Abstract Flow Drill Screws are self-piercing, self-tapping screws used for single sided joining of light metals, such as aluminum. This technology has been adopted by many automotive OEMs for use in metals. Thread forming profiles exist for material stackups that are made of entirely metals and entirely polymers/composites. This research evaluated the effectiveness of these thread profiles in dissimilar metal-on-composite stackups. Thread profiles designed for use in polymers/composites and aluminum were compared with a traditional machine screw thread profile for flow drill joining of 1mm and 2mm thick 6061-T6 aluminum to 3mm thick thermoset carbon fiber reinforced polymer. The three thread profiles were manufactured as M5x25mm flow drill screws in their commercially available configurations and materials. Two parameter sets from the FDS equipment manufacturer were evaluated, the first designed for use with the polymer thread forming profile, the second designed for use with the aluminum thread forming profile. The thread profiles were evaluated based on outputs of process time, peak torque, and lap shear strength. The polymer thread profile had shorter process times than the other 2 profiles but caused more damage to itself and its mating material. All 3 thread profiles exhibited greater shear strength when aluminum was used as the lower sheet material.

Abstract Minimally invasive surgery is a popular surgical method which allows surgeons to be able to employ small incisions to perform surgical procedures. Electrosurgical tissue joining is one method used in minimally invasive surgery to achieve hemostasis. The two major issues need to be concerned are the quality of the tissue joint and the potential thermal damage to the surrounding tissues. In this study, a new sequential compression concept was introduced. This concept uses a pair of smaller electrodes to sequentially compress and join multiple locations across the tissue to form a joint. Due to the smaller area of the electrode surface, a higher compression level can be achieved with a smaller compressive force. In this study, different joining times including 1.5, 1.6, 1.7, 1.9, 2.1 seconds with a 98% compression ratio were used to join porcine arterial tissue. In each test, three locations were sequentially compressed and joined. The resulting tensile strength, specific strength, energy consumption, and thermal dose from the sequential compression method were compared with those resulting from the nonsequential single compression method. For the 1.9-second joining time, the specific strength is 2.5 times higher than the highest specific strength achieved by the single joining method, meaning that only 40% of energy is needed to achieve an equivalent joint strength. A finite element model was built to estimate the temperature and thermal dose during the sequential joining process and also confirmed that the resulting thermal dose is much lower than the single joining method. The results validated that the sequential compression concept has the potential to minimize the thermal damage during the tissue joining process.

Abstract Ni nanoparticles were successfully used to join Inconel 718 via transient liquid phase (TLP) bonding in a vacuum environment. Ni nanoparticles of 20 nm, 29 nm, and 41 nm in diameter were synthesized by controlling the reducing agent injection rate and joined at up to 1050 °C and heating rate 5–15 °C/min. Based on the Gibbs-Thomson equation and surface melting models, joining using Ni nanoparticles occurs due to competing solid-state sintering and surface melting processes. It was found that faster heating rate; higher maximum bonding temperature, and larger particle size resulted in higher bonding strength. Using a faster heating rate suppresses the amount of solid-state particle-particle sintering that occurs at lower temperatures, where particle-Inconel 718 joining is less active. The suppression of particle-particle sintering as a function of particle diameter is also discussed. The maximum bonding strength achieved is 243 MPa. The fracture surface for Ni nanoparticle-bonded joints demonstrated intergranular fracture (low strength joints) and a combination of cleavage and microvoid coalescence (high strength joints).

Friction stir blind riveting (FSBR) is a recently developed manufacturing process for joining dissimilar lightweight materials. The objective of this study is to gain a better understanding of FSBR in joining carbon fiber-reinforced polymer composite and aluminum alloy sheets by developing a sensor fusion and process monitoring method. The proposed method establishes the relationship between the FSBR process and the quality of the joints by integrating feature extraction, feature selection, and classifier fusion. This study investigates the effectiveness of lower rank tensor decomposition methods in extracting features from multi-sensor, high-dimensional, heterogeneous profile data. The extracted features are combined with process parameters, material stack-up sequence, and engineering-driven features such as the peak force to provide rich information about the FSBR process. Sparse group lasso regression is adopted to select the optimal monitoring features. The selected features are fed into weighted classification fusion to estimate the quality of the joints. The fusion method integrates five individual classifiers with optimal weights. The correct classification rates resulted from various feature extraction and selection methods are assessed and compared. The proposed method can also be applied to other manufacturing processes with online sensing capabilities for the purpose of process monitoring and quality prediction.

