DENSE THICK ALLOY COATING WITHOUT LAYERED ORGANIZATIONAL STRUCTURE AND PREPARATION METHOD THEREOF

Abstract

A dense thick alloy coating with no layered organizational structure and a preparation method thereof are provided. The preparation method includes the following steps: step 1, placing a substrate in a plasma spraying apparatus, controlling a vacuum degree of the plasma spraying apparatus to be 0.2 mbar-0.5 mbar, controlling a length of a plasma flame of the plasma spraying apparatus to be 1000 mm-1200 mm, and controlling a diameter of the plasma flame to be 200 mm-300 mm; step 2, preheating, by using a plasma spray gun, the substrate to a temperature of 700° C.-1000° C. to obtain a preheated substrate; step 3, plasma spraying an alloy powder on the preheated substrate under a specific plasma spraying condition to obtain a sprayed substrate; and step 4, cooling the sprayed substrate after the plasma spraying, to thereby obtain the dense thick alloy coating without layered organizational structure.

Description

TECHNICAL FIELD

The present disclosure relates to the technical field of thermal spraying, and particularly to a dense thick alloy coating without layered organizational structure and a preparation method thereof.

BACKGROUND

Thermal spraying technology is a method that uses a heat source to heat a spraying material to a molten state or a semi-molten state, and sprays and deposits the spraying material on a surface of a pretreated substrate at a certain speed to form a coating. At present, commonly used thermal spraying methods include an ordinary flame spraying method, an arc spraying method, an atmospheric plasma spraying method, a high-velocity oxygen-fuel (HVOF) spraying method, a vacuum plasma spraying method, and a cold spraying method. A bonding between the coating prepared by the thermal spraying technology and the surface of the pretreated substrate is mechanical bonding, and the superposition of a thermal stress, a mechanical stress, and other residual stresses occurs with the increase of a thickness of the coating during forming the coating.

At present, the thermal spraying technology is the main process for preparing a high-temperature oxidation-resistant alloy coating, and other preparation processes for preparing the high-temperature oxidation-resistant alloy coating include a plasma surfacing process and a flame spray welding process. The prepared high-temperature oxidation-resistant alloy coating is metallurgically bonded with the surface of the substrate, and a molten pool needs to be formed on the surface of the substrate and a metallurgical reaction occurs during forming the high-temperature oxidation-resistant alloy coating. The research shows that an alloy coating prepared by the atmospheric plasma spraying method is a tensile stress coating, and a bonding strength of the tensile stress coating decreases with the increase of thickness of the tensile stress coating. A bonding strength of the tensile stress coating with a thickness of 0.2 millimeters (mm)-0.4 mm is about 30 megapascals (MPa)-50 MPa, and the bonding strength decreases to 5 MPa-15 MPa when the thickness is greater than 1 mm, and the tensile stress coating is easy to be peeled off locally due to stress superposition. An alloy coating prepared by the HVOF spraying method is a compressive stress coating with a higher bonding strength, and the bonding strength of the compressive stress coating does not decrease obviously with the increase of thickness of the compressive stress coating. However, an interior of the compressive stress coating is a layered organizational structure produced by the accumulation of droplets, therefore, a micro-crack is easy to be produced and the micro-crack can expand continuously, thereby resulting in cracking and peeling of the compressive stress coating; and it is impossible to prepare a thicker alloy coating without heat treatment to assist stress relief. Compared with the alloy coating prepared by the atmospheric plasma spraying method, a bonding strength of an alloy coating prepared by the vacuum plasma spraying method is greatly improved, but the alloy coating prepared by the vacuum plasma spraying method is still in a tensile stress state, so it is impossible to directly obtain a coating with a thickness more than 1 mm. At present, a spraying process in use for the vacuum plasma spraying method is as follows: (1) preparing an alloy coating with a thickness of 0.4 mm-0.8 mm by using the vacuum plasma spraying method on a part to obtain a coated part; (2) performing a vacuum heat treatment on the coated parts to relieve stress; (3) re-preparing an alloy coating with a thickness of 0.4 mm-0.8 mm by using the vacuum plasma spraying method on the basis of the original coating (i.e., the coating obtained in (1)); (4) re-performing a vacuum heat treatment to relieve stress; (5) repeating the above processes until a thickness of the coating meets a requirement. The thicker alloy coating prepared by this spraying process has obvious delamination caused by spraying interruption, and there is an obvious layered organizational structure caused by droplet accumulation in the coating, which can not meet usage requirements of harsh environments such as corrosion and high temperature, and at the same time, a production efficiency thereof is lower and the production cost thereof is higher, thus, the this spraying process is difficult to meet the requirements of mass industrial production.

