A composite coating for titanium alloy cast graphite mold and a preparation method and application thereof

Abstract

The application provides a composite coating for titanium alloy casting graphite mold and a preparation method and application thereof, the composite coating is composed of a metal bonding layer and a ceramic layer, the ceramic layer is divided into an upper ceramic layer and a lower ceramic layer, the lower ceramic layer is made of Al2O3, and the upper ceramic layer is made of ZrO2, Y2O3, CaO, MgO and CeO2, and the composition of the upper ceramic layer is: Y2O3: 28-32%, CaO: 4-8%, MgO: 2-4%, CeO2: 1-2%, and ZrO2: the balance; and the metal bonding layer is respectively connected with the graphite mold and the lower ceramic layer on both sides. The composite coating has better combination with the graphite mold substrate, good compatibility between the coating layers, low porosity, no interlayer cracks, and more excellent heat insulation effect, and the titanium alloy casting with more excellent surface quality can be obtained by using the graphite mold coated with the composite coating, the titanium alloy casting has a surface roughness Ra of less than or equal to 3.2 microns, a pollution layer thickness of less than or equal to 10 microns, and no surface cracks.

Description

TECHNICAL FIELD

[0001] The present application relates to coating materials and plasma spraying technology, in particular to a composite coating for graphite casting mold of titanium alloy and a preparation method and application thereof. BACKGROUND

[0002] Graphite is considered as one of the most suitable casting mold materials for titanium alloy casting due to its good thermal stability, weak reaction with titanium alloy, low thermal expansion coefficient and high melting temperature. However, due to the chilling effect, the surface alloy solution of the casting is discontinuous and cannot completely converge and fuse, resulting in cold shut and flow marks on the surface of the casting. At the same time, the active titanium liquid reacts with the graphite mold material to form a brittle contamination layer, and the graphite mold has poor yielding property, which hinders the shrinkage of the casting, causes the brittle contamination layer to crack, and finally leads to cracks on the surface of the casting, especially for thin-walled structure castings with complex shape. Therefore, it is of great significance to study how to improve the surface quality of titanium alloy castings. SUMMARY

[0003] In view of the above deficiencies of the prior art, the present application provides a composite coating for graphite casting mold of titanium alloy and a preparation method and application thereof. The composite coating has good bonding with the graphite mold substrate and excellent thermal insulation effect, and the graphite mold coated with the composite coating can obtain titanium alloy castings with higher surface quality.

[0004] The technical scheme of the present application is as follows:

[0005] A composite coating for graphite casting mold of titanium alloy, characterized in that the composite coating is composed of a metal bonding layer and a ceramic layer, the ceramic layer is divided into an upper ceramic layer and a lower ceramic layer, the lower ceramic layer is made of Al2O3, and the upper ceramic layer is made of ZrO2, Y2O3, CaO, MgO and CeO2, and the composition of the upper ceramic layer is: Y2O3: 28-32%, CaO: 4-8%, MgO: 2-4%, CeO2: 1-2%, and ZrO2: the balance.

[0006] The metal bonding layer is respectively connected to the graphite mold and the lower ceramic layer on both sides.

[0007] As a preferred technical scheme:

[0008] The metal bonding layer is made of Ni / Al alloy powder, and the mass percentage of each component material of the upper ceramic layer is: 60% ZrO2-30% Y2O3-6% CaO-3% MgO-1% CeO2.

[0009] The thickness of the metal bonding layer is 40-60 μm, the thickness of the lower ceramic layer is 20-40 μm, and the thickness of the upper ceramic layer is 60-80 μm.

[0010] The application also provides a preparation method of the composite coating, characterized by: spraying each coating on the surface of the inner cavity of the graphite mold in sequence by using a plasma spraying process, and then drying the graphite mold coated with the composite coating and placing it in a vacuum environment for stepwise baking.

