Date of Award
Doctor of Philosophy (PhD)
Dr. Garrett Pataky
Dr. Fadi Abdeljawad
Dr. Laura Redmond
Dr. Huijuan Zhao
Additive manufacturing (AM) is becoming a manufacturing process that is well established, even with all the resources and attention that has been brought to it, the field is still lacking some key understandings. Currently, there are certain aspects that are difficult to overcome. Some of the intrinsic obstacles include process-induced defects, such as porosity from lack of fusion and gaseous bubble entrapment, as well as complex thermal gradients. These defects can lead to altered material response especially when looking at the fatigue life. The fatigue behaviors of AM components can change from print to print as well as when compared to their wrought counterparts. In order for the AM field to have a greater confidence in their material response, the material needs to reach a standard that can be quantified and characterized consistently.
Studies have shown that surface treatments such as shot and shock peening can be performed on components to produce a longer fatigue life. However, the interaction of these treatments with the microstructural porosity in AM components is still relatively unknown. This work looks to understand how laser shock peening effects the low cycle fatigue life of AM specimens. The first part of this work was to determine the residual stresses that were created on an AM AlSi10Mg sample from femto-second laser shock peening (LSP). The residual stress was quantified to have a maximum compressive residual stress on the order of 300 MPa and to a depth ~0.5 to 1 mm using the contour method. A simulation was then conducted where the stress from the laser shock peening process were calculated for a dog bone specimen and then the model was elongated by applying a 1 kN load. The goal was to understand how LSP effects the critical pores near the specimen surface after loading. The results showed that the stresses around the pore were reduced significantly with the introduction of the LSP residual stresses.
The second part was low cycle fatigue testing of non-treated and treated AM AlSi10Mg specimens. Post-mortem fractography determined that gaseous pores were critical to the eventual failure of the specimens for the both the non-LSP and LSP cases. The fatigue life was independent of surface roughness present from the print, highlighting the severity of pores in fatigue. The use of LSP did not extend the fatigue life of the samples, however, the initial fatigue scatter results from the LSP samples showed a reduction in 78% when comparing the non-LSP to LSP condition.
The third part of the research was to implement high resolution digital image correlation coupled with electron backscatter diffraction to analyze how localized strains accumulate with respect to the microstructure. Two separate loading conditions were analyzed for both the non-LSP and the LSP condition. The average strain within each grain was calculated and compared between the two conditions. The results showed that the average strain range was higher for the non-LSP conditions as compared to the LSP conditions. For loading condition of 1 kN, there was a reduction of 71% in the average strain within a grain range. For the loading condition of 0.875 N, there was a reduction of 44% in the average strain within a grain range. The results were attributed to the reduction of the stress concentration factor around the pores.
Biddlecom, Jacob L., "The Effects of Laser Shock Peening on the Fatigue Life of Additive Manufactured AlSi10Mg" (2023). All Dissertations. 3328.