A composite coating for titanium alloy cast graphite mold and a method for preparing the same

Abstract

The application provides a composite coating for titanium alloy casting graphite mold and a preparation method thereof, so as to improve the surface quality of titanium alloy castings. The composite coating is composed of a metal bonding layer and three ceramic layers. The first ceramic layer is Al2O3, the second ceramic layer is composed of YSZ, and the third ceramic layer is composed of Sc2O3, La2O3, CeO2, Zr2O3, TiO2 and B powder. The spraying experiment is carried out on the graphite mold by using the plasma spraying technology, and the four coatings have their own spraying parameters. The composite coatings obtained by the plasma spraying process have good bonding strength between the coatings and between the coatings and the graphite mold substrate, the porosity of the coatings is low, there is no interlayer crack, and the thermal insulation effect is excellent. The graphite mold with the sprayed coating is subjected to pouring test, and the titanium alloy castings obtained have excellent surface quality, the surface roughness Ra of the titanium alloy castings is less than or equal to 3.2 microns, the thickness of the pollution layer is less than 10 microns, and the surface is free of cracks.

Description

Technical Field

[0001] This invention relates to coating materials and plasma spraying processes, specifically providing a composite coating for casting graphite molds of titanium alloy castings with complex variable curved surface flow channels and its preparation method. Background Technology

[0002] The cavities and flow channels of complex variable curved surface titanium alloy castings are extremely complex, and the casting process technology requirements for such castings are high. The surface and internal quality of key parts of the castings must meet the Class I B requirements specified in GJB2896A-2007, and the dimensional accuracy must reach the CT6 level specified in GB6414-1999.

[0003] Graphite mold casting is a common casting process for large-sized titanium alloy castings. However, due to the strong chilling effect of graphite molds, the alloy solution on the surface of the casting may experience solidification resistance and incomplete fusion, resulting in numerous defects such as cold shuts and flow marks on the casting surface. Molten titanium can react with the graphite mold material to form a brittle contamination layer. Because graphite molds have poor shrinkage properties, the shrinkage of the casting is hindered, causing the contamination layer to crack, ultimately resulting in surface cracks. Furthermore, defects such as cold shuts and flow marks within the mold cavity and runners cannot be removed by grinding, and internal cracks cannot be repaired by welding. Therefore, the surface quality of the casting can only be ensured by adopting appropriate casting process measures. Thus, researching how to improve the surface quality of titanium alloy castings is of great significance. Summary of the Invention

[0004] To address the shortcomings of traditional graphite mold casting processes, this invention provides a composite coating for graphite molds used in titanium alloy casting and its preparation method. The composite coating has good bonding with the graphite mold substrate and excellent heat insulation properties. Using a graphite mold coated with the composite coating can yield titanium alloy castings with higher surface quality.

[0005] The technical solution of this invention is as follows:

[0006] A composite coating for graphite molds used in titanium alloy casting is characterized in that: the composite coating consists of a metal bonding layer and a ceramic layer, with a graphite mold and a first ceramic layer on each side of the metal bonding layer; the ceramic layer has a three-layer structure, wherein the first ceramic layer is Al2O3, the second ceramic layer is a YSZ coating, and the third ceramic layer is composed of Sc2O3, La2O3, CeO2, Zr2O3, TiO2, and B powder.

[0007] As a preferred technical solution:

[0008] The metal bonding layer is made of NiCrAlY alloy powder with a particle size of 15–45 μm; the first ceramic layer is made of Al2O3 with a particle size of 20–45 μm; the second ceramic layer is composed of a conventional thermal barrier ceramic coating, which is composed of 8% yttrium-stabilized zirconium oxide (YSZ) with a particle size of 15–75 μm.

[0009] The third ceramic layer is composed of Sc, La, Ce doped Zr2O3 with the addition of TiO2 and a small amount of B powder. By mass percentage, its composition is as follows: Sc2O3: 5-10%, La2O3: 10-20%, CeO2: 10-15%, TiO2: 5%, B powder: 0.05%, balance: Zr2O3; wherein the particle size of B powder is 1-15 nm, and the particle size of other raw materials is 15-75 μm.

