Research And Development Of Monolayer Brazed CBN Wheels For High Efficiency Grinding Nickel-based Superalloy

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.