To improve the fuel economy, the automobile industry is vigorously shifting towards using a mix of lightweight materials which offers high strength-to-weight ratio. Dissimilar material joining is of critical importance in this area. Friction element welding (FEW) has been proposed for dissimilar materials, with the capability of joining high strength materials of varying thickness in minimal time with low input energy. A coupled thermal-mechanical finite element model is developed in this work to better understand the physical mechanisms involved in the process and predict the evolution of parameters such as temperature, stress, material flow, and weld quality. The Coupled Eulerian-Lagrangian (CEL) approach is adopted to capture the severe plastic deformation of both the tool and the workpiece. The material deformation and temperature evolution are analyzed at different steps, and good agreement are shown between the simulation results and the experimental data.

In this study, Friction stir welding (FSW) of aluminum alloy 6061-T6511 to TRIP 780 steel are analyzed under various process conditions. Two FSW tools with different sizes are used. To understand the underlying joining mechanisms and material flow behavior, nano-CT is applied for a 3D visualization of material distribution in the weld. With insufficient heat input, steel fragments are generally scattered in the weld zone in large pieces. This is observed in a combined condition of big tool, small tool offset and low rotating speed or a small tool with low rotating speed. Higher heat input improves the material flowability and generates a continuous strip of steel. The remaining steel fragments are much finer. When the volume fraction of steel involved in the stirring nugget is small, this steel strip can be in a flat shape near the bottom, which generally corresponds to a better joint quality and the joint would fracture in the base aluminum side. Otherwise, a hook structure is formed and reduces the joint strength. The joint would fail with a combined brittle behavior on the steel hook and a ductile behavior in the surrounding aluminum matrix.

Joining of dissimilar metals and alloys has been envisioned since a long time with specific high end applications in various fields. One such combination is austenitic stainless steel grade SS304 and commercial grade titanium, which is very difficult to join under conventional fusion process due to extensive cracking and failure caused by mismatch in structural and thermal properties as well as formation of the extremely brittle and hard intermetallic compounds. One of the methods proposed in literature to control the formation of intermetallics is by fast cooling fusion process like laser beam welding. The present study has been done on laser welding of titanium and stainless steel AISI 304 to understand the interaction of these materials during laser welding at different laser power and welding speed which could yield different cooling rates. Two types of cracks were observed in the weld joint, namely longitudinal cracks and transverse cracks with respect to the weld direction. Longitudinal cracks could be completely eliminated at faster welding speeds, but transverse cracks were found little influenced by the welding speed. The thermal history, i.e. melt pool lifetime and cooling rate of the molten pool during laser welding was monitored and a relation between thermo-cycle with occurrence of cracks was established. It is inferred that the longitudinal cracks are mainly due to the formation of various brittle intermetallic phases of Fe and Ti, which could be minimized by providing relatively less melt pool lifetime at high welding speeds. The reason of the transverse cracks could be the generation of longitudinal stress in weld joint due to the large difference in the thermal expansion coefficient of steel and titanium. In order to mitigate the longitudinal stress laser welding was carried out with a novel experimental arrangement which ensured different cooling rates of these two metals during laser welding. With this the tendency of transverse cracks also could be minimized significantly.

Multi-layered ultrasonic welding (USW) is widely used in joining of electrodes or tabs in lithium-ion batteries. To achieve quality joints and enhance the welding process robustness, an improved understanding of the joint formation is highly desirable. In this paper, USW of 4-layered Ni-coated Cu is studied to investigate the joint formation at a single interface and joint propagation from interface to interface under both ambient and preheated conditions. The results indicate that joint formation involves three major mechanisms: Ni-Ni bonding with minimal mechanical interlocking, Ni-Ni bonding with moderate mechanical interlocking, and a combination of Ni-Ni bonding, Cu-Cu bonding, and severe mechanical interlocking. Results also show that joints propagate from the interface close to the sonotrode side to that close to the anvil side. It is further observed that the joint formation can be accelerated and the joint strength can be improved with process preheating, especially at the interface closest to the anvil. The effect of preheating is most significant during the early stage of the process, and diminishes as process progresses. The favorable effects of preheating improve the robustness of multi-layered USW.

