Materials Science Forum Vols. 631-632

Abstract: Aiming at the modification of the sinterability of metal/ceramic powder mixtures, the influence of the homogeneity in microstructure on the sintering behavior is examined. Ni/Al2O3 specimens, with ratios ranging from 0/100 to 100/0, are prepared by compaction through two different procedures of mixing and granulating, to change the degree of particle dispersion. The sintering stress and the viscosity of the specimens are measured by sinter-compression tests to consider the sinterability from the viewpoint of both driving force and resistance. The sintering rate of the inhomogeneous compacts, with agglomeration of powder particles, is larger than that of the homogeneous compacts. This is because the viscosity becomes low, but the sintering stress does not change much by inhomogenizing the microstructure.

Abstract: Selective laser sintering (SLS) is a rapid prototyping technique which is used to manufacture plastic and metal models. The porosity of the final product obtained by SLS can be controlled by changing the energy density level used during the manufacturing process. The energy density level is itself dependent upon manufacturing parameters such as laser power, hatching distance and scanning speed. Through mechanical characterization techniques, it is possible to quantitatively relate the energy density levels to particular strength values. The present study is directed towards manufacturing functionally graded polyamide products by changing the energy density level in a predetermined manner. The mechanical properties of the functionally graded components are characterized by means of tensile testing. Both homogeneous and functionally graded specimens are produced and tested in order to examine the influence of the energy density level on the mechanical response and on the ultimate tensile and rupture strengths. Selective laser sintering is shown to possess the potential to produce functionally graded porous specimens with controlled variations in physical and mechanical properties.

Abstract: The authors have studied a new method of intermetallic coating using a small TIG welder. This method is based on a reaction between small liquid beads produced from very thin metal wire and the substrate metal surface. The authors designed a computer-aided 3-dimensional micro welder (3DMW) for a previous study on freeform fabrication of intermetallics, and have applied it to this study on intermetallic coating. In this study, a predetermined length of thin aluminum wire was fed onto the titanium substrate surface, and a spark was stricken from a thin electrode of a W-Ce2O3 alloy to make a small aluminum liquid bead on the titanium substrate surface and to simultaneously melt a small area of the substrate surface beneath the liquid bead. All process conditions had been programmed beforehand, including the length of the wire feeding per spark, the position of the electrode, electric power, movement of the stage holding the substrate, etc. The liquid bead containing aluminum and titanium rapidly solidified on the titanium substrate surface producing titanium aluminides on it. Repetition of the aluminum wire feeding, the electrode positioning and the spark striking produced a coating layer consisting of sub-layers of TiAl3, TiAl and Ti3Al from the surface side to the substrate side. Vickers hardness and wear resistance of the coated sample were remarkably improved.

Abstract: Three dimensional micro welding (3DMW) of a novel freeform fabrication process for metal or alloy components has been developed in our investigation group. A tungsten inert gas (TIG) welding machine is controlled by utilizing CAD/CAM processes. Various types of metal or alloy wires are fed automatically under a micro-arc torch to form tiny metallic beads. These micrometer order beads are joined continuously to build three dimensional structures. Near-net-shape components of metal or alloy compounds with high melting points can be fabricated automatically with minimized energy and resources. In this study, tungsten carbide-cobalt (WC-Co) and stainless steel (SUS304) micro beads of 1.0 mm in diameter were stacked alternately to fabricate cutting tools with graded structures by using the 3DMW. The microstructure and hardness were observed by scanning electron microscope (SEM), energy dispersive spectra (EDS) and Vickers hardness tester. The maximum hardness of micro bead was approximately 1300 HV and no crack or pore existed in the formed objects.

Abstract: Welding materials, that the principal chemical component is nickel, are used usually for the welding between copper and austenitic stainless steels. But many kinds of mechanical or physical properties of welds between two materials will change largely. In this study, Functionally Graded Piping Joints (FGPJ) have been manufactured as an experiment using copper and austenitic stainless steel (SUS304) powder by a process based on HIP. This composition has been confirmed by absorbed electron and characteristics X-ray images of each mixed layer for FGPJ to be uniform or continuous. The following items have been investigated and compared with the linear law of mixture rule: Vickers hardness, thermal expansion, and thermal conductivity at a one-dimensional non-steady state etc. Those physical properties have been identified to depend on the mixing ratios of copper and austenitic stainless steel (SUS304). Pretty good agreements have been obtained between the experimental data and the calculated values according to the linear law of mixture rule.