However, in the related art (such as Chinese patent publication NO. CN106048488B), during using the plasma surfacing process, a molten pool is first formed on a surface of a substrate, and then powder particles are sent to the molten pool for metallurgical reaction and is cooled together with the substrate to form a coating. The advantage of the coating is that the coating is completely dense and is metallurgically combined with the substrate; however, the disadvantage of the coating is that alloy elements such as yttrium and silicon in the coating are easy to burn during the solidification of the molten pool, which affects the high-temperature performance of the coating. Further, since a depth of the molten pool is larger, mechanical properties of single crystal, directionally solidified superalloy and other substrates are easier to be damaged. Therefore, the plasma surfacing process is not suitable for the preparation of high-temperature oxidation-resistant coatings on surfaces of hot-end components of aero-engines and gas turbines, and there is no application case. Moreover, the flame spray welding process is only suitable for self-fluxing alloys, such as NiCrBSi, and the types of coatings thereof are greatly limited.

SUMMARY

The present disclosure aims to provide a dense thick alloy coating without layered organizational structure and a preparation method thereof in order to overcome the defects in the related art, such as, local inclusion and pollutant aggregation caused by a layered organizational structure in a thick alloy coating structure, and a complicate production process. The preparation method of the present disclosure can realize continuous spraying to prepare a dense thick alloy coating with a thickness of 0.1 mm-3.5 mm, no layered organizational structure exists in the dense thick alloy coating, an edge of the dense thick alloy coating has no cracking and warping, and the preparation has a higher production efficiency, and can be suitable for the preparation of high-temperature oxidation-resistant coatings on surfaces of hot-end components of aero-engines and gas turbines.

The inventors of the present disclosure have found that at present, thick alloy coatings on the surfaces of the hot-end components of the aero-engines and the gas turbines are mainly prepared by the vacuum plasma spraying method. In the related art, a pressure in a vacuum chamber is above 50 millibars (mbar), and a length of a plasma flame generated by a spray gun is usually not more than 300 millimeters (mm) due to a higher environmental pressure. A flying distance of each of coating material particles in the plasma flame is short, the coating material particles are not sufficiently heated, there are a certain number of semi-melted particles, and surfaces of the coating material particles are oxidized in the ambient atmosphere, thereby resulting in a large number of layered organizational structures in the coating. Moreover, a pressure in the vacuum chamber is higher, a thermal conductivity of the vacuum chamber is good, an upper limit of a preheating temperature of a substrate is lower (300 degrees Celsius (C)-500° C.), and a cooling speed of droplets after deposition of the droplets is faster, which leads to a larger thermal stress of the coating. As such, when the coating is 0.4 mm-0.6 mm in thickness, it is necessary to interrupt the spraying and perform a stress-relieving annealing heat treatment, otherwise the coating will fall off as a whole. In addition, there will be obvious delamination in the coating due to spraying interruption. Also, if the HVOF method is performed in atmospheric environment, the melting of particles is more insufficient (compared with the vacuum plasma spraying method), and a cooling speed of droplets after deposition of the droplets is more faster (compared with the vacuum plasma spraying method), therefore, the HVOF method has the same problems as the existing vacuum plasma spraying method. Based on this, the present disclosure is proposed.

In order to achieve the above objectives, a first aspect of the present disclosure provides a method for preparing a dense thick alloy coating, which includes the following steps:

• • step 1, placing a substrate in a plasma spraying apparatus, controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 mbar to 0.5 mbar, controlling a length of a plasma flame of the plasma spraying apparatus to be in a range from 1000 mm to 1200 mm, and controlling a diameter of the plasma flame to be in a range from 200 mm to 300 mm;
  • step 2, preheating, by using a plasma spray gun, the substrate to a temperature in a range from 700° ° C. to 1000° ° C. to obtain a preheated substrate;
  • step 3, plasma spraying an alloy powder on the preheated substrate to obtain a sprayed substrate, where conditions of the plasma spraying includes: a spraying power is in a range from 55 kilowatts (kw) to 130 kw, preferably, 80 kw to 130 kw, a powder feeding speed is in a range from 10 grams per minute (g/min) to 30 g/min, a spraying distance is in a range from 450 mm to 800 mm, and a relative linear velocity between the plasma flame and a surface of the substrate is in a range from 1 meter per second (m/s) to 3 m/s; and
  • step 4, cooling the sprayed substrate after the plasma spraying, to thereby obtain the dense thick alloy coating without layered organizational structure.

In some embodiments, in the step 1, the controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 mbar to 0.5 mbar includes: vacuuming a vacuum chamber in which the substrate is placed of the plasma spraying apparatus to control a vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, filling argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 50 mbar to 100 mbar, re-vacuuming the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, and refilling argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be in the range from 0.2 mbar to 0.5 mbar.

In some embodiments, the step 1 further includes: before placing the substrate in the plasma spraying apparatus, performing a roughening treatment on the surface of the substrate.

