Research Investment casting with titanium oxide ceramic shell -shell technology for
Titanium Investment casting technology to meet the needs of the aerospace industry and developed , has been widely used in aerospace titanium titanium components and civilian structures manufacturing. Pedal Titanium Axle Practice has proved that the titanium alloy precision casting technology has become almost no margin of several forming processes to obtain maximum effectiveness of a process [ 1-3] . With the extensive application of titanium castings , its increasingly stringent quality and usability requirements , in addition, the level of costs also limit its application prospects is an important indicator . Graphite was low cost but poor quality of the casting surface , and is not suitable for the production of thin-walled complex parts [ 4 ] , but with the surface layer of refractory metal or yttrium oxide surface layer ceramic shell can be cast out of high quality titanium castings, but the cost is very high [ 5,6 ] . For this study the low -cost , suitable for the production of complex thin-walled shell technology for a new type of titanium oxide ceramic Investment casting is .
1 Materials and experimental methods
1.1 The main system shell material
Mold material : the material properties of the selected mode is shown in Table 1. The main use of imported wax wax casting specifically for titanium , gate part is low cost, performance can meet the requirements of homemade wax , titanium bar for sale respectively, after pressing, welding synthetic wax module.
Table 1 titanium casting mold materials with properties
Table1 Waxes used for making shells
Performance of imported wax homemade wax
Dropping Point / ℃ 69 ~ 71 70.5
Freezing point / ℃ 58 ~ 59 60 ~ 62
Linear shrinkage / % 1.3 0.4 0.6
Penetration / 1/10mm 12 ~ 14 11
Zhao hardness 21.1 19
Tensile strength / MPa 3.69 2.91
Adhesive wetting angle / ° 89 ~ 92 79 ~ 82
Density / g/mm3 0.91 ~ 0.94 0.95 ~ 1
Ash / % < 0.01 0.05
The surface layer and the adjacent surface layer of slurry : The main components of zirconia refractories , size of 30μm or less. Organic zirconium sol as a binder , the size of the refractory rubber match should be 10 ~ 15nm.
The surface layer and the adjacent surface layer of sanding : the main component of the surface layer is made of zirconium dioxide sand , the size of 150 ~ 300μm. To reduce costs, the use of adjacent surface layer is mainly composed of ZrO2.SiO2 zircon sand . The results show that the diffusion of Si is not seen in the casting , it is feasible the use of zircon sand .
Back layer system shell material: silica sol as a binder , using Morley powder and sand as refractories.
1.2 system shell method
The order of addition batter followed binders , refractory powder , trace additives. Minor amounts of additives to improve the stability of the coating role is to improve the wettability of the material and the mold , etc., and to prevent drying of the surface layer after the return water soluble additive is selected micro- organic material can be removed during the calcination . Stir batter than 2h at room temperature ( about 20 ℃) in the air , after the return of 1h to use .
O face the same batter pulp and raw materials , the production process is the same, only slightly smaller concentrations . Slurry feed back order binders , refractory powder. Also need to continuously stirred slurry back more than 2h , after the return of before use. Depending on the size of the backing layer casting , painted hanging normally takes 4-9 layers.
Drying : Drying time the surface layer and the adjacent surface layer are 24h, the temperature is 20 ℃, relative humidity of not less than 60% , the drying time of the backing layer is not less than 4h, the same as other conditions and the surface layer .
Dewaxing process : the use of microwave dewaxing , buy titanium bar the equipment used for the large cavity size , 1500W Adjustable microwave intensity.
Roasting process : After dewaxing gas shell fired at without protection roaster .
Optimization of two main parameters of the Shell Technology
In all system shell process parameters on shell quality and ultimately a greater impact on the quality of castings are: batter ingredients, namely powder -liquid ratio , dewaxing and roasting process parameters [ 7 ] , this research labs now there are devices on the above parameters in-depth study and optimize selection.
2.1 powder -liquid ratio across the layer of coating thickness and suspension rate impact
Table 2 powder-liquid ratio on the opposite layer coating thickness and coating suspension rates . The results show that the greater the surface pulp powder -liquid ratio , the greater the thickness of the surface coating , the coating suspension , the better. Generally believed that if the surface coating is too thin (<0.2mm), the shell will be affected by sanding the surface , the roughness increased. If the coating is too thick (> 0.6mm), the coating , poor mobility, easy accumulation , during the drying or firing cracks easily [ 7 ] , which in this study has also been confirmed in the present study therefore the powder in the liquid ratio should be controlled between 5.0:1 - 6.0:1 .
Table 2 in the powder -liquid ratio of coating performance
Table2 Effects of solid / liquid ratios on the properties of coating
4.0:1 5.0:1 5.5:1 powder -liquid ratio 6.0:1
Coating thickness / mm 0.11 0.25 0.36 0.65
Coating suspension rate / % 88.3 90.1 93.1 97.6
2.2 Effect of shell powder -liquid ratio and calcination temperature on the room temperature and high residual strength
Figure 1 shows the ratio of powder and liquid at room temperature and high temperature effect on residual strength . Strength of the sample is composed of five levels of plasma and surface sand composition , each drying 24h, continuous coating hanging , firing 4h. Flexural strength of the strength test .
