Analysis of grinding mechanics and improved grinding force model based on randomized grain geometric characteristics

Too high grinding force will lead to a large increase in specific grinding energy, resulting in high temperature in grinding zone, especially for the aerospace difficult cutting metal materials,seriously affecting the surface quality and accuracy. At present, the theoretical models of grinding force are mostly based on the assumption of uniform or simplified morphological characteristics of grains, which is inconsistent with the actual grains. Especially for non-engineering grinding wheel,most geometric characteristics of grains are ignored, resulting in the calculation accuracy that cannot guide practical production. Based on this, an improved grinding force model based on random grain geometric characteristics is proposed in this paper. Firstly, the surface topography model of CBN grinding wheel is established, and the effective grain determination mechanism in grinding zone is revealed. Based on the known grinding force model and mechanical behavior of interaction between grains and workpiece in different stages, the concept of grain effective action area is proposed. The variation mechanism of effective action area under the influence of grain geometric and spatial characteristics is deeply analyzed, and the calculation method under random combination of five influencing parameters is obtained. The numerical simulation is carried out to reveal the dynamic variation process of grinding force in grinding zone. In order to verify the theoretical model, the experiments of dry grinding Ti-6Al-4 V are designed. The experimental results show that under different machining parameters, the results of numerical calculation and experimental measurement are in good agreement, and the minimum error value is only 2.1 %, which indicates that the calculation accuracy of grinding force model meets the requirements and is feasible. This study will provide a theoretical basis for optimizing the wheel structure, effectively controlling the grinding force range, adjusting the grinding zone temperature and improving the workpiece machining quality in the industrial grinding process.

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Ai——Ⅰ.SLIP CHARACTERISTICS

Comparing with ordinary ploycrystalline materials sized to μm grade,the slip morphology of the coarse grained polycrystalline pure Al is characterized by:(1)several slip domains occur in a grain,and in same domain,several slip systems operate at same time or one after another intensely,a beautiful and neat slip pattern is forming on the specimen surface;(2)for high Σ-value coincident and random grain boundaries,the grain boundary affecting zone (GBAZ),bout 50—120μm wide,is favourable site to form intergranular crack at early fa- tigue life easily,and migration or slide of the boundaries were often observed.While low Σ-value near-coincident grain boundaries show a higher degree of slip continuity and strain compatibility than high Σ-value ones.Intergranular crack is not easily nucleated at low Σ-value near-coincident boundaries;and(3)due to suppression of grain boundary slip at triple grain boundary node,the high Σ-value and random grain boundary among the three boundaries of tricrystal crack easily during cyclic deformation.

Retarding the precipitation of η phase in Fe-Ni based alloy through grain boundary engineering

It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys,as they will deteriorate not only the mechanical property but also the hydrogen resistance.The present investigation shows that grain boundary engineering(GBE) can retard the formation and growth of ηphase in J75 alloy.After GBE treatment with 5% cold rolling followed by annealing at 1000℃ for 1 h,the fraction of special boundaries(SBs) increases from 38.4% in conventional alloy to 77.2% and the fraction of special triple junctions increases from 10% to 74%.During 800℃ aging treatment,quite amount of cellular η phases adjacent to random grain boundary(RGB) will be found in conventional alloy,and only a few small η phases have been observed in GBE treatment alloy subjected to the same aging treatment for long time.The reason for GBE in inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks.As nucleation and growth of η phases on SBs are difficult due to their lower Ti concentration and diffusion rate,and the disruption of RGB networks reduces supply of Ti atoms to the η phases significantly,which impedes their growth at RGB.