Abstract: This work aims to process W-Cu FGM for fusion material applications, in particular plasma facing components (PFC). Currently, PFCs are made by multi-material assembly and rely on tungsten armour tile (high heat flux side) which needs to be cooled by the attachment to a heat sink (Cu-alloys). The interfaces in multi-material assembly are unfavourable for the application (high working temperature, thermal fatigue). The use of FGM can be a new promising way. The aim of this study is to analyze the effect of a composition/grain size variation on the liquid phase migration during liquid phase sintering of FGM, in order to control the final composition profile. Bi-layer materials are processed by powder metallurgy route. W-Cu compositions with 10 and 20wt%Cu and with different W-particle sizes are processed starting with attritor-milled W-CuO powder mixtures which are reduced at 350°C. Analysis of copper liquid phase migration for different composition/grain size associations indicates that the phenomenon is driven by differential sinterability in the gradient. The copper liquid migration depends on the differences in sinterability, on the available liquid and open porosity in the structure beyond the melting point of copper. From these analyses, a way to control the gradient profile of W-Cu structure can be proposed.

Abstract: We designed micro-scale photonic crystal with or without graded lattice spacing composed of copper to control Terahertz (THz) waves. Designed structures were fabricated by using micro-stereolithography. By proper dewaxing and sintering process, pure copper photonic crystals were obtained. Transmission properties of THz waves propagating through the photonic crystals were measured by THz time-domain spectroscopy. Measured results showed good agreements with the simulated results.

Abstract: The diamond photonic crystals with the periodic arrangement of high dielectric constant (ε=100) were fabricated, and photonic band gap properties in the millimeter waveguides were investigated. Acrylic diamond lattice structures with TiO2 dispersion at 40 vol. % were fabricated by Micro-stereolithography. The forming accuracy was 10m. After sintering process, TiO2 diamond lattice structures are obtained. The relative density reached 96%. The millimeter wave transmittance properties were measured with network analyzer and W-band millimeter waveguide. The band gap was measured between 90 GHz and 110 GHz in the Γ-X <100> direction, which was well agreed with the results calculated by the plane wave expansion method and simulated by the Transmission Line Modeling method.

Abstract: Fabrication and terahertz wave properties of alumina micro photonic crystals with a diamond structure were investigated. The three-dimensional diamond structure was designed on a computer using 3D-CAD software. Acrylic diamond structures with alumina particles dispersion were formed by using micro-stereolithography. Fabricated precursors were dewaxed and sintered in the air. The electromagnetic wave properties were measured by terahertz time-domain spectroscopy. A complete photonic band gap was observed at the frequency range from 0.40 to 0.47 THz, and showed good agreement with the simulation results calculated by the plane wave expansion method. Moreover, a localized mode was obtained by introducing a plane defect between twinned diamond structures. The one-way transmission of the electromagnetic wave was realized by using this twinned photonic crystal with the graded diamond structure. They corresponded to the simulation by the transmission line modeling (TLM) method.

Abstract: This work explores the design of piezoelectric resonators based on functionally graded material (FGM) concept. The goal is to design single-frequency Functionally Graded Piezoelectric Resonators (FGPR) subjected to the following requirements: (i) an assurance of the specified resonance frequency, and (ii) for most acoustic wave generation applications, the FGPR is required to oscillate in the piston mode. Several approaches can be used to achieve these goals; however, a novel approach is to design the piezoelectric transducer by using Topology Optimization Method. Accordingly, in this work, the optimal material gradation of an FGPR is found, which maximizes a specified and single resonance frequency subjected to a volume constraint. To track the desirable piston mode, a mode-tracking method utilizing the modal assurance criterion (MAC) is applied. The continuous change of piezoelectric, dielectric, and elastic properties is achieved by using the graded finite element (GFE) concept, where these material properties are interpolated inside the finite element using interpolation functions. The optimization algorithm is constructed based on sequential linear programming (SLP), and the concept of the Continuum Approximation of Material Distribution (CAMD) is considered. The software is implemented in MATLAB language. In addition, to illustrate the method, a two-dimensional FGPR is designed with plane strain assumption. Performance of designed FGPR is compared with non-FGPR performance.