[0010] The optimal composition of the third ceramic layer is Sc2O3:7%, La2O3:15%, CeO2:13%, TiO2:5%, B powder:0.05%, and balance: Zr2O3. Titanium alloy castings prepared with this coating have the best surface quality.

[0011] The metal bonding layer is flanked by a graphite mold and a first ceramic layer, respectively.

[0012] The thickness of the metal bonding layer is 20-40 μm, the thickness of the first ceramic layer is 30-40 μm, the thickness of the second ceramic layer is 40-60 μm, and the thickness of the third ceramic layer is 50-60 μm.

[0013] The present invention also provides a method for preparing the composite coating, characterized in that: each coating is sequentially sprayed onto the inner surface of the graphite mold using a plasma spraying process, and then the graphite mold coated with the composite coating is dried and placed in a vacuum environment for step-by-step calcination.

[0014] Specifically: Before spraying the third ceramic layer, B powder (particle size 5-15nm), TiO2, Sc2O3, La2O3, CeO2, and ZrO2 powders are mixed in a mass ratio, and deionized water is added. The mixture is then ball-milled in a planetary ball mill for 3-4 hours. Polyvinyl alcohol is then added to the slurry and ball-milled for another 3-5 hours at a speed of 250-350 r / min to prepare a composite powder slurry for spray granulation.

[0015] Powder was prepared by spray granulation using a spray dryer. The process parameters were: (a) inlet temperature 250–280℃, outlet temperature 100–120℃; (b) atomizing wheel air supply pressure 1.4 × 10⁻⁶. 5 ~1.8×10 5Pa; (c) Feeding speed 40-60 r / min; screening of powder particles with a particle size range of 15-75 μm.

[0016] The plasma spraying process parameters are as follows:

[0017] The current is as follows: metal bonding layer: 400-450A; first ceramic layer: 350-400A; second ceramic layer: 450-500A; third ceramic layer: 500-550A;

[0018] The voltages are as follows: metal bonding layer: 55V; first ceramic layer: 40V; second ceramic layer: 60V; third ceramic layer: 60V;

[0019] The flow rates for H2 generation from plasma are as follows: metal bonding layer: 0.6–0.9 L / min; first ceramic layer: 0.5–0.8 L / min; second ceramic layer: 0.8–1.2 L / min; third ceramic layer: 0.8–1.2 L / min.

[0020] Spraying distance: Metal bonding layer: 100-150mm; First ceramic layer: 110-160mm; Second ceramic layer: 70-120mm; Third ceramic layer: 100-150mm.

[0021] The graphite mold coated with the composite coating was dried in an environment with a temperature of 18℃ and a relative humidity of 50%. After drying, it was placed in an oven for further drying and held at 110-120℃ for 2-4 hours. Then, the graphite mold coated with the composite coating was placed in a high-temperature vacuum heat treatment furnace for stepped calcination. First, it was held at 380℃ for 2 hours, then at 600℃ for 2 hours, and then the temperature was raised to 1030℃ and held for 2 hours. After furnace cooling to 300℃, the mechanical Roots pump and vacuum DC valve were turned off, the mechanical pump was turned off and air was introduced into the mechanical pump, the cooling water was stopped, and the mold was air-cooled to room temperature.

[0022] Based on the above composite coating, the present invention also provides a method for precision casting of titanium alloys, characterized by the following specific process:

[0023] (1) Preparation of graphite mold: According to the casting process drawings, the graphite electrode block is prepared into a graphite mold using moldless CNC machining technology, and the raw material is high-purity graphite;

[0024] (2) Coating with composite coating: The composite coating described in this invention is sprayed onto the inner surface of the prepared graphite casting mold using a plasma spraying process;

[0025] (3) Preheating and calcining graphite mold: The graphite mold coated with composite coating is placed in an environment with a temperature of 18℃ and a relative humidity of 50% for drying. After drying, it is placed in an oven for drying and kept at 110~120℃ for 2~4h. Then the graphite mold is placed in a high temperature vacuum heat treatment furnace for step calcination.

