Laser Peening of 2024-T3 Aluminum

Originally published by Lasers In Materials Processing, 1983, pp. 7-22.

Authored by Allan H. Clauer, Craig T. Walters, and Steve C. Ford.

INTRODUCTION
The effects of laser peening have been investigated in a number of metals and alloys with increases in hardness and tensile and fatigue strengths reported (1-7). A previous study of the fatigue response of laser shocked aluminum alloy plate containing simulated fastener holes showed marked increases in fatigue life in some cases. In addition, the study suggested certain process and geometry changes which would either further enhance the fatigue property improvements or aid in understanding and controlling the laser shock phenomenon influencing the properties (7). This paper describes the effects on fatigue life resulting from several different laser beam geometries and process conditions.

Both solid beam and annular beam geometries were used. The annular beam was added to determine whether a crack could be slowed down by encountering a laser shocked region. In addition, specimens were shocked from both sides simultaneously, from both sides consecutively with a momentum trap on the unirradiated side, and from one side only with a free surface opposite the irradiated side. The purpose of the momentum trap was to minimize the effect of the reflected wave from the surface opposite the irradiated surface. The one-side only shot without the momentum trap was to enable comparison to be made between the full effect of the tensile stress wave reflected from the surface opposite the irradiated side to the effects of a once through passage of the shock wave, i.e., with the momentum trap.

To understand the observed effects on the fatigue life, surface and in-depth residual stress distributions were determined for each of the laser beam geometries and process conditions. Both the residual stress and fatigue results are presented and discussed.

To download the entire article- as a pdf: The Effects of Laser Shock Processing on the Fatigue Properties of 2024-T3 Aluminum


Use of Laser Generated Shocks to Improve the Properties of Metals and Alloys

Originally published in Industrial Applications of High Power Laser Technology, Vol 86, 1977. This electronic reprint is made available with permission from SPIE.

Authored by Barry P. Fairand and Allan H. Clauer.

INTRODUCTION
Laboratory studies have established that the mechanical properties of different aluminum and iron base alloys can be improved by laser shock treatment. When the energy from a powerful pulsed laser is trained on the surface of a metal, a high amplitude stress wave is generated. This wave propagates into the material and alters its microstructure, which is the source of the observed improvement in the metal’s mechanical properties. The ability to generate stress waves in materials with short duration bursts of laser energy has been known for some time, (1-6) but it has only been in recent years that these stress waves have been shown to provide an effective method of altering the in-depth mechanical properties of metals. (7) Various methods have been used to increase the amplitude and duration of these stress waves in order to increase the depth
and degree of change introduced into the metal. (8-15) These techniques have generally taken the form of adding to the surface of the material various coatings and layers of material which may be opaque or transparent to the incident laser energy. The most effective method found up till now for increasing the efficiency of converting laser energy into mechanical stress wave energy has involved the use of transparent overlays. This technique has produced pressure with peak values several times greater than the Hugoniot elastic limit of most metals and alloys. When a pressure of this amplitude propagates through a material, the metal is plastically deformed in a manner similar to that observed in explosively shocked materials.

This paper discusses methods of generating high amplitude stress waves in materials with pulsed lasers and demonstrates by selected examples how these stress waves can be used to improve the properties of metals and alloys.

To download the entire article- as a pdf: Use of Laser Generated Shocks to Improve the Properties of Metals and Alloys


Laser Peening of Weld Zones in Aluminum Alloys

Originally published by Metallurgical Transactions A, 8A, 1871-1876 (1976). “Effect of Water and Paint Coatings on the Magnitude of Laser-Generated Shocks,” Optics Communications, 18 (4), 588-591 (1976).

Authored by Allan H. Clauer, Barry P. Fairand, and B. A. Wilcox

The feasibility of using a high energy, pulsed laser beam to shock-harden weld zones in 5086-32 and 6061-T6 aluminum sheet was investigated. The tensile strength, hardness, and microstructure of samples 0.3 cm thick were studied before and after laser shocking. After laser shocking, the tensile yield strength of 5086-H32 was raised to the bulk level and the yield strength of 6061-T6 was raised midway between the welded and bulk levels. The increases in ultimate tensile strength and hardness were smaller than the increases in the yield strength. The microstructures after shocking showed heavy dislocation tangles typical of cold working.

In many welded aluminum structures, the weld and its adjacent heat affected zone (HAZ) are a region of weakness having a lower strength than the rest of the structure. The strength of this region can be increased by a post-weld heat treatment or by mechanical working, such as rolling the weld bead or explosive shocking.1 However, these approaches are often either not practicable or are undesirable. Recently, another approach, laser induced shock hardening of the weld zone, has become a possibility. It was demonstrated earlier that a high energy, pulsed laser beam could strengthen 7075 aluminum alloy sheet,2 and the feasibility of using the laser to shock harden weld zones in 5086 and 6061 aluminum alloys is demonstrated in this paper. The use of a laser beam is attractive because the hardening can be localized to the desired region, is rapid, and can be easily adapted to numerical control.

Alloy 5086 is solid-solution strengthened (Al, 4.0 Mg, 0.45 Mn, 0.15 Cr, wt pct), is not age hardenable, and is widely used in the cold-worked condition. Therefore, the HAZ is subject to softening by recrystallization or recovery and can only be returned to the pre-weld strength levels by strain hardening. Alloy 6061 is age hardenable (Al, 1.0 Mg, 0.27 Cu, 0.6 Si, 0.2 Cr, wt pct) and is often used in the age hardened, T6, condition. Welding this alloy not only introduces a softer weld metal into the structure, but it also causes over-aging and resolutioning in the HAZ. The weld strength can be raised by a postweld heat treatment, but this isn’t always possible in welded structures. An alternative is to raise the strength of the weld zone by strain hardening, i.e., shock hardening.