What are the common industrial metal surface treatment technologies?

Introduction to Metal Surface Treatment Processes

Metal surface treatment, is the use of physics, chemistry, metallurgy and heat treatment and other disciplines of technology on the surface of the substrate to change the condition and nature of the surface of the workpiece, in order to achieve the intended performance requirements of the process method.

The role of surface treatment:

Improves metal surface hardness, corrosion resistance, wear resistance and reduces the coefficient of friction;
Mitigates, removes and repairs changes and damage to the surface of the material;
Saving energy, reducing costs and improving the environment;
Improvement of appearance and other decorative effects;
Other special functional requirements;

Metal surface treatment process classification

Metal Surface Treatment	Modified processing	-Changes the surface state of the substrate through physical and chemical methods; the chemical composition of the substrate remains unchanged.

	Alloying	-Physically, adding materials to the substrate to form an alloying layer.

	Conversion Membrane Processing	-Chemical reaction between the additive material and the substrate to form a conversion film by chemical means.

	coated	-Adds a film to the surface of the substrate, the substrate is not involved in the formation of the film

I. Surface modification technology

1.1, surface hardening

Surface quenching refers to the use of rapid heating to strengthen the surface austenitization after quenching to strengthen the surface of the parts heat treatment method without changing the chemical composition of the steel and the organization of the heart. The main methods of surface quenching are flame quenching and induction heating, and the commonly used heat sources are oxyacetylene or oxypropane and other flames.

1.2 Laser Surface Enhancement

Laser surface strengthening is to use a focused laser beam to the surface of the workpiece, in a very short period of time will be very thin surface layer of the workpiece material heated to the phase change temperature or melting point of the temperature above, and in a very short period of time cooling, so that the workpiece surface hardening and strengthening. Laser surface strengthening can be divided into laser phase change strengthening treatment, laser surface alloying treatment and laser cladding treatment.

Laser surface enhancement has a small heat-affected zone, small deformation, easy to operate, and is mainly used for locally enhanced parts, such as punching dies, crankshafts, camshafts, camshafts, spline shafts, precision instrument guides, high-speed steel cutting tools, gears, and cylinder liners of internal combustion engines.

Role:

Improve the mechanical strength of parts as well as wear resistance, fatigue resistance and corrosion resistance;
For surface matting and removing oxidized skin;
Elimination of residual stresses in cast, forged and welded parts, etc.

1.3. Polishing

Polishing is a processing method that utilizes mechanical, chemical, or electrochemical actions to reduce the surface roughness of a workpiece in order to obtain a bright (which can also be used to eliminate gloss), flat surface. Currently a large number of applications in various industries, such as cell phones and other consumer electronics, home appliances, automotive and so on.

process

Polishing process is divided into many kinds, there are mechanical polishing, electrolytic polishing, chemical polishing and nano-polishing, etc. Among them, mechanical polishing is more widely used, and the process mainly includes: local grinding, overall grinding, rough polishing, fine polishing, cleaning and inspection.

Comparison of various polishing processes

1.4. Other technologies indicate modified technologies

Shot Peening - a large number of high-speed movement of the projectile sprayed onto the surface of the parts, as if countless small hammers hammering the metal surface, so that the parts of the surface layer and the sub-surface layer of a certain amount of plastic deformation and to achieve the strengthening of a technology.

Rolling - is a surface treatment process in which a hard roller or roller is applied to the surface of a rotating workpiece at room temperature and moved in the direction of the bus bar to plastically deform and harden the surface of the workpiece in order to obtain an accurate, polished and reinforced surface or a specific pattern.

Drawing - is the surface treatment method of forcing metal through a mold under external force, where the metal cross-sectional area is compressed and the required cross-sectional area shape and size is obtained is called metal drawing process.

II. Alloying technology

2.1 Carburizing and nitriding

The workpiece will be placed in a specific medium heating insulation, so that the medium in the active atoms penetrate into the surface layer of the workpiece so as to change the surface layer of the chemical composition and organization of the workpiece, and then change the performance of the heat treatment process.

Compared with surface quenching, chemical surface heat treatment not only changes the surface organization of steel, but also changes its chemical composition. According to the infiltration of different elements, chemical heat treatment can be divided into carburization, nitriding, multiple co-infiltration, infiltration of other elements. The chemical heat treatment process includes three basic processes: decomposition, absorption and diffusion.

