Thermal spraying technology its application in repairing nodular cast iron crankshaft

Thermal spraying technology uses special equipment a specific heat source to heat metal non-metallic materials to a molten semi-melted state, blow them into tiny particles with high-speed airflow spray them on the surface of the part to form a covering layer to improve the corrosion resistance of the part Surface engineering technology with performance, wear resistance, heat resistance, etc. The diversity of thermal spraying methods, the versatility of preparing coatings, the economy of use are the outstanding features of thermal spraying technology. the wear resistance resistance of medium large steel components To the preparation of special functional coatings in the high-tech field, thermal spraying technology has played a unique role.

The use of this technology can make the surface of the material have wear resistance, corrosion resistance, high temperature oxidation resistance, heat insulation, sealing other properties; sprayable materials include metals, plastics, ceramics, composite materials, etc., are now used in aviation, aerospace, ships, vehicles , Tractors, petrochemicals, textiles, mold manufacturing other fields are widely used. This article focuses on the thermal spray repair process of ductile iron crankshafts for vehicles, studies thermal spray technology its process application in parts repair.

The load that the vehicle crankshaft bears in use is complicated, easy to wear, high production costs, expensive to replace. In order to better extend the life of the vehicle crankshaft increase economic benefits, it is often desirable to repair the worn crankshaft to continue Purpose of use. Among many repair processes, thermal spraying is a reasonable effective method for repairing various parts, which can reasonably effectively improve the hardness, wear resistance, corrosion resistance, etc. of the substrate surface, extend the service life of the parts .

1 Thermal spraying process test

1.1 Principle method of thermal spraying

The whole process of heating the thermal spraying material the whole process of combining the particles the surface of the base material is the key to the thermal spraying repair coating. Although the thermal spraying has many process methods unique characteristics, the whole spraying process coating The formation principle the structure of the coating are basically the same. the thermal spray material entering the heat source to the formation of the coating, the following 4 stages are generally required:

(1) Heating melting of spray material: For thermal spray repair wire, when its end enters the high temperature zone of the heat source, it is heated melted.

(2) Atomization of molten spray material: The molten droplets formed at the end of the repair wire are detached the end of the wire under the action of an external compressed air flow the jet of the heat source itself, are atomized into fine droplets to be sprayed forward

(3) Jet flight of droplet fine particles: During the entire flight of the atomized particles, the premise is accelerated to form a particle stream.As the flight distance increases, the particle's movement speed gradually decreases.

(4) The whole process of collision, deformation, solidification accumulation of moving particles on the surface of the matrix material: When particles with a certain environmental temperature speed contact the surface of the matrix material, the particles collide strongly with the surface of the matrix material, the kinetic energy of the particles It is converted into heat energy a part of its energy is transferred to the matrix material. At the same time, the particles deform along the uneven surface, lose heat condense, shrink adhere to the surface of the matrix material in a flat shape. Because of the sprayed particle beam Continuously hit the surface of the base material, continuously produce the whole process of collision→deformation→condensation→shrinkage, so that the deformed particles the surface of the base material, as well as the deformed particles the deformed particles, have a bonding effect with each other, thereby forming a coating. .

In the spraying process, the particles will chemically react with the surrounding medium during the process of melting, accelerating, flying, contacting the surface of the matrix material, so that the sprayed material will appear oxide after spraying, due to the continuous stacking of particles some particles The rebound loss of the particles will inevitably form some pores cavities between the particles. Therefore, the spray coating is composed of deformed particles, pores oxides.

There are many methods of thermal spraying. The method commonly used for parts repair is generally flame wire spraying. Flame wire spraying is an earlier thermal spraying method. The spraying principle is to send the wire into the burning flame at a certain controlled speed. The end of the heated wire is melted, the molten stream is sprayed, atomized, accelerated by compressed air to form a coating on the surface of the base material. Due to the insufficient enthalpy of the molten particles in this method, the coating the surface of the base material are It is mainly mechanically bonded, its bonding strength is low. At the same time, due to the uneven fusing spraying of the wire, the properties of the coating are uneven, the structure is loose, porous, the internal stress is large.

1.2 Test materials test equipment

The matrix material is the nodular cast iron crankshaft material QT700-02, the metallographic microstructure of the matrix is lower bainite; the selected sample material is a worn nodular cast iron crankshaft test block, the appropriate surface is selected as the research object. Domestic T8A steel wire; use oxygen-acetylene flame for thermal spray repair test.

Experimental research equipment: Grinding sandblasting are used to roughen the surface of the sample; high-temperature resistance heating furnace is used to preheat the test block; thermal spraying uses an air compressor with a displacement of 1.5m3/h, SQP-1 Type spraying machine; 4XB type metallographic microscope is used for metallographic microstructure analysis; M-200 wear tester is used for surface wear resistance measurement of sprayed coating; energy spectrum analysis grain determination methods are used to analyze its structure grain Size etc.

1.3 Test method

When using oxygen-acetylene flame for thermal spraying repair, the process flow is: surface treatment of the repaired workpiece → preheating → thermal spraying bottom layer → thermal spraying working layer → spraying post-treatment.

(1) Surface treatment of the repaired workpiece

In order to improve the bonding strength of the coating material the crankshaft base material, so that the sprayed particles the microscopically uneven base material surface tightly bite, for this reason, the surface of the repaired workpiece must be clean, rough fresh. It includes surface cleaning surface roughness. Two processes are processed.

Surface cleaning includes a series of processes such as degreasing, oil removal, rust removal, etc., to make the surface of the crankshaft base material clean, fresh, metallic. Surface treatment often uses pickling, sandblasting rust removal to remove oxide scale; organic solvents alkaline water are used to remove Oil stains; use 300～320℃ heating heat preservation for 2～2.5h to remove oil stains in the pores of old crankshaft parts.