[0026] (4) Alloy melting: Install the titanium alloy electrode onto the electrode rod of the vacuum consumable electrode solidification furnace, clamp the mold, close the furnace door, and evacuate the furnace body. When the vacuum degree is ≤3×10 -1 When Pa, the electrode begins to melt, with the melting current controlled between 8000A and 18000A and the voltage controlled between 35 and 40V;

[0027] (5) Casting alloy: After the alloy raw material is melted to the required weight, the centrifugal disc of the casting mold is rotated, and the speed of the centrifugal disc is controlled at 100-250 r / min. The crucible is then turned over to pour the alloy liquid into the casting mold to obtain a titanium alloy casting.

[0028] The present invention has the following advantages:

[0029] (1) Compared with conventional single coating materials, the composite coating material of the present invention has a denser structure, good compatibility between coatings, low porosity, no interlayer cracks, and better thermal insulation effect.

[0030] (2) The multi-layer ceramic structure can effectively alleviate coating damage. The outermost ceramic coating has a low thermal conductivity and high phase stability, which can play a role in heat preservation and protection of the inner layer, reducing defects such as flow marks and contamination layers on the casting surface. The solution described in this invention can significantly improve the surface quality of titanium alloy castings. The surface roughness Ra of the cast titanium alloy castings is ≤3.2μm, the contamination layer thickness is ≤8μm, and there are no surface cracks.

[0031] (3) The main feature of this invention is that it adopts a method of using a multi-layer ceramic coating plus a metal bonding layer. The multi-layer ceramic layer improves the bonding strength of the coating and avoids the bonding layer from falling off due to mismatch of thermal expansion coefficients and oxidative expansion.

[0032] (4) The present invention uses plasma spraying process to prepare the coating, which has the characteristics of controllable coating thickness and high spraying efficiency, and the bonding strength between the coating and the graphite substrate is ≥55MPa.

[0033] (5) Using a graphite mold coated with the composite coating described in this invention can improve the surface quality of castings, and is particularly suitable for preparing titanium alloy castings with complex variable curved surface flow channels. Attached Figure Description

[0034] Figure 1 Schematic diagram of the composite coating structure for graphite casting molds used in titanium alloy casting.

[0035] Figure 2 Vacuum calcination curve of composite coated graphite casting mold.

[0036] Figure 3 The composite coating prepared in Example 2.

[0037] Figure 4 The casting samples prepared in Example 3 are: (a) uncoated; (b) without the outermost third ceramic layer; (c) composite coating of Example 1; and (d) composite coating of Example 2. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments. However, the scope of protection of the present invention is not limited to the embodiments. If those skilled in the art make some non-essential improvements and adjustments to the present invention based on the above-described invention, they shall still fall within the scope of protection of the present invention.

[0039] Example 1

[0040] Preparation of composite coatings:

[0041] (1) The powder used for the first ceramic layer: Al2O3 powder with a particle size range of 20-45 μm; the powder used for the second ceramic layer: YSZ powder with a particle size range of 15-75 μm; and the composition of the third ceramic layer consists of the following four groups:

[0042] ①Sc2O3:5%,La2O3:10%,CeO2:10%,TiO2:5%,B powder:0.05%, balance:Zr2O3;

[0043] ②Sc2O3:7%, La2O3:15%, CeO2:13%, TiO2:5%, B powder:0.05%, balance: Zr2O3;

[0044] ③Sc2O3:10%, La2O3:20%, CeO2:15%, TiO2:5%, B powder:0.05%, balance: Zr2O3;

[0045] ④Sc2O3:7%,La2O3:15%,CeO2:13%,TiO2:5%, balance:Zr2O3.

[0046] The powder particle size is between 55 and 75 μm, while powder B is nano-sized with a particle size between 1 and 15 nm. The powder is prepared by spray granulation using a spray dryer. The main process parameters are: (a) inlet temperature 250℃, outlet temperature 100℃; (b) atomizing wheel air supply pressure 1.4 × 10⁻⁶. 5 Pa; (c) Feed rate 40 r / min. Powder particles of each coating particle size range were screened out respectively.

