Metal Surface Engineering Technology: The Ultimate Guide

Surface phenomena and surface changes are common in nature.

In engineering, almost all parts are inevitably in contact with the environment, and it is the surface of the parts that are in direct contact with the environment.

When the surface is in the process of environmental interaction, corrosion, abrasion, oxidation, and erosion will often occur, which will cause damage or failure of components soaring, and then cause damage or failure of components.

Therefore, the surface is the first line to prevent equipment failure.

Surface engineering refers to the improvement of the morphology and chemical composition of the solid metal surface or non-metal surface through surface strengthening, surface modification or multiple surface engineering techniques after surface pretreatment.

Systematic engineering of organization structure and stress state to obtain the required surface properties.

Metal surface strengthening technology

01 Surface deformation strengthening

Surface deformation strengthening is to produce compression deformation on the metal surface through mechanical means (rolling, shot peening, etc.) to form a hardened layer on the surface.

The depth of the deformation hardening layer can reach 0.15-1.5mm,

The methods of surface deformation strengthening mainly include: shot peening, surface rolling technology, and hole extrusion strengthening.

During the compression process, the following two changes will occur in the deformation hardened layer:

(1) From the perspective of organizational structure, the density of dislocations in the strengthening layer is extremely high, and the distortion of the crystal lattice is large.

Under the action of alternating stress, dislocations with opposite signs will cancel each other when they meet, and dislocations with the same sign will be rearranged.

At this time, although the dislocation density in the strengthening layer has decreased, finer sub-grains will gradually form.

(2) From the point of view of the stress state, due to the unbalanced degree of metal deformation between the surface layer and the inner layer, when the surface layer metal is plastically extended to the surroundings, it will be hindered by the inner layer metal, forming a higher macroscopic residual pressure in the strengthening layer stress.

1. Shot peening strengthening

Shot peening, also known as controlled shot peening, is to spray a high-speed projectile stream onto the surface of the part to plastically deform the surface of the part, thereby forming a strengthened layer with a certain thickness.

Due to the presence of compressive stress on the surface of the part, part of the stress can be offset when the part is under load, thereby improving the fatigue strength of the part. Shot peening is shown in Figure 1.

Schematic diagram of shot peening strengthening process

Figure 1 Schematic diagram of shot peening strengthening process

At room temperature, small hard projectiles sprayed at high speed are used to hit the surface of the workpiece, causing the surface layer to produce elastic and plastic deformation at the recrystallization temperature.

As shown in Figure 2, and presents a large residual compressive stress,

Because when each steel shot hits the metal part, it is like a miniature rod fading and hitting the surface, making small indentations or depressions.

In order to form depressions, the metal surface layer must be stretched.

Under the surface layer, the compressed grains try to restore the surface to its original shape, thereby creating a hemisphere under the action of a high degree of compression.

Numerous depressions overlap to form a uniform residual compressive stress layer, thereby improving surface fatigue strength and stress corrosion resistance.

Plastic deformation of the shot peening surface

Figure 2 Plastic deformation of the shot peening surface

Shot peening can also be used to remove oxide skin, rust, sand and old varnish from medium and large metal products with a thickness of 2mm or less or from castings and forgings that are not required to maintain accurate dimensions and contours, as a method of cleaning the surface before coating (plating).

Shot peening is a cold treatment process that is widely used to improve the anti-fatigue properties of metal parts that are in long-term service under high stress conditions, such as aircraft engine compressor blades, fuselage structural parts, and automotive transmission system parts.

According to the speed of the projectile, shot peening is divided into ordinary shot peening and supersonic surface shot peening.

The projectile speed of the supersonic spray gun is 300~500m/s, and as the part rotates, it can achieve shot peening on the entire surface of the part.

(1) Equipment for shot peening

According to the way to drive the shot, the shot peening machine can be divided into mechanical centrifugal shot peening machine and pneumatic shot peening machine two categories.

In addition, the shot peening machine has a dry spray and wet spray.

Dry spray shot peening machine working conditions are poor, wet spray shot peening machine is to mix the projectile into suspension, and then spray the projectile, so working conditions have improved.

① Mechanical centrifugal shot peening machine in the high-speed rotation of the blade and the impeller under the action of centrifugal force was accelerated to throw.

This type of shot peening machine has small peening power and high manufacturing costs.

It is mainly used for workpieces with high peening strength, few varieties, large batches, simple shapes and large sizes, as shown in Figure 3.

Mechanical centrifugal shot peening machine

  • 1 – Impeller
  • 2 – Impeller steering
  • 3 – Projectile prior to contact with blade
  • 4 – Pellet delivery tube
  • 5 – Hopper
  • 6 – Compressed air
  • 7 – Jet pipe
  • 8 -90° bent nozzles
  • 9 – Projectiles

Figure 3 Mechanical centrifugal shot peening machine

②Pneumatic centrifugal shot peening machine with compressed air as the driving force to accelerate the projectile to a higher speed, and then the projectile hit the surface of the workpiece to be sprayed.

The machine can control the air pressure to control the intensity of the shot peening, which has flexible operation.

One machine can spray multiple parts.

It is suitable for parts with low shot peening strength, large variety, small batch size, complex shape and small size, but high power consumption and low productivity, as shown in Figure 4.

Pneumatic centrifugal shot peening machine

  • 1 – Parts;
  • 2 – Valves;
  • 3 – Air filters;
  • 4 – Piping;
  • 5 – Nozzle;
  • 6 – Shot pipe;
  • 7 – Shot tank;
  • 8 – Dust extraction pipe;
  • 9 – Transfer port

Figure 4 Pneumatic centrifugal shot peening machine

(2) Types of projectiles

Wire cutting pellets: The commonly used wire is diameter d = 0.4 ~ 1.2mm, hardness to 45 ~ 50HRC.

The organization is the best tempered M or B.

Cast steel pellets: pill size 0.2 ~ 1.5mm;

After annealing, the hardness is 30 ~ 57 HRC.

It is fragile with large consumption, but the price is cheap.

The quality of the cast steel shot is related to the carbon content, the general carbon content in 0.85% ~ 1.2%, manganese content in 0.65% ~ 1.2%.

Glass shot: it contains 60% SiO2 with a hardness of 46 ~ 50HRC.

It has a big brittle, which is suitable for the occasion that parts have lower hardness than that of the bullet.

Ceramic shot: High hardness but high brittleness, high residual compressive stress can be obtained after shot blasting.

Liquid shot: includes SiO₂ particles and Al₂O₃ particles. The SiO₂ particles are mixed with water and sputtered with compressed air during blasting.

2.Surface Rolling Technology

Surface rolling technology is a process in which rolling balls or rollers roll or squeeze the surface of the processed part under a certain pressure to cause plastic deformation and form a strengthening layer, as shown in Figure 5.

Schematic diagram of surface roll strengthening

Fig. 5 Schematic diagram of surface roll strengthening

The depth of the surface modification layer of the surface rolling technology can reach more than 5mm, which is only suitable for some simple shape flat parts, shaft parts and groove parts etc., but it cannot be applied to the surface of complicated parts.

Surface rolling technology has many incomparable advantages.

For example, the surface rolling technology only changes the physical state of the material, but does not change the chemical composition of the material;

The surface rolling technology adopts simple tools and processes, and the processing efficiency is high;

Rolling rolling technology is a non-cutting processing technology,

In the process of processing, there will be no waste chips or waste liquid, less pollution to the environment, in line with the development concept of “green manufacturing”.

