Laser Shock Peening Process to Prevent Stress Corrosion Cracking of 5xxx Aluminum Alloys

Since their introduction in the 1920s, 5xxx series aluminum (Al) alloys have been widely employed for marine applications. The 5083-H116 Al alloy plates, in particular, are still used in the hull construction of many military and commercial vessels. However, the high amount of magnesium (Mg) contained in Al-Mg solids leads to thermodynamic instability. When these Al alloys are exposed to elevated temperatures, the Al3Mg2 phase is precipitated on the grain boundaries, making them susceptible to stress corrosion cracking (SCC). By introducing compressive residual stress on the surface of metallic materials, laser shock peening (LSP) has been demonstrated to improve mechanical properties, wear resistance, and fatigue life; and it is considered to be a very promising method for effectively preventing SCC problems. Currently, most LSP can only be carried out in a lab environment using Nd:Glass or Nd:YAG lasers with high pulse energy (over 1 J). The cumbersome equipment, complicated procedures, terrible efficiency, high cost, and rigorous requirements for the operating environment greatly limit the application and potential use of LSP. This study investigated an LSP processing technology for 5xxx series Al alloys using a low pulse energy Nd:YAG laser. Through systematic experiments and tests, the feasibility and effects of this technology were confirmed. After careful parameter optimization, a high level compressive residual stress (about -250 MPa) was achieved on 5083-H116 Al alloy plates. After LSP, the surface hardness and the yield strength of the Al material were enhanced about 20% and 10%, respectively. Moreover, the in-lab test results indicated that the service life of 5083-H116 Al alloy was extended obviously in an SCC-susceptible environment. These results are significant because a peening effect comparable to conventional high-energy LSP techniques was achieved using a Nd:YAG laser with a much lower pulse energy (0.65 J). Based on these results, a portable industrial LSP system was designed, built, and tested, which is capable of operation in harsh environments for on-site repair of marine vessels with an SCC problem and prevention of SCC in the future. Advisor: Yongfeng L