Metal Surface Engineering: The Ultimate Guide

Surface phenomena and changes are a frequent occurrence in nature. In engineering, nearly all parts come into contact with the environment, and it is the surface of these parts that is directly exposed to the environment. During environmental interaction, the surface may experience corrosion, abrasion, oxidation, and erosion, which can lead to damage or failure […]

Metal Surface Engineering

Table Of Contents

Surface phenomena and changes are a frequent occurrence in nature. In engineering, nearly all parts come into contact with the environment, and it is the surface of these parts that is directly exposed to the environment.

During environmental interaction, the surface may experience corrosion, abrasion, oxidation, and erosion, which can lead to damage or failure of the components. As a result, the surface acts as the first line of defense against equipment failure.

Surface engineering involves improving the morphology and chemical composition of solid metal or non-metal surfaces through surface strengthening, modification, or a combination of surface engineering techniques, after undergoing surface pretreatment. The goal of surface engineering is to systematically engineer the organization structure and stress state to achieve desired surface properties.

Metal surface strengthening technology

01 Surface deformation strengthening

Surface deformation strengthening involves creating compression deformation on the metal surface through mechanical means such as rolling or shot peening, resulting in a hardened layer on the surface. This layer can have a depth of 0.15-1.5mm.

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

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

(1) In terms of organizational structure, the dislocation density in the strengthening layer is very high, and the crystal lattice is severely distorted. When alternating stress is applied, dislocations with opposite signs will cancel each other out when they collide, and dislocations with the same sign will be rearranged. This results in a decrease in dislocation density in the strengthening layer, while finer sub-grains form gradually.

(2) In terms of stress state, due to the unbalanced degree of metal deformation between the surface layer and the inner layer, when the metal in the surface layer is plastically extended to the surrounding area, it is hindered by the inner layer metal, leading to a higher macroscopic residual stress in the strengthening layer.

1. Shot peening strengthening

Shot peening, also referred to as controlled shot peening, is a process in which a high-speed stream of projectiles is directed at the surface of a part, causing plastic deformation and forming a strengthened layer of a specific thickness.

Because the surface of the part experiences compressive stress, some of this stress can be offset when the part is under load, thus improving its fatigue strength. Shot peening is illustrated 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 are directed at high speeds towards the surface of the workpiece, causing elastic and plastic deformation at the recrystallization temperature. This results in a large residual compressive stress, as seen in Figure 2.

Each steel shot creates small indentations or depressions in the metal surface as it hits the part, similar to a miniature rod striking the surface. In order to form these depressions, the metal surface layer must be stretched.

Underneath the surface layer, compressed grains attempt to restore the surface to its original shape, creating a hemisphere under high compression. The overlapping of numerous such depressions forms a uniform residual compressive stress layer, thereby improving surface fatigue strength and resistance to stress corrosion.

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 metal products with a thickness of 2mm or less, or from castings and forgings that do not need to maintain precise dimensions and shapes, serving as a method of cleaning the surface prior to coating or plating.

Shot peening is a cold treatment process that is widely used to enhance the anti-fatigue properties of metal parts that are subjected to high stress over long periods, such as aircraft engine compressor blades, fuselage structural parts, and components of the automotive transmission system.

Shot peening is divided into ordinary shot peening and supersonic surface shot peening based on the speed of the projectiles. The projectile speed of the supersonic spray gun is between 300-500m/s, and as the part rotates, shot peening can be performed on its entire surface.

(1) Equipment for shot peening

The shot peening machine can be classified into two categories: mechanical centrifugal shot peening machine and pneumatic shot peening machine, based on the method of driving the shot.

Moreover, shot peening machines can be either dry spray or wet spray.

The working conditions of the dry spray shot peening machine are unfavorable, while the wet spray shot peening machine improves the conditions by mixing the projectiles into suspension before spraying them.

① The mechanical centrifugal shot peening machine operates by accelerating the projectiles under the action of centrifugal force due to the high-speed rotation of the blade and the impeller.

However, this type of shot peening machine has limited peening power and high production costs. It is mainly used for workpieces with high peening strength, limited variety, large batch sizes, simple shapes, and large sizes, as illustrated 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

② The pneumatic centrifugal shot peening machine uses compressed air as the driving force to propel projectiles at high speeds. The projectiles then impact the surface of the workpiece, achieving the desired shot peening effect.

This machine has the advantage of adjustable air pressure, which allows for flexible control over the intensity of shot peening. Additionally, it can handle multiple parts at once.

This machine is best suited for parts with low shot peening strength, diverse shapes and sizes, small batch sizes, and complex geometries. However, it is important to note that it has high power consumption and low productivity, as depicted 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 has a diameter range of 0.4mm to 1.2mm and a hardness of 45 to 50 HRC. The best tempering method is M or B.

Cast steel pellets: The size of the pellets ranges from 0.2mm to 1.5mm. After annealing, the hardness ranges from 30 to 57 HRC. Although it is fragile and has a high consumption rate, its price is low. The quality of cast steel shot is related to its carbon content, which generally ranges from 0.85% to 1.2%, and its manganese content, which ranges from 0.65% to 1.2%.

Glass shot: It consists of 60% SiO2 and has a hardness of 46 to 50 HRC. It is highly brittle, making it suitable for applications where the parts have a lower hardness than the shot.

Ceramic shot: It has high hardness and high brittleness, and high residual compressive stress can be obtained after shot blasting.

Liquid shot: This type of shot contains SiO₂ particles and Al₂O₃ particles. The SiO₂ particles are mixed with water and propelled by compressed air during the blasting process.

2.Surface Rolling Technology

Surface rolling is a technology that involves the use of rolling balls or rollers to apply pressure to the surface of a processed part. This pressure results in plastic deformation, which forms a strengthening layer on the surface of the part. This process is depicted in Figure 5.

Schematic diagram of surface roll strengthening

Fig. 5 Schematic diagram of surface roll strengthening

The depth of the modified layer created by surface rolling technology can reach over 5mm, making it ideal for flat parts with simple shapes, shafts, and grooves, but it cannot be used on complex parts.

Surface rolling technology offers numerous benefits that are unmatched. For instance, it only alters the physical state of the material without changing its chemical composition. Additionally, the technology uses simple tools and processes, leading to high processing efficiency.

As a non-cutting processing technology, rolling technology produces no waste chips or liquids, making it environmentally friendly and in line with the “green manufacturing” concept.

Furthermore, the technology eliminates tensile stress caused by cutting and puts the surface of the part into a state of compressive stress. This residual compressive stress can prevent cracks from expanding and improve the part’s fatigue life.

In conclusion, this technology has been widely adopted in various industries and has generated substantial economic benefits.

(1) Mechanism

① Microstructure Mechanism:

After cutting, the surface of the metal is left with cutting marks from the tool. At a microscopic level, the surface of the metal is observed to be uneven. Rolling processing is a type of pressure finishing, and the metal surface undergoes strong plastic deformation under the pressure of the hob.

