Tailoring residual stress profile of Selective Laser Melted parts by Laser Shock Peening

Abstract

Selective laser melting Laser shock peening 3D Laser shock peening Residual stress profile 15-5 PH stainless steel 316L stainless steel a b s t r a c t The paper describes a new approach in controlling and tailoring residual stress profile of parts made by Selective Laser Melting (SLM). SLM parts are well known for the high tensile stresses in the as -built state in the surface or subsurface region. These stresses have a detrimental effect on the mechanical properties and especially on the fatigue life. Laser Shock Peening (LSP) as a surface treatment method was applied on SLM parts and residual stress measurements with the hole -drilling method were performed. Two different grades of stainless steel were used: a martensitic 15-5 precipitation hardenable PH1 and an austenitic 316L. Different LSP parameters were used, varying laser energy, shot overlap, laser spot size and treatments with and without an ablative medium. For both materials the as-built (AB) residual stress state was changed to a more beneficial compressive state. The value and the depth of the compressive stress was analyzed and showed a clear dependence on the LSP processing parameters. Application of LSP on SLM parts showed promising results, and a novel method that would combine these two processes is proposed. The use of LSP during the building phase of SLM as a "3D LSP" method would possibly give the advantage of further increasing the depth and volume of compressive residual stresses, and selectively treating key areas of the part, thereby further increasing fatigue life.

3D Laser Shock Peening – A new method for the 3D control of residual stresses in Selective Laser Melting

“…This is in agreement with the previous investigation done on PH1 stainless steel, where it was also observed that (i) a larger spot size leads to deeper CRS, and (ii) a smaller spot size leads to higher max RS [31]. As already discussed in [31], result (i) comes from a geometrical effect associated to the use of a too small spot size, which results in a strong 2D attenuation of shockwaves, and therefore to a decreased plastically affected depth of the LSP treatment [27,35,50]. Result (ii) is in agreement with [51], and this effect can be explained from the increased number of impacts by a smaller spot size on a given surface area.…”

“…Regardless of the chosen LSP parameters, TRS of the AB state are systematically converted to CRS. Smaller spot sizes lead to larger maximum CRS which is in agreement with previous results obtained on a different material [31]. This is especially evident for the 80% overlap case where reducing the spot size from 5 to 1 mm led to an increase of 45% of UTS.…”

“…Initial investigation on the application of LSP as a conventional surface treatment method on parts made by SLM has shown that LSP is able to convert the TRS into more beneficial CRS in the subsurface region [31]. The residual stresses were successfully transformed for all considered LSP parameters.…”

“…Residual stress profile displaying the most relevant parameters: Max CRS -maximum amount of CRS; Depth of max CRS -depth at which the maximum CRS is observed; Depth of CRS -depth at which a transition from CRS to TRS occurs. shock waves [27,31,53]. Higher overlap expectedly led to both higher maximum CRS and deeper CRS, but at the cost of an increased LSP treatment time.…”

Fracture Toughness Prediction under Compressive Residual Stress by Using a Stress-Distribution T-Scaling Method

“…Some examples of such preloading types are warm prestressing (WPS) [1][2][3][4][5][6][7][8][9][10], shot peening, and more recently, laser peening [11][12][13]. One of the common contributors to the benefits of these preloads is CRS, introduced at the crack tip.…”