外文翻译--机械加工件表面层物理学性能改变的主要成因及处理措施

毕业设计 （ 论文 ）译文 题目名称 ： 机械加工件表面物理学性能 改变的主要成因及处理措施 院系名称 ： 班 级 ： 学 号 ： 学生姓名 ： 指导教师 ： 中原工学院毕业设计译文 1 Appendix 5 The main causes and treatment of the changes of the physical properties in machining 1. The main reasons A metals machinability is its ease of achieving a required production of machined components relative to the cost. It has ma ny aspects, such as energy (or power) consumption, chip form, surface integrity and finish, and tool life. Low energy consumption, short (broken) chips, smooth finish and long tool life are usually aspects of good machinability. Some of these aspects are directly related to the continuum mechanical and thermal conditions of the machining process. In principle, they may be predicted by mechanical and thermal analy- sis (but at the current time some are beyond prediction). Other aspects, principally tool life, depend not only on the continuum surface stresses and temperatures that are generated but also on microstructural, mechanical and chemical interactions between the chip and the tool. The required tool hardnesses to avoid the yielding shown in Figure 3.19 have been obtained by a method due to Hill (1954).The requirement that the tool does not yield at its apex, together with force equilibrium in the tool, limits the difference between the rakeface contact stress and the zero stress on the clearance face and hence places a maximum value on the allowable rake face contact stress With the cylindrical polar coordinate system shown in Figure A5.1(a), in which the origin is at the tool apex and the angular variable q varies from 0 on the rake face to b on the clearance face 1) Hardening of the three main factors. (1) The geometry 中原工学院毕业设计译文 2 Cutting edge radius increases, the chip deformation increases, the radial cutting force increases, the face of the workpiece after the extrusion tool, increasing friction, plastic deformation of the workpiece increases, the hardening increases, increasing the depth of hardened layer.Therefore, increasing the rake angle and cutting edge radius can reduce the chip deformation decreases, reducing the degree of hardening. (2) the impact of cutting Cutting the greatest impact on the work hardening is the cutting speed and feed.In the middle and low-speed stage, increasing the cutting speed, flank and shorten the role of the workpiece, so that the expansion of the depth of plastic deformation decreases, and the cutting speed increases to improve the workpiece material yield limit, plasticity decreases.In addition, the cutting speed in this range so that the cutting temperature will increase and strengthen the role of hardening response, hardening depth decreases.When the cutting speed is greater than 90m / min, the cutting heat in the surface layer on the role of time has been shortened, weakened back, hardening increased. As the feed rate increases, cutting force also increases, plastic deformation of the surface layer of the metal increases, the hardening worse.But when the feed rate is too small (eg, f: 0. 05 0. 08mm), may be less than the thickness of the cutting tool edge radius, tool and workpiece friction time increased, so that instead of hardening increases. (3) of the processed material The lower hardness of workpiece material, the greater plasticity, hardening after cutting more serious.Depending on the workpiece material and processing conditions, the use of appropriate cutting fluid to help reduce the hardening phenomenon. 2） Surface residual stress generation are the following reasons: (1) Cold plastic deformation caused by residual stress.Role in the cuttingforce in cutting process, the metal layer of a sharp cutting plastic deformation than the volume of the metal surface increases, volume increases, but the changes are connected to the inner layer of metal barriers and residual in the surface layerstress. (2) The plastic deformation caused by thermal residual stress.cutting process,the role of heat in the cutting, processing, production of surface layer ofthermal expansion, but the low temperature metal matrix and hinder thethermoplastic deformation of the metal surface leaving the surface compressive stresses.After cutting, the surface layer temperature decreases,the contraction and obstruction by the matrix tensile stress.Therefore, thehigher the grinding temperature, the greater the thermoplastic 中原工学院毕业设计译文 3 deformation, thegreater the residual tensile stress, and even lead to grinding surface cracks. (3) The microstructure of the residual stress caused by the change.Cutting, when the surface temperature is higher than the metal phase transition temperature, can cause changes in microstructure of the metal surface.Different temperatures at different depths, the phase transition is not the same.Because the density of the different microstructure is different from leaving would inevitably lead to changes in the volume of surface residual stress. 