Owing to the high porosity and low thermal conductivity of organic aerogels, they have been widely used to develop lightweight heat insulation materials. However, their applications in ...high‐temperature environments such as in aerospace applications are usually limited by the heat resistance or ablative property of the organic matrix. In this study, we prepared a novel organic aerogel for high‐temperature insulation by the sol–gel method and freeze‐drying. The matrix is the thermosetting silicon‐containing polyarylacetylene (PSA) resin with excellent comprehensive properties, namely, high heat stability, low volatile component content, and high‐temperature ceramic performance. The obtained PSA aerogel retains the heat resistance (Td5 = 634.8°C) and ablative properties (Yr800 = 92.6%) of the PSA resin, and the internal regular macropores and high porosity (70%) result in an extremely low weight (0.1200 g cm−3), low thermal conductivity (0.039 W m−1 K−1)), and efficient heat insulation. Thus, this is an ideal material for high‐temperature insulation in the aerospace field.
The thermal conduction of PSA aerogel consists of three parts: gas thermal conduction, solid thermal conduction and radiation heat transfer. The high porosity reduces the solid thermal conduction to have a low thermal conductivity.
The selective laser melting (SLM) process parameters significantly affect the bonding of melting powders and the substrate or deposited layer, and the microstructure of end-use components. In this ...study, single track, double track and cubic sample SLM experiments were carried out to investigate the effect of process parameters on the surface morphology of SLM-fabricated AlSi10Mg alloy. The hierarchical microstructures discriminated by the Si phase are observed in SLM-processed AlSi10Mg samples. The formation mechanism of the hierarchical microstructures is elucidated. The formula proved that the solidification rate (R) increases gradually from the boundary to the center of the melt pool. Coarse zones are formed by the instantaneous existence of an extremely high ratio of thermal gradient (G) and solidification rate at the melt pool boundary, where solidification microstructure grows planar. With the heat propagating, a gradual change of the G/R ratio appears and the microstructure turns to columnar-dendritic growth, creating the fine zones.
The densification behavior, surface morphology and attendant microstructural characteristics of the selective laser melting (SLM) processed AlSi10Mg alloy affected by the processing parameters were ...systematically investigated. Increasing the laser scanning speed or hatch spacing will deteriorate the metallurgical bonding between melt pools, resulting in the increase of irregular pores. Scanning speed and hatch spacing affect the liquid metallurgical bonding of melt pools in different ways. By manipulating scanning speed, the shape of the melt pool changes, resulting in different extents of metallurgical bonding. Whereas hatch spacing influences the resultant metallurgical bonding by controlling the overlapping rate between neighboring scan tracks simply. The formations of the hierarchical microstructures which discriminated by the Si phase are elucidated. Coarse zones are formed by the instantaneous existence of extremely high ratio of thermal gradient (G) and solidification rate (R) at the melt pool boundary, where solidification microstructure grows planar. Fine zones are formed by columnar-dendritic growth of microstructure. During the solidification process, the contraction forces that generated by the trapped gas in the pores and gravity are applied to the liquid around irregular pores and, forms the porous microstructures different from that in dense areas eventually. The tensile tests reveal that the tensile properties of SLM-processed samples are significantly affected by the formation of porosity.
In this study, the effects of laser power and scanning speed on the relative density, microstructure and microhardness of selectively laser melted (SLM) 1.2738 mold steel were systematically ...investigated. The deposited energy density, which can express the change in these process parameters and the relative density with one curve, were found more reliable than volume energy density (VED) on the design of process parameters. With the same VED value, samples formed by the combination of a high laser power and scanning speed possess a higher densification than that formed by low laser power and scanning speed. High power may lead to keyhole pore formation. In the top of the molten pool, ultrafine cellular structure was formed, causing that the microhardness value of the top section was always higher than that of the side section.
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•The S-shaped phenomenon appears in the diagram of potential energy vs temperature.•An optimization method is proposed to obtain suitable Tdamp value for simulation.•A new method is ...proposed to judge the melting point of metal.•The lower cooling rate is beneficial to the crystallization of titanium atoms.
Molecular dynamics (MD) was used to study the evolution of crystal structure of titanium (Ti) under different temperature damping parameter (Tdamp) values and heating/cooling rates. In the heating process, when the temperature reaches the melting point, the temperature of the system with lager Tdamp value decreases with the increase of average atomic potential energy. However, an increase in heating rate will cause the melting point to rise slightly. In the cooling process, a larger Tdamp value or a lower cooling rate is more conducive to the crystallization of Ti, which corresponding to the higher crystallization temperature.
