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.
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.
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.
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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