3) the factors that affect the residual stress (1) In the cutting process, where the impact of hardening, hot forging and metallurgical structure changes in the factors that will cause the surface residual stress.Factors influenced the workpiece material, cutting speed, rake angle, and a factor in the different cutting conditions, the impact is not the same.For example, the Turning with positive rake angle of 45 steel, regardless of changes in cutting speed, workpiece surface always produces the residual tensile stress, mainly due to poor performance of 45 hardened steel, cutting the heat factor plays a leading role, there is no residual pressurestress conditions. (2) In the grinding process, severe plastic deformation, the surface temperature is high, thus, thermal factors and surface plastic deformation on the workpiece residual stress effects are great.First, the grinding depth of the surface residual stress on the nature of the workpiece, the size greatly.When it is small, the grinding temperature is low, plastic deformation plays a dominant role, the workpiece surface residual compressive stress； when it increases, the grinding heat increased, although the increased plastic deformation in the workpiece surface residual tensile stress, whenit increases to a certain extent, the plastic deformation will gradually dominate the residual stress is gradually increasing.Second, the workpiece material and heat treatment on the nature of residual stress, grinding cracks have a great relationship, in general, depends on the nature of the surface residual stress the strength of the workpiece material, thermal conductivity, plastic and other factors.The higher strength of the material, the worse the thermal conductivity, lower plasticity, grinding metal surface residual tensile stress the greater the tendency to produce greater the possibility of grinding cracks. 4) Changes in metal microstructure and surface grinding burn (1) the surface microstructure changes and the reasons for grinding burn machining 中原工学院毕业设计译文 4 process, when the role of cutting heat to the workpiece surface layer temperature exceeds the critical temperature of phase change materials, the microstructure of the workpiece surface will change,In general the cutting process, because the chip can take most of the cutting heat, cutting heat has little effect on the workpiece.In the grinding process, due to the high speed grinding (typically 35 80m / s range), the grinding pressure than large, flat area on the cutting grinding heat generated by the cutting process than the general several times larger, and the grinding chip is very small, very little heat away (less than 20% of total calories), most of the heat is passed to the workpiece surface so that the workpiece surface with a high temperature.Often cause severe surface changes in the microstructure of metal, the metal surface hardness and strength decrease, resulting in residual stress and even lead to micro cracks, which is the phenomenon of grinding burn, it will seriously affect the use of performance parts. (2) measures to improve the process of grinding burn a) The reasonable choice of grinding parameters.Cylindrical grinding mill as an example to the amount of the impact of burns.Radial feed increases, different depths below the surface and the surface temperature will rise, burns increased. At this point, such as increasing the grinding speed, it will increase the degree of surface burns.Increased vertical feed the workpiece, grinding wheel and workpiece contact time shortened, reduced thermal effect, grinding burn relief.To compensate for the increase of the feed leaving the vertical surface roughness increases the defect, can be wide grinding wheel.Circular workpiece feed speed increases, the surface of the grinding zone temperature increased, the heat of the reaction time shortened, so that burns to reduce, but will increase the surface roughness.Commonly used to improve the grinding wheel speed to compensate.Therefore, in order to reduce the grinding burn can have a smaller surface roughness, can improve grinding speed and feed rate of workpiece circumference. b) the workpiece material.Workpiece material hardness and strength of the higher, the greater the toughness, the smaller the thermal conductivity, the grinding heat when grinding the more relevant the higher the temperature of the grinding zone.However, the hardness of workpiece material is too low, easy to plug the wheel, the grinding effect is not good. c) The correct choice of wheel.Wheel hardness is too high, not falling after blunt abrasive, easy to cause burns.Therefore, the choice of particle size large, soft grinding wheel grinding wheel can improve the performance, but also easy to plug debris will help 中原工学院毕业设计译文 5 prevent burns. 