In addition, the surface rolling technology can eliminate the tensile stress caused by the cutting of the part surface, and make the surface of the part in a state of compressive stress.

The residual compressive stress can not only close the crack tip but also inhibit the crack tip expansion, thereby further improving the fatigue life of the part.

This technology has been widely used in industry and has produced huge economic benefits.

(1) Mechanism

① Microstructure mechanism.

After cutting, there are cutting traces of the tool on the surface of the metal.

Observed at the microscopic level, it can be seen that the surface of the metal is uneven.

Rolling processing is a kind of pressure finishing, and the metal surface will undergo strong plastic deformation under the action of the hob.

According to the related theories of engineering materials, the basic way of plastic deformation of metal is slip, that is, the relative slip of the crystal relative to another part along a certain crystal plane and crystal direction.

Under the action of external force, the crystal slips continuously, and the crystal grains gradually rotate from soft orientation to hard orientation during the deformation process.

The grains are bound to each other and hinder the deformation of the grains.

Since most of the metals used in industry are polycrystalline, the metals can withstand large plastic deformation without being damaged.

The continuous slippage of the crystal grains in the metal will increase the dislocation density of the crystal grains and the crystal lattice distortion.

Dislocations with opposite signs cancel each other out, and dislocations with the same sign are rearranged into smaller sub-grains.

The finer the crystal grains, the higher the dislocation density, and the greater the deformation and dispersion.

Therefore, it is not easy to produce local stress concentration, so that the yield strength and fatigue performance of the rolled metal material are significantly improved.

② Surface quality mechanism.

The quality of the metal surface is often measured by surface roughness, which is one of the main factors causing stress concentration.

The rough surface is easy to form sharp cuts, causing stress concentration, and the source of fatigue often appears at the stress concentration.

Under the action of alternating stress, stress concentration promotes the formation and propagation of fatigue cracks.

The rougher the surface and the sharper the tip incision, the more serious the stress concentration.

Rolling strengthening is to use the rolling effect of the roller on the surface of the workpiece to make the surface metal of the workpiece produce plastic flow, and fill it into the original residual low concave trough, thereby reducing the roughness of the workpiece surface, eliminating residual tool marks, and reducing stress concentration.

In turn, the fatigue life of the workpiece is improved.

③ Mechanism of residual compressive stress.

As early as the 1930s, it was discovered that allowing residual compressive stress on the surface of the part to prolong the fatigue life of the workpiece.

The condition for crack propagation on the surface of metal materials is that the applied alternating load reaches a certain limit (that is, when the stress intensity reaches the critical stress intensity of the material itself).

Rolling can reduce the original micro-cracks on the surface, and can also generate residual compressive stress, thereby increasing the fatigue life of the parts.

(2) Process parameters that affect the rolling effect

The main process parameters that affect the surface rolling effect are: rolling pressure, rolling times and rolling speed.

The rolling pressure is the force of the roller pressing on the surface of the workpiece, which has a great influence on the fatigue strength of the workpiece.

However, the current research on it is not mature enough, and there is no mathematical formula that can accurately calculate the optimal rolling pressure.

The optimal rolling pressure is also related to factors such as the strength of the part itself, the size of the part, and the diameter of the roller.

In production, the best rolling pressure is determined through process tests;

The number of rolling is the number of times that the roller presses the same position of the workpiece, which has a great influence on the fatigue strength of the workpiece.

When the number of times is small, the surface of the workpiece fails to reach the due plastic deformation.

When the number of times is large, the workpiece will have contact fatigue, and the surface will fall off in severe cases;

The rolling speed is the rotation speed of the workpiece during rolling,

It has little effect on the fatigue strength of the workpiece, but it affects the efficiency of rolling processing.

If the speed is too high, it will cause greater plastic deformation, and if the speed is too slow, it will reduce production efficiency.

In production, it is necessary to determine the appropriate rolling speed according to the actual situation.

(3) Hole extrusion reinforcement

Hole extrusion is a surface strengthening process using specific tools (rods, bushings, open and close the die, etc.) to continuously, slowly and evenly squeeze the hole wall or periphery of the workpiece, which makes it form a certain thickness of plastic deformation layer to improve the surface fatigue strength and resistance to stress corrosion.

Commonly used process methods: rod extrusion, bushing extrusion, stamping die extrusion, spinning extrusion, as shown in Figure 6.

Process method of hole extrusion strengthening

  • (a) 1-Hydraulic press; 2-Clamp; 3-Squeeze bar; 4-Parts; 5-Base
  • (b) 1 — Parts; 2 — Bushings; 3 — Extrusion rods; 4 — Drawing guns
  • (c) 1-Hydraulic presses; 2-Indentation dies; 3-Parts; 4-Bases
  • (d) 1 – hole arm drill; 2 – collet; 3 – extrusion head; 4 – parts; 5 – base

Figure 6 Process method of hole extrusion strengthening

Hole extrusion strengthening is mainly for workpieces whose inner holes have fatigue resistance requirements or cannot be achieved by other methods, such as important parts on airplanes;

Impression die extrusion is suitable for strengthening key bearing parts such as large parts and skins; spinning extrusion is suitable for strengthening inner holes of large parts such as landing gear.

02 Plasma diffusion technology

Plasma is composed of a large number of free electrons and ions and behaves as an ionized gas that is approximately neutral on the whole.

Plasma chemical heat treatment technology, also known as plasma diffusion technology (PDT) or particle bombardment diffusion technology, uses ions generated by gas glow discharge in a low vacuum environment to bombard the surface of the workpiece to change the composition, structure and performance of the metal surface Process.

Compared with ordinary gas thermal diffusion technology, ion thermal diffusion has the following characteristics:

(1) Ion bombardment sputtering will remove the oxygen (passivation) film or impurities on the surface of the workpiece and improve the surface activity of the workpiece, which makes it easy to adsorb the elements to be infiltrated and accelerate the thermal diffusion rate;

(2) Plasma can activate the reaction gas and reduce the chemical reaction temperature;

(3) The structure of the heat-expanded layer and the thickness of the layer can be controlled by adjusting the process parameters;

(4) It has no pollution to the environment, which is an environmentally friendly treatment process.

Plasma can be divided into high temperature plasma and low temperature plasma.

Aurora, fluorescent lamp, electric arc, iodine tungsten lamp, etc. belong to low-temperature plasma, and fusion and solar core belong to high-temperature plasma.

The temperature of heavy particles in low-temperature plasma (also called non-equilibrium plasma) is close to normal temperature, while the temperature of electrons is as high as 10³~10⁴K.

The phenomenon of changing a gas from an insulator to a conductor is called gas discharge.

The conditions for gas discharge are: a certain electric field strength; the presence of charged particles in the gas.

In the electric field, the charged particles move in a directional motion.

A series of physical and chemical changes occur between charged particles and gas atoms, charged particles and electrodes, that is, collisions between charged particles cause gas excitation and ionization;

The collision causes the electrons in the atom to transition from the normal energy level to a higher energy level and become a metastable excited atom;

When the excited electron returns to the ground state, it releases energy in the form of photons (glow).

If the energy of the charged particle impact is large, it may knock an electron in the atom away from the atom (ionization).

1. The mechanism of ion nitriding

(1) Kolbel ion sputtering nitriding model

High-energy nitrogen ions bombard the cathode to sputter Fe atoms out of the cathode surface.

