Materials Science Forum Vol. 941

Abstract: Ternary metal vanadates have recently emerged as promising photoelectrode materials for sunlight-driven water splitting. Here, we show that highly active nanostructured BiVO₄ films can be deposited onto fluorine-doped tin oxide (FTO) substrates by a facile sequential dipping method known as successive ionic layer adsorption and reaction (SILAR). After annealing and deposition of a cobalt phosphate (Co-Pi) co-catalyst, the photoelectrodes produce anodic photocurrents (under 100 mW cm^-2 broadband illumination, 1.23 V vs. RHE) in pH 7 phosphate buffer that are on par with the highest reported in the literature for similar materials. To gain insight into the reason for the good performance of the deposited films, and to identify factors limiting their performance, incident photon-to-electron conversion efficiency spectra have been analyzed using a simple diffusion–reaction model to quantify the electron diffusion length (L_n; the average distance travelled before recombination) and charge separation efficiency (η_sep) in the films. The results indicate that η_sep approaches unity at sufficiently positive applied potential but the photocurrent is limited by significant charge collection losses due to a short L_n relative to the film thickness. The Co-Pi catalyst is found to improve η_sep at low potentials as well as increase L_n at all potentials studied. These findings help to clarify the role of the Co-Pi co-catalyst and show that there could be room for improvement of BiVO₄ photoanodes deposited by SILAR if L_n can be increased.

Abstract: 3D printing has been getting increasing attention from both industry and academy in recent years. Unlike traditional construction processes, 3D cementitious material printing requires more accurate control on the pumping flow rate, which is known to be affected by the material rheology. In this paper, the effect of fresh rheology on the flow rate of cementitious material was studied experimentally in 3D cementitious material printing process. The material viscosity was measured via a large gap vane viscometer at different time points after mixing, and printing tests were conducted at the same time period to measure the flow rate based on volume conservation principle. Experiments showed that the flow rate was significantly affected by rheology change with respect to time. An open loop control method was then implemented to harmonize the flow volume per unit length during printing processes to improve printing quality.

Abstract: Additive manufacturing of titanium components offers several advantages compared to conventional production technologies such as higher material utilization efficiency and increased geometric possibilities. In comparison to laser powder bed processes, arc-based additive manufacturing processes have the additional advantage of an almost unlimited assembly space, higher deposition rates and an improved utilisation factor of raw materials. Disadvantages of wire-based methods are the restricted availability of different types of wire consumables, the fact that the wire feed rate is directly coupled to the heat input and the lack of possibility to create multi-material structures in-situ.Within this work, the 3D Plasma Metal Deposition (3DPMD) method, based on a plasma powder deposition process is introduced. 3DPMD has some special advantages compared to the established plasma powder process and other additive processes. For example, up to four powders, which can differ in terms of material and powder fraction, can be mixed within one layer. This allows a targeted adaption of local properties (microstructure, mechanical properties, wear resistance, porosity, etc.) to the targeted load type and level. The tailored introduction of reinforcement particles, e.g. tungsten or titanium carbides, into the component is a simple example.The study aims to demonstrate the suitability of the 3DPMD for the production of titanium components in layer-by-layer design. Various demonstrators are prepared and analysed. The microstructures, the porosity and the hardness values of the different structures are analysed.In summary, 3DPMD offers the possibility to produce titanium structures with and without reinforcement particles. Using automated routines, it is possible to generate metallic structures directly from the CAD drawings using welding robots. Microstructures and properties are directly related to the process and, therefore, structure-process-property relationships are discussed within this work.

Abstract: A way to produce 3D scaffold is via laser stereolithography. We propose a method of direct laser writing for micro-stereolithography in which we use as light source a low power blue diode laser with a wavelength of 448nm. The material chosen for scaffold fabrication is a polyethylene glycol diacrylate (PEGDA) solution at concentration of 75% in ethanol. We chose a short PEGDA molecule with a molecular weight of 575 g/mol, in order to obtain a better control over the polymerization. We used Irgacure 819 as photoinitiator to initiate the photopolymerization. The absorption of the Irgacure 819 almost drops to zero at the excitation wavelength, so the efficiency of the photopolymerization is strongly reduced. Since the intensity of the light reduces by a factor 5 within a penetration depth, equal to the depth of focus of the optical system, we achieve a fine control of the vertical and lateral photopolymerization of the solution. The threshold for effective polymerization is not reached outside that region.

