RESIDUAL STRESS INVESTIGATIONS IN PROCESSED LAYERED METALLIC MATERIALS THROUGH SURFACE SENSITIVE CHARACTERIZATION METHODS

Claire Leeanne Adams

doi:10.7273/000007307

Residual stresses are naturally induced in various material processes and can originate mechanically or thermally. They are internal and remain in the material even after processing or loading is removed. Their location, magnitude, and type (i.e., tensile or compressive) can either be beneficial or detrimental to the mechanical behavior of the material. Layered materials and dissimilar metals create complex microstructures and interfaces, complicating residual stress profiles and affecting mechanical properties. There are challenges, however, with measuring residual stresses since they are internal, require a stress-free reference, are inferred from strain, and their values may differ depending on the measurement technique used. This research completed three studies in which layered metallic specimens were produced through three different processing methods and residual stresses were measured with three different characterization techniques. The first study involves commercially pure titanium additively manufactured samples through the directed energy deposition process. Two scanning speeds (500, 1000 mm/min) and three scanning patterns (one cross-hatched and two unidirectional) were explored while laser power remained constant. The top, middle, and bottom of the specimens were characterized with electron backscatter diffraction (EBSD) to investigate microstructure, phase, and kernel average misorientation (KAM). A face centered cubic (FCC) phase was identified, as well as a Kurjumow-Sachs orientation relationship between this FCC phase and the parent beta phase, suggesting the material consisted of a complete beta structure at high temperatures and then transitioned to a combination of FCC and HCP during cooling. Energy dispersive spectroscopy was also used to investigate composition, where the FCC phase was presumed not to be a hydride or oxide. Small amounts of nitrogen were identified, though no clear relationship was found between nitrogen and the FCC phase. A larger amount of the FCC phase was found in unidirectional scanning patterns for the slower scanning speed specimens, while the cross-hatched pattern for both scanning speeds showed a lower amount of FCC, indicating that the scanning pattern could impact the formation of the FCC phase. Additionally, this FCC phase could be influenced or activated by the heating of the additive process, suggesting scanning speed could be a cause. The highest KAM averages belong to the FCC phase of the faster scanning speed specimens due to faster cooling in the additive process, causing more deformation in the material. Residual stresses were also investigated through a cross-correlation EBSD (CC-EBSD) software called ATEX. The general trend suggests that FCC regions contain higher stress while HCP regions contain lower stress, corresponding to KAM map trends. The FCC phase contains more local lattice curvature and higher GND density, relating to higher local plastic strain. The second study consists of metal sheets of alternating layers of aluminum alloys 1050 and 5052, manufactured through a severe plastic deformation method known as the friction assisted lateral extrusion process (FALEP). An additional shear force drives this process to produce homogenous ultrafine-grained structures in a single step. There were variations in the thicknesses of layers after the shearing process, which may be due to the differences in hardness between the two alloys and a reflection of how FALEP interacts with multi-material specimens. Residual stresses were measured across all layers for both longitudinal (σ11) and transverse (σ22) directions with X-ray diffraction using the cos(α) method. Flow stress values for stress calculations were determined through high-pressure compressive shearing. Microstructure was investigated with EBSD, and grain size distributions were determined. The average grain size for AA1050 was approximately 0.75 μm, while AA5052 showed a bimodal distribution for grain sizes slightly below and above 1 μm, with the average being approximately 1.14 μm. Pole figures were also created, showing a shear texture. Residual stress profiles displayed an oscillating pattern for both stress components. Layered specimens showed slight compressive stresses near interfaces for the σ11 component. The magnitude of longitudinal RS values (±40 MPa) are larger than that of the transverse component (±20 MPa), with an uncertainty of ±9 MPa. The third study involves titanium alloy Ti6Al4V diffusion bonded to vanadium. EBSD and EDS were performed to investigate microstructure and determine composition. Microstructures in the bulk of both materials show vanadium grains are approximately 3X larger than Ti6Al4V grains, and pole figures show both materials have a rolling texture. There were four microstructural regions of interest across the interface: primary α-Ti, α-Ti transformation, β-Ti, and V. It was found that this β-Ti phase had formed along the interface due to V acting as a beta stabilizer. Parent grain reconstruction shows that the α-Ti transformation structure originates from β-Ti grains rather than primary α-Ti grains. EDS results show a change in composition across the β-Ti region, making it difficult to obtain an accurate reference specimen for stress calculations from nanoindentation. There are clear differences in hardness for primary α-Ti, β-Ti, and V regions, and this hardness decreases moving across the interface from primary α-Ti to V. Residual stresses were measured using nanoindentation, CC-EBSD with ATEX software, and XRD using the sin2Ψ method. The general residual stress trend between indentation and CC-EBSD results consists of compressive stress in the primary α-Ti region and tensile stress in β-Ti and V regions, with β-Ti having a slightly larger magnitude of stress than V. Residual stress occurs from the mismatch in microstructures (BCC to HCP), interstitials from diffusion bonding, pile up of dislocations at the interface, and differences in properties between materials (i.e. coefficients of thermal expansion). Further analyses are necessary to understand trends associated with XRD measurements.

RESIDUAL STRESS INVESTIGATIONS IN PROCESSED LAYERED METALLIC MATERIALS THROUGH SURFACE SENSITIVE CHARACTERIZATION METHODS

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