Doctor of Philosophy, The Ohio State University, 2024, Welding Engineering

Li-ion batteries (LIB) have been spotlighted as a promising power source to replace traditional fuel gases owing to their high energy density, lightweight, and greenhouse gas emission free characteristics. Nevertheless, the catastrophic failure of the LIB is usually connected to safety issues, and the solutions must be addressed from the perspective of materials and designs. The representative materials for the current collectors in LIB are the commercial pure-grade aluminum (Al) and copper (Cu) foils because of their high electrical conductivities, electrochemical stabilities, and low density. However, they degrade during the multiple charge/discharge cycles when the applied voltage exceeds their corrosion potentials. The current flows generate the resistance heating during the charge/discharge cycles at the joint between the foil stacks and the lead tab, increasing the cell temperature. This accelerates the degradation of the foil and reduces the life cycle of the LIB. To reduce the electrical resistance, the increase in conductive area is desirable. Therefore, securing the large joint area with minimal weld discontinuities not only improves the mechanical properties helps but also impedes the resistance heating in LIB. The conventional resistance spot welding (RSW) process has not been widely used to produce the joint between the current collectors and the lead tab so far because of the high thermal, electrical conductivity and thinness of the Al and Cu weld stacks. These aspects generally lead to the smaller weld nugget size, and the sticking of the weld stacks to the electrodes. A recently developed hybrid joining process, known as ultrasonic-assisted resistance spot welding (URW), shows the great effects on increasing the weld nugget size as well as mechanical properties in various pairs of similar and dissimilar metal sheets by the microstructure modification. In the present study, multiple thin pure aluminum (Al) and copper (Cu) foils and tab stacks are (open full item for complete abstract)

Committee: Xun Liu (Advisor); Glenn Daehn (Committee Member); Avraham Benatar (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2024, Welding Engineering

Whether a vehicle runs on fossil fuel, electricity, or hydrogen, one method that increases the energy efficiency of any vehicle is to decrease the weight of the vehicle body structure by using multi-material design. Multi-material design uses strong materials such as advanced high-strength steels (AHSS) for high load bearing parts, and lightweight materials such as aluminum (Al) or magnesium (Mg) for parts that experience lower loads. Traditional resistance spot welding (RSW) is unable to create adequate dissimilar metal joints between Al and steel or Mg and steel because of the differences in physical properties and the formation of brittle intermetallics (IMCs). A variety of alternative processes have been developed to solve the dissimilar metal joining challenge, but many of them require the purchase of new machinery and use complex consumables for each joint. Ultrasonic interlayered Resistance Spot Welding (Ulti-RSW) is a recently developed joining process that uses existing RSW machinery and a cheap consumable interlayer. Ulti-RSW has proven feasibility in creating strong joints between dissimilar metals such as Al and press-hardened boron steel that are difficult to join using other joining processes. To date, the qualification of Ulti-RSW joints has been completed for quasi-static shear tension testing, but there are many other qualifications that need to be met before Ulti-RSW can be incorporated into the automotive industry. The overarching goal of this dissertation is to further qualify the Ulti-RSW process in the areas of fatigue, mode 1 loading, and corrosion, and to advance the fundamental understanding of joint microstructure and properties when joining aluminum and magnesium to steel. The specific research tasks and accomplishments are summarized as follows. The Ulti-RSW joints were fatigue tested and compared to other joining processes in the literature. Previously, there was no adequate method of comparing fatigue results across various joining (open full item for complete abstract)

Committee: Wei Zhang (Advisor); Xun Liu (Committee Member); Desmond Bourgeois (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2022, Welding Engineering

