【作者】 毛聪；

【导师】 周志雄；

【作者基本信息】 湖南大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 与其它材料去除方法相比,磨削加工的主要局限性在于其比磨削能很高,且绝大部分磨削能转化为热能,传递到工件、砂轮、磨屑和磨削液等物质中,这往往会引起磨削区的温度急剧升高。磨削温度过高将会造成工件表层出现各种形式的热损伤,如铁基材料的烧伤、二次回火、白层、残余应力等。为了控制和防止磨削高温及热损伤的出现,有必要对磨削温度进行深入的研究。本文通过实验分析、理论推导和数值仿真等手段,对平面磨削温度场及热损伤进行了深入全面的研究,所做的工作主要包括:(1)对磨削温度进行了系统的实验研究。根据实验结果,深入分析了磨削参数、磨削方式、比磨削能和砂轮属性等对磨削温度的影响,讨论了磨削温度对工件表面质量的影响。实验发现:磨削温度随着切深、砂轮速度和工件速度的增大而升高,其中切深对温度的影响最大,砂轮速度次之,工件速度的影响最小;磨削温度随比磨削能的增大而升高;在相同磨削参数下,顺磨的磨削温度比逆磨的高,CBN砂轮磨削后的工件表面温度比氧化铝砂轮磨削后的低;磨削温度升高会引起工件表面形貌恶化,表面粗糙度增大,而当磨削温度不足以使工件表面出现烧伤时,其对表面粗糙度的影响不大;在磨削液或空气的冷却作用下,磨削区工件中心部位的温度高于其两侧附近的温度,使得中心区的热膨胀量较大,故其实际材料去除厚度大于两侧的去除厚度,所以磨削后工件表面沿宽度方向呈中间低两侧高的“凹”型曲面。(2)对砂轮-工件接触长度进行了深入的理论研究和实验研究。研究发现,由于工件、砂轮以及磨粒在磨削力和磨削温度等因素的作用下会产生弹性变形和热变形,使得实际接触长度大于几何长度,当工件速度较大或切深较小时,二者的差距更加明显。增大工件速度、砂轮直径和切深,接触长度增大。讨论了磨削力、磨削温度与接触长度之间的关系:当磨削温度升高时,工件变软,使得接触长度增大;同时,接触长度增大导致磨削区散热条件恶化,引起磨削温度升高;此外,随着磨削力的增大,砂轮、工件和磨粒的变形增大,从而接触长度增大。(3)对磨削温度场和热量分配进行了深入的理论分析。根据磨削过程中磨粒与工件的相互作用,探讨了磨削热源分布综合模型。根据砂轮-工件的实际接触状态,以及未变形切屑形态,将砂轮-工件的几何接触圆弧面假定为工件的热源面,流入工件的热流密度沿着热源面呈抛物线分布,建立了新的磨削温度场数学模型,实验表明通过该模型获得的理论值与实验值吻合较好。分析了磨削区热量的传递途径,并讨论了各种传热方式的热流密度,建立了新的磨削热量分配的数学模型。(4)提出了一种新的平面磨削温度场三维数值仿真方法。该方法考虑了工件材料物理性能参数与温度的非线性关系,以及磨削区热流密度与未变形切屑厚度的比例关系,根据砂轮与工件的接触状态,提出了流入工件的热流密度呈抛物线分布。采用该数值仿真方法进行了实例仿真,获得了工件的温度场及温度的变化历程,实验发现采用该方法获得的仿真结果与实验结果吻合较好。讨论了不同工件材料物理性能值、热流密度和热源模型的选取方法对仿真结果的影响。根据仿真获得的温度分布情况,参照工件材料金相转变的临界温度,可以预测工件的热影响程度。(5)对磨削白层性质及其影响因素进行了深入的研究。实验发现:磨削淬硬钢时产生的白层比退火钢产生的白层更厚且硬度更高,淬硬钢的亚表层因回火而出现软化黑层,而退火钢却没有这一现象;与45钢相比,轴承钢含碳量高,产生的白层更厚且硬度更高;白层厚度随切深的增大而增大;在较小的工件速度下,增大工件速度,白层变厚,当工件速度超过某一临界值时,进一步增大工件速度白层厚度反而变小了;由于磨削过程中工件各个区域的传热情况不同,导致工件磨削起始区的白层厚度最小,中间区次之,磨削终止区的厚度最大。更多还原