Fe atoms combine with N atoms to form FeN, which is redeposited on the surface of the workpiece (backscattering).

FeN in a metastable state is decomposed in the order of FeN→Fe₂-₃N→Fe₄N, and the decomposed active N atoms penetrate into the surface or near surface of the steel.

At the same time, a nitrided layer of Fe₂-₃N (ε phase) and Fe₄N (γ’ phase) is formed from the outside to the inside of the steel surface, which is shown in Figure 7.

Kolbel ion sputtering nitriding model

Figure 7 Kolbel ion sputtering nitriding model

(2) New ion nitriding model

The new DC ion nitriding model is shown in Figure 8, and the ion nitriding device is shown in Figure 9.

New DC ion nitriding model

Figure 8 New DC ion nitriding model

Schematic diagram of ion nitriding device

  • 1- DC power supply;
  • 2- Vacuum chamber;
  • 3- Workpiece;
  • 4- Temperature controller;
  • 5- Vacuum gauge;
  • 6- Vacuum pump;
  • 7- Flow meter; 8-Air supply system

Figure 9 Schematic diagram of ion nitriding device

2. Ion nitriding process

(1) It should put the cleaned workpiece into the ion nitriding furnace and vacuumize it to about 1Pa;

(2) It should input a small amount of nitrogen-containing gas and switch on the DC high-voltage power supply to make the gas glow and discharge;

(3) It should sputter and clean the surface of the processed workpiece;

(4) It should adjust the air pressure and voltage, then heat the workpiece to the required processing temperature, and start nitriding;

(5) It should keep the heat for a certain period of time to reach the required thickness of the nitriding layer;

(6) When the power is cut off, the workpiece is cooled to below 200°C in vacuum, and the surface of the workpiece after nitriding is silver gray.

3. Tissue types and influencing factors of ion nitriding

Nitriding is performed in a temperature environment less than 590°C (eutectoid temperature),

As the nitrogen potential increases, the structure of the nitrided layer is from the outside to the inside: ε→ε+γ’→γ’+diffusion layer→α diffusion layer, as shown in Figure 10.

Surface structure morphology of 38CrMoAl steel after nitriding

Fig. 10 Surface structure morphology of 38CrMoAl steel after nitriding (560℃×5h)

The main factors affecting the ion nitriding layer are as follows:

(1) Nitriding temperature: As the temperature increases, the thickness of the nitriding layer increases.

When the temperature is less than 550℃, the γ’phase ratio increases with the increase in temperature;

When the temperature is more than 550℃, the ε phase ratio increases with the increase of temperature

(2) Nitriding time: the initial stage of nitriding (<30min), the nitriding speed is much greater than the gas nitriding speed,

As time increases, the infiltration rate slows down and gradually approaches the gas nitriding rate.

(3) Nitrogen gas: ammonia, nitrogen + hydrogen, etc. are commonly used.

(4) Nitrogen gas pressure, voltage and current density: the higher the gas pressure, the thicker the nitriding layer;

The greater the discharge power, the thicker the nitriding layer;

The greater the current density, the thicker the nitriding layer.

4. The performance of the ion nitriding layer

The indicators for evaluating the performance of the ion nitriding layer mainly include the following aspects:

(1) Hardness: The hardness of the nitriding layer depends on the nitriding temperature, the type of alloying elements in the steel and the steel type.

(2) Fatigue strength: Nitriding can increase the fatigue strength of the workpiece, and it increases with the increase of the thickness of the diffusion layer.

(3) Toughness: In the nitrided layer, only the diffusion layer has the best toughness, followed by the single-phase compound layer (ε phase or γ’phase), and the γ’+ε mixed phase is the worst.

(4) Wear resistance: Compared with other nitriding methods, ion nitriding has the best wear resistance to rolling friction.

The ion nitriding process of commonly used steel grades is shown in Table 1.

Table 1 Ion nitriding process of commonly used steel grades

Steel grade Process parameters Surface hardness

(HV0.1)

Compound

layer depth

(µm)

Total coating depth

(mm)

Temperature (oC) Time (h) Pressure (Pa)
38CrMoaIa 520~550 8~15 266~532 888~1164 3~8 0.30~045
40Cr 520~540 6~9 266~532 750~900 5~8 0.35~0.45
42CrMo 520~560  8~15 266~532 750~900 5~8 0.35~0.40
3Cr2w8V 540~550 6~8 133~400 900~1000 5~8 0.20~0.90
4Cr5MoVI 540~550  6~8 133~400 900~1000 5~8 0.20~0.30
Crl2MiV 530~550 6~8 133~400 841~1015 5~7 0.20~0.40
QT60-2 570 8 266~400 750~900 ___  0.30

03. Laser surface treatment technology

Laser surface treatment technology refers to the use of the unique performance characteristics of the laser beam to process the surface of the material and form a treatment layer with a certain thickness, which can significantly improve the mechanical properties, metallurgical properties, and physical properties of the material surface.

So as to improve the wear resistance, corrosion resistance and fatigue resistance of parts and workpieces, it is an efficient and mature surface treatment technology.

1. Features

(1) After laser beam treatment, the chemical uniformity of the material surface is very high, the crystal grains are fine, so the surface hardness is high, the wear resistance is good, and the high surface performance is obtained without losing the toughness.

(2) Low input heat and small thermal deformation.

(3) High energy density and short processing time.

(4) The treatment part can be arbitrarily selected, such as deep holes, grooves and other special parts that can be treated by laser.

(5) The process does not require vacuum and no chemical pollution.

(6) During the laser treatment process, the surface layer undergoes martensite transformation and residual compressive stress exists, which improves its fatigue strength.

2. Laser surface treatment equipment

Laser surface treatment equipment includes: laser, power meter, light guide focusing system, worktable, numerical control system and software programming system.

3. The principle and characteristics of laser surface treatment technology

A laser is an electromagnetic wave with the same phase, a certain wavelength, and a strong directivity.

The laser beam is controlled by a series of mirrors and lenses, so the laser beam can be focused into a beam with a small diameter (only 0.1mm in diameter), which can achieve extremely high power density (10⁴~10⁹W/cm²).

According to the laser intensity and radiation time, the interaction between the laser and the metal can be divided into the stages of absorption of the light beam, energy transfer, metal structure change and laser action cooling.

The laser surface treatment technology uses a high-power density laser beam to heat the surface of the material in a non-contact manner, relying on the thermal conductivity of the surface of the material to achieve the purpose of cooling, so as to achieve its surface strengthening process.

Its advantages in material processing are as follows:

(1) The energy transfer is convenient, and the surface of the workpiece can be selectively strengthened locally;

(2) Its energy is concentrated, the processing time is short, the heat-affected zone is small, and the deformation of the workpiece is small after laser processing;

(3) It can handle workpieces with complex surface shapes and is easy to realize automation;

(4) The modification effect is more significant than the ordinary method, the speed is high, the efficiency is high, and the cost is low;

(5) It can usually only process some thin sheet metal and is not suitable for processing thicker plates.

4. The type of tissue after laser surface treatment

Because the laser heating rate is extremely fast, the phase change process is carried out under a large degree of superheat, so the nucleation rate of the crystal nucleus is very high.

Due to the short heating time, the diffusion of carbon atoms and the growth of grains are restricted, so the obtained austenite grains are smaller.

The cooling rate is also faster than using any quenching agent, so it is easy to obtain hidden needles or fine needle martensite structure.

