Laser Welding Basics You Should Know

Case Study Introduction

The rear shelf panel of the Volkswagen Passat B6 model is composed of three parts, which were initially welded using resistance spot welding. Volkswagen replaced the four robotic arms and five resistance welding guns required for spot welding with one laser welding machine (including an optical galvo scanning device), accomplishing the same 35 spot welds.

The result of this improved process showed that while resistance spot welding took 35 seconds, laser scanning welding only took about 13 seconds, improving the welding speed by approximately three times. Moreover, the laser weld spots were much stronger, significantly enhancing the welding quality of the car body.

Interactions Between Lasers and Materials

Laser Beam Welding (LBW) is an efficient, precise welding method that uses a laser beam with extremely high energy density as a heat source. Compared with traditional welding methods, laser welding has the advantages of high energy density, strong penetration, high precision, and strong adaptability.

As a product of modern high technology, laser welding has become an indispensable processing technology in modern industrial development.

With the rapid development of aerospace, electronic components, automotive manufacturing, medical, and nuclear industries, the structures of product parts are becoming more complex, the demands on material performance are constantly increasing, and the requirements for processing precision and joint quality are becoming increasingly strict.

At the same time, enterprises have higher requirements for the production rate of processing methods and working environment. Traditional welding methods can no longer meet these requirements, and high-energy beam welding methods represented by laser beams are increasingly valued and widely used.

Reflection and Absorption of Lasers

A laser is a beam of light with good monochromaticity, strong directionality, and high brightness, produced by the radiation emitted by an excited laser active substance, or working substance, amplified through light.

After being focused by a transmission or reflection mirror, a beam with a diameter of less than 0.01 mm and a power density of up to 106-1012W/cm2 can be obtained, which can be used as a heat source for welding, cutting, and material surface treatment.

The reflection and absorption of lasers on the welded surface are essentially the result of the interaction of the electromagnetic field of the light wave with the material. When a laser is irradiated onto the surface of a workpiece, a portion is reflected, and a portion is absorbed by the workpiece. The reflective ability of metal to a laser beam is related to the density of free electrons it contains.

The greater the density of free electrons, or the greater the electrical conductivity, the higher the reflectivity of the laser. Gold, silver, copper, aluminum, and their alloys reflect lasers much more than other metal materials.

The thermal effect of laser welding depends on the degree to which the workpiece absorbs the energy of the laser beam, usually characterized by the absorption rate. The absorption rate of metal to a laser mainly depends on the laser wavelength, the properties of the metal, temperature, surface conditions, and laser power density. Generally, the absorption rate of metal to a laser increases with increasing temperature and electrical resistivity.

Bright metal surfaces strongly reflect lasers. At room temperature, the absorption rate of materials to a laser is less than 10%, but it will dramatically increase above the melting point.

The thermal conductivity of metal materials, surface conditions, laser wavelength, and incident angle all have a certain impact on the absorption rate. For example, increasing surface roughness or forming a high-absorption thin film can reduce laser reflection loss.

The absorption rate of the original surface of pure aluminum is 7%, which drops to 5% after electrolytic polishing and rises to 20% after sandblasting; when there is an oxide layer on the surface, it is 22%.

In addition, the power density of the laser beam also significantly affects the absorption rate of the laser. During laser welding, when the power density of the laser spot exceeds a certain value (greater than 106W/cm2), the photon bombardment causes the metal surface to vaporize, and the absorption rate of the metal to the laser changes. In terms of material absorption of lasers, metal vaporization is a boundary.

When the material has not vaporized, whether in the solid or liquid phase, its absorption of the laser only changes slowly with the increase in surface temperature; once vaporization occurs, the evaporated metal forms a plasma, preventing the remaining energy from being reflected by the metal.

If the welded metal has good thermal conductivity, a greater depth of fusion will be obtained, forming a small hole, thereby significantly increasing the laser absorption rate.

Material Heating

When laser photons strike a metal crystal, the photons collide inelastically with the electrons, transferring their energy to the electrons and causing them to transition from a low-energy level to a high-energy level. At the same time, the electrons inside the metal are continuously colliding with each other. The average time interval between two collisions of an electron is on the order of 10-13 s.

Therefore, the electrons that have absorbed photons and are at a high energy level will transfer energy through collisions with other electrons and interactions with the lattice. The energy of the photon is finally converted into the thermal vibration energy of the lattice, causing the material temperature to increase and changing the temperature on the surface and inside the material.

Melting and Vaporization of Materials

During laser processing, the conversion of absorbed light energy into heat energy is completed in an extremely short time (about 10-9s). During this time, heat energy is only confined to the area of the material that is exposed to laser radiation, and then, through heat conduction, the heat transfers from the high-temperature area to the low-temperature area.

During laser welding, the time it takes for the material to reach the melting point is on the microsecond level. In pulse laser welding, when the power density absorbed by the material’s surface is 105W/cm2, it takes a few milliseconds to reach the boiling point. When the power density is greater than 106W/cm2, the welded material will evaporate dramatically.

