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Application of laser cleaning in pre welding pretreatment

Laser welding is a common and mature field in the application of laser technology. The development of China"s manufacturing industry makes our demand for welding will continue to grow and may become the next flashpoint. Welding quality is very important for laser welding. The welding quality is in addition to laser welding process and laser selection.
The kind of welding material and so on, and also have a direct relationship with the surface state of the welded material. Then, how to do the pretreatment before laser welding, laser cleaning as a new generation of laser technology family, will be on the stage, clear away the pollutants before welding, and also a "super clean and lossless" welding base material.
In simple terms, the high frequency is generated by a certain wavelength of laser shock, the surface attachment shattered; laser generated by the instantaneous high temperature attachment stripping the substrate surface, so as to achieve the purpose of cleaning.

The objects of laser cleaning include contamination layer, paint layer, rust layer and adhesive layer on any material surface. Its applications include mold cleaning, metal surface coating and welding point removal, oxides, oil and grease removal, restoration and preservation of historical relics and so on.
Laser cleaning is achieved by overcoming the adhesion between the substrate and the surface attachment. The main mechanisms of action include the following aspects:
1 laser pressure: the pressure in the attachments, in the cleaning process, because generally compared to atmospheric pressure, pressure to several orders of magnitude, so the effect is negligible;
2 liquid splash: attachment surface does not meet the gasification temperature of liquid film to form the moment, while the surface gas blowing force moment expansion forces and process gas jet, forcing the liquid attachment splash, thereby eliminating the effect;
3 removal of vaporized evaporation: laser energy causes the substrate to vaporize or vaporize;
4 thermal stress removal: the thermal expansion of the attachment due to a sharp change in temperature produces a huge acceleration which leads to appreciable thermal effects. The adsorption effect was achieved by thermal stress;
5 plasma: instantaneous high temperature laser produced high energy density will produce high temperature gas between the attachment and the base material, the gas to absorb laser energy to form high temperature plasma, plasma absorption energy moment expansion explosion, explosion induced shock wave break adhesion layer;
6 soft: soft laser ablation ablation was defined as low power density laser irradiation, the surface of the material for a variety of mechanisms caused by mass transfer material, erosion or dissipation phenomenon. Soft ablation usually occurs in the laser cleaning of organic adhesion layer, and its action is mainly chemical reaction.
The pulsed Nd:YAG laser cleaning process depends on the characteristics of the light pulses generated by the laser, based on the photophysical reactions caused by the interaction between the high-intensity beam, the short pulse laser, and the contamination layer.

Its physical principles can be summarized as follows:
1 The beam emitted by the laser is absorbed by the contaminated layer on the surface to be processed;
2 The absorption of large energy forms a rapidly expanding plasma (highly ionized unstable gas) that produces shock waves;
3 Shock waves cause debris to be removed and removed;
4 The pulse width must be short enough to avoid heat accumulation that destroys the treated surface;
5 Experiments show that when the metal surface has oxide, the plasma is produced on the metal surface.
The plasma is produced only when the energy density is above the threshold, depending on the contaminated layer or oxide layer that has been removed. This threshold effect is very important for effective cleaning in the case of safety of the base material. There are second thresholds for the emergence of plasmas. If the energy density exceeds this threshold, the base material will be destroyed. In order to effectively clean the material under the premise of ensuring the safety of the base material, the laser parameters must be adjusted according to the situation, so that the energy density of the light pulse is strictly between the two thresholds.
The first homemade laser cleaning equipment in China  
Application of laser cleaning in pre welding pretreatment
Different from the traditional blasting, sandblasting, sanding and other cleaning processes, laser cleaning efficient, fast, non-contact cleaning methods, and China"s industrial green environmental protection development direction is consistent. In the rail, ships, automobiles, military equipment and other aspects, for pre weld cleaning, painting and cleaning and other fields have broad prospects for development. To iron Road rail, for example, the railway network can be said to be a crucial component of China"s transport network, by the end of 2015, China"s total operating railways up to 121 thousand kilometers, ranking second in the world. Among them, the high-speed rail mileage of over 19 thousand kilometers, ranking first in the world. China high-speed running speed of 160~320 km/h, all use of CWR (except for special sections of tunnels, bridges, switches, platform); seamless railway can save more than 15% of the cost of maintenance, prolong the service life of the rail 25%. Now the seamless rail has become the main track of the railway network in China, seamless railway line in China reached more than 70%. The key process of seamless rail production is rail welding, and the rail cleaning technology is very important to the welding.
The corrosion of rail surface consists of paper label, oxide skin, rust, paint, grease and dust. There are many problems with the traditional grinding wheel grinding:
1 Deep grinding ensures cleanliness, but destroys surface topography of rail surface. Uneven transition zone and gap exist between the rail, rail and wheel hub. The train shake is serious, and the train running at high speed produces micro tremor, which reduces the comfort of passengers;
2 Reduce the amount of grinding, ensure the appearance of the arc surface, but lead to serious corrosion residues. Wide range of welding current changes (instability + A (200~1600)). The welding current changes greatly, which makes the weld seam difficult to detect, such as coarse grains and gray spots, and seriously hidden dangers;
3 At present, polishing cleaning rust residue 20%~30%, causing the welding failure rate of 10%; the repair process is very complicated, the cost of 30 thousand yuan / head, restricted the production process of seamless railway track laying;
4 A large amount of abrasive consumables are consumed, which has caused dust pollution to the environment and seriously endangered the health of the workers
5 The maintenance and repair work of the railway is heavy, and laser cleaning technology can replace manual grinding and cleaning, and promote the process of on-line automatic repair of highly efficient automatic railways in our country.

Laser cleaning can avoid the above problems and has the following advantages:
1 The cleaning light source can adapt the shape of the rail surface, reduce the damage of the base material to the maximum extent, and the corrosion clearance rate is greater than 99%, and the base material damage is less than 0.01 mm;
2 Laser cleaning can achieve selective cleaning of pollutants, improve cleanliness, and protect substrate;
3 No supplies of green cleaning technology to eliminate dust pollution, significantly reduce cleaning costs;
4 The cleaning light source can be integrated with the robot by flexible optical fiber transmission;
5 Greatly improve the quality of cleaning, improve the yield of welding, rail welding strength and reliability, and eliminate potential safety problems.
Laser welding is a vast market, pretreatment with the corresponding welding requirements will also increase, the technology of laser cleaning can be achieved without damage to the substrate and no two pollution before welding processing, ensure the welding quality is a good helper.


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