By observing the type of tissue, the steel surface treated by the laser beam can be distinguished,

Low carbon steel can be divided into two layers: the outer layer is a completely quenched zone, and the structure is hidden needle martensite;

The inner layer is an incompletely quenched area, which retains ferrite.

Medium carbon steel can be divided into four layers:

The outer layer is white bright hidden needle martensite with a hardness of 800HV, which is more than 100 higher than the general quenching hardness;

The second layer is hidden needle martensite plus a small amount of troostite, the hardness is slightly lower;

The third layer is hidden needle martensite plus mesh troostite, plus a small amount of ferrite;

The fourth layer is cryptoneedle martensite and complete ferrite mesh.

High carbon steel can also be divided into two layers:

The outer layer is cryptographic martensite; the inner layer is cryptographic martensite plus undissolved carbides.

Cast iron can be roughly divided into three layers: the surface layer is the dendritic crystals obtained from melting and solidification, and this area decreases with the increase of the scanning speed;

The second layer is the eutectic structure of cryptoneedle martensite plus a small amount of residual graphite and phosphorus;

The third layer is martensite formed at a lower temperature.

5. The classification of laser surface treatment technology

(1) Laser phase transition hardening

Laser phase transition hardening, also known as laser quenching, refers to irradiating the surface of the workpiece with a high-energy density laser beam, so that the part that needs to be hardened instantly absorbs a large amount of light energy and converts it into heat immediately.

As a result, the temperature of the laser action zone rises sharply, and the structure type rapidly changes to austenite.

After rapid cooling, very fine martensite and other structures are obtained.

Its characteristics are as follows:

① The surface of the material can be heated and cooled at high speed.

The heating speed can reach 10⁴~10⁹℃/s, and the cooling speed can reach 10⁴℃/s, which is beneficial to improve the scanning speed and production efficiency.

② The surface hardness of the workpiece after laser quenching is high, generally 5%~20% higher than the conventional quenching hardness.

After the treatment, a very fine hardened layer structure can be obtained.

③ Because the laser heating speed is fast, the heat affected zone is small, and the quenching stress and deformation are small.

It is generally believed that the laser quenching treatment hardly produces deformation, and the phase transformation hardening can produce compressive stress greater than 4000MPa on the surface, which helps to improve the fatigue strength of the parts;

However, the deformation of parts whose thickness is less than 5mm cannot be ignored.

④ Parts with complex shapes and parts that cannot be processed by other conventional methods, such as parts with grooves, can be partially hardened.

⑤ The laser quenching process cycle is short and the production efficiency is high.

The technological process is easy to realize computer control because of high automation, so it can be incorporated into the production line.

⑥ Laser quenching relies on its own thermal conductivity to make it self-cooling by conduction on the surface and inside without cooling medium, and has no pollution to the environment.

(2) Laser surface cladding

Laser surface cladding is a surface strengthening method that rapidly heats and melts alloy powder or ceramic powder and the surface of the substrate under the action of a laser beam, and then cools itself when the beam is removed.

Its characteristics are as follows:

①It has a fast cooling rate (up to 10⁶℃/s), and the structure has the typical characteristics of rapid solidification;

②It has small heat input and distortion, low coating dilution rate (generally less than 5%), and metallurgical bonding with the substrate;

③There are almost no restrictions on the choice of powder, especially for low-melting-point metal surface deposition with high-melting-point alloy;

④It can carry out selective area welding, consumes less material, and has excellent cost performance;

⑤Beam aiming can weld hard-to-reach areas;

⑥The process is easy to realize automation.

(3) Laser surface alloying

Laser surface alloying refers to the rapid melting and mixing of a thin layer on the surface of the base material and external alloying elements under the irradiation of a high-energy laser beam to form a surface melting layer with a thickness of 10~1000μm.

The cooling rate of the molten layer during solidification can reach 10⁵~10⁸℃/s, which is equivalent to the cooling rate that can be achieved by quenching technology.

In addition, due to the physical phenomena such as diffusion and surface tension effect in the molten layer liquid, the surface alloy layer of predetermined depth and chemical composition can be formed on the surface of the material in a short time (50μs~2ms).

The biggest feature of the laser surface alloying process is that the composition, structure and performance changes occur only in the melting zone and a small affected zone, the thermal effect on the matrix can be reduced to a minimum, and the deformation caused is also minimal.

This kind of process can not only meet the needs of surface use, but does not sacrifice the overall characteristics of the structure.

The melting depth is controlled by laser power and irradiation time.

An alloy layer with a thickness of 0.01~2mm can be formed on the surface of the base metal.

Due to the high cooling rate, the segregation is minimized and the crystal grains are significantly refined.

(4) Laser shock hardening

When a short pulse (tens of nanoseconds), high peak, high power density (>10W/cm²) laser beam irradiates a metal target, the metal surface absorption layer absorbs the laser energy and explosively vaporizes, resulting in high temperature (>10000K) ), high-pressure (>1GPa) plasma,

When the plasma is constrained by the confinement layer, a high-intensity pressure shock wave will be generated, which will act on the metal surface and then propagate into the metal.

When the peak pressure of the shock wave exceeds the dynamic yield strength of the processed material, strain hardening occurs on the surface of the material, and large compressive stress will remain in the material.

This new type of surface strengthening technology is laser shock strengthening. Because its strengthening principle is similar to shot peening, it is also called laser shot peening.

Laser shock strengthening has the characteristics of deep strain influence layer, controllable impact area and pressure, little influence on surface roughness, and easy automation.

Compared with shot peening, the residual compressive stress layer obtained by laser shock treatment can reach 1mm, which is 2 to 5 times that of shot peening.

Strengthening techniques such as extrusion and impact strengthening can only be performed on flat or regular surfaces of revolution.

In addition, laser shock strengthening can well maintain the surface roughness and dimensional accuracy of the strengthening position.

(5) Amorphization of laser surface

Laser surface amorphization is to use the ultra-high-speed cooling conditions of the laser molten pool to form an amorphous layer with special properties on the surface of certain alloys.

Compared with other amorphization methods, laser amorphization can form a large area of the amorphous layer on the surface of the workpiece, and the composition of the amorphous layer can also be expanded.

04. Electron beam surface treatment technology

The process of using high-energy electron beams to bombard the surface of the material to increase the temperature and change the composition and structure to achieve the required performance is called electron beam surface treatment.

It uses high-speed moving electrons in the electric field as the energy carrier, and the energy density of the electron beam can reach up to 10⁹W/cm².

The characteristics of electron beam surface treatment are:

Because the electron beam has a higher energy density, the size range and depth of heating are larger;

The equipment investment is low and the operation is simpler (no need to “blacken” before processing like laser beam processing);

Due to the vacuum conditions, the size of the parts is limited.

1. The principle of electron beam surface treatment technology

The electron beam is a stream of high-energy electrons, which are generated by the cathode filament.

When the negatively charged electron beam flies to the high-potential positive electrode at high speed, it is accelerated by the accelerator and focused by the electromagnetic lens to increase the power of the electron beam.

After the second focus, its energy density is highly concentrated, and it rushes to a very small area on the surface of the workpiece at a very high speed.

Most of the kinetic energy carried by the electron beam is converted into heat energy, so the temperature of the impacted part of the material surface will rise to several thousand degrees Celsius within a fraction of a microsecond, causing the material to melt or even vaporize instantly.