Principles of Laser Welding

Laser welding was one of the earliest developed fields of laser industrial applications. There are two basic modes of laser welding: heat conduction welding and deep penetration welding.

Heat Conduction Welding

The power density of the laser spot in heat conduction welding is less than 105W/cm2. During welding, the surface of the workpiece converts the absorbed laser energy into heat energy, causing the surface temperature to rise and melt. Then, through heat conduction, the heat energy is transferred to the interior of the metal, causing the melting zone to rapidly expand.

It then cools and solidifies to form a weld spot or weld seam, with the shape of the weld pool being approximately hemispherical. This welding mechanism is called heat conduction welding, and its welding process is similar to that of tungsten inert gas (TIG) welding, as shown in Figure 2-1a.

The characteristic of heat conduction welding is that the power density of the laser spot is small, and a large portion of the laser is reflected by the metal surface. The absorption rate of the laser is low, the depth of melt is shallow, and the weld spot is small. It is mainly used for precision welding and processing of thin plates (thickness less than 1 mm) and small parts.

Two Basic Modes of Laser Welding
Figure 2-1 Two Basic Modes of Laser Welding

a) Heat Conduction Welding
b) Deep Penetration Welding
1— Plasma Cloud
2— Melted Material
3— Keyhole
4— Weld Depth

Deep Penetration Welding

The power density of the laser spot in deep penetration welding is greater than 106W/cm2. The metal surface is rapidly heated under the irradiation of the laser beam, and its surface temperature rises to the boiling point in a very short time (10-8~10-6s), causing the metal to melt and vaporize.

The metal vapor produced leaves the surface of the melt pool at a certain speed, thereby applying an additional pressure to the liquid metal in the melt pool, causing the surface of the melted metal to sink downward, creating a small pit under the laser spot, as shown in Figure 2-1b.

When the laser beam continues to heat at the bottom of the hole, the produced metal vapor both pushes the liquid metal at the bottom of the hole to deepen the hole further and squeezes the melted metal towards the periphery of the melt pool. As the heating process continues, the laser can directly shine into the bottom of the hole, forming a long, narrow hole in the liquid metal.

Once the recoil pressure of the metal vapor produced by the beam energy balances with the surface tension and gravity of the liquid metal, the hole will no longer continue to deepen, forming a stable deep hole to achieve welding, hence the name laser deep penetration welding.

When the power density of the spot is very large, the produced hole will penetrate the entire thickness of the plate, forming a deep penetration weld seam (or spot). In continuous laser welding, the hole moves forward along the welding direction relative to the workpiece. The metal melts in front of the hole, then flows around the hole towards the back, re-solidifying to form a weld seam.

The laser beam in deep penetration welding can penetrate deep into the workpiece, creating a weld seam with a high depth-to-width ratio. If the laser power is large enough and the material is relatively thin, then the hole formed by laser welding will penetrate the entire thickness of the plate, and some of the laser can be received on the backside.

This method is known as thin plate laser keyhole effect welding. To weld through, a certain laser power is required. Usually, a laser power of 1kW is needed to penetrate through 1mm of plate thickness.

Features and Applications of Laser Welding

Features of Laser Welding

Compared to conventional arc welding methods, laser welding has the following features:

1) The power density of the focused laser beam can reach 105~107W/cm2 or even higher, providing fast heating, a narrow heat-affected zone, low welding stress, and small deformation, making it suitable for deep penetration welding, high-speed welding, precision welding, and micro welding.

2) It can achieve weld seams with a high depth-to-width ratio. The depth-to-width ratio of laser welding has now exceeded 12:1, and when welding thick pieces, one can achieve full penetration without a groove.

3) It is suitable for welding materials that are difficult to weld using conventional welding methods, such as refractory metals, highly heat-sensitive materials, and workpieces with vastly different thermal physical properties, dimensions, and volumes; it can also be used for non-metal materials, such as ceramics, acrylic glass, etc.

4) The laser beam can reach parts that are difficult to weld using conventional methods with the help of a reflecting mirror. YAG lasers and semiconductor lasers can be transmitted through optical fibers, providing good accessibility, especially suitable for microparts and long-distance welding.

5) It can weld workpieces inside a sealed container through a transparent medium, such as welding beryllium alloys in sealed glass containers.

6) The laser beam is not affected by electromagnetic interference, does not have X-ray protection issues, and does not require vacuum protection.
Laser welding also has some disadvantages, such as difficulty in welding metals with high reflectivity, relatively high requirements for workpiece processing, assembly, positioning, and high initial equipment investment.

Table 2-1 lists the comparison of laser welding with traditional welding processes.