In some embodiments, a roughness Ra of the surface of the substrate is in a range from 6 μm to 8 μm through the roughening treatment.

In some embodiments, in the step 2, the preheating, by using a plasma spray gun, the substrate includes: performing blowing and preheating the surface of the substrate by using a flame from the plasma spray gun, where a power of the plasma spray gun is controlled to be in a range from 60 kw to 150 kw, a blowing distance for the blowing is in a range from 200 mm to 500 mm, a blowing speed for the blowing is in a range from 200 millimeters per second (mm/s) to 1000 mm/s, and a preheating time for the preheating is in a range from 3 minutes (min) to 8 min.

In some embodiments, in the step 3, the alloy powder satisfies the following requirements: a particle size of the alloy powder is smaller than 40 micrometers (μm), and an oxygen content of the alloy powder is less than 600 parts per million (ppm)

In some embodiments, in the step 3, the alloy powder includes at least one selected from the group consisting of NiCoCrAlY, NiCrAlY, NiAl, NIAlW, CoCrW, and CoCrMoSi.

In some embodiments, in the step 4, a cooling speed for the cooling is in a range from 5 degrees Celsius per minute (C/min) to 40° C./min, and a cooling time for the cooling is in a range from 20 min to 60 min.

In some embodiments, the cooling is performed through multiple gradient cooling stages, and a difference between a cooling speed of a latter gradient cooling stage of the multiple gradient cooling stages and a cooling speed of a former gradient cooling stage of the multiple gradient cooling stages is in a range from 5° C./min to 20° C./min.

A second aspect of the present disclosure provides a dense thick alloy coating without layered organizational structure, which is prepared by any preparation method described in the first aspect.

In some embodiments, no layered organizational structure exists in the dense thick alloy coating, an edge of the dense thick alloy coating has no cracking and warping, a thickness of the dense thick alloy coating is in a range from 0.1 millimeters (mm) to 3.5 mm; and the dense thick alloy coating satisfies the following requirements: a porosity of the dense thick alloy coating without layered organizational structure is less than 1%, an oxygen content of the dense thick alloy coating without layered organizational structure is less than 0.1%; and a bonding strength of the dense thick alloy coating without layered organizational structure is greater than 70 megapascals (MPa).

According to the present disclosure, the substrate is preheated to a higher temperature by setting a specific lower vacuum degree and larger flame of plasma spraying, and by combination of a spraying power, a spraying distance, a powder feeding speed and a relative linear velocity, so that a dense thick alloy coating with a thickness of 0.1 mm-3.5 mm, no layered organizational structure and no edge warping and cracking can be prepared by continuous spraying, and the coating performance and manufacturability are obviously superior to the existing process for preparing a thick alloy coating by intermittent spraying assisted by heat treatment. The coating meets the requirements of high temperature, marine corrosion and other harsh use environments, and has important applications in aero-engines and gas turbines.

In the present disclosure, a pressure in the vacuum chamber is reduced to 0.2 mbar-0.5 mbar during high-power plasma spraying, and a length and a diameter of the plasma flame generated by the plasma spray gun are expanded to 1000 mm-1200 mm and 200 mm-300 mm, respectively, so that a flying distance of alloy powder particles in the plasma flame can be greatly increased (the spraying distance is controlled to be 450 mm-800 mm), the alloy powder particles are fully heated, the alloy powder particles are completely melted and the molten state has good identity, and the surfaces of the alloy powder particles are basically free of oxidation, thus eliminating the layered organizational structure in the coating. Further, the pressure in the vacuum chamber is greatly reduced, a thermal insulation is improved, the preheating temperature of the substrate can be increased to 700° C.-1000° C., a cooling speed of droplets is slowed down after deposition of the droplets, and coatings deposited each time are fully fused, thus eliminating the layered organizational structure and reducing the thermal stress of the coatings. In addition, a metallurgical transition layer is formed between the coating and the substrate, which greatly improves a bonding strength of the coating. Also, the high uniformity of melting state of the alloy powder is realized by controlling the spraying power and the powder feeding speed, and the full fusion between deposited coatings is realized by controlling the spraying distance and the spraying linear velocity. Therefore, continuous sprayings can be performed to obtain a coating with a thickness of 0.1 mm-3.5 mm. It is not necessary to interrupt spraying to carry out stress-relieving annealing heat treatment, and thus the production efficiency thereof is greatly improved.

The dense thick alloy coating prepared by the method of the present disclosure has the advantages that the porosity of the coating is less than 1%, the oxygen content is less than 0.1%, there is no delamination in the coating, there is no layered organizational structure after being enlarged by 400 times, and the edge thereof is free from cracking and warping, and the bonding strength thereof is greater than 70 MPa. Compared with the coating in the related art, the performance of the coating of the present disclosure, such as, bonding strength and spraying process, has been greatly improved, which can meet the use requirements of more severe working conditions such as corrosion and high temperature and the requirements of large-scale production.

