Laser Peening Improves Fatigue Life of As-Printed Additive Manufacturing Parts

Additive Manufacturing (AM) is a transformative technology that can be utilized in rapid prototyping and the manufacturing of complex part geometries that cannot be produced by traditional subtractive processes. However, the additive manufacturing process typically results in components having lower material properties, particularly fatigue resistance, compared to traditionally manufactured components.  This decrease in performance often stems from multiple deficiencies that include voids, rough printed surface finish, heat affects, and limited microstructure control. Some of these deficiencies can be mitigated through post-processing techniques, or combinations of treatments. The surface finish of an AM part is typically improved through standard machining processes – cutting, grinding, polishing, etc.  Although machining the surface of AM components can improve fatigue life, the resulting improvements can be modest, and it negates many of the time advantages offered by the AM process.  Another process typically used to post process AM components is Hot Isostatic Pressure (HIP) to collapse voids entrapped within the printed component. Collectively, these treatments add substantial time and cost to AM parts. One potential singular treatment that can mitigate many of the issues is laser peening. By providing a deep, compressive residual stress at the part surface, the deleterious effects of surface finish and voids can be mitigated.

AM Ti-6Al-4V test coupons were laser peened in the as printed condition to determine the benefits of laser shock peening (LSP) on the fatigue life. These test coupons were LSP processed without the application of an opaque overlay (LSP-Bare) and compared against samples that were hot-isostatic pressed (HIP) and machined on the surface after printing.  The compressive residual stresses were measured in these test coupons and shown in comparison to the HIP and machined surfaces in Figure 1.  Laser peening shows that residual stresses were generated on the surface and reached a maximum compressive stress of -600MPa and maximum dept of 0.8mm.  These surface stresses extend well beyond the residual stresses of the baseline data.

AM Ti-6Al-4V 4-point test coupons were fatigue tested to evaluate the fatigue life improvement provided by laser peening and compared to the base line data as shown in Figure 2.  The LSP-Bare process showed an average fatigue life improvement of ~13x compared to un-peened coupons.  Large variation in fatigue life was observed with the limited dataset and traced to large variations in surface porosity. Even at the worst case, a minimum 7x improvement in the fatigue life was observed with LSP over the baseline.  Even with variations in surface porosity, , laser peening was able to induce high magnitude and high depth compressive residual stresses that enhanced the fatigue life of the AM coupons.  Investigations of the fracture surfaces of the failed specimens indicate that the fatigue cracks in the machined specimens initiated on the surface and the crack ignition point for the laser peened specimens was subsurface.

While this initial work demonstrates that laser peening the as manufactured surface of an AM test coupon without an opaque overlay generates compressive residual stresses and improves fatigue life, additional work is needed to further define the benefits of laser peening to improve the fatigue life of AM components.