Materials Science Forum Vols. 492-493

Abstract: It is difficult to generate any user-defined three dimensional gradient to tailor the functional properties of a component. Problems are not only the lack of local material design tools, but also a suitable manufacturing process. The implementation of the concept of local composition control into the Solid Freeform Fabrication (SFF) process 3D-Printing is described, which leads to geometrical complex parts out of tailored materials. Suspensions of different functional inks containing a binder and carbon black nano-particles are dispensed into droplets through multiple jets – like inkjet printing a halftone image on a paper – but into a metal powder bed to generate layer by layer graded green parts. In this case the tailored preforms are then sintered, while the nano-particle additions from the functional ink act locally as alloying elements in the steel matrix to combine e.g. both, toughness and hardness in the part. This work concentrates on the realisation of the new process and shows first results taking the generation of carbon-graded steel parts as an example.

Abstract: Dense FGMs with gradually changing fraction of very hard and light-weight B4C particles were fabricated employing an original Reactive Forging (RF) combustion synthesis method. In RF, a self-sustained reaction (thermal explosion, TE) is ignited by placing a powder blend between preheated press rams and the high temperature achieved is utilized for consolidation of the combustion product. The reported FGMs were prepared by stacking layers of highly exothermic Ti-C-Ni powder blend diluted with varying fractions of non-reacting B4C particles. Only a limited amount of B4C could be added as its diluting action reduces the overall exothermicity of the blend. In some cases, B4C particles were precoated in order to prevent their interaction with Ti during combustion. Temperature evolution in (Ti-Ni-C)-B4C powder compacts corresponding to different FGM layers was measured separately and in the stacked form. The better understanding of combustion process in B4C-diluted blends coupled with well-devised geometrical design allowed us to fabricate FGMs with high volume fractions of B4C in the surface layer.

Abstract: The purpose of the present work is to study the formation of the compositional gradient during the fabrication of FGMs by a centrifugal in-situ method. Al-Al2Cu FGMs were fabricated by the centrifugal in-situ method using eutectic Al-33mass%Cu alloy, and the microstructures of fabricated FGMs were studied. It was found that the Al-Al2Cu FGMs could be fabricated by the centrifugal in-situ method from eutectic alloy. Based on the experimental results, the formation mechanism of the compositional gradient during the fabrication of FGM by the centrifugal in-situ method in the A-B alloy could be summarized as follows; 1) Partial separation of A and B elements in the liquid state occurs due to the density difference. 2) A compositional gradient is formed before the crystallization of the primary crystal. 3) The primary crystals in the matrix appear to depend on local chemical composition. 4) The primary crystals migrate according to density difference, and a further compositional gradient is formed.

Abstract: CeO2-stabilised ZrO2-based functionally graded materials (FGMs) can be successfully produced using electrophoretic deposition and pressureless sintering in air. A cylindrical Al2O3/ZrO2 FGM, for instance, shows an ideal structure for applications such as drill blanks. It has a central hole with a diameter less than 0.5 mm, a tough Ce-ZrO2 core with a diameter of about 3 mm, a gradient layer of about 1 mm, and a hard Al2O3-rich surface layer. The Ce-ZrO2 core shows a Vickers hardness between 10 and 11 GPa and an excellent toughness (>10 MPa m1/2). In the gradient layer, hardness and toughness vary continuously along the radius. The Al2O3-rich surface layer has a hardness of 15.2 GPa but a modest toughness of 2 MPa m1/2. Annealing experiments of the air-sintered FGM in inert atmosphere (Ar + 5 vol % N2) allows a significant toughness enhancement, especially in the surface layer (up to 8 MPa m1/2). The experimental results indicate that a proper controlled reduction of the Ce-TZP phase allows a modification of the toughness of the Ce-TZP phase. The influence of the annealing cycle on the properties of the FGM are reported and elucidated.

Abstract: In this study, the development of a functionally graded material (FGM) with hard outer surfaces and a tougher inner core was envisaged. The applicability of electrophoretic deposition (EPD) for the processing of FGM materials by continuously changing the suspension composition is shown. Optimisation of the colloidal processing technique was combined with hot pressing experiments on homogeneous composites in the Al2O3-ZrO2-Ti(C,N) system in order to create a very hard functionally graded material with beneficial residual stresses. Finally, the residual stress distribution was briefly discussed using an existing analytical model.

Abstract: Sub-micrometer scaled CGO (Ce0,9Gd0,1O1,95) powder is used in order to manufacture Ni/CGO functionally graded green bodies with high green densities. Thereby the near-shape manufacturing of Ni/CGO cermets by gel casting of CGO and Ni powder mixtures and electrophoretic impregnation of sub-micro scaled ceria doped particles into a Ni/CGO mesoporous matrix is described. Solids content of dispersed suspensions is varied as well as the mixing ratio of sub-micro sized and micro sized particles. The influence on green density and sintering behavior is investigated in order to achieve maximum CGO densities in combination with maximum ionic conductivity for potential use in SOFC. Shrinkage rates from sintering experiments with constant rates of heating (CRH) are analyzed to determine most suitable co-sintering conditions of Ni-Ni/CGO composite anode materials.

Abstract: NaX zeolite crystals of 50 µm were grown by a continuous crystallization with seed crystal (10 ㎛) in a mother solution having a 3.5Na2O : Al2O3 : 2.1SiO2 : 1000H2O composition. The seed crystal morphology of NaX zeolite shows that an octahedron is formed composed of 8 equilateral triangles which have an average lattice constant of a = 24.9911 Å with a SiO2/Al2O3 molar ratio of 2.1~2.4. In order to grow crystals of zeolite X to an appropriate size by the continuous method, the mother solution was supplied with various seed contents (3~20 wt%) in an autoclave at 90°C after 7, 12, 16, 19 and 24 days.

Abstract: The reaction at narrow holes method (RANH method) has been proposed for fabricating fiber reinforced metal (FRM), such as an intermetallic compound fiber / metal matrix composite. This study clarifies a microstructure at a fiber / metal matrix interface of FRM fabricated by using a combination of pure-copper and pure-aluminum in the RANH method. Pure-aluminum fiber was inserted into a narrow hole drilled in the copper matrix. The assembly comprising the pure-aluminum fiber and the pure-copper matrix was heated to a temperature greater than eutectic temperature of the copper-aluminum binary alloy. A molten aluminum reacted with copper to form an annular reacted region consisting of g1 intermetallic compound in a single phase near the edge of the narrow hole. The g1 intermetallic compound has very high hardness on the order of 800-900 HV. The annular reacted region may have a high tensile strength and may work as a reinforcing metal fiber in FRM.