Principle of laser welding
Laser welding involves directing a high-intensity laser beam onto a metal surface. The laser interacts with the metal, causing it to absorb the laser energy and convert it into heat. This heat causes the metal to melt and eventually cool and solidify, resulting in a welded joint.
There are two mechanisms of laser welding:
1. Heat conduction welding:
When a laser is directed onto a material surface, some of the laser energy is reflected while the rest is absorbed by the material. This absorbed energy is converted into heat, which causes the material to heat up and melt.
The heat from the surface layer of the material continues to transfer through heat conduction to the deeper layers of the material until the two pieces being welded are joined together.
Pulse laser welding machines are commonly used for this process, and the depth-to-width ratio is typically less than 1.
Drawing pipe welding – continuous welding
2. Laser deep penetration welding
When a high-power density laser beam is directed onto a material surface, the material absorbs the light energy and converts it into heat energy. As a result, the material heats up, melts, and vaporizes, producing a large amount of metal vapor.
The reaction force from the exiting vapor pushes the molten metal around, creating pits. With continuous laser irradiation, the pits penetrate deeper into the material.
When the laser is turned off, the molten metal around the pits flows back and solidifies, resulting in the two pieces being welded together.
This process is commonly used in continuous laser welding machines, and the depth-to-width ratio is typically greater than 1.
Characteristics of laser welding
- Laser welding is known for its fast welding speed, large welding depth, and minimal deformation of the materials being welded.
- Laser welding can be performed at room temperature or under specific conditions, and the equipment required is relatively simple. For example, when a laser passes through an electromagnetic field, the beam will not shift. Additionally, laser welding can be performed in vacuum, air, and some gas environments, and can even weld through transparent materials such as glass.
- Laser welding can weld refractory materials such as titanium and quartz, as well as dissimilar materials with excellent results.
- With high-power laser welding machines, the power density is very high, and the depth-to-width ratio can reach 5:1.
- Laser welding is capable of micro-welding due to its ability to produce a small focused spot that can be accurately positioned. This feature makes it ideal for the assembly and welding of micro and small workpieces produced in large quantities.
- Laser welding can reach inaccessible parts for non-contact long-distance welding, providing greater flexibility for welding operations.
- Laser beams can be split in terms of energy and time, allowing for multi-station simultaneous welding and time-sharing welding, which greatly improves production efficiency and equipment utilization.
Laser welding classification
There are two types of laser welding: pulse laser welding and fiber continuous laser welding, which are classified based on the type of laser used.
Here are the differences between the two methods:
Continuous welding pattern
Pulse welding
Pulse welding spot superposition
| Welding mode | Pulse welding | Continuous welding |
|---|---|---|
| Penetration | Small | Big |
| Power consumption | Big | Small |
| Weld quality and appearance | Normal | Well |
Laser welding classified by laser welding method
According to the product combination, it is divided into the following:
Butt welding typically requires no gap or, if necessary, a gap of less than 0.05mm. The thinner the product being welded, the more stringent the requirements for the gap.
In the case of penetration welding, it is important to ensure a firm bond between the upper and lower layers. As the upper layer material becomes thinner, tighter fitting is required to achieve the desired result.
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 |
| Thermal deformation | Less | More | Commonly | More | Less |
| Weld spot | Less | 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 replacing tungsten electrode | Copper electrode | Solder | Faster |
| Welding speed | Faster | Slow | / | / | / |
| 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 | Aluminum | Gold | Silver | Copper |
| Stainless steel | easy | |||||||||
| Die steel | easy | easy | ||||||||
| Carbon steel | easy | easy | easy | |||||||
| Alloy steel | easy | easy | easy | easy | ||||||
| Nickel | easy | easy | easy | easy | easy | |||||
| Zinc | easy | easy | easy | easy | easy | easy | ||||
| Aluminum | hard | hard | hard | hard | slightly difficult | hard | easy | |||
| Gold | hard | hard | hard | hard | hard | hard | hard | slightly difficult | ||
| Silver | hard | hard | hard | hard | hard | hard | hard | hard | hard | |
| Copper | slightly difficult | hard | hard | hard | slightly difficult | hard | slightly difficult | hard | hard | easy |
Welding characteristics of steel
Steel is an alloy of iron and carbon, with a carbon content ranging between 0.04% and 2.3%. To ensure the steel’s toughness and plasticity, the carbon content typically does not exceed 1.7%.