The automotive industry wants to advance integrated computational materials engineering (ICME) approaches that combine models of joining processes and microstructural evolution for prediction of material property gradients and ultimately the mechanical performance of multi-sheet lap joints. Despite the increasing demand for computational optimization within vehicle structures and the increased use of low-density materials, modern integrated modeling frameworks of automotive lap joining are often limited to the resistance spot welding (RSW) of conventional steels. Moreover, important phenomena in steel weldments, like decomposition of austenite on-cooling, tempering of martensite, and microstructure-dependent flow stress and damage properties are too material-specific for universal application. In this research, generalized metallurgical and mechanical modeling strategies are investigated for increased applicability to a wider range of steels and joining processes. The study evaluates: the reliability of heat transfer predictions within state-of-the-art numerical models of RSW, the accuracy of existing austenite decomposition models, the readiness of steel time-temperature-transformation (TTT) diagram tools containing CALPHAD-calculated parameters, the generality of a recently developed martensite tempering model, and the determination of RSW fusion and heat-affected zone flow stress and fracture behavior. Results show that state-of-the art finite element models of RSW that are validated using experimental weld nugget dimensions have a propensity to underpredict cooling rates. A JMAK and additivity rule approach calibrated with experimental TTT diagram data exhibited the greatest accuracy when predicting AHSS austenite decomposition; however, calibrations using calculated TTT diagrams better facilitated material optimization. Generalized parameters within a JMAK-type model of martensite tempering successfully predicted HAZ softening within martensitic and dual-phase (open full item for complete abstract)

Committee: Antonio Ramirez (Advisor); Avraham Benatar (Committee Member); Boian Alexandrov (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2018, Welding Engineering

Multi-materials vehicle structures, employing light-weight metals such as advanced high strength steels (AHSS), aluminum alloys, can satisfy the ever-increasing requirement of light-weighting and fuel efficiency, as well as maintaining or improving the crash resistance of vehicles. The present research provides a fundamental understanding of the process-microstructure-mechanical properties of resistance and ultrasonic spot welding of light-weight metals. The dissertation consists of three main parts: (1) study of the relationship of process-microstructure-mechanical properties for resistance spot welded two sheets (2T) and complex stack-ups of ultra-high strength grade of AHSS, (2) development of a novel technique, namely Ultrasonic Plus Resistance Spot Welding, for dissimilar metal joining of Al to steel, and (3) investigation of the bonding mechanism of USW of Al by in-situ relative vibration measurement. In the first part, softening in subcritical heat affected zone of resistance spot welded hot-stamped boron steels is investigated by weld microstructure characterization and tempering kinetics of martensite. The local constitutive behavior of the potential failure locations is extracted and incorporated into performance model to investigate its effect on the accuracy of deformation and failure prediction. A major challenge for RSW of complex stack-ups with large thickness ratio is the limited nugget penetration into the thin sheet at the outside of the stack-up. The effect of welding current, electrode force, electrode material/size on nugget formation and the possible ways to improve nugget penetration into the thin sheet are investigated for 3T and 4T stack-ups of AHSS. The second part of the dissertation is focused on the development of a new dissimilar metal joining method, namely ultrasonic plus resistance spot welding (abbreviated as U+RSW) for Al/Steel. The bonding mechanisms have been investigated through numerical simulation to validate the (open full item for complete abstract)

Committee: Wei Zhang (Advisor); Carolin Fink (Committee Member); Xun Liu (Committee Member) Subjects: Automotive Materials; Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2017, Materials Science and Engineering

Hot stamped boron steels, such as Usibor® 1500, have been increasingly used in automotive structural components for light-weighting and impact resistance. Classified as an ultra-high strength steel, these alloys have superior strength with tensile strengths exceeding 1500 MPa. The rapid heating and cooling thermal cycle during resistance spot welding can significantly alter the martensitic base metal microstructure, resulting in formation of coarse-grained and subcritical heat-affected zones (CGHAZ and SCHAZ) with inferior mechanical properties. The martensitic CGHAZ is adjacent to the weld nugget and experiences the most time above the AC3, which allows for austenite grain growth. The SCHAZ is next to the unaffected base metal and does not reach the AC1¬ during welding, thus the base metal microstructure is over-tempered into cementite and ferrite. The present research aims at developing the fundamental knowledge of plastic deformation and fracture behaviors of ultra-high strength steel resistance spot welds. As a resistance spot weld comprises highly inhomogeneous microstructure, the overall research approach is based on studying the local (or microstructure-dependent) mechanical properties for individual regions in the weld as well as their interactions with weld geometry on the deformation behavior. Specifically, optimal welding parameters are determined to produce welds of appropriate nugget diameter for 2T Usibor 1500 with a gauge thickness of 1.5 mm. Micro-hardness mapping and metallographic analysis allow for characterization of the weld metal, CGHAZ, SCHAZ, and base metal of the spot weld. Quasi-static tensile testing with digital image correlation (DIC) is used to determine the local stress-strain behaviors of each region using bulk microstructural samples created in a Gleeble® thermal-mechanical simulator. Conventional and innovative resistance spot weld mechanical testing methods are used to generate more knowledge on the deformation of joints un (open full item for complete abstract)

