Z.J. Luo

Bio: Z.J. Luo is an academic researcher from Northwestern Polytechnical University. The author has contributed to research in topics: Forging & Finite element method. The author has an hindex of 5, co-authored 8 publications receiving 76 citations.

Fundamental aspects of hot isostatic pressing: An overview

Abstract: Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Modeling of grain size effect on micro deformation behavior in micro-forming of pure copper

Abstract: In micro-scaled plastic deformation process such as micro-forming, material grain size effect is difficult to reveal and investigate using conventional material models. Finding a way to study and model the grain size effect on micro-scaled deformation behavior is a non-trivial issue that needs to be addressed in greater depth. In this study, the grain size effect is investigated through micro-compression of pure copper. The deformation behaviors, including inhomogeneous material flow and the decrease of flow stress with the increase of grain size for the same size of specimens, are studied. It is revealed that when the specimen is composed of only a few grains, the grains with different sizes, shapes and orientations are unevenly distributed in the specimen and each grain plays a significant role in micro-scaled plastic deformation and leads to inhomogeneous deformation and the scatter of experimental data. Furthermore, it is found that the decrease of flow stress is caused by the dwindling of grain boundary strengthening effect when the grain size is increased. Based on the experiment results and the proposed composite model, the methodologies to estimate grain properties and model grain size effect are developed. Through Finite Element (FE) simulation, the grain size effect on deformation behavior and the scatter of flow stress are modeled. The results of the physical experiment and the proposed modeling methodologies provide a basis for understanding and further exploration of micro-scaled plastic deformation behavior in micro-forming process.

Experimental studies and numerical modeling of the specimen and grain size effects on the flow stress of sheet metal in microforming

Abstract: In this research, the interactive effect of grain and specimen sizes on the flow stress of sheet metal in microforming is investigated via the tensile test of pure copper and numerical modeling. Models based on different assumptions are proposed to analyze the size effect phenomenon. It is found that the flow stress decreases linearly with the decrease of the ratio of specimen to grain sizes. The grain boundary thickness decreases and its volume fraction increases with the decrease of grain size. The variation of grain boundary thickness is not proportional to the variation of grain size. Furthermore, the fraction of grain boundary increases with the strain and the ratio of specimen to grain sizes. Based on the FE simulation, it is found that the simulated flow stress, which is modeled based on the identified grain boundary thicknesses using the proposed models, has a good agreement with the experimental result. In addition, the size effect on flow stress is also analyzed based on the surface layer model. The methodology to identify the surface and internal grain properties is proposed based on the experimental result. The identified properties are applicable in modeling of the interactive effect of specimen and grain sizes on flow stress. This research thus provides an in-depth understanding of the plastic deformation behavior in microforming process.

Size effect on material surface deformation behavior in micro-forming process

Abstract: In micro-forming, when the billet material size is decreased to micro-scale, the mechanical behaviors of material change and the so-called size effect occurs. The design and fabrication of micro-parts by micro-forming cannot be conducted via leveraging on the knowledge of macro-forming process to micro-forming since the size effect is a barrier to this knowledge transfer. Material surface deformation behavior, which plays a significant role on interfacial friction, needs to be investigated in development of micro metal-formed parts. In this study, the effects of specimen size, grain size and asperity size of material on the surface deformation behavior have been investigated extensively via compression of pure copper cylinder. It is found that the real contact areas (RCAs) at the tooling-workpiece interface are concentrated at the outer rim, while the close lubricant pockets (CLPs) are located at the inner region. The fraction of RCA does not decrease in proportion to the decrease of specimen size. This leads to the increase of interfacial friction force. Furthermore, it is also observed that the efficiency of lubricant increases with the increase of the asperity size, due to the fact that more lubricant is trapped in the asperity valley resulting in the increase of the CLP fraction. In addition, the research further shows the size effect could affect the surface stress evolution which in turn affects the surface material properties of the formed part. Based on the study of the end surface area of the compressed specimen, it is found that the interfacial friction decreases with the increase of grain size. It is believed that the decrease of friction force is due to the decrease of grain boundary strengthening effect and the increase of the fraction of surface grain to facilitate the material flow.