[0011] Before spraying the upper ceramic layer, ZrO2, Y2O3, CaO, MgO and CeO2 powders are mixed according to a mass ratio, and deionized water is added at the same time, and then the mixture is ball milled in a planetary ball mill for 3-4 hours, and then polyvinyl alcohol is added to the slurry and the ball milling is continued for 3-5 hours, and the rotation speed of the ball mill is 250-350 r / min, to prepare a composite powder slurry for spray granulation. The spray granulation is performed by using a spray dryer, and the main process parameters are: (a) the inlet temperature is 250-280℃, and the outlet temperature is 100-120℃; (b) the air supply pressure of the atomizing wheel is 1.4×10 5 -1.8×10 5 Pa; (c) the feeding speed is 40-60 r / min; and the powder particles with a particle size range of 20-100 μm are screened.

[0012] The plasma spraying process parameters are as follows:

[0013] The current is: 350-400 A for the lower ceramic layer, 400-450 A for the upper ceramic layer, and 350-400 A for the metal bonding layer.

[0014] The voltage is: 40-45 V for the lower ceramic layer, 45-50 V for the upper ceramic layer, and 40-45 V for the metal bonding layer.

[0015] The H2 flow generated by the plasma is: 0.2-0.6 L / min for the lower ceramic layer, 0.8-1.2 L / min for the upper ceramic layer, and 0.2-0.6 L / min for the metal bonding layer.

[0016] The spraying distance is: 130-170 mm for the lower ceramic layer, 130-170 mm for the upper ceramic layer, and 180-220 mm for the metal bonding layer.

[0017] The graphite mold coated with the composite coating is placed in an environment with a temperature of 18℃ and a relative humidity of 50% for drying, and then it is placed in an oven for drying after drying, and is kept at a temperature of 110-120℃ for 2-4 hours. Then the graphite mold is placed in a high-temperature vacuum heat treatment furnace for stepwise baking, first kept at 380℃ for 2 hours, then kept at 600℃ for 2 hours, and then the temperature is raised to 1030℃ for 2 hours, and then the furnace is cooled to 300℃, the mechanical Roots pump and the vacuum direct current valve are closed, the mechanical pump is filled with air, the cooling water is stopped, and the air cooling is stopped until the temperature reaches room temperature.

[0018] A titanium alloy precision casting method, characterized in that the specific process is as follows:

[0019] (1) Preparation of graphite mold: according to the casting process drawing, the graphite electrode block is prepared into a graphite mold by using a moldless numerical control machining technology, and high-purity graphite is used as the raw material;

[0020] (2) Coating of composite coating: a composite coating is sprayed on the inner cavity surface of the prepared graphite mold by using a plasma spraying process;

[0021] (3) Preheating and baking of graphite mold: the graphite mold coated with the composite coating is placed in an environment with a temperature of 18 DEG C and a relative humidity of 50% for drying, and after drying, it is placed in an oven for drying, and is kept at 110-120 DEG C for 2-4 h, and then the graphite mold is placed in a high-temperature vacuum heat treatment furnace for stepwise baking;

[0022] (4) Alloy melting: the titanium alloy electrode is installed on the electrode rod of the vacuum consumable electrode skull furnace, and the mold is clamped, and after the furnace door is closed, the furnace body is vacuumed, when the vacuum degree is less than or equal to 3*10 -2 Pa, the electrode starts to melt, the melting current is controlled at 8000A-18000A, and the voltage is controlled at 35-40V;

[0023] (5) Alloy pouring: after the alloy raw material is melted to the pouring weight, the mold centrifugal disc is rotated, the rotation speed of the centrifugal disc is controlled at 100-250r / min, the alloy liquid is poured into the mold by turning the crucible, and the titanium alloy casting is obtained.

[0024] The present application has the following advantages:

[0025] 1. The composite coating material has a more compact structure compared with the conventional single coating material, and has good adaptability between the coating layers, low porosity, no interlayer cracks, and more excellent heat insulation effect.

[0026] 2. The scheme can significantly improve the surface quality of the titanium alloy casting, the surface roughness Ra of the titanium alloy casting obtained by pouring is less than or equal to 3.2 microns, the pollution layer thickness is less than or equal to 10 microns, and there is no surface crack.

