Expanding the use of laser welding

While laser welding is one of the oldest applications for industrial lasers, its implementation has been limited in some important material processing applications. Recent advances in both the laser source and the system technology are set to create new opportunities for laser welding.

Insufficient beam quality and wavelength, limited maximum average and peak power, and an inability to respond to high-speed controls reduced the effectiveness of previous generations of lasers, such as lamp pumped solid state (pulsed and continuous wave) and CO₂ lasers.

Today’s high power continuous wave (CW), and quasi-continuous wave (QCW) fiber lasers – with integrated control of the laser with the motion system and related system hardware – address these legacy problems, allowing manufacturers to expand their use of laser welding.

Understanding the capabilities of laser welding aluminum and copper alloys, as well as combinations with other materials, is necessary, especially with electrical vehicle (EV) battery manufacturing.
 

Battery materials joining

EV batteries use a combination of materials, posing challenges when joined.

Typical EV battery packs include:

• Aluminum alloys
• Pure copper
• Aluminum to copper
• Copper to stainless steel
• Copper to pure nickel
• Aluminum to pure nickel
• Nickel-coated steels

Components carrying electric currents inside a lithium-ion (Li-ion) battery are made of copper or aluminum alloys, as are the external buss bars that join to the outside terminals connecting a series of cells. Aluminum alloys and pure copper need to be laser welded 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 cell assembly welding step is seam sealing of the aluminum cans. This creates a barrier for the internal electrolyte, and welding of tab material to negative and positive terminals creates the pack’s electrical contact.

The battery has to operate reliably for a set life cycle – usually 10 or more years – so effective and high quality laser and processing parameters must produce consistent and defect-free weld joints.
 

 

Interactions, challenges

Copper conducts heat energy up to 10x faster than most steel. Aluminum 3000 series alloys conduct heat around 4x faster than steels. Copper and aluminum are also more reflective than steels to the wavelengths of most industrial lasers. With both copper and aluminum, only a small amount of laser energy is absorbed. This energy dissipates rapidly into the bulk of the material, making it difficult to maintain a molten pool. (See figure 1, Pg. 57)

Successful welding of these materials requires high peak power pulsed laser or high average power continuous wave laser to produce sufficient energy for good quality welds. Aluminum and copper have incompatible melting temperatures with 3000 series aluminum alloy melting at 643°C and pure copper melting at 1,083°C. That difference leads to poor weldability and the formation of brittle intermetallics (CuAl2), producing weld cracks when the joint cools.
 

New process

Fiber laser welding, using a special process, may offer aluminum-to-copper advantages, though brittle intermetallics may form. The weld using fiber laser is narrow and the volume of intermetallics may be reduced to acceptable limits. To accomplish this, it may be possible to offset the welding beam in one direction, allowing measured control regarding the composition of the resulting alloy. Compared to conventional lamp-pumped Nd:YAG and CO2 lasers, fiber laser (CW and QCW) offer beam quality or focusability – the ability to achieve a small focus diameter with a given optical element resulting in a quality weld joint.

 

Positive results Both continuous wave (CW) and quasi-continuous wave (QCW) fiber lasers are capable of welding electric vehicle (EV) battery materials. However, the challenge lies in developing laser and processing parameters that will produce acceptable quality welds. A combination of laser and processing parameters effect weld quality. Parameters include: Laser output – Modulated output from high-power CW lasers produces welds with controlled heating and cooling, reducing the freezing range of the weld metal, minimizing tendencies for solidification cracking in aluminum alloys, and reducing formation of brittle intermetallics during the dissimilar material welding process. Power density – High initial reflectivity of materials can be overcome with sufficiently intense laser-focused beams. The absorption rate increases as the temperature of the material rises. Aluminum and copper require higher power intensity than steel. Welding with reduced power density can lead to back-reflection problems. Focus position – When welding reflective materials, the focus position of the laser beam should be at the workpiece to reduce back reflection and to produce consistent weld penetration. Shield gas and cleaning – Cleaning aluminum alloy surfaces prior to welding with adequate inert gas shielding eliminates porosity that stems from evolutions of hydrogen gas during solidification. Hydrogen can originate from lubricants, moisture in the atmosphere, and surface oxides or the presence of hydrogen in the parent material. Welding speed – High welding speed can produce partial penetration welds, slow welding speed can produce full penetration welds, and very slow welding speed can lead to excessive drop through the back bead. With pulsed welding, pulse rate, spot diameter, and weld speed have to match to produce the required overlap. Hermetic welds are between 70% to 80% overlap, and for non-hermetic welds, between 50% to 60% overlap. Weld joint type (overlap, butt, etc.) – EV battery welds are usually performed without filler material, so gaps must be filled with metal from the adjacent area. Generally, a gap no more than 10% of the thickness of the thinnest component is allowed. This can be relaxed for materials ≥1mm, but might need to be reduced to 5% or lower for materials ≤0.2mm. Gap problems can be concave weld beads failing to bridge the gap between components. It is more difficult to start a laser weld in a large-gap area, compared to seam welds through a short section of high-gap areas where bridging can sometimes be maintained.

 

Cross-jet nozzle

New developments have improved laser accuracy. For example, a focusing lens assembly with a cross-jet feature provides a high velocity gas barrier – preventing molten metal spatter and fumes from the weld zone from contaminating the protective lens cover slide. Critical to this design is the cross-jet does not contaminate the welding shield gas.

The cross-jet nozzle can be used with shield-gas-delivery-devices including welding shoes and coaxial gas nozzle tips. The shielding gas shoe provides a controlled atmosphere while the weld zone is molten and cooling. This is important when trying to minimize porosity in the weld. The design of the focusing lens/shield gas assemblies for laser welding allows them to be changed quickly to vary the focused spot size.

 

 

SmartRamp

Another feature, SmartRamp, uses the integrated control of the laser and motion within the laser system’s control. For closed circular and rectangular welds, laser power is typically ramped down at the end of the weld after the start point has been overlapped, leaving a depression at the end point. This depression can be visually undesirable or cause a leak point. With SmartRamp, laser parameters are controlled with the Laserdyne system control to eliminate the weld end point.

Managing laser power ramping and laser pulse shaping with sub-millisecond resolution controls weld temperature and cooling of the weld and heat-affected zone, improving weld quality by reducing porosity, cracking, and intermetallics.

Ramp-down cooling improves the quality of battery weld joints by allowing slow cooling, resulting in a long-term weld joint.
 

Validation

Prima Power Laserdyne on laser welding EV battery materials covers a broad range of processing and laser parameters to examine crack sensitivity and porosity. With correct laser and processing parameters, it is possible to produce quality welds in 3000 series aluminum alloys, pure, and with dissimilar joints.

 

Prima Power Laserdyne
www.primapower.com

 

About the author: Dr. Mohammed Naeem is the senior manager, business development, at Prima Power Laserdyne and can be reached at or 763.433.3700.