Fine Cellular Structure Research Articles

Effect of scanning speed on the microstructure and mechanical behavior of 316L stainless steel fabricated by selective laser melting

In this work, the tensile behavior and microhardness of 316L stainless steel fabricated by selective laser melting under different process parameters were investigated. The ultimate tensile strength decreased slightly with increasing energy input, while the opposite tendency was observed for the elongation to failure. Microstructure characterizations were performed to relate the pore morphology, melting pool geometry, solidification cell structure, and grain sizes to the mechanical performance of as-built samples. Fine grains with high fraction of low-angle grain boundaries and fine cellular structures with nano-inclusions are observed in the sample fabricated with a high scanning speed (1000 mm/s). As a result, the sample shows high ultimate tensile strength of up to 707 MPa, while maintaining a total elongation of 30%. The sample fabricated with a low scanning speed (800 mm/s) shows high ductility with total elongation to failure of 55%. The improved ductility is mainly attributed to the elimination of residual pores and melting pool boundaries, which result in brittle features in the as-built samples. The results indicate that selective laser melting may act as a physical metallurgy method to modify the microstructure, and thus improve the mechanical performance of metallic materials.

Influence of the viscosity of poly(methyl methacrylate) on the cellular structure of nanocellular materials

AbstractThree different grades of poly(methyl methacrylate) (PMMA) with different rheological properties are used for the production of nanocellular materials using gas dissolution foaming. The influences of both the viscosity of the different polymers and the processing parameters on the final cellular structure are studied using a wide range of saturation and foaming conditions. Foaming conditions affect similarly all cellular materials. It is found that an increase of the foaming temperature results in less dense nanocellular materials, with higher cell nucleation densities. In addition, it is demonstrated that a lower viscosity leads to cellular polymers with a lower relative density but larger cell sizes and smaller cell nucleation densities, these differences being more noticeable for the conditions in which low solubilities are reached. It is possible to produce nanocellular materials with relative densities of 0.24 combined with cell sizes of 75 nm and cell nucleation densities of 1015nuclei cm−3using the PMMA with the lowest viscosity. In contrast, minimum cell sizes of around 14 nm and maximum cell nucleation densities of 3.5 × 1016nuclei cm−3with relative densities of 0.4 are obtained with the most viscous one. © 2019 Society of Chemical Industry

Effects of pulsed laser surface treatments on microstructural characteristics and hardness of CrCoNi medium-entropy alloy

ABSTRACTA hot-swaged/annealed CrCoNi medium-entropy alloy (MEA) was surface-treated by pulsed laser at two different powers (400 and 200 W). Microstructural characteristics of the laser-modified zones were characterised and analysed by energy dispersive spectroscopy, electron backscatter diffraction and electron channelling contrast imaging techniques. Results show that melting and rapid solidification occur on the surfaces of both laser-treated specimens, and profuse annealing twins existing in the initial microstructures are essentially eliminated in the melting zone (MZ) with plentiful low-angle boundaries appearing. Meanwhile, the initial equiaxed grains are replaced by new grains in the MZ with either granular or columnar appearance (in the 2D cross-sectional views). These grains are comprised of fine cellular structures with relatively uniform sizes (∼1–2 μm in width/diameter), the formation of which is related to the segregation of Cr during solidification. With the laser power decreasing from 400 to 200 W, the volume of the MZ and grain sizes in its interior are reduced. This is due to less heat supply and faster cooling caused by reducing the laser power. Hardness tests reveal that the surfaces of both the laser-treated specimens are slightly softened (by 10–20%), and quantitative analyses suggest that this is mainly related to grain coarsening and the disappearance of annealing twins in the MZ.

Effect of Graphene Surface Functional Groups on the Mechanical Property of PMMA Microcellular Composite Foams

The functional groups on graphene sheets surface affect their dispersion and interfacial adhesion in polymer matrix. We compared the mechanical property of polymethymethacrylate (PMMA) microcellular foams reinforced with graphene oxide (GO) and reduced graphene oxide (RGO) to investigate this influence of functional groups. RGO sheets were fabricated by solvent thermal reduction in DMF medium. UV-Vis, FT-IR and XPS analyses indicate the difference of oxygen-containing groups on GO and RGO sheets surface. The observation of SEM illustrates that the addition of a smaller number of GO or RGO sheets causes a fine cellular structure of PMMA foams with a higher cell density(about 1011 cells/cm3) and smaller cell sizes (about 1–2 µm) owing to their remarkable heterogeneous nucleation effect. Compared to GO reinforced foams, the RGO/PMMA foams own lower cell density and bigger cell size in their microstructure, and their compressive strength is lower even when the reinforcement contents are the same and the foam bulk density is higher. These results indicate that the oxygen-containing groups on GO sheets’ surface are beneficial to adhere CO2 to realize a larger nucleation rate, and their strong interaction with PMMA matrix improves the mechanical property of PMMA foams.

