Preparation Of Porous Ni-based Alloy By Laser Sintering And Analysis Of Sintering Properties

The in-situ reforeced porous nickel-based alloy was prepared by direct laser sintering of a mixed powder system consisting of a self-fluxing nickel-based alloy powder and certain content pore-forming agent. Several powder systems with different components were prepared by using a high energy ball milling equipment, and the sintering of these mixed powders was performed on laser processing equipments. Sintered samples were characterized by XRD, SEM, EDX and microhardness testing. Two sintering methods were used and the influences of the type and adding content of pore-forming agents on pore morphology, reinforcing phase, vickers hardness as well as sintering properties were studied. Formation mechanisms were also elucidated.It revealed that porous structures were formed after the squash-sintering of nickel-based alloy powder mixed with TiH₂. Some in-situ strengthening phases such as CrB and M23C6 were synthesized after sintering, and CrB presented needles-like shape. Hydrogen atoms (H) originated from TiH₂ under effect the laser beam produced gas pores in this sintering process.When TiH₂ and graphite (C) were added simultaneously, a new in-situ strengthening phase TiC was synthesized after squash-sintering. Furthermore, porosity and hardness of samples were improved with the increase of the adding content of pore-forming agents. Large numbers of gas pores showed homogeneous size and spatial distribution when the weight percentage of pore-forming agent (TiH₂) was 8%, and a good porous structure had formed. Morphological evolution of pores was caused by a combined effect of the adding content of pore-forming agents, the Marangoni Effect, the interaction between melt and bubbles and the merging of bubbles during growing up. Open and closed pores were both formed by a powder spreading-sintering; porosity and number of closed pores were increased with the rise of the adding content of pore-forming agents, and the porous structures were essentially affected by liquid wetting ability.Inorganic salts acting as pore-forming agents decomposed completely and molten slag layer formed during sintering could protect the molten pool from oxidation so as to improve the spreading ability of melt. The number of pores on the surface increased with the rise of the addition amount of NaHCO3. Several irregular large-sized pores appeared on the surface of samples with an addition of 5 wt.% alkaline mode MgCO3 after sintering. Inorganic salts could hardly form pores in the matrix but on the surface under coequal conditions. The stability of molten pool and addition content of salts had significantly affected morphologies of pores. Connected sintered gaps leaded to a relative high porosity after powder spreading-sintering.