Ultrasonic Spot Research Articles

Effect of Fe-Al intermetallics on fatigue properties of aluminum to steel dissimilar spot welds

Lightweight structures made of aluminum alloys and high strength steels are increasingly used for improving vehicle fuel efficiency. One barrier for joining aluminum and steel is the formation of brittle Fe-Al intermetallics that can have deleterious effects on joint ductility and strength under static loading conditions. However, it is unclear how the intermetallics affect the joint fatigue properties. In this study, the effect of intermetallics on fatigue properties of dissimilar metal joints between a 6xxx aluminum alloy and a high-strength steel is investigated on welds created by ultrasonic interlayered resistance spot welding (Ulti-RSW). Joint efficiency calculations are used to normalize the peak load so the fatigue properties of Ulti-RSW are compared with a wide range of joining processes in the literature. During fatigue testing the failure mode transitions from interfacial or button pull-out failure during low cycle fatigue loading (below 100,000 cycles) to through-thickness failure during high cycle (above 100,000 cycles). Based on the stress intensity factors calculated using a simple 2D fracture mechanics model, the transition in failure mode is attributed to the plastic deformation near the notch tip. Overall, this study shows that the intermetallics have little effect on the high-cycle fatigue properties of dissimilar spot joints.

A Review on the Recent Trends in Forming Composite Joints Using Spot Welding Variants

Traditional resistance spot welding (RSW) has been unsuccessful in forming quality composite joints between steel– or aluminum–polymer-based composites. This has led to the development of spot welding variants such as friction stir spot welding (FFSW), ultrasonic spot welding (USW), and laser spot welding (LSW). The paper reviewed the differences in the bonding mechanisms, spot weld characteristics, and challenges involved in using these spot welding variants. Variants of RSW use series electrode arrangement, co-axial electrodes, metallic inserts, interlayers, or external energy to produce composite joints. FFSW and USW use nanoparticles, interlayers, or energy directors to create composite spot welds. Mechanical interlocking is the common composite joint mechanism for all variants. Each spot welding variant has different sets of weld parameters and distinct spot weld morphologies. FFSW is the most expensive variant but is commonly used for composite spot weld joints. USW has a shorter welding cycle compared to RSW and FFSW but can only be used for small components. LSW is faster than the other variants, but limited work was found on its use in composite spot weld joining. The use of interlayers in FFSW and USW to form composite joints is a potential research area recommended in this review.

An investigation on metallurgical features and joint characteristics of ultrasonic spot welded Al-Ni sheets

Several potential benefits in joining thin, dissimilar and highly conductive metals in a solid-state environment make ultrasonic welding (USW) a popular method for building Li-ion battery packs. The USW method is highly complex and relies on multiple factors. In this study, aluminum (AA1100) and nickel (UNS N02200) dissimilar sheets were ultrasonically welded using various process parameters. Plastic deformation, microstructural development, and the corresponding mechanical characteristics of the joints were evaluated qualitatively and quantitatively. The joint strength and critical intensity factor values seemed to rise until the welding duration of 0.3 s, following which there was a subsequent drop in their values. Based on the mechanical analysis, it is observed that the optimal condition records an amplitude of 68 µm, a weld time of 0.3 s, and a clamping pressure of 0.24 MPa. It showed that the joint performance was significantly impacted by the plastic deformation and microstructural changes at the weld region. On the Al side, substantial plastic deformation was observed, and it was primarily attributed to the sonotrode knurls’ penetration. The Al sheet’s thickness was reduced by more than 40%. The microhardness readings at the welding region experienced diminution due to the acoustic softening effect. The cross-section and fracture surfaces of the Al/Ni weld exhibited several distinct microstructural characteristics, such as atomic diffusion, crimped interface, dimple pattern, and swirl pattern flow. Additionally, the diffusion zone is also investigated using X-ray diffraction analysis. These observations offer valuable insights for enhancing weld quality and controlling the overall welding process.

