Welding Publication
Tim Morris is Welding Market Development Manager (Timothy.Morris@nlight.net), Brian Victor is Director of Industrial Applications at nLIGHT.
The battery market is approaching the tipping point. When battery prices fall below the industry consensus of $100/kWh, electric vehicles (EVs) will match internal combustion engine vehicles (ICEs) on the purchase price. The passenger vehicle market is the most significant volume application for batteries and has been pushing advances in battery technology and manufacturing efficiency. As EV and hybrid vehicle sales continue to ramp, improvements in performance and price are compelling other battery markets to accelerate. Battery advances offer more valuable solutions for clean energy storage and peak shaving for the stationary power market. Delivery trucks and public transit in the commercial vehicle market and two-wheeled transit for personal mobility will draft off the improvements in energy density, charge cycle, and reliability driven by the passenger vehicle market. Price parity between EVs and ICEs is expected to occur in only a few years.
Laser integrators, system builders, and laser enthusiasts understand how exciting this rapidly growing battery market is for the laser industry. New laser processing solutions solve the increasing demands in battery packaging efficiency, weight reduction for pack energy density, and faster cycle times for higher production volumes. Historically, lasers have been used in battery welding applications for decades. Battery manufacturers in medical and aerospace have utilized legacy pulsed YAG laser welding, ultrasonic welding, or resistance spot welding to join thin copper, aluminum, stainless, and nickel components. While able to join battery materials effectively, these legacy processes have slower cycle times or require larger weld tab geometries that are incompatible with EVs' production demands and weight limitations.
Fiber lasers have quickly overtaken these legacy processes in the electrified transportation market due to superior processing value (production speed versus operating cost). Fiber lasers are used to cut the anode and cathode foils, weld foil tabs, seal prismatic cans, assemble current interrupt devices, bus cells into modules, and join modules into structural packs. As battery packaging efficiency demands increase, single-mode fiber lasers are becoming more popular than multimode lasers due to the narrower weld size, lower heat input, and higher intensity.
Challenges for laser welding
Just like every manufacturing process, battery welding still has several challenges confronting manufacturers. Evolving product designs and shifts in the supply chain can impact the quality and repeatability of the parts to be welded, leading to significant part- to-part variability.
Copper and aluminum are more challenging to weld than steel, stainless steel, and nickel, of the metals used in battery production.
Both have high heat conductivity, low liquid viscosity, and an affinity for picking up gases in the molten weld pool. To produce fusion welds in copper and aluminum, the welding process must be very fast with low heat input to avoid overheating nearby components. The fusion process must be stable at high speeds despite any joint variability to prevent weld spatter and porosity.
For structural connections, welds need to be as strong as possible without requiring an increased joint area for the welding process as this additional weight reduces energy density. The weld area needs to be wide enough for current carrying capacity while limiting the distortion that large melt volumes can cause for electrical connections. For combination structural-electrical joints, the welding process must work within the
constraints of the product design to achieve the best balance. These challenges combined with extreme cycle speed and quality requirements can lead to laser welding engineers' very narrow operating window.
Single-mode beam shaping for the battery processing toolbox
Single-mode fiber lasers have proven to be an effective solution for these challenging welding conditions. The single-mode beam's small spot size and high-power density can produce deep penetration welds at very high speeds with little heat input and low distortion. The intensity available from single-mode fiber lasers can easily overpower the reflectivity of copper and aluminum during keyhole welding.
Single-mode fiber lasers have established their reputation as a versatile and valuable battery processing tool, from foil tab connections and small structural welds to thicker electrode connections.
In recent years with the introduction of fiber lasers with tunable beam shapes, battery integrators have shown the value of ring-shaped beams for welding thicker aluminum such as prismatic cell cap-to-can (FIG 1), multicell module bus bars, module housings, structural pack frames, and aluminum EV body welding.
