Michael P. Sealy
Date of this Version
C. Kanger, 2018. Structural Integrity of 420 Stainless Steel From Coupling Directed Energy Deposition and Laser Shock Peening, Masters Thesis.
Additive manufacturing (AM) is used for prototyping and full production of high value components. The ability to manufacture application specific designs more quickly than traditional manufacturing has broad appeal to aerospace, medical, and automotive industries. However, most AM metals exhibit insufficient mechanical properties compared to wrought alloys. Since the majority of AM processes add material in a controlled manner layer-by-layer, an opportunity arises to improve mechanical properties by cold working each layer using hybrid additive manufacturing techniques. Hybrid additive manufacturing processing refers to the fully coupled combination of printing with two or more manufacturing processes that synergistically affects quality and performance of a printed part. Combining printing with a secondary process such as peening allows mechanical properties to be printed layer-by-layer. This work focuses on combining powder-based directed energy deposition (DED) with laser peening to investigate the cumulative surface integrity of 420 stainless steel coupons. In this work, the cumulative surface integrity from cyclically printing and peening is referred to as structural integrity. The objective of this study was to use laser peening between printed layers to induce a complex structural integrity consisting of beneficial mechanical properties (e.g., work hardening, grain refinement, and compressive residual stress) throughout the build volume. Specifically, two experiments and twelve samples were printed, peened, then printed again in various combinations and conditions to show what, if any, combination of layer peening frequency (i.e., number of layers printed between peening treatments) and laser peening treatment conditions can withstand the intense heat from additional printing. Measuring microhardness, microstructure, and residual stress assessed the ability of laser peening to cold work individual layers that would later be exposed to the harsh thermal effects of directed energy deposition. It was found that layer peening frequency affected on residual stress formation, microhardness, and microstructure. Specifically, a cyclic pattern of residual stress and microhardness were achieved within the bulk material of treated samples. These experiments set the foundation for further development of coupling laser peening and DED printing that will ultimately lead to the ability to design and manufacture tailored mechanical properties throughout a build volume.
Advisor: Michael P. Sealy