Preparation method of piston surface composite coating

Abstract

The invention discloses a preparation method of a piston surface composite coating. The preparation method is characterized by comprising the following steps: firstly, presetting a ceramic layer in dot matrix distribution on the top of a piston by using cold spraying powder; then an oxidation tool is used for preparing a composite ceramic coating fused with the dot matrix ceramic coating in gaps of the dot matrix ceramic coating; and finally, annealing treatment is performed. The cold spraying powder comprises the following components in percentage by weight: 5-10% of hexamethyldisiloxane and 0-1% of absolute ethyl alcohol, with the balance being solid components; wherein the solid component is a mixture of nano aluminum oxide, silicon carbide and aluminum powder; according to the oxidation tool, a base is arranged at the bottom of a piston, a reaction tank jacket is arranged at the upper end, and the reaction tank jacket is tightly combined with the upper edge contact part of the piston. Cold spraying is used as pretreatment, various functional particles with large performance difference can be introduced more easily, and excellent performance is obtained; the reaction tank jacket with the brim structure can prevent the electrolyte from being contacted with the side surface of the piston, so that the risks of oxidation and ablation of the side surface of the piston are reduced, and the influence of subsequent processes on the production efficiency is avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of surface treatment, and particularly relates to a piston surface composite coating preparation method. BACKGROUND

[0002] The piston top micro-arc oxidation technology has entered the practical stage in the field of high-performance internal combustion engines, and is mainly used as a strengthening means for the surface of aluminum pistons to improve the heat resistance, wear resistance and ablation resistance of the top surface. In the process, a ceramic layer mainly composed of alpha-Al2O3 and gamma-Al2O3 with a thickness of 20-80 microns is generated in a silicate-phosphate composite electrolyte in a constant voltage or constant current mode, and the hardness can reach 1200-1600 HV, so that the instantaneous temperature resistance of the piston top surface is increased by 150-200 DEG C. Domestic and foreign engine manufacturers superimpose micro-arc oxidation with internal cooling oil channels, ceramic ring inserts, anode oxidation ring grooves and other schemes to form a "multi-layer heat insulation-wear resistant" system, which has been applied in mass production of passenger car gasoline engines, gas engines and light diesel engines, and the service life is increased by more than 30%. However, the high-thickness film layer easily causes the increase of the roughness of the top surface, the poor matching of thermal expansion, and the easy cracking and peeling of the coating, and thus the porosity needs to be reduced through the use of nano-particle composite electrolyte, pulse energy regulation and post-treatment sealing.

[0003] The above solution method which only optimizes the oxidation process still has many limitations for aluminum alloy pistons which are used in a cyclic thermal shock environment for a long time, such as that in a conventional oxidation process, the functional particles in the electrolyte have a large difference in properties from the matrix, the proportion of the functional particles participating in the oxidation film formation is low, the internal stress of the oxidation film is large, and the risk of cracking and peeling of the coating is large. Therefore, it is necessary to develop a coating preparation method with good ablation resistance and thermal shock resistance. SUMMARY

[0004] The first technical problem to be solved by the present application is to provide a piston surface composite coating preparation method for piston end face protection and coating with strong thermal shock resistance, which uses ablation-resistant cold spraying powder to prepare an ablation-resistant pre-layer on the end face of an aluminum alloy piston, and uses a piston local oxidation tooling to inhibit the edge ablation effect,

[0005] The technical solution adopted by the present application to solve the above technical problem is as follows: a composite ceramic coating preparation method, characterized by: first, using cold spraying powder to pre-arrange a dot-matrix distributed ceramic layer on the top of the piston; then using an oxidation tooling to prepare a composite ceramic coating layer which is fused with the dot-matrix ceramic coating layer in the gap of the dot-matrix ceramic coating layer; and finally performing annealing treatment.

[0006] Further, the ablation-resistant cold spraying powder is prepared from the following raw materials in the following mass percentages:

[0007] Solid component: balance;

[0008] Assistant one: 5-10%;

[0009] Assistant two: 0-1%;

[0010] The solid component is a mixture of nano-alumina, nano-silicon carbide and nano-aluminum powder;

[0011] Assistant one is hexamethyl disiloxane;

[0012] Assistant two is anhydrous ethanol.

[0013] Further preferably, the cold spraying powder is prepared from the following mass percentage of raw materials:

[0014] Alumina: 30-40%;

[0015] Silicon carbide: 10-20%;

[0016] Aluminum powder: 40-50%;

[0017] Hexamethyl disiloxane: 5-10%;

[0018] Anhydrous ethanol: 0-1%;

[0019] Each solid phase particle diameter: ≤50nm.

