Date of this Version
Klein G. (2022) Experimental Investigation on Thermally Induced Residual Stress Redistribution from Interlayer Laser Shock Peening During Additive Manufacturing. Masters Thesis. University of Nebraska-Lincoln. p. 1-77
The field of additive manufacturing (AM) has rapidly developed and expanded over the past decade. The geometric freedom provided by the process is unparalleled by any conventional means of manufacturing, opening the door to truly abstract design. Currently, the further adoption of AM practices is limited by a key issue: tensile residual stress. Tensile residual stresses are detrimental to part performance as they significantly reduce a material’s strength, fatigue life, corrosion resistance, and reliability, and are unfortunately an intrinsic quality of AM parts. AM parts are built by rapidly melting and re-solidifying hundreds of thin layers to gradually create the final product. This layer-by-layer approach exposes AM parts to extreme thermal gradients during their production, forcing the bulk into a state of tension. To mitigate this, a solution of hybrid AM has been proposed. Hybrid AM utilizes intermittent secondary processes to modify the residual stress state of parts. This research focused on the viability of using laser shock peening (LSP) treatments as the secondary process in hybrid AM as LSP can create high magnitude compressive residual stresses (CRSs) that extend deeply into the bulk. There exist significant barriers to the coupling of these two processes. Water plays an important role in LSP but is devastating to the AM process. The lack in understanding of the effects that thermal loads associated with the deposition of additional layers have on the modified residual stress states of hybrid AM parts is problematic as well. There were two objectives of this work: (1) assess the feasibility of nonconventional LSP treatments for hybrid AM applications and (2) quantify the losses in magnitude and depth of CRSs due to additional layer deposition (thermal cancellation). Results indicated that water is too critical to exclude from LSP processes for hybrid AM. Two distinct regions of thermal effects were identified for hybrid AM parts. In total, residual stress redistribution reached 550 µm beyond the LSP treated layer, however the number of additionally deposited layers affected only the first 160 µm. This work provides the first quantification of the response that hybrid AM parts have to thermal loads.
Advisor: Michael P. Sealy