According to engineering materials theories, the basic mechanism of plastic deformation in metal is slip, which occurs when the crystal slides relative to another part along a certain crystal plane and direction. Under external force, the crystal continues to slip, and the crystal grains gradually rotate from soft orientation to hard orientation during the deformation process. The grains are interlocked, hindering the deformation of each grain.

Since most metals used in industry are polycrystalline, they can withstand significant plastic deformation without being damaged. The continuous slippage of the crystal grains in the metal increases the dislocation density and crystal lattice distortion. Dislocations with opposite signs cancel each other out, while dislocations with the same sign are rearranged into smaller sub-grains. The finer the crystal grains, the higher the dislocation density, resulting in greater deformation and dispersion.

This means that it is difficult to produce local stress concentrations, improving the yield strength and fatigue performance of the rolled metal material.

② Surface Quality Mechanism:

The quality of the metal surface is often measured by its roughness, which is one of the main causes of stress concentration. A rough surface is more likely to form sharp cuts, causing stress concentration and making it a common source of fatigue.

Under alternating stress, stress concentration promotes the formation and propagation of fatigue cracks. The rougher the surface and the sharper the incision tip, the more serious the stress concentration.

Rolling strengthening involves using the rolling effect of the roller on the surface of the workpiece to cause plastic flow in the surface metal and fill in original low concave troughs, reducing the roughness of the workpiece surface and eliminating residual tool marks. This reduces stress concentration and improves the fatigue life of the workpiece.

③ Mechanism of Residual Compressive Stress:

As early as the 1930s, it was discovered that residual compressive stress on the surface of a part could prolong its fatigue life. The propagation of cracks on the surface of metal materials occurs when the alternating load applied reaches a certain limit, which 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 generate residual compressive stress, thereby increasing the fatigue life of the parts.

(2) Process parameters that affect the rolling effect

The key process parameters that impact the surface rolling outcome are: rolling pressure, number of rolls, and rolling speed.

Rolling pressure refers to the force applied by the roller on the surface of the workpiece and has a significant impact on its fatigue strength. However, current research on this is limited, and there is no precise mathematical formula to calculate the optimal rolling pressure. It is also influenced by factors such as the strength of the part, its size, and the roller’s diameter. In practice, the best rolling pressure is determined through trial and error.

The number of rolls refers to the number of times the roller presses the same location on the workpiece, and it has a significant impact on the fatigue strength of the workpiece. If the number of rolls is too low, the workpiece surface may not reach the desired plastic deformation. If the number of rolls is too high, the workpiece may experience contact fatigue, and the surface may deteriorate severely in severe cases.

Rolling speed is the rotation speed of the workpiece during the rolling process and has little effect on its fatigue strength, but it affects the efficiency of the rolling process. If the speed is too high, it may cause excessive plastic deformation, and if it is too slow, it may reduce production efficiency. In production, it is essential to determine the appropriate rolling speed based on the specific conditions.

(3) Hole extrusion reinforcement

Hole extrusion is a surface strengthening process that involves the use of specific tools, such as rods, bushings, and dies, to gradually and consistently apply pressure to the walls or periphery of a hole in a workpiece. This process results in the formation of a plastic deformation layer of a specific thickness, which improves the surface’s fatigue strength and resistance to stress corrosion.

There are several commonly used methods for hole extrusion, including rod extrusion, bushing extrusion, stamping die extrusion, and spinning extrusion, as illustrated 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 primarily used for workpieces that require improved fatigue resistance in their inner holes and cannot be achieved through other methods, such as critical components in airplanes.

Impression die extrusion is ideal for strengthening critical bearing parts, such as large components and skins, while spinning extrusion is best suited for enhancing the inner holes of large components, such as landing gear.

02 Plasma diffusion technology

Plasma is a mixture of free electrons and ions that acts like an ionized gas and is largely neutral. Plasma chemical heat treatment technology, also known as plasma diffusion technology (PDT) or particle bombardment diffusion technology, uses ions produced through gas glow discharge in a low vacuum environment to bombard the surface of the workpiece and alter its composition, structure, and performance.

Compared to traditional gas thermal diffusion technology, ion thermal diffusion has several advantages:

(1) Ion bombardment sputtering removes the oxygen (passivation) film or impurities from the surface of the workpiece, thereby improving its surface activity and making it easier to absorb the infiltrating elements. This accelerates the thermal diffusion rate.

(2) Plasma can activate the reaction gas and lower the chemical reaction temperature.

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

(4) It is an environmentally friendly process that does not pollute the environment.

Plasma can be divided into high temperature plasma and low temperature plasma. Examples of low-temperature plasma include aurora, fluorescent lamps, electric arcs, and iodine tungsten lamps, while fusion and the solar core belong to high-temperature plasma.

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

The transformation of a gas from an insulator to a conductor is referred to as gas discharge and requires a certain electric field strength and the presence of charged particles in the gas. In an electric field, charged particles move in a directional motion, leading to a series of physical and chemical changes between charged particles and gas atoms, and between charged particles and electrodes. This results from collisions between charged particles that cause gas excitation and ionization.

The collision causes electrons in atoms to transition from their normal energy level to a higher energy level, resulting in a metastable excited atom. When the excited electron returns to its ground state, it releases energy in the form of photons (glow). If the energy of the charged particle impact is large enough, it can knock an electron away from the atom, causing ionization.

1. The mechanism of ion nitriding

(1) Kolbel ion sputtering nitriding model

High-energy nitrogen ions impact the cathode, causing sputtering of Fe atoms from its surface. The Fe atoms then react with N atoms to form FeN, which is redeposited on the surface of the workpiece (backscattering).

The metastable FeN undergoes decomposition in the following sequence: FeN → Fe₂-₃N → Fe₄N. During this process, the decomposed N atoms penetrate into the surface or near-surface of the steel.

Simultaneously, a nitrided layer of Fe₂-₃N (ε phase) and Fe₄N (γ’ phase) is formed on the steel surface, from the outside towards the inside, as depicted in Figure 7.

Kolbel ion sputtering nitriding model

Figure 7 Kolbel ion sputtering nitriding model

(2) New ion nitriding model

A diagram of the new Direct Current (DC) ion nitriding model can be found in Figure 8, and the design of the ion nitriding device is depicted 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) The cleaned workpiece should be placed in the ion nitriding furnace and vacuumized to a pressure of approximately 1Pa.

(2) A small quantity of nitrogen-rich gas should be introduced, and the DC high-voltage power supply should be activated to cause the gas to glow and discharge.

(3) The surface of the workpiece should be sputtered and cleaned.

(4) The air pressure and voltage should be adjusted, the workpiece should be heated to the required processing temperature, and nitriding should commence.