[0027] 3. The composite coating has multiple functions at different stages of pouring: in the early stage of pouring, the coating has a heat insulation effect, which can significantly reduce the heat loss of the melt, thereby effectively improving the filling, forming and feeding capacity of the titanium liquid; during the pouring process, the coating can improve the surface quality of the cavity, reduce the defects caused by the particles falling off from the cavity surface due to thermal shock or physical impact and entering the melt; in the late stage of pouring, the coating forms a temperature gradient with the graphite mold, which can realize sequential solidification and feeding.

[0028] 4. Compared with the manual painting method, the casting surface coating prepared by the plasma spraying technology of the application shows more excellent coating hanging property, and the bonding strength between the coating and the graphite matrix is ≥50 MPa. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Schematic diagram of the composite coating structure of the titanium alloy casting graphite mold.

[0030] Figure 2 Vacuum baking curve of the composite coating graphite mold.

[0031] The figure mark: 1, metal bonding layer, 2, ceramic lower layer, 3, ceramic upper layer. DETAILED DESCRIPTION

[0032] Example 1

[0033] Preparation of the composite coating:

[0034] (1) The powder used for the ceramic upper layer: 60% ZrO2-30% Y2O3-6% CaO-3% MgO-1% CeO2, and the powder used for the ceramic lower layer: Al2O3 powder, with a particle size range of 20-100 μm.

[0035] The ceramic upper layer powder is prepared by the spray granulation method: ZrO2, Y2O3, CaO, MgO, and CeO2 powders are mixed in a mass ratio of 60:30:6:3:1, and an appropriate amount of deionized water is added, and then ball milling is carried out in a planetary ball mill for 3 h, and then polyvinyl alcohol is added to the slurry and ball milling is continued for 3 h, and the rotation speed of the ball mill is 300 r / min, to prepare a composite powder slurry. The spray granulation method is used to prepare the powder by using a spray dryer, and the main process parameters are: (a) the inlet temperature is 250℃, and the outlet temperature is 100℃; (b) the atomizing wheel gas supply pressure is 1.4×10 5 Pa; (c) the feeding speed is 40 r / min. The powder particles with a particle size range of 20-100 μm are screened.

[0036] (2) The atmospheric plasma spraying technology is used to prepare the composite coating on the surface of the high-purity graphite mold for titanium alloy casting: the graphite matrix surface is first pretreated by sandblasting.

[0037] The plasma spraying process parameters are as follows:

[0038] The current is: 350 A for the ceramic lower layer, 410 A for the ceramic upper layer, and 350 A for the metal bonding layer Ni / Al. The voltage is: 40 V for the ceramic lower layer, 45 V for the ceramic upper layer, and 40 V for the metal bonding layer Ni / Al. The plasma generated H2flow is: 0.2 L / min for the ceramic lower layer, 0.8 L / min for the ceramic upper layer, and 0.3 L / min for the metal bonding layer Ni / Al. The spraying distance is: 130 mm for the ceramic lower layer, 140 mm for the ceramic upper layer, and 180 mm for the metal bonding layer Ni / Al.

[0039] The coating thickness is: 40 μm for the metal bonding layer, about 20 μm for the Al2O3ceramic lower layer, and about 60 μm for the ceramic upper layer.

[0040] (3) After the coating is prepared, the graphite mold is placed in a special environment (temperature 18°C, relative humidity 50%) for drying, and after drying, it is placed in an oven for drying, and is kept at 110°C for 2 h, and then is high-temperature baked according to the baking curve shown in Figure 2 The baking equipment used is a high-temperature vacuum heat treatment furnace.

[0041] It is detected that the porosity of the composite coating prepared in this embodiment is 2.5%, and the bonding force between the coating and the substrate is 52 MPa.

[0042] Example 2

[0043] The graphite mold prepared in the example is used to prepare a titanium alloy casting:

[0044] Alloy melting: the titanium alloy electrode is installed on the electrode rod of the vacuum self-consumption electrode skull furnace, the mold is clamped, and after the furnace door is closed, the furnace body is vacuumed. When the vacuum degree is ≤3 x 10 -2 Pa, the electrode starts to melt, the melting current is controlled at 10000 A, and the voltage is controlled at 38 V.

