Two-Step Laser Welding of Metal Foil Stacks to Thick Substrates
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Solution Overview
Problem
Joining multiple thin metal foils to a thicker metal tab in lithium-ion battery production is challenging due to disparate thicknesses, requiring strong and durable connections with low electrical resistance, which existing methods like precision resistance welding and ultrasonic welding struggle to achieve without causing defects such as micro-cracks or excessive heat accumulation.
Innovation Solution
A two-step laser welding process where the metal foil stack is first gently laser-welded to itself to prevent spatter and cracking, followed by a more powerful weld to connect it to the metal substrate, using conduction welding initially and keyhole welding for a robust joint, with a removable clamp to avoid welding to it.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If precision resistance welding is used to join thin metal foils to a thick metal tab, then electrical connection is achieved, but micro-cracks are caused that propagate under stress and weaken the joint
Solution Approach 1:
The welding process is divided into two distinct stages: first welding the thin foils to each other at lower power, then welding the assembled stack to the thick tab at higher power. This segmentation allows each welding operation to be optimized for its specific thickness requirements, preventing micro-crack propagation while ensuring reliable electrical connection.
Solution Approach 2:
The thin metal foils are pre-welded to each other before being welded to the thick metal tab. This preliminary action creates a stable foil stack that can better withstand the higher power welding process, preventing micro-crack formation in the thin foils during the final welding operation.
2Strength
If ultrasonic welding is used to join metal foils to metal substrate, then mechanical attachment is achieved, but excessive mechanical compression is required that must be maintained prior to assembly
Solution Approach 1:
The patent replaces the mechanical compression-based ultrasonic welding process with a laser-based thermal welding process. This substitution eliminates the need for complex compression fixtures and continuous mechanical pressure, allowing the workpieces to be held in position without excessive compression forces.
3Strength
If keyhole laser welding is used to weld through the full thickness of foil stack and tab, then a strong weld is formed, but excessive heat accumulation occurs
Solution Approach 1:
The laser welding process is segmented into two power levels: a first welding operation at lower power to weld thin foils together, and a second welding operation at higher power to weld the stack to the thick tab. This segmentation distributes heat input over time and space, preventing excessive heat accumulation while still achieving strong welds through the full thickness.
Solution Approach 2:
The thin foils are pre-welded to each other before the final welding to the thick tab. This preliminary welding action reduces the total thickness that must be penetrated in the final high-power welding operation, thereby reducing heat accumulation while maintaining weld strength.
4Ease of manufacture
If a single laser welding operation is used to join disparate thicknesses, then process simplicity is maintained, but welding quality deteriorates due to thickness mismatch
Solution Approach 1:
The patent applies segmentation to the welding process by dividing it into two operations with different power settings: a first operation for thin foils and a second operation for the thick tab. This resolves the contradiction by showing that a simple two-step process can achieve high welding quality, outweighing the minor increase in process complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method ensures strong, durable, and low-resistance electrical connections between the metal foils and substrate, minimizing defects and heat accumulation, while allowing for precise power delivery and efficient assembly.
Implementation Method 1
Conduction welding occurs at lower laser powers and lower power densities. Absorbed laser power heats the irradiated material, melting material in each part to be joined, which flows, mixes, and then solidifies.
Implementation Method 2
Keyhole welding occurs at higher laser powers and higher power densities that are sufficient to vaporize some of the irradiated material. Pressure of the vaporized material on surrounding melted material opens a channel through the melted material, having a characteristic narrow and deep profile, which allows deep penetration of the laser beam.
Data Source
AI summary
A method for laser welding a stack of metal foils to a metal substrate includes securing the stack of metal foils between a surface of the metal substrate and a removable clamp such that a side of the stack, formed by edges of the foils, is located on an interior portion of the surface, and the clamp is set back from the side of the stack. A first laser welding step interconnects the foils with an initial laser-weld joint by serially tracing a plurality of lateral paths along the foil edges with a laser beam. A second laser welding step connects the stack of interconnected foils to the substrate by tracing, with a laser beam, a path along the interface between the initial laser-weld joint and the substrate surface. This two-step laser welding process circumvents the difficulties of welding together materials with highly disparate thicknesses in a single laser-welding operation.