Figure 1. Cap-to-can weld of an aluminum prismatic battery cell using a multimode ring- shaped fiber laser beam (CFX series), 2.5kW 200mm/s.
nLIGHT has recently extended this tunable beam shaping functionality to single-mode fiber lasers combining the benefits of ring-shapes with the intensity of single-mode beam quality. nLIGHT's AFX series fiber lasers offer the capability to dynamically change the laser power distribution from a single-mode 15-µm diameter fiber up to a 40- µm ring fiber and a variety of intermediate shapes (FIG 2). This tunability offers versatility to accomplish various welding tasks for battery materials from one flexible laser source.
Figure 2. AFX fiber laser range of beam shapes from single-mode Gaussian spot to a 40-µm outer diameter ring.
The highest-intensity single-mode beam shape for welding copper provides the deepest penetration and fastest welding speed by operating in keyhole welding mode. When copper welding speeds are high using the small single-mode beam, weld discontinuities are avoided resulting in smooth, narrow welds with a high aspect ratio (FIG 3). This tiny spot size enables keyhole welding for the fastest productivity. Often battery welding systems utilize galvo scanners for remote welding to allow the most control over the beam shape. The flexibility provided by a scanner enables fast straight-line speeds and beam oscillation patterns at slower speeds. This oscillation, wobbling, or stirring of the beam can increase the weld width without reducing the local speed of the keyhole, thus avoiding any slow speed weld discontinuities.
The larger ring beam shapes are advantageous for welding aluminum joints for maintaining a stable weld pool and keyhole during processing. At three times larger diameter than the single-mode beam, the intensity of this 40-µm ring beam at 1kW is still sufficiently high to form a vapor cavity in the aluminum for applications like bus bar and foil tab welding. By shifting more power to the ring, the keyhole shape transforms from a narrow capillary to a wide-bottom cylinder allowing vapor and entrapped gases to escape before solidification. This combination of power density and a wide beam shape results in a wide, smooth weld surface with little to no spatter. Additionally, this wider beam shape produces a broader weld bead without the penetration spiking often seen when welding aluminum with Gaussian beam shapes at slow speeds.
Figure 3. Copper and aluminum tab welds (1) 585W, 140mm/s, 125µm copper on aluminum; (2) 625W, 255mm/s, 125µm aluminum on copper; (3) 750W, 175mm/s, 250µm copper on aluminum; (10) 700W, 140mm/s, two 125µm copper collector tabs to copper electrode.
When welding ferrous, nickel, or dissimilar alloy combinations, the intermediate beam shapes provide the flexibility to change the balance of power between the intense single-mode core and the wide ring beam tailored for each application. Most of the energy is often directed to the center core for the deepest penetration at high speeds. If the speed is reduced to produce a more comprehensive weld or due to machine acceleration and path geometry, more power can be shifted to the ring. With the intermediate beam shapes (FIG 2), the single-mode core provides the penetrating power, and the energy in the ring vaporizes the metal around the keyhole, widening the top opening of the vapor cavity. This fraction of ring power alters the keyhole shape to be more cone-shaped than a narrow capillary.
If the speed is further reduced, more power is shifted to the ring to prevent a momentary collapse of the keyhole and further widen the keyhole opening. This wider keyhole opening results in a more stable process with fewer spatter ejections and weld surface fluctuations. The versatility in spatial profile provided by AFX single-mode beam shaping fiber lasers enables a broad range of processing conditions from a single laser source.
Summary
As the battery market repeatedly breaks performance, price, and manufacturing scale records, battery producers are looking to add new tools and capabilities to their kit of welding solutions for new challenges in production speed and product evolution. While most of the manufacturing challenges facing battery producers are not new to laser processing, the scale and complexity of system builders' tasks are unique. Laser beam
shaping has proven to be a valuable tool in the laser process engineer's toolbox. The innovative combination of beam shaping and single-mode beam quality in a single laser source adds the processing flexibility to control keyhole stability and weld quality while maintaining the high intensity necessary for welding battery materials. With declining battery prices and increasing production volumes, the laser industry's horizon for battery applications and the EV market is specifically exciting.
AFX product images