Tribological properties of selective laser melted Al 12Si alloy

Abstract The effect of scanning strategy and heat-treatment on the tribological properties of Al 12Si fabricated using selective laser melting (SLM) were evaluated and compared with their cast counterparts (CC). The SLM sample shows a mesostructure and in addition, the microstructure consists of the Al rich cells surrounded by Si network in the fine cellular structure. Annealing leads to the precipitation of Si and disintegration of the cellular structure, resulting in the reduction of hardness. The as-prepared SLM specimens show the least wear rate compared to heat treated SLM and CC specimens. SLM specimens built with single melt (SM) and checkerboard (CB) scanning strategies have similar hardness, the wear rate of in the former is significantly higher due to high porosity in it.

Grain size dependence, mechanical properties and surface nanoeutectic modification of Al2O3-ZrO2 ceramic

Abstract The present work aims to provide fundamental insights into the grain size dependence and mechanical behavior of hot-pressed Al2O3-ZrO2 ceramic at its eutectic composition, and further to explore the hardening effect of laser-induced surface nanoeutectic layer. The underlying correlations between densification behavior, grain size distribution and mechanical properties were elucidated. Sintering at 1550 °C promotes the densification without extensive grain growth, and in this case the sample exhibits a critical density of 99.3 %. The average grain size is tailored into a range of 0.6–0.9 μm, and the measured flexural strength and toughness reach 1100 MPa and 11 MPa·m1/2, respectively. The metastable t-ZrO2 grains indeed play a pivotal role in energy dissipation at the crack tip through crack deflection and branching. In addition, the mechanical behavior is reasonably explained through constructing a multilevel toughening mechanism map associated with grain size distribution of ZrO2. Particularly, surface nanocrystallized Al2O3-ZrO2 eutectic layer with a thickness of 1000 μm free of pores and cracks is achieved by a rapid laser melting process. The outmost laser-modified nanoeutectic layer exhibits a fine cellular structure with an interphase spacing of only 105 nm and a hardness of as high as 26.1 GPa, which provides a promising potential in enhancing significantly the hardness and wear resistance for applications as sliding ceramic components.

Overcoming the Challenge of Producing Large and Flat Nanocellular Polymers: A Study with PMMA

Although nanocellular polymers are interesting materials with improved properties in comparison with conventional or microcellular polymers, the production of large and flat parts of those materials is still challenging. Herein, gas dissolution foaming process is used to produce large and flat nanocellular polymethylmethacrylate samples. In order to do that, the foaming step is performed in a hot press. The methodology is optimized to produce flat samples with dimensions of 100 × 100 × 6 mm3, relative densities in the range 0.25–0.55 and cell sizes around 250 nm. Additionally, foaming parameters are modified to study their influence on the final cellular structure, and the materials produced in this paper are compared with samples produced by using a most conventional approach in which foaming step is conducted in a thermal bath. Results obtained show that an increment in the foaming temperature leads to a reduction in relative density and an increase of cell nucleation density. Moreover, differences in the final cellular structure for materials produced by both foaming routes are studied, proving that although there exist some differences, the mechanisms governing the nucleation and growing are the same in both processes, leading to the production of homogeneous materials with very similar cellular structures.