(5) The workpiece should be maintained at the required temperature for a specified duration to achieve the desired nitriding layer thickness.

(6) Upon power cut-off, the workpiece should be cooled to below 200°C in a vacuum environment. The surface of the nitrided workpiece will appear silver gray.

3. Tissue types and influencing factors of ion nitriding

Nitriding is carried out in a temperature range that is lower than 590°C (the eutectoid temperature). As the nitrogen content increases, the structure of the nitrided layer changes from the exterior to the interior as follows: ε → ε + γ’ → γ’ + diffusion layer → α diffusion layer, as illustrated in Figure 10.

Surface structure morphology of 38CrMoAl steel after nitriding

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

The primary factors that impact the ion nitriding layer are as follows:

(1) Nitriding Temperature: The thickness of the nitriding layer increases with an increase in temperature.

  • When the temperature is below 550°C, the ratio of γ’ phase increases with the temperature.
  • When the temperature exceeds 550°C, the ratio of ε phase increases with the temperature.

(2) Nitriding Time: During the initial stage of nitriding (<30 minutes), the nitriding rate is much faster compared to gas nitriding. As time progresses, the infiltration rate decreases and eventually reaches the gas nitriding rate.

(3) Nitrogen Gas: Commonly used nitrogen gases include ammonia, nitrogen + hydrogen, etc.

(4) Nitrogen Gas Pressure, Voltage, and Current Density:

  • The thicker the nitriding layer, 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.

4. The performance of the ion nitriding layer

The performance of the ion nitriding layer is mainly evaluated based on the following indicators:

(1) Hardness: The hardness of the nitriding layer is determined by the nitriding temperature, the type of alloying elements present in the steel, and the type of steel.

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

(3) Toughness: In the nitrided layer, the diffusion layer has the highest toughness, followed by the single-phase compound layer (either ε phase or γ’ phase), and the γ’ + ε mixed phase has the lowest toughness.

(4) Wear Resistance: Compared to other nitriding methods, ion nitriding offers the best wear resistance against rolling friction.

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

Table 1 Ion nitriding process of commonly used steel grades

Steel gradeProcess parametersSurface hardness
layer depth
Total coating depth
Temperature (oC)Time (h)Pressure (Pa)

03. Laser surface treatment technology

Laser surface treatment technology involves utilizing the distinct characteristics of laser beams to process the surface of a material and form a treatment layer of specific thickness. This leads to significant enhancements in the mechanical, metallurgical, and physical properties of the material’s surface. As a result, it improves the resistance to wear, corrosion, and fatigue of parts and workpieces, making it an efficient and well-established surface treatment technology.

1. Features

(1) Laser beam treatment results in high chemical uniformity on the material’s surface, fine crystal grains, and increased surface hardness. This leads to improved wear resistance and high surface performance without sacrificing toughness.

(2) The process has low input heat and minimal thermal deformation.

(3) It features high energy density and a fast processing time.

(4) The treatment can be applied to specific parts such as deep holes, grooves, and other intricate areas that can be reached by laser.

(5) The process does not require a vacuum or result in chemical pollution.

(6) During the treatment, the surface layer undergoes martensite transformation and retains residual compressive stress, which enhances its fatigue strength.

2. Laser surface treatment equipment

Laser Surface Treatment Equipment comprises:

  • A laser
  • A power meter
  • A light guide focusing system
  • A worktable
  • A numerical control system
  • Software programming system.

3. The principle and characteristics of laser surface treatment technology

A laser is a type of electromagnetic wave that has the same phase, a specific wavelength, and a strong directional quality. The laser beam is controlled by a series of mirrors and lenses, which allow it to be focused into a beam with a small diameter (as small as 0.1mm) and high power density (ranging from 10⁴ to 10⁹W/cm²).

The interaction between the laser and metal can be divided into several stages based on the laser intensity and duration of radiation, including: absorption of the light beam, energy transfer, alteration of the metal structure, and laser action cooling.

Laser surface treatment technology uses a high-power density laser beam to heat the material surface in a non-contact manner, relying on the thermal conductivity of the surface to cool and achieve surface strengthening.

This technology offers several advantages for material processing:

  • Convenient energy transfer, allowing for selective strengthening of the surface of the workpiece.
  • High concentration of energy, resulting in a short processing time, small heat-affected zone, and minimal workpiece deformation.
  • Ability to handle workpieces with complex surface shapes and easy automation.
  • More significant modification effects compared to traditional methods, with high speed, efficiency, and low cost.
  • Typically only suitable for processing thin sheet metal and not ideal for thicker plates.

4. The type of tissue after laser surface treatment

The laser heating process is incredibly fast, leading to a high degree of superheat during the phase change process. This results in a high nucleation rate for crystal nuclei.

Due to the short heating time, carbon atom diffusion and grain growth are limited, leading to smaller austenite grains.

The cooling rate is also faster than with any quenching agent, making it easier to achieve a hidden needle or fine needle martensite structure.

The type of tissue formed on the steel surface treated with the laser beam can be determined by observation.

Low carbon steel can be divided into two layers: the outer layer is a completely quenched zone with a hidden needle martensite structure, while the inner layer is an incompletely quenched area that retains ferrite.

Medium carbon steel has four layers: the outer layer is a white, bright hidden needle martensite with a hardness of 800HV, which is over 100 higher than the general quenching hardness. The second layer is a combination of hidden needle martensite and a small amount of troostite, with a slightly lower hardness. The third layer is a combination of hidden needle martensite, mesh troostite, and a small amount of ferrite. The fourth layer is composed of cryptoneedle martensite and a complete ferrite mesh.

High carbon steel also has two layers: the outer layer is cryptographic martensite and the inner layer is a combination of cryptographic martensite and undissolved carbides.

Cast iron can be roughly divided into three layers: the surface layer is made up of dendritic crystals from melting and solidification and decreases with an increase in scanning speed. The second layer is the eutectic structure of cryptoneedle martensite and 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 referred to as Laser Quenching, involves the use of a high-energy density laser beam to irradiate the surface of the workpiece. The targeted area instantly absorbs the light energy and converts it into heat, causing a sharp rise in the temperature of the laser action zone and a rapid change in the structure type to austenite. After a quick cooling process, the result is very fine martensite and other structures.