[0045] Alloy pouring: after the alloy raw material is melted to the pouring weight, the centrifugal disc of the mold is rotated, the rotating speed of the centrifugal disc is controlled at 100 r / min, the alloy liquid is poured into the graphite mold by turning the crucible, and a titanium alloy casting is obtained. The titanium alloy casting poured has excellent surface quality, no surface cracks, the thickness of the pollution layer is ≤10 μm, and the surface roughness Ra of the casting is ≤3.2 μm.

[0046] Example 3

[0047] Compared with Example 1, the following changes are made in this embodiment:

[0048] 1. The powder used for the ceramic upper layer: 64% ZrO2-28% Y2O3-4% CaO-2% MgO-2% CeO.

[0049] 2. The thickness of the bonding layer is 50 μm, the thickness of the lower ceramic layer is 30 μm, and the thickness of the upper ceramic layer is 70 μm.

[0050] 3. The powder is prepared by spray granulation using a spray dryer, and the main process parameters are as follows: (a) the inlet temperature is 265°C, and the outlet temperature is 110°C; (b) the air supply pressure of the atomizing wheel is 1.6 x 10 5 Pa; (c) the feeding speed is 45 r / min; and the powder particles with a particle size in the range of 20-100 μm are screened.

[0051] 4. The plasma spraying process parameters are as follows:

[0052] The current is: 370 A for the lower ceramic layer, 430 A for the upper ceramic layer, and 380 A for the metal bonding layer.

[0053] The voltage is: 42 V for the lower ceramic layer, 47 V for the upper ceramic layer, and 42 V for the metal bonding layer.

[0054] The H2 flow rate for plasma generation is: 0.4 L / min for the lower ceramic layer, 1.0 L / min for the upper ceramic layer, and 0.4 L / min for the metal bonding layer.

[0055] The spraying distance is: 150 mm for the lower ceramic layer, 150 mm for the upper ceramic layer, and 200 mm for the metal bonding layer.

[0056] The castings poured using the graphite mold prepared in this example have no surface cracks, the thickness of the surface contamination layer is ≤10 μm, and the surface roughness Ra is ≤3.2 μm.

[0057] Comparative Example 1

[0058] The following changes are made in this example compared with Example 1:

[0059] 1. The powder used for the upper ceramic layer is: 67% ZrO2-10% Y2O3-10% CaO-8% MgO-5% CeO.

[0060] 2. The thickness of the bonding layer is 40 μm, the thickness of the lower ceramic layer is 50 μm, and the thickness of the upper ceramic layer is 80 μm.

[0061] The castings poured using the graphite mold prepared in this example have a large number of surface cracks, the thickness of the surface contamination layer is 15 μm, and the surface roughness Ra is ≤6.3 μm.

[0062] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the spirit and essence of the present application shall be covered within the protection scope of the present application.

[0063] Furthermore, descriptions of well-known structures and techniques are omitted so as not to unnecessarily obscure the concept of the present application.

Claims

1. A composite coating for casting titanium alloys in graphite molds, characterized by: The composite coating is composed of a metal bonding layer and a ceramic layer, the ceramic layer is divided into an upper ceramic layer and a lower ceramic layer, the lower ceramic layer is made of Al2O3, and the upper ceramic layer is made of ZrO2, Y2O3, CaO, MgO and CeO2, and the upper ceramic layer has a composition of Y2O3: 28-32%, CaO: 4-8%, MgO: 2-4%, CeO2: 1-2%, and ZrO2: the balance in terms of mass percentage; The metal bonding layer is made of Ni / Al alloy powder, and the upper ceramic layer has a composition of Y2O3: 30%, CaO: 6%, MgO: 3%, CeO2: 1%, and ZrO2: the balance in terms of mass percentage.

2. The composite coating for casting titanium alloys in graphite molds according to claim 1, characterized in that: The metal bonding layer has a thickness of 40-60 microns, the lower ceramic layer has a thickness of 20-40 microns, and the upper ceramic layer has a thickness of 60-80 microns.

3. A method of producing a composite coating according to claim 1 or 2, characterized in that: Each coating is sprayed in the inner cavity surface of the graphite mold in sequence by using a plasma spraying process, and then the graphite mold coated with the composite coating is dried and placed in a vacuum environment for stepwise baking.

