Laser welding has a place in electric vehicles, aerospace, ship and rail transportation, construction, the energy sector, semiconductors, consumer electronics, medical device manufacturing and more. Even the fusion of dissimilar materials, which is difficult with conventional welding techniques, can be easily solved with the flexibility and precision of laser welding, and has even become the preferred solution. This process, often referred to as "dissimilarity welding," is an important part of achieving modern engineering goals.
The production of batteries and electrical components for e-mobility applications is driving greater interest in laser welding of dissimilar materials such as copper and aluminum.
Dissimilar welding allows for wider design freedom when selecting different materials with good properties such as electrical and thermal conductivity, ductility, relative density, melting point and hardness, but traditionally requires adhesives or mechanical methods to bond together.
Although the technique has elements in common with conventional welding, it offers a unique opportunity to increase the degree of design freedom, the variety of material combinations, thereby reducing manufacturing and assembly costs and improving component or system performance.
However, welding different materials requires careful consideration of laser wavelength, average power, beam profile, pulse width and peak power. Laser system parameters must also be customized for specific material combinations and applications.
The most important and fastest growing application area is the production of batteries and electrical components for electric vehicles. The demand for electric vehicles (EVs) has increased dramatically in the last two years, and welding dissimilar materials is at the heart of making EVs more efficient and environmentally friendly.
While dissimilarity welding has much in common with conventional welding, optimizing the quality and speed of the weld is more challenging. The flexibility of laser welding systems offers unique solutions to expand new applications and opportunities. (Contributed by Tomo Express)
Matthew Philpott, Chief Marketing and Sales Officer of NUBURU, a leading innovator of high-power and high-brightness industrial blue laser technology, said, "Electric vehicles are projected to account for more than 20% of the market in the next 5 to 10 years, and consumer electronics will account for between 10% and 15%."
The manufacture of lithium-ion (Li-ion) batteries requires the ability to weld aluminum to copper in a foil-to-electrode or electrode-to-electrode weld. In cylindrical batteries, the copper electrode lugs must be welded to a steel can.
In battery pack manufacturing, the cells are usually already assembled and engineers must implement a design that connects the cells to provide optimal energy. Current lithium-ion batteries are made of nickel-plated cold-rolled steel. However, welding a less resistive metal, such as aluminum or copper, to the standard stainless steel terminals of a lithium-ion battery reduces its resistance, so less energy is wasted in heat loss.
Enhanced electric vehicle battery performance is a major factor in the steady growth of electric vehicle sales," said Mark L. Boyle, senior manager of product engineering and applications at AMADA WELD TECH. Better performance stems in part from recent developments in dissimilar metal welding, which improves efficiency by increasing energy storage, reducing size and maintaining reliability."
In addition, the shipbuilding industry provides another example where dissimilar welding is delivering unique value. The industry routinely uses steel-aluminum welded interfaces to optimize weight distribution, resulting in lower CO2 emissions and increased stability. In particular, welding a steel hull to an aluminum superstructure can reduce deadweight.
Blue light laser welding of copper sheets. Green and blue lasers are often better suited for welding highly reflective metals such as copper and aluminum, providing lower heat input and improved process stability of >1 µm. (Photo by NUBURU)
"In addition to reducing CO2 emissions and energy consumption, the center of gravity of the vessel can be lowered through intelligent arrangement of the material, thus improving transport stability." Rabi Lahdo, researcher in the Metal Welding and Cutting Group at the Hanover Laser Center, said.
Although materials with similar properties usually produce more reliable welds, major players such as AMADA WELD TECH are receiving an increasing number of requests to weld dissimilar materials.
"Commercially, choosing a different material may reduce manufacturing costs and improve the performance of a component or device." Mark L. Boyle said, "When this happens, the choice of dissimilar metals can be used as a competitive advantage in the marketplace to provide a better product at a lower price."
01 Challenges and Considerations-
When fusing materials such as steel or copper with aluminum, changes in the material's melting point and coefficient of thermal expansion can lead to the formation of brittle intermediate portions that weaken the weld joint.
"Metals have different melting and fusion temperatures, different light absorption coefficients (especially at certain laser wavelengths), and different thermal diffusivities. This makes it difficult to melt them to the right degree at the same time." NUBURU's Philpott says, "This is most noticeable in highly reflective metals, which can have very different absorption coefficients in the infrared."
