Friction stir welding (FSW) has emerged as a promising solid-state process with encouraging results, particularly when used on high-strength aerospace aluminum alloys that are generally difficult to weld. Laser peening has been applied to the mechanical and fatigue properties of welded joints. Laser peening introduces a compressive residual stress at the surface that can extend several millimeters or deeper into the material. These residual stresses resulting from laser peening can be significantly higher and deeper than for conventional shot peening, resulting in superior mechanical and fatigue properties in FSW.

An increasing number of applications are using FSW to join materials that were not considered weldable using conventional fusion welding methods. However, the rigid clamping configuration required to clamp the parts during the FSW process, along with the heating cycle the material experiences during welding, can result in high residual stresses in the weld. These residual stresses, along with the reduction in properties from the welding process, are likely to affect the mechanical and fatigue properties and therefore influence the in-service performance of structural components.

The weld strength in some cases can be improved by post-weld heat treatment. However, this is not always an option in welded structural components. Consequently, laser peening was investigated as a means for improving the mechanical and fatigue properties in FSW.

For the laser peening process, different peening layers were used in an effort to identify the optimum number of peening layers capable of producing superior properties. The laser peening process was applied over the desired treatment area in a raster fashion. The laser-peened samples displayed an approximate increase of 60 percent to the yield strength of the material.

Conventional shot peening exhibited only a slight improvement to the tensile properties when compared to the unpeened FSW specimens. A subsequent investigation also revealed that laser peening resulted in a reduction in the grain size on the surface of the processed part, which may also explain some of the increase in tensile properties.

However, the increase in mechanical properties from the laser peening was mainly attributed to the strain hardening, which can be explained by the generation of dislocations under the effect of the plastic deformation from the high energy laser peening. The resulting increase in dislocations tends to increase the flow resistance of the material to plastic deformation.

After the laser peening was applied, tensile residual stresses introduced during the welding process were found to become compressive. In general, the crack growth behavior in friction stir welded coupons is a function of microstructure, residual stresses, and specimen geometry. The results in this study indicate a significant reduction in fatigue crack growth rates using laser peening compared to the native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material. In contrast, shot peened specimens did not result in a significant reduction to fatigue crack growth.

Significant hardness improvement was achieved by processing the FSW 2195 aluminum alloy samples with laser peening. The laser-peened samples processed using six layers exhibited a hardness increase of around 28 percent on the top surface, and a 21-percent increase on the bottom side of the weld nugget region. The hardness levels due to laser peening increased proportionally with the number of peening layers.

Corrosion behavior of laser peened FSW samples was investigated by submerging several specimens (some laser peened and some unpeened) in a sodium chloride solution for 60 days. After comparing the samples, it was noticed that the corrosion pits size and number were large on the unpeened surfaces, whereas they were much smaller on the laser treated surface.

This work was done by Omar Hatamleh of Johnson Space Center. MSC-24551-1

NASA Tech Briefs Magazine

This article first appeared in the February, 2014 issue of NASA Tech Briefs Magazine.

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