Ultrasonic welding (USW) is one of the joining technologies that can be applied to short carbon fiber thermoplastic composites. In this study, the USW of Nylon 6 reinforced by short carbon fibers created using injection molding is used to investigate the USW process without energy directors. In addition to process parameters and performance parameters, a new category of parameters is introduced to characterize the behavior of base materials to control USW without energy directors. These parameters, named morphological parameters, are the degree of crystallinity (DoC) and the ratio of the crystalline phases of Nylon 6 (α/γ ratio). One method of controlling the morphological parameters is annealing. A design of experiments is carried out using 5 replicates and 7 annealing temperatures above the glass transition temperature (T g ) and below the melting temperature (T m ) of Nylon 6 to investigate the influence of annealing on the morphological parameters. The DoC and α/γ ratio are measured for each replicate by utilizing differential scanning calorimetry and X-ray diffraction. The results show that the DoC becomes uniform and the α/γ ratio increases after annealing. Consequently, the variation in weld strength decreases and the average weld strength increases by controlling the morphological parameters through annealing.

Spot welding is the predominant joining process for the sheet metal assemblies. The assemblies, during this process, are mainly bent and deformed. These deformations, along with the single part variations, are the primary sources of the aesthetic and functional geometrical problems in an assembly. The sequence of welding has a considerable effect on the geometrical variation of the final assembly. Finding the optimal weld sequence for the geometrical quality can be categorized as a combinatorial Hamiltonian graph search problem. Exhaustive search to find the optimum, using the finite element method simulations in the computer-aided tolerancing tools, is a time-consuming and thereby infeasible task. Applying the genetic algorithm to this problem can considerably reduce the search time, but finding the global optimum is not guaranteed, and still, a large number of sequences need to be evaluated. The effectiveness of these types of algorithms is dependent on the quality of the initial solutions. Previous studies have attempted to solve this problem by random initiation of the population in the genetic algorithm. In this paper, a rule-based approach for initiating the genetic algorithm for spot weld sequencing is introduced. The optimization approach is applied to three automotive sheet metal assemblies for evaluation. The results show that the proposed method improves the computation time and effectiveness of the genetic algorithm.

Projection welding of steel weld nuts to advanced high strength steel (AHSS) in automotive applications allows for the reliable mounting of critical components with different thicknesses to the vehicle body. However, the galvanized coatings commonly used on AHSS result in electrode surface degradation during welding. In this study, the electrode degradation and its effect on the mechanical properties of welded steel nuts and AHSS sheets are investigated. Two common electrode materials are tungsten/copper and beryllium-free class III copper—both display the formation of an oxidized alloy surface layer and pitting as weld number increases. Unlike resistance spot welding, where electrodes grow in the contact area diameter as they degrade, projection welding electrodes do not experience this type of mechanical degradation. Instead, increased resistance at the electrode interface with increasing weld number results in higher temperatures at the weld interface and a larger fusion zone size, which is responsible for an observed 30% increase in weld strength over the span of 10,000 welds.

Zinc-coated advanced high strength steels (AHSS) used in automotive applications are susceptible to liquid metal embrittlement (LME) during resistance spot welding (RSW). This study examines the impact of multiple pulse welding schedules on LME severity in welds of TRIP1100. Two different types of pulsing methodologies have been proposed to reduce LME severity: applying a pre-pulse before the welding current to remove the zinc coating and pulsing during the welding current to manage heat generation. However, the mechanisms by which these methods affect LME severity have not been fully explored. This work showed that a welding schedule consisting of two equal length pulses resulted in the least severe LME because it reduced the amount of free zinc available for LME without creating too much tensile stress. The majority of pre-pulse welding schedules caused an increase in LME cracking due to the additional heat introduced into the weld. However, a 4 kA (low current) pre-pulse applied for 3 cy (low time) reduced LME cracking by almost 30%. The pre-pulse allowed zinc to diffuse into the coating and stabilize the zinc, without introducing too much additional heat into the weld. These results indicate that multiple pulse welding schedules may be successfully used to reduce LME cracking, although the mechanisms by which they impact LME are more complicated than previously thought.

Ultrasonic welding has been widely used in joining plastic parts since it is fast, economical, and suitable for automation. It also has great potential for joining thermoplastic composite structures in the aerospace and automotive industries. For a successful industrial application of ultrasonic composite welding, it is necessary to have effective weld quality prediction technology. This paper proposes a model for weld quality prediction by establishing a correlation between ultrasonic wave transmission and welding process signatures. The signatures, welding power, and force are directly related to the weld quality. This model is used to predict the weld quality with three contact conditions and validated by experiments. The results show that the quality model performs well when a centralized and consistent contact condition is achieved. The model provides a process physics-based solution for the online weld quality prediction in ultrasonic welding of carbon fiber composite.

Sensor signals acquired during the manufacturing process contain rich information that can be used to facilitate effective monitoring of operational quality, early detection of system anomalies, and quick diagnosis of fault root causes. This paper develops a method for effective monitoring and diagnosis of multisensor heterogeneous profile data based on multilinear discriminant analysis. The proposed method operates directly on the multistream profiles and then extracts uncorrelated discriminative features through tensor-to-vector projection, and thus, preserving the interrelationship of different sensors. The extracted features are then fed into classifiers to detect faulty operations and recognize fault types. The developed method is demonstrated with both simulated and real data from ultrasonic metal welding.

