Materials Science Forum Vol. 1016

Abstract: Additively manufactured (AM) 316L stainless steel (SS) often contains cellular dislocation structure which is a distinct microstructural feature compared with those fabricated traditionally, like casting and forging. The role of this unique cellular dislocation structure on the mechanical properties of the AM 316L SS needs to be determined to guide its further performance improvement. In this study, the effect of cellular dislocation structure on the strength of AM 316L SS was investigated via micro-mechanical compression test. Single crystalline micro-pillars were firstly prepared from both the as-built and annealed AM 316L SS bulk specimens, with and without cellular dislocation structure relatively. The results show a significant increase of the yield strength of the micro-pillars with the cellular dislocation structure. The micro-pillars containing cellular dislocation structure with different sizes and morphologies have been studied to evaluate the effect of cellular dislocation structure on the strength of AM 316L SS.

Abstract: In producing syngas, which offers environmental benefits, dry reforming of methane (DRM) could promote the installation of the future carbon tax. This reaction has been already extensively studied and nowadays, no stable catalysts are enough efficient to scale up the process to its industrialization. It has been suggested that basic sites can affect the performance of catalyst. It is known that magnesium promotes the performance of catalyst. In order to understand the effect of Mg for dry reforming of methane, NiO-MgO-ZrO₂ catalysts were studied. The activity was carried out at 700 °C in a fixed-bed micro-reactor under CH₄:CO₂:Ar=1:1:8. It was shown that the introduction of Mg led to an unexpected decrease in the activity when compared to non-promoted catalyst. It was also shown that the surface area, pore-volume, pore diameter, and weak basicity decreased when the Mg was introduced into NiO-ZrO₂ catalyst. All these properties can cause a decrease in the activity, selectivity, and stability of NiO-MgO-ZrO₂ catalyst for DRM.

Abstract: Dense oxide coated AZ 31 magnesium alloy surfaces were uniformly formed using a radio frequency magnetron sputtering method. The magnesium oxide thin film thickness was about 240 nm. XRD results of the film indicated that film of magnesium oxide single phase was deposited. The surface of the film was uniform and no crack was observed. The Vickers hardness measured by the nanoindenter was about Hv80 and Hv200 for the AZ31 substrate and the sample coated with the thin film, respectively. The dynamic hardness of the AZ31 substrate and the sample coated with the thin film were almost the same. In the curve at the time of pressurization, a step was observed in the sample coated with the thin film. On the other hand, many steps were observed in the data curve for the thin film deposited on the glass substrate.

Abstract: The most popular spraying technologies include: flame spraying, arc spraying and plasma spraying. Spraying technologies allow for the formation of coatings with a desired chemical composition and thickness. However, such coatings characterize by numerous imperfections associated with the nature of the process itself. It is obvious that some of imperfections in thermal spraying coatings can be eliminated by choosing the right parameters of the process. However, in order to improve the properties and eliminate material discontinuous, it is necessary to conduct the remelting process. Research in most cases, showed that the reduction of porosity and simultaneous increase in hardness are possible only by remelting the coating using different heat sources. By adjusting the technological parameters and remelting speed, it is possible to precisely control the depth of the remelted material and thus the properties of final coatings. The paper presents the remelting processes of thermal spraying coatings in relation to technologies, properties and applications.

Abstract: The processing of binary alloys consisting of ferromagnetic Fe and antiferromagnetic Cr by severe plastic deformation (SPD) with different chemical compositions has been investigated. Although the phase diagram exhibits a large gap in the thermodynamical equilibrium at lower temperatures, it is shown that techniques based on SPD help to overcome common processing limits. Different processing routes including initial ball milling (BM) and arc melting (AM) and a concatenation with annealing treatments prior to high-pressure torsion (HPT) deformation are compared in this work. Investigation of the deformed microstructures by electron microscopy and synchrotron X-ray diffraction reveal homogeneous, nanocrystalline microstructures for HPT deformed AM alloys. HPT deformation of powder blends and BM powders leads to an exorbitant increase in hardness or an unusual fast formation of a σ-phase and therefore impede successful processing.

