Mini Encyclopedia: Laser Welding Principle & Process Applications
Energy Levels
Matter is composed of atoms, and atoms consist of a nucleus and electrons. Electrons orbit around the nucleus. The energy of electrons in an atom is not arbitrary.
Quantum mechanics, which describes the microscopic world, tells us that electrons occupy fixed energy levels. Different energy levels correspond to different electron energies: orbits farther from the nucleus have higher energy.
In addition, each orbit can hold a maximum number of electrons. For example, the lowest orbit (closest to the nucleus) can hold up to 2 electrons, while higher orbits can hold up to 8 electrons, and so on.
Transition
Electrons can move from one energy level to another by absorbing or releasing energy.
For instance, when an electron absorbs a photon, it may jump from a lower energy level to a higher one. Similarly, an electron at a higher energy level can drop to a lower level by emitting a photon.
In these processes, the energy of the absorbed or emitted photon always equals the energy difference between the two levels. Since photon energy determines the wavelength of light, the absorbed or emitted light has a fixed color.
Principle of Laser Generation
Stimulated Absorption
Stimulated absorption occurs when atoms in a low-energy state absorb external radiation and transition to a high-energy state. Electrons can jump from low to high energy levels by absorbing photons.
Stimulated Emission
Stimulated emission means that electrons at a high energy level, under the “stimulation” or “induction” of a photon, transition to a low energy level and emit a photon with the same frequency as the incident photon.
The key feature of stimulated emission is that the generated photon is identical to the original one: same frequency, same direction, and completely indistinguishable. In this way, one photon becomes two identical photons through one stimulated emission process. This means light is strengthened or amplified — the basic principle of laser generation.
Spontaneous Emission
Spontaneous emission occurs when electrons at a high energy level drop to a lower level without external influence, emitting light (electromagnetic radiation) during the transition. The photon energy is E=E2−E1, the energy difference between the two levels.
Conditions for Laser Generation
Laser Gain Medium
Laser generation requires a suitable gain medium, which can be gas, liquid, solid, or semiconductor. The key is to achieve population inversion in the medium, a necessary condition for laser output. Metastable energy levels are highly beneficial for population inversion.
Pumping Source
To achieve population inversion, the atomic system must be excited to increase the number of particles at the upper energy level.
Common methods include:
- Electrical pumping: gas discharge using high-kinetic-energy electrons
- Optical pumping: irradiation by pulsed light sources
- Thermal pumping, chemical pumping, etc.
These methods are collectively called pumping. Continuous pumping is required to maintain more particles at the upper level than at the lower level for stable laser output.
Resonator
With a suitable gain medium and pumping source, population inversion can be achieved, but the stimulated emission intensity is too weak for practical use. Further amplification is needed, which is provided by an optical resonator.
An optical resonator consists of two highly reflective mirrors placed parallel at both ends of the laser:
- One total reflection mirror
- One partial reflection & partial transmission mirror
The total reflection mirror reflects all incident light back along its original path. The partial reflection mirror reflects photons below a certain energy threshold back into the medium, while photons above the threshold transmit out as amplified laser light.
Light oscillates back and forth in the resonator, triggering a chain reaction of stimulated emission, amplifying like an avalanche to produce high-intensity laser output.
What is a Pump Lamp?
A xenon lamp is an inert gas discharge lamp, usually straight-tube shaped. It generally consists of electrodes, a quartz tube, and filled xenon (Xe) gas.
Electrodes are made of metal with high melting point, high electron emission efficiency, and low sputtering. The lamp tube is made of high-strength, high-temperature-resistant, high-transmittance quartz glass, filled with xenon gas.
What is an Nd:YAG Laser Rod?
Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) is the most commonly used solid laser material.
YAG is a cubic crystal with high hardness, excellent optical quality, and high thermal conductivity. Trivalent neodymium ions replace some trivalent yttrium ions in the crystal lattice, hence the name neodymium-doped yttrium aluminum garnet.
Characteristics of Laser
Good Coherence
Light from ordinary sources is chaotic in direction, phase, and timing, and cannot be focused to a single point even with a lens.
