摘要
砂带磨削作为一种较新的机械加工技术,具有加工效率高、适应性强、使用成本低等优点,广泛应用于现代制造业的各个领域中。近年来,随着新的磨削材料出现以及磨削设备的研制,砂带磨削加工无论在精密加工技术还是在高效磨削技术上均有长足的发展。在超精密加工方面,一些先进工业国家已发展出纳米级的砂带研磨加工技术,砂带磨削在超精密加工的应用前景十分广阔。当前,宇航、光学仪器和精密仪器等领域对元器件的质量要求很高,如金属基片的加工需要达到纳米级的表面粗糙度。研究超精密砂带磨削的磨削机理对改善超精密磨削加工工艺和提高磨削加工质量有十分重要的意义。
弹性砂带磨削和刚性砂轮的磨削加工机理是有所区别的。当前磨削加工的研究重点主要集中在固结砂轮的磨削过程的分析,对具有弹性磨削的砂带磨削机理还有待进一步的分析和研究。从这种现状出发,本文一方面针对普通砂带磨削加工中磨削温度进行了分析和研究。通过分析恒压磨削的弹性磨削特性,提出了一种适合弹性恒压磨削的有限元热学模型;并通过实验验证了模型的可靠性。另一方面,针对超精密砂带磨削加工,本文着重探讨了恒压磨削形式的超精密磨削加工机理,采用分子动力方法,较为全面地研究了纳米尺度下单磨粒的恒压磨削加工过程。
归纳起来,本文主要创新性成果包括:
(1)基于分子动力学方法,建立了单磨粒的纳米恒压磨削分子动力模型,研究了单晶铜材料的工件在不同磨削速度和磨削压力下,磨削力、磨削温度分布以及工件应力分布的变化规律。本文提出的纳米恒压磨削分子动力模型以多种势函数相结合的方式(原子嵌入法势函数、Tersoff势函数及Morse势函数)来确定原子间的作用;模型中磨粒被施加恒定的磨削压力,并由带阻尼的恒速驱动源驱动。模拟实验结果发现,磨削过程中三个轴向磨削力变化皆有振动现象,但振动幅度较为稳定;磨粒行进阻力的大小与磨削压力大致为线性关系;当磨削速度达到一定程度后,磨粒所受阻力逐渐趋于平稳。等效应力模拟结果表明,磨粒与工件的接触区域存在效大的应力集中;当磨削压力达到一定程度,接触区域的下方及前方也有应力集中现象。温度分布的分析结果发现,在不同压力下,最高磨削温度较为稳定;另外,磨削高温主要位于磨屑堆积处。上述的分析结果表明,在纳米磨削加工中,对磨粒施加的压力不宜过高,否则容易造成更深层的应力集中;在同样磨削压力下,可通过增加磨削速度来提高磨削稳定性及磨削效率。
(2)提出原子邻接变化率的概念,利用原子邻接变化率分布和中心对称参数分布,研究了各向异性单晶金属材料在纳米磨削加工过程中工件表层的范性形变规律。原子邻接变化率用来分析原子所在位置的发生形变的强烈程度。中心对称参数则十分适合分析面缺陷型形变。通过结合这两种参数的分布变化,可以有效地研究磨削过程的工件表层的形变情况。模拟结果发现,当磨削压力达到一定程度时,面心立方结构的单晶材料很容易产生{111}滑移,发生滑移的晶面和滑移的强度与磨粒行进的方向以及滑移面的相互位置有关,在磨粒前方及两侧的{111}晶面很容易发生滑移形变。另外,原子间相互作用的强弱也会影响磨削加工所产生的形变。原子间作用较弱的单晶镁材料,磨削过程中其表层下并无明显形变;而原子间强作用力的钨材料工件,只有接触区域附近出现塑性形变,工件表层下主要呈现弹性形变。上述分析表明,在对面心立方单晶金属材料的纳米磨削加工中,磨削方向应避免对{111}滑移面施加较大的压力,同时应选择低致密度晶面作为被磨削面避免磨粒的受力发生振荡。
(3)研究提出了适合用于砂带恒压磨削的移动热源有限元热学模型,并结合模拟实验及实际实验分析了砂带磨削加工中磨削温度的分布规律。有限元热学模型是根据弹性接触轮与工件间的接触压力分析以及磨削热分配比分析建立的。研究首先通过分析接触轮与工件的压力分布,通过改进了Signorini接触模型来获得得到工件表面压力分布。磨削热分配比则根据磨屑热分析和微观磨粒与工件接触模型确定。有限元模拟结果和实际测量结果相比,有限元模拟结果的最高磨削温度误差在3%~5%之间,单点温度的连续变化的误差约在4%。总体上,本文建立的有限元模型较为可靠。由于该有限元热学模型综合考虑了磨粒材料、磨粒粒度以及磨削速度等磨削参数对磨削温度的影响,可用于预测磨削过程的温度分布,为优化磨削工艺设计提供指导,避免发生磨削烧伤。
Belt grinding process is a new machining technology with high efficiency, adaptability and low cost. It is widely used in various fields of modern manufacturing. Recently, with the development of new abrasive material and grinding equipments, much progress has been made in both high precision and high efficiency belt grinding technology. For example, some advanced countries have developed nano belt grinding techniques for ultra-precision processing. Ultra processes using belt grinding method will receive more and more attention, since it can be use to produce components which require high precision and high quality efficiency. One example is that some metal substrates, such as non-corrosive steel substrates and monocrystal metal substrates, used in optical instruments are required at nano-scale surface roughness to get better performance. Therefore, it is very useful to study the working mechanism of ultra belt grinding processes for the purposes of designing grinding process and optimizing its quality and efficiency.