2. Equipment for electron beam surface treatment technology

The equipment of electron beam surface treatment technology consists of the following five systems:

①The electron gun system emits high-speed electron flow;

②The vacuum system guarantees the vacuum degree required by the system;

③The control system controls the size, shape, and direction of the electron beam;

④The current system supplies high and low voltage stabilized current;

⑤The transmission system controls the movement of the worktable.

3. Features of electron beam surface treatment technology

①The workpiece is heated in a vacuum chamber without oxidation or decarburization.

The surface phase change strengthening does not require a cooling medium.

Relying on the cooling behavior of the matrix itself, “green surface strengthening” can be realized.

②The energy conversion rate of the electron beam is about 80%~90%, the energy is concentrated, the thermal efficiency is high, and the local phase transformation strengthening and surface alloying can be realized.

③Due to the concentration of heat, the heat point of action is small, and the thermal stress formed during heating is small.

And because the hardened layer is shallow, the structure stress is small, and the surface transformation strengthening distortion is small.

④The one-time input of electron beam surface treatment equipment is less than that of laser (about 1/3 of laser), and the cost of the electron beam is only half of that of the laser.

⑤The structure of the equipment is simple, the electron beam is rotated and scanned by magnetic deflection, and no workpiece rotation, movement and light transmission mechanism are required.

⑥The electron beam surface treatment has a wide range of applications, which can be applied to the surface treatment of various steel, cast iron and other materials, and it is also suitable for parts with complex shapes.

⑦Electron beam is easy to excite X-ray, so care should be taken to protect it during use.

4. The classification of electron beam surface treatment technology

The classification of electron beam surface treatment technology is shown in Figure 11.

Classification of electron beam surface treatment technologies

Figure 11 Classification of electron beam surface treatment technologies

(1) Electron beam surface phase change strengthening

For metals with martensitic transformation process, the key to the process is parameter control: the average power density of the electron beam spot is 10⁴~10⁵W/cm², the heating rate is 10³~10⁵℃/s, and the cooling rate can reach 10⁴~10⁶ ℃/s.

The rapid fusion of the electron beam causes supersaturated solid solution strengthening, and forms ultrafine martensite, the hardness increases, and the surface shows residual compressive stress, thereby improving the wear resistance of the material.

(2) Electron beam surface remelting treatment

Electron beam remelting can redistribute the chemical elements of the alloy and reduce the degree of micro-segregation of certain elements, thereby improving the surface performance of the workpiece.

Since the electron beam remelting is carried out under vacuum conditions, it is helpful to prevent surface oxidation.

Therefore, the electron beam remelting treatment is particularly suitable for the surface treatment of magnesium alloys and aluminum alloys with high chemical activity.

(3) Electron beam surface alloying

Generally, elements such as W, Ti, B, Mo and their carbides are selected as alloying elements to improve the wear resistance of materials;

Choosing elements such as Ni and Cr can improve the corrosion resistance of the material;

The appropriate addition of Co, Ni, Si and other elements can improve the alloying effect.

(4) Amorphization treatment of electron beam surface

Increase the average power density of the electron beam to 10⁶~10⁷W/cm² and shorten the action time to about 10-⁵s, so that the metal produces a large temperature gradient between the substrate and the molten surface.

After stopping the electron beam irradiation, the rapid cooling rate of the metal surface (10⁷~10⁹s-¹) far exceeds the cooling rate of conventional amorphous preparation (10³~10⁶s-¹),

The obtained amorphous structure is compact, and has excellent fatigue and corrosion resistance.

(5) Annealing of the thin layer of electron beam surface

When the electron beam is used as a heat source for annealing a thin layer on the surface, the required power density is much lower than that of the above method, thereby reducing the cooling rate of the material.

For metal materials, this method is mainly applied to the surface treatment of thin strips.

In addition, electron beam annealing has also been successfully applied to semiconductor materials.

5. Application of electron beam surface strengthening technology

After the surface of the die steel is strengthened by electron beam, the outermost layer of the material melts.

When the thickness of the surface remelted layer reaches about 10μm, melting will cause the surface microhardness to decrease;

The surface carbide particles dissolve the matrix solid solution chromium and energy increase, resulting in supersaturated solid solution strengthening, and the formation of ultra-fine martensite.

The microhardness of the sample is increased from 955.2HK to 1169HK, and the relative wear resistance is increased by 5.63 times.

The more bombardment times, the deeper the affected zone and the greater the increase in microhardness.

05. EDM surface treatment technology

The basic principle of the electric spark surface treatment technology is that the energy storage power source passes through the electrode to generate spark discharge between the electrode and the parts at a frequency of 10-2000 Hz.

The conductive material as the electrode is melted into the surface of the workpiece to form an alloyed surface strengthening layer to improve the physical and chemical properties of the surface of the workpiece.

The performance of the EDM surface strengthening layer mainly depends on the base material itself and the electrode material. The commonly used electrode materials are TiC, WC, ZrC, NbC, Cr3C2, cemented carbide, etc.

1. EDM surface treatment technology process

Figure 12 is a schematic diagram of the electrical spark surface treatment technology process.

When the distance between the electrode and the workpiece is large, the power supply charges the capacitor through the resistor R, and the electrode is driven by the vibrator to approach the workpiece, as shown in Figure 12(a);

When the gap between the electrode and the workpiece is close to a certain distance, the air in the gap is ionized under the action of a strong electric field, resulting in spark discharge, as shown in Figure 12(b);

When the metal of the electrode and the workpiece is partially melted or even vaporized at the part where the discharge occurs, the electrode continues to approach and contact the workpiece.

At this time, the spark discharge stops, and a short-circuit current flows through the contact point to continue heating there.

Because the electrode is pressed against the workpiece with proper pressure, the molten materials are bonded and diffused to form alloys or new compounds, as shown in Figure 12(c);

The electrode leaves the workpiece under the action of the oscillator, as shown in Figure 12(d).

Schematic diagram of EDM surface strengthening process

Figure 12 Schematic diagram of EDM surface strengthening process

(1) Physical chemical metallurgy process under high temperature and high pressure.

The high temperature generated by the spark discharge makes the electrode material and the matrix material on the surface of the workpiece partially melt,

The pressure generated by the thermal expansion of the gas and the mechanical impact force of the electrode later make the electrode material and the matrix material fuse and physically and chemically interact.

The action of ionized gas elements such as nitrogen and oxygen produces a special alloy on the surface of the substrate.

(2) High temperature diffusion process.

The diffusion process takes place both in the melting zone and at the liquid-solid phase boundary.

Due to the very short diffusion time, the diffusion of liquid elements into the matrix is limited, and the diffusion layer is very shallow, but the matrix and alloy layer can also achieve better metallurgical bonding.

(3) Rapid phase change process.

Due to the rapid heating and rapid cooling of the heat-affected zone, the part near the melting zone of the workpiece matrix undergoes an austenitization and martensitization transformation, which refines the grains, increases the hardness, and generates residual compressive stress.

It is beneficial to improve fatigue strength.

2. The characteristics of electric spark surface treatment technology

(1) Advantages

①The equipment is simple and the cost is low;

②The combination of strengthening layer and substrate is very firm;

③The internal temperature of the workpiece is not heated or the temperature is very low, there is no change in structure and performance, and the workpiece will not be annealed and deformed;

④Low energy consumption and low material consumption;

⑤There is no size limit on the processing object, especially suitable for local processing of large workpieces;

⑥Significant surface strengthening effect;

⑦It can be used to repair the workpiece with excessive wear;

⑧ Simple operation and easy to master.