BRIEF DESCRIPTION OF DRAWINGS

In order to explain the technical solutions of the embodiments of the present disclosure more clearly, accompanying drawings required to be used in the embodiments are briefly introduced hereinafter. It should be understood that the accompanying drawings merely show some embodiments of the present disclosure, so the accompanying drawings should not be regarded as limiting the scope of protection of the present disclosure. For the skilled in the art, other relevant drawings can be obtained according to these accompanying drawings without creative labor.

FIG. 1 is a photo of a spraying process of an embodiment 1 of the present disclosure.

FIG. 2A is a microscopic picture of a coating in the embodiment 1 of the present disclosure with a magnification of 20 times, in which a black line is an interface between the coating and a substrate;

FIG. 2B is a microstructure picture of the coating in the embodiment 1 of the present disclosure with a magnification of 400 times.

FIG. 3 is a microstructure picture of a coating of a comparative example 1.

FIG. 4 is a microstructure picture of a coating of comparative example 2.

FIG. 5 is a microscopic picture of a coating of comparative example 3 with a magnification of 20 times.

DETAILED DESCRIPTION OF EMBODIMENTS

Endpoints values of ranges and any values disclosed herein are not limited to exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For a numerical range, one or more new numerical ranges can be obtained by combining of the endpoint values of the numerical range, combining of an endpoint value of the numerical range and an individual point value, and combining of individual point values, and the one or more new numerical ranges should be regarded as specifically disclosed herein.

A first aspect of the present disclosure provides a method for preparing a dense thick alloy coating, which includes the following steps:

• • step 1, placing a substrate in a plasma spraying apparatus, controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 mbar to 0.5 mbar, controlling a length of a plasma flame of the plasma spraying apparatus to be in a range from 1000 mm to 1200 mm, and controlling a diameter of the plasma flame to be in a range from 200 mm to 300 mm;
  • step 2, preheating, by using a plasma spray gun, the substrate to a temperature in a range from 700° ° C. to 1000° ° C. to obtain a preheated substrate;
  • step 3, plasma spraying an alloy powder on the preheated substrate to obtain a sprayed substrate, where conditions of the plasma spraying includes: a spraying power is in a range from 55 kilowatts (kw) to 130 kw, preferably, 80 kw to 130 kw, a powder feeding speed is in a range from 10 grams per minute (g/min) to 30 g/min, a spraying distance is in a range from 450 mm to 800 mm, and a relative linear velocity between the plasma flame and a surface of the substrate is in a range from 1 meter per second (m/s) to 3 m/s; and
  • step 4, cooling the sprayed substrate after the plasma spraying, to thereby obtain the dense thick alloy coating without layered organizational structure.

It should be understood that the steps 2 and 3 are all performed under the conditions of the step 1.

According to the present disclosure, the dense thick alloy coating without layered organizational structure is prepared through continuous praying by using a specific plasma spraying process in a state of lower vacuum degree and higher substrate temperature, where the vacuum degree is controlled to be 0.2 mbar-0.5 mbar, and the length and the diameter of the plasma flame are respectively 1000 mm-1200 mm and 200 mm-300 mm, so that the surface of the substrate can be completely covered by a large-scale plasma flame in the state of lower vacuum degree, and the substrate can be uniformly preheated to 700° C.-1000° C. During plasma spraying, due to the larger length and the larger diameter of the plasma flame, long-distance spraying can be realized, and the spraying distance is controlled to be 450 mm-800 mm and a suitable spraying speed can be reached. However, a preheating temperature for thermal spraying of parts in the related art is generally about 200° C.-300° C., and it is impossible to realize long-distance continuous spraying. The present disclosure solve that problems that the thick alloy coating prepared by the related art has delamination, obvious layered organizational structure in the coating, lower production efficiency and finished product rate, higher production cost, obvious influence on a crystal structure and mechanical properties of the substrate. The coating of the present disclosure is suitable for industrial continuous production, especially for the production of thick alloy coatings of hot-end components of aero-engines and gas turbines.

Moreover, compared with the related art such as Chinese patent publication NO. CN106048488B, which adopts the solution that powder particles are sent to a molten pool for metallurgical reaction and then cooled together with the substrate to form a coating, in the present disclosure, a molten pool is not formed in the formation process of the coating, and the powder particles are fully melted in a low-vacuum flame and then hit the surface of the substrate at a high speed to form the coating, and then the coating can be solidified quickly without performing a metallurgical reaction. At the same time, the bonding between the coating and the substrate is a mechanical bonding in a micro-metallurgical state, and an element diffusion depth is less than 0.1 mm, which will not damage the mechanical properties of the substrate.

