Wayne Cai

A Variational Method of Robust Fixture Configuration Design for 3-D Workpieces

Fixtures are used to locate and hold workpieces during manufacturing. Because workpiece surface errors and fixture set-up errors (called source errors) always exist, the fixtured workpiece will consequently have position and/or orientation errors (called resultant errors). In this paper, we develop a variational method for robust fixture configuration design to minimize workpiece resultant errors due to source errors. We utilize both first-order and second-order workpiece geometry information to deal with two types of source errors, i.e., infinitesimal errors and small errors. Using the proposed variational approach, other fundamental fixture design issues, such as deterministic locating and total fixturing, can be regarded as integral parts of the robust design. Closed-form analytical solutions are derived and numerical examples are shown. By employing the nonlinear programming technique, simulation software called RFixDesign is developed. This paper presents a new procedure for robust fixture configuration design that contributes especially to fixture designs where deformation is not influential.

Ultrasonic Welding Simulations for Multiple Layers of Lithium-Ion Battery Tabs

Ultrasonic welding is a solid-state bond created using ultrasonic energy. It has been used in the semiconductor industry for several decades, and more recently, in the automotive industry such as for lithium-ion battery welding. Although there existed numerical simulations for ultrasonic welding, the models were limited to two-layer and like materials stackups. In this study, finite element theories are introduced and simulation procedure is established for multiple sheets and dissimilar metal ultrasonic welding. The procedures require both abaqus/Standard and abaqus/Explicit to simulate the coupled mechanical-thermal phenomena over the entire weld duration with moderate computational cost. The procedure is verified and used to simulate selected specific cases involving multiple sheets and dissimilar materials, i.e., copper and aluminum. The simulation procedure demonstrates its capability to predict welding energy, distortion, and temperature distribution of the workpieces. Case studies of ultrasonic welding simulations for multiple layers of lithium-ion battery tabs are presented. The prediction leads to several innovative ultrasonic welding process designs for improved welding quality.

Transient Temperature and Heat Flux Measurement in Ultrasonic Joining of Battery Tabs Using Thin-Film Microsensors

Procеss physіcs undеrstаndіng, rеаl tіmе monіtorіng аnd control of vаrіous mаnufаcturіng processes, such аs bаttеry mаnufаcturіng, аrе crucіаl for product quаlіty аssurаncе. Whіlе ultrаsonіc wеldіng hаs bееn usеd for joining bаttеries in еlеctrіc vеhіclеs, the welding physics and process attributes, such as the heat generation and heat flow during the joining process, іs stіll not wеll understood lеаdіng to tіmе-consumіng trіаl-аnd-еrror bаsеd procеss optіmіzаtіon. Thіs study іs to іnvеstіgаtе thеrmаl phеnomеnа (і.е. transient tеmpеrаturе аnd hеаt flux) by using mіcro thіn fіlm thеrmocouplе (TFTC) аnd thіn fіlm thеrmopіlе (TFTP) аrrаys (referred to as micro sensors in this report) аt thе vеry vіcіnіty of thе ultrаsonіc wеldіng spot. Micro sеnsors were first fаbrіcаtеd on the buss bаrs. A series of experiments were then conducted to investigate the dynamic heat generation during the welding process. Expеrіmеntаl rеsults showеd that TFTCs еnаblеd thе sеnsіng of trаnsіеnt tеmpеrаturеs wіth much hіghеr spаtіаl аnd tеmporаl rеsolutіons thаn convеntіonаl thеrmocouplеs. It was further found that the TFTPs were more sensitive to thе trаnsіеnt heat generation process during welding thаn TFTCs. Morе sіgnіfіcаntly, the hеаt flux chаngе rаtе was found to be able to provіdе better іnsіght for the process. It provided evidence indicating thаt thе ultrаsonіc welding procеss іnvolvеs thrее dіstіnct stаgеs, і.е., frіctіon hеаtіng, plаstіc work аnd dіffusіon bondіng stаgеs. Thе hеаt flux chаngе rаtе thus hаs sіgnіfіcаnt potеntіаl to identify the in-situ welding quality, in the context of welding procеss monіtorіng аnd control of ultrаsonіc wеldіng procеss. The weld samples were examed using scanning electron microscopy (SEM) and energy dispersive X-ray spectropy (EDS) to study the material interactions at the bonding interface as a function of weld time, and have successfully validated the proposed three-stage welding theory. As a case study, TFTCs were fabricated on Si wafers for sensor insertion units, which were successfully inserted into a pre-machined slot in the welding anvil as a temperature sensing device. Dynamic temperature rises during welding were successfully measured with excellent repeatability.

