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Laser Cutting Machine

In the current metal plate and pipe cutting, laser cutting machine is undoubtedly the best choice. It can not only cut various shapes, but also has small cutting gap, high precision and high cutting efficiency. Today, it is very cost-effective.

5 Metal Surface Modification Technologies You Should Know

1. Electroplating

1. Definition and principle of electroplating

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

Electroplating principle: in the salt solution containing the metal to be plated, with the plated base metal as the cathode, the cation of the metal to be plated in the plating solution is deposited on the surface of the base metal through electrolysis to form a coating.

As shown in Fig. 1.

schematic diagram of electroplating

Fig. 1 schematic diagram of electroplating

The purpose of electroplating is to obtain a surface layer with special properties different from the base material and improve the corrosion resistance and wear resistance of the surface.

The coating thickness is generally several microns to tens of microns.

Electroplating features: the electroplating process and equipment are relatively simple, the operating conditions are easy to control, the coating materials are widely used and the cost is low. Therefore, it is widely used in industry and is an important method for material surface treatment.

2. Classification of coatings

There are many kinds of coatings, which are classified as follows according to their performance:

(1) Protective coating:

For example, zinc, zinc nickel, nickel, cadmium, tin and other coatings are used as anti-corrosion coatings resistant to atmosphere and various corrosive environments.

(2) Protective decorative coating:

For example, Cu Ni Cr coating has both decorative and protective properties.

(3) Decorative coating:

For example, Au and Cu Zn imitation gold coatings, black chromium, black nickel coatings, etc.

(4) Wear resistant and wear reducing coatings:

For example, hard chromium coating, loose hole coating, Ni SiC coating, Ni graphite coating, Ni PTFE composite coating, etc.

(5) Plating performance:

For example, Au coating and Ag coating not only have high conductivity, but also can prevent oxidation and avoid increasing contact resistance.

(6) Magnetic coating:

For example, the coatings with soft magnetic properties include Ni Fe coating and Fe Co coating; Hard magnetic properties include Co-P coating, Co-Ni coating, Co-Ni-P coating, etc.

(7) Weldable coating:

For example, Sn Pb coating, Cu coating, Sn coating, Ag coating, etc. It can improve weldability and is widely used in electronic industry.

(8) Heat resistant coating:

For example, Ni-W coating, Ni coating and Cr coating have high melting point and high temperature resistance.

(9) Coating for repair:

For some easily worn parts with high cost or parts with out of tolerance processing, electroplating repair size can save cost and prolong service life.

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

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

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

When the potential of the coating relative to the base metal is positive, the coating is cathode, which is called cathode coating, such as nickel coating and tin coating 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;

Multilayer metal coating, such as Cu Sn / Cr coating, Cu / Ni / Cr coating, etc;

Composite coating, such as Ni Al3O₃ coating, CO SiC coating, etc.

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

3. Basic composition of electroplating solution

The salts of main salt deposited metal mainly include: single salt, such as copper sulfate, nickel sulfate, etc;

Complex salts, such as sodium zincate, sodium cyanazinate, etc.

The main function of the coordination agent is to change the electrochemical properties of the plating solution and control the electrode process of metal ion deposition.

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

Common compounding agents include cyanide, hydroxide, pyrophosphate, tartrate, aminotriacetic acid, citric acid, etc.

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

For example, Na ν SO ₄ is added to the nickel plating solution.

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

The pH value of buffer is an important process parameter in weak acid or weak alkaline plating solution.

Add buffer to make the plating solution have the ability to adjust pH value by itself, so as to keep the pH value stable in the plating process.

The buffer must have sufficient amount to effectively control the acid-base balance.

Generally, 30 ~ 40g / L is added, such as boric acid in potassium chloride galvanizing solution.

The metal ions of anode activator are continuously consumed in the electroplating process.

Most plating solutions rely on soluble anode to supplement, so as to make the cathode precipitation of metal equal to the dissolution of anode and maintain the composition balance of plating solution.

Adding active agent can maintain the active state of anode without passivation and maintain normal dissolution reaction. For example, Cl – must be added to the nickel plating solution to prevent nickel anode passivation.

Special additives in order to improve the performance of the bath and improve the quality of the coating, it is often necessary to add some special additives.

Its dosage is less, generally only a few grams per liter, but the effect is remarkable.