In some embodiments, in the step 1, the controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 mbar to 0.5 mbar includes: vacuuming a vacuum chamber in which the substrate is placed of the plasma spraying apparatus to control a vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, filling an inert gas into the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 50 mbar to 100 mbar, re-vacuuming the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, and refilling an inert gas into the vacuum chamber to controls the vacuum degree of the vacuum chamber to be in the range from 0.2 mbar to 0.5 mbar. With this technical solution, the removal of water vapor and oxygen in the vacuum chamber can be quickly completed, which is more conducive to improving production efficiency of the coating.

In the present disclosure, those skilled in the art can select the inert gas according to actual needs, and the inert gas can be argon, or helium.

In some embodiments, the step 1 further includes: before placing the substrate in the plasma spraying apparatus, performing a roughening treatment on the surface of the substrate.

In some embodiments, a roughness Ra of the surface of the substrate is in a range from 6 μm to 8 μm through the roughening treatment. With this technical solution, the bonding state of the coating and the roughness of the surface of the substrate can be taken into account, which is more conducive to reducing a machining allowance of the coating after spraying on the premise of ensuring the quality of the coating.

The performing a roughening treatment on the surface of the substrate can include: cleaning and oil-removing first, then sand blowing and optional cleaning. It may also include surface roughening by machining, as long as the required surface roughness can be achieved. In an embodiment, the cleaning is ultrasonic cleaning, and conditions for the ultrasonic cleaning include: an ultrasonic frequency is 20 kHz-60 kHz, a power density is 0.5 watts per square centimeter (W/cm²)-3 W/cm², a cleaning temperature is 30° C.-70° C., and a cleaning time is 10 min-30 min. Those skilled in the art can choose the specific operation process of the sand blasting or the surface roughening by machining according to the requirements of the roughening treatment.

In the step 2, it can be understood that the length and the diameter of the plasma flame sprayed by the plasma spray gun are 1000 mm-1200 mm and 200 mm-300 mm respectively; and the vacuum degree during the preheating is 0.2 mbar-0.5 mbar. In the present disclosure, the plasma flame can completely cover the surface of the substrate in a lower vacuum state, and thus uniform preheating can be realized.

In some embodiments, in the step 2, the preheating, by using a plasma spray gun, the substrate includes: performing rapidly blowing and preheating the surface of the substrate by using the flame from the plasma spray gun, where a power of the spray gun is controlled to be 60 kw-150 kw, a blowing distance is 200 mm-500 mm, a blowing speed is 200 mm/s-1000 mm/s, and a preheating time is 3 min-8 min.

In the step 3, the present disclosure realizes the high uniformity of the melting state of the alloy powder through the proper control of the spraying power and the powder feeding speed, and realizes the full fusion between the deposited coatings through the proper control of the spraying distance and the relative linear velocity, so as to realize that there is no layered organizational structure in the coating.

In an embodiment, the spraying power is 80 kw-130 kw. With this technical solution, it is more beneficial to improve the bonding strength of the coating and reduce the porosity of the coating.

In some embodiments, in the step 3, the alloy powder satisfies the following requirements: the particle size of the alloy powder is less than 40 μm (understandably, the particle size of all powders is less than 40 μm) and the oxygen content of the alloy powder is less than 600 ppm. With this technical solution, both the spraying process requirements and the powder cost can be considered, which is more conducive to reducing the coating production cost and improving the production efficiency.

The method of the present disclosure uses alloy powder with various compositions, such as at least one of binary, ternary, quaternary and multicomponent alloys based on cobalt and nickel. In some embodiments, in the step 3, the alloy powder includes at least one selected from the group consisting of NiCoCrAlY, NiCrAlY, NiAl, NIAlW, CoCrW, and CoCrMoSi. The composition of each element in the alloy powder can be selected by a person skilled in the art according to requirements and can be used in the present disclosure.

In some embodiments, the step 3 further includes drying the alloy powder before performing the plasma spraying.

In some embodiments, in the step 4, a cooling speed for the cooling is in a range from 5ºC/min to 40° C./min, and a cooling time for the cooling is in a range from 20 min to 60 min. In some embodiments, the cooling is performed until the sprayed substrate is cooled to below 100° C.

In some embodiments, the cooling is performed through multiple gradient cooling stages, and a difference between a cooling speed of a latter gradient cooling stage of the multiple gradient cooling stages and a cooling speed of a former gradient cooling stage of the multiple gradient cooling stages is in a range from 5° C./min to 20° C./min. With this technical solution, gradient staged cooling can fully reduce the thermal stress of the coating and achieve high production efficiency.

In the present disclosure, a cooling time of the latter gradient cooling stage can be the same as or different from that of the former gradient cooling stage, as long as the overall cooling time meets the requirements. In some embodiments, an absolute value of a difference between the cooling time of the latter gradient cooling stage and the cooling time the previous gradient cooling stage is in a range from 0 to 5 min.