There are some differences between wheel grinding process and belt grinding process. The current research of grinding process mainly focuses on wheel grinding process. In contrast, the related research on belt grinding processes under constant pressure needs further exploration. Based on our observation, we first focus on thermal analysis for constant pressure grinding using finite element method. Next, in order to study the mechanism of constant pressure grinding process in ultra-precision machining, we build up a model for better analyzing single grit nano-grinding process using molecular dynamics method.
The main contributions of this paper are as follows:
(1) We propose a molecular dynamics (MD) model for single grit constant pressure grinding process. Based on the model, we also investigate some major factors that affect the grinding process, such as grinding forces, stress and temperature distribution. In the MD modeling, we first model the interaction among atoms of the grit and the workpiece using embedded atom method (EAM), Tersoff and Morse potential functions. Then, the grit is set to be driven by a driving source with constant speed and damping connection. The simulation results indicate that in the grinding process, the axial grinding forces show some amplitudes of vibration, but within a stable range; the resistance force from the workpiece becomes stable as the grinding speed reaches at a certain high level. The equivalent stress experimental results indicate that the region of stress concentration occurs not only near the contact area but also below and in front of the contact area. The temperature results show that although different grinding forces act on the workpiece, the highest grinding temperatures are stable. In addition, the highest grinding temperature locates in the chip stack area. It is consistent with the macro phenomenon. The aforementioned results conclude that in nano grinding process, pressure forces should be controlled at a proper value to avoid deep stress concentration. In addition, a higher grinding speed can help to improve the grinding efficiency.
(2) Using MD method, we study the plastic deformation of anisotropy single metal crystal under different grinding directions using the atom adjacent changing ratio and central symmetry parameter distributions. The Adjacency changing ratio proposed in this paper is used to describe the deformation degree of atoms within a certain interval. Central symmetry parameter which describes the symmetry degree of atom neighbors is very suitable to distinguish crystal defect types, especially for planer defect analysis. The adjacency changing ratio distribution experimental results show that when the forces that act on the {111} crystal plane reach a certain level, the slipped phenomenon will occur. Meanwhile, its deformation direction and strength is related to pressure forces and grinding direction. In addition, the degree of deformation will vary under different strength of the interaction among metal atoms. For Mg material which has week atom interaction, the deformation mostly occurs in front of the grit. For W material with strong atom interaction, the plastic deformation is only found on the contact area while other area is mainly affected by elastic deformation. The results show that the higher pressure on the grit, the stronger stress concentration will occur in the deep place of the workpiece. In addition, in order to avoid oscillation of the grain and less deformation appeared, the low dense crystal face shoud be selected as grinding surface and the grinding direction should push less pressure on {111} crystal plane.
(3) We propose a finite element (FE) thermal model for constant pressure grinding process. This model is based on the analysis of the elastic contact pressure distribution and the grinding heat energy partition. In modeling the pressure distribution between the contact wheel and the workpiece, we improve the Signorini finite element model. The grinding energy partition is obtained based on energy analysis of the chip ground and single grit contact model of grinding process. The surface grinding experiments show that our FE model can produce satisfied results. Our model generates about 4% error rate in subsurface temperature and only 3~5% error rate in surface grinding temperature. It indicates that our model is reliable to some extend. Since we take into account several important factors, such as grinding speed, properties of abrasive belt and workpiece and elastic property of contact wheel, our FE thermal model and its simulation results can help to optimize the grinding process and predict the grinding thermal results so that the grinding burn can be avoided.
引文
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