Dynamic Response of Battery Tabs Under Ultrasonic Welding

Ultrasonic metal welding (USMW) for battery tabs must be performed with 100% reliability in battery pack manufacturing as the failure of a single weld essentially results in a battery that is inoperative or cannot deliver the required power due to the electrical short caused by the failed weld. In ultrasonic metal welding processes, high-frequency ultrasonic energy is used to generate an oscillating shear force (sonotrode force) at the interface between a sonotrode and few metal sheets to produce solid-state bonds between the sheets clamped under a normal force. These forces, which influence the power needed to produce the weld and the weld quality, strongly depend on the mechanical and structural properties of the weld parts and fixtures in addition to various welding process parameters, such as weld frequencies and amplitudes. In this work, the effect of structural vibration of the battery tab on the required sonotrode force during ultrasonic welding is studied by applying a longitudinal vibration model for the battery tab. It is found that the sonotrode force is greatly influenced by the kinetic properties, quantified by the equivalent mass, equivalent stiffness, and equivalent viscous damping, of the battery tab and cell pouch interface. This study provides a fundamental understanding of battery tab dynamics during ultrasonic welding and its effect on weld quality, and thus provides a guideline for design and welding of battery tabs from tab dynamics point of view.

A computational design-of-experiments study of hemming processes for automotive aluminium alloys

Abstract Hemming is a three-step sheet-folding process utilized in the production of automotive closures. It has a critical imact on the performance and perceived quality of assembled vehicles. Using a two-dimensional finite element model, this paper presents a design-of-experiments (DOE) study of the relationships between important hemming process parameters and hem quality for aluminium alloy AA 6111-T4PD flat surface-straight edge hemming. The quality measures include roll-in/roll-out of the hem edge as well as the maximum true strain on the exposed bent surface. The finite element (FE) model combines explicit and implicit procedures in simulating the three forming subprocesses (flanging, pre-hemming, and final hemming) along with the corresponding springback (unloading). The results show that the pre-hemming die angle and the flanging die radius have the greatest influence on hem edge roll-in/roll-out, while pre-strain and the flanging die radius impact the maximum surface strain significantly. The computational DOE results also provide the basis for process parameter selection to avoid hem surface cracking and particular insights for achieving acceptable formability.

Dynamic Stress Analysis of Battery Tabs Under Ultrasonic Welding

Ultrasonic metal welding is widely used for joining multiple layers of dissimilar metals, such as aluminum/copper battery tabs welding onto copper busbars. It is therefore important to have a robust product/process design using ultrasonic metal welding that ensures consistent welds with desired quality. In this work, the effects of longitudinal and flexural vibrations of the battery tab during ultrasonic welding on the development of axial normal stresses that occasionally cause cracks near the weld area are studied by applying a one-dimensional continuous vibration model for the battery tab. Analysis results indicate that fracture could occur near the weld area, due to low cycle fatigue as a result of large dynamic stresses induced by resonant flexural vibration of the battery tab during welding. This study provides a fundamental understanding of battery tab dynamics during ultrasonic welding and its effects on weld quality, and can be used to develop guidelines for product/process design of ultrasonically welded battery tabs.