In some embodiment, the number of the multiple gradient cooling stages is in a range from 4 to 7.

The cooling can be realized by backfilling inert gas (such as argon) into the vacuum chamber to cool down the sprayed substrate and adjust the vacuum degree.

The substrate in the present disclosure can be any part which needs to be provide with a high-temperature oxidation-resistant coating, such as the hot-end components of the aero-engines and the gas turbines.

A second aspect of the present disclosure provides a dense thick alloy coating without layered organizational structure, which is prepared by any preparation method described in the first aspect.

In some embodiments, no layered organizational structure exists in the dense thick alloy coating, an edge of the dense thick alloy coating has no cracking and warping, a thickness of the dense thick alloy coating is in a range from 0.1 millimeters (mm) to 3.5 mm; and the dense thick alloy coating satisfies the following requirements: a porosity of the dense thick alloy coating without layered organizational structure is less than 1%, an oxygen content of the dense thick alloy coating without layered organizational structure is less than 0.1%; and a bonding strength of the dense thick alloy coating without layered organizational structure is greater than 70 MPa.

The dense thick alloy coating has no layered organizational structure, which means that the dense thick alloy coating does not have characteristics of layered organizational structure such as aggregation of oxide, inclusion, tiny unmelted particles, and uneven pore distribution.

In the present disclosure, a metallurgical transition layer is formed between the dense thick alloy coating and the surface of the substrate, and a depth of the metallurgical transition layer is less than 0.1 mm, and the composition of the metallurgical transition layer includes any one selected from the group consisting of NiCoCrAlY, NiCrAlY, NiAl, NiAlW, CoCrW, and CoCrMoSi.

The present disclosure will be further described in detail with specific embodiments.

Embodiment 1

A preparation method of a thick MCrAlY coating on an outer ring block of a high-pressure turbine of a gas turbine includes the following steps:

• • step 1, placing the outer ring block of the high-pressure turbine into an ultrasonic cleaning machine, and using alcohol as a cleaning agent for ultrasonic cleaning and oil-removing, with a ultrasonic frequency of 40 kHz, a power density of 0.8 W/cm², a cleaning temperature of 50° C., and a cleaning time of 15 min;
  • step 2, taking out the outer ring block of the high-pressure turbine, drying the outer ring block of the high-pressure turbine by compressed air, and protecting an uncoated area by a sand blowing protective tape;
  • step 3, roughening a surface of a spraying area of the outer ring block of the high-pressure turbine; automatically blowing sand by using a press-in sand blowing gun with 50-mesh fused broken white corundum sand and clamped by a multi-axis manipulator, where the surface roughness Ra after the blowing sand is 6 μm-8 μm; removing the sand blowing protective tape after the blowing sand; and washing and cleaning the blown sand surface with high-pressure water;
  • step 4, putting NiCoCrAlY alloy powder of a type of KF-309 (with a particle size less than 40 μm, and an oxygen content less than 600 ppm) in a barrel, covering the barrel and putting barrel in a blast drying oven to perform drying at 70° ° C. for 2 hours (h), taking the barrel out and shaking the NiCoCrAlY alloy powder evenly, and then adding the NiCoCrAlY alloy powder into a powder feeder;
  • step 5, installing the outer ring block of the high-pressure turbine obtained in the step 3 on a spraying tool, connecting the spraying tool with a spraying guide rod in a transition chamber, debugging an automatic spraying program, checking a spraying distance and a process allowance of spraying flame (i.e., plasma flame), setting a infrared temperature measuring position, and closing a vacuum chamber and the transition chamber;
  • step 6, starting an automatic vacuuming procedure to vacuum the vacuum chamber to reduce a vacuum degree of the vacuum chamber to be 0.01 mbar, backfill argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be 50 mbar, re-vacuum the vacuum chamber to control the vacuum degree of the vacuum chamber to be below 0.01 mbar, and then backfill argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be 0.5 mbar; and controlling a length and a diameter of the plasma flame to be 1000 mm-1200 mm and 200 mm-300 mm respectively;
  • step 7, starting an automatic preheating program to open a gate valve between the vacuum chamber and the transition chamber and put the outer ring block of the high-pressure turbine in the vacuum chamber, and use the flame plasma of the plasma spray gun to quickly blow and preheat a surface of the outer ring block of the high-pressure turbine under a power of the plasma spray gun of 110 kw, a blowing distance of 400 mm, a blowing speed of 500 mm/s, and a preheating time of 5 min, to preheat the substrate to 950° C.
  • step 8, starting the automatic spraying program under the conditions that a spraying power of the plasma spray gun is 105 kw, a powder feeding amount (also referred to as powder feeding speed) is 22 g/min, a relative linear velocity between the plasma flame and a surface of the outer ring block of the high-pressure turbine is 2 m/s, a spraying distance is 500 mm, and spraying times are 200 times; and after spraying, closing the plasma spray gun, returning the outer ring block of the high-pressure turbine to the transition chamber, and closing the gate valve between the vacuum chamber and the transition chamber (a photo of the spraying process is shown in FIG. 1);
  • step 9, starting an automatic cooling program, with cooling rates of gradient cooling stages being 5° C./min(a period of 10 min), 10° C./min(a period of 15 min), 15° C./min(a period of 10 min), 20° C./min(a period of 10 min), 25° C./min(a period of 10 min) and 40° C./min(a period of 5 min) respectively; a flow rate of argon of 0.5 liters per minutes (L/min)-2 L/min, and a total cooling time of 60 min; and after the cooling, opening the transition chamber and taking out the outer ring block of the high-pressure turbine.

