Principle of laser welding
Laser welding is to radiate the high-intensity laser beam to the metal surface. Through the interaction between the laser and the metal, the metal absorbs the laser and converts it into heat energy, so that the metal melts and cools and crystallizes to form welding.
There are two mechanisms of laser welding:
1. Heat conduction welding:
When the laser irradiates the material surface, part of the laser is reflected and part is absorbed by the material, which converts the light energy into heat energy and heats and melts.
The heat of the material surface layer continues to transfer to the depth of the material in the form of heat conduction, and finally the two weldments are welded together.
It is commonly used in pulse laser welding machine, and the depth width ratio is less than 1.
Drawing pipe welding – continuous welding
When the laser beam with high power density irradiates the material surface, the material absorbs light energy and converts it into heat energy.
The material is heated, melted and vaporized to produce a large amount of metal steam.
Under the reaction force generated when the steam exits the surface, the molten metal liquid is pushed around to form pits.
With the continuous irradiation of the laser, the pits penetrate deeper.
When the laser stops irradiation, the molten liquid around the pit flows back, and after cooling and solidification, the two weldments are welded together.
It is commonly used in continuous laser welding machine, and the depth width ratio is greater than 1.
Characteristics of laser welding
1. Fast speed, large depth and small deformation.
2. It can be welded at room temperature or under special conditions, and the welding equipment is simple.
For example, when a laser passes through an electromagnetic field, the beam will not shift;
Laser can weld in vacuum, air and some gas environment,
It can be welded through glass or materials transparent to the light beam.
3. It can weld refractory materials such as titanium and quartz, and can weld heterosexual materials with good effect.
4. After laser focusing, the power density is high.
When welding with high-power laser welding machine, the depth width ratio can reach 5:1.
5. Micro welding is possible.
After focusing, the laser beam can obtain a very small spot and can be positioned accurately.
It can be applied to the assembly and welding of micro and small workpieces produced in large quantities.
6. It can weld inaccessible parts for non-contact long-distance welding, which has great flexibility.
7. Laser beam can realize energy splitting and time splitting, carry out multi station simultaneous welding and multi station time-sharing welding, and greatly improve the production efficiency and equipment utilization.
Laser welding classification
Laser welding can be divided into pulse laser welding and fiber continuous laser welding according to the classification of lasers.
The differences are as follows:
Continuous welding pattern
Pulse welding spot superposition
|Pulse welding||Continuous welding|
|Weld quality and appearance||Normal||
Laser welding classified by laser welding method
According to the product combination, it is divided into the following:
Butt welding gap requirements: no gap as far as possible, generally less than 0.05mm. And the thinner the product, the stricter the requirements.
Requirements for penetration welding gap: the upper and lower layers shall be bonded firmly as far as possible. The thinner the upper layer material, the tighter the fitting is required.
Comparison between laser welding and other welding methods
|Welding mode||Laser welding||Argon arc welding||Resistance welding||Brazing||Electron beam welding|
|Heat affected zone||Min||More||Commonly||More||Less|
|Weld quality and appearance||Well||Commonly||Commonly||Commonly||Preferably|
|Whether add solder||No||No||No||Yes||No|
|Welding environment||No requirement||No requirement||No requirement||No requirement||Vacuum|
|Consumables||/||Welding wire or replace tungsten electrode||Copper electrode||Solder||Faster|
|Degree of automation||High||Commonly||Commonly||Commonly||Commonly|
Pulse / continuous welding
Welding characteristics of metallic materials
|Difficulty||Stainless steel||Die steel||Carbon steel||Alloy steel||Nickel||Zinc||Aluminium||Gold||Silver||Copper|
|Copper||slightly difficult||hard||hard||hard||slightly difficult||hard||slightly difficult||hard||hard||easy|
Welding characteristics of steel
Steel is an iron carbon alloy with carbon content between 0.04% – 2.3%. In order to ensure its toughness and plasticity, the carbon content generally does not exceed 1.7%.
In order to improve the mechanical properties, process properties or some special properties of steel (such as corrosion resistance, heat resistance, wear resistance, etc.), some alloy elements (such as Mn, Si, Cr, Ni, Mo, W, V, Ti, etc.) are purposely added in smelting. This steel is called alloy steel.
