| Selective laser sintering(SLS) technology is used to make sand mould and sand core for the advantages of quick response speed, short manufacture period, good flexibility and stability, integrated manufacture and so on. It has a significant promotion of rapid trial-manufacturing level of larger and complicated casting structure manufactured. SLS is present a large potential in solving key casting structure manufacturing used in aviation, aerospace and vehicle industries. Nowadays,SLS has been used to make coated sand core and mould to cast aluminum and steel.Molten titanium has very high chemical activity, hence, it is easy to reaction with common casting mould, which leads a series of defects on the casting surface, such as thick oxide layer, adhere burnt sand and porosity,which is limited coated sand mould and core used to casting titanium alloy. So it is very important to promotion SLS technology in casting titanium alloy by research and development novel new kind of coated sand mould materials, which is suitable for casting titanium alloy.In this paper, combination of the characteristics of SLS technology and titanium casting, zirconium sand with good high temperature stability is selected as the primary mould material, Y2O3 as the filling mould material, novolak phenolic resin and yttrium sol as the binders. Based on the hot and cold ways to lining-up coated zirconium sand and laser sintered, postcured, then infiltrated by yttrium sol, Y2O3 sprayed and roasted treatment. Influence and reaction mechanism of laser power single scanning area and content of Y2O3 on the SLS coated zirconium sand primary strength are investigated. Effect of roasting temperature on SLS zirconium sand mould tension strength and the gas evolution of the sand mould are further researched. SLS zirconium sand mould was used to cast pure titanium. Reaction mechanism of SLS zirconium sand mould with titanium was discussed.The result shows that, when keep the same content of phenolic resin, zirconium sand mould strength made by hot way is obviously higher than cold way. Furthermore the Y2O3 scattered in the coated zirconium sand mould made by hot way is morewell-distributed. SLS zircon sand processing experiment results show that single layer scanning area can significantly affect the mechanical properties of the sintered structure. As the increasing of single layer scanning area, the sintered structure mechanical properties are decreased sharply. The formation structure strength is significantly decreased.The processing parameters of zircon sand are selected as follows: scanning width is 0.15 mm, preheated temperature is 55℃,powder thick is 0.2 mm and the laser power is 35 W scanning speed is 2000mm/s,single layer scanning area is 57cm2. After the processing strength of the formation is 2.14 Mpa, and the good comprehensive properties of the formation structure is obtained.Adding the amount of Y2O3 less than or equal to 0.5wt% in zircon sand has less influence on the forming of zircon sand SLS. With the increasing amount of Y2O3 powder, the initial strength of SLS coated zirconium sand decreases rapidly. When the amount of Y2O3 is equal to 1.5wt%, the initial strength of SLS coated zirconium sand is 0.15 MPa, and after curing on the temperature of 170 ℃, the tensile strength is1.67 MPa, which are 55.5% and 78.2% of the strength of zirconium sand without Y2O3 coating, respectively.Smooth gel film forming on the surface of zirconium sand due to Zircon sand yttrium sol impregnating strengthens the mold strength and surface finish. Y2O3 coating on the surface of SLS zircon sand and sand core, when the powder-liquid ratio is 0.667, has smooth surface, no cracking, uniform coating which thickness of is about0.03 mm, and has a good combination of coating and sand.Calcination temperature has a significant influence on the increase of gas of coating of SLS zircon sand of Infiltration of yttrium sol. With the increase of calcination temperature, the tensile strength of SLS zircon sand decreases slowly, and the gas volume decreases quickly. The tensile strength of 2.01 MPa and gas of coating of 5.21ml/g are obtained by the calcination temperature 250, which is better comprehensive performance.Titanium castings using SLS zircon sand mold(core) casting has smooth surface,and no casting defects. Y2O3 in the sand effectively prevents the contact between molten Ti and zircon sand particles, avoids the diffusion migration of Si, Zr and O elements in Zircon sand, and reduces the interfacial reaction degree. SEM and EDSanalysis also shows that, interfacial reaction thickness between SLS zircon sand mold and titanium casting is only about 3μm. Oxidation products on casting surface are TinO2n-1 and TiO2. The thickness of O diffusion enrichment solution layer is about1μm, and no Y element is not found in diffusion enrichment layer. Compared with the Y2O3 as a refractory material, the thickness of the interfacial reaction layer in investment casting decreases significantly. |
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