After testing, a thickness of NiCoCrAlY coating is 2.0 mm, a porosity thereof is less than 1%, an oxygen content thereof is less than 0.1%, and a bonding strength thereof is 82 MPa. A microstructure of the coating is shown in FIGS. 2A and 2B, and there is no delamination in the coating. After high magnification, there is no layered organizational structure common to the conventional thermal spraying coating.

Embodiment 2

The method of the embodiment 2 is similar to that of the embodiment 1, and the difference is that a vacuum degree in the vacuum chamber in the step 6 in the embodiment 2 is finally backfilled to 0.2 mbar.

There is no delamination in the obtained coating in the embodiment 2, and there is no layered organizational structure in the coating after high magnification. In the embodiment 2, the coating has a bonding strength of 85 MPa-90 MPa, a thickness of 2.0 mm, a porosity less than 0.1%, and an oxygen content less than 0.1%.

Embodiment 3

The method of the embodiment 3 is similar to that of the embodiment 1, and the difference is that the powder feeding amount in the step 8 in the embodiment 3 is 10 g/min.

There is no delamination in the obtained coating in the embodiment 3, and there is no layered organizational structure in the coating after high magnification. In the embodiment 3, the coating has a bonding strength of 80 MPa-85 MPa, a thickness of 2.0 mm, a porosity less than 0.05%, and an oxygen content less than 0.1%.

Embodiment 4

The method of the embodiment 4 is similar to that of the embodiment 1, and the difference is that a spraying power of the plasma spray gun in the step 8 in the embodiment 4 is 55 kw.

There is no delamination in the obtained coating in the embodiment 4, and there is no layered organizational structure in the coating after high magnification. In the embodiment 4, the coating has a bonding strength of 40 MPa-50 MPa, a thickness of 2.0 mm, a porosity less than 5%, and an oxygen content less than 0.1%.

Comparative Example 1

NiCoCrAlY coating is prepared by a method of Chinese patent application publication NO. CN107805775A, and the microstructure of the coating is shown in FIG. 3.

A coating prepared by a method of Chinese patent application publication NO. CN108179371A and the microstructure of the coating is shown in FIG. 4.

Comparative Example 2

The method of the comparative example 2 is similar to that of the embodiment 1, and the difference is that a vacuum degree in the vacuum chamber in the step 6 in the comparative example 2 is 30 mbar.

In the subsequent spraying process, a preheating temperature of the substrate can only reach about 500° C., and there is no delamination in the obtained coating. After high magnification, there is no layered organizational structure in the coating. The bonding strength of the coating is 40 MPa-50 MPa, the porosity thereof is less than 1%, and the oxygen content thereof is less than 0.5%. However, a maximum spraying thickness of the coating is about 0.7 mm, and when the thickness is more than 0.7 mm, an edge of the coating will warp, crack, or even fall off directly.

Comparative Example 3

The method of the comparative example 3 is similar to that of the comparative example 2, and the difference is that the power of the plasma spray gun in the step 8 in the comparative example 3 is 55 kW.

After the coating is enlarged by 20 times, there is a layered organizational structure in the coating, as shown in FIG. 5. The coating has a bonding strength of 45 MPa, a thickness of 2.0 mm, a porosity less than 5%, and an oxygen content less than 0.1%.

Comparative Example 4

The method of the comparative example 4 is similar to that of the embodiment 1, and the difference is that a preheating temperature in the step 7 in the comparative example 4 is 600° C.

There is no delamination in the obtained coating, and there is no layered organizational structure in the coating after high magnification. The coating has a bonding strength of 55 MPa −65 MPa, a thickness of 2.0 mm, a porosity less than 1%, and an oxygen content less than 0.1%. However, an edge of the coating is prone to have defects such as warping and cracking.