The following are the key characteristics of Laser Quenching:

  • High-speed heating and cooling of the material surface: The heating speed can reach 10⁴ to 10⁹℃/s, while the cooling speed can reach 10⁴℃/s, improving scanning speed and production efficiency.
  • High surface hardness: The workpiece surface hardness after laser quenching is higher, typically 5% to 20% higher than conventional quenching hardness. The treatment results in a very fine hardened layer structure.
  • Minimal deformation: The fast laser heating speed results in a small heat-affected zone, with minimal quenching stress and deformation. It is widely believed that laser quenching treatment produces little deformation, and the phase transformation hardening can produce compressive stress greater than 4000 MPa on the surface, improving the fatigue strength of parts. However, deformation in parts with a thickness less than 5mm cannot be ignored.
  • Partial hardening of complex shapes: Parts with complex shapes and those that cannot be processed by other conventional methods, such as parts with grooves, can undergo partial hardening.
  • Short process cycle and high production efficiency: The laser quenching process has a short cycle and high production efficiency. The technology is easy to incorporate into the production line due to its high level of automation and ability to be computer-controlled.
  • Environmentally friendly: Laser quenching relies on its own thermal conductivity for self-cooling by conduction on the surface and inside without the need for a cooling medium, making it environmentally friendly and producing no pollution.

(2) Laser surface cladding

Laser Surface Cladding is a process of enhancing the surface strength by rapidly heating and melting alloy or ceramic powder and the substrate surface with a laser beam. The beam is then removed, allowing the material to cool and solidify.

The following are its key characteristics:

  • It has a high cooling rate (up to 10⁶℃/s), resulting in a structure with the hallmark traits of rapid solidification.
  • The process involves minimal heat input and distortion, with a low dilution rate of the coating (typically less than 5%) and strong metallurgical bonding with the substrate.
  • There are minimal restrictions on the choice of powder, making it especially suitable for depositing low-melting-point metals on high-melting-point alloys.
  • Selective area welding can be performed, using less material and offering excellent cost-effectiveness.
  • The laser beam can reach hard-to-access areas for welding.
  • The process is easily adaptable to automation.

(3) Laser surface alloying

Laser surface alloying is a process that involves the rapid melting and mixing of a thin layer of the base material with external alloying elements using a high-energy laser beam. This results in the formation of a surface melting layer with a thickness ranging from 10 to 1000 μm.

The cooling rate of the molten layer during solidification can be as high as 10⁵ to 10⁸ ℃/s, comparable to the cooling rate achieved through quenching technology.

In addition, the physical phenomena of diffusion and surface tension in the molten layer liquid result in the formation of a surface alloy layer of a predetermined depth and chemical composition in a short time frame of 50 μs to 2 ms.

The main advantage of the laser surface alloying process is that the changes in composition, structure, and performance occur only in the melting zone and a small affected zone, minimizing the thermal effect on the matrix and minimizing deformation. This process meets the requirements of surface use without sacrificing the overall structural characteristics.

The depth of melting is controlled through adjustments to laser power and irradiation time. A surface alloy layer with a thickness of 0.01 to 2mm can be formed on the base metal.

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

(4) Laser shock hardening

When a high-peak, high-power-density laser beam with a pulse duration of tens of nanoseconds is directed at a metal target, the metal’s surface absorbs the laser energy and instantly vaporizes, resulting in a high-temperature, high-pressure plasma.

When the plasma is confined by a confinement layer, it generates a high-intensity pressure shock wave that impacts the metal surface and then propagates into the metal.

When the shock wave’s peak pressure exceeds the dynamic yield strength of the material, it causes strain hardening on the material’s surface, leaving behind large compressive stress in the material.

This process is known as laser shock strengthening, which is also referred to as laser shot peening. It has the benefits of a deep strain influence layer, controllable impact area and pressure, minimal impact on surface roughness, and easy automation.

Compared to shot peening, laser shock treatment can result in a residual compressive stress layer that is 2 to 5 times deeper, reaching 1mm.

In contrast, strengthening techniques such as extrusion and impact strengthening can only be applied to flat or regularly shaped surfaces.

Furthermore, laser shock strengthening can preserve the surface roughness and dimensional accuracy of the strengthened area.

(5) Amorphization of laser surface

Laser surface amorphization is the process of using the rapid cooling conditions of a laser-generated molten pool to form a special, amorphous layer on the surface of certain alloys.

Compared to other amorphization methods, laser amorphization can produce a large area of amorphous layer on the workpiece surface and can also expand the composition of the amorphous layer.

04. Electron beam surface treatment technology

The process of increasing the temperature of a material’s surface and altering its composition and structure to improve its performance through the use of high-energy electron beams is called Electron Beam Surface Treatment.

It employs high-speed electrons in an electric field as energy carriers, and the electron beam can have an energy density of up to 10⁹W/cm².

The following are the key features of Electron Beam Surface Treatment:

  • The electron beam has a higher energy density, resulting in larger heating ranges and depths.
  • The equipment investment is low and the operation is more straightforward (no need for pre-processing “blackening” as in laser beam processing).
  • The size of parts is limited due to the vacuum conditions.

1. The principle of electron beam surface treatment technology

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

As the negatively charged electron beam travels towards the high-potential positive electrode at high speed, it is accelerated by an accelerator and focused by an electromagnetic lens, enhancing the power of the beam.

After the second focusing, its energy density becomes highly concentrated, causing it to rush towards a small area on the surface of the workpiece at high speed.

Most of the kinetic energy carried by the electron beam is transformed into heat energy, resulting in the impacted part of the material surface to rapidly rise to several thousand degrees Celsius within a fraction of a microsecond. This causes the material to instantly melt or vaporize.

2. Equipment for electron beam surface treatment technology

The electron beam surface treatment technology equipment comprises five systems:

  • The electron gun system, which emits a high-speed flow of electrons.
  • The vacuum system, which maintains the required vacuum level.
  • The control system, which regulates the size, shape, and direction of the electron beam.
  • The current system, which provides stabilized high and low voltage current.
  • The transmission system, which manages the movement of the worktable.

3. Features of electron beam surface treatment technology

① The workpiece is heated in a vacuum chamber, resulting in no oxidation or decarburization. The surface phase change strengthening process does not require a cooling medium. Instead, relying on the cooling behavior of the matrix itself, “green surface strengthening” can be achieved.

② The electron beam has an energy conversion rate of approximately 80% to 90%, which allows for concentrated energy and high thermal efficiency. This makes it possible to achieve local phase transformation strengthening and surface alloying.

③ The electron beam’s concentrated heat results in a small heat point of action and minimal thermal stress during heating. Additionally, the shallow hardened layer leads to minimal structural stress and surface transformation strengthening distortion.

④ The cost of electron beam surface treatment equipment is less than that of laser equipment, with a one-time input of less than 1/3 of laser costs, and the cost of electron beam treatment being only half that of laser treatment.

⑤ The equipment structure is simple, with the electron beam being rotated and scanned through magnetic deflection. There is no need for workpiece rotation, movement, or light transmission mechanisms.

⑥ Electron beam surface treatment has a wide range of applications and can be used for the surface treatment of various materials, including steel and cast iron, as well as for parts with complex shapes.

