Patterned Peening for Strength and Ductility Optimization

Peening operations are vital to many applications and are continually diversifying. Each method has unique advantages and limitations, but one disadvantage common among traditional, impact-mediated peening operations is the randomized nature of impacts in terms of energy, trajectory, and position. Because of this randomness, these processes can only be controlled by setting global parameters, such as total surface coverage, air pressure, stand-off distance, shot size and geometry, etc. This allows for practical application, but it limits the fundamental understanding and optimization of these peening processes. The work presented here extends the utility of a high-energy peening process known as surface mechanical attrition treatment (SMAT), which uses relatively large diameter shot (e.g., 3 mm spheres or larger) and produces a deeper residual stress profile and enhanced surface grain refinement in comparison to traditional shot peening. Recent reports on SMAT have indicated that incomplete surface coverage (much less than 100%) can still produce significant increases in strength while losing relatively little ductility. Unfortunately, because of the stochastic nature of these impacts, the distribution of impact energy can be highly nonuniform and results in localized weaknesses. To further study the potential for low-coverage peening, we have developed a position and energy controlled SMAT process. This allows us to precisely place impacts and measure the energy of each impact. This removes the stochastic limitations in peening, and it allows us to quantitatively study impact effects and apply those principles more broadly. This has already resulted in unique outcomes in strengthening mechanisms, suppressing deformation-induced phase transformations, and outperforming randomized impacts. The processing methods, performance results (including fatigue), and the potential for commercial application will be discussed.