It can be seen from the above embodiments and comparative examples that no layered organizational structure can be realized in the coating by adopting the technical solutions of the embodiments of the present disclosure, a thick alloy coating can be prepared, a thickness of the coating can be greater than a thickness limit (i.e., 0.7 mm) of the conventional preparation. In addition, an edge of the coating has no defects such as warping and cracking. However, for the comparative examples of the present disclosure, either the coating has a layered organizational structure therein, or the thickness cannot exceed the conventional thickness limit, or the edge of coating has defects such as warping and cracking.

Further, it can be seen from the embodiments 1 and 4 that by adopting the technical solutions of the embodiments of the present disclosure, the bonding strength of the coating can be further improved and the porosity of the coating can be reduced.

The exemplary embodiments of the present disclosure have been described in detail above, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, including the combination of various technical features in any other suitable way. These simple modifications and combinations should also be regarded as the contents disclosed by the present disclosure and belong to the scope of protection of the present disclosure.

Claims

1. A preparation method of a dense alloy coating without layered organizational structure, wherein no layered organizational structure exists in the dense alloy coating, an edge of the dense alloy coating has no cracking and warping, a thickness of the dense alloy coating is in a range from 2 millimeters (mm) to 3.5 mm, and the preparation method comprises:

step 1, placing a substrate in a plasma spraying apparatus, controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 millibars (mbar) to 0.5 mbar, controlling a length of a plasma flame of the plasma spraying apparatus to be in a range from 1000 mm to 1200 mm, and controlling a diameter of the plasma flame to be in a range from 200 mm to 300 mm;

step 2, preheating, by using a plasma spray gun, the substrate to a temperature in a range from 700 degrees Celsius (° C.) to 1000° C. to obtain a preheated substrate;

step 3, plasma spraying an alloy powder on the preheated substrate to obtain a sprayed substrate, wherein conditions of the plasma spraying comprise: a spraying power is in a range from 80 kilowatts (kw) to 130 kw, a powder feeding speed is in a range from 10 grams per minute (g/min) to 30 g/min, a spraying distance is in a range from 450 mm to 800 mm, and a relative linear velocity between the plasma flame and a surface of the substrate is in a range from 1 meter per second (m/s) to 3 m/s; the alloy powder satisfies the following requirements: a particle size of the alloy powder is smaller than 40 micrometers (μm), and an oxygen content of the alloy powder is less than 600 parts per million (ppm); and the alloy powder comprises at least one selected from the group consisting of NiCoCrAlY, NiCrAlY, NiAl, NiAlW, CoCrW, and CoCrMoSi; and

step 4, cooling the sprayed substrate after the plasma spraying, to thereby obtain the dense alloy coating without layered organizational structure.

2. The preparation method according to claim 1, wherein in the step 1, the controlling a vacuum degree of the plasma spraying apparatus to be in a range from 0.2 mbar to 0.5 mbar comprises:

vacuuming a vacuum chamber in which the substrate is placed of the plasma spraying apparatus to control a vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, filling argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 50 mbar to 100 mbar, re-vacuuming the vacuum chamber to control the vacuum degree of the vacuum chamber to be in a range from 0.005 mbar to 0.01 mbar, and refilling argon into the vacuum chamber to control the vacuum degree of the vacuum chamber to be in the range from 0.2 mbar to 0.5 mbar.

3. The preparation method according to claim 1, wherein the step 1 further comprises: before placing the substrate in the plasma spraying apparatus, performing a roughening treatment on the surface of the substrate.

4. The preparation method according to claim 3, wherein a roughness Ra of the surface of the substrate is in a range from 6 μm to 8 μm through the roughening treatment.

5. The preparation method according to claim 1, wherein in the step 2, the preheating, by using a plasma spray gun, the substrate comprises:

performing blowing and preheating the surface of the substrate by using a flame from the plasma spray gun, wherein a power of the plasma spray gun is controlled to be in a range from 60 kw to 150 kw, a blowing distance for the blowing is in a range from 200 mm to 500 mm, a blowing speed for the blowing is in a range from 200 millimeters per second (mm/s) to 1000 mm/s, and a preheating time for the preheating is in a range from 3 minutes (min) to 8 min.

6. The preparation method according to claim 1, wherein in the step 4, a cooling speed for the cooling is in a range from 5 degrees Celsius per minute (° C./min) to 40° C./min, and a cooling time for the cooling is in a range from 20 min to 60 min.

7. The preparation method according to claim 6, wherein the cooling is performed through a plurality of gradient cooling stages, and a difference between a cooling speed of a latter gradient cooling stage of the plurality of gradient cooling stages and a cooling speed of a former gradient cooling stage of the plurality of gradient cooling stages is in a range from 5° C./min to 20° C./min.

8. (canceled)

9. The preparation method according to claim 1, wherein

a porosity of the dense alloy coating without layered organizational structure is less than 1%, an oxygen content of the dense alloy coating without layered organizational structure is less than 0.1%; and a bonding strength of the dense alloy coating without layered organizational structure is greater than 70 megapascals (MPa).