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Spring 2009 Seminar Series
Monday, 4/13, at 3:30 p.m.
Room 264 MacQuigg Labs
Adam Pilchak
PhD Candidate advised by Dr. James Williams
Department of Materials Science and Engineering
The Ohio State University
The Effect of Friction Stir Processing on the Microstructure and Fatigue Behavior of Investment Cast Ti-6Al-4V
Abstract
Due in part to the rising cost of titanium, investment casting is becoming increasingly attractive as a manufacturing route because it is a near net shape process and results in minimal waste. Investment casting is typically not considered, however, as a manufacturing process for fatigue critical components because the coarse, fully lamellar microstructure has poor fatigue crack initiation resistance. Therefore, this study employed friction stir processing to locally refine the microstructure on the surface of cast and hot isostatically pressed Ti-6Al-4V plates.
Depending on processing parameters, the peak temperature in the stir zone was either above or below the β transus and the resulting microstructures consisted of either ≈1 μm equiaxed α grains or ≈25 μm prior β grains containing a colony α + β microstructure. The changes in microstructure were characterized with scanning and transmission electron microscopy and electron backscatter diffraction. The texture in the stir zone was nearly random for all processing conditions, however several components of ideal simple shear textures were observed in both the α (hcp) and β (bcc) phases which provided insight into the operative recrystallization mechanisms.
The effect of friction stir processing on crack initiation resistance was assessed using high cycle fatigue tests conducted in four-point bend which put only the stir zone in maximum tension. The results indicate that at constant stress amplitude, there is between a one and two order of magnitude increase in fatigue life after friction stir processing. This improvement has been verified with a statistically significant number of tests. High resolution fractography coupled with quantitative tilt fractography and electron backscatter diffraction has also provided a direct link between microstructure, crystallography and fracture topography.
These techniques have been used extensively to study the early stages of post-initiation crack growth in Ti-6Al-4V, especially at low values of ΔK values in the as-cast material. Limited experiments have been performed on other microstructures to assess the generality of the detailed results obtained for the fully lamellar material. The results show the general applicability to other microstructures in α + β titanium alloys and that fracture topography depends strongly on ΔK, crystallographic orientation and microstructural length scale. This work was supported by the Office of Naval Research.
Bio
Adam received his bachelors degree in Materials Science and Engineering from Michigan State University in December of 2005 and joined The Ohio State University in January of 2006. His main research interests include the deformation behavior of titanium alloys, particularly during fatigue and dwell fatigue loading, as well as good old fashioned physical metallurgy. Adam also has a strong interest in electron backscatter diffraction and its use for studying the microscopic aspects of fracture. He has given seven talks at conferences and has authored 10 articles in peer reviewed journals, with additional publications appearing in conference proceedings. In his spare time, Adam enjoys spending time with his wife, Jennifer, cooking, playing guitar, writing Matlab code and throwing tennis balls for his untiring, unrelenting dogs, Winston and Shelly.
