Choosing the Right Laser Welding Equipment for Your Project

Composition of Laser Welding Equipment

Laser welding equipment primarily consists of a laser, an optical system, a mechanical system, a control and monitoring system, a beam detector, and some auxiliary devices, as illustrated in Figure 2-4. The lasers used for welding mainly fall into two categories: YAG solid-state lasers and CO2 gas lasers. The optical system includes a guiding and focusing system, protection devices, and others.

The mechanical system is primarily the worktable and the computer control system (or CNC worktable). The control and monitoring system chiefly serves to monitor the welding process and quality. The beam detector is used to monitor the output power of the laser, and some can even measure the energy distribution on the cross-sectional area of the beam to determine the beam mode.

Figure 2-4 Schematic Diagram of Laser Welding Equipment Composition

Laser

The laser is a device that generates stimulated radiation and amplifies it. It is the core of laser welding equipment. According to the state of the working substance in the laser, lasers are divided into solid, liquid, and gas lasers. The lasers used for welding and cutting are mainly solid-state lasers and CO2 gas lasers.

High-power semiconductor diode lasers, which are promising, will replace YAG solid-state lasers and CO2 gas lasers in some welding fields as their reliability and service life improve and their prices decrease.

(1) Solid-state Laser

The solid-state laser mainly consists of a laser working substance (ruby, YAG or neodymium glass rod), a concentrator, a resonator (total reflection mirror and output window), a pump lamp, a power supply, and a control device. The YAG solid-state laser used for laser welding, whose working substance is a neodymium-doped yttrium aluminum garnet crystal, has an average output power of 0.3~3kW, and the maximum power can reach 4kW.

The YAG laser can work in continuous or pulsed states, or in Q-switched states. The characteristics of YAG lasers with three types of output are presented in Table 2-3. The structure of a typical Nd:YAG solid-state laser is shown in Figure 2-5.

Table 2-3 Characteristics of Different Output Modes of YAG Solid-State Lasers

Output ModesAverage Power
/kW
Peak Power
/kW
Pulse DurationPulse Repetition FrequencyPulse Energy
/J
Continuous0. 3 ~ 4
Pulsed≈4≈500.2 ~20ms1 ~500Hz≈100
Q-switched≈4≈100<1μs≈100kHz10-3
Figure 2-5: Typical Structure of an Nd:YAG Solid-State Laser
  1. High Voltage Power Supply
  2. Energy Storage Capacitor
  3. Trigger Circuit
  4. Pumping Lamp
  5. Laser Active Medium
  6. Focusing Device
  7. Total Reflecting Mirror
  8. Partial Reflecting Mirror
  9. Laser Beam

The wavelength of the YAG laser output is 1.06μm, which is 1/10 of the CO2 laser. The shorter wavelength is conducive to the focusing and fiber transmission of the laser and the absorption by the metal surface, which is an advantage of the YAG laser.

However, the YAG laser needs to use a light pump, and the service life of the pump lamp is short, so it needs to be replaced frequently. The YAG laser generally outputs multimode beams, with irregular modes and large divergence angles.

(2) Gas Laser

Most of the gas lasers used for welding and cutting are CO2 lasers, whose working gas mainly consists of CO2, N2, and He gases.

The CO2 molecule is the particle that generates the laser; the role of the N2 molecule is to resonate and exchange energy with the CO2 molecule, excite the CO2 molecule, increase the number of CO2 molecules on the high energy level of the laser, and accelerate the relaxation process of the CO2 molecule; the main role of He is to pump the particles on the lower energy level of the laser.

When the He molecule collides with the CO2 molecule, the CO2 molecule quickly returns to the base pole from the lower energy level of the laser. He has good thermal conductivity, so it can transfer the heat in the gas of the laser working chamber to the tube wall or heat exchanger, greatly improving the output power and efficiency of the laser. The best working gas components for CO2 lasers with different structures are not the same.