An experimental investigation on feasibility of submerged ultrasonic spot welding of thermoplastics

Although conventional methods such as mechanical fastening, adhesive bonding and hot air welding have proven effective in dry conditions, they exhibit diminished efficacy in submerged environments. Hence, a thermoplastic welding technique with minimal dependence on surrounding media is essential. Ultrasonic spot welding (USW) represents a promising approach to thermoplastic joining, offering high efficiency and low operating costs. In this study, we investigate the efficacy of water-submerged ultrasonic spot welding (S-USW) for joining amorphous polyvinyl chloride (PVC) to PVC and semi-crystalline polypropylene (PP) to PP under submerged conditions. Our experimental results show that S-USW leads to a remarkable 39% and 21% increase in lap-shear strength for PVC/PVC and PP/PP welds, respectively, as compared to traditional USW techniques. We corroborate these findings with additional metrics such as Shore-D hardness tests, optical microscopy and scanning electron microscopy imagery, which collectively confirm the improved efficacy of S-USW over USW for joining PVC and PP.

Microstructure and mechanical properties of Al-Zn-Mg-Cu alloy joints welded by ultrasonic spot welding with soft specification

The high-strength lightweight Al-Zn-Mg-Cu alloy sheets were welded using ultrasonic spot welding with soft specification. The peak tensile shear failure load reached 7.3 kN, which satisfies the requirements of AWS D17.2 for spot-welded specimens. This is due to the fact that the material undergoes sufficient plastic deformation during the welding process with soft specification, thereby improving metallurgical bonding. Moreover, the dynamic recrystallization resulting from plastic deformation makes the grain size of the interface significantly refined. The grains with a size of 10 μm in the base metal are transformed into recrystallized grains with a size of 2 μm at most, which is in favor of improving the mechanical properties of the joint. Finally, even under soft specification conditions, excessive welding time can lead to macroscopic damage to the joint, initiation of cracks, and destruction of the grain boundary structure.

Improvement on microstructure and mechanical properties of Cu/Al ultrasonic spot welded joints assisted by periodic additional force

This study utilizes active control of welding clamping force through periodic additional force (PAF) to assist in fabricating ultrasonic spot-welded copper-aluminum joints. The microstructural characteristics and mechanical properties of the copper-aluminum ultrasonic spot welded with periodic additional force joints were systematically investigated. Periodic additional force was applied to the welding interface, facilitating plastic deformation and flow. This can transform the relatively smooth weld interface into a wave- and swirl-like surface with a maximum peak height of 12 μm. Simultaneously, the mutual diffusion length of copper and aluminum at the weld interface increased from 2 μm to 8 μm. Additionally, the weld interface of ultrasonic spot welded with periodic additional force exhibited typical {111}〈112〉 or {111}〈110〉 shear textures, confirming a strong interface bonding. The effect of the periodic additional force on the thermomechanically affected zone (TMAZ) was also examined. It was discovered that lower periodic additional forces enhanced dislocation density and decreased anisotropy, leading to improved mechanical characteristics. The utilization of periodic additional force in assisting ultrasonic spot welded can significantly enhance the lap shear failure force by up to 59.98%. The paper innovatively proposes a control method of reverse periodic additional force, offering a strategy for the preparation of reliable aluminum-copper joints.

Ultrasonic spot welding of open-cell Cu foam and Al plate: A study on the quality of joints

As a type of novel material, open-cell copper (Cu) foam has been widely applied in electrode production of batteries, due to its functional and structural properties. The use of ultrasonic welding technique to join Cu foam and aluminium (Al) plate has been proven to be feasible, but quality evaluation of the joints remains a blank to be filled in. In this study, orthogonal test design is used to investigate the effects of welding pressure, welding amplitude, welding energy and the interaction among the three parameters on the shear-tensile strength of ultrasonically welded joints of Cu foam/Al plate, and it is influenced by the three parameters in the following order: welding pressure > welding energy > welding amplitude. Based on the results of the orthogonal test, a Cu foam/Al plate ultrasonic welding joint evaluation system is established, based on which, qualitative and quantitative analysis on the quality of the joints classified the joints into three grades. The optimal parameter combination of the welding joint of Cu foam/Al plate is obtained as follows: 0.5 MPa, 65 %, 130 J, under which the porous structure of the joint is complete while its tensile strength is equal to or even better than the base material.