⑦ Electron beams are easily excitable and can produce X-rays, so it is important to take care to protect against them 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 undergoing martensitic transformation, the success of the process lies in controlling the parameters. The average power density of the electron beam should be between 10⁴ and 10⁵ W/cm², while the heating rate should range from 10³ to 10⁵ ℃/s. The cooling rate should be able to reach 10⁴ to 10⁶ ℃/s.

The rapid fusion of the electron beam creates a supersaturated solid solution, strengthening the material and forming ultrafine martensite. This increases the hardness of the material and leaves residual compressive stress on the surface, thus enhancing its wear resistance.

(2) Electron beam surface remelting treatment

Electron beam remelting has the ability to redistribute the chemical elements of alloys and reduce the micro-segregation of certain elements, resulting in improved surface performance of the workpiece.

Additionally, since the remelting process is conducted in a vacuum environment, it helps prevent surface oxidation.

As a result, electron beam remelting is an ideal treatment for the surface improvement of chemically active magnesium and aluminum alloys.

(3) Electron beam surface alloying

Typically, elements such as tungsten (W), titanium (Ti), boron (B), molybdenum (Mo) and their carbides are selected as alloying elements to enhance the wear resistance of materials.

The addition of elements like nickel (Ni) and chromium (Cr) can improve the corrosion resistance of the material.

Furthermore, the appropriate combination of elements such as cobalt (Co), nickel (Ni), silicon (Si) and others can enhance the overall alloying effect.

(4) Amorphization treatment of electron beam surface

By increasing the average power density of the electron beam to a range of 10⁶ to 10⁷ W/cm² and shortening the exposure time to approximately 10-⁵ seconds, a substantial temperature gradient can be created between the substrate and the molten surface of the metal.

Once the electron beam irradiation has been stopped, the cooling rate of the metal surface, at 10⁷ to 10⁹ s-¹, significantly exceeds the cooling rate in conventional amorphous preparation processes, which is in the range of 10³ to 10⁶ s-¹.

As a result, the obtained amorphous structure is dense and boasts excellent resistance to both fatigue and corrosion.

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

When the electron beam is utilized as a heat source for annealing a thin layer on the surface, the necessary power density is much lower than that required for the previous method, resulting in a slower cooling rate of the material.

This method is mainly used for the surface treatment of thin strips made of metal materials.

Moreover, electron beam annealing has also been effectively applied to semiconductor materials.

5. Application of electron beam surface strengthening technology

After the die steel’s surface is strengthened through electron beam treatment, the outermost layer of the material undergoes melting. When the thickness of the remelted layer reaches around 10 μm, this melting results in a decrease in surface microhardness.

The surface carbide particles dissolve into the matrix solid solution chromium and cause an increase in energy, leading to supersaturated solid solution strengthening and the formation of ultra-fine martensite. This results in an increase in sample microhardness from 955.2 HK to 1169 HK and an increase in relative wear resistance by 5.63 times.

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

05. EDM surface treatment technology

The electric spark surface treatment technology is based on the principle that an energy storage power source is passed through an electrode to create a spark discharge between the electrode and the workpiece at a frequency ranging from 10 to 2000 Hz.

The electrode, made of conductive material, melts onto the surface of the workpiece to form an alloy layer that enhances the physical and chemical properties of the surface.

The effectiveness of the EDM surface strengthening layer is influenced by both the base material and the electrode material. Common electrode materials include TiC, WC, ZrC, NbC, Cr3C2, and cemented carbide.

1. EDM surface treatment technology process

Figure 12 illustrates the process of electrical spark surface treatment technology.

In Figure 12(a), when the distance between the electrode and the workpiece is large, the power supply charges the capacitor through resistor R, and the electrode is brought closer to the workpiece through the action of a vibrator.

Figure 12(b) shows that when the gap between the electrode and the workpiece reaches a certain distance, the strong electric field ionizes the air in the gap, resulting in spark discharge.

The discharge causes partial melting or even vaporization of the metal at the point of contact between the electrode and the workpiece. The electrode continues to approach and make contact with the workpiece, causing the spark discharge to stop and a short-circuit current to flow through the contact point for further heating.

In Figure 12(c), proper pressure from the electrode onto the workpiece ensures that the molten materials bond and diffuse to form alloys or new compounds.

Finally, as shown in Figure 12(d), the electrode is separated from the workpiece through the action of an oscillator.

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 causes the electrode material and the matrix material on the surface of the workpiece to partially melt. The pressure generated by the thermal expansion of the gas and the mechanical impact force of the electrode then causes the electrode material and the matrix material to fuse and undergo physical and chemical interaction. The interaction 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 occurs in both the melting zone and at the boundary between the liquid and solid phases.

Because of the brief diffusion time, the diffusion of liquid elements into the matrix is limited, resulting in a shallow diffusion layer. However, this allows for improved metallurgical bonding between the matrix and the alloy layer.

(3) Rapid phase change process.

The heat-affected zone of the workpiece matrix experiences rapid heating and cooling, causing the portion near the melting zone to undergo an austenitization and martensitization transformation. This process refines the grain structure, increases hardness, and creates residual compressive stress.

These effects are beneficial in improving fatigue strength.

2. The characteristics of electric spark surface treatment technology

(1) Advantages:

① The equipment is simple and cost-effective;

② The bonding between the strengthening layer and substrate is strong and reliable;

③ The workpiece’s internal temperature remains low or unchanged, preventing changes in structure and performance and avoiding annealing and deformation;

④ Low energy and material consumption;

⑤ Processing objects have no size limitations, making it ideal for local processing on large workpieces;

⑥ The surface strengthening effect is significant;

⑦ It can be used to repair excessively worn workpieces;

⑧ Easy to operate and master.

(2) Disadvantages:

① The surface strengthening layer is shallow, typically only 0.02-0.5mm deep;

② The surface roughness will not be very low;

③ Small holes and narrow grooves are challenging to process, resulting in poor uniformity and continuity of the surface strengthening layer.

Metal surface modification technology

01. Electroplating

1.Definition and principle of electroplating

Electroplating is a surface treatment process that utilizes electrochemical principles to deposit a specific type of metal coating onto the surface of the item being plated.

The Principle of Electroplating: The base metal to be plated serves as the cathode in a salt solution containing the desired metal for plating.

The process of electrolysis then causes the cations of the desired metal in the plating solution to be deposited onto the surface of the base metal, resulting in the formation of a plating layer (as depicted in Figure 13).

Electroplating principle

Figure 13 Electroplating principle

The purpose of electroplating:

Through electroplating, a surface layer with properties that differ from the base material can be obtained. This layer can improve the surface’s resistance to corrosion and wear.

Typically, the thickness of the coating ranges from a few microns to several tens of microns.

Features of electroplating:

The equipment used in the electroplating process is relatively straightforward and the operating conditions are easily controlled.

Due to the wide range of coating materials available and its relatively low cost, electroplating has become a widely utilized method for material surface treatment in various industries.