Stress fields created by different coefficients of thermal expansion during cooling can also weaken welds and can lead to weld joint failure. These hard, brittle structures, called "intermetallic phases," form in the transition zone between the weld metal and the base metal. This is a phenomenon that can plague any welding method.
Cross-section of a steel and aluminum dissimilarity weld. (LZH contribution)
The formation of intermetallic phases, such as FeAl2, Fe2Al5, FeAl3 in the steel-aluminum system and Cu9AL4, CuAl2, Cu4Al3 in the copper-aluminum system, is due to the limited solubility of the elements," says Sarah Nothdurft, Head of the Joining and Cutting Metals Group at the Laser Center Hannover. Such phases also exhibit significantly higher resistivity compared to the base material."
Careful selection of the laser's operating parameters, such as combining high welding speeds, low heat loads, and precise control of the melting process, allows engineers to mitigate some of these issues.
"While the formation of intermetallic compounds is inevitable, their brittleness is not." Alexei Markevitch, Market Development Manager at IPG Photonics, said, "The right process formulation can minimize the formation of these compounds and maximize their malleability, resulting in structurally sound, more conductive, and more stable welds."
02 Applications for welding different materials-
Attention to proper mixing ratios and proper matching arrangements can further enhance the performance of dissimilar weld joints. For example, an I-seam with a lap weld opening has proven to be advantageous. In this method, a steel plate is placed on an aluminum plate. In order to minimize intermetallic phases, the welding is performed through the steel plate and only to the aluminum plate.
Oliver Seffer, a researcher in the Metal Welding and Cutting Group at the Hanover Laser Center, says: "Due to the low aluminum content, the proportion of such brittle phases in the final microstructure is relatively low."
03 Laser Parameter Considerations-
The choice of laser technology depends on the material to be welded. Dissimilar weld ports for glass and metals may require a CO2 laser system. Welding aluminosilicate glass and various metals may benefit from a femtosecond laser system, while welding aluminum alloys and technical glass can often be successful with a picosecond laser source.
The goal is to minimize heat input, eliminate spatter, improve process stability and provide a wide window of process parameters while welding at the highest possible speed.
"While steel alloys absorb well in the near-infrared region, even metals with high reflectivity, such as aluminum and copper, are mostly processed with 1 µm lasers." IPG's Markevitch says, "This is because the absorption depends on the metal temperature and phase. At room temperature, copper and aluminum absorb about 5% at 1 µm and 40% to 50% at 515 nm, with higher absorption at blue wavelengths."
"All absorptivities increase for heated metals, and IR jumps at the melting point," he says, "and molten metals absorb all wavelengths very well. Thus, a high enough IR power density overcomes the high reflectivity."
However, in shallow conduction welding of foils or certain welding geometries involving thicker materials, the use of high-intensity infrared lasers can lead to overheating, material damage, or process instability at the point of the fast absorption transition. As a result, in some cases, green or blue lasers are more suitable for copper welding because they offer lower heat input and improved process stability at >1 µm.
Rabi Lahdo says that lowering the required output intensity attenuates turbulence in the molten pool, which improves process stability. "The increase in process stability is accompanied by an improvement in the quality of the hybrid weld opening, and spatter formation is suppressed."
In keyhole welding of thicker materials, starting with micro-bond holes of hundreds of micrometers, infrared lasers are typically more effective than green or blue lasers, resulting in less heat input, as well as better weld quality and faster speeds.
Tunable mode beam lasers eliminate spatter while quickly achieving high quality weld openings in dissimilar materials. These lasers emit a core beam enclosed in an individually controllable ring beam. Busbar welding applications for melting aluminum and copper can be achieved using an infrared single mode beam (above). However, the Tunable Mode Laser (below) exhibits complete control of spatter by enclosing the single-mode beam within an external annular beam. Such systems are capable of spatter-free copper busbar welding at speeds up to 60 m/min and depths of fusion >0.65 mm.
"The single-mode beam brightness of up to 2 kW overcomes the reflective nature of the bright metal to create stable small-hole welds with a depth of fusion that can be much deeper than the width of the weld," said Ken Dzurko, global senior key account manager at the ThruFast Laser Technology Center in Santa Clara, Calif.
"The rapid oscillation of the beam inhibits the formation of intermetallic compounds, thus limiting the duration of the melting phase at the weld opening." He said, "In addition, the high beam brightness increases welding efficiency and greatly reduces the heat-affected zone, thereby producing a higher weld volume at a lower average power input."