[0020] Finally, the preparation process of the powder is: according to the formula, first mix and stir the solid powder uniformly for standby, then prepare the liquid mixture of assistant one and assistant two, and finally mix the solid mixture and the liquid mixture within 12h before use, and stir uniformly to obtain the cold spraying powder.

[0021] Further, the cold spraying method is: the compressed gas is nitrogen, the powder spraying speed is 700-900m / s, the powder deposition speed is 1-2μm / min; the average thickness of the cold spraying layer is 10-15μm.

[0022] Further, the micro-arc oxidation electrolyte is a mixture of sodium hexametaphosphate, sodium tungstate, isopropyl alcohol and deionized water, and the mass percentage of each component is 10-20%, 2-8%, 0.5-2%, and the balance is deionized water.

[0023] Further, the micro-arc oxidation process is:

[0024] Oxidation mode: double pulse, constant current oxidation

[0025] Positive current: 35-50A; frequency: 1000-3000Hz; duty cycle: 20-50%;

[0026] Negative current: 5-15A; frequency: 800-1500Hz; duty cycle: 10-30%;

[0027] Positive pulse number ratio 100: (20-50);

[0028] Oxidation time: 25-40 min;

[0029] Electrolyte temperature: 10-15℃.

[0030] Further, the annealing process is achieved by adjusting the workpiece power parameters to control the workpiece self-heating, and the stage is:

[0031] Power supply mode: multi-stage, single pulse, constant current;

[0032] The frequency is 300-800 Hz, the duty cycle is 80-100%, the power-on time is 5-10 min, and the current is uniformly reduced to 0A in multiple stages, and the current reduction amplitude of each stage is 2-5A;

[0033] Further, the electrolyte used in the annealing process has the same composition as the micro-arc oxidation electrolyte, and the electrolyte temperature is normal temperature, without refrigeration.

[0034] Further, the oxidation tool for locally micro-arc oxidation of the piston comprises a base arranged at the bottom of the piston to be processed, an anode connecting rod arranged on the side of the base, and an upwardly protruding base boss on the base, wherein the base boss is assembled and connected with the groove at the bottom of the piston.

[0035] A reaction tank cover is arranged at the upper end of the piston, and a cathode is arranged inside the reaction tank cover, and a cathode connecting rod is arranged on the cathode.

[0036] Further, the base is made of aluminum alloy, and the outer diameter of the base is equal to the outer diameter of the piston, and the outer shape of the base boss is the same as the inner contour structure of the groove at the bottom of the piston, which can be closely fitted and positioned.

[0037] Further, the reaction tank cover is a grass hat structure without a top, and the reaction tank cover is divided into three layers, which are the uppermost vertical section, the middle small horn mouth and the bottom large horn mouth, the angle between the inner wall and the outer wall of the small horn mouth is 2-5°, the angle between the inner wall and the outer wall of the large horn mouth and the horizontal plane is 30-60°, a rubber gasket is arranged on the inner wall of the small horn mouth, and the contact part between the reaction tank cover and the upper edge of the piston is closely combined, and when the electrolyte is injected into the inside of the reaction tank cover, the electrolyte will not penetrate and flow to the side wall of the piston along the contact part.

[0038] Finally, the material of the reaction tank cover is aluminum alloy, and the material of the base is aluminum alloy, and the outer diameter of the base is equal to the outer diameter of the piston, the material of the cathode is stainless steel, the diameter of the cathode is 0.5-0.7 times the diameter of the piston, the cathode is below the upper edge of the reaction tank cover, and an insulating layer is arranged on the outer layer of the anode connecting rod.

[0039] The small horn mouth upper end inner circle diameter is less than the outer diameter of the piston, and the lower end inner circle diameter is greater than the outer diameter of the piston, and the large horn mouth lower end outer diameter is 1.5-2 times of the diameter of the piston.

[0040] The method for using the tool is that electrolyte is continuously injected into the reaction tank, so that the electrolyte is above the ring cathode, then overflows from the upper edge, and then the power is turned on, so that the micro-arc oxidation reaction occurs in the cylindrical space formed by the piston top and the reaction tank sleeve.

[0041] Compared with the prior art, the advantages of the present application are that:

[0042] I. Cold spraying as a pretreatment can more easily introduce a variety of functional particles with large performance differences on the surface of the workpiece, and then through micro-arc oxidation treatment, the content of functional particles in the composite coating can be effectively improved to obtain excellent performance.