2.Classification of coating

Coatings can be classified into several types based on their performance:

  • Protective Coatings: Zinc, zinc-nickel, nickel, cadmium, tin, and other coatings can provide anti-corrosion protection against various corrosive environments and the atmosphere.
  • Protective-Decorative Coatings: Copper-nickel-chromium (Cu-Ni-Cr) coatings are both decorative and protective.
  • Decorative Coatings: Gold and copper-zinc (Cu-Zn) imitation gold coatings, black chromium, black nickel coatings, etc. are used for decorative purposes.
  • Wear-Resistant and Anti-Friction Coatings: Hard chromium coatings, loose hole coatings, nickel-silicon carbide (Ni-Sic) coatings, nickel-graphite coatings, nickel-PTFE composite coatings, etc. are used to reduce wear and friction.
  • Electrical Performance Coatings: Gold (Au) and silver (Ag) coatings, etc. have high conductivity and prevent oxidation, avoiding an increase in contact resistance.
  • Magnetic Coatings: Soft magnetic coatings include nickel-iron (Ni-Fe) coatings and iron-cobalt (Fe-Co) coatings; hard magnetic coatings include cobalt-phosphorus (Co-P) coatings, cobalt-nickel (Co-Ni) coatings, cobalt-nickel-phosphorus (Co-Ni-P) coatings, etc.
  • Solderability Coating: Tin-lead (Sn-Pb) coating, copper (Cu) coating, tin (Sn) coating, and silver (Ag) coating, etc. improve solderability and are widely used in the electronics industry.
  • Heat-Resistant Coating: Nickel-tungsten (Ni-W) coating, nickel (Ni) coating, and chromium (Cr) coating, etc. have high melting points and high-temperature resistance.
  • Repair Coating: Electroplating can be used to repair expensive wear parts or processing out-of-tolerance parts, saving costs and extending the service life.

For example, layers of Ni, Cr, and Fe can be electroplated for repairs. Based on the electrochemical properties between the coating and the base metal, the coating can be divided into anodic and cathodic coatings.

An anodic coating occurs when the potential of the coating relative to the base metal is negative. An example of this is a zinc coating on steel. On the other hand, a cathodic coating occurs when the potential of the plating layer relative to the base metal is positive. Examples of this are nickel-plated and tin-plated layers on steel.

In terms of combination form, coatings can be divided into single-layer coatings (such as Zn or Cu layers), multi-layer metal coatings (such as Cu-Sn/Cr coatings, Cu/Ni/Cr coatings, etc.), and composite coatings (such as Ni-Al₂O₃ coatings, Co-SiC coatings, etc.).

When classified according to coating composition, coatings can be further divided into single metal coatings, alloy coatings, and composite coatings.

3. The basic composition of electroplating solution

The primary metal salts found in salt deposits include:

Single salts, such as copper sulfate and nickel sulfate;

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

The complexing agent forms a complex with the metal ions being deposited and primarily serves to alter the electrochemical properties of the plating solution and regulate the metal ion deposition process.

The complexing agent is a crucial component of the plating solution and has a significant impact on the coating quality.

Common complexing agents include cyanide, hydroxide, pyrophosphate, tartrate, nitrilotriacetic acid, and citric acid, among others.

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

For instance, adding Na2SO4 to a nickel plating solution.

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

The buffer is an important process parameter in weakly acidic or weakly alkaline baths.

The buffer is added to give the plating solution the ability to adjust its pH value and maintain a stable pH during the plating process.

The buffer must be present in sufficient amounts to effectively control the acid-base balance, usually added in an amount of 30-40g/L, such as boric acid in a potassium chloride zinc plating solution.

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

Most electroplating solutions rely on soluble anodes to supply metal ions, ensuring that the amount of metal deposited on the cathode is equal to the amount of metal dissolved from the anode, keeping the composition of the plating solution balanced.

The addition of an activator can maintain the activity of the anode without passivation and sustain the normal dissolution reaction.

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

Special additives are added to improve the performance of the plating bath and the quality of the coating. This step is crucial in electroplating.

The amount of additives added is usually small, only a few grams per liter, but the impact is significant.

There are various types of these additives, which can be classified into:

(1) Brightener – improves the brightness of the coating.

(2) Grain refining agent – alters the crystallization conditions of the coating, refines the crystal grains, and makes the coating dense.

For example, the addition of a condensate of epichlorohydrin and amines to a zincate zinc plating bath can change the coating from spongy to dense and bright.

(3) Leveling agent – improves the micro-dispersion capability of the plating solution and smooths the micro-rough surface of the substrate.

(4) Wetting agent – reduces the interfacial tension between the metal and the solution, making the coating better adhere to the substrate and reducing pinholes.

(5) Stress relief agent – reduces the stress in the coating.

(6) Coating hardener – improves the hardness of the coating.

(7) Masking agent – eliminates the influence of trace impurities.

4.Basic steps of the electroplating process

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

5.Factors affecting electroplating quality

(1) Plating Solution:

The key factors that determine the quality of the plating solution include the solubility of the main salt, the ion coordination, the presence of any additional salts, the pH value, the potential for hydrogen evolution, and the current parameters such as current density, current waveform, additives, temperature, and stirring. The properties of the base metal and its surface processing state also play an important role, as does the pretreatment process.

(2) Electroplating Method: Rack Plating

Rack plating is a method of electroplating metals such as tungsten (W), molybdenum (Mo), titanium (Ti), and vanadium (V), which cannot be electroplated from an aqueous solution individually. By co-depositing these metals with iron group elements like iron (Fe), cobalt (Co), or nickel (Ni), alloys can be formed, resulting in an appearance that cannot be achieved with a single metal.

(3) Conditions for Depositing Alloy

To successfully deposit an alloy, two conditions must be met:

① At least one of the two metals must be able to be deposited from an aqueous solution of its salt.

② The deposition potential of the two metals must be very close to each other.

02. Chemical plating

Electroless plating is a type of surface processing method that utilizes chemical reactions to deposit metal onto the surface of a substrate to form a plating layer. Unlike electroplating, which uses an electrical current to drive the deposition process, electroless plating relies on chemical reactions to reduce metal ions in the solution to metal.

There are three methods for carrying out electroless plating.

1.Displacement deposition

The process of immersion plating in engineering involves the replacement of deposited metal ions on the surface of the workpiece with metal M2 (such as Cu), which is more positive than the metal to be plated (M1, such as Fe) present in the solution.

Coating thickness is limited as the deposition stops once metal M1 is completely covered by metal M2.

Displacement deposition is used in processes such as iron immersion copper plating, copper immersion mercury plating, and aluminum zinc plating.

Immersion plating can be challenging to produce practical coatings, and as a result, it is frequently used as a supplementary process for other plating methods.

2.Contact deposition

In addition to M1 and M2, there is a third metal, M3, involved in the plating process.