This paper aims at providing a state-of-the-art review of an increasingly important class of joining technologies called solid-state welding. Among many other advantages such as low heat input, solid-state processes are particularly suitable for dissimilar materials joining. In this paper, major solid-state joining technologies such as the linear and rotary friction welding, friction stir welding, ultrasonic welding, impact welding, are reviewed, as well as diffusion and roll bonding. For each technology, the joining process is first depicted, followed by the process characterization, modeling and simulation, monitoring/diagnostics/NDE, and ended with concluding remarks. A discussion section is provided after reviewing all the technologies on the common critical factors that affect the solid-state processes such as the joining mechanisms, chemical and materials compatibility, surface properties, and process conditions. Finally, the future outlook is presented.

Friction self-piercing riveting (F-SPR) process has been proposed to join low ductility lightweight materials, and has shown advantages over fusion welding, solid state welding and traditional mechanical joining processes in joining dissimilar as well as low ductility materials. Because of the thermo-mechanical nature of F-SPR process, the formation of the joint is determined by riveting force and softening degree of the materials. However, it is still not clear that how exactly the riveting force and generated frictional heat jointly influence the mechanical interlocking formation and inhibit cracks during F-SPR process. To address these issues, in current study, F-SPR process was applied to join 2.2 mm-thick AA6061-T6 aluminum alloy to 2.0 mm-thick AZ31B magnesium alloy. The correlation of riveting force, torque responses as well as energy input with joint quality were investigated systematically under a wide range of process parameter combinations. It was found that a relatively greater final peak force and higher energy input were favorable to produce sound joints. Based on that, a two-stage method was proposed to better control the energy input and riveting force. It was found that the joints produced by the two-stage method exhibited significantly improved lap-shear strength, i.e., 70% higher than traditional SPR joints and 30% higher than one-stage F-SPR joints. This research provides a valuable reference for further understanding the F-SPR joint formation and process optimization.

Ultrasonic welding is a solid-state joining process which uses ultrasonic vibration to join materials at relatively low temperatures. Ultrasonic powder consolidation is a derivative of the ultrasonic additive process which consolidates powder material into a dense solid block without melting. During ultrasonic powder consolidation process, metal powder under a compressive load is subjected to transverse ultrasonic vibrations resulting in a fully-dense consolidated product. While ultrasonic powder consolidation is employed in a wide variety of applications, the effect of critical process parameters on the bonding process of powder particles during consolidation is not clearly understood. This study uses a coupled thermo-mechanical finite element analysis technique to investigate the effect of critical process parameters including vibrational amplitude and base temperature on the stress, strain, and particle temperature distribution during the ultrasonic powder consolidation process. The study finds that during this process, the ultrasonically vibrating tool imparts cyclic vibratory shear stress on the particles. The simulation also revealed that the particle temperature just reaches the recrystallization point. Higher vibration amplitude imparted higher frictional heat on the particles, thereby aiding the consolidation process. The simulation study also showed indications of thermal softening and restricted grain boundary sliding during the ultrasonic powder consolidation process. The outcomes of this study can be used to further the industrial applications of ultrasonic powder consolidation process as well as other ultrasonic welding based processes.