【Abstract】 Grinding process is often selected for the final machining in the production of components because of its ability to obtain a high surface quality with fine tolerance and roughness. Compared with other machining processes, grinding process requires extremely high specific energy, and almost all of the energy is converted into heat which is conducted into the workpiece, the wheel, the chips and the coolant if used. This usually results in rise for the temperature of both the wheel and workpiece. The high temperature in the grinding zone will cause various types of thermal damage to the workpiece, such as burning, retempering, white layer, residual tensile stresses, etc. Therefore, the investigation of the grinding temperature is necessary for controlling the high temperature and preventing the thermal damage of the workpiece. In this paper, the temperature field and the thermal damage in surface grinding are investigated deeply through experimental analysis, theoretical calculation, and numerical simulation.(1) The effects of the grinding parameters, the grinding manners, the specific grinding energy and the grinding wheel properties on the grinding temperature are analyzed deeply on the basis of the experiment. The effect of the grinding temperature on the workpiece surface quality is also discussed. It is found that the grinding temperature increases with the cutting depth, the wheel speed and the table speed. The cutting depth is primary and the wheel speed is secondary factor for the increasing of the grinding temperature. The temperature of the down-grinding is higher than that of the up-grinding, and the temperature increases with the specific grinding energy. The temperature in the workpiece surface which is ground by CBN wheel is higher than that by aluminium oxide wheel. The relation between the grinding temperature and the morphology of the ground surface is discussed, as well as the relation between the grinding temperature and the surface roughness of the ground surface. The grinding temperature has little effect on the surface roughness of the ground surface, provided that the grinding temperature is not high enough to cause an evident burnout on the ground surface. The temperature, thereby the thermal expansion in the central zone of the workpiece is higher than that in the side zone under grinding fluid. Therefore, the actual workpiece material removal layer in the central zone is thicker than that in the side zone, and the workpiece surface is concave after grinding. (2) The contact length between the wheel and workpiece is studied. It is found that the real contact length is larger significantly than the geometric contact length for the deformations of the workpiece, the wheel and the wheel grains caused by grinding forces and heat, especially at a small cutting depth or a high table speed. The contact length increases on the conditions of larger cutting depth, higher table speed or bigger wheel diameter. The relations among the grinding force, grinding temperature and the contact length are discussed. A high temperature makes the wheel and the workpiece soften seriously, consequently the real contact length increases. At the same time, with the increase of the contact length, the dissipate heat condition of the grinding zone deteriorates, thus the grinding temperature increases. Wheel, workpiece and wheel grains deform subjected to the grinding force, thus the real contact length increases.(3) The theoretical calculations of the grinding temperature field and energy partitioning are carried out. Based on the interaction of the wheel grains with the workpiece, the grinding heat flux integrated model is discussed. The effects of the parameters in the model on the temperature field, the rate of the temperature change and the position of the maximum grinding temperature are described. The theoretical model of the grinding temperature is analysised on the basis of the moving heat source model which is proposed by Jaeger. A new mathematic model of energy partitioning in the grinding zone is proposed, and the effects of the material properties for the grinding wheel and the workpiece, the grain effective contact flat radius, the grinding parameters, grinding fluid, grinding forces and equivalent diameter of grinding wheel on the energy partitioning are considered in the model.(4) A new three dimensional numerical simulation technique of the temperature field in surface grinding is proposed. According to the contact condition between the grinding wheel and the workpiece, the physical properties of the workpiece material are considered non-linear according to the temperature and the heat flux in the grinding zone is considered proportional to the undeformed chip thickness in this technique. The circular arc heat source model in which the heat flux entering the workpiece is assumed to have a parabolic distribution is used to simulate the temperature field. A good agreement is found between the simulational results and experimental observations. The heat affected grade of the workpiece can be predicted from the temperature fields derived from the simulation, considering the critical temperatures for tempering, martensitic and austenitic transformation.(5) The properties and the influence factors of the white layer in the grinding are investigated. The influences of heat treatment, carbon content and grinding conditions on the white layer formation are discussed. It is found that the white layer in the hardened steel is thicker and harder than that in the annealed steel, and there is not a softer transition zone in the annealed steel. Higher carbon content tends to increase white layer thickness at the larger cutting depth, while no difference is observed at the smaller cutting depth. Increased carbon content tends to increase white layer hardness. The white layer thickness increases as the cutting depth increases. The table speed increases the white layer thickness at the low table speed. However, it is contrary at the high one. The white layer in the central zone of the workpiece is thicker than that in the entrance zone, but it is thinner than that in the exit zone.更多还原