The two most important types of chemical surface heat treatment are carburizing and nitriding.

comparison	carburizing	nitrides
goal	Improve the surface hardness, wear resistance and fatigue strength of the workpiece, while maintaining good toughness within the substrate.	Improve the surface hardness, wear resistance and fatigue strength of the workpiece, and improve corrosion resistance.
makings	Containing 0.1-0.25%C low carbon steel, high carbon heart toughness is reduced	Medium carbon steel containing Cr, Mo, Al, Ti, V.
methodologies	Gas carburizing method, national body carburizing method, vacuum carburizing method	Gas Ammoniation, Ionic Hydrogenation
temp	900-950°C	500 - 570°C
film thickness	0.5 - 2 mm	Not more than 0.6-0.7mm
use	Widely used in mechanical parts such as gears, shafts, camshafts, etc. for airplanes, automobiles and tractors.	It is used for parts with high requirements for wear resistance and precision and heat, wear and corrosion resistant parts. Such as small shafts of instruments, light load gears and important crankshafts, etc.

III. Metal surface conversion film technology

3.1 Blackening and phosphating

Blackening:
A process in which steel or steel parts are heated to the proper temperature in air-water vapor or chemicals to form a blue or black oxide film on their surface. Also known as bluing.
Phosphatization:
Workpiece (steel or aluminum, zinc parts) immersed in phosphate solution (some acid phosphate-based solution), deposited on the surface to form a layer of insoluble crystalline phosphate conversion film process, called phosphate.

name (of a thing)	processing condition	color	Membrane properties	corrosion resistance	appliance
darken	Boiling in concentrated alkali solution produces iron tetraoxide oxide film	blue-black	Film thickness 0.5-1.6 microns, good adsorption, can be impregnated with oil or filling treatment	differ from	Thin film, does not affect the equipment size of the parts; high requirements for surface finish called polished precision parts, after blackening the surface is bright and black, can be protected and decorative
phosphide	Chemical treatment in phosphate solution to produce a crystalline phosphate film	Gray or gray-black	Film thickness 3-20 microns, good adsorption, can be impregnated with oil or filled with treatment	general	Mainly used for corrosion protection of steel parts (guns, etc.) and pre-painting treatment to increase the adhesion and protection of paint and steel workpieces.

3.2. Anodizing

It mainly refers to the anodic oxidation of aluminum and aluminum alloy. Anodic oxidation is the aluminum or aluminum alloy parts immersed in acid electrolyte, under the action of the external current as the anode, on the surface of the parts to form and the substrate firmly combined with the corrosion-resistant Al2O3 (alumina) film layer. This oxide film has special characteristics such as protective, decorative, insulating and wear-resistant.

Process.

Single color, gradient color: polishing/sandblasting/drawing→degreasing→cleaning and other pretreatment→anodizing→neutralization→dyeing→sealing→drying

Two-color:

①Polishing/sandblasting/drawing →degreasing →cleaning and other pretreatment →masking →anodizing 1 →anodizing 2 →sealing →drying

②Polishing/sandblasting/drawing→degreasing→cleaning and other pretreatment→anodizing1 →radium engraving→anodizing2 →sealing→drying

Technical features.

Boost strength;
Achieve any color except white;
Realization of nickel-free sealing holes to meet the requirements of nickel-free in Europe, the United States and other countries.

Technical difficulties and key points for improvement:

The yield level of anodizing is related to the cost of the final product, and the focus of improving the yield of oxidizing is on the suitable amount of oxidizing agent, the suitable temperature and current density, which requires structural parts manufacturers to explore and seek breakthroughs in the production process.

Applications:

Commonly used in automobiles, aircraft, some special parts of the protective treatment and handicrafts and daily-use hardware products of decorative treatment.

IV. Surface Coating Technology

4.1 Thermal spraying

Thermal spraying is the metal or non-metallic materials heated and melted, by compressed gas continuous blowing spray to the surface of the parts, the formation of a strong bond with the substrate coating, from the surface of the parts to obtain the required physical and chemical properties.

Thermal spraying technology can be utilized to improve the abrasion resistance, corrosion resistance, heat resistance and insulation of materials.

Applications: Almost all fields including aerospace, atomic energy, electronics and other cutting-edge technologies.

Powder Coating:

The powder coating is sprayed onto the surface of the workpiece by powder spraying equipment (electrostatic spraying machine), under the action of electrostatic force, the powder will be evenly adsorbed on the surface of the workpiece to form a powder coating; the powder coating is baked at high temperature and cured by leveling, and then turns into the final coating with various effects (the effects of different kinds of powder coatings).

