The Ti-25Ta alloy was fabricated by LPBF using mechanically mixed powders. It was found that the relative density, phase composition, and surface roughness were quite different with different LPBF preparation parameters. The Ti-25Ta alloy samples have a high relative density (>99.9%) with better surface roughness at faster scan speed, and thereby lower laser energy density. However, the amount of unmelted Ta particles increased at a lower energy density. In addition, the energy input of the LPBF process had a great influence on the phase formation of Ti-25Ta due to element composition differences. The α' phase was mainly dominated in Ti-25Ta samples under lower laser energy density while changing to the α" phase gradually with the increase of laser energy density. The SEM image and XRD analysis were applied to show the characteristics. The micro-hardness of Ti-25Ta samples under various LPBF parameters was tested for proving the difference in the phase composition.
Laser welding was used to conduct autogenous welding of AlSi10Mg alloy fabricated by laser selective melting (SLM) technology and lap welding of the as-cast cover plate and SLM plate with similar composition. After welding, the weld was treated by three heat treatment processes (T5 single aging +T6 solution aging + annealing), and the microstructure as well as mechanical properties of the weld in each heat treatment state was analyzed. The results show that the microstructure of overlapped SLM welds is finer than that of autogenous welds, and the properties of overlapped SLM welds are better and more sensitive to heat treatment. After annealing and T5 single aging treatment, the dendritic structure with acicular eutectic silicon as network distribution at the edge of α-Al matrix of typical weld microstructure does not change, and the grain is coarsened to a certain extent. After solid solution treatment in T6 state, the Si network breaks, spheroidize and grows, and the eutectic Si distribution is more uniform, the number of pores in the weld increases and the volume of pores increases. After single aging treatment, the weld microhardness is increased, while annealing and solid solution treatment will reduce the weld microhardness.
Due to the formation of protective borosilicate scale during high-temperature oxidation, Mo-62Si-5B (at.%) alloy is deemed to be the promising candidate of high-temperature oxidation resistant coatings. Nevertheless, it faces the challenges on the application on surface engineering due to the difficulty of powder fabrication. In the present study, the pre-alloyed powder was obtained by mechanical crushing from Mo-62Si-5B bulk alloy fabricated by vacuum induction levitation melting. Subsequently, the original powder was further sieved by 60 mech sifter for the compatibility of laser cladding. The size distribution, morphology, oxygen content and phase composition of the powder were characterized. The results show that the D(50) of the powder is 130.55 μm and the average particle size is 124.65 μm. There are MoSi2 and MoB2 phases distributed in the powder with irregular morphologies, which is accord with the bulk Mo-62Si-5B alloy. The oxygen content of the powder is lower than 0.11%, meeting the requirements of the powder for laser cladding. A laser cladded layer was prepared on Nb-Si based alloy substrate by using the powders, which exhibits dense structure free of voids and cracks. The study proves the feasibility of pre-alloyed Mo-62Si-5B powder, which may give guidance for producing Mo-Si-B system oxidation-resistant coating by laser cladding or thermal spraying.
In order to further explore the application of W and W alloy fabricated by selective laser melting (SLM), W with different geometrical morphologies, support structure and second phase combination were prepared, and the corresponding microstructure characteristics were also investigated. The grain morphology and size distribution were significantly depend on the heat dissipation conditions caused by different geometrical morphologies, support structure and second phase combination. With the specimen size increases from 1D-2 to 3D, the average grain size increases, the percentage of large grains increases, and the dislocation density decreases. Because no remelting occurred in 2D specimen due to no overlap in the corresponding position, more prone to epitaxial growth and formed elongated cellular grains. Increase the height of support structure could decrease the cooling rate, especially the center area, which induced the grain size along with the reduction of cracks. The crack in pure W during SLM was related to the high thermal stress caused by high cooling rate as well as the recrystallization and epitaxial growth of W phase during SLM. Adding the second phase such as Cu or Cu10Sn could reduce the grain size of W phase remarkably, and crack was severely restrained in W phase simultaneously. This could be attributed to that grain refinement of W phase could decrease the DBTT and the second phase combination also breaks the epitaxial growth of W phase.
With the development of additive manufacturing technology, it provides an efficient method for preparing complex structured NiTi alloy specimens. Different additive manufacturing technologies have different requirements for powder particle size. In order to satisfy the requirements of additive manufacturing technology for powders. This study aimed to produce spherical NiTi powders suitable for additive manufacturing by electrode induction melting gas atomization (EIGA). Scanning electron microscopy, X-ray diffractometry and differential scanning calorimetry were used to investigate the surface and inner micro-morphology, phase constituent and martensitic transformation temperature of the surface and inner of the NiTi powders with different particle sizes. The results show that the powder mean particle size D50 was 75 μm, flowability was 19.3 s/50 g, apparent density was 3.40 g·cm–3, and the oxygen content of the powder only 0.005% higher than the raw materials. That the grain of powder becomes finer gradually with decreasing particle size. Ingot and all the powders exhibit a main B2 phase. Particles with different particle sizes have experienced different cooling rates during atomization. Various cooling rates cause different grain size inside the powder; in particular, the transformation temperature decreases with decreasing particle size. This study provides a basis for preparing high quality AM NiTi parts.
Selective laser melting (SLM) technology has received great attention in recent years for its application in the fabrication of Cu-Sn-based devices used in a wide range of industries, such as aerospace, ocean engineering, etc. However, the SLMed Cu-Sn alloys have different microstructure and properties, compared with the alloys made by traditional process, especially after heating treatments. In this paper, the effects of heat-treatment processing parameters on the microstructure and mechanical strength have been investigated for the SLMed Cu-10Sn alloy. A dense Cu-10Sn alloy bulk specimen was obtained by optimizing the SLM processing, and the relative density of the specimen reached above 99%. The grain morphology was mainly the columnar dendrite and inter-dendritic phases formed along the solidification direction. Tensile testing and detailed microstructural characterization were carried out on specimens in the as-SLMed and heat-treated conditions. The strength and plasticity of the SLMed specimen are much higher than that of the casted Cu-l0Sn alloy, mainly because of the grain refinement in the grain of the SLMed specimen. After the 800°C solution treatment, and the 400°C aging treatment, the microstructure of the specimen transformed from the columnar grain to equiaxed grain, the dislocations reduced, and a lot of twins generated obviously. Therefore, the yield strength (σ0.2) of the heat-treated specimen was decreased compared to the as-SLMed specimen. However, the UTS and the elongation were increased, due to the interaction between twins and equiaxed grain.
Selective laser melting (SLM) is one of the important additive manufacturing technologies which can produce high quality parts with complex geometry. The main objective of current study is to investigate laser welding of 2 mm thick AlSi10Mg parts fabricated by SLM, and to emphasis on the weld shape, microstructure, pore distribution and hardness of the welded joints. The results show that the cross section of the welds is multi-pass welding appearance, and the cross-sectional area of the welded joints increases from 1.58 mm2 to 3.66 mm2 with increasing heat input from 51 J/mm to 135 J/mm. The increased heat input causes the grain size of the welds to increase from 1.1 μm to 2.5 μm. Circular pores are observed under different heat inputs, and the maximum pore diameter increases from 92.0 μm to 155.4 μm with the increase of heat input. The hardness of the weld zone is lower than that of the base metal, whereas the increase in heat input causes the average hardness to decrease from 86.7 HV to 77.8 HV. These preliminary results demonstrate that it is the feasible to joining SLM AlSi10Mg parts by using laser welding process.
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