[0043] II. Adding a large amount of nano-particle aluminum powder in the cold spraying powder can make the mixture coating more closely combined with the aluminum piston, and the hexamethyldisiloxane in the preposition layer can provide in-situ Si atoms for the micro-arc oxidation reaction in the second step, compared with adding Si elements in the electrolyte, the content of SiO2 in the composite ceramic layer can be increased and the distribution can be more uniform.

[0044] III. The piston has high dimensional accuracy and large thermal deformation, and by adjusting the content of hexamethyldisiloxane and anhydrous ethanol in the spraying powder, the spraying temperature can be controlled through evaporation and thermal decomposition, thereby reducing the influence of the spraying treatment on the dimensional tolerance of the piston.

[0045] IV. The hat brim structure of the reaction tank sleeve can prevent the electrolyte from contacting the side surface of the piston, thereby reducing the risk of oxidation of the side surface of the piston and avoiding the influence of the subsequent polishing process on the production efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is a structure diagram of the oxidation tool provided by the present application;

[0047] Figure 2 is a sectional view of the oxidation tool provided by the present application;

[0048] Figure 3 is a front sectional view of the reaction tank sleeve provided by the present application;

[0049] Figure 4 is a base structure diagram provided by the present application;

[0050] In the above diagram: 1, reaction tank sleeve, 2, cathode connecting rod, 3, cathode, 4, base, 5, anode connecting rod, 6, piston, 7, vertical section, 8, inclined section, 9, rubber gasket, 10, outer edge portion, 11, base boss. DETAILED DESCRIPTION

[0051] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0052] An ablation-resistant cold spray powder system, the powder system being formulated from the following raw materials in the indicated mass percentages:

[0053] Solid composition: Balance;

[0054] Additive 1: 5-10%

[0055] Additive 2: 0-1%;

[0056] The solid components are a mixture of nano-alumina, silicon carbide and aluminum powder;

[0057] Additive 1 is hexamethyldisiloxane;

[0058] Additive 2 is anhydrous ethanol.

[0059] More preferably, the cold spray powder is formulated from the following raw materials in the indicated mass percentages:

[0060] Alumina: 30-40%;

[0061] Silicon carbide: 10-20%;

[0062] Aluminum powder: 40-50%;

[0063] Hexamethyldisiloxane: 5-10%;

[0064] Anhydrous ethanol: 0–1%;

[0065] Diameter of each solid particle: ≤50nm.

[0066] Finally, the powder preparation process is as follows: the ingredients are prepared according to the formula. First, the solid powder is mixed and stirred evenly for later use. Then, a liquid mixture of additive one and additive two is prepared. Finally, the solid mixture and liquid mixture are mixed and stirred evenly within 12 hours before use to obtain the required cold spray powder.

[0067] like Figure 1 As shown, a piston local oxidation tooling includes a piston 6, a base 4 at the bottom of the piston 6, a reaction tank jacket 1 at the upper end of the piston 6, and a cathode 3 in the area formed by the reaction tank jacket 1 and the top of the piston 6.

[0068] Typically, the piston 6 has a groove at its bottom, and the base 4 has an anode connecting rod 5 on its side. The anode connecting rod 5 is connected to the positive electrode of the micro-arc oxidation power supply. The upper part of the base 4 has a boss 11. The outer contour of the boss 11 is the same as the inner contour of the groove at the bottom of the piston 6, forming a tight fit during assembly. The cathode 3 has a ring structure, and the cathode 3 has a cathode connecting rod 2. The cathode connecting rod 2 is connected to the negative electrode of the micro-arc oxidation power supply.

[0069] The outer casing 1 of the reaction tank is a topless straw hat structure. The outer casing 1 of the reaction tank is divided into three layers: the uppermost vertical section 7, the middle small flared mouth 8, and the bottom large flared mouth 10. The angle between the inner and outer walls of the small flared mouth 8 is 2 to 5°, and the angle between the inner and outer walls of the large flared mouth 10 and the horizontal plane is 30 to 60°. A rubber gasket 9 is provided on the inner wall of the small flared mouth 8.

[0070] In this oxidation fixture, the contact area between the outer sleeve 1 of the reaction tank and the upper edge of the piston 6 is tightly sealed. When electrolyte is injected into the inner part of the outer sleeve 1 of the reaction tank, the electrolyte will not seep through the contact area to the side wall of the piston 6.

[0071] In this oxidation fixture, the outer casing 1 of the reaction tank is made of aluminum alloy, the base 4 is made of aluminum alloy and its outer diameter is equal to that of the piston 6, the cathode 3 is made of stainless steel and its diameter is 0.6 times that of the piston 6, the cathode 3 is located below the upper edge of the outer casing 1 of the reaction tank, and the outer layer of the anode connecting rod is provided with an insulating layer.