Microstructure and mechanical properties of ultrasonic spot welding of AA7075-T6 and A380 casting aluminum alloy

Ultrasonic spot welding (USW) was employed to join AA7075-T6 and casting Al alloy A380. Under a constant welding power, the influence of the welding time duration (from 1 s to 3 s) on peak temperature, microstructure, hardness, and mechanical strength of the joints were studied. It was found that the peak temperature increased with welding time. A sound solid-state metallurgical bonding was obtained at the joint interface. The cracks occurred at the edge of the joints were eventually eliminated as the welding time increased. A layer of refined grains formed in A380 adjacent to the bonding line. Hardness reduction occurred in AA7075 at the joint interface and the degree of hardness reduction and the volume increased with the peak temperature. By increasing welding time, lap shear tensile tests show that the peak load increased from 5.3 kN to 6.55 kN. The corresponding interfacial shear strength ranged from 96.29 MPa to 99.72 MPa, lower than the shear strength of both base materials (331 MPa for AA7075 and 185 MPa for A380). All joint samples exhibited interfacial fracture after lap shear tensile testing, with a thin layer of A380 remnant attached to AA7075 at the center of the fractured surface.

Free Standing Electrodes: From Roll Product to 41-Layer Pouch Cells

Processes to produce free standing electrodes for Lithium-Ion Batteries (LIB), using lithium iron phosphate (LFP) as active material for the cathode and artificial graphite for the anode, are being developed. The production of these electrodes is realized by using a slot die to produce a liquid film which solidifies in a precipitation bath and is wound up after moving through washing baths. This procedure consumes significantly less energy compared to conventional coating processes, as the drying needs are reduced, and the electrodes have a broader spectrum of use cases due to their high flexibility.In this work, the development process of the manufacturing procedures of pouch cells using free standing electrodes will be presented. This includes the characterization of the electrodes, usability trials with research pilot line equipment, and cycling tests of built pouch cells of up to more than 7 Ah per device.The screening of electrodes through cycling in coin cells showed a specific discharge capacity of > 120 mAhg-1 for 50 cycles at 0.2C.This innovative and sustainable electrode production process required adjustments throughout the whole production process of a LIB, starting with the slot die geometry to reduce thick edges of the released film. The electrodes’ surfaces were investigated regarding homogeneity and roughness using a digital microscope. Gas adsorptioneasurements showed the specific surface of cathodes at around 3.6 m²g-1. Challenges during calendaring of the free-standing electrodes included thickness deviations of the pristine electrodes and uneven tension throughout the width of the samples.After laminating the free-standing electrodes with current collectors, they were electrochemically and mechanically analysed. Therefore, additional adjustments had to be made to attach tabs on multiple layers of these flexible electrodes through a single ultrasonic welding spot. All electrodes were successfully processed inside a dry room (dew point < -40 °C) using industrially relevant pilot scale equipment (punching, stacking, electrolyte filling, sealing).The manufactured pouch cells all showed an initial OCV of around 400 mV and were formed prior to cycling. During three formation cycles at 0.1C, followed by a constant Voltage step, the two-layered pouch cells showed a specific discharge capacity of . These cells reached roughly 90 mAhg-1 of discharge capacity after 20 cycles at 0.2C, but higher C-rates reduced the capacity and cell life significantly. Measuring the cells’ resistance after cycling at 3.3 V using 10 measurement points at 1 kHz revealed the high inner resistance of > 2 Ohm. Post-mortem analysis revealed lithium deposition throughout the whole surface of the anode. In the following upscaling process, the number of layers was increased to 11 layers (2 Ah cells) and 41 layers (7 Ah cells). These pouch cells showed similar performances, specific discharge capacities of around 110 mAhg-1 during formation, but with decreased cycling capabilities at C-rates higher than 0.1C. The main cause for this behaviour is the high inner resistance of these cells.In this work we demonstrate that free-standing electrodes can be handled at pilot scale in a comparable manner as conventionally coated electrodes. The produced cells were all successfully cycled but showed higher inner resistances and lower capacities at high C-rates (> 0.5C). The proof-of-concept was therefore achieved, further improvement of the cell performance will be focused on in the future. Figure 1

