Influence of surface topography evolution of grinding wheel on the optimal material removal rate in grinding process of cemented carbide

Most of the reported studies on the optimization of grinding parameters do not consider the evolution of the surface topography of grinding wheels, and the established empirical models will no longer apply when the surface conditions of the grinding wheel changes. In this paper, an integrated model based on the surface topography of grinding wheel is established. The grinding process of cemented carbide is simulated using the established model, and the simulation results are analyzed to obtain the surface roughness model and the specific grinding energy model based on the undeformed chip thickness distribution. Subsequently, the grinding constraint models are defined according to the two grinding constraints—surface roughness and specific grinding energy. Through inversion analysis, the maximum material removal rate of the given grinding wheel surface conditions satisfying the defined grinding constraints are obtained, and the influence rules of the grinding wheel surface conditions on the maximum material removal rate are analyzed. Then the grinding wheel surface conditions are adjusted by changing the radial dressed height of the grinding wheel and the arrangement distance of the grains in wheel circumferential direction to improve the maximum material removal rate of the grinding wheel. Finally, the optimization results are verified through grinding tests of cemented carbide.     

基于有限元/离散元耦合的大理石高速划擦过程仿真

针对大理石的磨粒加工过程,借助有限元/离散元耦合方法建立了单颗磨粒的高速划擦仿真模型,并通过在实体单元中插入零厚度内聚力单元,控制内聚力单元的失效来实现大理石加工过程中裂纹的萌生及扩展过程。基于所建立的单颗磨粒划擦模型,对比了划擦深度和划擦速度对大理石去除过程的影响。结果表明:大理石划擦过程中划擦力和加工表面损伤层厚度随划擦速度和划擦深度的增加而显著增大,且划擦力与损伤层厚度存在正相关性。      

Microstructure and fracture mechanism of a flash butt welded 380CL steel

High strength low alloy (HSLA) steels have been widely used in the manufacture of wheel rim on account of the higher strength and larger elongation compared to the traditional low carbon steels. This experimental investigation was aimed to evaluate the microstructure and fracture mechanism of a flash butt welded 380CL steel by comparing the failed wheel rim and the survived wheel rim. The results showed that the continuous banded structure in the HAZ and the serious Widmanstatten ferrite in the weld were the major reasons for the failure of the wheel rim. The micro-hardness of two joints was similar in the overall profile. For the joint of the failed rim, the micro-hardness at the weld was the highest at 254 HV and the lowest micro-hardness value was in the HAZ. The high yield ratio of the failed wheel rim resulted in poor formability in the flaring process. The fracture mechanism of the failed rim was the mixture of ductile and brittle fracture modes. The crack initiation was the fusion zone of the weld having highest micro-hardness, then the cracking propagated along the weld metal to the middle of the wheel rim, a limited deviation was found in the terminal of the crack. The large ferrite grain in the HAZ was the main reason for the brittle–ductile transition.     

Polishing of polycrystalline diamond by the technique of dynamic friction,part 4: Establishing the polishing map

Through a systematic experimental investigation into the polishing of polycrystalline diamond composites by the dynamic friction technique, this paper identified three major regimes of polishing conditions: the regime capable of a low material removal, that enabling a safe, high removal processing, and that of an unsafe but ultra-high material removal. The study concluded that a higher polishing pressure–speed combination results in a high material removal rate, but with a greater risk of workpiece cracking. At a too low-pressure–speed combination, on the other hand, material removal may not take place. Based on the systematic experimental measurements, the paper established a polishing map to characterise the polishing conditions. It was found that using the polishing parameters determined by this polishing map, a quality surface finish can be obtained efficiently in duration about 10 times shorter than that of the abrasive polishing process currently used in industry.     

Diamond polishing

The empirical know-how of single crystalline diamond polishing has been developed over centuries in the diamond gem cutting industry. Since the 1950s new and varied uses and potential applications for synthetically produced diamond have been consistently proposed and developed. This innovation process continues with the availability of ever better, more specialized and less costly single crystalline and polycrystalline diamond materials. Yet, the potential exploitation of this hardest of materials is still in its infancy. Polishing is a critical and limiting step for advancing diamond applications in terms of cost effective processing and the achievable material surface finish. The current state-of-the-art of polishing single crystalline and polycrystalline diamond materials is reviewed based on the published literature. The material removal process during traditional mechanical polishing using diamond grit and polishing wheels is strongly anisotropic and depends upon crystal planes and polishing directions. Wear debris analyses and molecular dynamic simulations led to the understanding that this anisotropy is primarily caused by a mechanically induced transition from diamond to an amorphous carbon phase rather than by microchipping as previously thought. Mechanical polishing also leads to subsurface damage and limits the achievable surface finish for single crystalline diamond. Advanced techniques are discussed to improve the polished crystal's surface quality. Mechanical polishing of polycrystalline diamond films and freestanding plates is particularly slow due to the intrinsic structure variations in such materials. To overcome these limitations faster polishing techniques have been developed and are reviewed and compared. These techniques introduce additional chemical and physical means of material removal extending the capabilities of mechanical polishing. There is no single method that can address all requirements, but the available variety affords the careful selection of an optimal process for a given task. Finally, while diamond polishing is a subject of interest since centuries, it still remains a very important research area required to unfold the promise of diamond as a technical material.     

螺带式混凝土搅拌机混合特性及DEM模拟

采用离散单元法模拟新拌混凝土于搅拌机中的混合过程,研究了双筒螺带式混凝土搅拌机的混合效率。 用Hertz-Mindlin with JKR接触方法建立新拌混凝土离散元模型,模拟了坍落度实验、L箱实验和流变仪实验,将模拟结果与实验结果进行对比,校准模型参数,采用混合系数定量研究了不同初始装填方式下搅拌机的混合效率。结果表明,采用上下装填方式时搅拌机混合效率较高;对任一初始装填方式,左部区域与右部区域、前部区域与后部区域间混合效率无明显差别,而上部区域混合效率比底部区域高,底部出料口处存在搅拌盲区,采用舍弃初始出料的方法可提高新拌混凝土性能。转速较高时混合效率较高,相同旋转圈数时,混合效率基本相同。     

Photoluminescence enhancement of quantum dots with different emission wavelengths using oxide shell-isolated Au nanoparticles

Journal of Materials Science - Improving the quantum yield of non-toxic InP/ZnS QDs is essential for its commercial application in illumination and displays. In this work, single size Au...     

The anisotropy of surface deformation of sapphire: Continuous indentation of triangular indenter

An attempt is made to explain the anisotropy of indentation-induced deformation and fracture of sapphire for different crystallographic planes indented by a variously oriented triangular diamond indenter. The anisotropic surface deformation and fracture of the indented crystal are discussed using a new effective resolved shear stress (ERSS) model. The patterns of the surface traces of twins and cracks are predicted on the basis of the ERSS-calculations performed for all the slip and twinning systems which are potentially activated during room temperature indentation. The theoretical prediction of the ERSS model agrees very well with the observed hardness anisotropy. The present work is the first application of the continuous indentation test to the anisotropic surface deformation of single crystals.     

金刚石锯片胎体与大理石摩擦磨损的离散元模拟研究

建立圆锯片胎体与大理石摩擦磨损的离散元模型,并通过压痕试验验证胎体模型的模型参数,模拟并观察不同载荷下胎体与大理石磨损过程中的磨损量的动态变化规律。结果表明:随着载荷增加,胎体的磨损量不断增加,然后逐渐趋于平稳,磨损率呈先增加后下降的趋势。离散元模拟与试验结果具有较好的一致性,验证了离散元法模拟磨损是可行的。