| Selective laser melting(SLM) is an important branch of the rapid prototyping manufacturing technology which can deposit near net shape parts with fine microstructure, excellent performance and uniform components by melting metallic powder with high energy laser beam, especially for complex inner structure parts which formed from titanium alloy, high temperature alloys in the aerospace field. Due to their excellent characteristics including high speci?c strength, good fracture toughness, light weight as well as strong corrosion resistance, titanium(Ti) alloys have been particularly concerned and widely applied in aeronautical. With the development of aerospace technology in our country, higher temperature materials will be put into use andthe requirements on the high temperature material properties will be stricter, mainly reflected in further heat resistant temperature.Near alpha titanium alloy powder was investigated in this paper to find out the influence of different process parameters on forming quality, density and so on and got optimized process parameters through experiments. The mechanism of crack and suppression method have been studied and verified by experimentsbecause it is easy to crack when forming titanium alloy in SLM. The main works are as follows:(1) Explores of different process parameters forming titanium alloy by SLM have been studied and finally found that process parameters was divided into three zones when the line energy density range from 0.28J/mm to 0.9J/mm. Single molten pool with good quality and better appearance can be formed when the line energy density range from 0.36J/mm to 0.7J/mm and monolayer morphology with better appearance can be formed when thescanning interval was 70?m. The relative density increased with the increasing of line energy density when it was lower than 0.36J/mm and the biggest relative density was 99.34%. When the line energy range from 0.36J/mm to 0.55J/mm the relative density stabled at 98.8% since a number of several microns' pores appeared. When the line energy density exceeded 0.55J/mm coarse pores which were about 50?m in width appeared together with micron pores and cracks. As a result, the relative density decreased sharply and even reached to 95.44%.(2) The mechanism of crack and suppression method had been studied and found that uneven notches arose from the sidewall of the specimen during fabrication. At the same time, small metal balls formed by splash as well as Ti3 O,TiO,TiC which were hard and brittle embedded in the matrix of the deposition. Under the function of residual stress, the cracks originated in the sidewall notches and small metal balls formed by splash during the manufacturing process. And they expanded in the deposition along these hard and brittle compounds. Analysis shows that process optimization and preheating together with slow cooling can reduce the residual stress effectively, thereby inhibiting or even eliminate cracking. Through the experiments, when the laser power is 140 W, scanning speed is 450mm/s, layer thickness is 0.03 mm, scanning space is 0.07 mm, molding temperature is 350 ? during SLM and incubated 1h, slow cooling to room temperature, the number of cracks will reduce significantly.In conclusion, the optimum parameters of SLM process of near alpha titanium alloy powders were obtained. At same time the mechanism of crack and suppression method have been studied and verified by experiments. A certain foundation was built for the fabrication of crack free parts by SLM. |
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