There are many kinds of such additives, which can be divided into:

(1) Brightener – can improve the brightness of the coating.

(2) Grain refiner – can change the crystal state of the coating, refine the grain and make the coating dense.

For example, in zincate zinc plating solution, adding additives such as the condensation of epichlorohydrin and amine, the coating can change from spongy to dense and bright.

(3) Leveling agent – it can improve the micro dispersion ability of the plating solution and flatten the micro rough surface of the substrate.

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

(5) Stress relieving agent – can reduce the stress of 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 electroplating process

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

5. Factors affecting electroplating quality

(1) Plating solution:

  • Solubility of main salt, ions and additional salt;
  • PH value;
  • Hydrogen evolution;
  • Current parameters: current density, current waveform;
  • Additive;
  • Temperature;
  • Stir;
  • Base metal: property and surface processing state;
  • Pretreatment.

(2) Plating method: hanging plating.

Metals such as W, Mo, Ti and V that cannot be plated separately from aqueous solution can be co deposited with iron group elements (Fe, CO and Ni) to form alloys, so as to obtain the appearance that a single metal cannot get.

(3) Conditions for alloy deposition:

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

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

2. Electroless plating

Electroless plating refers to a surface processing method in which the metal ions in the solution are reduced to metal by chemical method without external current, and deposited on the surface of the substrate to form a coating.

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

There are three ways to complete the process.

1. Displacement deposition

Using the potential of the plated metal M ₁ (such as Fe) is more negative than that of the deposited metal M ₁ (such as Cu), the deposited metal ions are replaced on the workpiece surface from the solution.

This method is called dip coating in engineering.

When the metal M ₁ is completely covered by the metal M ₁, the deposition stops, so the coating is very thin.

Iron immersion copper plating, copper immersion mercury plating and aluminum galvanizing are such displacement deposits.

It is difficult to obtain practical coating by dip coating, which is often used as an auxiliary process for other kinds of plating.

2. Contact deposition

In addition to the plated metal M ₁ and deposited metal M ₁, there is a third metal M ₃.

In the solution containing M З ion, connect the two metals of M З – M З, and the electrons flow from m З with high potential to M З with low potential, so as to reduce and deposit M З on M З.

When the contact metal M ₁ is also completely covered by M ₁, the deposition stops.

When electroless nickel plating is carried out on functional materials without autocatalysis, contact deposition is often used to trigger nickel deposition.

3. Reductive deposition

The process of reducing metal ions to metal atoms by the free electrons released by the oxidation of 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

Electroless plating in engineering mainly refers to this kind of reduction deposition electroless 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 that 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 occurs only when it is in contact with the catalytic surface.

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

(4) The reduced metal 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 kinds of metals and alloys for electroless plating, such as Ni-P, Ni-B, Cu, Ag, PD, Sn, in, Pt, Cr and various Co based alloys, but electroless nickel plating and electroless copper plating are the most widely used.

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

3. Thermal spraying technology and thermal spray welding technology

Thermal spraying technology and thermal spray welding technology are technologies that use heat energy (such as oxygen acetylene flame, arc, plasma flame, etc.) to melt coating materials with special properties and apply them on workpieces to form coatings.

It can prepare relatively thick coating (0.1 ~ 10mm), which is mainly used in manufacturing composite layer parts and repairing.

1. Thermal spraying technology

(1) Principle and characteristics of thermal spraying technology

Various heat sources are used to heat, melt or semi melt the coating materials, and then the coating materials are dispersed and refined with high-speed gas and hit the substrate surface at high speed, so as to form the coating process, as shown in Fig. 2.

Fig. 2 basic process diagram of thermal spraying

Thermal spraying process mainly includes: melting of spraying materials; Atomization of spraying materials; Flight of spraying materials; Impact and solidification of particles.

(2) Coating material

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

  • There is a wide liquid phase area, which is not easy to decompose or volatilize at spraying temperature;
  • Good thermal stability; Good performance;
  • Good wettability; Good solid fluidity (powder);
  • The coefficient of thermal expansion is appropriate.
  • Coating materials can be divided into wire and powder according to the shape of spraying materials.

(3) Bonding mechanism of thermal sprayed coatings

① Mechanical bonding: after the molten particles hit the surface of the substrate, they are spread into a flat liquid thin layer, embedded in the undulating surface and form mechanical bonding.