Classification by chemical composition:
(1) Carbon steel:
a. Low carbon steel (C ≤ 0.25%);
b. Medium carbon steel (C ≤ 0.25 ~ 0.60%);
c. High carbon steel (C ≤ 0.60% ~ 2.11%).
The higher the carbon content, the easier it is to produce explosion holes in the molten pool.
(2) Alloy steel:
a. Low alloy steel (total alloy element content ≤ 5%);
b. Medium alloy steel (total alloy element content > 5 ~ 10%);
c. High alloy steel (total alloy element content > 10%).
The weldability of alloy steel depends on the alloy elements, and the weldability similar to the melting point characteristics of stainless steel is good.
(3) Stainless steel
It refers to steel resistant to weak corrosive media such as air, steam and water and chemical corrosive media such as acid, alkali and salt.
Stainless steel is divided into martensitic steel, ferritic steel, austenitic steel, etc.
Martensitic stainless steel, martensitic stainless steel is mainly low carbon or high carbon steel with chromium content in the range of 12% – 18%, and the main alloy elements are iron, chromium and carbon.
Among the stainless steels, martensitic stainless steel has the worst weldability.
The welded joints are usually hard and brittle with cold cracking tendency.
When welding stainless steel with carbon content greater than 0.1%, preheating and tempering can reduce the tendency of crack and embrittlement, such as 403, 410, 414, 416, 420, 440A, 440b and 440C.
Austenitic stainless steel refers to stainless steel with austenitic structure at room temperature.
The steel contains about 18% Cr and Ni.
It has stable austenite structure when 8% ~ 10% and C is about 0.1%.
The laser welding performance is generally good. Due to the addition of sulfur and selenium to improve the mechanical properties of austenitic stainless steel, the tendency of solidification crack increases.
The thermal conductivity of austenitic stainless steel is only 1 / 3 of that of carbon steel, the absorption rate is slightly higher than that of carbon steel, and the welding penetration depth is about 5 ~ 10% of that of ordinary carbon steel.
Laser welding has small heat input and high welding speed. It is very suitable for the welding of Cr Ni series stainless steel.
The common austenitic stainless steel is 201301302303304.
The weldability of stainless steel is the best, and the welding pool is the most perfect!
(4) 200 series – Cr Ni Mn
Austenitic stainless steel, 300 series – chromium nickel
The meaning of each letter:
- CR means chromium
- Ni stands for nickel
- Mn stands for manganese
- 1 indicates carbon content (0 in 304 is not carbon free, but carbon content is less than 0.1%, belonging to low carbon)
- 201: 1Cr17Mn6Ni5N, indicating austenitic stainless steel 201 containing 1% carbon, 17% manganese, 17% chromium and 6% nickel;
- 304: 0Cr19Ni9 (0Cr18Ni9), indicating austenitic stainless steel 304 containing less than 0.1% carbon, 18% / 19% chromium and 9% nickel;
201 stainless steel contains manganese, which is easy to oxidize and rust when the air is wet, salty, heavy and poorly maintained (of course, the situation is much better than iron products, and the oxidation and rust of stainless steel can be treated again by wire drawing, polishing, etc., unlike iron products, the electroplating layer on the surface can not be treated after corrosion).
304 stainless steel does not contain manganese, contains more chromium and nickel, and is not easy to oxidize and rust;
The price of 201 stainless steel is 3-4 times that of iron (chrome plated or sprayed furniture materials), and the price of 304 stainless steel is more than half or nearly twice that of 201 stainless steel.
304 surface is white with metal luster. It’s like a plastic plate.
Ferritic stainless steel, mainly ferritic stainless steel. The chromium content is 11% ~ 30%, with body centered cubic crystal structure.
This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of high thermal conductivity, low expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and so on. For example: 430.
Compared with austenitic and martensitic stainless steels, laser welded ferritic stainless steels have the least tendency to produce hot and cold cracks.
Welding of automobile steering system structure – continuous welding
Welding characteristics of aluminum alloy
High surface reflectivity and high thermal conductivity require high power density during welding, so it is difficult to form a stable molten pool.
Many aluminum alloys contain volatile elements, such as silicon and magnesium, and there are many pores in the weld.
The viscosity of liquid aluminum is low and the surface tension is also very low. The liquid metal in the molten pool is easy to overflow and affect the weld formation.
