The present article describes a possible mechanism behind the microstructure development in aluminum (Al) alloy (AlSi10Mg: Al–10wt%Si–0.3wt%Mg) parts additively manufactured by selective laser melting (SLM) combined with powder bed system (one of the most conventional additive manufacturing processes for metals). It introduces the thermodynamic calculation applied to the studied alloy to understand the microstructure development during the SLM process and then elaborates microstructural features of SLM-fabricated Al alloy parts characterized by electron microscopy and electron back-scattered diffraction analyses. The aforementioned results are utilized to present the formation process of microstructure through the rapid solidification during the SLM process. An attempt was made to quantify the cooling rate and molten pool dimensions using the preliminary two-dimensional finite element model. The analyzed thermal gradient and cooling rates are presented as well.
The constant increase in power and heat flux densities encountered in electronic devices fuels a rising demand for lightweight heat sink materials with suitable thermal properties. In this article, I introduce the thermal conductivity and thermal expansion coefficient (TEC) of discontinuous pitch-based carbon fiber or graphite flake reinforced aluminum (Al) matrix composites with aluminum-silicon alloy (Al-Si) alloy fabricated through hot pressing. The small amount of Al-Si alloy contributed to enhance the sintering process in order to achieve fully-dense composites. Carbon fiber provides the reducing of TEC while the conservation of thermal conductivity and weight of Al. On the other hand, graphite flake enhanced thermal conductivity Al although TEC of Al was constant despite graphite flake addition. It seems that these results were attributed to the graphite sheet bridging close to Al/Graphite flake interface. This simple process using small quantity of Al-Si alloy allows the low-cost fabrication of Al based lightweight heat sink material.
As the problems on environment, energy and resources become more serious, the importance of porous ceramics for environmental cleaning/purification and energy conversion/storage is increasing. This paper introduces the microstructure control of porous ceramics using a pyrolytic reactive sintering method. In the latter half, some examples of “precise reactive sintering by controlling the composition of polymorphs”, “high-temperature in situ observation of pyrolytic reactive sintering”, and “microfiltration application using porous ceramics” will be demonstrated.
This review considers the microstructural evolution during sintering, which involves the pinch-off of pore channels and the formation of closed pores, from a point of view of mechanics. The microscopic shrinkage, i.e., the relative motion of adjacent particles, is driven by the sintering force and the mechanical force transmitted by the contact. The concept of sintering force, which has been proposed originally for sintering of two particles, is extended to the sintering of multi-particles. Here, we focus on the sintering by coupled grain boundary diffusion and the surface diffusion, which is the most important mechanism for sintering of fine powders at relatively low temperatures. We show that the shrinkage rate of a closed pore is also proportional to the sintering force. The mechanics of sintering in particle-scale will help to understand the relationship between the microstructure and the continuum mechanics of sintering.
The characterization of the processing-induced defects is a significant step for developing defect-free processing, which is important to the assurance of the mechanical reliability of brittle ceramics. Recent advances in multiscale X-ray computed tomography which consists of micro-CT and nano-CT enables us to observe the three-dimensional (3D) internal structure non-destructively. Micro-CT reveals the distribution of defects in the entire body of a sample, while nano-CT reveals the shape of the identified defect. This powerful imaging tool was used to reveal the complicated 3D morphology of defects evolved during sintering of alumina. The hierarchical packing structure of granules was the origin of several types of strength-limiting defects, which could not be eliminated due to the differential sintering of heterogeneous microstructures. This imaging technique of internal defects provides a link between the processing and the fracture strength for the development of structural materials.