When M2 ions are present in the solution, the two metals M1 and M3 are connected and electrons flow from M3 (which has a high potential) to M1 (which has a low potential), causing the reduction and deposition of M2 onto M1.

Deposition stops when the contact metal M1 is completely covered by M2.

When electroless nickel plating is performed on functional materials without autocatalytic properties, contact deposition is often used to initiate nickel deposition.

3.Reduction deposition

The process of converting metal ions into metal atoms through the reduction of the reducing agent, which releases free electrons through oxidation, is known as reduction deposition.

The reaction equation can be represented as follows:

Reductant oxidation

Rn+ → 2e- + R(n + 2)+

Metal ion reduction

M2+ + 2e- → M

The term “chemical plating in engineering” mainly refers to the process of reduction deposition chemical plating.

The following are the conditions for electroless plating:

  • The reducing agent in the plating solution has a significantly lower reduction potential than the deposited metal, allowing the metal to be reduced and deposited on the substrate.
  • The plating solution is prepared in such a way that it does not undergo spontaneous decomposition, and metal deposition only occurs when it comes into contact with a catalytic surface.
  • The reduction rate of the metal can be controlled by adjusting the pH and temperature of the solution, which in turn adjusts the plating rate.
  • The metal precipitated through reduction has catalytic properties, allowing the redox deposition process to continue and the coating to be continuously thickened.
  • The reaction product does not interfere with the normal progression of the plating process, meaning that the solution has a sufficient service life.

There are many types of metals and alloys that can be used for electroless plating, including Ni-P, Ni-B, Cu, Ag, Pd, Sn, In, Pt, Cr, and many Co-based alloys, with electroless nickel plating and electroless copper plating being the most commonly used.

Electroless plating generally has good corrosion resistance, wear resistance, brazing properties, and other special electrical or magnetic properties, making it an effective method for improving the surface properties of materials.

03. Thermal spray technology, thermal spray welding technology

Thermal spray technology and thermal spray welding technology utilize thermal energy sources (such as an oxygen-acetylene flame, electric arc, or plasma flame) to melt specialized coating materials and apply them onto a workpiece, forming a protective layer.

This technology is known for its ability to create relatively thick coatings (ranging from 0.1 to 10mm) and is primarily utilized in the manufacturing and repair of composite layer components.

1.Thermal spray technology

(1) Principles and Characteristics of Thermal Spraying Technology

In thermal spraying, various heat sources are utilized to heat the coating material to a melted or semi-melted state. The melted material is then dispersed and refined using a high-speed gas, which impacts the surface of the substrate at high velocity to form a coating, as illustrated 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 is comprised of four main stages:

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

(2) Coating materials

Thermal spraying has specific requirements for coating materials, which must meet the following conditions:

  • A wide liquid phase zone that is not prone to decomposition or volatilization at the spraying temperature.
  • Good thermal stability.
  • Good performance and wettability.
  • Good solid fluidity (for powder materials).
  • An appropriate coefficient of thermal expansion.

Coating materials can be divided into two categories based on their shape: wire and powder.

(3) Combination mechanism of thermal spray coating

①Mechanical bonding: In this type of bonding, particles in a molten state collide with the surface of the substrate and spread out into a thin, flat liquid layer. The layer becomes embedded in the substrate’s undulating surface, forming a mechanical bond.

②Metallurgical bonding: This type of bonding is achieved through diffusion and welding between the coating and the substrate surface.

③Physical bonding: When molten particles moving at high speeds collide with the substrate surface, if the distance between the two sides of the interface falls within the range of the atomic lattice constant, the particles bond together through van der Waals forces.

(4) The formation process of the coating

① The spray material is heated until it reaches a molten state.

② The material is then atomized into small droplets and directed at high speed towards the surface of the substrate.

The stronger the impact of the particles on the substrate and the greater their kinetic energy, the stronger the bond of the resulting coating.

③ Upon impact with the substrate surface, the molten, high-speed particles deform and eventually condense to form a coating.

The formation of the coating is depicted in Figure 15.

Schematic diagram of the coating formation process

Figure 15 Schematic diagram of the coating formation process

The coating structure consists of flat particles of varying sizes, unmelted spherical particles, inclusions, and pores.

The presence of pores can be attributed to the following reasons:

  • Low kinetic energy of unmelted particles upon impact.
  • The shadowing effect caused by variations in the spray angle.
  • The solidification shrinkage and stress release effect.

Well-controlled pores in the coating can bring several benefits, such as the ability to store lubricants, improved thermal insulation performance, reduced internal stress, and increased thermal shock resistance.

However, an excessive number of pores can have negative effects on the coating, such as reduced corrosion resistance, increased surface roughness, and decreased bonding strength, hardness, and wear resistance.

It is therefore crucial to carefully regulate the number of pores in the coating during its preparation.

2.Thermal spray welding technology

(1) Principles and Characteristics of Thermal Spray Welding Technology

Thermal spray welding technology is a process of strengthening a surface by using heat to remelt or partially melt the coating material and then condense it onto the surface of the substrate to create a surface layer with a metallurgical bond with the substrate, also known as sintering.

Compared to other surface treatment methods, the structure produced by thermal spray welding is dense with minimal metallurgical defects and a high bonding strength with the substrate. However, it has a limited range of material selection. The substrate deformation is greater compared to thermal spraying and the composition of the thermal spray layer is altered from the original composition.

(2) Classification of Thermal Spray Welding Technology

Thermal spray welding technology is mainly divided into two categories: flame spray welding and plasma spray welding.

① Flame Spray Welding: The process involves spraying powder onto the surface of the substrate, then heating the coating directly with a flame to remelt 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 is characterized by its simple equipment and process, high bonding strength between the coating and substrate, and good erosion resistance of the coating.

② Plasma Spray Welding: This method uses a plasma arc as the heat source to heat the substrate to form a molten pool on the surface. At the same time, the spray welding powder material is introduced into the plasma arc and preheated in the arc column. The powder is then sprayed into the molten pool by the flame, fully melted, and discharges gas and slag. After the spray gun is removed, the alloy molten pool solidifies to form a spray welding layer.

Features of Plasma Spray Welding:

  • High production efficiency
  • Ability to spray weld refractory materials
  • Low dilution rate
  • Good process stability
  • Easy to automate
  • Smooth spray-welded layer
  • Uniform composition and structure
  • Ability to achieve thicker coatings
  • Precise control of the process

(3) The Difference between Thermal Spray Welding Technology and Thermal Spray Technology:

① Workpiece surface temperature: The surface temperature of the workpiece during spraying is less than 250°C, while the temperature during spray welding must be greater than 900°C.

② Bonding state: The spray coating is mainly held together through mechanical bonding, while the spray-welded layer is formed through metallurgical bonding.

③ Powder material: Spray welding uses self-fluxing alloy powder, while there are no restrictions on the powder used in thermal spraying.