Hot cracks may occur in the welding pool of some aluminum alloys during solidification. The formation of cracks is related to cooling time and weld protection.
The higher the purity of aluminum, the better the welding. The welding effect within 3-Series aluminum is OK. The aluminum welding pool with low purity will produce explosion holes and cracks.
Characteristics of laser welding process
There are many process parameters that affect the quality of laser welding, such as power density, beam characteristics, defocus, welding speed, laser pulse waveform and auxiliary blowing.
1. Power density
Power density is one of the key parameters in laser welding.
With high power density, the metal can be quickly heated to the melting point in a few microseconds to form a good melt welding.
The power density is determined by the peak power and solder joint area.
Power density = peak power ÷ solder joint area
When welding highly reflective materials such as aluminum and copper, it is necessary to improve the power density, that is, set a large current or power and weld near the focus as much as possible.
2. Laser pulse waveform
Laser pulse waveform is very important in laser welding (especially for sheet welding).
When the high-intensity laser beam strikes the material surface, 60% ~ 90% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity will change with the surface temperature.
The reflectivity of metal varies greatly during a laser pulse.
When the metal material is in the solid state, the reflectivity of the laser is large.
Once the material surface melts, the reflectivity decreases and the absorption increases, which can slowly reduce the current or power.
Therefore, the pulse waveform is usually as follows:
3. Defocus amount
Defocus amount refers to the distance that the workpiece surface deviates from the focal plane.
The defocus position directly affects the keyhole effect during tailor welding.
There are two defocusing modes: positive defocusing and negative defocusing.
If the focal plane is above the workpiece, it is positive defocus, otherwise it is negative defocus.
When the positive and negative defocuses are equal, the power density of the corresponding plane is approximately the same, but in fact, the shape of the molten pool is different.
When negative defocusing, greater penetration can be obtained, which is related to the formation process of molten pool.
The experimental results show that the laser heating is 50 ~ 200 μ S, the material begins to melt, form liquid phase metal and partially vaporize to form high-pressure steam, which is sprayed at a very high speed and emits dazzling white light.
At the same time, the high concentration gas moves the liquid metal to the edge of the molten pool and forms a depression in the center of the molten pool.
During negative defocus, the internal power density of the material is higher than that of the surface, which is easy to form stronger melting and gasification, so that the light energy can be transmitted to the deeper part of the material.
Therefore, in practical application, negative defocus should be used when the penetration is large, and positive defocus should be used when welding thin materials.
the point with the smallest spot and the largest energy; Spot welding can be used, or when the energy is small and the spot is required to be small;
Negative defocus position:
the spot is slightly larger, the farther away from the focus, the larger the spot, which is suitable for deep penetration continuous welding and deep penetration spot welding;
Positive defocus position:
the spot is slightly larger, and the farther away from the focus, the greater the spot. It is suitable for continuous welding of convenience surface seal welding or occasions with low penetration requirements;
4. Welding speed
The welding speed determines the welding surface quality, penetration, heat affected zone and so on.
The penetration can be improved by reducing the welding speed or increasing the welding current.
The method of reducing the welding speed is usually used to improve the penetration and prolong the service life of the equipment.
5. Auxiliary blowing
Auxiliary blowing is an essential process in high power laser welding.
On the one hand, it is to prevent metal material sputtering from polluting the focusing mirror (coaxial protective gas);
On the other hand, it is to prevent excessive accumulation of high-temperature plasma generated in the welding process and prevent the laser from reaching the material surface (side blowing);
The third aspect is to blow protective gas to isolate the air, so as to protect the welding pool from oxidation.
The type of auxiliary gas and blowing air volume have a great impact on the welding results, and different blowing methods will also have a certain impact on the welding quality.
6. Optical fiber and welding joint configuration
optical fiber diameter = 0.6mm, focusing focal length = 120mm, collimating focusing = 150mm
Focus diameter = 0.6×120/150 = 0.48mm
The specific configuration is determined according to the material, thickness, penetration and fit clearance of the product.
Long focus features:
1. The working distance is large, which can avoid the interference of the fixture, reduce the influence of the fluctuation of the product height, and reduce the pollution of splashes to the protective lens.
2. If the same penetration is achieved, the power of the equipment needs to be increased.