[0047] (2) A composite coating was prepared on the surface of a high-purity graphite mold for titanium alloy casting using atmospheric plasma spraying technology:

[0048] The graphite substrate surface is first pretreated by sandblasting.

[0049] The graphite mold to be sprayed is preheated at a temperature of 200℃.

[0050] The plasma spraying process parameters are as follows:

[0051] The current is as follows: metal bonding layer 450A, first ceramic layer 400A, second ceramic layer 500A, third ceramic layer 500A. The voltage is as follows: metal bonding layer 55V, first ceramic layer 40V, second ceramic layer 60V, third ceramic layer 60V. The H2 flow rate for plasma generation is as follows: metal bonding layer 0.9L / min, first ceramic layer 0.8L / min, second ceramic layer 0.9L / min, third ceramic layer 0.9L / min. The spray distance is as follows: metal bonding layer 112mm, first ceramic layer 130mm, second ceramic layer 86mm, third ceramic layer 110mm.

[0052] The coating thicknesses are as follows: the metal bonding layer is 40 μm thick, the Al2O3 ceramic layer is approximately 40 μm thick, the YSZ ceramic layer is approximately 60 μm thick, and the third ceramic layer is approximately 60 μm thick.

[0053] (3) After preparing the coating, the graphite mold is placed in a special environment (temperature 18℃, relative humidity 50%) for drying. After drying, it is placed in an oven for further drying and kept at 110℃ for 2 hours. Then, it is processed according to... Figure 2 The calcination curve shown was subjected to high-temperature calcination using a high-temperature vacuum heat treatment furnace. The resulting composite coatings showed good compatibility and no obvious cracks.

[0054] Testing revealed that the outermost ceramic coating in the composite coating prepared in this embodiment had porosities of 2.8%, 2.9%, and 3.5%, respectively, and the bonding strength between the 3.9% coating and the substrate was 57 MPa, 59 MPa, 55 MPa, and 52 MPa, respectively. The optimal composition of the third ceramic layer was Sc2O3:7%, La2O3:15%, CeO2:13%, TiO2:5%, B powder:0.05%, and the balance being Zr2O3. The addition of B powder facilitates full melting of the coating under a certain spraying power, improving the coating bonding strength and reducing porosity.

[0055] Example 2

[0056] (1) The first ceramic layer uses Al2O3 powder with a particle size range of 15–45 μm; the second ceramic layer uses YSZ powder with a particle size range of 15–75 μm; the third ceramic layer is divided into two groups:

[0057] ①Sc2O3:7%,La2O3:15%,CeO2:13%,TiO2:5%,B powder:0.05%, balance:Zr2O3;

[0058] ②Sc2O3:7%,La2O3:15%,CeO2:13%,TiO2:5%,B powder:0.05%, balance:Zr2O3.

[0059] The oxide powder particles range from 30 to 55 μm. In group ①, powder B is nano-sized with a particle size between 1 and 15 nm. In group ②, powder B is micron-sized with a particle size between 10 and 20 μm. The powders were prepared by spray granulation using a spray dryer, with the main process parameters the same as in Example 1.

[0060] (2) A composite coating was prepared on the surface of a high-purity graphite mold for titanium alloy casting using atmospheric plasma spraying technology:

[0061] The graphite substrate surface is first pretreated by sandblasting.

[0062] The graphite mold to be sprayed is preheated at a temperature of 200℃.

[0063] The plasma spraying process parameters are the same as in Example 1, except that the current of the third ceramic layer is 550A and the voltage is 60V, while other parameters remain unchanged.

[0064] (3) After preparing the coating, the graphite mold is placed in a special environment (temperature 18℃, relative humidity 50%) for drying. After drying, it is placed in an oven for further drying and kept at 110℃ for 2 hours. Then, it is processed according to... Figure 2 The roasting curve shown is subjected to high-temperature roasting, and the roasting equipment used is a high-temperature vacuum heat treatment furnace.

