Research and Application of Laser Processing in Equipment Manufacturing and Maintenance

Abstract Through review of the research and application status of laser welding, laser cutting and laser cladding and repair in foreign countries in recent years, the main features of advanced laser processing technology are discussed, and the advanced laser processing is vigorously promoted in the manufacture and maintenance of military equipment. The importance of technology and potential economic and military value.
Keywords laser welding laser cutting laser cladding equipment manufacturing and repair

Since the birth of the world's first laser in 1960, laser technology has been developed, which has greatly stimulated the development of laser-related technologies. Laser processing technology is one of them. Compared with the traditional processing heat source, the laser has the characteristics of high brightness, high directionality, high monochromaticity and high coherence. Therefore, laser processing is a new type of high-energy beam processing technology, which improves product quality and labor. Productivity, automating production and pollution, and reducing material consumption are playing an increasingly important role. According to the global statistics for the year 2000, the assigned amount of industrial lasers by application area is [1]: laser cutting accounts for 32%, marking accounts for 30%, laser welding accounts for 13%, microprocessing accounts for 13%, and laser drilling accounts for 4 %, other 8%. Visible laser processing is a representative advanced manufacturing technology today, providing a new practical means for material processing and structural manufacturing. The potential application of laser processing technology in naval equipment manufacturing and maintenance will be described below in terms of laser welding, laser cutting, laser cladding and repair.

1 Laser welding

Laser welding is to irradiate the laser beam with very small and high intensity laser light to the surface of the workpiece. Through the interaction between the laser and the material, the base metal in the active area is quickly melted and vaporized to achieve welding. Many tests and practical applications show that laser welding has many advantages: high welding speed, small weld seam, large welding penetration, narrow heat-affected zone, small welding distortion, no pollution during operation, easy automation, and automatic welding. Complex shapes; no subsequent steps required. The use of single heat source laser welding also has deficiencies. For example, the laser has short acting time on the base metal and the cooling rate is fast. It may generate defects such as porosity, porosity, and cracks in the weld seam. Since the laser spot diameter is small, the heat affected area is small. The requirements for the interface of the base material of the base material to be welded are high, and the assembly precision is demanded; the surface state and temperature of the material influence the absorption effect of the laser surface on the material surface.

In order to eliminate or reduce the defects of single heat source laser welding, on the basis of maintaining the advantages of laser heating, the heating characteristics of other heat sources are used to improve the heating of the laser to the workpiece, thereby forming a laser hybrid welding of the laser together with other heat sources. Laser and arc, plasma arc, high frequency induction heat source hybrid welding and dual laser beam welding [2].

Fig.1 Schematic diagram of laser arc hybrid welding

The laser hybrid welding process applied in the shipbuilding industry is laser and gas metal arc welding [3, 4]. Fig. 1 is a schematic diagram of the principle. Hybrid laser and arc welding is the arc in the vicinity of the laser beam, the use of a larger range of thermal effects of the arc, the arc of the parent metal to be preheated, so that the temperature of the parent material is increased, the material of the laser absorption rate, ease the laser Welding interface requirements. At the same time, since the laser beam has a focusing and guiding effect on the arc, the welding penetration is greatly increased, and the arc welding speed and welding quality can be improved. In addition, the arc heat effect range is large, the heat-affected zone is enlarged, the temperature shave is reduced, the cooling rate is reduced, and the generation of pores or cracks is reduced or eliminated.

In order to improve the competitiveness of the shipbuilding industry and meet the needs of customers, to reduce one of the key processes of the hull structure - the total amount of welding work and welding caused heat deformation of the sheet metal, the Federal Ministry of Education and Research funding of the Meyer Werft shipyard from 1994 Since the beginning of the year, the "Metallic Sandwich Planes" laser welding method has been studied and evolved into the European SANDWICH research program [5]. In 1998, the Italian Fincantier Shipyard established a laser welding station capable of welding up to 16m with 18KW CO2 laser and a plate thickness of 20mm [4]. From 1999 to 2001, a new advanced laser processing workshop was established in Meyer Werft. Using automated module production methods, steel structures with different strength grades and thicknesses are used to manufacture steel structures. The laser composite welding station can produce flat sections of 20m by 20m. Laser hybrid welding has achieved a combination of high flexibility, high production efficiency and reduced thermal deformation [5].

Europe, the United States and other countries use laser/gas metal arc welding for the construction of their ships. For example, tests on tensile, impact, dynamic tearing, and explosion of HY-80 steel laser composite welded specimens are appropriate. Under the welding process conditions, the performance of the welded joint meets the performance requirements of the US military standard for HY-80 steel [6].

Japanese scientists used laser welding to repair underwater nuclear reactor pressure vessels [7]. The process diagram is shown in Figure 2. Laser welding is performed under a pressure of 0.4 MPa and using gas protection. The Nd:YAG laser with a laser power of 3 to 4 kW is used to fill SUS308L stainless steel at a speed of 4.2 to 33.3 mm/s, and a 10 mm thick SUS304 stainless steel is welded. A deep-welded joint with no welding defects was obtained.

Figure 2 Underwater laser welding process diagram [7]

Researchers at the Hong Kong Polytechnic University used a 2kW continuous Nd:YAG laser to remelt the surface of manganese-nickel-aluminum bronze (MAB) propellers. It was found through research that the laser surface remelted propeller surface was 3.5wt% The cavitation resistance of NaCl in artificial seawater has increased by 5.8 times, even exceeding that of nickel aluminum bronze (NAB). Therefore, laser surface remelting can improve the cavitation erosion resistance and corrosion resistance of the propeller [8].