The output power range of the CO2 laser is large, with the minimum output power being a few milliwatts and the maximum being a few hundred kilowatts of continuous laser power. The theoretical conversion efficiency of the CO2 laser is 40%, and its electro-optical conversion efficiency can reach 15% in actual applications, which is higher than that of solid-state lasers.

The wavelength of the CO2 laser is 10.6μm, which is infrared light. It can propagate a long distance in the air with very little attenuation. Therefore, the CO2 laser has been widely used in various fields such as medicine, communication, material processing, and weapon equipment.

Based on different structural forms, CO2 lasers used in thermal processing can be divided into four types: sealed, transverse flow, axial flow, and slab.

1) Sealed CO2 Laser

Its main structure is made of glass tubing, filled with a mixture of CO2, N2, and He gases in the discharge tube. By applying a DC high voltage between the electrodes and glow discharging through the mixed gas, the CO molecules are excited to generate a laser, which is output from the window. To obtain a larger power, multiple discharge tubes are often used in series or parallel. The structure of the sealed CO laser is shown in Figure 2-6.

Figure 2-6: Schematic Diagram of a Sealed CO2 Laser Structure
  1. Plane Reflecting Mirror
  2. Cathode
  3. Cooling Tube
  4. Gas Storage Tube
  5. Return Gas Tube
  6. Anode
  7. Concave Reflecting Mirror
  8. Water Inlet
  9. Water Outlet
  10. Excitation Power Supply

2) Transverse Flow CO2 Laser. The mixed gas flows through the discharge area, and the gas directly exchanges heat with the heat exchanger, so the cooling effect is good, allowing a large electrical power input. The output power of the discharge tube per meter can reach 2~3kW. The structure of the transverse flow CO laser is shown in Figure 2-7.

Figure 2-7: Schematic Diagram of a Transverse Flow CO2 Laser Structure
  1. Flat Plate Anode
  2. Folding Mirror
  3. Rear Cavity Mirror
  4. Cathode
  5. Discharge Area
  6. Sealed Casing
  7. Output Reflecting Mirror
  8. High-Speed Fan
  9. Air Flow Direction
  10. Heat Exchanger

3) Axial Flow CO2 Laser

Its main feature is that the direction of gas flow and discharge is coaxial with the laser beam. The gas flows in the discharge tube at a speed close to the speed of sound, approximately 150 m/s, and each meter of discharge tube length can output 0.5~2kW of laser power. The structure of the fast axial flow CO2 laser is shown in Figure 2-8.

Figure 2-8: Schematic Diagram of a Fast Axial Flow CO2 Laser Structure
  1. Rear Cavity Mirror
  2. High Voltage Discharge Area
  3. Output Mirror
  4. Discharge Tube
  5. High-Speed Fan
  6. Heat Exchanger

4) Slab CO2 Laser

Figure 2-9 shows the schematic diagram of a slab CO2 laser structure. Its main characteristics are good beam quality, low gas consumption, reliable operation, maintenance-free, and low operating cost. Currently, the output power of the slab CO2 laser has reached 3.5 kW.

Figure 2-9: Schematic Diagram of a Slab CO2 Laser Structure

1-Laser Beam
2-Beam Shaper
3-Output Key
4、6- Cooling Water
5- RF Excitation
7-Rear Cavity Mirror
8-RF Excitation Discharge
9-Waveguide Electrode

See Table 2-4 for the features and applications of lasers used in welding.