[0065] The tests showed that the porosity of the composite coating prepared in group ① of this embodiment was 1.5%, and the porosity of the composite coating prepared in group ② was 1.8%. The bonding strength between the coating and the substrate was 65 MPa and 62 MPa, respectively.

[0066] Compared to Example 1, the oxide powder particle size in this example was reduced from 55-75 μm in Example 1 to 30-55 μm in Example 2. In the plasma spraying parameters, the current was 500 A in Example 1 and 550 A in Example 2, thus increasing the spraying power. Due to the increased spraying power and reduced powder particle size, the porosity and bonding strength of the coating in Example 2 were better than those in Example 1. Figure 3As shown, the composite coatings prepared in group ① of this embodiment exhibit good compatibility and no obvious cracks. The effect of B powder particle size on coating performance was also compared; increasing the B powder particle size increased the porosity and slightly reduced the coating bonding strength.

[0067] Example 3

[0068] Using the optimal group from Example 1 (the composition of the third ceramic layer is group ②), the optimal group from Example 2 (the composition of the third ceramic layer is group ①), the graphite castings prepared without a third ceramic coating, and the castings prepared without any coating:

[0069] (1) Alloy smelting: Install the titanium alloy electrode onto the electrode rod of the vacuum consumable electrode solidification furnace, clamp the mold, close the furnace door, and evacuate the furnace body. When the vacuum degree is ≤3×10 -1 When Pa, the electrode begins to melt, with the melting current controlled at 10000A and the voltage controlled at 38V;

[0070] (2) Alloy casting: After the alloy raw material is melted to the required weight, the centrifugal disc of the casting mold is rotated, and the speed of the centrifugal disc is controlled at 100 r / min. The crucible is then flipped to pour the alloy liquid into the graphite casting mold to obtain the titanium alloy casting.

[0071] (3) The alloy casting obtained in Example 1 had a contamination layer thickness ≤ 8 μm and a surface roughness Ra ≤ 3.2 μm. The alloy casting obtained in Example 2 had a contamination layer thickness ≤ 6 μm and a surface roughness Ra ≤ 3.2 μm. This indicates that reducing the powder particle size and increasing the spraying power is beneficial to improving the surface quality of the casting. The alloy casting without a third ceramic coating had a contamination layer thickness ≤ 10 μm and a surface roughness Ra ≤ 3.2 μm, resulting in relatively poor surface quality. Finally, the casting without any spray coating showed significantly poor surface quality with numerous flow marks, as detailed in the following figures. Figure 4 .

[0072] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

[0073] Furthermore, descriptions of well-known structures and techniques are omitted herein to avoid unnecessarily obscuring the concepts of the present invention.

Claims

1. A composite coating for graphite molds used in titanium alloy casting, characterized in that: The composite coating consists of a metal bonding layer and a ceramic layer. The ceramic layer has a three-layer structure, wherein the first ceramic layer is Al2O3, the second ceramic layer is a YSZ coating, and the third ceramic layer is composed of Sc2O3, La2O3, CeO2, Zr2O3, TiO2, and B powder. The metal bonding layer is made of NiCrAlY alloy powder; the mass percentage of each component material in the third ceramic layer is: Sc2O3: 5%~10%, La2O3: 10~20%, CeO2: 10~15%, TiO2: 5%, B powder: 0.05%, balance: Zr2O3; wherein the particle size of B powder is 1~15nm.

2. The composite coating for titanium alloy casting graphite molds according to claim 1, characterized in that: The third coating layer has the following composition: Sc2O3: 7%, La2O3: 15%, CeO2: 13%, TiO2: 5%, B powder: 0.05%, and the balance: Zr2O3.

3. The composite coating for titanium alloy casting graphite molds according to claim 1, characterized in that: The thickness of the metal bonding layer is 20~40μm, the thickness of the first ceramic layer is 30~40μm, the thickness of the second ceramic layer is 40~60μm, and the thickness of the third ceramic layer is 50~60μm.

4. A method for preparing a composite coating for a graphite casting mold of titanium alloy as described in any one of claims 1 to 3, characterized in that: The graphite casting mold is coated with various layers sequentially using a plasma spraying process. After drying, the graphite casting mold coated with the composite coating is placed in a vacuum environment for step-by-step calcination.