Process:

Loading → electrostatic dust removal → spraying → low temperature leveling → baking

Technical characteristics:

Pros:

1, rich color, high-gloss, matte optional;

2. Lower cost, suitable for building furniture products and the shell of heat sinks, etc;

3, high utilization rate, 100% utilization, environmental protection;

4. Strong ability to mask defects;

5, can imitate the wood grain effect.

4.2, Vacuum plating, vapor deposition

Vapor phase deposition technology is a new coating technology in which a vapor phase substance containing a deposition element is physically or chemically deposited on the surface of a material to form a thin film.

Depending on the principle of the deposition process, vapor deposition technology can be categorized into two main types: physical vapor deposition (PVD) and chemical vapor deposition (CVD).

Physical vapor deposition (PVD)

Physical vapor deposition is a technology that uses physical methods to vaporize materials into atoms, molecules, or ionize them into ions under vacuum conditions and deposits a thin film on the surface of the material through a vapor phase process.

Physical deposition technology mainly consists of three basic methods: vacuum evaporation plating, sputter plating, and ion plating.

Physical vapor deposition has the advantages of a wide range of applicable substrate materials and membrane layer materials; a simple process, saving materials, no pollution; the membrane layer obtained by the membrane base adhesion, uniform thickness of the membrane layer, dense, less pinholes.

Widely used in machinery, aerospace, electronics, optics and light industry to prepare wear-resistant, corrosion-resistant, heat-resistant, electrically conductive, insulating, optical, magnetic, piezoelectric, slippery, superconducting and other films.

Chemical Vapor Deposition (CVD)

Chemical vapor deposition is a method of forming a thin film of a metal or compound on the surface of a substrate by the interaction of a gas mixture with the surface of the substrate at a certain temperature.

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

4.3. Electroplating

Electroplating is an electrochemical and redox process. Take nickel plating as an example: the metal parts are immersed in a solution of metal salt (NiSO4) as the cathode and the nickel metal plate as the anode, and a nickel metal plating layer is deposited on the parts after connecting the DC power supply.

Plating methods are categorized into general plating and specialty plating.

General Plating	galvanized	It is mainly used for anti-corrosion on steel parts, and is the plating type with the largest output. It is widely used in light industry, electromechanics, agricultural machinery and national defense because of its low cost, good corrosion resistance and storage resistance.
	cadmium plating	Cadmium plating on steel parts, in the marine and hot and humid atmospheric environment, its protection performance is better than galvanized, mostly used in aviation, navigation and electronic industry parts protection.
	tinned	Tin corrosion products are harmless to humans, and easy to braze, tin plating is widely used in food Yau Tau packaging products, drinking utensils, tableware and electronic industry, many need to braze the parts.
	coppering	Copper plating is often used as an intermediate layer for other plating layers to improve the bonding between the surface plating and the base metal.
	nickel-plated	It has a wide range of applications and can be used for both protective decorative and functional aspects. The former is mainly used for electrical appliances, hardware, automobiles, etc.; the latter is used for the repair of easy-to-wear products.
	chrome	Chrome plating has high hardness, excellent wear resistance and low coefficient of friction, and is used to protect decorative plating, prevent rust and beautify the appearance.

Specialty Plating	brush plating	Brush plating without plating tank, rotating workpiece connected to the negative pole of the DC power supply, the plating pen connected to the positive pole, the front part of the plating pen wrapped in degreasing cotton, pouring the plating solution immersed in the degreasing cotton sleeve, the plating solution in the metal of the positive ions under the action of the electric field force in the surface of the workpiece (cathode) to obtain the electrons and deposition of plating on the surface, the formation of plating layer. Advantages: simple equipment, flexible operation, can be localized plating and field work.
	barrel plating	Roller plating is to put a certain number of small parts in a special roller, in the rolling state to indirectly conductive way to make the surface of the parts deposited on a variety of metals or alloys plating, in order to achieve the surface protection and decoration and a variety of functional purposes of a plating process. Advantages: labor saving, good surface quality, low fluctuation of coating thickness, etc.
	rack plating	Hanging plating, also known as suspended plating, is a plating method in which parts are mounted on hangers for plating deposition, and is generally used for plating of large-sized parts (e.g., wheels).