【作者】 丁文锋；

【导师】 徐九华；

【作者基本信息】 南京航空航天大学 ， 机械制造及其自动化， 2006， 博士

【摘要】 立方氮化硼(CBN)磨料具有硬度极高、热稳定性好、化学惰性大、导热性好的优点,所以由该种磨粒制作的各型固结磨料砂轮已在高效磨削镍基高温合金、钛合金等难加工材料中得到了应用,并取得了显著效果。然而,大量的应用实践也表明,由于传统的多层烧结与单层电镀CBN砂轮中磨粒与胎体或镀层材料仅仅通过机械镶嵌作用结合,存在磨粒把持强度低、容屑空间狭小等缺陷,使得重负荷磨削过程中砂轮工作面容易出现磨粒过早脱落、甚至电镀砂轮的局部结合剂层从基体表面剥离的现象,这势必制约了CBN超硬磨料磨削性能的充分发挥,也限制了加工效率与质量的进一步提高。凭借磨粒、钎料、基体在高温钎焊过程中的化学和冶金反应对磨粒提供的超强把持效果,本课题研制了磨粒呈有序排布的新型单层钎焊CBN成型砂轮,并将其应用于高效磨削镍基铸造高温合金。本文完成的创新性研究工作主要包括:(1)在确保Ag-Cu-Ti液相合金中活性元素Ti活度的前提下,兼顾控制钎料层新生相数量和克服活性元素的消耗效应,通过冶金热力学分析对钎料成分进行了优选。结果表明,含Ti 5%的Ag-Cu共晶合金是综合性能比较优异的钎料,后续试验已显示此种合金组分制作的单层钎焊CBN砂轮确实可取得预期效果。(2)利用扫描电镜、能谱仪和X射线衍射仪分析了钎焊工艺参数对界面新生化合物和微观结构的影响。结果显示,当钎焊温度920℃、保温时间5 min时可获得较佳的磨粒表面新生化合物形貌,界面层由CBN/TiB2/TiB/TiN/含Ti合金层的过渡结构组成。磨粒抗压强度和冲击强度测试结果表明钎焊前后磨粒强度无明显变化。(3)对界面层新生相的形核顺序与生长过程进行了动力学分析。结果表明,新生相按TiN→TiB2→TiB的顺序依次形成,其生长过程主要受新生TiN控制,而活性元素Ti的引入和扩散则是界面层生长的主要因素。(4)依据型槽工件要求设计制作了磨粒呈有序排布的单层钎焊CBN成型砂轮,并将其用于镍基铸造高温合金K424槽类工件的模拟加工。结果表明,单层钎焊CBN砂轮在高效磨削过程中磨削力和温度得到了有效控制,工件加工表面完整性也得到明显改善。单层钎焊CBN砂轮的磨损机理主要是磨耗磨损,试验中未出现磨粒脱落,这表明采用钎焊工艺制作单层CBN砂轮能提高对磨粒的把持力,有助于充分发挥超硬磨料本身的耐磨特性。更多还原

【Abstract】 Cubic boron nitride (CBN) abrasive grains have some distinguished properties, such as extremely high hardness, superior thermal stability, large chemical inertia and excellent thermal conductivity. Therefore CBN wheels have been utilized and taken good effects in grinding difficult-to-machine materials, i.e. nickel-based superalloy and titanium alloy. Much grinding practice has discovered that, however, the shortcomings of low holding strength for grains and limited storage space for chips are always accompanied with the traditional multiplayer sintered and monolayer electroplated CBN abrasive tools. They are mainly attributed to the merely mechanical embedding effects between the grains and the connecting layer. Under such circumstance, the unexpected phenomena, including the premature fallout of the grains in good condition and the tearing-up behavior of the regional connecting layer, easily appear in the heavy load grinding process. They not only restrict the display of the outstanding wear resistance ability of CBN superabrasive grains, but also hinder the improvement of the machining efficiency and the product quality.Depending on the strong joining effects induced by the chemical and metallurgical behavior among the abrasive grains, the filler alloy and the metal substrate during high temperature brazing, the monolayer brazed CBN profile grinding wheels with rhythmed grain distribution are developed. Subsequently the special abrasive tools were evaluated in the high efficiency grinding process of nickel cast superalloy.The main creative contents in this paper are as follows:1) For insuring the activity of the active element, namely Ti, in the liquid Ag-Cu-Ti alloy, the composition of the filler alloy is optimized according to the metallurgical thermodynamic theory. Thus, the quantity of the new-formed inner compounds of the filler metal could be controlled, meanwhile the expenditure of the active element is overcome. The results show that, the Ag-Cu eutectic alloy containing 5%(wt) Ti is the ideal one with the excellent integrated performance, which has been approved true in the fabrication experiments of monolayer brazed CBN grinding wheels.2) The new-formed reaction products and the interfacial microstructure under different conditions have been detected by scanning electron microscope (SEM), energy dispersion spectrometer (EDS), and X-ray diffraction (XRD). It has been observed that the perfect microstructure of the new-formed compounds could be acquired with brazing temperature 920℃for dwell time 5min. A special layer, i.e. CBN/TiB2/TiB/TiN/filler containing Ti has existed in the interface, which provides good transitional property in the chemical bond and the physical performance for the brazed joints of CBN grain and filler alloy. Both the tests of the compressive strength and the impact strength of the grains reveal that the remarkable change does not occur during brazing.3) The formation sequence and corresponding mechanism of the interfacial phases between CBN and Ti were discussed through the kinetic analysis. The results demonstrate that the new phases come forth in a sequence of TiN→TiB2→TiB. Moreover, the formation process of the interfacial layer is mainly dominated by the TiN phase. The application and diffusion of active element Ti in Ag-Cu-Ti alloy is the preliminary factor.4) The monolayer brazed CBN profile grinding wheels with rhythmed grain distribution are designed and fabricated according to the requirements of the grooves in the aircraft components. The grinding performance has been evaluated in the high efficiency grinding process of a special nickel cast superalloy, K424. During machining, not only the grinding force and temperature could be controlled effectively, but also the surface quality of the machined parts could be improved significantly. On the other hand, the abrasion wear is the leading wear behavior for the brazed CBN grinding wheels. The disadvantageous phenomenon, for example, the fallout of CBN grains, has never happened in the experiment, which indicates that strong joining to the grains has been realized in the brazed abrasive tools. Thus, the excellent abrasion resistance of CBN grains could be exhibited completely.更多还原