Tailored blanks characterized by variable thickness were friction stir welded (FSWed) with the aim to obtain constant joint properties along the weld seam, regardless of the thickness change. To pursue this goal, the heat input was kept constant by in-process control of tool rotation. A dedicated numerical model of the process was used to determine the tool rotation values as a function of the sheet thickness. The mechanical properties and the microstructure of the FSWed joints, produced with varying process parameters, were studied. It was found that the proposed approach can produce joints with uniform properties along the weld line in terms of stress–strain curve shape, joint strength, elongation at failure, and microstructure.

Welding-induced buckling is a special type of welding distortion occurring during thin plate butt welding and was investigated using both experimental and computational approaches for this benchmark investigation. In addition, the characteristic parameter and its magnitude for the occurrence of welding-induced buckling were also presented. Fundamental theories of the inherent deformation, finite strains, and eigenvalues of the structure stiffness matrix were considered to investigate welding-induced buckling. A series of experiments on thin plate butt welding with different heat inputs were conducted, and buckling behavior was observed from the deformed shape and the distribution of out-of-plane welding distortion. Transient nonlinear thermal elastic–plastic finite element (TEP FE) and elastic finite element (FE) analyses were conducted to predict welding-induced buckling, and the results were in good agreement with the measurement data. Criteria for the occurrence of welding-induced buckling were proposed and discussed. Inherent deformation was considered as a characteristic parameter of buckling behavior during welding, and its critical magnitude was calculated using a loading incremental method and eigenvalue analysis with good agreement.

High temperature, short welding time, and low relative motion generate high bond quality in ultrasonic metal welding (USMW). Friction is considered to be the main heat source during USMW. Hence, a comprehensive and accurate understanding of friction heating has become particularly valuable for designing USMW processes and devices. However, stick, slip, and separation states may appear alternately in the welding zone between superimposed workpieces during USMW vibrations; hence, a strong nonlinear process is created. Furthermore, the structural dynamics and the heat transfer are highly coupled because material properties depend on temperature. In this research, we propose a fast and accurate numerical methodology to calculate the friction heating through a multiphysical approach integrating a nonlinear contact model, a nonlinear structural dynamics model, and a thermal model. The harmonic balance method and the finite element method are utilized to accelerate the simulation. Several experiments were performed with aluminum and copper workpieces under different clamping forces and vibration amplitudes to confirm the presented numerical method, resulting in a good match.

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

Impact welding is a material processing technology that enables metallurgical bonding in the solid state using a high-speed oblique collision. In this study, the effects of thickness of the flier and collision angle on weld interface morphology were investigated through the vaporizing foil actuator welding (VFAW) of AA1100-O to AISI 1018 Steel. The weld interfaces at various controlled conditions show wavelength increasing with the flier thickness and collision angle. The AA1100-O flier sheets ranged in thickness from 0.127 to 1.016 mm. The velocity of the fliers was directly measured by in situ photon Doppler velocimetry (PDV) and kept nearly constant at 670 m/s. The collision angles were controlled by a customized steel target with a set of various collision angles ranging from 8 deg to 28 deg. A numerical solid mechanics model was optimized for mesh sizes and provided to confirm the wavelength variation. Temperature estimates from the model were also performed to predict local melting and its complex spatial distribution near the weld interface and to compare that prediction to experiments.

Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (α- and β-phase). It is shown that both the spherulite size of the α-phase and the amount of β-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).

In this study, the friction stir welding (FSW) of aluminum alloy 6061-T6511 to TRIP 780 steel is analyzed under various process conditions. Two FSW tools with different sizes are used. To understand the underlying joining mechanisms and material flow behavior, nano-computed tomography (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.

The complexity in weld profile caused by abrupt change in polarity in square waveform welding is investigated through the development of a model capable to accurately predict weld profile. A semi-analytical model is conceived wherein characteristic attributes of a composite parabolic–elliptic function, which represent the weld profile, are obtained through nonlinear regression (NLR). The proposed model is demonstrated for its efficacy in the prediction of weld profile over a wide range of welding parameters, vis-à-vis, welding current, frequency, electrode negative (EN) ratio, and welding velocity. The investigation suggests that the center and outer cores of welding arc remains more active during positive and negative polarity, respectively, that leads to distinct macroscopic zones in weld cross section and thus, necessitates a composite profile for representation of weld profile. The intersection of the zones forms a metallurgical notch which the investigation offers a method to estimate and thus control. Unlike the convention continuous arc welding, the waveform arc welding caters welding at higher velocity without compromising the weld penetration and almost abolishing the metallurgical notch as well.