Deformation of cellular components of bone forming cells when exposed to a magnetic field

It has been suggested that alignment control of biological macromolecules and structures is possible when the diamagnetic energy is sufficient to overcome the thermal energy. Strong magnetic fields of several Tesla (T) have previously been shown to enable the observation of the magnetic orientations of protein fibers and suspended or adherent cells. In the present study, the fine structure of a bone-forming osteoblast cell line was observed in real-time under strong static magnetic fields of up to 5 T. The magnetic field was applied normal to the adhering surface, and induced slight deformation of cellular shape under bright field illumination. A newly developed dark field observation method presented more precise analysis of the inner structures of the cells in strong magnetic fields. Specifically, structures with a round appearance were clearly visualized around the cell nucleus using the electromagnetic illumination. In addition to the magnetically induced swelling of the round structures—which showed differences in diameter of several micrometers—color changes for these intracellular components were also detected using this technique. A high-resolution analysis of fine cellular structure based on the magnetic orientation of intracellular components including microtubules is proposed.

Discharge in a Subthreshold Microwave Beam as an Unusual Type of Ionization Wave

Ionization−overheating instability of a non-self-sustained discharge in air in a subthreshold microwave field creates a self-sustained discharge with a fine cellular structure, whose UV radiation, in turn, generates a new non-self-sustained discharge on the microwave beam path, where ionization−overheating instability arises again. It is shown that, at microwave intensities in the range of 3–18 kW/cm2, the propagation velocity of the ionization wave, including the region of the self-sustained discharge and the forward region of the non-self-sustained discharge developing in its UV halo, is proportional to the third power of the microwave field strength, while the maximum temperature in the discharge is inversely proportional to the microwave field strength.

Investigation of the mold-filling phenomenon in high-pressure foam injection molding and its effects on the cellular structure in expanded foams

Mold filling plays a crucial role in determining the cells' structural morphology and uniformity in high-pressure foam injection molding. This process affects the formation, deformation, and dissolution of premature cells, which nucleate in the melt/gas mixture during the mold filling. We investigated the mold-filling phenomena and the cell structure development in the high-pressure foam injection molding process with mold opening. Polystyrene was used as the matrix, and carbon dioxide and nitrogen were used as physical blowing agents. We adopted an in-situ visualization technique, which enabled us to monitor and study the formation and evolution of the cells during the mold-filling and melt-packing phases. It was found that both the shape and the size of cells, which had nucleated during the mold filling process inside the melt, were changed by the packing condition selected. Statistical analyses have shown that the packing pressure and its duration were the most influential parameters affecting the size and the shape of cells that had nucleated in the melt before the mold opening. A greater packing pressure and a longer packing time resulted in a smaller cell size with a larger deformation in the melt during mold filling. A proper packing condition eventually removed all of the cells and re-dissolved them back into the melt before mold opening. The effect of the mold-filling parameters on the final cellular structure was also investigated, using carbon dioxide and nitrogen as physical blowing agents. A complete dissolution of the premature cells during the mold-filling and packing phases resulted in a more uniform and finer cellular structure after mold opening. The obtained results and the developed knowledge provide a deeper insight into the fundamental aspects of cellular structure development in foam injection molding process, which can be used to produce highly expanded, ultra-light foams with uniform cell structure.

Microstructure evolution of as-cast AlN/AZ91 composites and room temperature compressive properties

The microstructures of as-cast AlN/AZ91 composites prepared by different in-situ nitridation reaction times are characterized. Increasing nitridation reaction times resulted in morphological changes of the primary α-Mg phase from coarse dendritic structures to fine cellular structures, the volume fraction decrease of eutectic β-Mg17Al12 phase, and the eutectic structure transformation from partial divorced eutectic to fully divorced eutectic. During the room temperature compression of as-cast AlN/AZ91 composites, the (0002) basal plane texture perpendicular to the compression direction was weakened effectively, and compression strength and plasticity were simultaneously improved compared with as-cast AZ91 alloy. When the nitridation reaction time is 2 h, i.e. the volume fraction of in-situ AlN particles is 1.5%, the compressive yield strength of 110 ± 2 MPa, the ultimate compressive strength of 371 ± 8 MPa and the compressive fraction elongation of 22.1 ± 0.4% are achieved in the in-situ AlN/AZ91 composites.