Evaluation of ultrasonic spot welded Al-Cu dissimilar sheets: Mechanical performance, microstructural changes and thermal analysis

Automotive lithium-ion battery manufacturing industries are increasingly using ultrasonic welding (USW) to join thin dissimilar metals and foils. USW has good joint efficiency, no formation of the heat-affected zone, high production rate, shorter welding time, and sustainability over conventional fusion welding techniques. However, this process is less distinct and precise control of weld surface conditions to get good quality welds. In this regard, experimental and thermal analysis significantly helps to control the state of the weld interface during the joining process. In this study, the objective is to enhance the mechanical performance of aluminum (Al) and copper (Cu) joints under different surface conditions such as lubricating, normal, electrolytically, and emery polished. According to the lap shear tensile load result, the joint strength decreases significantly with the rise in weld time after 0.7 s. The lubricating surface provides the maximum lap shear tensile load of 1507 N among all other surface conditions. Fracture surface analyses are conducted to determine the quality of welds under varying surface conditions. Additionally, a three-dimensional (3D) finite element model (FE) for ultrasonic welding of two highly conductive metal sheets is also being investigated for the purposes of analysing the thermal behaviour around the weld interface. It reveals that the thermal profile of the interface differs depending on the surface condition. The numerical framework is confirmed by comparing the predicted interface temperature variations at different faying surface conditions with the experimental results.

Enhancing joint strength: investigating different interlayer effects on Ultrasonic spot–welded NiTi/304 stainless steel

Enhancing joint strength: investigating different interlayer effects on Ultrasonic spot–welded NiTi/304 stainless steel

Modal Analysis of Ultrasonic Spot Welding for Lightweight Metals Joining

Ultrasonic spot welding (USW) represents one of the unique solid-state joining methods for lightweight materials such as magnesium alloy and aluminum alloy. However, the sonotrode vibration may have a detrimental impact on the sheet material and the existing welds, depending on the component geometry and vibration frequency. In this study, a modal analysis tool based on steady-state dynamics was developed for ultrasonic spot welding which features a cyclic load applied to the sheets during the joining process. Through predicting relative motion and shear stress at the faying surfaces, coupon geometry and weld spacing are identified as two major factors that affect the welding reliability and joint quality in USW. The model was validated via welding experiments on aluminum alloy and magnesium alloy and relevant characterization of temperature distribution, joint strength as well as fracture location.

Experimental Analysis on Ultrasonic Resistance Spot Welding of Aluminum Alloys

A recently developed hybrid joining process known as ultrasonic resistance spot welding (URW) was used on various pairs of similar and dissimilar aluminum (Al) alloys with different thicknesses, including AA5182–AA5182, AA6111–AA6111, AA7075–AA6111, and AA7075–AA5182, and comprehensively studied. Compared to conventional resistance spot welding (RSW), URW of the alloys showed consistently enhanced mechanical behavior in lap shear and crosstension tests. This can be attributed to the multiple perspectives on microstructure improvements. For different stacks of Al alloys and welding conditions, nugget formation was promoted with a larger nugget size in URW. In the nugget center, ultrasonically assisted (UA) vibration facilitated the formation of an equiaxed crystal zone. At the nugget boundary, URW showed a narrower coarse columnar zone and partially melted zone, which are associatedwith the lowest hardness in the weld. Specifically in dissimilar Al welds, UA vibration moved the nugget more centered toward the weld interface. These microstructure improvements indicated UA vibration can homogenize temperature and elemental distribution, which modifies solidification behavior.