A hybrid friction stir resistance spot welding (RSW) process is applied for joining aluminum alloy 6061 to TRIP 780 steel. Compared with conventional RSW, the applied current density is lower and the welding process remains in the solid state. Compared with conventional friction stir spot welding (FSSW) process, the welding force is reduced and the dissimilar material joint strength is increased. The electrical current is applied in both a pulsed and direct form. With the equal amount of energy input, the approximately same force reduction indicates that the electro-plastic material softening effect is insignificant during FSSW process. The welding force is reduced mainly due to the resistance heating induced thermal softening of materials. With the application of electrical current, a wider aluminum flow pattern is observed in the thermo-mechanically affected zone (TMAZ) of weld cross sections and a more uniform hook is formed at the Fe/Al interface. This implies that the aluminum material flow is enhanced. Moreover, the Al composition in the Al–Fe interfacial layer is higher, which means the atomic diffusion is accelerated.

Friction stir scribe (FSS) welding as a recent derivative of friction stir welding (FSW) has been successfully used to fabricate a linear joint between automotive Al and steel sheets. It has been established that FSS welding generates a hook-like structure at the bimaterial interface. Beyond the hook-like structure, there is a lack of fundamental understanding on the bond formation mechanism during this newly developed FSS welding process. In this paper, the microstructures and phases at the joint interface of FSS welded Al to ultra-high-strength steel were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that both mechanical interlocking and interfacial bonding occurred simultaneously during the FSS welding process. Based on SEM observations, a higher diffusion driving force in the advancing side was found compared to the retreating side and the scribe swept zone, and thermally activated diffusion was the primary driving force for the interfacial bond formation in the scribe swept region. The TEM energy-dispersive X-ray spectroscopy (EDXS) revealed that a thin intermetallic compound (IMC) layer was formed through the interface, where the thickness of this layer gradually decreased from the advancing side to the retreating side owing to different material plastic deformation and heat generations. In addition, the diffraction pattern (or one-dimensional fast Fourier transform (FFT) pattern) revealed that the IMC layer was composed of Fe 2 Al 5 or Fe 4 Al 13 with a Fe/Al solid solution depending on the weld regions.

Multilayered 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 four-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 multilayered USW.

Energy directors (EDs) have been widely used in ultrasonic welding (UW) of polymers and polymer-based composites. They help concentrate the welding energy and localize the weld at the location where the EDs are present. However, the utilization of EDs increases manufacturing cost and time, especially for complex parts and structures. This paper presents a method for UW of carbon fiber reinforced composite without using EDs. A reusable annular clamp (called a blankholder) is used as part of the weld tool to apply a variable force (called blank holding force) on the composite sheets during the UW. The effect of the blank holding force (BHF) on the weld formation is investigated. The results show that the duration of the BHF had significant impact on the weld formation. There is a critical duration with which a localized weld can form. Suitable durations of BHF at different levels of welding energy are determined by experiments. The main function of the BHF is to create an initial melting area by improving the contact condition. The initial melting area will act as an ED to concentrate the welding energy, and therefore, promotes the formation of a localized weld.

Thermal history and residual stresses in dissimilar friction stir welding (FSW) of AA2024 and AZ31 were studied under different tool offsets using a coupled Eulerian–Lagrangian (CEL) finite element model and a mechanical model. Welding experiments and residual stresses' measurements were conducted to validate the models. Comparisons between the experimental and numerical results indicated good agreement. The maximum temperature in the welded zone was predicted to be slightly lower than 400 °C, regardless of offset, and that its location shifted with tool offset from the advancing side (AS) to the retreating side (RS). Longitudinal residual stresses changed from tensile under the tool shoulder to compressive beyond this region and it appeared to be the dominant stress component. The transverse stresses were tensile and of lower magnitude. Both the longitudinal and transverse residual stresses have their maximum values within the weld zone near the end of the weld length. For both peak temperatures and residual stresses, higher values were obtained at the AS with no tool offset and 1 mm offset to the AS, and at the RS with 1 mm offset to the RS. Lower residual stresses and better weld quality were obtained with tool offset to the aluminum side.

In this study, weldability of ultrasonic welding of 4-mm-thick fiber carbon/nylon 66 composite in lap configuration was investigated. Ultrasonic welding tests were performed, and the weld appearance, microstructure, and fractography of the welded joints were examined using optical and scanning electron microscope. The transient temperatures near the faying surfaces and horn-workpiece interfaces were recorded to understand the weld growth mechanism. It was found that it is feasible to join 4-mm-thick lapped carbon fiber reinforced nylon 66 composite with ultrasonic welding. Under the ultrasonic vibration, the weld initiated and grew at the faying surfaces, while the weld indentation developed at the horn-workpiece interface. The pores observed in the regions between the heat-affected-zone (HAZ) and the fusion zone (FZ), and the severe weld indentation on the surface of upper workpieces decreased the loading capacity of the ultrasonic welded (UW) joints and caused the welded carbon/nylon 66 composite fractured prematurely. The strengths of the ultrasonic welds were determined by the balance of positive effect of the weld area and negative effects of the weld indentation and porosity near the FZ. To ensure the joint strength, it is necessary to apply the proper weld schedules (i.e., welding time and horn pressure) in ultrasonic welding of 4-mm-thick carbon fiber reinforced nylon 66 composite, which were developed based on the joint strength criterion.