Confinement and absorption layer free nanosecond laser shock peening of tungsten and its alloy.

Traditionally, nanosecond laser shock peening (ns-LSP) of metals requires an additional application of an absorption layer (black paint) and more importantly a confinement layer (typically water or transparent material) on the workpiece for introduction of compressive stresses. In this paper, we demonstrate for the first time, to the best of our knowledge, introduction of compressive stresses in pure tungsten and its alloy TAM7525 (75% tungsten and 25% copper) without any absorption and confinement layer for ns-LSP. Peak compressive stresses of -349 MPa and -357 MPa were measured in pure tungsten and TAM7525, respectively, when a 0.25-cm area was irradiated by a Yb:YAG laser (1030 nm) operating at ∼5 J, ∼2 ns with circular polarization. These peak compressive stresses (without confinement layer) compare well to those with tungsten ns-LSP done with water as confinement layer at twice the energy at 10-ns pulse duration. Furthermore, compared to femtosecond laser shock peening (fs-LSP) of aluminum at atmospheric pressure, the depth of compressive stresses recorded in tungsten and its alloy (∼7 times denser than aluminum) is nearly four times more in the case of confinement layer free nanosecond laser shock peening (CLF-ns-LSP).