Another factor affecting the use of laser energy is light scattering by the metal vapor plume, which is proportional to the fourth power of the wavelength. 1070nm lasers scatter 18 times less than 515nm lasers and 30 times less than 455nm lasers. The high scattering rates of blue and green lasers in metal vapor plumes easily offset their slightly higher absorption rates in molten materials.
Today, most manufacturers choose continuous-wave 1 µm lasers, which lead the way in processing speed, quality and cost reduction. But all wavelengths offer advantages, depending on the specific situation. For example, NUBURU's Philpott believes that a wavelength shift to blue or green light is worth exploring in applications that benefit from increased absorption.
"The beam delivery for blue or green light lasers (e.g., scanners, processing heads, beam control and other auxiliary components) is similar to that used for NIR lasers." Philpott says, "As a result, conversion from infrared to blue or green light is very easy, and the method of managing the plume is similar, so there are no problems due to absorption or scattering."
Today's laser systems are limited to 3kW at 515nm and 4kW at 455nm, and because of the limited beam quality of blue lasers, beam focusability and processing efficiency are also limited.
"When welding copper using laser beam wavelengths in the visible range, especially in the blue light spectrum, there is currently a lack of sufficient laser beam power and required beam quality," says Rabi Lahdo, "Achieving high beam quality is the biggest challenge when using laser diodes to generate laser radiation . In addition, visible lasers are more prone to damage optics than infrared sources, which shortens lifetime and increases costs."
Despite the challenges, Philpott anticipates further improvements in soldering performance and value as blue light diodes continue to improve in availability and performance.
"There is no reliability or cost risk associated with operating lasers within the design tolerances of the optics," he said. "That said, customers may experience short optics life with a particular laser supplier's product; however, if a manufacturer does not release a product without the optics devices properly validated, then this can happen with any wavelength."
04
-Specialization of laser systems-
Continuous-wave fiber lasers can weld aluminum and copper with proper control of the beam profile. The development of core-ring beam profiles and more powerful scanning systems has significantly improved the quality and potential of hybrid weld ports over the past decade.
During small hole welding of copper and aluminum, the holes become unstable at high welding speeds. One way to eliminate this instability is to slow down the welding speed, but this is usually not desirable. Instead, another method is to use a galvanometer to add oscillation to the laser beam to agitate the molten pool. This improves convection in the melt stream to prevent small holes from collapsing. It usually produces an excellent quality weld, but slows down the welding process further.
A third way to eliminate spatter during high-speed welding is to use an Adjustable Mode Beam (AMB) laser, which emits a core beam surrounded by a ring beam. The core beam power and intensity determines the depth of penetration of the small holes, while the energy of the ring beam stabilizes the small holes to minimize or completely eliminate undesirable spatter, cracks and porosity.
The smallest cores are single-mode beams with a diameter of 14 µm. Multimode cores are typically 50 or 100 µm in diameter, and ring beams are typically up to 300 µm in diameter.
"The use of core-ring fiber lasers is an active development area in infrared dissimilarity laser welding and is sought after by all the major players," says Markevitch, market development manager at IPG Photonics. "The AMB laser with a single-mode core was chosen for its its versatility, high welding speeds, and inherent ability to minimize the formation of brittle intermetallic compounds."
A 3 kW single-mode core AMB laser with 3 kW of additional power in a ring laser is capable of spatter-free copper busbar welding at 60 m/min with greater than 0.65 mm penetration.
Current commercial green or blue lasers cannot achieve the same processing speed and quality, says Markevitch. But as he points out, weld consistency can still be affected by variations in the gap between materials or material contamination. With the tendency for busbar thicknesses to decrease, clamping and fixturing becomes a challenge. Insufficient weld melt depth may result in higher resistance and lower mechanical strength, while excessive melt depth or piercing may make EV battery cells a fire hazard.
"Typical material thicknesses for busbar lap welds are 200 to 300µm, less than 1mm," says Markevitch, "Immediately below the thin lap weld is a thermally-sensitive organic electrolyte, which may decompose at >60°C."
Aluminum melts at 660°C, copper at 1,085°C, and steel alloys at 1,500°C. Two metals with very different melting temperatures need to be melted without damaging the lithium salts containing flammable organogel or battery components (such as seals, gaskets, and spacers) below.
In-line process control based on spectral process emission or OCT can provide real-time non-destructive weld depth measurements. This allows corrective action to be taken to achieve a constant melt depth.
