Interaction of Laser-Induced Stress Waves With Metals

Originally published by Applications of Laser Material Processing, 1979.

Authored by A. H. Clauer B. P. Fairand.

ABSTRACT
The effect of high intensity laser induced stress waves on the hardness and tensile strength of 2024 and 7075 aluminum and on the fatigue properties of 7075 aluminum were investigated. Laser shocking of these alloys increases the hardness of the underaged 2024-T351 but has little or no effect on the peak aged 2024-T851 and 7075-T651 or the over-aged 7075-T73. The largest increases in tensile strength were observed in 7075-T73, lesser increases in 2024-T351 and none in 2024-T851 or 7075-T651. The fretting fatigue life of fastener joints of 7075-T6 was increased by orders of magnitude by laser shocking the region around the fastener hole before drilling and assembling. Also the fatigue crack propagation rates were significantly decreased by laser peening.

INTRODUCTION
The effects of high amplitude stress waves on the micro-structure and properties of metals and alloys have been the subject of numerous investigations. These stress waves are often generated by explosive charges or impact between a projectile and the target specimen. One of the interesting effects is that the shock waves can develop significant plastic strains in the metal with a smaller equivalent change in the specimen dimensions than required by more conventional metal working processes. This led to investigations of the effects of shock treatments on strength and hardness, fracture toughness, stress corrosion cracking, thermomechanical processing, and other properties (1-4). While in some of these areas the benefit derived from shock deformation compared to conventional working is not always clear, it is established that shock waves can produce high dislocation densities with attendant effects on material properties. However, systems design considerations associated with handling explosives or using driver plates for materials processing make application of shock processing difficult.

Another source of high intensity stress waves is the high energy pulsed laser. This potential was first recognized and explored in the early nineteen sixties (5,6). Subsequent work established the conditions for major enhancement of the amplitude of the stress waves, making it possible to plastically deform metals when irradiating in air at standard conditions (7-11). This stimulated interest in using these laser induced pressure waves to alter the properties of materials in a manner similar to high explosive and flyer plate shock deformation of metals and alloys.

This paper presents the results of investigations on the effects of laser induced stress waves on the hardness, tensile strength, and fatigue life of several aluminum alloys. Although some of the results are from a limited number of experiments, they are of sufficient interest in showing the potential of laser shock processing that they are presented here.

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