Spot Weld Layout Optimization of Tube Crash Performance With Manufacturing Constraints

Spot weld layout is critical to structural performance of vehicle and its design is also subject to manufacturing constraints. In this study, using thin-walled tube crash as an example, we establish the relation between structural performance and weld layout design with manufacturing constraints from resistance spot welding. First, a straight tube crash performance is evaluated as a function of flange width, weld distance to flange corner, and weld pitch, without consideration of manufacturing constraints. All these parameters exhibit certain influence on the deformation mode and the energy absorption capacity. Then, an S-shaped tube is studied in the design optimization of weld layout by adding manufacturing constraints. The proposed approach can determine optimized results by simultaneously considering crash performance and manufacturing constraints. It is also concluded that weld layout has more significant influence on crash performance in straight tubes than in S-shaped tubes.

Three-Dimensional Numerical Simulations of Curved Edge-Curved Surface Hemming of Aluminum Alloy

Hemming is a manufacturing process to fold a sheet onto itself or another sheet. The dimensional defects (roll-in/rollout, warp/recoil, distortion due to springback, etc.) of hems critically impact the perceived quality of automotive exteriors. This paper summarizes the procedures and the results of three-dimensional (3D) numerical simulations on curved edge-curved surface hemming of aluminum alloy AA6111-T4PD. A solid-element model is built in ABAQUS using explicit quasi-static finite element (FE) procedure for flanging, pre-hemming and final hemming, and implicit procedure for the corresponding preloading and resulting springback at high simulation cost. Aiming at improving the computational efficiency, various approaches have been taken and tested including using shell elements as alternatives, developing simplified simulation procedure by combining pre- and final hemming in explicit scheme, and further simplification by neglecting intermediate springback analysis. The same conditions are analyzed using shell elements in LS-DYNA, but only final hemming springback is considered. The results of the simplified models are compared with the results of ABAQUS solid-element model with complete procedure. Both accuracy and efficiency of the models are presented and discussed.Copyright

Ultrasonic Welding Simulations of Multiple, Thin and Dissimilar Metals for Battery Joining

Ultrasonic welding is a solid-state bond created using ultrasonic energy. It has been used in the semiconductor industry for several decades, and more recently, in the automotive industry such as for battery welding. Even though there existed several numerical simulations on ultrasonic welding, the models were too simplistic, in both theory and welding configuration, to present the multiple sheet, dissimilar metal ultrasonic welding. In this study, theories and a finite element procedure for the ultrasonic welding process are developed. The procedure invokes both Abaqus/Standard and Abaqus/Explicit to simulate the mechanical-thermal coupled phenomena over the entire weld duration with moderate computational cost. The procedure is verified and used to simulate selected specific cases involving multiple sheets and dissimilar materials, i.e., copper and aluminum. The simulation procedure demonstrates its capability to predict welding energy and temperature distribution of the workpieces, towards the goal of improving welding quality.Copyright

SPOT WELD LAYOUT OPTIMIZATION WITH MANUFACTURING CONSTRAINTS FOR VEHICLE STRUCTURAL PERFORMANCE

Spot weld layout on thin walled vehicle structures is an influencing factor to the structural performance such as NVH, durability and crashworthiness. The weld layout is also subject to manufacturing constraints such as minimum weld pitch, thickness and curvature of flanges, and accessibility of weld gun. Using an S-shaped thin walled tube as an example, this paper presents a study of spot weld layout optimization considering both structural performance and manufacturing constraints for reducing design iterations between the performance design and the manufacturing design. First, several complex manufacturing constraints, including minimum spot weld pitch, maximum curvature of flange, etc., are mathematically characterized. Then, with and without typical manufacturing constraints, the weld layout is optimized respectively for crash performance and torsion performance of the structure. The effects of adding manufacturing constraints on the spot weld layout optimization are evaluated. The analysis results reveal that the crash performance responses are generally less sensitive to the spot weld layout while the torsion stiffness is closely related to the spot weld layout. To analyze why the crash performance is less dependent on the weld layout, a detailed study is further conducted to reveal the relation between the weld layout and the crash performance of S-shaped thin walled tube. It shows that the parameters for assessing the structural crash performance have distinct sensitivity to the spot weld layout design. For instance, the peak impact force is generally sensitive to the spot welds placed in the curved segment and the total energy absorption capacity is mainly determined by the curvature design of the tube instead of the spot weld layout design.Copyright