Effects of La on Microstructure and Corrosion Behavior of AlSi5Cu1Mg Alloy

The microstructures and corrosion behavior of AlSi5Cu1Mg alloy modified with different contents of La were investigated. Optical microscopy observations showed that the microstructures of AlSi5Cu1Mg–xLa (x = 0, 0.3, 0.6, and 0.9 wt%) alloy transformed from coarse cellular to fine cellular as the content of La increased from 0 to 0.9 wt%. The results of electrochemical measurement and immersion test in 3.5 wt% NaCl solution showed that the corrosion resistance of AlSi5Cu1Mg–xLa alloy strongly depended on its microstructures. The AlSi5Cu1Mg–0.6La sample exhibited the highest corrosion resistance due to its fine cellular structure and the cathode phases coated by the La element. The covering film of La hindered the corrosion electron flowing between the cathode and the anode. As a result, the corrosion current density of the AlSi5Cu1Mg–0.6La alloy was only about 40% that of the matrix alloy in the electrochemical measurement.

Effect of Pulsed Nitrogen Plasma and Nitrogen Ion Fluxes on the Structure and Mechanical Properties of Vanadium

The effect of powerful pulsed fluxes of nitrogen plasma and nitrogen ions generated in the PF-4 Plasma Focus setup (LPI; energy flux density of plasma pulse was 108–1010 W/cm2) on the modification of vanadium surface is studied. Melting and ultrafast solidification result in a fine cellular structure (cell size of 100–200 nm) in a thin surface layer in samples. There are irradiation regimes causing directional crack growth after solidification and cooling of the surface layer and the formation of a block microstructure with a block size of several tens of micrometers. The thickness of the melted layer in the samples is 2–4 μm. Cracks propagate to a depth of 5–20 μm. It is established that irradiation by pulsed nitrogen plasma and high-energy nitrogen ions changes the microhardness of the vanadium surface layers. The microhardness increases by a factor of three with the number of plasma pulses and the distance between a sample and the anode of the Plasma Focus (PF) setup. The increase in the microhardness is in agreement with the refinement of coherent scattering regions, the increase in lattice microstrain e, and the formation of vanadium nitrides. Pulsed fluxes of nitrogen plasma and nitrogen ions decrease the lattice parameter much greater than cold working (rolling) does. The lattice parameter decreases when the total irradiation power is increased (the number of pulses increases or the distance between a sample and the anode of the PF setup decreases). Such changes seem to be caused by the action of the residual macrostresses induced by pulsed plasma irradiation. In addition, X-ray diffraction analysis showed a change in the texture of the surface layer after ion-plasma treatment of coldworked vanadium samples in the PF setup.

Effect of Recycling on the Cellular Structure of Polylactide in a Batch Process

The aim of the work is to demonstrate the possibility of using recycled biodegradable material as a cellular material with a reduced weight. An experimental study on the influence of recycling on the properties and foam ability of polylactide (PLA) was carried out. The influence of recycling on the polymer crystallinity, thermal and viscoelastic properties was investigated. During the batch process the cellular structure in PLA and recycled PLA were created. A higher degree of crystallinity, a lower viscosity value and a lower melt strength for recycled PLA, as compared to original material, were observed. For the recycled PLA, a fine cellular structure and low density (0.60 g/cm3) were obtained.

Unusual Fabrication of Lightweight Injection-Molded Polypropylene Foams by Using Air as the Novel Foaming Agent

We report a novel approach to prepare injection-molded foams using air as the foaming agent, which is realized through our newly developed foam injection molding technology. Microcellular polypropylene (PP) foams exhibiting cell size of ∼7 μm and cell density of 1.63 × 109 cells/cm3 were successfully obtained employing air foaming. Fourier transform infrared spectroscopy and rheological results illustrated that the molecular structures and melt properties of PP were greatly altered in air foaming, resembling a reactive foaming process. Furthermore, compared with nitrogen and carbon dioxide foaming, air foaming gave us a more flexible PP foam with finer cellular structure.

Investigation on Microstructure, Micro segregation and Mechanical Properties of ATIG welded Alloy C-276

This research article reports the activated flux tungsten inert gas (ATIG) welding of alloy C-276. Microstructures of the welded joint were studied using optical and scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) analysis showed that segregation of Mo and W lead to the formation of the secondary phases. The microstructure results show that fine cellular structure was observed in the fusion zone. Studies using scanning electron microscopy revealed the formation of secondary phases in the grain boundaries. Strength and ductility of the welded joint was evaluated using tensile test. It was inferred that the joint strength was almost matching to the strength of the base metal. It was deduced that the formation of the secondary phases had reduced the ductility and toughness of the weld joint.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ УЛЬТРАДИСПЕРСНЫХ ЧАСТИЦ МОНОКАРБИДА ВОЛЬФРАМА НА СТРУКТУРУ НАПЛАВЛЕННОГО МЕТАЛЛА