Carbon fiber reinforced plastic (CFRP) are advanced engineering materials which are recognized as the most sought-after composite for several industrial applications including aerospace and automotive sectors. CFRP have superior physical and mechanical properties such as lightweight, high resilience, high-durability and high strength-to-weight ratio. CFRP composites stacked up with titanium to form multi-layered material stacks to enhance its load bearing capability. Traditional methods of stacking up CFRP and titanium involves using either high strength adhesives or rivets and bolts. The laminate structures joined by these methods often tend to fail during high load-bearing applications. Conventional metal welding technologies use high heat causing high thermal stresses and microstructural damages. Ultrasonic welding is a solid-state joining process, which has the capability of welding dissimilar materials at relatively low temperatures using ultrasonic vibration. Ultrasonic additive manufacturing (UAM) process is an ideal method to weld CFRP and Titanium. During the ultrasonic welding process, two dissimilar materials under a continuous static load are subjected to transverse ultrasonic vibrations, which results in high stress and friction between the two surfaces. This research focuses on the study of ultrasonically welding CFRP and Titanium stacks using UAM process. The study involves experimentation performed on an in-house built UAM setup. Finite element analysis is performed to understand the distribution stresses and strains during the UAM process. In this study, CFRP and Titanium layers are successfully welded using UAM process without causing any melting or significant heating. The finite element analysis study revealed that during UAM process, CFRP/Titanium stacks are subject to repeated cyclic shear stress reversals resulting in a strong weld joint. The stress-strain diagram during the process showed a considerable increase in plastic strain during the UAM process. The outcomes of this study can be used to further the industrial applications of the ultrasonic additive process as well as other ultrasonic welding based processes involving dissimilar materials.

To increase the fuel economy of their fleets, automotive OEMs are turning to lightweighting their vehicles through multi-material bodies. This involves forming and joining of materials with high strength to weight ratios such as aluminum and advanced high strength steels. These metals come with the downside of decreased formability and increased springback compared to conventional automotive steels. Electrical augmentation has been shown to decrease springback and increase formability in sheet forming and represents a potential solution to the use of new lightweight metals. Applied electricity is traditionally measured as a current density, however this measure struggles to represent elevated strain rate manufacturing processes. This paper examines other predictors of electrically assisted process performance such as electrical energy and power through comparison of nominally equivalent waveforms. It is found that energy is a better predictor of process performance than current density, but is dependent on the ability to predict process temperature. The leading predictive electrically assisted temperature model is examined in depth through testing of 13 different parameter sets. It is found that the model is unable to predict the correct temperature at a high current density and that the transient stress drop cannot predicted for any of the electrical cases.

The popularity of additive manufacturing for producing porous bio-ceramics using vat photopolymerization in the recent years has gained a lot of impetus due to its high resolution and low surface roughness. In this study, a commercial vat polymerization printer (Nobel Superfine, XYZprinting) was used to create green bodies using a ceramic suspension consisting of 10 vol.% of alumina particles in a photopolymerizable resin. Four different sizes of cubical green bodies were printed out. They were subjected to thermal processing which included de-binding to get rid of the polymer and thereafter sintering for joining of the ceramic particles. The porosity percentage of the four different sizes were measured and compared. The lowest porosity was observed in the smallest cubes (5 mm). It was found to be 43.3%. There was an increase in the porosity of the sintered parts for the larger cubes (10, 15 and 20 mm). However, the difference in the porosity among these sizes was not significant and ranged from 61.5% to 65.2%. The compressive testing of the samples showed that the strength of the 5-mm cube was the maximum among all samples and the compressive strength decreased as the size of the samples increased. These ceramic materials of various densities are of great interest for biomedical applications.

Electrosurgical tissue joining is an effective way to create hemostasis, especially in surgical procedures performed in the minimally-invasive manner. The quality of tissue joints and potential thermal damage to the surroundings are the two main concerns when using electrosurgical tissue joining tools. A more robust method for quality control is still needed. In this study, we developed an experimental setup to join tissues and performed tensile tests to evaluate the quality of the tissue joint, while also monitoring the process parameters including voltage, current, impedance, temperature and thermal dose. Three joining times (4, 6, and 8 seconds) and three compression levels (80%, 90%, and 95%) were used to join porcine arterial tissues. It was found that 95% compression can form a strong joint with a shorter joining time and less energy, but the joint strength decreases when the joining time is extended to 8 seconds. A lower compression level can still form a quality joint but requires longer joining time and energy which could lead to more thermal damages. A new index, specific strength (mmHg/J), which is defined as the ratio between tensile strength and the consumed energy, is proposed. Specific strength offers a new way to estimate the required joining time to achieve sufficient joining strength while minimizing the energy consumption to reduce thermal damages.