(2) Disadvantages

①The surface strengthening layer is shallow, generally the depth is only 0.02~0.5mm;

②The surface roughness will not be very low;

③Small holes and narrow grooves are difficult to handle, and the uniformity and continuity of the surface strengthening layer is poor.

Metal surface modification technology

01. Electroplating

1.Definition and principle of electroplating

Electroplating is a surface treatment process that uses electrochemical properties to deposit a desired form of metal coating on the surface of the plated article.

Electroplating principle: In the salt solution containing the metal to be plated, the base metal to be plated is used as the cathode.

Through electrolysis, the cations of the metal to be plated in the plating solution are deposited on the surface of the base metal to form a plating layer, which is shown in Figure 13.

Electroplating principle

Figure 13 Electroplating principle

The purpose of electroplating:

It can obtain a surface layer that is different from the base material and has special properties to improve the corrosion resistance and wear resistance of the surface.

The thickness of the coating is generally a few microns to several tens of microns.

Features of electroplating:

The electroplating process equipment is relatively simple, and the operating conditions are easy to control,

Wide range of coating materials and low cost makes it widely be used in industry, which is an important method of material surface treatment.

2.Classification of coating

There are many types of coatings, which is classified as follows according to performance:

(1) Protective coating: zinc, zinc-nickel, nickel, cadmium, tin and other coatings can be anti-corrosion coatings resistant to the atmosphere and various corrosive environments.

(2) Protection-decorative coatings: Cu-Ni-Cr coatings, which are both decorative and protective.

(3) Decorative coatings: Au and Cu-Zn imitation gold coatings, black chromium, black nickel coatings, etc.

(4) Wear-resistant and anti-friction coatings: hard chromium coatings, loose hole coatings, Ni-Sic coatings, Ni-graphite coatings, Ni-PTFE composite coatings, etc.

(5) Electrical performance coatings: Au and Ag coatings etc., which not only have high conductivity, but also prevent oxidation and avoid increasing contact resistance.

(6) Magnetic coatings: soft magnetic coatings include Ni-Fe coatings and Fe-Co coatings; hard magnetic coatings include Co-P coatings, Co-Ni coatings, Co-Ni-P coatings, etc.

(7) Solderability coating: It has Sn-Pb coating, Cu coating, Sn coating and Ag coating etc., which can improve solderability and is widely used in the electronics industry.

(8) Heat-resistant coating: Ni-W coating, Ni coating and Cr coating etc. with high melting point and high temperature resistance.

(9) Plating layer for repairing: for some expensive wear parts, or processing out-of-tolerance parts, it uses electroplating to repair the size, which can save costs and extend the service life.

For example, Ni, Cr, Fe layers can be electroplated for repair.

According to the electrochemical properties between the coating and the base metal, it can be divided into:

Anodic coating and cathodic coating.

When the potential of the coating relative to the base metal is negative, the coating is an anode, which is called an anodic coating, such as a zinc coating on steel;

When the potential of the plating layer relative to the base metal is positive, the plating layer is a cathode, which is called a cathodic plating layer, such as a nickel-plated layer and a tin-plated layer on steel.

If divided according to the combination form of the coating, the coating can be divided into:

Single layer coating, such as Zn or Cu layer;

Multi-layer metal coating, such as Cu-Sn/Cr coating, Cu/Ni/Cr coating, etc.;

Composite coating, such as Ni-Al₂O₃ coating, Co-SiC coating, etc.

If classified according to coating composition, it can be divided into single metal coating, alloy coating and composite coating.

3. The basic composition of electroplating solution

The main salt deposit metal salts mainly include:

Single salts, such as copper sulfate, nickel sulfate, etc.;

Complex salts, such as sodium zincate, sodium zinc cyanide, etc.

The complexing agent forms a complex with the deposited metal ions, and its main function is to change the electrochemical properties of the plating solution and control the electrode process of metal ion deposition.

The compounding agent is an important component of the plating solution and has a great influence on the quality of the coating.

Commonly used compounding agents are cyanide, hydroxide, pyrophosphate, tartrate, nitrilotriacetic acid and citric acid etc.

The role of conductive salt is to improve the conductivity of the plating solution, reduce the tank end voltage and increase the process of current density.

For example, adding Na2SO4 to the nickel plating solution.

Conductive salts do not participate in the electrode reaction, and acids or bases can also be used as conductive substances.

Buffer is an important process parameter in weak acid or weak alkaline bath.

Buffer is added to make the plating solution have the ability to adjust the pH value, so as to keep the pH value stable during the plating process.

The buffer must have a sufficient amount to effectively control the acid-base balance, generally adding 30-40g/L, such as boric acid in potassium chloride zinc plating solution.

The anode activator is continuously consumed by metal ions during the electroplating process.

Most electroplating solutions rely on soluble anodes to supplement, so that the amount of metal deposited on the cathode is equal to the amount of anode dissolved, and the composition of the plating solution is balanced.

The addition of an active agent can maintain the active state of the anode without passivation and maintain the normal dissolution reaction.

For example, Cl- must be added to the nickel plating solution to prevent passivation of the nickel anode.

Special additives is to improve the performance of the bath and the quality of the coating, which is a necessary step in electroplating.

Its addition amount is small, generally only a few grams per liter, but the effect is significant.

There are many types of these additives, which can be divided into:

(1) Brightener-it can improve the brightness of the coating.

(2) Grain refining agent—It can change the crystallization condition of the coating, refine the crystal grains, and make the coating dense.

For example, if additives such as the condensate of epichlorohydrin and amines are added to zincate zinc plating bath, the coating can change from sponge to dense and bright.

(3) Leveling agent—It can improve the micro-dispersion ability of the plating solution and smooth the micro-rough surface of the substrate.

(4) Wetting agent-can reduce the interfacial tension between the metal and the solution, make the coating better adhere to the substrate, and reduce pinholes.

(5) Stress relief agent-can reduce the stress of the coating.

(6) Coating hardener-can improve the hardness of the coating.

(7) Masking agent—can eliminate the influence of trace impurities.

4.Basic steps of the electroplating process

The basic steps of the electroplating process include: liquid phase mass transfer, electrochemical reduction, and electrocrystallization.

5.Factors affecting electroplating quality

(1) Plating solution

Main salt solubility, ion coordination, additional salt; pH value; hydrogen evolution;

Current parameters: current density, current waveform; additives; temperature; stirring;

Base metal: properties, surface processing state; pretreatment.

(2) Electroplating method: rack plating

Metals such as W, Mo, Ti, and V that cannot be electroplated separately from an aqueous solution can be co-deposited with iron group elements (Fe, Co, Ni) to form alloys;

This results in an appearance that cannot be obtained with a single metal.

(3) Conditions for depositing alloy

① At least one of these two metals can be deposited from the aqueous solution of its salt.

②The deposition potential of the two metals co-deposited must be very close.

02. Chemical plating

Electroless plating refers to a surface processing method that uses chemical methods to reduce metal ions in the solution to metal, and deposits on the surface of the substrate to form a plating layer.

During electroless plating, the electrons required to reduce metal ions are directly generated in the solution through chemical reactions.

There are three ways to complete the process.

1.Displacement deposition

The use of the metal to be plated M1 (such as Fe) is more negative than the deposited metal M2 (such as Cu) to replace the deposited metal ions on the surface of the workpiece from the solution, which is called immersion plating in engineering.