Alloy steel is produced by intentionally adding alloying elements, such as Mn, Si, Cr, Ni, Mo, W, V, Ti, etc., during the smelting process. These alloying elements can be used to improve the mechanical properties, process properties, or other special properties of the steel, such as corrosion resistance, heat resistance, and wear resistance.
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
Stainless steel refers to a type of steel that is resistant to weak corrosive media such as air, steam, water, and chemically corrosive media such as acid, alkali, and salt. It is divided into different types, including martensitic steel, ferritic steel, and austenitic steel.
Martensitic stainless steel is typically low-carbon or high-carbon steel with a chromium content ranging between 12% and 18%, and the main alloying elements are iron, chromium, and carbon. However, it has the worst weldability among all stainless steels. The welded joints are often hard and brittle, with a tendency for cold cracking. To reduce the likelihood of crack and embrittlement, preheating and tempering are recommended when welding stainless steel with a carbon content greater than 0.1%, such as 403, 410, 414, 416, 420, 440A, 440B, and 440C.
Austenitic stainless steel, on the other hand, refers to stainless steel with an austenitic structure at room temperature. This type of steel contains about 18% chromium and nickel, and has a stable austenite structure when the chromium content is between 8% and 10%, and the carbon content is about 0.1%. It generally has good laser welding performance. However, the addition of sulfur and selenium to improve its mechanical properties increases the tendency of solidification cracking.
Austenitic stainless steel has a lower thermal conductivity than carbon steel, with an absorption rate slightly higher than that of carbon steel. The welding penetration depth is only about 5-10% of that of ordinary carbon steel. Nevertheless, laser welding, which has a small heat input and high welding speed, is well-suited for welding Cr Ni series stainless steel. Some common austenitic stainless steel types include 201, 301, 302, 303, and 304.
Overall, stainless steel has good weldability, with a well-formed welding pool.
(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 makes it prone to oxidation and rust in wet, salty, and poorly maintained environments (although it is still much better than iron products, and can be treated with wire drawing or polishing after oxidation and rust).
Unlike iron products, the surface electroplating layer cannot be treated after corrosion.
On the other hand, 304 stainless steel does not contain manganese, but has a higher chromium and nickel content, making it more resistant to oxidation and rust.
The price of 201 stainless steel is 3-4 times that of iron-based (chrome-plated or sprayed) furniture materials, while the price of 304 stainless steel is more than half or nearly twice that of 201 stainless steel.
The surface of 304 stainless steel is white with a metallic luster, similar to a plastic plate.
Ferritic stainless steel, with a body-centered cubic crystal structure, typically contains 11% – 30% chromium, and does not contain nickel (though it may contain small amounts of Mo, Ti, Nb, and other elements).
This type of steel has high thermal conductivity, low expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance.
One example is 430 stainless steel.
Compared to austenitic and martensitic stainless steels, ferritic stainless steels have the least tendency to produce hot and cold cracks when laser-welded.
Welding of automobile steering system structure – continuous welding
Welding characteristics of aluminum alloy
Due to high surface reflectivity and high thermal conductivity, welding aluminum requires high power density, which makes it difficult to form a stable molten pool.
Many aluminum alloys contain volatile elements such as silicon and magnesium, leading to the formation of many pores in the weld.
The low viscosity and surface tension of liquid aluminum make it easy for the liquid metal in the molten pool to overflow, affecting the weld formation.
Some aluminum alloys may experience hot cracking during solidification, which is related to the cooling time and weld protection.
The higher the purity of aluminum, the better the welding quality.
Welding within the 3-Series aluminum is generally acceptable, while low-purity aluminum welding may produce explosion holes and cracks.
Characteristics of the laser welding process
There are numerous process parameters that impact the quality of laser welding, including power density, beam characteristics, defocus, welding speed, laser pulse waveform, and auxiliary gas flow.