[0072] In this oxidation fixture, the inner diameter of the upper end of the small flared mouth 8 is smaller than the outer diameter of the piston 6, and the inner diameter of the lower end is larger than the outer diameter of the piston 6, while the outer diameter of the lower end of the large flared mouth 10 is 1.5 times the piston diameter.

[0073] Finally, when the piston local oxidation fixture is working, the electrolyte is continuously injected into the interior of the reaction tank, so that the electrolyte covers the annular cathode 3 and then overflows from the upper edge. Then the oxidation power supply is turned on, and a micro-arc oxidation reaction occurs in the cylindrical space formed by the top of the piston 6 and the outer sleeve 1 of the reaction tank.

[0074] A method for preparing a composite ceramic coating comprises the following steps: first, a ceramic layer with a dotted distribution is pre-placed on the piston end using the above-mentioned cold spray powder; then, a composite ceramic coating fused with the cold spray coating is prepared in the gaps of the dotted ceramic coating using the above-mentioned oxidation tool; finally, post-treatment is performed to eliminate the internal stress of the coating.

[0075] The cold spraying process is as follows: the compressed gas is nitrogen, the powder spraying speed is 700-900 m / s, the powder deposition speed is 1-2 μm / min, and the average thickness of the cold spray coating is 10-15 μm.

[0076] The above-mentioned oxidation fixture is used as follows: a constant-temperature electrolyte is continuously injected into the reaction zone at the top of the reaction tank jacket 1 and piston 6, and then overflows from the upper edge of the reaction tank jacket 1, flows down along the reaction tank jacket 1, and returns to the electrolyte tank; the power is turned on, and a micro-arc oxidation reaction occurs in the area at the top of the reaction tank jacket 1 and piston 6.

[0077] The composition and mass ratio of the micro-arc oxidation electrolyte are as follows:

[0078] Sodium hexametaphosphate: 10-20%

[0079] Sodium tungstate: 2-8%

[0080] Isopropanol: 0.5–2%

[0081] The remainder is deionized water.

[0082] The above micro-arc oxidation process is as follows:

[0083] Oxidation mode: Dual-pulse, constant-current oxidation;

[0084] Positive current: 35-50A; Frequency: 1000-3000Hz; Duty cycle: 20-50%;

[0085] Negative current: 5–20A; Frequency: 800–1500Hz; Duty cycle: 10–30%;

[0086] The ratio of positive to negative pulses is 100:(20-50);

[0087] Oxidation time: 25–40 min;

[0088] Electrolyte temperature: 10~15℃.

[0089] The final post-processing is achieved by adjusting the electrical parameters of the workpiece to control its self-heating.

[0090] The post-processing power supply mode is: multi-stage, single pulse, constant current;

[0091] Within each phase:

[0092] The power supply frequency is 300–800 Hz.

[0093] Duty cycle: 80-100%;

[0094] Power-on time: 5-10 minutes;

[0095] The current drops uniformly to 0A, with an average decrease of 2-5A in each stage.

[0096] The electrolyte used in the above post-treatment has the same composition as the electrolyte for micro-arc oxidation. The temperature of the post-treatment electrolyte is room temperature, that is, it is not cooled by a refrigeration unit.

[0097] The present invention will be further illustrated below through specific embodiments and comparative examples:

[0098]

[0099]

[0100] The above-mentioned cyclic thermal shock test cycle is as follows: the workpiece is heated to 200°C, held at that temperature for 5 minutes, and then quickly immersed in deionized water at room temperature to cool to room temperature.

[0101] The number of cyclic thermal shocks is the number of cycles that cause new cracks to form on the workpiece.

[0102] The number of cycles of thermal shock resistance of the composite coating shows that the solution provided by this invention can significantly improve the service performance of the coating.

[0103] The above description is merely a partial preferred embodiment of the present invention. Any person skilled in the art can modify the above-described technical solutions or modify them into equivalent technical solutions. Therefore, any simple modifications or equivalent transformations made based on the technical solutions of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a composite coating on a piston surface, characterized in that: First, a ceramic layer with a dotted distribution is pre-placed on the piston end using cold spray powder; Then, an oxidation tool was used to prepare a composite ceramic coating that was fused with the cold-sprayed coating in the gaps of the lattice ceramic coating; Finally, post-treatment is performed to eliminate internal stress in the coating.

