Current situation and development trend of the hot

Current situation and development trend of laser shock processing technology

laser shock processing (LSP) technology uses strong pulse laser production. It needs to be pointed out that shock waves are generated to strengthen materials, and residual compressive stress layers of 1mm and above are generated on the surface of materials. In 1997, this technology was successfully applied to the single blade of aeroengine for the first time in the United States, which greatly improved the resistance to foreign body damage and high cycle fatigue performance of the blade. So far, the basic application field of this technology has been expanding, and it has become one of the most popular surface strengthening technologies. This paper mainly introduces the latest application examples of laser shock processing technology in aerospace and nuclear industry, such as the application in the strengthening of engine blisk, wing panel forming, fuselage small hole strengthening, etc. in addition, it is also applied to the fatigue resistance of welded joints and the life extension of nuclear power equipment

latest application

1 laser shock treatment of Blisk

blisk structure is an important measure to improve engine performance, simplify structure, reduce weight and improve reliability. The anti foreign material damage ability and fatigue performance of the blade strengthened by laser shock are greatly improved. Even when the edge notch of the strengthened blade is less than 3 mm, its service life is still equivalent to that of the intact unreinforced blade. Due to the improvement of the performance of single blades, the probability of scrapping the whole blade disc due to the damage of a single blade is reduced. After the in-service unreinforced blisk blades have small cracks, they can be remanufactured by laser shock treatment, and the fatigue strength still meets the design requirements. Compared with the single blade, the interference and accessibility between blades need to be considered in the laser shock treatment of the integral blade disc, and the rapid coating technology and quality monitoring technology need to be developed [1]. Since 2003, the U.S. Air Force has applied laser shock processing technology to Aeroengine blisks (see Figure 1). By 2009, 75% of the blisks on F22 fighters have been laser shock treated

laser shock treatment of Blisk

2 laser shock treatment of welded structures

the mechanical properties and residual stress of welded joints may cause brittle fracture, fatigue fracture, stress corrosion damage and reduce the stability of the structure. The research results of NASA Johnson Space Center show that after laser shock treatment, the yield strength and tensile strength of aluminum alloy friction stir welded joints are significantly improved (the yield strength of 2195 aluminum alloy friction stir welded joints is increased by 60% and the tensile strength is increased by 11%, see Figure 2), and there is grain refinement in the impact zone. Beijing Institute of aeronautical manufacturing engineering applied laser shock treatment to strengthen the welded joints of laser welding and electron beam welding, which significantly improved the original welding stress distribution. Laser shock treatment technology is expected to become a key technology to solve the large dispersion of fatigue properties of high-energy beam welded joints

compared with shot peening, laser shock treatment has great advantages in the application of welded joints. Taking aluminum alloy as an example, the cold work hardening degree of shot peening is 30%~40%, and that of laser shock treatment is 4%~9%[2]. According to Bauschinger effect, the residual compressive stress produced by laser shock treatment is more stable under cyclic load, as shown in Figure 2. In addition, laser shock treatment can produce a surface quality very close to the welded joint, which is conducive to fatigue performance

3 laser shock forming of wing panels

when the thin-walled structure is subjected to single-sided laser shock processing, the thin-walled structure will bend and deform to the side of the unreinforced surface. Both surfaces are in the state of compressive stress, and the thin-walled structure can be formed by controlling the impact parameters. This technology is called "laser shock forming"

the overall wing panel structure is large, the profile is complex, and there are stiffeners inside the panel, so the wing panel forming has become a major problem in aircraft manufacturing in China. The integral wall panel of ARJ21 wing is formed by shot peening, but compared with the shot peening technology, the forming curvature of laser shock forming is larger (see Figure 3), the residual compressive stress is deeper, and it is easier to control the forming parameters

in 2008, the wing panel of Boeing airliner adopted laser shock forming technology. Boeing purchased laser shock forming equipment from mic company, which adopts double optical transmission, and the underground transmission optical path is up to 45m [3]. With the development of laser shock forming technology, this technology will be widely used

4 hole structure strengthening

fastening hole is a typical stress concentration structure on aircraft, which is easy to produce cracks under fatigue load, especially the size is small（ φ The strengthening effect of shot peening and cold extrusion process for hole structure or blind hole below 6 mm is not ideal or difficult to achieve. As a new surface strengthening technology, laser shock processing has great advantages in strengthening small-size holes, special-shaped holes, blind holes and so on. The laser beam is focused into a circular spot, and the area around the small hole is impacted, resulting in residual compressive stress on the strengthened surface and subsurface [3]. By adjusting the energy and shape of the laser spot to meet different strengthening effects, as shown in Figure 5, the residual compressive stress obtained by the strengthening method of three ring spots from the inside out is deeper and wider, and the fatigue result is better. In addition, the latest research results of Beijing Institute of Aeronautical Manufacturing Engineering show that for 7050 aluminum alloy, the fatigue performance of the hole can also be greatly improved by laser shock treatment on its surface first and then drilling. Another major advantage of laser shock processing for structural strengthening of small holes is that it can meet the on-site strengthening, and the accessibility constitutes a leading enterprise with a production and marketing range of more than 40gwh