Abstract: Selective laser melting (SLM) is one of the new additive manufacturing techniques in which complex parts can be created directly by selectively melting layers of powder. If the productivity of the process is too fast, defects (porosity, partially melted powder, spatters …) are generated inside the fabricated parts and can deteriorate the mechanical properties of the product. A new Laser Boost strategy with a larger melting area and a productivity of 43.20 cm3/h has been compared to a Linear Classic strategy. Ti-64 alloy samples were elaborated with both strategies to study their influence on microstructure and mechanical properties. Laser Boost strategy leads to the formation of Ti-64 prior β grains that are larger than the Linear Classic strategy. Mechanical properties obtains are similar with both strategies with a maximum strength average around 1250MPa and an elongation at failure between 3 and 9%. A thermal post-processing by Hot Isostatic Pressure have been carried out on samples made by Laser Boost to increase the ductility of the material up to 15%.

Abstract: An aluminum silicon-based alloy Al-7wt%Si-2wt%Fe, was processed by severe plastic deformation technique in high-pressure torsion (HPT) at room temperature under a pressure of 6.0 GPa and rotational speed of 1.0 rpm with various numbers of turns up to five. Microstructure evolution, especially iron-containing intermetallic phases, was observed using an optical microscope and a scanning electron microscope (SEM). The microstructure results demonstrate that the large strains introduced by HPT at ambient temperature cause fragmentation of iron-intermetallic particles. The degree of fragmentation increases with increasing numbers of turns so that the intermetallic particles decreased in size with increasing imposed strain. In addition, the wear properties were evaluated using ball-on-disc dry sliding testing for both the as-cast material and the alloy processed by HPT using micro-tribometer UMT-2 (CETR Co., USA) following the ASTM G99-05 (2010) standard. The wear tests were conducted on the surface of the samples at 1.5 mm from the disc center under a normal load of 5 N with a rotational speed of 60 rpm and sliding time of 10 min. The friction coefficient and wear volume loss were examined to evaluate the effect of HPT on wear resistance. The results show that the samples processed by HPT have lower average values for the COF and wear volume loss than that of unprocessed samples.

Abstract: A stress softening known as the Mullins effect is observed usually in rubberlike material after the first load. This paper describes an experimental test method for defining the nonlinear properties of rubber materials used for finite element analysis. Experimental observations have shown that the Mullins effect induces a permanent set and some anisotropy. To test the Mullins effect the mechanical preconditioning is suggested to stabilize the properties of rubber material. A stress-strain curve will change significantly when the rubber material is strained greater than the previous stabilized level. Therefore, material properties at maximum strain level are obtained to predict behavior of rubber products. To obtain the rubber material constants used for finite element analysis to understand the characteristics of automotive rubber parts, mechanical properties tests such as uniaxial tension, equ-biaxial tension and pure shear tests are required. When the load was repeatedly applied to the rubber specimen, the stress-strain relationship was greatest in the first and second cycles, and the larger the strain range, the more the stress was reduced. The material constants were obtained using the stress-strain data after the rubber specimen was stabilized. The value of stiffness decreased as the maximum strain range increased. The static stiffness of an automotive engine mount is calculated by nonlinear finite element analysis using the experimentally determined material constants and compared with the experimental results considering the mechanical preconditioning effect resulting in a good correlation.

Abstract: Modeling and simulation in meso-scale have been used to investigate the complicated physics involving powder behaviors during the forming process in electron beam selective melting. But it is difficult to quantitatively predict the forming quality of parts due to the huge calculation amount for multi-layer and multi-pass cases. In this paper, we presented a novel frame for investigating the side surface roughness of fabricated components based on single-track simulation. An integrated model of selective beam melting with considering the random distribution powder particles was developed. The surface morphologies of simulated tracks were used to calculated the surface roughness, and the calculated results were validated by experiment data from independent literature. The effects of heat input were discussed in detail. It reveals that the side roughness increases with a lower heat input. The mechanism behind is the fluctuation of tracks induced by the asymmetry of molten pool, which is related to the coalescence of melting particles and unevenness of powder distribution.

Abstract: The aim of this work was to analyze the recrystallization behavior of cold rolled Aluminum/graphene composites during annealing. The Aluminum/graphene composite was cold rolled firstly, and then annealed at different temperature (250°C, 300°C, 350°C, 400°C) and for various time (1 h, 2 h, 8 h, 32 h). Full recrystallization did not occur until the annealing temperature was above 300 °C. With annealing temperature increasing from 250 to 300°C, the hardness of the composites decreased from 49.6 to 27.6 HV. Grain growth were not observed at high annealing temperature and longer annealing time, which suggested that Graphene has strong pinning effect on the grain boundary of Aluminum.