1. Power density
Power density is a critical parameter in laser welding.
A high power density can rapidly heat the metal to its melting point in microseconds, resulting in a high-quality weld.
The power density is determined by the peak power and the area of the solder joint.
Power density = peak power ÷ solder joint area
When welding highly reflective materials such as aluminum and copper, it is necessary to increase the power density. This can be achieved by using a higher current or power, and welding as close to the focal point as possible.
2. Laser pulse waveform
The laser pulse waveform is a critical factor in laser welding, particularly for sheet welding.
When the high-intensity laser beam interacts with the material surface, 60% to 90% of the laser energy is lost due to reflection, and the reflectivity changes with surface temperature.
The reflectivity of the metal changes significantly during a laser pulse.
When the metal is in a solid state, the laser’s reflectivity is high.
However, when the material surface melts, the reflectivity decreases, and the absorption increases, allowing for a gradual reduction in current or power.
Therefore, the pulse waveform is usually designed to accommodate these changes, such as:
3. Defocus amount
The term “defocus amount” refers to the deviation of the workpiece surface from the focal plane.
The position of the defocus directly impacts the keyhole effect during tailor welding.
There are two modes of defocusing: positive and negative.
If the focal plane is located above the workpiece, it is considered positive defocus, and if it is located below the workpiece, it is considered negative defocus.
When positive and negative defocuses are equal, the power density of the corresponding plane is roughly the same, but the shape of the molten pool is different.
Negative defocus can result in greater penetration, which is related to the formation of the molten pool.
Experimental results show that when the laser heating reaches 50 to 200 μS, the material starts to melt, forming liquid phase metal and partially vaporizing to form high-pressure steam. This results in a high-speed spray of dazzling white light.
At the same time, the high-concentration gas moves the liquid metal to the edge of the molten pool, creating a depression in the center of the pool.
During negative defocus, the internal power density of the material is higher than that of the surface, leading to stronger melting and gasification. This allows the light energy to be transmitted to the deeper part of the material.
Therefore, in practical applications, negative defocus should be used when deep penetration is required, and positive defocus should be used when welding thin materials.
Focus position:
The smallest spot with the highest energy can be achieved through spot welding. Conversely, when a small spot is required and the energy is low, spot welding can also be used.
Negative defocus position:
A slightly larger spot is appropriate for deep penetration continuous welding and deep penetration spot welding. As the distance from the focus increases, the spot size becomes larger.
Positive defocus position:
A slightly larger spot is suitable for continuous welding of surface seal welding or situations where low penetration is needed. As the distance from the focus increases, the size of the spot also increases.
4. Welding speed
The quality of the welding surface, penetration, heat-affected zone, and other factors are determined by the welding speed.
Penetration can be improved by either reducing the welding speed or increasing the welding current.
Reducing the welding speed is commonly used to improve penetration and increase the lifespan of the equipment.
5. Auxiliary blowing
Auxiliary blowing is a crucial process in high-power laser welding.
Firstly, it helps prevent metal sputtering from contaminating the focusing mirror by using coaxial protective gas.
Secondly, it prevents the buildup of high-temperature plasma generated during the welding process and stops the laser from reaching the material surface through sideblowing.
Thirdly, it uses protective gas to isolate the air and protect the welding pool from oxidation.
The choice of auxiliary gas and the volume of blowing air greatly influence the welding results, and different blowing methods can also have a significant impact on the welding quality.
6. Optical fiber and welding joint configuration
For example, if the optical fiber diameter is 0.6mm and the focusing focal length is 120mm with a collimating focusing of 150mm, the focus diameter can be calculated as follows:
Focus diameter = 0.6 x 120/150 = 0.48mm
The specific configuration is determined based on the material, thickness, penetration, and fit clearance of the product.
Features of Long Focus:
- The working distance is considerable, which enables the avoidance of interference from the fixture, reduces the impact of product height fluctuations, and minimizes the contamination of splashes to the protective lens.
- To achieve the same level of penetration, the power requirements of the equipment will be higher.
