Laser welding aluminum enables low-cost EV battery manufacturing

To reduce CO₂ emissions and health risks from particulate and exhaust emissions, and to achieve energy independence, several countries and municipalities have announced plans to phase out internal combustion engines in favor of electric vehicles (EVs). However, high cost of battery manufacturing hinders the adoption of the new technology. The battery pack is one of the most expensive parts of the EV powertrain. It requires a more efficient manufacturing technology to reduce the overall cost.

Aluminum (Al) has the highest electrical conductance per unit mass and the lowest cost per unit of electrical conductivity of all the practical conductive materials. Average weight of Al used in an automotive traction battery is about 50kg, with more than 20% representing current-conducting parts such as cables and busbars. In the past, the use of Al in EV batteries has been limited due to lack of appropriate joining technology.

Recent advances in laser welding enable wide use of Al for current-conducting and structural parts. Laser welding provides precise and measured heat input, while the combination of real-time advanced process control and nondestructive evaluation of manufacturing quality guarantees 100% good welds. It can be fully automated and provides the shortest cycle time and lowest production costs of any joining method.

Automotive battery cells come in three form factors: cylindrical, pouch, and prismatic. A typical challenge in welding cells is maintaining controlled weld depth in thin materials in the presence of very high temperature gradients. In the case of cylindrical Li-ion cells, such as 18650 and 21700 cells, a strong metallurgical bond must be formed between the 0.3mm-to-0.4mm thick steel cell enclosure (cell casing) and the 0.2mm-to-0.5mm thick Al busbar. A high quality weld nugget, with high mechanical strength and low electrical resistance, must be made on the cell’s positive and negative terminals. The process must be replicated precisely on millions of joints in large scale production.

To form a high-quality weld nugget, steel and Al must be melted. The solidus and liquidus temperatures of low carbon steel, forming the cell enclosure, are approximately 1,480°C and 1,520°C respectively, while the melting temperature of pure Al, commonly used for busbars, is approximately 660oC.

The interior of the cell is filled with a heat-sensitive electrolyte, a solution of lithium (Li) salts in an organic solvent that may decompose above 60°C. In addition, temperatures higher than 100°C may damage cell elements such as seals, gaskets, and separators. Thus, the heat-sensitive electrolyte and cell elements are separated from molten steel by a 0.3mm-to-0.4mm thick steel enclosure, requiring temperature gradients >7,500°C/mm. To prevent any cell puncture or damage during welding, lasers with exceptional beam quality and power stability serve as a viable solution (See Fig. 1, page 8).

Another challenge, found in both cylindrical and pouch cell modules, is the need to weld dissimilar metals (iron [Fe] to Al in cylindrical cell modules and copper [Cu] to Al in pouch cell modules). Welding dissimilar metals may form brittle intermetallics, such as Fe-Al in cylindrical cells and Al-Cu in pouch cells. Laser welding can achieve extremely fast heating and cooling rates, reaching 106-to-108 degeees C per second, resulting in amorphous and nanometer size grain structures of non-stoichiometric compounds in the intermetallic weld nugget.

Formation of intermetallic compounds in the Al-Cu weld is inevitable; however, brittle fracture of the weld isn’t (See Fig. 2, page 9). The force-extension curve obtained during mechanical testing of the laser weld is highly nonlinear, indicating substantial plastic deformation. High weld strength triggers global plasticity in the material around the weld.

Welding prismatic cells requires formation of large weld nuggets to achieve low electrical resistance and high mechanical strength. The challenge is to maximize the weld contact area and process throughput, while limiting heat input and maintaining consistent weld quality. Welding thick sections, up to 6mm, joining stacks of multiple Al plates and dissimilar materials (Al to Cu or steel) present additional challenges. Multi-kilowatt fiber lasers allow flexible adjustments of temporal and spatial beam properties, meeting these demands. To maximize the contact area, the laser beam can be rapidly wobbled, typically in a circular pattern. Wobbling the laser beam widens the weld seam and stabilizes the weld pool, reducing molten spatter. Typical welding speeds using the wobbler reach 6m/min to 8m/min. Another solution, adjustable mode beam (AMB) lasers, delivers laser power through a combination of a narrow central core and wider ring beams, with the same result, but at speeds up to 36m/min. Wobbling and AMB can be used simultaneously to produce the widest interfaces with the highest quality at the highest welding speeds (See Fig. 3, page 9).

Li-ion batteries require a battery management system (BMS) to control the cell voltage. The BMS wiring harness, usually made as a flex printed circuit with copper traces, must be reliably welded to Al busbars (See Fig.4, page 10).

Perhaps the biggest challenge in EV battery manufacturing is productivity. For example, a production volume of 100,000 EVs per year with 100kWh battery packs comprised of 21700-type cylindrical cells requires about 1.1 billion welds per year, or approximately 150 welds per second per 8 hour shift, 5 days a week.

The automated laser welding production line is by far the least expensive solution to achieve this throughput. Other joining methods, such as ultrasonic welding, wire bonding, resistance spot welding, or mechanical joining, join one cell at a time. These welding machines operate in the stop-and-go regime: the machine welds in the first position, then moves to the next position, stops and welds the second joint, etc., limiting the joining rates to 2 welds/sec. The target throughput requires dozens of welding machines working in parallel, leading to highly expensive manufacturing, substantial maintenance, and quality control issues.

In contrast, a fully automated CNC laser welding machine powered by a multi-kilowatt single-mode fiber laser with a fast scanning head can form more than 50 welds per second. Laser welding can be done “on the fly” with laser scanning head moving, while the laser beam is steered rapidly and accurately by galvanometer mirrors, forming precise welds in milliseconds.

The CNC machine is seamlessly integrated into a fully automated production line including accumulating palletizing conveyor running through the machine. The conveyor’s lift-and-locate station becomes part of the machine, minimizing load/unload time.

Assembled battery modules travel on the conveyor to the welding machine, are automatically presented for welding, welded, tested, and removed for further assembly. Full process automation enables high productivity.

To monitor stability of the welding process and ensure 100% yield of good parts, an automated non-destructive inline weld process monitoring system can be integrated with the scanning laser head to provide real-time weld depth monitoring (See Fig. 5, page 10).

Fiber laser welding is a highly stable process, capable of forming millions of high-quality welds. Its reliability is better than a few defects per billion, enabling mass production with the throughput of millions of welds per year. Laser welding emerges as an enabling technology for high throughput EV battery production; it’s the most flexible manufacturing tool suitable for full automation, resolving challenges of Al welding quality and throughput. More than 10x faster than other welding methods, laser welding requires proportionally fewer workstations, resulting in substantial manufacturing cost savings and enabling wide use of Al for current-conducting and structural elements in EV powertrains. This brings considerable savings in battery material and manufacturing costs, enabling manufacturers to meet the ever-increasing demand for safe and reliable EVs.

IPG Photonics https://www.ipgphotonics.com

About the lead author: Leonid Lev is vice president for Advanced Industrial Solutions at IPG Photonics. He can be reached at llev@ipgphotonics.com or 508.506.2663.