Table 2-4: Features and Applications of Lasers for Welding

Lasers:Wavelength
/μm:
Operating ModeRepetition Rate
/Hz
Output Power or Energy RangePrimary Uses
Ruby Laser0.69Pulsed0~11~100JSpot welding, punching
Nd:Glass Laser1.06Pulsed0~101~100JSpot welding, punching
YAG Laser1.06Pulsed                             
Continuous
0~4001~100J                                0~2kWSpot welding, punching                                      welding, cutting, surface treatment
Sealed CO2 Laser10.6Continuous0~1kWWelding, cutting, surface treatment
Transverse Flow CO2 Laser10.6Continuous0~25kWWelding, surface treatment
Fast Axial Flow CO2 Laser10.6Continuous, Pulsed0~50000~6kWWelding, cutting

Beam Transmission and Focusing System

The beam transmission and focusing system, also known as the external optical system, is used to transmit and focus the laser beam onto the workpiece. Figure 2-10 shows schematics of two types of beam transmission and focusing systems. Mirrors are used to change the direction of the beam, while spherical mirrors or lenses are used to focus it. In solid-state lasers, optical glass is commonly used to make mirrors and lenses.

For CO2 laser welding equipment, due to the longer wavelength of the laser, mirrors are often made of copper or other high-reflection metals, and lenses are made of GaAs or ZnSe. Transmission focusing is used for mid to low power laser processing equipment, while reflection focusing is used for high power laser processing equipment.

Figure 2-10 Schematic of Beam Transmission and Focusing System

a) Transmission Focusing
b) Reflection Focusing
1—Laser Beam
2—Planar Mirror
3—Lens
4—Spherical Mirror

Beam Detector

The beam detector is primarily used to measure the output power or energy of the laser and control these parameters through a control system. A motor drives a rotating reflector needle to rotate at high speed. When the laser beam passes through the rotating path of the reflector needle, a small portion of the laser (<0.4%) is reflected by the needle’s reflecting surface, attenuated through a germanium lens, focused, and falls onto an infrared laser probe.

The probe converts the light signal into an electrical signal, which is amplified by the signal amplification circuit and read by a digital millivoltmeter. Since the electrical signal from the probe is proportional to the detected laser energy, the reading of the digital millivoltmeter is proportional to the laser power, and the displayed voltage corresponds to the laser power.

Gas and Power Source

Current CO2 lasers use a mixture of CO2, N2, and He (or Ar) as the working medium, with a volume ratio of 7:33:60. He and N2 are both auxiliary gases, and the mixed gas can increase the output power by 5 to 10 times. However, due to the high cost of He, its cost should be considered when choosing. To ensure the stable operation of the laser, a fast-response, high-stability electronic control power supply is generally used.

Worktable and Control System

A servo motor-driven worktable is available for placing workpieces for laser welding or cutting. The control system for laser welding often uses a numerical control system.

Principles for Choosing a Laser Welding Machine

In the early days, pulse solid-state lasers were used for spot welding small parts, forming seam welding by overlapping welding points, and the welding process was mostly heat-conduction welding. In the 1970s, the advent of high-power CO2 lasers marked a new era for the application of lasers in welding and industry, and laser welding has been increasingly used in the automotive, steel, shipbuilding, aviation, light industry, and other industries.

In recent years, high-power YAG lasers have made breakthroughs, and YAG lasers with an average power of around 4kW for continuous or repetitive frequency output have appeared. They can be used for deep penetration welding. Because of their short wavelength, metals have a high absorption rate for this type of laser, and the welding process is less affected by the plasma, thus, they have a promising application prospect.

When choosing or buying laser welding equipment, comprehensive considerations should be made based on the size, shape, and material of the workpiece, the characteristics, technical specifications, application range, and economic benefits of the equipment.

For the welding of micro-parts and precision parts, a low-power welding machine can be chosen; for spot welding, a pulse laser welding machine can be chosen; for spot welding of metal wires with diameters below 0.5mm, wires and plates, or between thin films, especially micrometer-sized fine wires and foils, a low-power pulse laser welding machine should be chosen.

As the thickness of the weldment increases, a welding machine with larger power should be chosen. In addition, one-time investment, the consumption level of electricity, cooling water, and working gas, the price of consumable and fragile parts, and the purchase of spare parts should also be considered.

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