Committee: Wei Zhang PhD (Advisor); Menachem Kimchi (Advisor); David Phillips PhD (Committee Member) Subjects: Automotive Engineering; Automotive Materials; Engineering; Materials Science; Metallurgy; Transportation

Doctor of Philosophy, The Ohio State University, 2005, Welding Engineering

Small scale resistance spot welding (SSRSW) is extensively used in electronic and biomedical devices manufacturing. The process has its own features that are quite different from normal scale RSW process. These features bring difficulties in the reliable monitoring and control of the process. A real-time model-based feedback control with model adaptation has been proposed for the quality assurance of the process. Several aspects of the model were studies. Monitoring is the first step for a successful control. An optical sensor was used for workpiece thickness and electrode displacement measurement. A load cell was placed at the bottom of lower electrode seat to record the clamping and change of clamping force in welding. The welding current and voltage were also measured. From these signatures it is an easy task to monitor the occurrence of expulsion in the process. The weld nugget size can also be related to electrode displacement. Specifically, through observing the electrode separation using a high-speed video camera, it is confirmed that the nugget size is more closely related to high-speed part of the electrode displacement curve. The way the optical sensor was implemented introduced measurement noise. However, the true displacement can be recovered by identification of the sensor holder system model followed by feeding of measured displacement to this model. This way a reliable displacement measurement was obtained that is suitable for the utilization of a real-time control. The interaction between the primary process and the welding machine affects welding quality. Future real-time control must consider this interaction, because this interaction also influences the measured value of the process variables. A more flexible machine will allow the electrodes to separate more under the pushing force of heated materials than a more rigid welding machine. Due to the interaction between different part of welding machine and the primary process, some measurable proces (open full item for complete abstract)

Committee: Dave Farson (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2003, Welding Engineering

This dissertation study has developed a fundamental understanding of the contact resistance behavior using the virtual contact volume concept and the equivalent contact resistivity definition. In this research, an integrated experimental-numerical approach was used to demonstrate the proposed equivalent contact resistivity versus temperature relationship. Such relationship was further used to study the expulsion behavior of the resistance spot welding aluminum alloy. A concept of using the constriction ring was demonstrated to be effective in preventing weld expulsion in resistance spot aluminum welds. The a-spot model simulated a single ball contact to define the equivalent contact resistivity. For a specific loading range, the generic equivalent contact resistivity versus temperature relationship, consisting of a proportional term and a exponential term, was derived. The proportionality term represents the increasing contact resistivity with temperature reflecting the Wiedemann-Franz-Lorentz behavior. The exponential term represents the softening effect of the contact surface as temperature increases. With the generic equivalent contact resistivity versus temperature relationship established, an a-spot welding which incorporated the contact pairs with the resistivity relationship leaving three factors as parametric variables and an experimental a-spot welding model were conducted. Both the numerical and experimental analyses show the same relationship that is defined by the theoretical hypothesis of a-spot model. The parametric study was conducted to quantify the parametric factors based on comparisons on nugget size and electrical potential drops across the electrodes. For an ideal combination of parameters at the Cu/Al interface and the Al/Al faying surface, both the comparisons of weld nugget size and potential drops are satisfactory. The mechanical analysis proposed a concept that expulsion would occur when the crack tip is within the solidus nugget zone at an (open full item for complete abstract)

Committee: Chon Tsai (Advisor) Subjects: Engineering, Mechanical