitride ceramics. Understanding the features of temperature variation can improve the performance of LAMill. Based on the analysis of operating temperature, this paper aims to provide guidelines on parameter selection for LAMill from three aspects: laser?Csilicon nitride interaction mechanism, effect of parameters on temperature, and evaluation of surface quality of the machined workpieces. First, the laser?Csilicon nitride interaction mechanism is explored via heating experiments. It is found that the formation of silica bubbles at the thin top layer of the workpiece can slightly increase the temperature of silicon nitride workpieces due to the heat energy released from the oxidation process. Then, the trends of temperature variations in LAMill are obtained through a parametric study. The key parameters such as laser power, laser beam diameter, feed rate, and preheat time are highlighted. At last, the surface quality of the machined workpieces under different operating temperatures is evaluated in terms of edge chipping, surface finish, and surface residual stress. It is shown that high operating temperature leads to low cutting force, good surface finish, small edge chipping, and low residual stress. In addition, the temperature range for brittle-to-ductile transition should be avoided since the cutting force decreases slowly due to the rapid increase of fracture toughness.     

Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid

A surface modification method by electrical discharge machining (EDM) with a green compact electrode has been studied to make thick TiC or WC layer. Titanium alloy powder or tungsten powder is supplied from the green compact electrode and adheres on a workpiece by the heat caused by discharge. To avoid the production process of the green compact electrode, a surface modification method by EDM with powder suspended in working fluid is proposed in this paper. After considering flow of working fluid in EDM process, the use of a thin electrode and a rotating disk electrode are expected to keep powder concentration high in the gap between a workpiece and an electrode and to accrete powder material on the workpiece. The accretion machining is tried under various electrical conditions. Titanium powder is suspended in working oil like kerosene. TiC layer grows a thickness of 150 μm with a hardness of 1600 Hv on carbon steel with an electrode of 1 mm in diameter. When a disk placed near a plate rotates in viscous fluid, the disk drags the fluid into the gap between the disk and the plate. Therefore, the powder concentration in the gap between a workpiece and a rotational disk electrode can be kept high. A wider area of the accretion can be obtained by using the rotational electrode with a gear shape.     

基于模板的大型复杂曲面超声C扫方法

针对大型复杂曲面工件通常采用不完全定位安装,超声C扫查路径生成过程重复耗时的问题,提出了一种基于模板的扫查方法.该方法首先对第1个扫查工件表面进行采样,重构生成初始曲面模型,再通过仿形测量修正初始曲面模型,生成高精度CAD(computer aided design)模板.下一个扫描工件在合理获取少量测量点后,根据模板曲面的几何性质进行精确重定位,并利用模板快速生成该工件的扫查路径.仿真与实验结果表明,该方法可以快速地生成高精度扫查路径,大大缩短了非扫查准备时间,提高了检测效率.     

In-process prediction of the hardened layer in cylindrical traverse grind-hardening

Grind-hardening is an innovative manufacturing process that takes advantage of the high amount of heat generated in the contact zone to produce a martensitic phase transformation in the subsurface layer of the workpiece. However, for a successful industrial implementation of the process, the closed loop control of the hardening depth is essential. Firstly, in this paper, cylindrical traverse grinding tests and metallographic analysis are conducted, and a grinding parameter that enables the in-process control of the hardness penetration depth (HPD) is proposed. Secondly, a nondestructive method based on the Barkhausen noise technique is presented as a quality control procedure for the HPD estimation.     

Improving waviness of bore in precision hard turning by pressurized coolant

Precision hard turning is an emerging innovated machining technology for machining hardened workpiece (60~62 HRC) with characteristic of better flexibility, high production rate, and cost saving. However, one of problem in this process is that it is difficult to remove the thermal distortion of the workpiece induced by the heat generated during the process, especially in the case of precision hard turning of bore. In this paper, pressurized coolant was used in a precision hard turning of bore, with polycrystalline cubic boron nitride as the tool materials, to reduce the thermal distortion of the machined part such as form error and to improve its waviness. The pressurized coolant jet was directly delivered under the chip on the rake face. Experimental result indicated that form error induced by the heat has been reduced, and waviness of the bore was improved with use of pressurized coolant. Crater wear is smaller under pressurized coolant compared to dry cut due to lower rake face temperature and shorter contact length between chip and workpiece.     

变速感应淬火的示教再现控制

针对机床轴类工件外锥面或内锥孔进行连续感应淬火时存在的淬火硬度和有效硬化层不均匀等质量问题，采用电液比例阀调节淬火移动速度的方法保持淬火加热温度并通过示教再现控制保证工艺的稳定性与再现性。