Effects of material combination for ultrasonic spot welding on microstructure and strength of Al/Al, Al/Cu, and Al/Ni dissimilar joints

The dissimilar joining of Al/Cu and Al/Ni is required to reduce the weight of electronic and electrical devices. Ultrasonic spot welding (USW) is expected to be applied to dissimilar materials because of the low heat input during the joining process. However, to obtain high strength via USW, the relationship among the welding parameters, joining strength, and microstructure for different material combinations must be investigated. In this study, Al/Al, Al/Cu, and Al/Ni are joined via USW by systematically varying the welding parameters; subsequently, their tensile shear load and microstructure are investigated. Results show that the vibration amplitude and welding time impose greater effects on the joint strength than the welding pressure for all material combinations. High-strength joints that fractured at the Al upper plate are obtained by optimizing the welding parameters for each material combination. Al/Ni bonding had a narrower margin of welding time to obtain a higher tensile shear load compared with other material combinations. This is due to the high heating rate and maximum temperature of Al/Ni joints, which cause the excessive deformation of the Al upper plate during prolonged welding. Texture analysis showed that recrystallized grains with GOSS orientation was remained in the high-strength dissimilar joints.

New Input Factors for Machine Learning Approaches to Predict the Weld Quality of Ultrasonically Welded Thermoplastic Composite Materials

Thermoplastic composites (TCs) enjoy high popularity in the field of engineering. Due to this popularity, there is a growing need to assemble this material with the help of fast and efficient joining processes. One joining process, which has seen increased use, is the process of ultrasonic welding. To make reliable statements about the quality of the joined material, some kind of quality assurance has to be made. In terms of ultrasonic spot welding, there are already some documented approaches for observing or predicting the joining quality, but some of these most promising parameters for quality assurance are difficult to measure in the process of continuous ultrasonic welding. This is why new parameters are investigated for their potential to improve the prediction of ultrasonic-welded TCs’ quality. Thermography and sound emission data have been found to have a correlation with the produced weld quality and are fed into different machine learning algorithms. Despite the relatively small dataset, trained algorithms reach binary classification rates of over 90%, indicating that the newly discovered parameters show the potential to improve the quality assurance of ultrasonic-welded TCs in the future. This improvement may enable the establishment of the ultrasonic welding of TCs in manufacturing.

Ultrasonic Spot Weld inspection system based on Industrial Robotic, Artificial Intelligence and Artificial Vision

Nowadays, spot welding is still one of the most frequently used techniques for bonding metal plates, for example, in an automotive chassis between 3000 and 4000 spot welds can be found. Using this technique is possible to obtain at a low cost, easily automated and versatile welds with high quality and speed of operation but, the inspection and quality control process needs to be improved so that it is possible to reach automatic systems that allow maximizing the speed of inspection and guarantee, in turn, the repeatability and accuracy of the results, eliminating possible deviations introduced by the operator. In this paper an inspection system, based on the ultrasonic propagation, is described. The main challenges faced are presented, such as the location of the spot weld, using artificial vision, the definition and positioning of the probes until a reliable signal, and the final analysis through a development based on artificial intelligence. The results achieved in real parts, have shown that this robotic system, has a better defect detection capacity than the systems currently on the market, and is applicable to the inspection on assembly lines, optimizing not only the inspection times but also improving the results and therefore the quality of the process.

Insights into microstructure evolution and interface joining mechanism of ultrasonic spot welded Cu/Cu joint

To understand the underlying microstructure evolution and joining mechanism of ultrasonic spot welded Cu/Cu joint, the microstructure and crystallography analysis were investigated. The grain size, grain boundary, misorientation angle, Kernel Average Misorientation, Schmid factor and crystal orientation along the weld thickness were heterogeneous and unsymmetrical. The changes of corresponding microstructural and crystallographic features were more obvious for the top sheet comparing with the bottom sheet. The {111} <110> texture, nano-sized equiaxed induced by discontinuous dynamic recrystallization and refined elongated grains because of continuous dynamic recrystallization were formed at the welding interface. The nucleated fine grains between the elongated grains attributing to continuous dynamic recrystallization were beneficial to the grain boundary migration and consequent the achievement of the interfacial metallurgical bonding.