Statement of problem (relevance): the paper describes the problem of structure formation in welding low-carbon low-alloy steels. The weld metal structure forms in a complex thermal environment that predetermines the operating performance of the metal. Emergence of large columnar crystals in welds poses a potential hazard, as such structures may under certain conditions concentrate the tension and trigger destruction. The paper dwells upon the possibility of controlling the primary structure of welded metal by injecting nano- and ultrafine particles of a refractory material into the weld pool. The objective is to study how nano- and ultrafine particles of tungsten monocarbide WC) affect the structure of welded metal. Methods used: the researchers have carried out a laboratory experiment consisting in making electrodes with a coating containing a varying amount of ultrafine WC powder, then making welded-metal samples and studying the samples by optical microscopy (a Micromed-Met microscope) as well as measuring the Vickers hardness (a HV-1000 device). The novelty of this research consists in gathering new data on how the concentration of ultrafine WC powder in the electrode coating affects the structure of welded metal. The results: the structure of metal welded by electrodes with a coating containing 0, 0.02, or 0.2 % of WC powder (% of drymix weight) features columnar crystals surrounded by a grid of peripheral ferrite formed along the primary-grain boundaries. Needle-like inclusions of Widmanstatten ferrite sprouting from the crystal edges to the crystal center are observed along with bainitic structures. Electrodes with a coating containing 0.4% of WC powder produce welded metal of a finer cellular structure in the form of equiaxed crystals. The hardness of the welded-metal samples is within 184 to 189 HV. Practical significance: the research has produced data necessary for creating new coated electrodes for welding and surfacing low-carbon low-alloy steels.

Optimization of cellular structure of aluminum foams produced by powder metallurgy method

This paper describes a new method for optimizing the cellular structure of AlMg4Si8 alloy foams manufactured via powder metallurgy. The cellular structure was improved by pretreating TiH2 with a layer of Sn powder, which was then remixed with an AlMg4Si8 alloy used as the raw material for the preparation of the precursor. TiH2 was completely encapsulated in the molten Sn at an early stage of foaming when the matrix alloy was still solid, so that liberated H2 was fully captured when TiH2 began to decompose. The interpore flow of H2 along the cracks in the solid matrix was avoided, improving the utilization ratio of TiH2 as a blowing agent. After determining that 4.0 wt% Sn cladding was capable of almost completely encapsulating 0.5 wt% TiH2, the final cellular structure was optimized and regular circular pores were obtained.

Enhancing the strength of pre-made foams for foam concrete applications

Chemical and mechanical foaming techniques are commonly used in foam concrete technology for developing lightweight construction materials. The characteristics of the foam before the lightweight structure sets and maintains its shape has a great impact on the properties of foamed concretes. The tendency of the foams to coalesce and collapse during the preparation process brings some challenges in controlling the properties of cellular structures. Consequently, it is critical to improve the stability of fresh foams in order to produce high quality cellular structures using a predictable and reliable approach. Aggregating the liquid film around bubbles is known to be effective in improving the stability of foams, but the impact of this stabilizing method has not been investigated for foam concrete applications. In this paper, Xanthan gum (with a thickening capacity) has been utilized as the foam stabilizer to aggregate the liquid film. This stabilizing method is shown to significantly enhance the pore size distribution of foam concretes. The resulting pre-made foams are remarkably more stable than the control foam, and the mechanical properties of the final cellular structure are considerably improved (about 34% in mechanical foaming and 20% in the chemical foaming technique).

Issues of Lattice Structures Production via Metal Additive Manufacturing

Metal additive manufacturing (MAM) is used in the production of parts, where a product is built layer by layer. MAM includes Direct Laser Metal Sintering (DMLS), which allows the production of complex metal parts directly from 3D software models without using sometimes expensive tools such as moulds, dies and cutting tools. New possibilities in the production of complicated components are made available using this advanced manufacturing technology. Nevertheless, this technology has limits, resulting from the method of melting in the powder bed. Therefore, this paper investigates the ability to produce fine cellular lattice structures. Some structures with self-supporting cell units were selected for experimentation and were produced with identical cell size and volume fraction. Based on this, a suitable topology was established for the production of fine structures with small volume fractions.