The study of laser welding of dissimilar metals such as aluminum and steel, which have large differences in melting points and large differences in coefficient of linear expansion, thermal conductivity, and heat capacity, shows that as long as a suitable laser welding process is developed, high bonding strength and good weld quality can be obtained. Dissimilar materials welded joints [9,10].

2 Laser cutting

Laser cutting is the use of high power density laser directly focused on the surface of the cutting part, to generate enough to make the material to be cut melting temperature or even vaporization, supplemented by blowing gas blowing, so as to achieve the purpose of separating material. Compared with other conventional processing methods, such as water cutting, oxy-acetylene, and laser cutting, the kerf formed is narrow and the quality (accuracy) is high, which can improve the part dimensional accuracy and material utilization rate; the kerf is of good quality and has no slag, The edges are vertical and the surface is smooth. The laser cutting has high energy density and the heat affected zone is small. The laser cutting workpiece has no mechanical deformation and is easy to be combined with automation equipment. It is easy to realize cutting automation and no tool wear. Therefore, the efficiency is high and the speed is high. High flexibility. Laser plate cutting has been practical, without sawdust, can cut any pattern in any direction, no tool wear and noise, in the processing accuracy, yield and variability are superior to the traditional processing methods.

Figure 3 Odense Shipyard Laser Processing Workshop [11]

Laser cutting technology has a history of more than 30 years. According to 2000 statistics [1], there are 40,000 laser processing systems used worldwide for industrial cutting, of which the United States accounts for nearly 30,000, Germany and Japan. Second only to the United States, each has nearly 5,000 laser cutting systems. In some European shipyards, in order to meet the modular construction needs of the hull, laser cutting workshops were established. Figure 3 is the laser cutting shop of the Odense Steel Shipyard [11] under the guidance of CNC machine tools. Its scope of work is 1m×4m×16m. James Harris and Milan Brandt [12] used Nd:YAG lasers to perform cutting tests on 16-50 mm low-carbon steel plates, and the incisions under various process conditions are shown in Fig. 4. It can be seen that high-quality kerfs can be obtained completely under suitable process parameters.

Fig. 4 Effect of laser power and cutting speed on the incision of low carbon steel thick plate [12]

Bender Shipbuilding, Alabama, USA, studied the new “LASOX” cutting process using a combination of CO2 lasers with less than 2 kW of power and (boosted) oxygen [13], successfully cutting 50 mm thick steel plates; cutting at 38 mm thickness The steel plate research test shows that each steel plate cuts an average time of 40 minutes than flame cutting, which greatly reduces the operating cost. The company stated that once the existing shortcomings of this technology are overcome, this laser-assisted cutting technology will be used for the construction of submarines, which will make the construction cost of a new generation of steel ships cheaper.

3 Laser Cladding and Repair

Laser surface alloying and cladding is the use of lasers with high energy density to irradiate the surface of the material. The base material and the added material scanned by the laser beam are quickly melted and rapidly solidified, and a highly alloyed, high-performance surface is produced in situ on the surface of the metal material. Reinforcement layer of new technologies. The use of laser surface alloying and cladding technology can effectively improve the hardness, yield strength, fatigue strength, fatigue crack propagation resistance and wear fatigue life of metal materials; in the easily worn or easily corroded parts of machines and equipment, lasers are used. The surface cladding has a cladding layer with excellent overall performances such as wear resistance, corrosion resistance, and heat resistance, so that the service life of the machine and equipment can be greatly prolonged; wear and tear can be achieved under the condition of ensuring the original part size and material properties. The surface of the part is repaired to realize the reuse of waste.

The research and development of laser surface alloying and cladding technology has a history of nearly 30 years. It is the most advanced and most advanced modern advanced surface treatment technology in recent years. Compared with traditional surface treatment technologies such as surfacing, spray coating, thermal spraying, and spray welding, the laser surface treatment technology has the following advantages: the cladding layer and the substrate can form a solid metallurgical bond, and the interface has high bonding strength; A low dilution rate cladding layer is obtained; the heat affected zone and the small thermal deformation make the workpiece deformation small and the degree of automation is high; the laser surface treatment is a rapid solidification process, easy to get a fine grain structure or a new equivalent that cannot be obtained by conventional treatment. Of course, there are also deficiencies, there are two main points: one is that the cladding layer may be cracked, especially when the cladding layer has a high hardness; the other is that the high power lasers used for purchase and maintenance have higher costs and increase the product cost. However, as long as appropriate technological measures are taken, cracks in the cladding layer can be avoided; for the repair of precision parts and large parts, the high value of the part itself can overcome the adverse effects of cost factors. Therefore, the laser surface Alloying and cladding are known as "green remanufacturing technology."

Figure 5 Laser Clad Repaired Shaft [15]

Abroad, laser surface alloying and cladding technology has been applied to practical production [14,15]. Figure 5 is a typical transmission shaft. It cannot be used because of spline wear and it is scrapped, but it is prolonged after laser cladding repair. After being treated as a waste, the entire repaired part is not deformed after laser repair, and it can be used without any heat treatment. Figure 6 shows the device for laser welding the gear journal. In Australia, a laser cladding technology center was established to provide technical support and process operation training for the application of laser cladding technology.

Figure 6 Laser cladding gear journals [15]

4 Conclusion

The advanced laser processing technology has been widely used in the civilian field. The laser processing technology is developing in the direction of intelligence and integration. Laser processing shows low cost, high efficiency, and high flexibility. Introducing it to equipment manufacturing and maintenance will enrich equipment maintenance support methods and methods, improve maintenance support capabilities, and bring potential economic benefits.