Date of Award

5-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Grujicic, Dr. Mica

Committee Member

Tong , Dr. Chenning

Committee Member

Ochterbeck , Dr. Jay

Abstract

Friction Stir Welding (FSW) is a solid-state metal-joining process. Within FSW, a (typically) cylindrical tool-pin (threaded at the bottom and terminated with a circular-plate shape shoulder, at the top) is driven between two firmly-clamped plates (placed on a rigid backing support). Due to a high normal downward pressure applied to the shoulder and due to frictional sliding and plastic-deformation, substantial amount of heat is generated at the tool/work-piece interface and in the region underneath the tool shoulder. Thermally plasticized work-piece material is then extruded around the traveling tool and forged into a welding-joint behind the tool. Due to its solid-state character and lower process temperatures, FSW possesses a number of advantages in comparison to the conventional fusion welding processes. In the present work, advanced computational methods and tools are used to investigate three specific aspects of the FSW process: (a) material flow and stirring/mixing: Within the numerical model of the FSW process, the FSW tool is treated as a Lagrangian component while the workpiece material is treated as a Eulerian component. The employed coupled Eulerian/Lagrangian computational analysis of the welding process was of a two-way thermo-mechanical character (i.e. frictional-sliding/plastic-work dissipation is taken to act as a heat source in the thermal-energy balance equation) while temperature is allowed to affect mechanical aspects of the model through temperature-dependent material properties. The workpiece material (AA5059, solid-solution strengthened and strain-hardened aluminum alloy) is represented using a modified version of the classical Johnson-Cook model (within which the strain-hardening term is augmented in order to take into account for the effect of dynamic recrystallization) while the FSW tool material (AISI H13 tool steel) is modeled as an isotropic linear-elastic material. Within the analysis, the effects of some of the FSW key process parameters are investigated (e.g. weld pitch, tool tilt-angle and the tool pin-size). The results pertaining to the material flow during FSW are compared with their experimental counterparts. It is found that, for the most part, experimentally observed material-flow characteristics are reproduced within the current FSW-process model; (b) modifications of the existing workpiece material models for use in FSW simulations: Johnson-Cook strength material model is frequently used in finite element analyses of various manufacturing processes involving plastic deformation of metallic materials. The main attraction to this model arises from its mathematical simplicity and its ability to capture the first order metal-working effects (e.g. those associated with the influence of the extent of plastic deformation, rate of deformation and the attendant temperature). However, this model displays serious shortcomings when used in the engineering analyses of various hot-working processes (i.e. those utilizing temperatures higher than the material recrystallization temperature). These shortcomings are related to the fact that microstructural changes involving: (i) irreversible decrease in the dislocation density due to the operation of annealing/recrystallization processes; (ii) increase in grain size due to high-temperature exposure; and (iii) dynamic recrystallization-induced grain refinement, are not accounted for by the model. In the present work, an attempt is made to combine the basic physical-metallurgy principles with the associated kinetics relations in order to properly modify the Johnson-Cook material model, so that the model can be used in the analyses of metal hot-working and joining processes. The model is next used to help establish relationships between process parameters, material microstructure and properties in FSW welds of AA5083 (a non-age-hardenable, solid-solution strengthened, strain-hardened/stabilized Al-Mg-Mn alloy); and (c) FSW-joint failure mechanisms under ballistic impact loading conditions: A critical assessment is carried out of the microstructural changes, of the associated reductions in material mechanical properties and of the attendant ballistic-impact failure mechanisms in prototypical Friction Stir Welding (FSW) joints found in armor structures made of high-performance aluminum alloys (including solution-strengthened and age-hardenable aluminum alloy grades). It is argued that due to the large width of FSW joints found in thick aluminum-armor weldments, the overall ballistic performance of the armor is controlled by the ballistic limits of its weld zones (e.g. heat affected zone, the thermo-mechanically affected zone, the nugget, etc.). Thus, in order to assess the overall ballistic survivability of an armor weldment, one must predict/identify welding-induced changes in the material microstructure and properties and the operative failure mechanisms in different regions of the weld. Towards that end, a procedure is proposed in the present work which combines the results of the FSW process modeling, basic physical-metallurgy principles concerning microstructure/property relations and the fracture mechanics concepts related to the key blast/ballistic-impact failure modes. The utility of this procedure is demonstrated using the case of a solid-solution strengthened and cold-worked aluminum alloy armor FSW-weld test structure.

Share

COinS