Abstract: Micro-channel heat exchangers (MCHXs) manufactured by Zess & Lin Industries provide highly effective heat transfer and are used in a growing number of critical applications. MCHXs consist of stainless steel or high temperature Nickel-based alloy plates with micro channels that are chemically etched or machined into each plate. These traditional extractive manufacturing methods of chemical etching and machining used in manufacturing MCHX plates are difficult and costly as a large percentage of expensive alloy is lost during manufacturing.

Abstract: Additive Manufacturing technologies allow for the direct fabrication of lightweight structures with improved properties. In this context, Fused Deposition Modelling (FDM) has also been considered to design 3D multifunctional scaffolds with complex morphology, tailored biological, mechanical and mass transport properties. As an example, poly (ε-caprolactone) (PCL), surface-modified PCL and PCL-based nanocomposite scaffolds were fabricated and analysed. The effects of structural and morphological features (i.e., sequence of stacking, fiber spacing distance, pore size and geometry), surface modification and nanoparticles on the in vitro biological and mechanical performances were investigated.

Abstract: The steel 18Ni300 is widely used for tooling of injection moulding and die casting industries. Additive manufacturing (AM) technology is applicable to manufacture dies with “ideal” design without construction of manufacturing reality. Selective laser melting (SLM) processed materials have finer microstructure due to steeper temperature gradient and more rapid cooling conditions than conventional casting process during solidification. This difference may make different heat treatment behavior in obtaining optimal properties of the 18Ni300 maraging steel manufactured by SLM. Heat treatment is one of the most processes to improve microstructure, mechanical properties and performance of tooling dies. This work studies evolution of microstructure and properties during heat treatment, by X-ray diffraction, optical and scanning electron microscopy (SEM). The results show that the SLMed materials with only aging treatment have comparable strengths and hardness to those of conventionally cast materials with both solution and aging treatment. For the SLMed materials, with increase of aging time and/or temperature, the formed reverted austenite (γ-Fe) fraction increases, while aging precipitation hardening decreases. This is more apparent at aging temperatures of higher than 540°C. The combined effects of softening by formation of reverted austenite (γ-Fe) and age hardening induced by precipitation are discussed.

Abstract: In this research, a composition optimized In718 superalloy powder suitable for additive manufacturing has been developed by using the vacuum induction melting gas atomization (VIGA) and the powder sieving process. VIGA which combines the vacuum induction melting (VIM) and gas atomization (GA) processes uses high pressure inert gas to atomize the metal melt formed by VIM to form metal droplets. These metal droplets are solidified to form metal powders during the falling process in the atomized chamber. After the sieving process, the mean particle size D₅₀ of the powder is less than 35 μm and the particle size distribution (PSD) ranges from 10 to 55 μm (D₁₀~D₉₀). Besides, the produced powder has high flowability (I_Carr ≦15), which is suitable for selective laser melting (SLM) additive manufacturing (AM). After the SLM process, the tensile tests are conducted at room temperature and high temperature of 650°C. The results show that the high temperature properties of the optimized In718 superalloy are superior to the commercial In718 superalloy.

Abstract: Cold spraying (CS) is a solid-state layer-by-layer deposition technique, allowing to fabricate complex-shaped metallic components, such as metal matrix composites (MMCs). It has been demonstrated that introduction of a nacre-like nanolaminated architecture in man-made MMCs can perfectly beat the conflict between strength and ductility (toughness). In this work, the fully dense carbon nanotubes (CNTs) reinforced Al6Si (hereafter called CNT/Al6Si) MMC with the nanolaminated architecture is successfully realized by flake powder metallurgy followed by CS. It is revealed that the nanolaminated architecture, containing nanosized grains, is properly created in the CNT/Al6Si flaky powder by ball milling, which is then conserved in the CS-processed component. The harmful excessive formation of Al₄C₃ due to interfacial reaction is limited and structural integrity of initially incorporated CNTs is well retained.

Abstract: In this study, LMD-CLAD® process (Direct Laser Additive manufacturing) is developed for alumina and Al₂O₃-Y₂O₃-ZrO₂ ternary eutectic compositions. Powder flowability, laser-material interaction and thermal gradient control have been investigated. Powder granules of aforementioned compositions have been designed by spray-drying. Particle size distribution, Hall funnel test and SEM observations have been performed. Flowability has been improved by 20% in order to match with the LMD-CLAD® process by adjusting their density, size and surface quality. Otherwise, optical absorption of the ceramics has been increased up to 90% thanks to the addition of doping ions. With such a flowability improvement, laser powder deposition tests were successful and enabled us to investigate the effect of laser parameters and thermal environment on deposited beads state.