Ultrasonic spot-welding of AA 6061-T6 aluminium alloy: Optimization of process parameters, microstructural characteristics and mechanical properties of spot joints

The ultrasonic spot welding (USW) is used to develop the lap joints of AA 6061-T6 aluminium alloy because it is difficult to spot weld using resistance spot welding (RSW) and laser beam spot welding (LBSW) processes due to its high electrical conductivity, thermal conductivity and light reflectivity respectively. The main objective of this investigation is to optimize the USW parameters specifically welding time (s), amplitude (%) and pressure (bar) for enhancing the tensile shear fracture load (TSFL) bearing capability of AA 6061-T6 aluminium alloy lap joints for automotive applications. The statistical response surface methodology (RSM) was utilized for generating strength prediction models (SPM) and validated using analysis of variance (ANOVA). The RSM is widely used for optimizing the process parameters as it provides greater information such as optimum conditions, direct effect of process parameters and performance prediction from limited number of experiments compared to other optimization techniques. The response surfaces were created using RSM and analyzed. The effect of USW parameters on macrostructure, microstructure and TSFL of USW joints was studied. From the results, it was observed that the USW joints created using the welding amplitude of 100%, welding time of 21 s and welding pressure of 6 bar exhibited greater TSFL capacity of 5.08 kN. The SPM accurately predicted the TSFL of USW joints within 5% error on 95% confidence. Welding pressure disclosed major effect on TSFL of USW joints followed by welding amplitude and time. The greater TSFL of USW joints is imputed to the better coalescence of weld surfaces and refined microstructure.

To measure is to know: Evaluating indirect measurement techniques for observing the damage tolerance behaviour of spot welded thermoplastic composites

In thermoplastic composite joints with a circular ultrasonic spot weld, the damage growth is located at the interface between the joined components. This means that any damage in these joints is invisible from the outside. This experimental study compares three different in-situ methods to measure the damage growth indirectly. The motivation lies in the potential benefits of using multi-spot welded joints for increased damage tolerance and the need to prove the damage arresting and damage progression behaviour for certification. The current study focused on measuring the damage in single-spot welded Single Lap Shear (SLS) joints during fatigue test with the global specimen compliance, the local out-of-plane displacement and the local strain on the surface. Digital Image Correlation (DIC) measurements were used to obtain the local quantities. The results showed that the local quantities are better suited to obtain detailed information on the damage state. The local surface strain showed more distinct features that facilitate the recognition of damage locations. The benefits and challenges of all methods are discussed as well as the difference in the level of detail, generalization and the prior knowledge necessary to obtain the damage state.

Investigation on the mechanical properties of AA5052/AZ61A by ultrasonic welded joints

In this research, Ultrasonic spot welding (USW) was used as a solid-state joining method to strengthen the joints. Magnesium-alloy (AZ61A) and aluminum-alloy (AA5052) welded structures are commonly found in lightweight vehicles. Under this process, plastic deformation was observed in the weld joint at an energy level of 1550 J, with increasing welding energies beyond 1550 J, and a reaction layer of intermetallic compounds (IMCs) forms, with a thickness of about 17 µm. The γ-Mg17Al12 + β-Mg2-Al3 phase comprises the majority of the IMCs; the β-Mg2-Al3 phase exhibits a hardness of 6.7 GPa and a classic pop-in phenomenon, while the γ-Mg17Al12 cubic structure phase has an average hardness of 3.5 GPa. For welding energies exceeding 1550 J, the rapid formation of these phases can negatively impact the joint's performance. The observed fractures feature multiple secondary cracks, which could indicate a cleavage fracture, although further analysis would be necessary to confirm this.