Friction stir blind riveting (FSBR) is a novel and highly efficient joining technique for lightweight metal materials, such as aluminum alloys. The FSBR process induced large gradients of plastic deformation near the rivet hole surface and resulted in a distinctive gradient microstructure in this domain. In this study, microstructural analysis is conducted to analyze the final microstructure after the FSBR process. Dynamic recrystallization (DRX) is determined as the dominant microstructure evolution mechanism due to the significant heat generation during the process. To better understand the FSBR process, a two-dimensional Cellular Automaton (CA) model is developed to simulate the microstructure evolution near the rivet hole surface by considering the FSBR process loading condition. To model the significant microstructure change near the rivet hole surface, spatial distributed temporal thermal and mechanical loading conditions are applied to simulate the effect of the large gradient plastic deformation near the hole surface. The distribution grain topography and recrystallization fraction are obtained through the simulations, which agree well with the experimental data. This study presents a reliable numerical approach to model and simulate microstructure evolution governed by DRX under the large plastic deformation gradient in FSBR.

This study presents a detailed analysis of friction stir riveting (FSR) processes that are used for joining similar as well as dissimilar materials. It covers the operating principle of FSR methods along with the insight into various process parameters responsible for successful joints formation. The paper further evaluates the research in friction stir-based riveting processes which unearth the enhanced metallurgical and mechanical properties for instance microstructure modification, local mechanical properties and improved strength, corrosion and fatigue resistance. The results of the study show that use of FSR process yields refined microstructures and improved mechanical properties in materials, which will entail a significant rise in the usage of friction stir-based riveting processes.

The requirement of increased fuel economy standards has forced automakers to incorporate multi-materials into their current steel dominant vehicles in order to lightweight their fleets. Technologies such as Self Piercing Rivets and Flow Drill Screws are currently implemented for joining aluminum to high-strength steels but only one-technology is viable for joining aluminum to ultra-high-strength steels without pre-holes, namely Friction Element Welding. This study is aimed at investigating how variations in the cleaning and welding steps of the Friction Element Welding process influence joint quality. A design of experiment was conducted to understand the influence of key process parameters (endload, spindle RPM, and relative distance) during these steps on the pre-defined joint quality metrics of head height, weld zone diameter, under-head fill area, temperature, and microhardness. It is found that cleaning step parameters have the greatest influence on process time and energy consumption, while welding step parameters greatly influence maximum torque on the element, head height, and underhead fill, with both cleaning force and weld force influencing weld diameter, all parameters influence temperature.

This paper studies an electrically assisted friction stir spot welding (FSSW) process for joining aluminum alloy 6061-T6 to TRIP 780 steel. The electrical current shows to reduce the axial plunge force and assist the material flow of the aluminum matrix during the welding process. When electrical pulses and direct current (DC) with the same energy input are applied, the results show insignificant differences. Bulk material flow can be observed in the weld cross sections. A more uniform hook is generated at the Fe/Al interface after applying the current. Besides, the diffusion of aluminum atoms into the steel matrix is enhanced. Regarding the weld quality, electrically assisted FSSW improves the joint lap shear strength when compared with regular FSSW process.

Arteriovenous fistula is the joining of an artery to a vein to create vascular access for dialysis. The failure or maturation of fistula is affected by the vessel wall shear stress (WSS), which is difficult to measure in clinic. A computational fluid dynamics (CFD) model was built to estimate WSS of a patient-specific fistula model. To validate this model, a silicone phantom was manufactured and used to carry out a particle imaging velocimetry (PIV) experiment. The flow field from the PIV experiment shows a good agreement with the CFD model. From the CFD model, the highest WSS (40 Pa) happens near the anastomosis. WSS in the vein is larger than that in the artery. WSS on the outer venous wall is larger than that on the inner wall. The combined technique of additive manufacturing, silicone molding, and CFD is an effective tool to understand the maturation mechanism of a fistula.