The surface roughness treatment is carried out by a combination of grinding sandblasting. After the rough treatment, a necessary surface cleaning process is carried out. The purpose is to further clean the surface of the repaired workpiece, ensure its surface quality, improve the spraying material the crankshaft base material. The bonding strength between the two, the coating thickness is reserved.

(2) Preheating of the repaired workpiece

The repaired workpiece should be preheated before spraying repair. The main purpose is to increase the contact temperature between the surface of the workpiece the molten particles, so as to facilitate the deformation of the molten particles, the mutual engagement with the crankshaft matrix material, the increase of the deposition rate of the sprayed material; Reduce the internal stress of the crystal lattice the condensation speed of the molten particles, increase the interatomic bonding force between the sprayed material the crankshaft base material; reduce the shrinkage difference the internal stress of the sprayed material the crankshaft base material during condensation. For ductile iron crankshaft materials, 130 ～150℃ is suitable. In order to make the temperature of the workpiece uniform no moisture is generated on the surface of the workpiece, the resistance furnace is generally used for preheating, the spray gun can also be used for preheating.

(3) Thermal spray primer

In order to increase the bonding strength between the spray coating the crankshaft base material, a layer of nickel-clad aluminum aluminum-clad nickel powder can be sprayed as a thermal spray primer before the formal spraying repair. The general spraying thickness is about 0.10 ~ 0.15mm, neutral Flame spraying is economical has good bonding strength.

(4) Thermal spray working layer

Thermal spraying for crankshaft repair is mainly composed of four processes, namely, the melting of metal spraying materials, the atomization of liquid metal, the flight transition of liquid metal particles, the adhesion of metal particles. According to the specific conditions of spraying heat source, spraying material, spraying equipment, etc., The spraying distance is generally 100-150mm, the spraying angle is generally 60-90°. In order to study the quality of metal thermal spraying, different processes have been tested the thermal spraying distance coating thickness. The spraying distance is 100, 4 kinds of 120, 140, 160mm, etc., choose 0.25, 0.5, 1, 1.5mm, etc. for the coating thickness. During the spraying process, the working temperature is controlled within the range of 200～220℃ to prevent the temperature being too high Affect the bonding strength of the sprayed material the crankshaft base material, cause the deformation of the workpiece, change the structure performance of the crankshaft base material.

(5) Treatment after spraying

The flame thermal spray coating has a porous structure, which only has an adverse effect on the performance of the coating working under corrosive media conditions, has little effect on the wear-resistant coating performance of the ductile iron crankshaft. However, for the ductile iron crankshaft after spraying Mechanical finishing is required. Generally, finishing grinding are used. The amount of feed for finishing should be small to effectively prevent turning cracks; when grinding, you should try to choose sharp grinding wheels , In order to improve the surface quality of the crankshaft; oil immersion treatment after processing. 2 Test results analysis

2.1 Organization performance of workpiece repair layer

After thermal spraying repair, the surface layer of the ductile iron crankshaft is composed of three parts of 28 Hunan Institute of Engineering Journal 2004, namely the surface coating, the Ni-containing roughening layer the ductile iron surface layer. The metallographic analysis shows that the repaired The surface coating of the crankshaft is composed of a metal surface layer, gray pores black spots. The gray pores are oxides the black spots are holes; the energy spectrum analysis shows that the surface coating is mainly composed of FeO, Fe3O4 F. Gray. The hole is caused by the formation of an oxide film on the surface layer of the particles by oxidation of the metal droplets formed during the thermal spraying process when the heated melted T8A steel wire is subjected to the action of compressed air. After the high-speed flying particles of the film hit the roughened layer, they are squeezed into a disc shape, holes are formed at the particle interval.

After thermal spraying repairs, a metal coating is formed on the surface of the crankshaft. The mechanical properties of the coating show that the coating strength σb is 80～81MPa, the hardness is 30～31HRC, the bonding strength σb between the coating the substrate is 44～45MPa. , The tensile strength of the coating is significantly higher than the bonding strength of the coating the substrate, which can effectively meet the strength requirements after the crankshaft is repaired. At the same time, the wear test results conducted on the M-200 wear tester show that the thermal spray coating has abrasion resistance The performance is equivalent to the wear resistance requirements of ductile iron crankshafts, which can meet the performance requirements of crankshafts.

2.2 Effective process parameters of spray coating

Through metallographic analysis, it can be seen that the crystal grains of the thermal spray repair layer present an irregular shape. According to the oxidation characteristics of the thermal spray coating, if the part surrounded by the oxide is regarded as a crystal grain, the average crystal grain diameter is about 8.8~ 9.6μm, the oxidation rate of the coating is about 28-31%, the porosity is about 8.5-11.3%; the test results show that as the spraying distance increases, the grain size of the coating decreases, the oxidation rate of the coating the porosity increases; as the thickness of the coating increases, the strength of the coating the base material does change much, but its elongation decreases the bonding performance becomes worse.

Comprehensive test results show that when the thermal spraying distance is 120mm the coating thickness is 0.5mm, the thermal spraying layer exhibits good tensile strength, hardness, wear resistance the bonding performance of the coating the base material; The average diameter of the crystal grains is about 9.1μm, the porosity is about 10.2%, the oxidation rate is about 30%.

3 concluding remarks

Thermal spraying technology is an advanced surface treatment method, which has broad application prospects in the repair of worn parts. Through thermal spraying repair, the surface performance of ductile iron crankshaft meets its performance requirements. After thermal spraying, the mechanical Finishing meets the size surface accuracy requirements of the crankshaft, can keep it in service, effectively saving production costs improving economic benefits.