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Norton, Seth J.Development of a gleeble based test for post weld heat treatment cracking in nickel alloys
Master of Science, The Ohio State University, 2003, Welding Engineering
The purpose of this work was to use a thermo-mechanical simulator to develop a simple test that will quantify susceptibility to the various forms of postweld heat treatment cracking, including stress-relief cracking and strain-age cracking. Materials evaluated include Waspaloy and Alloy 718. Samples are initially given an elevated temperature thermal exposure that simulates the weld HAZ. Upon cooling from elevated temperature, the sample is loaded such that yield strength magnitude stresses are present at room temperature. The sample is then heated to a selected PWHT temperature and held for up to 8 hours. Acquisition of tensile force data during PWHT shows an initial relaxation of stresses, followed by a rise in stress as precipitation reactions proceed. Hot ductility tests were performed at various PWHT temperatures and times. From the reduction in area measurements a mathematical model was developed to relate ductility to PWHT temperature and time. The alloys tested show a significant dip in ductility at elevated temperature. Waspaloy samples exhibited lower ductility at elevated temperatures than did Alloy 718, confirming the higher susceptibility of Waspaloy to strain-age cracking. SEM fractography and optical microscopy were used to analyze the failed samples. Minimum ductility samples revealed ductile intergranular fracture paths in Waspaloy and a tendency for transgranular cracking in Alloy 718. Intergranular micro-cracks were evident in a coarse grained heat affected zone in both alloys. Grain size was shown to effect ductility and fracture mode. Smaller grained simulated HAZ samples exhibited more ductility and a tendency for transgranular ductile fracture. Larger grains resulted in an increase in intergranular fracture and a reduction in elevated temperature ductility. Alloy 718 showed evidence of bending of crystal lattices to accommodate stresses. Both alloys showed indications of an effect of grain boundary orientation on cracking susceptibility.


John Lippold (Advisor)


Waspaloy; Alloy 718; strain-age cracking; Gleeble testing; post weld heat treatment cracking; stress-releif cracking

Makiewicz, Kurt TimothyDevelopment of Simultaneous Transformation Kinetics Microstructure Model with Application to Laser Metal Deposited Ti-6Al-4V and Alloy 718
Master of Science, The Ohio State University, 2013, Materials Science and Engineering
Laser based additive manufacturing has become an enabling joining process for making one-of-a-kind parts, as well as, repairing of aerospace components. Although, the process has been established for more than a decade, optimization of the process is still performed by trial and error experimentation. At the same time, deployment of integrated process-microstructure models has remained as a challenge due to some of the reasons listed below: (1) lack of good process models to consider the laser-material interactions; (2) inability to capture all the heat transfer boundary conditions; (3) thermo-physical-mechanical properties; and (4) robust material model. This work pertains to the development of robust material model for predicting microstructure evolution as a function of arbitrary thermal cycles (multiple heating and cooling cycles) that can be integrated into a process model. This study focuses on the development of a material model for Ti-6Al-4V and Alloy 718. These two alloys are heavily used in turbine engines and undergo complex phase transformations, making them suited to developing a material model for laser metal deposition (LMD). The model uses simultaneous transformation kinetics (STK) theory to predict the transformation of one parent phase into several products. The model uses calculated thermodynamic properties of the alloys for portions of the respective transformation characteristics. Being a phenomenological model there are several user defined calibration parameters to fit the predicted output to experimental data. These parameters modify the nucleation and growth kinetics of the individual transformations. Analyses of experimental LMD builds are used to calibrate the material model. A Ti-6Al-4V build made on a room temperature substrate showed primarily colony alpha morphology in the bottom half of the build with a transition to basketweave alpha in the top half. An increase in hardness corresponding to the microstructural transition was observed. This sample had an average of 340 HV hardness. Analysis of the calculated thermal profiles at the location of the morphology transition showed a transition from cooling below the beta transus to cooling above the beta transus. The Ti-6Al-4V STK model was calibrated using the experimental data from this sample. The substrate of a second build was heated above the Ti-6Al-4V beta transus. This build showed predominantly basketweave alpha without a microstructural transition. Large prior beta grains (>1mm) were observed growing epitaxially from the substrate. These large grains promoted the basketweave formation. Hardness testing showed an average of 344 HV. Samples built in this way were also fatigue tested in the as built condition. Results show that they match previous builds that had been stress relieved. A third build was performed at room temperature on a substrate with large prior beta grains. This build showed basketweave morphology like the second build even though the substrate was not thermally controlled. The hardness for this build averaged 396 HV which is ~50 HV higher than the previous two. This build shows that it may be possible to produce better mechanical properties by controlling the beta grain size rather than heating the substrate. Eighteen Alloy 718 builds were made using proprietary processing conditions. All of these builds were analyzed for nano-scale γ’ and γ’’ precipitates. Two of the builds were similar but had different laser powers. The low laser power build did not show nano-scale precipitates. The higher power build did show small amounts (<3%) of nano-scale precipitates and a corresponding increase in hardness at their locations. The higher power build was used to develop the STK model for Alloy 718. Sixteen of these builds were part of a design of experiments and are referred to as DOE samples. Eight of them have a single layer while the other eight have multiple layers. They were examined for nano-scale precipitates. The amounts of precipitates were correlated to hardness values and thermal profiles.


Sudarsanam Babu (Advisor); Wolfgang Windl (Committee Member)


Aerospace Materials; Materials Science; Metallurgy


Simultaneous Transformation Kinetics; STK; Microstructure Modeling; Laser Additive Manufacturing; Laser Metal Deposition; aerospace repair; Ti-6Al-4V; Inconel 718; Alloy 718; Additive Manufacturing; LAM; LMD;