Table 2-1 Comparison of Laser Welding with Traditional Welding Processes

Performance FeaturesLaser WeldingElectron Beam WeldingTungsten Inert Gas WeldingResistance Spot WeldingFriction Welding
Welding QualityExcellentExcellentGoodFairly GoodGood
Welding SpeedHighHighMediumMediumMedium
Heat InputLowLowVery HighMediumMedium
Assembly Requirements for Welded JointsHighHighLowLowMedium
Melt DepthLargeLargeMediumSmallLarge
Range of Dissimilar Materials for WeldingWideWideNarrowNarrowWide
Range of Workpiece Geometric DimensionsWideMediumWideWideNarrow
ControllabilityVery GoodGoodFairly GoodFairly GoodMedium
Degree of AutomationExcellentMediumFairly GoodExcellentGood
Initial CostHighHighLowLowMedium
Operating and Maintenance CostMediumHighLowMediumLow
Processing CostHighVery HighMediumMediumLow

As can be seen from Table 2-1, the powerful competitor of laser welding is electron beam welding. Compared with electron beam welding, laser welding does not require a vacuum chamber, the size and shape of the workpiece can be unrestricted, and it is easy to achieve automated processing, does not produce X-rays, and is convenient for observation and adjustment.

However, electron beam welding can achieve a greater depth of melt, and is clearly more advantageous for thick plate welding. In recent years, modern laser welding technology has begun to develop towards thick plate, high adaptability, high efficiency, and low cost.

With the emergence of new materials and structures, laser welding technology will gradually replace some traditional welding processes and occupy an important position in industrial production.

Applications of Laser Welding

Since the United States used a ruby laser to drill holes in diamonds in the 1960s, laser processing technology has undergone decades of development and has become a common technique in modern industrial production. In the 1970s, the advent of high-power CO2 lasers in the kilowatt range ushered in a new era of laser applications in welding.

In recent years, laser welding has been increasingly applied in industries such as vehicle manufacturing, steel, energy, aerospace, and electronics. Practice has proven that not only is the production rate of laser welding higher than traditional welding methods, but the quality of welding has also significantly improved.

Starting from the 1980s, laser welding technology entered the field of automotive manufacturing, as shown in Figures 2-2 and 2-3. Laser welding is mainly used for body welding, frame structures, and component welding, and traditional resistance spot welding has gradually been replaced by laser welding. The use of laser welding technology not only enhances the aesthetics of the workpiece surface but also reduces the use of sheet metal.

Figure 2-2 Laser Welding of the Car Body Roof and Sides
Figure 2-3 Fixed Welding of the Left and Right Side Frame Outer Plates, Bottom Plates, and the Side Frames to the Roof Cross Beam of the Car Body

As there is no deformation at the welding location of the parts, no post-weld heat treatment is needed, which also improves the rigidity of the car body. For example, during the assembly of a certain car model, conventional resistance spot welding requires a 100mm wide flange, while laser welding only requires a 1.0~1.5mm wide flange.

According to calculations, this alone can reduce the average weight of each car by 50kg. Currently, FAW Volkswagen uses laser welding technology to varying degrees in the manufacturing process of most of its brands, such as Bora, Sagitar, and Magotan.

In the construction of power stations and the petrochemical industry, there are a large number of tube-to-tube and tube-to-plate joints. High-quality single-side welding and double-side forming welds can be obtained with laser welding. In shipbuilding, the filler metal laser welding method is used to weld large-thickness plates.

The joint performance is superior to conventional arc welding, which reduces the manufacturing cost of the product and improves the reliability of component operation. This is beneficial for extending the service life of the ship. Laser welding is also used in the welding of motor stator cores, thin plates, or thin steel strips. Some application instances of laser welding are shown in Table 2-2.

Table 2-2 Some Application Examples of Laser Welding

Application IndustriesApplication Examples
AerospaceEngine casings, wing spacers, membrane boxes, etc.
Electronics and InstrumentsInternal leads of integrated circuits, electron guns of image tubes, speed control tubes, instrument filaments, etc.
Mechanical ManufacturingPrecision springs, parts of dot matrix printers, thin-wall corrugated metal pipes, thermocouples, electro-hydraulic servo valves, etc.
Iron and Steel MetallurgyWelding silicon steel sheets with a thickness of 0.2~8mm and a width of 0.5~1.8m, carbon structural steel, and stainless steel.
Automobile ManufacturingAutomobile chassis, transmission devices, gears, combination of shaft and dial parts in ignitors, etc.
Medical DevicesPacemakers and lithium-iodide batteries used in pacemakers, etc.
Food ProcessingFood cans (laser welding replaces traditional tin soldering or contact high-frequency welding, featuring non-toxicity, fast welding speed, material saving, good-looking joints, and excellent performance), etc.
Other FieldsGas turbine wheels, heat exchangers, dry battery zinc canister shells, nuclear reactor parts, etc.

In recent years, a variety of new high-power lasers have emerged in industrial production, causing a significant impact on traditional welding technology. Laser welding technology has begun to develop in a more diversified, practical, and efficient direction, such as laser brazing, laser-arc hybrid welding, laser pressure welding, etc., further expanding the application scope of laser welding.

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