② Metallurgical bonding: diffusion and welding appear on the surface of coating and substrate, which is called metallurgical bonding.

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

(4) Coating formation process

① The spraying material is heated to a molten state;

② The spraying material is atomized into small droplets and strikes the substrate surface at high speed.

The greater the particle kinetic energy and impact deformation, the better the combination of the coating formed;

③ The 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 Fig. 3.

Fig. 3 schematic diagram of coating formation process

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

The reasons for the existence of pores are: the low impact kinetic energy of unmelted particles;

Shielding effect caused by different spraying angles;

Solidification shrinkage and stress release effect.

Proper pores can store lubricant, improve the thermal insulation performance of the coating, reduce internal stress and improve the thermal shock resistance of the coating.

However, too many pores will destroy the corrosion resistance of the coating and increase the roughness of the coating surface, so as to reduce the bonding strength, hardness and wear resistance of the coating.

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

2. Thermal spray welding technology

(1) Principle and characteristics of thermal spray welding technology

Thermal spray welding technology is a surface metallurgical strengthening method, also known as fusion bonding, which uses a heat source to remelt or partially melt the coating material on the substrate surface and condense on the substrate surface to form a surface layer with metallurgical bonding with the substrate.

Compared with other surface treatment processes, the microstructure obtained by thermal spray welding is dense, there are few metallurgical defects and high bonding strength with the matrix. However, the selection range of materials used is narrow, the deformation of the substrate is much larger than that of thermal spray, and the composition of the thermal spray welding layer is different from the original composition.

(2) Classification of thermal spray welding technology thermal spray welding technology mainly includes flame spray welding, plasma spray welding, etc.

① Flame spray welding: first spray powder on the surface of the substrate, and then directly heat the coating with flame to remelt the coating on the surface of the substrate.

The surface of the substrate is completely wetted, and there is mutual element diffusion at the interface to form a firm metallurgical bond.

Flame spray welding features: simple equipment; Simple process;

The bonding strength between coating and substrate is high;

The erosion wear resistance of the coating is good.

② Plasma spray welding: the plasma arc is used as the heat source to heat the matrix to form a molten pool on its surface.

At the same time, the spray welding powder material is sent into the plasma arc.

The powder is preheated in the arc column and is in a molten or semi molten state.

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

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

Characteristics of plasma spray welding: high production efficiency;

It can spray weld refractory materials, low dilution rate, good process stability, easy to realize automation, flat and smooth spray welding layer, uniform composition and structure, larger coating thickness, and the test process can be accurately controlled.

(3) Difference between thermal spray welding technology and thermal spraying technology

① Workpiece surface temperature: < 250 ℃; Spray welding shall be more than 900 ℃.

② Bonding state: the spraying layer is mainly mechanically bonded;

Spray welding layer is a metallurgical combination.

③ Powder material: self fluxing alloy powder for spray welding.

The spraying powder is not limited.

④ Coating structure: the spray coating has pores, and the spray welding layer is uniform and dense without pores.

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

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

4. Chemical conversion membrane technology

Chemical conversion film technology is the process of forming a stable compound film on the metal surface by chemical or electrochemical means.

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

It uses a certain metal to contact with a specific corrosive solution, and the two react chemically under certain conditions.

Due to the concentration polarization, cathodic and anodic polarization, a layer of insoluble corrosion product film with good adhesion is formed on the metal surface.

These films can not only protect the base metal from the influence of water and other corrosive media, but also improve the adhesion and aging resistance to the organic film.

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

1. Phosphating treatment

Phosphating is the process of putting steel materials into phosphate solution to obtain a layer of phosphate film insoluble in water.

The phosphating process of iron and steel materials is as follows: chemical degreasing → 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 phosphate, zinc phosphate, etc. it is gray white or gray black crystal.

The film is firmly bonded to the base metal and has high resistivity.

Compared with oxide film, phosphating film has higher corrosion resistance, especially in atmosphere, oil and benzene medium, but it has poor corrosion resistance in acid, alkali, ammonia, seawater and steam.

The main methods of phosphating treatment are impregnation, spraying and combination of impregnation and spraying.

According to the solution temperature, phosphating can be 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 process workpieces of any shape and obtain phosphating films of different thicknesses.