2. Treatment measures Analysis of the surface layer of the physical and mechanical properties, the use of machined parts has great influence on performance and service life, enhance and improve the processing of the surface layer of the physical and mechanical properties, mainly adopt the following measures: 1) the final process to select the appropriate processing method Residual stress on the surface of the workpiece or machine parts have a direct impact on work performance, and the nature of the surface residual stress depends on the final machining process the workpiece processing methods.Therefore, the final workpiece machining method selection process should consider the specific working conditions of the workpiece and the possible failure modes.By the alternating stress of the workpiece, starting from the improved fatigue strength of the workpiece, the workpiece should be selected to avoid residual stress residual stresses in the final processing methods. For the relative sliding of two parts, the sliding surface will gradually produce wear and tear.There are many reasons caused by sliding wear, both the mechanical effect of sliding wear, but also bonding, diffusion and oxidation wear the role of physical and chemical factors.When the work surface compression stress suffered more than the material allowable stress, the metal surface will wear.To enhance the capacity of the workpiece to resist sliding friction, the final processing procedures should be chosen in the surface residual tensile stress of the processing methods. For the two parts relative to rolling, the relative motion there mechanical or rolling surface friction, there is also bonding, diffusion, oxidation and other physical and chemical aspects of the combined effects, rolling will gradually wear the same face.but the decisive factor causing rolling wear is the subsurface depth h at the maximum tensile stress. 2) Rolling Process Processing is the use of a rolling wheel or ball high hardness at room temperature, under the squeeze on the surface, to produce plastic deformation, after rolling, so that the convex surface on the original peak to the adjacent concave valley fillto, reduce the surface roughness, and surface of the metal lattice distortion as a result chilled layer and the residual stress and improve the carrying capacity of the workpiece and fatigue strength. 中原工学院毕业设计译文 6 3. Conclusion Understand the physical properties of surface machining causes, and control, treatment measures, according to the role of the workpiece using different processing methods, resulting in better surface quality, and improve product performance. 中原工学院毕业设计译文 7 附录 5.1 机械加工件表面层物理学性能改变的主要成因及处理措施 1.主要原因 一种金属材料的可加工性是能够简单实现所需要求的加工工件的相对成本。它包括许多方面，例如能源（或动力）的消耗。磨屑的形成，表面的完整性，和刀具的寿命。低的能量消耗，短的（或断的磨屑），光洁度和具有较长寿命的刀具通常表现出良好的切削性能的方面。这些方面的一部分直接关系到机械的连续运行和加工过程的热学条件。原则上，这些方面可通过机械的力学性能和热分析进行预测 (但目前有些是超出预测的 )。还有其他方面，主要是刀具的寿命，不仅取决于 表面的应力和连续变化的温度，而且也取决于切削和刀具之间的微观结构、机械和化学性能。 在 1954 年，希尔提出一个方法，可以避免如图 3.19 所示从而得到所需的工具的硬度。按照 原来的说法，工具不会在其屈服点达到力平衡，工具表面的接触应力和间隙上的零点应力之间的差异是有限制的，前刀面上所允许的最大受力值为表面接触应力。 如下图所示圆柱极坐标系，其中，坐标原点在工具的顶端，在前刀面的表面上，角变量的变化可以从 0 到 b。 1） .影响加工硬化的因素主要有三方面。 ( 1) 具几何形状的影响 刃口半 径增大 ,切屑变形增大 , 径向切削分力增大 ,后刀面对工件的挤压、 摩擦作用加剧 , 工件塑性变形增大 ,硬化程度增大 , 硬化层深度也增大。因此 ,增大刀具前角与减小刃口半径都能减小切屑变形 ,减小硬化程度。 中原工学院毕业设计译文 8 ( 2) 切削用量的影响 切削用量中 ,对加工硬化影响最大的是切削速度和进给量。在中、 低速阶段 , 增大切削速度 , 后刀面与工件的作用时间缩短 , 使塑性变形的扩展深度减小 , 且切削速度增大使工件材料的屈服极限提高 , 塑性降低。此外 , 在此范围中切削速度增大还会使切削温度升高 , 加强硬化的回复作用 ,硬化层深度减小。当 切削速度大于 90m/ min 时 , 切削热在工件表面层上的作用时间也缩短了 , 回复作用减弱 , 硬化程度增加。随着进给量增大 , 切削力也增大 , 表面层金属的塑性变形增大 ,硬化程度加剧。但当进给量过小时 ( 如 f: 0. 050. 08mm) ,可能使切削厚度小于刀具刃口半径 ,此时刀具与工件摩擦力加剧 ,使加工硬化现象反而增大。 ( 3) 被加工材料的影响 工件材料硬度越低 ,塑性越大 , 切削后硬化程度越严重。根据不同的工件材料和加工条件 ,采用合适的切削液 ,有助于减轻加工硬化现象。 2) 表层残余应力的产生有以 下几方面原因 : (1) 冷态塑性变形引起的残余应力。切削过程中在切削力作用下 ,金属切削层产生剧烈的塑性变形 , 使金属表层的比容积增大 ,体积增大 , 但其变化受到与之相连的里层金属的阻碍而在表面层产生残余压应力。 (2) 热态塑性变形引起的残余应力。切削过程中 , 在切削热的作用下 ,加工表面的表面层产生热膨胀 ,但金属基体温度较低 , 阻碍表层金属的热塑变形而使表层产生压应力。切削结束后 , 表面层温度降低 ,其收缩又受到基体的阻碍而产生拉应力。所以磨削温度越高 ,热塑变形就越大 , 残余拉应力也越大 , 甚至会导致磨削表面产 生裂纹。 (3) 金相组织变化引起的残余应力。切削时 ,当工件表面温度高于金属相变温度 ,会引起金属表层金相组织变化。不同深度处温度不同 ,其相变也不相同。由于不同的金相组织的密度不同 , 必然引起体积的变化而使表层产生残余应力。 3) 影响残余应力的因素 (1) 在切削加工中 , 凡影响加工硬化、 热塑性变形及金相组织变化的因素 ,都会引起表面残余应力。影响较大的因素有工件材料、切削速度、 刀具前角等 ,且一种因素在不同的切削条件下 ,其影响是不相同的。例如用正前角车刀车削 45 钢时 ,无论切削速度如何变化 ,工件表层始终 产生的是残余拉应力 , 主要原因是 45 钢淬火性能差 ,切削中热因素起了主导作用 ,没有产生残余压应力的条件。 (2) 在磨削加工中 ,塑性变形严重 ,工件表面温度高 , 因而 , 热因素与塑性变形对工件表层残余应力的影响都很大。首先 , 磨削深度对工件表层残余应力的性质、 大小有很大影响。当其较小时 , 磨削温度较低 ,塑性变形起主导作用 , 工件表层产生残中原工学院毕业设计译文 9 余压应力 ； 当其增大 ,磨削热随之增加 ,虽塑性变形加剧 , 在工件表层产生残余拉应力 ,当其增大到一定程度后 , 塑性变形又会逐渐占据主导地位 ,残余压应力逐渐增大。其次 , 工件材 料及其热处理状态对残余应力的性质、 磨削裂纹产生也有很大关系 , 一般而言表层残余应力的性质取决于工件材料的强度、 导热性、 塑性等因素。材料的强度越高 ,导热性越差 , 塑性越低 , 磨削时金属表层产生残余拉应力的倾向就越大 ,产生磨削裂纹的