Fiber laser welding technology is contributing to lower cost and higher reliability batteries for the latest generation of electric vehicles. Fiber laser welding reliably produces consistently high-quality welds for electric vehicle batteries where aluminum, copper, nickel-coated steel and combinations of these materials are welded together.
This article provides an update on the latest laser welding developments as they apply to these newest battery designs, including data to document the quality of laser welds of similar and dissimilar metal combinations.
According to an observer of the 2018 CES (Consumer Electronics Show), “automakers are reallocating and prioritizing their research and development budgets toward electrification of vehicles.”
Bloomberg writer Tommaso Ebhardt quoted Sergio Marchionne, CEO of Fiat Chrysler Automobiles NV, as saying, “by 2025, fewer than half the cars sold will be fully combustion-powered, as gas and diesel give way to hybrid, electric, and fuel-cell drivetrains.”
For this to occur, there will be significant investment to make electric vehicles more attractive and affordable for mainstream buyers. These investments will include reducing vehicle cost and increasing vehicle reliability and lifetime of the components, particularly the battery technology that powers electric vehicles.
Following is a review of recent product and process developments related to laser welding of electric vehicle battery components.
Diverse Weld Requirements
Discussing ‘’Electric Vehicle Battery Welding” implies a broad subject considering the number of variants. First, there are three main styles of batteries: cylindrical, prismatic and pouch shape. Second, there are a number of materials. There are also several types of welds.
The electric current-carrying components inside a lithium-ion battery are made of copper or aluminum alloys, as are the external buss bars that need to be joined to the outside terminals that connect a series of cells. Aluminum alloys (3000 series) and pure copper, which are the main materials, need to be laser welded either in similar or dissimilar configurations to produce electrical contact to the positive and negative outside terminals.
Overlap, butt, and fillet welded joints make the various connections within the battery. The final welding steps in the cell assembly are seam sealing of the aluminum cans, which creates a barrier for the internal electrolyte, and welding of tab material to negative and positive terminals. It also creates the electrical contacts for the pack.
Quality through Process Control
As consumers, we expect batteries for electric vehicles to have a relatively long life, typically a minimum of ten years. This is a standard that automotive companies are targeting with their new battery designs. For this to be accomplished, all components and assemblies must be of even higher reliability than current car batteries.
To achieve a ten-year battery life, welding processes must meet the challenges, whether or not the joints involve the same or dissimilar metals. Factors with laser welding that must be considered in creating highly reliable welds of similar and dissimilar metal combinations include:
Physical material properties
- Differences in absorption/reflectivity of the laser beam
- Differences in heat conductivity
Metallurgical material properties
- Solubility of material/alloying elements
- Differences in melting temperatures
- Thermal expansion
- Formation of intermetallic phases
One of the advantages of modern fiber laser systems is the degree of control of laser parameters that is possible. Laser pulse parameters (pulse duration, peak power, and even the shape—peak power as a function of time—of the pulse) can be controlled on a pulse-by-pulse basis. While simpler systems will call a pre-programmed set of conditions, the “real-time” control that is possible goes above and beyond this providing more control of the process and, as a result, metallurgical weld quality.
Newer fiber laser welding systems provide control of the following parameters:
- Average power
- Peak power
- Laser output
- Continuous wave (CW)
- Modulation (sine wave, square wave)
- Pulsed (various shapes possible), including pulse duration and frequency on a pulse-by-pulse basis
- Welding speed
- Percent overlap of laser pulses
- Focus position relative to the weld joint
- Shield gas type
- Shield gas flow rate
Dissimilar Metal Welds
Welds of aluminum to copper require considerable care, certainly more than those of copper and nickel. Welds of aluminum and copper can contain intermetallic compounds of the two metals that can be quite brittle. The brittle compounds reduce the weld strength and its ability to bend without cracking. Cracking of the weld will result in battery damage and eventual battery failure.
Welding tests carried out with aluminum to copper revealed that no single parameter controls weld quality. Rather, it is a combination of both laser and processing parameters that have a significant effect on the weld quality of these joints. Laser output and power density (optical power per unit area) are key parameters when welding highly reflected materials like copper and aluminum in a precise laser spot position on the workpiece. Also, offsetting the laser beam in one direction or the other relative to the joint is used to control the resulting composition of the alloy and weld properties.
Fiber laser welding, with its characteristic high cooling rates, has two advantages over many lower intensity welding processes for which cooling is slower. First, high solidification rates have been shown to yield finer weld microstructures and smaller precipitates. Continued high cooling rates of the solid material minimize time for growth of the undesirable intermetallic compounds with cooling.
Research has shown that intermetallic compounds less than 20 µm in diameter have negligible effect on the structure.