④ Coating structure: The thermal spray coating has pores, whereas the plasma spray-welded layer is uniform, compact, and non-porous.

⑤ Bearing capacity: The plasma spray-welded layer has the ability to bear impact loads and higher contact stresses.

⑥ Dilution rate: The dilution rate of the plasma spray-welded layer is around 5% to 10%, while the dilution rate of the thermal spray coating is close to zero.

04. Chemical conversion film technology

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

This technology is primarily utilized for anti-corrosion and surface embellishment of workpieces, and can also enhance the wear resistance of these components.

It involves contact between a specific metal and a corrosive liquid phase, under specific conditions, resulting in a chemical reaction.

This reaction, due to concentration polarization and anode-cathode polarization, creates a layer of insoluble corrosion products with strong adhesion on the metal surface.

These films protect the base metal from corrosive substances such as water and can also improve the adhesion and aging resistance of organic coating films.

The two main forms of conversion film technology used in production are phosphating treatment and oxidation treatment.

1.Phosphating treatment

Phosphating is a process in which steel materials are treated with a phosphate solution to form a water-insoluble phosphate film. The steps involved in this process are as follows:

  • Chemical oil removal
  • 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, among others, which appear as gray-white or gray-black crystals. The film forms a strong bond with the base metal and has high resistivity. Compared to an oxide film, the phosphating film has higher resistance to corrosion.

The phosphating film offers good resistance to corrosion in atmospheric, oily, and benzene media, but has poor resistance to acids, alkalis, ammonia, seawater, and steam.

The main methods for phosphating treatment are dipping, spraying, and a combination of dipping and spraying. Phosphating is further categorized based on the temperature of the solution into room temperature, medium temperature, and high temperature phosphating.

The impregnation method is ideal for high, medium, and low temperature phosphating processes. This method can handle workpieces of any shape, resulting in phosphating films of different thicknesses, with simple equipment and stable quality.

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

The spray method is suitable for medium and low-temperature phosphating processes and is ideal for large-area workpieces, such as car shells, refrigerators, washing machines, and other large items used as paint primers or for cold deformation processing. This method has a short processing time and fast film formation speed, but it can only produce thin to medium-thick phosphating films.

2.Oxidation treatment

(1) Steel Oxidation Treatment

The steel oxidation treatment, also known as bluing, is a process of exposing the steel workpiece to an oxidizing solution to form a dense and firm Fe3O4 film on the surface. The film has a thickness of around 0.5 to 1.5 micrometers.

Bluing does not usually affect the precision of parts and is often used for decorative protection of tools and instruments. This treatment can improve the corrosion resistance of the workpiece surface, relieve residual stress, reduce deformation, and enhance its surface appearance.

The most commonly used method for steel oxidation treatment is the alkaline method. The composition and process conditions of the oxidizing solution can be selected based on the material and performance requirements of the workpiece.

A commonly used solution is composed of 500 g/L sodium hydroxide, 200 g/L sodium nitrite, and the balance being water. The solution should be processed for 6 to 9 minutes at a temperature of around 140°C.

(2) Aluminum and Aluminum Alloy Oxidation Treatment

① Anodization

Anodization is a method in which the workpiece is placed in an electrolyte and subjected to an electric current to produce an oxide film with high hardness and strong adsorption properties. The most commonly used electrolytes are 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 treated with hot water to transform the oxide film into water-containing alumina, which is closed due to volume expansion. It can also be sealed with a potassium dichromate solution to prevent corrosive substances from penetrating the substrate through the crystalline crevices of the oxide film.

② Chemical Oxidation

Chemical oxidation is a method in which the workpiece is immersed in a weak alkali or weak acid solution to produce an oxide film that is firmly bonded to the aluminum substrate. This method is mainly used to enhance the corrosion resistance and wear resistance of the workpiece, as well as for surface decoration of aluminum and aluminum alloys, such as anti-rust aluminum for construction and decorative films for signs.

05. Vapor Deposition Technology

Vapor deposition technology is a new form of coating technology that involves the deposition of vapor-phase substances containing deposition elements onto the surface of a material to form a thin film. This process can be achieved through either physical or chemical methods.

Based on the principles behind the deposition process, vapor deposition technology can be categorized into two types: physical vapor deposition (PVD) and chemical vapor deposition (CVD).

1.Physical vapor deposition

Physical Vapor Deposition (PVD) is a technology that uses physical methods to vaporize materials into atoms, molecules, or ions under vacuum conditions. The vaporized material is then deposited onto the surface of a material through a gas phase process, forming a thin film.

There are three main methods of PVD: vacuum evaporation, sputtering deposition, and ion plating.

Vacuum evaporation involves evaporating film-forming materials to vaporize or sublimate and depositing them onto the surface of a workpiece. The heating method used depends on the melting point of the evaporation material and can include resistance heating, electron beam heating, or laser heating. Vacuum evaporation has the advantage of simple equipment and process, but the low kinetic energy of the vaporized particles leads to weak bonding between the coating and substrate, resulting in poor impact and wear resistance.

Sputtering deposition involves ionizing argon under vacuum to form argon ions that are accelerated and bombard a cathode. The sputtered particles are deposited onto the surface of the workpiece, forming a film. This method has the advantage of a wide range of applicable materials and good throwing ability, but it also has the disadvantages of slow deposition speed and expensive equipment.

Ion plating uses gas discharge technology under vacuum to ionize evaporated atoms and deposit them onto the surface of a workpiece, along with high-energy neutral particles. This method results in high-quality coatings with strong adhesion, good leveling ability, and fast deposition speed. However, the equipment used for ion plating is complex and expensive.

PVD has a wide range of applicable base materials and film materials, and it is a simple process that saves material and has no pollution. The resulting film has strong adhesion, uniform film thickness, and few pinholes. PVD is widely used in industries such as machinery, aerospace, electronics, optics, and light industry to prepare wear-resistant, corrosion-resistant, heat-resistant, conductive, insulating, optical, magnetic, piezoelectric, and superconducting films.

2. Chemical vapor deposition

Chemical Vapor Deposition (CVD) is a process in which a mixture of gases interacts with the surface of a substrate at a specific temperature, resulting in the formation of a metal or compound film on the substrate’s surface.

The following are some of the key characteristics of CVD:

  • There are various types of deposits that can be classified into deposited metals, semiconductor elements, carbides, nitrides, borides, etc.
  • The composition and crystal form of the film can be controlled over a wider range.
  • It is capable of evenly coating parts with complex shapes.
  • The deposition rate is fast and the film produced is dense and strongly bonded to the substrate.
  • CVD is well-suited for mass production.

Due to its good wear resistance, corrosion resistance, heat resistance, and special properties in electrical and optical fields, CVD has been widely used in industries such as machinery manufacturing, aerospace, transportation, and coal chemicals.

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Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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