Figure 5 distribution of residual compressive stress around small holes

5 Application in nuclear industry

in recent years, the nuclear power industry has developed rapidly, but the aging of nuclear power equipment has not been effectively solved. The stress of pressure vessel weld is the key to solve the aging problem of nuclear power equipment. Toshiba company of Japan adopts laser shock treatment without absorption layer, which greatly improves the corrosion resistance of weld after laser shock treatment. At present, all Toshiba nuclear power plants have applied this technology, and developed underwater laser equipment and optical fiber transmission technology, which can φ Strengthen the inner wall of the 9.5mm pipe [4]. In addition, the storage of nuclear waste and prevention of leakage are also very important. A large amount of nuclear waste must be stored in special containers and sealed by welding. The YMP project in the United States uses laser shock treatment to strengthen the alloy22 weld of the nuclear waste storage container, and the depth of the residual compressive stress layer in the strengthened area is more than 5mm. Its goal is to ensure that the nuclear waste storage container will not leak due to stress corrosion within 10000 years. Nuclear power is the direction of great development in China in the future, and laser shock processing technology will be vigorously applied in the nuclear industry

6 applications in other aspects

in addition to applications in military and nuclear power fields, laser shock processing can also be applied in civil aviation, automotive industry, petrochemical industry, marine shipping and medical industry

after laser shock treatment, the anti FOD ability and aircraft endurance ability of the civil aircraft engine blades are improved. By the end of 2008, Boeing and Airbus had strengthened more than 35000 wide chord fan blades; Laser shock treatment technology is used in the welding area of oil and natural gas transmission pipelines in the United States, which improves the stress corrosion fatigue life of pipelines and is expected to generate billions of dollars in economic benefits; At present, most of the medical implants used in human body are made of titanium alloy, but the fretting fatigue performance of titanium alloy is very poor. Laser shock treatment can increase the fretting fatigue life of titanium alloy by 10-25 times

the latest technological development

1 beam movement strengthening mode

the previous laser shock processing generally adopts the strengthening mode of laser beam fixation and workpiece movement, which is relatively convenient for the implementation of the flow constraint layer. However, for some large parts (such as pipes) that are difficult to hold, as well as some assembled parts, the strengthening method of moving parts is difficult to implement, and the way of fixing parts and moving beam must be adopted. The beam scanning system developed by American mic company can realize the rapid positioning and rotation of laser beam (see Figure 6)

2 square spot

mic and lspt have begun to use square spot for laser shock processing, mainly because the directional spot has the advantage of avoiding the stress cavity phenomenon caused by the circular spot in the center of the impact area. In order to ensure full coverage impact, the overlapping rate of circular light spot must be more than 20%, while that of square light spot is only 3%. The strengthening efficiency is significantly improved, and the surface roughness is better. At present, the new laser shock processing equipment is more inclined to directly output square light spot. Similarly, a series of optical lens combinations can be added to the path of circular beam to realize beam shaping and output square light spot

3 improve the low cycle fatigue performance of cylindrical fillers made by eldorodo century plastics company with PEEK

the latest research results show that laser shock treatment can also improve the low cycle fatigue performance of parts, such as applied to rotating surfaces, complex surfaces, grooves and other parts. The residual compressive stress produced by laser shock treatment is deep, the thermal stability of strengthening effect is good, and combined with other strengthening technologies, it is expected to improve low cycle fatigue by more than 2 times

conclusion

laser shock processing is a new surface strengthening technology, which plays an irreplaceable role in some occasions and has a huge potential economic market. At present, only the United States has applied this technology to the actual production field, and has produced significant economic benefits and national defense benefits. At present, there is no application example in China, but it has a good research foundation, and this technology will surely embark on the journey of industrialization in China

references

[1] David W S, Allan h C. applications of laser peening to titanium alloys Pressure Vessels and Piping Division Conference,San Diego, CA,2004.

[2] Omar H, Jed L, Royce ser Peening and shot peening effects on fatigue life and surface roughness of friction stir welded 7075-T7351 aluminium. Fatigue Fract Engng Mater Struct,2007(30): .

[3] Zou Shikun, Cao ziwen Laser peening of aluminum alloy 7050 with fastener holes. Chinese Optics Letters,2008, 6(2): .

[4] Sano Y, Adachi T. Enhancement of surface property by low-energy laser peening without protective coating. Key Engineering Materials Vols. 2007(): . (end)