Microstructure-based three-dimensional characterization of chip formation and surface generation in the machining of particulate-reinforced metal matrix composites

Particulate-reinforced metal matrix composites(PRMMCs)are difficult to machine due to the inclusion of hard,brittle reinforcing particles.Existing experimental investigations rarely reveal the complex material removal mechanisms(MRMs)involved in the machining of PRMMCs.This paper develops a three-dimensional(3D)microstructure-based model for investigating the MRM and surface integrity of machined PRMMCs.To accurately mimic the actual microstructure of a PRMMC,polyhedrons were randomly distributed inside the matrix to represent irregular SiC particles.Particle fracture and matrix deformation and failure were taken into account.For the model’s capability comparison,a two-dimensional(2D)analysis was also conducted.Relevant cutting experiments showed that the established 3D model accurately predicted the material removal,chip morphology,machined surface finish,and cutting forces.It was found that the matrix-particle-tool interactions led to particle fractures,mainly in the primary shear and secondary deformation zones along the cutting path and beneath the machined surface.Particle fracture and dilodegment greatly influences the quality of a machined surface.It was also found that although a 2D model can reflect certain material removal features,its ability to predict microstructural variation is limited.

The Rising Importance of Precision Engineering

Precision engineering is the discipline that encompasses the design,development,fabrication,and measurement of parts of a mechanical,optical,or electronic system,in software or in fixtures and other structures.For example,a precision instrument or machine should be able to function or perform at a high accuracy that is many orders of magnitude smaller than the size of the instrument or machine itself.It is critical that such accuracy and accurate performance are repeatable and stable over a designated period.Precision engineering also involves the generation of new knowledge-bases and creation of new technologies which will advance the innovation of such machines,instruments,or systems.

High-fidelity replication of thermoplastic microneedles with open microfluidic channels

Development of microneedles for unskilled and painless collection of blood or drug delivery addresses the quality of healthcare through early intervention at point-of-care.Microneedles with submicron to millimeter features have been fabricated from materials such as metals,silicon,and polymers by subtractive machining or etching.However,to date,large-scale manufacture of hollow microneedles has been limited by the cost and complexity of microfabrication techniques.This paper reports a novel manufacturing method that may overcome the complexity of hollow microneedle fabrication.Prototype microneedles with open microfluidic channels are fabricated by laser stereolithography.Thermoplastic replicas are manufactured from these templates by soft-embossing with high fidelity at submicron resolution.The manufacturing advantages are(a)direct printing from computeraided design(CAD)drawing without the constraints imposed by subtractive machining or etching processes,(b)high-fidelity replication of prototype geometries with multiple reuses of elastomeric molds,(c)shorter manufacturing time compared to three-dimensional stereolithography,and(d)integration of microneedles with open-channel microfluidics.Future work will address development of open-channel microfluidics for drug delivery,fluid sampling and analysis.

Precision glass molding： Toward an optimal fabrication of optical lenses

It is costly and time consuming to use machining processes, such as grinding, polishing and lapping, to produce optical glass lenses with complex features. Precision glass molding （PGM） has thus been developed to realize an efficient manufacture of such optical components in a single step. However, PGM faces various technical challenges. For example, a PGM process must be carded out within the super-cooled region of optical glass above its glass transition temperature, in which the material has an unstable non-equilibrium structure. Within a narrow window of allowable tempera- ture variation, the glass viscosity can change from 10s to 10t2 Pa-s due to the kinetic fragility of the super-cooled liquid. This makes a PGM process sensitive to its molding temperature. In addition, because of the structural relaxa- tion in this temperature window, the atomic structure that governs the material properties is strongly dependent on time and thermal history. Such complexity often leads to residual stresses and shape distortion in a lens molded, causing unexpected changes in density and refractive index. This review will discuss some of the central issues in PGM processes and provide a method based on a manufacturing chain consideration from mold material selection, property and deformation characterization of optical glass to process optimization. The realization of such optimization is a necessary step for the Industry 4.0 of PGM.

Mechanical properties and deformation mechanisms of surface-modified 6H-silicon carbide

The effect of amorphous film on the deformation mechanism and mechanical properties of 6 H-SiC were systematically explored by a combination of both experiments and molecular dynamic(MD)simulations in nanoindentation.The experimental results showed that the plastic deformation of surface-modified6 H-SiC is mainly accommodated by dislocation activities in the subsurface and an amorphous layer with uniform thickness.The MD results indicated that the amorphous layer on the surface of the residual indentation mark consists of both amorphous SiO_(2)and SiC due to direct amorphization.In addition,the amorphous SiO_(2)film undergoes densification and then ruptures with the indentation depth increases.The modulus and hardness increase with increasing the indentation depth at the initial stage but will reach their stable values equivalent to monocrystalline 6 H-SiC.