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Collision welding or impact welding is a solid-state welding technique which enables unique materials joining opportunities. The high velocities and short timescales of the welding process lead to extreme peak collision pressures, high strain rates, low heat inputs, steep thermal gradients, and narrow thermo-mechanically affected zones (TMAZ). These features can be advantageous in the joining of traditionally unweldable materials such as 2XXX or 7XXX series aluminum alloys, and for the joining of dissimilar metals which would form unwanted phases under traditional fusion welding. However, these same features can also make impact welding challenging to control and characterize. Analytic process limits have been developed to calculate limiting flyer velocities and impact angles for welding success as functions of the weld member properties based on the physical mechanisms that enable joining.The maximum impact velocity for a weld is determined by a dynamic solidification cracking mechanism: reflected dynamic tensile stresses arrive to the nascent weld interface prior to full interface cohesion, which is mediated by the presence of interfacial melting. However, current analytic models for the upper limit of the collision welding window were developed for autogenous welds, which develop symmetric stress conditions at impact. In this thesis, we develop alternative formulations to the analytical upper limit of the welding window which better support dissimilar welding. Shock-informed calculation of the asymmetric stress and thermal partitioning between dissimilar weld members is achieved through the application of modified Rankine-Hugoniot relations. We compare the application of the shock-informed upper limit to the existing upper limits in the context of historical data.The shock upper limit is further validated experimentally through the use of The Ohio State University Impulse Manufacturing Laboratory’s (IML) Laser Impact Welding (LIW) and Vaporizing Foil Actuator Welding (VFAW) facilities. LIW use a short duration pulse laser, both with and without chemical augmentation, to launch metal flyers to impact welding targets at velocities up to 1 km/s through the generation of ablation and detonation pressures behind the flyer in conjunction with a transparent pressure confinement backer. VFAW generates pressure pulses for flyer acceleration through capacitive discharge into consumable aluminum foils, which burst due to extremely rapid superheating well beyond the melting point.We briefly describe the IML LIW facilities including weld sample configuration, velocity profiles, and weld diagnostics including photon Doppler velocimetry (PDV) flyer acceleration the impact plane. We investigate the upper welding limit of dissimilar welds using commercially pure titanium flyers and tin targets as a model system for which the upper weld limit is within range of the IML facility flyer velocities.VFAW of a medium entropy alloy system (equimolar CrCoNi) is also utilized to demonstrate instability-driven defects’ role in failure initiation in welds that survive the impact process. These investigations show the preservation of bulk microstructures to enable advanced materials joining in conjunction with the mechanisms that can generate TMAZ defects and their associated failure modes. Together, they provide a pathway to designing weld processes to produce higher quality impact welds in thin weld members where shock propagation and instability-driven localized melting dominate the stress and thermal interactions which generate TMAZ flaws and failures.

The process of laser impact welding utilizes impact welding and laser-driven flyers to form solid-state, metallurgical welds between similar or dissimilar metallic flyers and targets. With chemically augmented laser impact welding, stronger and thicker metal flyers and targets can be welded together. Using a high-powered laser, a laser pulse is shot through a transparent tamping layer onto a translucent layer of chemical liquid and the bare surface of a metallic flyer. The energy from the laser pulse detonates the chemical augment and the pressure created from the explosion is confined by the tamping layer. This pressure is directed towards the flyer that is then driven to velocities in the hundreds of meters per second within 20 microseconds. Under the correct conditions, high speed and acceptable impact angle between the flyer and target, jetting will occur. The jet cleans the surface of the flyer and target of oxides, and the two surfaces will form a solid-state, metallurgical bond.Using a chemical augment, thicker, stronger flyers and targets can be welded compared to unaugmented laser impact welding. With the chemical augment, a 3J, 8.1ns laser pulse can weld a 0.5mm Al2024-T3 flyer to a 0.5mm Al2024-T3 target. To explore the capabilities of chemically augmented laser impact welding, two chemical augments were used as candidates for the process. Various tamping materials and thicknesses were also investigated along with variance in the laser spot diameter. The velocities of flyers were measured using Photon Doppler Velocimetry and a thicker tamping layer produced higher velocities and larger deformations than thinner tamping layers did with the same parameters. The strength of the welds between 0.5mm Al2024-T3 flyers and targets were also measured using a tensile test. Over two-thirds of the welded samples failed by nugget pullout during these tensile tests, validating the strength of the welds formed. Micrographs of a welded sample were also collected to observe the welded and unwelded regions. Finally, future work to advance chemically augmented laser impact welding is discussed.