In this study, deficiencies in using the laser energy density (LED) to combine different process parameters to represent the heat input in selective laser melting (SLM) process are proposed. The ...change in melt pool shape could not always be clarified by the LED input, since the vary of LED input caused by altering hatch spacing cannot change the melt pool shape. In addition, the microstructure of SLM fabricated AlSi10Mg material was observed, and this paper gives a further elucidation on the formation of the hierarchical microstructures that discriminated by the morphology of Si phase. Microhardness is enhanced by the supersaturated α-Al matrix, and the hardness value of coarse zones and fine zones show different performance.
Molecular dynamics simulation is used to explore the changes in melting point of Fe nanomaterials under different initial configurations. What reveals in this simulation is that an S-curve with ...different curvature appears in the diagram owing to the hindering of total energy supply in larger damping coefficient by changing the temperature-damping coefficient (T-damp). In addition, the change in simulated theoretical melting points are found by comparing the initial configurations of different particles under the same conditions.
A matrix resin poly(silicon-containing arylacetylene vinyl)s (PSAV) containing vinyl at both branch and terminal chains underwent synthesis via the zinc powder catalytic method using m-diacetylene ...benzene and dichloromethylvinylsilane as raw materials. Vinyl in the PSAV resin was crosslinked by the free radical initiator dibenzoyl peroxide to obtain a crosslinked network structure resin (PSAV-L). This approach sought to improve the thermal properties and other related properties of the matrix resin. A series of tests, such as rotated rheometer, FTIR, DSC, TGA, Py-GC-MS and universal testing machine, characterised processing property, curing behaviour, thermal properties and mechanical properties. The rheological curve shows that PSAV-L resin has a wide processing window (40–134.5°C), endowing the resin with excellent processing performance. Thermal curing behaviour indicates that PSAV-L resin can start curing at a lower temperature, namely, 32°C earlier than PSAV resin. TGA analysis shows that the degradation temperature at 5% weight loss (Td5) of PSAV-L resin stands at 579.4°C, 45.4°C higher than that of PSAV resin due to the fact that the crosslinking of vinyl gives PSAV-L resin a network structure. The flexural strength, flexural modulus and ILSS of the quartz fibre cloth reinforced PSAV-L resin composite (QF/PSAV-L) are 184.68 MPa, 15.50 GPa and 12.40 MPa. The PSAV-L resin exhibits the comprehensive properties of good processing performance, low curing temperature, excellent thermal performance and high mechanical properties.
15-5PH, a martensitic, precipitation hardening stainless steel, can easily form a martensite structure during rapid solidification, especially in the selective laser melting (SLM) process. In our ...study, the 15-5PH/Gr composites with different mass ratios (0 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%) of graphene nanoplates (GNPs) were successfully prepared using SLM technology. Through the observation of optical microscopy images and Raman spectra, it is discovered that the GNPs are uniformly dispersed in the 15-5PH/Gr composites. Based on this phenomenon, a flow model is established to explain the process of the secondary dispersion of the GNPs in the molten pool at high temperatures, and then we come up with a mechanism for it. Besides, it is found that GNPs have a strong inhabiting effect on phase transformation of 15-5PH/Gr composites during the rapid solidification of the SLM process, and the inhibition will be enhanced as the GNPs content increases. The mechanical properties of metal matrix composites (MMCs) are significantly influenced by the changes in phase and additive content. According to the mechanical property tests on the 15-5PH/Gr composites, it is found that their ductility and impact toughness have maximum values of 41.1 ± 1.3% and 194.9 ± 3.1 J/cm2 respectively, and the friction coefficient of the 15-5PH/Gr composites have a minimum value of 0.34. Compared with the SLM-ed 15-5PH, the improvement of mechanical properties of the 15-5PH/Gr composites on ductility, impact toughness and wear resistance are increased by 130.9%, 97.3% and 45.2% respectively.
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The Al2O3/P20 composites were successfully manufactured by selective laser melting (SLM) from powder mixture of Al2O3 and P20 mold steel with different Al2O3 contents (0.5 wt%, 1 wt%, 2 wt%) in this ...work. The surface morphology analysis indicated that Al2O3 particles were partially or fully melted for agglomerates during the SLM process with segregating between contiguous molten tracks. The model of Al2O3 diffusing during the SLM process was built for illustrating the generated mechanism of Al2O3 agglomerates. Moreover, the densification behavior of specimens became poor (the relative density declined from 99.3% to 94.2%) as the Al2O3 contents increased. The microstructure within samples had some obvious variations including coarsening martensite microstructure, tempering microstructure transformation, reverted austensite and precipitation of fine carbide particles. The result of Rockwll hardness test manifested that 1 wt% obtained the maximum value of 57.6 HRC, while 2 wt% had the minimum value of 45.9 HRC. Tribological test revealed that coefficient of friction (COF) and wear rate significantly declined when the Al2O3 contents were 1 wt% and 2 wt%, which emphasized the superior wear resistance property.
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