5. The method for preparing the composite coating for titanium alloy casting graphite molds according to claim 4, characterized in that: Before spraying the third ceramic layer, B powder, TiO2, Sc2O3, La2O3, CeO2, and ZrO2 powders are mixed in a mass ratio, and deionized water is added. The mixture is then ball-milled in a planetary ball mill for 3-4 hours. Polyvinyl alcohol is then added to the slurry and ball-milled for another 3-5 hours at a speed of 250-350 r / min to prepare a composite powder slurry for spray granulation. Powder was prepared by spray granulation using a spray dryer. The process parameters were: (a) inlet temperature 250~280℃, outlet temperature 100~120℃; (b) atomizing wheel air supply pressure 1.4×10⁻⁶. 5 ~1.8×10 5 Pa; (c) Feeding speed 40~60r / min; screening of powder particles with a particle size range of 15~75μm.

6. The method for preparing the composite coating for titanium alloy casting graphite molds according to claim 4, characterized in that: The plasma spraying process parameters are as follows: The current is as follows: metal bonding layer: 400~450 A; first ceramic layer: 350~400 A; second ceramic layer: 450~500 A; third ceramic layer: 500~550 A; The voltages are as follows: metal bonding layer: 55 V; first ceramic layer: 40 V; second ceramic layer: 60 V; third ceramic layer: 60 V; The plasma-generated H2 flow rates are as follows: metal bonding layer: 0.6~0.9 L / min; first ceramic layer: 0.5~0.8 L / min; second ceramic layer: 0.8~1.2 L / min; third ceramic layer: 0.8~1.2 L / min; Spraying distance: Metal bonding layer: 100~150 mm; First ceramic layer: 110~160 mm; Second ceramic layer: 70~120 mm; Third ceramic layer: 100~150 mm.

7. The method for preparing the composite coating for titanium alloy casting graphite molds according to claim 4, characterized in that: The graphite mold coated with the composite coating was placed in an environment with a temperature of 18 ℃ and a relative humidity of 50% for drying. After drying, it was placed in an oven for drying and kept at 110~120 ℃ for 2~4 h.

8. The method for preparing the composite coating for titanium alloy casting graphite molds according to claim 4, characterized in that: The graphite mold coated with the composite coating was placed in a high-temperature vacuum heat treatment furnace for step-by-step calcination. First, it was held at 380℃ for 2 hours, then at 600℃ for 2 hours, and then the temperature was raised to 1030℃ and held for 2 hours before being cooled in the furnace to 300℃.

9. A method for precision casting of titanium alloys with complex variable-curvature flow channels based on the composite coating described in any one of claims 1-3, characterized in that, The specific process is as follows: (1) Preparation of graphite mold: According to the casting process drawings, the graphite electrode block is prepared into a graphite mold using moldless CNC machining technology, and high-purity graphite is used as the raw material; (2) Coating with composite coating: A composite coating is sprayed onto the inner surface of the prepared graphite casting mold using a plasma spraying process; (3) Preheating and calcining graphite mold: The graphite mold coated with composite coating is placed in an environment with a temperature of 18 ℃ and a relative humidity of 50% for drying. After drying, it is placed in an oven for drying and kept at 110~120 ℃ for 2~4 h. Then the graphite mold is placed in a high temperature vacuum heat treatment furnace for step calcination. (4) Alloy melting: Install the titanium alloy electrode onto the electrode rod of the vacuum consumable electrode solidification furnace, clamp the mold, close the furnace door, and evacuate the furnace body. When the vacuum degree is ≤3×10 -1 When Pa, the electrode begins to melt, with the melting current controlled at 8000A~18000A and the voltage controlled at 35~40V; (5) Casting alloy: After the alloy raw material is melted to the required weight, rotate the centrifugal disc of the mold. The speed of the centrifugal disc is controlled at 100~250r / min. Turn the crucible over and pour the alloy liquid into the mold to obtain the titanium alloy casting.

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