2. The method for preparing a composite coating on the piston surface according to claim 1, characterized in that... The cold spray powder is formulated from the following raw materials in the indicated mass percentages: Additive 1: 5-10% Additive 2: 0-1%; Solid composition: Balance; The solid components are a mixture of alumina, silicon carbide and aluminum powder; Additive 1 is hexamethyldisiloxane; Additive 2 is anhydrous ethanol.

3. The method for preparing a composite coating on the piston surface according to claim 2, characterized in that... The cold spray powder is formulated from the following raw materials in the indicated mass percentages: Alumina: 30-40%; Silicon carbide: 10-20%; Aluminum powder: 40-50%; Hexamethyldisiloxane: 0.045–0.18%; Anhydrous ethanol: 0–1%; Diameter of each solid particle: ≤50nm; The powder preparation process is as follows: First, the solid powder is mixed and stirred evenly for later use. Then, a liquid mixture of additive one and additive two is prepared. Finally, the solid mixture and the liquid mixture are mixed and stirred evenly within 12 hours before use to obtain the required cold spray powder.

4. The method for preparing a composite coating on a piston surface according to claim 3, characterized in that... The cold spraying method is as follows: the compressed gas used is nitrogen, the powder spraying speed is 700-900 m / s, the powder deposition speed is 1-2 μm / min, and the average thickness of the cold spray coating is 10-15 μm.

5. The method for preparing a composite coating on a piston surface according to claim 1, characterized in that... The micro-arc oxidation electrolyte in the oxidation tooling is a mixture of sodium hexametaphosphate, sodium tungstate, isopropanol and deionized water, with the following mass percentages of each component: 10-20%, 2-8%, 0.5-2%, and the remainder being deionized water. The micro-arc oxidation process is as follows: dual-pulse, constant-current oxidation; positive current 35-50A, frequency 1000-3000Hz, duty cycle 20-50%; negative current 5-20A, frequency 800-1500Hz, duty cycle 10-30%; positive-to-negative pulse ratio 100:20-50; oxidation time 25-40min; electrolyte temperature: 10-15℃.

6. The method for preparing a composite coating on a piston surface according to claim 1, characterized in that... The post-treatment involves adjusting the workpiece's electrical parameters to achieve self-heating and annealing of the composite coating, thereby eliminating internal stress. The electrical parameters in this stage are multi-stage, single-pulse, and constant current. In each stage, the power frequency is 300–800 Hz, the duty cycle is 80–100%, the energizing time is 5–10 min, and the current is uniformly reduced to 0 A. The electrolyte used in the post-treatment has the same composition as the micro-arc oxidation electrolyte, and the temperature of the post-treatment electrolyte is room temperature.

7. The method for preparing a composite coating on a piston surface according to claim 1, characterized in that... The oxidation tooling for localized oxidation of the piston includes: A base is set at the bottom of the piston to be processed. An anode connecting rod is provided on the side of the base. The upper part of the base is an upward-protruding base boss, which is assembled and connected to the groove at the bottom of the piston. A reaction tank jacket is located at the upper end of the piston. Inside the reaction tank jacket is a cathode, and a cathode connecting rod is provided on the cathode.

8. The method for preparing a composite coating on a piston surface according to claim 7, characterized in that... The base is made of aluminum alloy, and its outer diameter is equal to that of the piston. The outer contour of the base boss is the same as the inner contour of the piston bottom groove, which can fit together for positioning.

9. The method for preparing a composite coating on a piston surface according to claim 7, characterized in that... The reaction tank outer casing has a topless straw hat structure and consists of three layers: a vertical section at the top, a small flared opening in the middle, and a large flared opening at the bottom. The angle between the inner and outer walls of the small flared opening is 2-5°, and the angle between the inner and outer walls of the large flared opening and the horizontal plane is 30-60°. A rubber gasket is provided on the inner wall of the small flared opening. The reaction tank outer casing is tightly connected to the upper edge of the piston. When electrolyte is injected into the reaction tank outer casing, the electrolyte will not seep through the contact area to the side wall of the piston.

10. The method for preparing a composite coating on a piston surface according to claim 7, characterized in that... The outer casing of the reaction tank is made of aluminum alloy, the base is made of aluminum alloy, and its outer diameter is equal to that of the piston. The cathode is made of stainless steel, and its diameter is 0.5 to 0.7 times that of the piston. The cathode is located below the upper edge of the outer casing of the reaction tank. The outer layer of the anode connecting rod is provided with an insulating layer. The inner diameter of the upper end of the small flared mouth is smaller than the outer diameter of the piston, and the inner diameter of the lower end is larger than the outer diameter of the piston. The outer diameter of the lower end of the large flared mouth is 1.5 to 2 times the diameter of the piston.

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