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What Is Laser Welding?

Laser Welding Define

Laser welding is an efficient and precise welding method that uses a high-energy-density laser beam as a heat source. Laser welding is one of the important aspects of the application of laser material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process is of thermal conduction type, that is, the surface of the workpiece is heated by laser radiation, and the surface heat diffuses to the interior through thermal conduction. By controlling the width, energy, peak power and repetition frequency of the laser pulse and other parameters to melt the workpiece and form a specific molten pool. Due to its unique advantages, it has been successfully used in the precision welding of micro and small parts.

China’s laser welding is at the world’s advanced level, and it has the technology and ability to use laser to form complex titanium alloy components over 12 square meters, and has invested in the prototype and product manufacturing of many domestic aviation scientific research projects. In October 2013, Chinese welding experts won the Bruker Award, the highest academic award in the field of welding, and China’s laser welding level has been recognized by the world.

With the development of science and technology, laser welding has appeared in recent years. So what is laser welding?

How Laser Welding Works

Laser technology uses a polarizer to reflect the beam generated by the laser and concentrate it in a focusing device to generate a beam of enormous energy. If the focus is close to the workpiece, the workpiece will melt and evaporate within a few milliseconds. This effect can be used in welding processes High power CO2 and The emergence of high-power YAG lasers has opened up a new field of laser welding. The key to laser welding equipment is high-power lasers. There are two main categories. One is solid-state lasers, also known as Nd:YAG lasers. Nd (neodymium) is a rare earth element, YAG stands for yttrium aluminum garnet, and its crystal structure is similar to ruby. The wavelength of Nd:YAG laser is 1.06μm. The main advantage is that the generated beam can be transmitted through optical fiber, so it can save the complicated beam transmission system. It is suitable for flexible manufacturing systems or remote processing, and is usually used for workpieces with high welding precision requirements. The automotive industry commonly uses Nd:YAG lasers with output powers of 3-4 kW. The other type is gas laser, also known as CO2 laser. Molecular gas is used as working medium to generate infrared laser with a uniform size of 10.6μm. It can work continuously and output high power. The standard laser power is between 2-5 kilowatts.

First of all, what is a laser? The world’s first laser beam was generated in 1960 by using a flash bulb to excite ruby ​​grains. Due to the limited heat capacity of the crystal, only a very short pulsed beam can be generated and the frequency is very low. Although the peak energy of the instantaneous pulse can be as high as 106 watts, it is still a low energy output.

Improve and develop new technology of laser welding

With the progress of the times, the technology of laser welding is also constantly developing. The following technologies help to expand the application scope of laser welding and improve the automatic control level of laser welding.

1.Filled wire laser welding

Laser welding generally does not fill the welding wire, but it has high requirements on the assembly gap of the weldment, which is sometimes difficult to guarantee in actual production, which limits its application range. The use of filler wire laser welding can greatly reduce the requirements for assembly gaps. For example, for an aluminum alloy plate with a thickness of 2mm, if no filler wire is used, the gap between the plates must be zero to obtain good forming. Good shaping. In addition, filler wire can also adjust the chemical composition or perform multi-layer welding of thick plates.

2.Beam rotary laser welding

The method of rotating the laser beam for welding can also greatly reduce the requirements for weldment assembly and beam alignment. For example, when 2mm thick high-strength alloy steel plates are butted, the allowable seam assembly gap is increased from 0.14mm to 0.25mm; while for 4mm thick plates, it is increased from 0.23mm to 0.30mm. The allowable error in the alignment of the center of the beam and the center of the weld is increased from 0.25mm to 0.5mm.

3.On-line inspection and control of laser welding quality

The detection of laser welding process by the light, sound and charge signals of plasma has become a hot research topic at home and abroad in recent years, and a few research results have reached the level of closed-loop control.

The Work Equipment Of Laser Welding

It consists of an optical oscillator and a medium placed between the mirrors at both ends of the cavity of the oscillator. When the medium is excited to a high-energy state, it begins to generate in-phase light waves and reflects back and forth between the mirrors at both ends, forming a photoelectric tandem effect, amplifying the light waves, and obtaining enough energy to start emitting laser light.

Laser can also be interpreted as a device that converts primary energy such as electrical energy, chemical energy, thermal energy, light energy or nuclear energy into electromagnetic radiation beams of certain specific light frequencies (ultraviolet light, visible light or infrared light). Transformation of forms is easily carried out in certain solid, liquid or gaseous media. When these media are excited in atomic or molecular form, a laser beam with nearly the same phase and near single wavelength is generated. Due to the same phase and single wavelength, the difference angle is very small, and the distance that can be transmitted is quite long before being highly concentrated to provide functions such as welding, cutting and heat treatment.

The Classification Of Laser

There are two main types of lasers used for welding, namely CO2 laser and Nd:YAG laser. Both CO2 laser and Nd: YAG laser are infrared light invisible to the naked eye. The light beam generated by the Nd: YAG laser is mainly near-infrared light with a wavelength of 1.06 Lm. The thermal conductor has a high absorption rate for this wavelength. For most metals, its reflectivity is 20% to 30%. The beam in the near-infrared band can be focused to a diameter of 0.25 mm as long as a standard optical mirror is used. The beam of CO2 laser is far-infrared light with a wavelength of 10. 6Lm. Most metals have a reflectivity of 80% to 90% for this light. Special optical mirrors are required to focus the beam into a diameter of 0. 75 – 0. 1mm . Nd: YAG laser power can generally reach about 4000~6000W, and now the maximum power has reached 10000W. The CO2 laser power can easily reach 20 000W or more.

The high-power CO2 laser solves the problem of high reflectivity through the pinhole effect. When the surface of the material irradiated by the light spot melts, a pinhole is formed. This vapor-filled pinhole is like a black body, almost absorbing the energy of the incident light. Equilibrium temperature reaches around 25 000 e, and the reflectivity drops rapidly within a few microseconds. Although the development focus of CO2 lasers is still focused on the development of equipment, it is not about improving the maximum output power, but how to improve the beam quality and its focusing performance. In addition, in the high-power welding of CO2 lasers above 10 kW, if argon shielding gas is used, a strong plasma is often induced, which makes the penetration depth shallow. Therefore, helium gas, which does not generate plasma, is often used as a protective gas during high-power CO2 laser welding.

The application of diode-laser combinations for exciting high-power Nd:YAG crystals is an important development topic, which is bound to greatly improve the quality of laser beams and lead to more efficient laser processing. Using direct diode arrays to excite lasers with output wavelengths in the near-infrared region, the average power has reached 1 kW, and the photoelectric conversion efficiency is close to 50%. The diode also has a longer lifetime (10 000 h), which is beneficial to reduce the maintenance cost of the laser equipment. Development of a diode-pumped solid-state laser device (DPSSL).