The equipment is simple and the quality is stable.

Thick phosphating film is mainly used for anti-corrosion treatment of workpieces and enhancing surface antifriction.

Spray method is suitable for medium temperature and low temperature phosphating process, and can deal with large-area workpieces, such as automobile shell, refrigerator, washing machine and other large workpieces as paint base and cold deformation processing.

This method has the advantages of short treatment time and fast film-forming speed, but only thin and medium thickness phosphate film can be obtained.

2. Oxidation treatment

(1) Oxidation treatment of iron and steel

The oxidation treatment of steel, also known as bluing, is to put the steel workpiece into some oxidizing solution to form a thickness of about 0.5 ~ 1.5 on its surface μ m dense and firm Fe3O4 film.

Bluing usually does not affect the precision of parts and is often used for the decoration and protection of tools and instruments.

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

Alkaline method is the most widely used oxidation treatment.

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

The common solution consists of 500g / L sodium hydroxide, 200g / L sodium nitrite and residual water.

It is treated for 6 ~ 9min when the solution temperature is about 140 ℃.

(2) Oxidation treatment of aluminum and aluminum alloys

① Anodic oxidation

Anodizing is a method of putting the workpiece in the electrolyte and then energizing to obtain an oxide film with high hardness and strong adsorption.

Common electrolytes include sulfuric acid with concentration of 15% ~ 20%, chromic acid with concentration of 3% ~ 10% and oxalic acid with concentration of 2% ~ 10%.

The anodic oxide film can be boiled with hot water to turn the oxide film into aqueous alumina and closed due to volume expansion.

It can also be treated with potassium dichromate solution to prevent the corrosive solution from corroding the matrix through the crystallization gap of the oxide film.

② Chemical oxidation

Chemical oxidation method is to put the workpiece into the solution of weak alkali or weak acid to obtain the oxide film firmly bonded with the matrix aluminum.

It is mainly used to improve the corrosion resistance and wear resistance of workpieces.

It is also used for the surface decoration of aluminum and aluminum alloy, such as antirust aluminum for construction, decorative film of signs, etc.

5. Vapor deposition technology

Vapor deposition technology is a new coating technology that deposits the vapor phase substances containing deposition elements on the surface of materials by physical or chemical methods to form thin films.

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

1. Physical vapor deposition

Physical vapor deposition (PVD) refers to the technology of vaporizing materials into atoms, molecules or ions by physical methods under vacuum, and depositing a thin film on the surface of materials through vapor process.

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

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

According to the different melting points of evaporation materials, there are many heating methods, such as resistance heating, electron beam heating, laser heating and so on.

Vacuum evaporation is characterized by simple equipment, process and operation.

However, due to low kinetic energy of vaporized particles, weak adhesion between coating and substrate and loose coating, the impact resistance and wear resistance are not high.

Sputter plating is a method of ionizing argon by glow discharge in vacuum.

The argon ions produced accelerate the bombardment of the cathode under the action of electric field, and the sputtered particles are deposited on the surface of the workpiece to form a film;

It has the advantages of large kinetic energy of gasified particles, wide range of applicable materials (including matrix materials and coating materials) and good uniform plating ability, but the deposition speed is slow and the equipment is expensive.

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

It is characterized by high coating quality, strong adhesion, good uniform plating ability and fast deposition speed, but it has the disadvantages of complex and expensive equipment.

Physical vapor deposition has a wide range of suitable matrix materials and film materials;

Simple process, material saving and pollution-free;

The obtained film has the advantages of strong adhesion to film base, uniform film thickness, compactness and less pinholes.

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

2. Chemical vapor deposition

Chemical vapor deposition (CVD) is a method of forming metal or compound films on the substrate surface by the interaction between mixed gas and substrate surface at a certain temperature.

The characteristics of chemical vapor deposition are: there are many kinds of sediments, which can be divided into deposited metals, semiconductor elements, carbides, nitrides, borides and so on;

The composition and crystal form of the film can be controlled in a large range;

It can evenly coat parts with complex geometry;

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

Easy to achieve mass production.

Chemical vapor deposition film has been widely used in mechanical manufacturing, aerospace, transportation, coal chemical industry and other industrial fields because of its good wear resistance, corrosion resistance, heat resistance, electrical, optical and other special properties.

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