Pulse shaping, or controlling output power as a function of time within a laser pulse, can be very beneficial when laser welding copper and aluminum. The single sector “square wave” pulse is often adequate when welding standard ferrous alloys. However, when welding reflective or dissimilar materials, pulse shaping has a measurable effect on the quality and consistency of the welds. Following are two typical pulse shapes used to improve the welding process and the quality of lithium-ion batteries.
Materials with high electrical conductivity, such as pure copper, tend to also have high reflectivity to infrared and near infrared (Yb fiber laser) wavelengths. In fact, for some high-power fiber laser systems, laser beam delivery mirrors are produced from copper.
For welding copper, an enhanced spike pulse shape helps to prevent back reflection from the workpiece as well as to produce consistent weld penetration along the length of the weld.
The first 0.5–5 msec can be of a much higher peak power than the following sectors. Once the initial spike starts melting the surface, the absorption increases (reflectivity decreases) about 20 times so that the laser pulse energy for the rest of the pulse can be much lower.
Aluminum-based alloys which are prone to cracking or porosity, such as 1000 series or 3000 series used for battery cans, also benefit from shaped pulses.
For these materials, a ramp-down pulse shape has been shown to prevent cracking and reduce porosity. The extended, step down of power within a pulse controls cooling of the weld fusion zone avoiding weld defects.
When welding with the “ramp-down” shape, combining the normal single welding sector (main sector) with sectors of lower peak power will slowly reduce the laser energy going into the weld nugget, allowing slow cooling. In some applications, it is possible to have over 10 sectors in this type of pulse shape but usually 3–5 sectors is sufficient.
The pulse shape is also useful for sealing the hole used for filling the 3000 Al alloy battery package with electrolyte. The leading edge spike counters the initial reflectivity and initiates melting. The remaining portion of the pulse melts material while avoiding significant vaporization (spatter).
Compared to a typical square wave, the shaped pulse makes coupling more consistent, reduces back reflection, and reduces weld spatter from the process.
Hermetic Sealed Welds
Sealing of batteries and assemblies within batteries that are filled with electrolyte requires special care to avoid contaminating the electrolyte and to ensure a leak-tight weld. Hermetic welds are most effectively produced using pulsed laser conditions to control heat input to the joint. For a hermetic weld, the rule of thumb is to have a pulse overlap of 70% minimum with 80% overlap being more typically used.
Prima Power Laserdyne has developed a Process Calculator that assists process engineers in selecting laser parameters that will give both the required width, penetration, and pulse overlap.
To avoid weld leaks, the Laserydyne SmartTechnique called SmartRamp eliminates the depression at the end of a closed weld. SmartRamp involves integrated control of the laser and motion parameters to avoid the depression. By eliminating the depression at the end point of a closed weld the likelihood of pinhole leaks is eliminated. This is especially critical with thin materials that require a hermetically sealed weld.
For materials such as 3000 aluminum series used for battery cans which require hermetic sealing, shaped pulses are also part of a robust manufacturing process.
For this material, a ramp-down pulse shape has been shown to reduce porosity for aluminum alloys prone to this type of defect. The extended, step down of power within a pulse can control cooling of the weld fusion zone. The ramp-down pulse shape also can have a huge benefit in reducing cracks and porosity. Although the standard and enhanced spike pulse shapes show better results in terms of weld penetration, the cooling cycle is extremely high. As a result, there may be occluded gas and cracks formed during solidification. A ramp-down pulse controls the cooling rate and solidification cracking is greatly reduced as well as formation of porosity.
Controlling Weld Width
Many applications benefit from the narrow profile associated with laser welds. However, this is not always the case. The small size of the focused laser beam can create stringent requirements for joint fit-up. In some cases, the requirements are not consistent with other manufacturing processes. Such is often the case for formed sheetmetal components.
There are other applications for which a larger weld is preferred. Wider welds are required either for mechanical strength (wider interface of a lap weld, for example) or, in the case of welding battery buss bars, for electrical properties. Welds of buss bars require butt joints between aluminum and copper of a larger cross section due to the high current handling requirements.
For this application, a wobbling technique gives the required weld width while preserving the flexibility of the system for making narrow welds in other areas. Wobbling involves superimposing laser beam motion onto the motion required to track the weld joint. Various patterns can be used, though a common pattern is a circular motion.
Results of tensile tests of commonly fiber laser welded copper and aluminum alloys are included in the accompanying table. For similar material combinations, the tensile strength is close to that of the parent material and the tensile fracture occurred in the base material. For dissimilar joints, the tensile strength was limited by formation of brittle intermetallics and fracture occurred in the fusion zone.
Extensive testing has shown that fiber laser welding has been proven effective in producing high-quality, robust welds in a range of lithium-ion battery materials. Control of the laser weld metallurgy and dimensions are possible with the enhanced control of fiber laser welding process parameters available with today’s fiber laser welding systems.
Mohammed Naeem is senior manager, applications engineering & technology development of Prima Power Laserdyne. Terry L. VanderWert is retired president of Prima Power Laserdyne.