When the metal M1 is completely covered by the metal M2, the deposition stops, so the coating is very thin.

Iron immersion copper, copper immersion mercury, and aluminum zinc plating all  use displacement depositions.

Immersion plating is difficult to obtain practical coatings, so it is often used as an auxiliary process for other plating species.

2.Contact deposition

In addition to the plated metal M1 and the deposited metal M2, there is a third metal M3.

In a solution containing M2 ions, the two metals M1 – M3 are connected, and electrons flow from M3 with high potential to M₁ with low potential, so that M2 is reduced and deposited on M1.

When the contact metal M1 is also completely covered by M2, the deposition stops.

When electroless nickel plating is performed on functional materials that do not have autocatalytic properties, contact deposition is often used to initiate nickel deposition.

3.Reduction deposition

The process of reducing metal ions to metal atoms by the free electrons released by the oxidation of the reducing agent is called reduction deposition.

The reaction equation is as follows:

Reductant oxidation

Rn+ → 2e- + R(n + 2)+

Metal ion reduction

M2+ + 2e- → M

The chemical plating in engineering also mainly refers to this kind of reduction deposition chemical plating.

The conditions of electroless plating are as follows:

(1) The reduction potential of the reducing agent in the plating solution is significantly lower than the potential of the deposited metal, so that the metal may be reduced and deposited on the substrate.

(2) The prepared plating solution does not produce spontaneous decomposition, and the metal deposition process only occurs when it contacts the catalytic surface.

(3) When adjusting the pH and temperature of the solution, the reduction rate of the metal can be controlled to adjust the plating rate.

(4) The metal precipitated by reduction also has catalytic activity, so that the redox deposition process can continue and the coating can be continuously thickened.

(5) The reaction product does not hinder the normal progress of the plating process, that is, the solution has sufficient service life.

There are many types of metals and alloys for electroless plating, such as Ni-P, Ni-B, Cu, Ag, Pd, Sn, In, Pt, Cr and many Co-based alloys, but the most widely used are electroless nickel plating  and electroless copper plating.

Electroless plating generally has good corrosion resistance, wear resistance, brazing properties and other special electrical or magnetic properties, so this kind of surface treatment process can well improve the surface properties of the material.

03. Thermal spray technology, thermal spray welding technology

Thermal spray technology and thermal spray welding technology are both technologies that use thermal energy (such as oxygen-acetylene flame, electric arc, plasma flame, etc.) to melt coating materials with special properties and apply them to the workpiece to form a coating.

It has the characteristics of being able to prepare a relatively thick coating (0.1~10mm), and is mainly used in the manufacture of composite layer parts repair.

1.Thermal spray technology

(1) Principles and characteristics of thermal spraying technology

Various heat sources are used to heat the coating material to melt or semi-melt,

The high-speed gas is then used to disperse and refine the coating material and hit the surface of the substrate at high speed to form a coating, as shown in Figure 14.

Schematic diagram of the basic process of thermal spraying

Figure 14 Schematic diagram of the basic process of thermal spraying

The thermal spraying process mainly includes:

  • Melting of spray material;
  • Atomization of spray materials;
  • Flight of sprayed materials;
  • Impact and solidification of particles.

(2) Coating materials

Thermal spraying has certain requirements for coating materials, and the conditions to be met:

  • There is a wide liquid phase zone, which is not easy to decompose or volatilize at the spraying temperature;
  • Good thermal stability;
  • Good performance; good wettability;
  • Good solid fluidity (powder);
  • The coefficient of thermal expansion is appropriate.

The coating material can be divided into wire and powder according to the shape of the spray material.

(3) Combination mechanism of thermal spray coating

①Mechanical bonding: the particles in the molten state hit the surface of the substrate and then spread into a flat liquid thin layer, which is embedded in the undulating surface and forms a mechanical bond.

②Metallurgical bonding: diffusion and welding between the coating and the substrate surface are called metallurgical bonding.

③Physical bonding: When the molten particles moving at high speed hit the surface of the substrate, if the distance between the two sides of the interface is within the range of the atomic lattice constant, the particles are bonded together by van der Waals force.

(4) The formation process of the coating

①The spray material is heated to a molten state;

②The spray material is atomized into tiny droplets and hits the surface of the substrate at high speed.

The greater the kinetic energy of the particles hitting the substrate and the greater the impact deformation, the better the bonding of the formed coating;

③The molten high-speed particles deform after impacting the surface of the substrate, and form a coating after condensation.

The formation process of the coating is shown in Figure 15.

Schematic diagram of the coating formation process

Figure 15 Schematic diagram of the coating formation process

The coating structure is composed of flat particles of different sizes, unmelted spherical particles, inclusions and pores.

Reasons for the existence of pores:

  • Low impact kinetic energy of unmelted particles;
  • The shadowing effect caused by different spraying angles;
  • Solidification shrinkage and stress release effect.

Appropriate pores can store lubricants, improve the thermal insulation performance of the coating, reduce internal stress and improve the thermal shock resistance of the coating, etc.

However, excessive pores will damage the corrosion resistance of the coating, increase the roughness of the coating surface, and reduce the bonding strength, hardness, and wear resistance of the coating.

Therefore, the number of pores should be strictly controlled during the preparation of the coating.

2.Thermal spray welding technology

(1) Principles and characteristics of thermal spray welding technology

Thermal spray welding technology is a surface metallurgical strengthening method that uses a heat source to remelt or partially melt the coating material on the surface of the substrate and condense on the surface of the substrate to form a surface layer that has a metallurgical bond with the substrate, which is also called sintering.

Compared with other surface treatment processes, the structure obtained by thermal spray welding is dense, with few metallurgical defects, and high bonding strength with the substrate.

However, the selection range of the materials used is narrow.

The deformation of the substrate is much greater than that of thermal spraying, and the composition of the thermal spraying layer is different from the original composition and other limitations.

(2) Classification of thermal spray welding technology

Thermal spray welding technology mainly includes flame spray welding and plasma spray welding.

① Flame spray welding:

First spray powder on the surface of the substrate, and then heat the coating directly with flame to re-melt the coating on the surface of the substrate.

The surface of the substrate is completely wetted, and elements diffuse at the interface to form a strong metallurgical bond.

Flame spray welding features: simple equipment; simple process; high bonding strength between the coating and the substrate; good erosion resistance of the coating.

②Plasma spray welding: use plasma arc as a heat source to heat the substrate to form a molten pool on the surface.

At the same time, the spray welding powder material is sent into the plasma arc, and the powder is preheated in the arc column and is in a molten or semi-melted state.

After being sprayed into the molten pool by the flame, it is fully melted and discharged gas and slag.

After the spray gun is removed, the alloy molten pool solidifies, and finally a spray welding layer is formed.

Features of plasma spray welding:

High production efficiency, spray welding of refractory materials, low dilution rate, good process stability, easy automation, smooth spray welding layer, uniform composition and organization, larger coating thickness and precise control of the test process.

(3) The difference between thermal spray welding technology and thermal spray technology

①The surface temperature of the workpiece: the surface temperature of the workpiece during spraying is less than 250℃; the spray welding should be more than 900℃.

②Combination state: The spray coating is mainly mechanically bonded; the spray-welded layer is metallurgical bonding.

③Powder material: self-fluxing alloy powder is used for spray welding, but spray powder is not restricted.

④Coating structure: the spray coating has pores, and the spray welding layer is uniform, compact and non-porous.