Figure 1 powder-liquid ratio of the residual strength of the opposite layer
Fig.1 Effects of S / L ratios on the remain flexural of
surface coats baked at different temperatures
The results showed that the powder -liquid ratio improved shell specimen bending strength . Calcination temperature is increased , the flexural strength increases with . Since 1100 ℃ ZrO2 occurs near the phase transition , so that the strength in the vicinity of this temperature increase is not significant , but the phase change process , the lattice of unstable energy increased activity increased , so should be avoided in the actual production of the calcined temperature. After calcination at 1200 ℃ by the high ratio of powder and liquid coatings ( 6.5:1 ) intensity decreased slightly reason: Because the coating is too thick , a slight increase in the calcination process of internal defects , thereby affecting its strength. From the results in Table 2 and Figure 1 show that the optimum value of 5.5:1 powder-liquid ratio of the surface layer material . The firing temperature is preferably at about 1200 ℃, but for small pieces of calcination temperature can be set at around 1000 ~ 1050 ℃.
2.3 Optimization of process parameters dewaxing
For this study cast casting spade 150mm × 100mm , thickness of 2.5mm, the microwave dewaxing process, the time relationship between the intensity of the microwave shown in Figure 2 and dewaxing .
Figure 2 microwave intensity and dewaxing time
(a) the inner surface intact ; (b) inner surface microcracks
Fig.2 Effects of different microwave intensities on dewaxing time
(a) no crack; (b) have cracks
Figure in a district that is less than 70% when the microwave intensity , dewaxed shell inner surface smooth , with little residual wax, no visible cracks. When the microwave intensity is too large (b zone ) , dewaxed shell intact , although overall , but there are a few cracks in the surface layer , with the microwave intensity increased the number of cracks increases , the reasons for this phenomenon may be due to over- heating rate fast , since the internal stress of the inner and outer layers of different thermal expansion coefficients of the time to produce slack . Therefore microwave intensity used in this study was 50% , dewaxing time about 8min. Through other experiments show that this dewaxing process useful in the present study not only the spade member is equally applicable to other shapes and sizes of parts.
3 smelting , casting and casting surface quality analysis
3.1 Melting and casting
Because of the high melting point of titanium alloy , titanium alloy bar chemical activity , smelting and casting must be performed under vacuum or inert gas . The experiment used a water-cooled copper crucible vacuum induction melting and casting furnace , the main process parameters are shown in Table 3 . Using Ti-6Al-4V alloy , the cast as an aerospace components, size of 150mm × 100mm, thickness 2.5mm, shell preheat temperature 300 ℃, gravity casting , casting the shell intact, no cracks , clean up after the castings shown in Figure 3 .
Table 3 Parameters melting titanium major
Table3 Main melting parameters of Ti alloy
Before pouring the vacuum degree / mbar 1.6 × 10-2
Before pouring water temperature / ℃ 37
When pouring the current intensity / A 870
When pouring voltage / V 565
When pouring electric power / kW 496
Frequency / kHz 7.91
Figure 3 titanium castings
Fig 3. A casting of titanium alloy
3.2 casting surface quality analysis
Birds casting surface silvery white , casting a clear outline , no surface flow marks and cold shut . Surface roughness Ra6.3μm.
Casting the X ray diffraction analysis that the casting surface Ti, ZrO2 and Ti2O, causes Ti2O ZrO2 is at a high temperature Ti reaction occurs with the reaction equation is as follows :
ZrO2 + Ti = Zr + TiOx + Oy
Zr and O to the diffusion of titanium castings , constitute the reaction layer .
EPMA sectional view of the casting for the detection , measurement and distribution line section of the zirconium -plane distribution , the results shown in Figure 4 . The reaction of zirconium castings depth of 25 ~ 50μm.
Figure 4 Distribution of zirconium wire probe and the surface distribution of photographs (a) casting microstructure ; (b) line distribution ; (c) surface distribution
Fig.4 Distribution of Zr (a) Microstructure; (b) Linear; (c) Planar
4 Conclusion
( 1) The main component of sand and zirconium dioxide powder as refractory organic zirconium sol as a binder , a kind of cost-effective suitable for titanium casting Casting Shell Technology .
( 2 ) a systematic study of the effects of powder -liquid ratio , calcination temperature, time, etc. microwave dewaxing process parameters on the coating properties and performance of the shell to determine the optimum process parameters.
( 3 ) the use of water-cooled copper crucible vacuum induction furnace casting out a complete and clear Spade aerospace castings dimensions of 150mm × 100mm, thickness of 2.5mm. Casting surface roughness Ra = 6.3μm, the thickness of the reaction layer is 25 ~ 50μm.
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