High-temperature metallic materials such as nickel-based and titanium alloys are attractive as skin structures for aerospace vehicles. They can allow significant performance improvement and mass reduction in aircraft. However, there are substantial challenges in welding and forming them affordably for service. This project examines the use of impulse-based methods, as enabled by the vaporizing foil actuator method, for the impact welding and precise shaping of alloys Ni - 718, Ni - 625, Ni - 230, and Ti 6242. The mechanical properties and weld microstructure of four similar and Six dissimilar VFA spot welding combinations are presented and analyzed. Microhardness measurements showed the absence of a heat affected zone (HAZ). The dissimilar Ni - Ni joints and Ni - Ti joints exhibited high loads to failure in lap-shear tests and show great potential for applications involving transition joints, repair welding, medical devices, and more. The VFA method is cheap, safe, fast, durable, and marks the advancement in the solid-state joining of dissimilar nickel and titanium systems.Nickel alloys typically exhibit low springback during quasi-static forming processes. However, the large amounts of strain hardening that occurs during these operations often requires a second annealing stress relief operation. Titanium alloys are commonly known to exhibit high springback levels due to the high strength to stiffness ratios of titanium alloys. Sheet metals components are usually shaped by hot or superplastic forming. This process is expensive and has long lead-times. This work examines an athermal process to relax or remove the residual stresses and elastic strains in sheet metals. All the materials explored, especially titanium showed significant improvements in shape conformance when processed through the VFA method. Recent shock-based calibration studies have provided some insight into the previously unconfirmed mechanism of springback relief. The driving hypothesis for this physical phenomenon is that modest shock waves plastically relieve elastic residual stresses and result in target shape conformance. To better understand this mechanism, VFA based shock processing experiments were used to change the curvature of pre-strained materials to a fully flat shape. It is speculated that the change in shape is a consequence of elastic stress relief caused by the propagation of planar shockwaves. A mechanics and shock physics based theory for shockwave interaction with residual stresses in a pre-strained sample is proposed. The possible pressures involved in this process were calculated from shock breakout velocity profiles captured by a photon doppler velocimetry system. The preliminary pressures are estimated to have satisfied the plastic yield criterion for this loading condition.

Explosive welding is a field with a wide variety of applications of great value, such as corrosion resistant cladding and bi-metallic joints. It occupies a special place in the available metal joining techniques. Dissimilar metal welding is possible in metal pairings that don’t support other conventional bonds, and it can produce superior area welds regardless of the metal parts to be joined. The objectives of this dissertation were to further the understanding of explosive welding in general, as well as the empirical understanding of welding of Aluminum 6061-O, and to investigate the use of LS-DYNA’s Multi-Material Arbitrary Lagrangian-Eulerian formulation as a potential tool for the design of explosive welds. In the course of the work, the theory on formation of bond interfacial waves was identified as an area where there was not an apparent consensus, and this was addressed in light of both recent works and information from this study.For this dissertation, an experimental program of explosive welding tests, mechanical weld verification, and metallurgical observation were undertaken in order to add to the data available for this type of welding. Nine different explosive welding tests were conducted covering four scenarios, which were combinations of different explosive thicknesses and flyer inclination angles. Tensile shear tests with digital image correlation were used to test the welds, and optical microscope, Scanning Electron Microscope, and Transmission Electron Microscope images were used to investigate the nature of the bond. The numerical investigation was conducted and compared to both experiment and initial modeling results.The results reinforce the need for well-developed and material specific welding windows, adding additional data for the joining of Aluminum 6061-O. The endorsement of the continuous Kelvin-Helmholtz jet wake as the source of instability was supported with modeling results. The Multi-Material Arbitrary Lagrangian Eulerian modeling with Euler-Lagrange Coupling was demonstrated to yield results comparable to research codes for welding parameters, to be able to capture jetting, and provide meaningful temperature results. Bond interfacial waves were characterized with some success as well, concluding that this modeling technique is a viable means to assist in the design of explosive welds.

Impact welding is a method for joining similar and dissimilar metals by a high-speed collision. In this study, a novel impact welding technology, Vaporizing Foil Actuator Welding (VFAW), is used to investigate the process-structure-property relationship of impact welding and expand its applications. VFAW has been shown to successfully weld aluminum alloy plates of over 6 mm thickness to steel in a laboratory environment. Mechanical testing results show that the joint lap shear strength is equivalent or superior to the shear strength of the base materials. A significant portion of this work contributes to understanding of process parameter effects on the weld interface morphology. The typical wave-like interface found in impact welds regularly shows a distinctive wavelength and amplitude, and is believed to indicate a high joint strength by possible mechanical interlocking as well as metallurgical bonding. The determining factors of the wave size and shape are investigated by manipulating process parameters such as material thicknesses and collision angle. From both experimental and numerical observations, the interfacial wavelength is found to be positively correlated with both material thicknesses and collision angle, but the correlation seems to weaken when the target thickness exceeds two times the flyer thickness. The relationships are validated by robust numerical simulations using Arbitrary Lagrangian-Eulerian (ALE) and Smooth Particle Hydrodynamics (SPH) methods, performed by the project collaborators. The relationships are explained in terms of propagation and interference of shock waves at the interface. Metallurgical characteristics of the weld interface are also studied using several advanced microscopy techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD), X-ray micro-computed tomography (µ-CT), energy-dispersive X-ray spectroscopy (EDXS), and nano-diffraction. In this study, grain structures in and near the weld interface of pseudo single grained Cu/Cu and Cu/Ag welds are revealed. Evidence of severe plastic deformation are easily found in both micro and nano-scale observations. In close proximity to the interface a thin (~25 nm) intermixed layer along the interface is observed, where compositional analysis confirms that both elements co-exist while maintaining crystallinity. The rigorous microscopic characterization conducted in this study attempts to aid in understanding the bonding mechanism during impact welding.