⑤ Bearing capacity: the spray welding layer can bear impact load and higher contact stress.

⑥Dilution rate: The dilution rate of the spray welding layer is about 5%~10%, and the dilution rate of the spray coating is almost zero.

04. Chemical conversion film technology

The chemical conversion coating technology is the process of forming a stable compound film on the metal surface through chemical or electrochemical methods.

Chemical conversion film technology is mainly used for anti-corrosion and surface decoration of workpieces, which can also be used to improve the wear resistance of workpieces.

It uses a certain metal to contact a certain corrosive liquid phase.

Under certain conditions, a chemical reaction occurs between the two.

Due to the concentration polarization and the anode and anode polarization, a layer of insoluble corrosion products with good adhesion is formed on the metal surface.

These films can protect the base metal from water and other corrosive media, and can also improve the adhesion and aging resistance of the organic coating film.

In production, the conversion film technology used mainly includes phosphating treatment and oxidation treatment.

1.Phosphating treatment

Phosphating is a process in which steel materials are put into a phosphate solution to obtain a water-insoluble phosphate film.

The process of phosphating iron and steel materials is as follows.

Chemical oil removing → hot water washing → cold water washing → phosphating treatment → cold water washing → post phosphating treatment → cold water washing → deionized water washing → drying.

The phosphating film is composed of iron phosphate, manganese phosphide and zinc phosphate etc., which is gray-white or gray-black crystals.

The film and the base metal bond very firmly and have a high resistivity.

Compared with the oxide film, phosphating film has higher corrosion resistance.

It has good corrosion resistance in the atmosphere, oily and benzene media, but it has poor corrosion resistance in acids, alkalis, ammonia, sea water and steam.

The main methods of phosphating treatment are dipping, spraying and combined dipping and spraying.

According to the temperature of the solution, phosphating is divided into room temperature phosphating, medium temperature phosphating and high temperature phosphating.

The impregnation method is suitable for high temperature, medium temperature and low temperature phosphating process.

It can handle workpieces of any shape, and can obtain phosphating films of different thicknesses, with simple equipment and stable quality.

The thick phosphating film is mainly used for the anti-corrosion treatment of the workpiece and enhance the anti-friction of the surface.

The spray method is suitable for medium and low-temperature phosphating processes, and can treat large-area workpieces, such as car shells, refrigerators, washing machines and other large workpieces as paint primers and cold deformation processing.

This method has short processing time and fast film formation speed, but it can only obtain thin and medium-thick phosphating films.

2.Oxidation treatment

(1) Oxidation treatment of steel

The oxidation treatment of steel is also called bluing,

It is a process to put the steel workpiece in some oxidizing solution to form a dense and firm Fe3O4 film with a thickness of about 0.5~1.5μm on the surface.

Blueing usually does not affect the precision of parts, and is often used for decorative protection of tools and instruments.

It can improve the corrosion resistance of the surface of the workpiece, help eliminate the residual stress of the workpiece, reduce deformation, and make the surface shiny and beautiful.

The alkaline method is most used for oxidation treatment.

The solution composition and process conditions used in the oxidation treatment of steel can be determined according to the material and workpiece performance requirements.

The commonly used solution is composed of 500g/L sodium hydroxide, 200g/L sodium nitrite and the remainder of water.

When the solution temperature is about 140℃, it needs to be processed for 6-9min.

(2) Oxidation treatment of aluminum and aluminum alloys

①Anodization

Anodization is a method in which the workpiece is placed in an electrolyte and then energized to obtain an oxide film with high hardness and strong adsorption.

The commonly used electrolyte is sulfuric acid with a concentration of 15% to 20%, chromic acid with a concentration of 3% to 10%, and oxalic acid with a concentration of 2% to 10%.

The anodic film can be boiled with hot water to turn the oxide film into water-containing alumina, which is closed due to volume expansion.

It can also be sealed by treatment with potassium dichromate solution to prevent the corrosive solution from corroding the substrate through the crystalline crevice of the oxide film.

②Chemical oxidation

Chemical oxidation is a method in which the workpiece is placed in a weak alkali or weak acid solution to obtain an oxide film that is firmly bonded to the aluminum substrate.

It is mainly used to improve the corrosion resistance and wear resistance of workpieces, and also used for surface decoration of aluminum and aluminum alloys, such as anti-rust aluminum for construction and decorative film for signs.

05. Vapor Deposition Technology

Vapor deposition technology refers to a new type of coating technology in which vapor phase substances containing deposition elements are deposited on the surface of the material by physical or chemical methods to form a thin film.

According to the different principles of the deposition process, vapor deposition technology can be divided into two categories: physical vapor deposition (PVD) and chemical vapor deposition (CVD).

1.Physical vapor deposition

Physical vapor deposition (PVD) refers to a technology that uses physical methods to vaporize materials into atoms, molecules or ionize into ions under vacuum conditions, and deposit a thin film on the surface of the material through a gas phase process.

The physical deposition technology mainly includes three basic methods: vacuum evaporation, sputtering deposition and ion plating.

Vacuum evaporation is a method of evaporating film-forming materials to vaporize or sublimate and deposit them on the surface of the workpiece to form a thin film.

Depending on the melting point of the evaporation material, the heating methods include resistance heating, electron beam heating, laser heating, etc.

The characteristics of vacuum evaporation are simple equipment, process and operation.

However, due to the low kinetic energy of vaporized particles, the bonding force between the coating and the substrate is weak, and the coating is looser, so impact resistance and wear resistance are not high.

The sputtering deposition is a method in which argon is ionized by glow discharge under vacuum, the generated argon ions are accelerated to bombard the cathode under the action of an electric field, and the sputtered particles are deposited on the surface of the workpiece to form a film;

Its advantages are large kinetic energy of gasification particles, wide range of applicable materials (including matrix materials and coating materials), good throwing ability, but slow deposition speed and expensive equipment.

Ion plating is a method of using gas discharge technology under vacuum to ionize the evaporated atoms into ions and deposit them on the surface of the workpiece together with a large number of high-energy neutral particles produced at the same time.

Its characteristics are high coating quality, strong adhesion, good leveling ability and fast deposition speed, but it still has disadvantages such as complex and expensive equipment.

Physical vapor deposition has a wide range of applicable base materials and film materials;

The process is simple, which also saves material and has no pollution;

The obtained film has the advantages of strong adhesion, uniform film thickness, compactness and few pinholes etc.

It has been widely used in the fields of machinery, aerospace, electronics, optics and light industry etc. to prepare wear-resistant, corrosion-resistant, heat-resistant, conductive, insulating, optical, magnetic, piezoelectric and slippery superconducting films.

2. Chemical vapor deposition

Chemical vapor deposition (CVD) refers to a method in which a mixed gas interacts with the surface of a substrate at a certain temperature to form a metal or compound film on the surface of the substrate.

The characteristics of chemical vapor deposition are:

There are many types of deposits, which can be divided into deposited metals, semiconductor elements, carbides, nitrides, borides, etc.;

It can control the composition and crystal form of the film in a larger range;

It can evenly coat parts with complex geometric shapes;

The deposition speed is fast, the film is dense, and it is firmly combined with the substrate;

It is easy to realize mass production.

Because the chemical vapor deposition film has good wear resistance, corrosion resistance, heat resistance, electrical, optical and other special properties, it has been widely used in machinery manufacturing, aerospace, transportation, coal chemical industry and other industrial fields.

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