Laser light is highly coherent: it has a pure frequency, propagates in the same direction in perfect phase, and can be focused to a tiny spot with highly concentrated energy.
Excellent Directionality
Laser has far better directionality than any other light source, behaving almost as a parallel beam. Even when aimed at the Moon (about 384,000 km away), the spot diameter is only about 2 km.
Good Monochromaticity
Laser light from stimulated emission has an extremely narrow frequency range. In simple terms, laser has excellent monochromaticity — its “color” is extremely pure. Monochromaticity is critical for laser processing applications.
High Brightness
Laser welding uses the excellent directionality and high power density of laser beams. The laser is focused into a tiny area via an optical system, forming a highly concentrated heat source in a very short time, melting the material and forming stable weld spots and seams.
Advantages of Laser Welding
Compared with other welding methods, laser welding offers:
- High energy concentration, high welding efficiency, high precision, and large depth-to-width ratio of welds.
- Low heat input, small heat-affected zone, minimal residual stress and deformation.
- Non-contact welding, flexible fiber-optic transmission, good accessibility, and high automation.
- Flexible joint design, saving raw materials.
- Precisely controllable energy, stable welding results, and excellent weld appearance.
Laser Welding Processes for Metal Materials
Stainless Steel
- Good results can be achieved with ordinary square-wave pulses.
- Design joints to keep weld spots away from non-metallic materials.
- Reserve sufficient welding area and workpiece thickness for strength and appearance.
- Ensure workpiece cleanliness and dry environment during welding.
Aluminum Alloys
- High reflectivity requires high laser peak power.
- Prone to cracking during pulse spot welding, reducing strength.
- Material composition may cause spattering; use high-quality raw materials.
- Better results with large spot size and long pulse width.
Copper & Copper Alloys
- Higher reflectivity than aluminum; requires even higher laser peak power.
- Laser head should be tilted at an angle.
- Copper alloys (brass, cupronickel, etc.) are more difficult to weld due to alloying elements; careful parameter selection is required.
Common Defects in Laser Welding & Solutions
Incorrect parameters or improper operation often cause welding defects, including:
- Surface spattering
- Internal weld porosity
- Welding cracks
- Welding deformation
Weld Spatter
Spatter is mainly caused by excessively high laser power density: the workpiece absorbs too much energy in a short time, leading to severe material vaporization and violent molten pool reaction.
Spatter damages appearance, assembly accuracy, and welding strength.
Causes
- Excessively high laser peak power.
- Inappropriate welding waveform, especially for high-reflectivity materials.
- Material segregation leading to local high energy absorption.
- Contamination or non-metallic impurities on the workpiece surface.
- Low-melting-point substances between or under workpieces, generating gas during welding.
- Closed hollow structures causing gas expansion and spattering.
Solutions
- Optimize parameters: reduce peak power or use spike waveforms.
- Use qualified, high-quality raw materials.
- Strengthen pre-weld cleaning to remove oil and impurities.
- Optimize welding structure design.
Internal Porosity
Porosity is the most common defect in laser welding. The fast thermal cycle and short molten pool lifetime prevent gas from escaping, forming pores.
Common types: hydrogen pores, carbon monoxide pores, and keyhole collapse pores.
Welding Cracks
Cracks severely reduce weld strength and service life. Laser welding’s fast heating and cooling increase cracking risk.
Most laser welding cracks are hot cracks, common in aluminum alloys and high-carbon / high-alloy steels.
Prevention
- For brittle materials, add preheating and slow-cooling waveforms to reduce cracking.
- Optimize joint design to reduce welding stress.
- Select materials with lower cracking tendency under equivalent performance.
Welding Deformation
Deformation often occurs in thin sheets, large-area workpieces, or multi-spot welding, affecting assembly and performance. It is caused by uneven heat input and inconsistent thermal expansion / contraction.
Solutions
- Optimize parameters to reduce heat input: increase peak power while reducing pulse width.
- Lower welding speed and pulse frequency to reduce heat per unit time.
- Optimize welding sequence to ensure uniform heating.
Post time: Feb-25-2026