4. The method of claim 3, wherein the composite coating is prepared by: Before spraying the upper ceramic layer, the ZrO2, Y2O3, CaO, MgO and CeO2 powders are mixed in a mass ratio, deionized water is added, and ball milling is performed in a planetary ball mill for 3-4 hours, and then polyvinyl alcohol is added to the slurry for further ball milling for 3-5 hours at a ball mill speed of 250-350 r / min to prepare a composite powder slurry for spray granulation.

5. The method of claim 4, wherein the composite coating is prepared by: The powder is prepared by spray granulation using a spray dryer, the main process parameters being: (a) inlet temperature 250-280°C, outlet temperature 100-120°C; (b) atomizing wheel air supply pressure 1.4 x 10 5 ~1.8 x 10 5 Pa; (c) feed speed 40-60 r / min; the powder particles are sieved to a particle size range of 20-100 μm. The powder is prepared by spray granulation using a spray dryer, the main process parameters being: (a) inlet temperature 250-280°C, outlet temperature 100-120°C; (b) atomizing wheel air supply pressure 1.4 x 10 5 ~1.8 x 10 5 Pa; (c) feed speed 40-60 r / min; the powder particles are sieved to a particle size range of 20-100 μm.

6. The method of claim 3, wherein the composite coating is prepared by, The plasma spraying process parameters are as follows: Current: 350-400 A for the lower ceramic layer, 400-450 A for the upper ceramic layer, and 350-400 A for the metal bonding layer; Voltage: 40-45 V for the lower ceramic layer, 45-50 V for the upper ceramic layer, and 40-45 V for the metal bonding layer; Plasma generated H2 flow rate: 0.2-0.6 L / min for the lower ceramic layer, 0.8-1.2 L / min for the upper ceramic layer, and 0.2-0.6 L / min for the metal bonding layer; Spraying distance: 130-170 mm for the lower ceramic layer, 130-170 mm for the upper ceramic layer, and 180-220 mm for the metal bonding layer.

7. The method of claim 3, wherein the composite coating is prepared by: The graphite mold coated with the composite coating is placed in an environment with a temperature of 18℃ and a relative humidity of 50% for drying, and then placed in an oven for drying and heat preservation at 110-120℃ for 2-4 hours.

8. The method of claim 3, wherein the composite coating is prepared by, The graphite mold coated with the composite coating is placed in a high-temperature vacuum heat treatment furnace for stepwise baking, first heat preserved at 380℃ for 2 hours, then at 600℃ for 2 hours, and then the temperature is raised to 1030℃ for 2 hours before the furnace is cooled to 300℃.

9. A method of precision casting a titanium alloy based on the composite coating according to claim 1 or 2, characterized in that, The specific process is as follows: (1) Preparation of graphite mold: according to the casting process drawing, the graphite mold is prepared by using moldless numerical control machining technology, and high-purity graphite is used as raw material; (2) Coating of composite coating: the composite coating is sprayed on the inner cavity surface of the prepared graphite mold by using plasma spraying process; (3) Preheating and baking graphite mold: the graphite mold coated with the composite coating is placed in an environment with a temperature of 18℃ and a relative humidity of 50% for drying, and after drying, is placed in an oven for drying, and is kept at a temperature of 110-120℃ for 2-4h, and then the graphite mold is placed in a high-temperature vacuum heat treatment furnace for stepwise baking; (4) Fusion alloying: titanium alloy electrode is installed on the electrode rod of vacuum consumable electrode skull furnace, the mold is clamped, the furnace body is vacuumized after the furnace door is closed, when the vacuum degree is ≤3×10 -2 Pa, the electrode starts to melt, the melting current is controlled at 8000A-18000A, and the voltage is controlled at 35-40V; (5) Pouring the alloy: after the alloy raw material is melted to a pouring weight, the centrifugal disc of the mold is rotated, the rotating speed of the centrifugal disc is controlled at 100-250r / min, the crucible is turned over to pour the alloy liquid into the mold, and a titanium alloy casting is obtained.

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