Laser peening without protective coating (LPPC) has been applied to water-immersed SUS304 (Type 304) and SUS316L (Type 316L) austenitic stainless steels. The surface residual stress of both materials was converted from tensile to compressive of several hundreds of megapascals by LPPC with a Q-switched and frequency-doubled Nd:YAG laser. The depth of the compressive residual stresses after LPPC exceeded 1 mm from the surface. Accelerating stress corrosion cracking (SCC) tests in a high-temperature and corrosive-water environment showed that LPPC completely prevented the SCC initiation of sensitized SUS304. SCC tests of pre-cracked samples were also performed for SUS304, which indicated that LPPC inhibits the propagation of the small pre-cracks. Rotating bending tests demonstrated that the fatigue strength of SUS316L with LPPC is enhanced by 1.4–1.7 times compared to that of the reference material at 108 cycles.

Retardation of crack initiation and growth in austenitic stainless steels by laser peening without protective coating

The laser shock peening (LSP) process using a Q-switched pulsed laser beam for surface modification has been reviewed. The development of the LSP technique and its numerous advantages over the conventional shot peening (SP) such as better surface finish, higher depths of residual stress and uniform distribution of intensity were discussed. Similar comparison with ultrasonic impact peening (UIP)/ultrasonic shot peening (USP) was incorporated, when possible. The generation of shock waves, processing parameters, and characterization of LSP treated specimens were described. Special attention was given to the influence of LSP process parameters on residual stress profiles, material properties and structures. Based on the studies so far, more fundamental understanding is still needed when selecting optimized LSP processing parameters and substrate conditions. A summary of the parametric studies of LSP on different materials has been presented. Furthermore, enhancements in the surface micro and nanohardness, elastic modulus, tensile yield strength and refinement of microstructure which translates to increased fatigue life, fretting fatigue life, stress corrosion cracking (SCC) and corrosion resistance were addressed. However, research gaps related to the inconsistencies in the literature were identified. Current status, developments and challenges of the LSP technique were discussed.

/pdf/laser-peening-process-and-its-impact-on-materials-properties-1up080uvlh.pdf

Laser Peening Process and Its Impact on Materials Properties in Comparison with Shot Peening and Ultrasonic Impact Peening

https://escholarship.org/content/qt9jj8t067/qt9jj8t067.pdf?t=mj53rl

On the effect of deep-rolling and laser-peening on the stress-controlled low- and high-cycle fatigue behavior of Ti-6Al-4V at elevated temperatures up to 550 C

The aim of this paper was to address the effects of multiple laser peening and laser peening intensity used in laser shock peening (LSP) on the residual stress, micro-hardness and three-point bending fatigue performance of Ti–6Al–4V alloy. The multiple laser peening was accomplished by using the successive laser shocks at the same spot and the laser peening intensity was changed through changing the number of overlapped laser spots. The microstructure, which was characterized by highly tangled and dense dislocation arrangements due to high strain rate, can be found near the surface of the laser-peened specimen. By comparing with the as-received specimen, high micro-hardness and compressive residual stress were introduced at the surface of the laser-peened specimen. With increasing the number of overlapped laser spots, the fatigue life of the laser-peened specimen increased, reached a local maximum and then decreased. The specimen treated by using three overlapped laser spots exhibited the highest fatigue life. When the number of overlapped laser spots was kept to be three, the LSP treatments with one single laser shock and two successive laser shocks respectively provided a 22.2% and 41.7% increase in the fatigue strength as compared with the as-received specimens.

Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening

Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 1.2 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto a water-immersed type aluminum samples. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the higher the pulse density the larger the zone size with compressive residual stress. Densities of 900, 1350 and 2500 pulses/cm 2 with infrared (1064 nm) radiation are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. Fatigue crack growth rate is compared in specimens with and without LSP process. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness in the 6061-T6 aluminum alloy.

Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy

The effect of laser shock peening on the fatigue behavior of 2024-T3 aluminum alloy with a fastener hole, multiple crack stopholes and single-edge notch was investigated. Laser shock peening (LSP) was performed under a ‘confined ablation mode’ using a Nd:glass laser at a laser power density of 5 GW cm−2. The fatigue crack initiation life and fatigue crack growth rates of the Al alloy, with different preexisting notch configurations, were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for improving the fatigue performance of Al alloys having various preexisting notch configurations.

Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes

The cyclic stress response characteristics of several SiC particulate reinforced aluminum composites and their unreinforced counterpart at room temperature were investigated. The as-extruded specimens were cyclically deformed with fully-reversed loading under a range of constant plastic-strain amplitudes. The test results showed that the cyclic response characteristics of the reinforced and unreinforced materials were different. During most of the fatigue life, the unreinforced aluminum mainly exhibited cyclic hardening, while the composites showed gradual softening. The dislocation substructure developed during cyclic deformation was analyzed using transmission electron microscopy. The discrepency between the dislocation substructure obtained from processing compared to its development during cyclic strain loading is thought to give rise to the observed cyclic stress response behavior.

Plastic-strain cyclic response of SiC particulate reinforced aluminum composites

The effect of laser shock peening on the fatigue behavior of 2024-T3 aluminum alloy with a fastener hole, multiple crack stopholes and single-edge notch was investigated. Laser shock peening (LSP) was performed under a ‘confined ablation mode’ using a Nd:glass laser at a laser power density of 5 GW cm 2 . The fatigue crack initiation life and fatigue crack growth rates of the Al alloy, with different preexisting notch configurations, were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for improving the fatigue performance of Al alloys having various preexisting notch configurations. © 2001 Elsevier Science B.V. All rights reserved.

http://web.nchu.edu.tw/pweb/users/ycho22081/research/5067.pdf

Nanlin Han

Papers

Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes

Plastic-strain cyclic response of SiC particulate reinforced aluminum composites

Short communication Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes