Joining and Deformation Processes with Corrosion Resistance
Creators: Brandal, Grant Bjorn
Publisher: name: Columbia University
Year: 2016
Subjects: subject: Intermetallic compounds--Thermal properties
subject: Metals--Stress corrosion
subject: Laser peening
subject: Corrosion and anti-corrosives
subject: Mechanical engineering
subject: FOS: Mechanical engineering
subject: Materials science

description: Dissimilar metal joining was performed with the main goal being maximization of the strength of the joined samples, but because of some potential applications of the dissimilar joints, analyzing their corrosion behavior also becomes crucial.  Starting with materials that initially have suitable corrosion resistance, ensuring that the laser processing does not diminish this property is necessary.  Conversely, the laser shock peening processing was implemented with a complete focus on improving the corrosion behavior of the workpiece.  Thus, many commonalities occur between these two manufacturing processes, and this thesis goes on to analyze the thermal and mechanical influence of laser processing on materials’ corrosion resistances.
      Brittle intermetallic phases can form at the interfaces of dissimilar metal joints.  A process called autogenous laser brazing has been explored as a method to minimize the brittle intermetallic formation and therefore increase the fracture strength of joints.  In particular, joining of nickel titanium to stainless steel wires is performed with a cup/cone interfacial geometry.  This geometry provides beneficial mechanical effects at the interface to increase the fracture strength and also enables high-speed rotation of the wires during irradiation, providing temperature uniformity throughout the depth of the wires.  Energy dispersive X-ray spectroscopy, tensile testing, and a numerical thermal modelling are used for the analysis.
      The material pair of nickel titanium and stainless steel have many applications in implantable medical devices, owing to nickel titanium’s special properties of shape memory and superelasticity.  In order for an implantable medical device to be used in the body, it must be ensured that upon exposure to body fluid it does not corrode in harmful ways.  The effect that laser autogenous brazing has on the biocompatibility of dissimilar joined nickel titanium to stainless steel samples is thus explored.  While initially both of these materials are considered to be biocompatible on their own, the laser treatment may change much of the behavior.  Thermally induced changes in the oxide layers, grain refinement, and galvanic effects all influence the biocompatibility.  Nickel release rate, polarization, hemolysis, and cytotoxicity tests are used to help quantify the changes and ascertain the biocompatibility of the joints.
      To directly exert a beneficial influence on materials’ corrosion properties laser shock peening (LSP) is performed, with a particular focus on the stress corrosion cracking (SCC) behavior.  Resulting from the combination of an applied load on a susceptible material exposed to a corrosive environment, SCC can cause sudden material failure.  Stainless steel, high strength steel, and brass are subjected to LSP and their differing corrosion responses are determined via cathodic charging, hardness, mechanical U-bend, Kelvin Probe Force Microscopy, and SEM imaging.  A description accounting for the differing behavior of each material is provided as well as considerations for improving the effectiveness of the process.
      SCC can occur by several different physical processes, and to fully encapsulate the ways in which LSP provides mitigation, the interaction of microstructure changes induced by LSP on SCC mechanisms is determined.  Hydrogen absorbed from the corrosive environment can cause phase changes to the material.  Cathodic charging and subsequent X-ray diffraction is used to analyze the phase change of sample with and without LSP processing.  Lattice dislocations play an important role, and transmission electron microscopy helps to aid in the analysis.  A finite element model providing spatially resolved dislocation densities from LSP processing is performed.