The number of electric vehicles (EVs) in global circulation has increased six-fold between 2013 and 2017 (the last year of available data) according to the International Energy Agency (IEA). In the United States alone, approximately 760,000 EVs traverse roads and highways.
One force behind steadily increasing EV sales is the improved performance of the batteries used in these vehicles. Primarily lithium ion, these batteries must hold sufficient energy to propel vehicles at high speeds for hours on end. Designing lighter, smaller, safer and more efficient batteries is proving to be instrumental in continuing global EV sales trends. Advancements in welding technologies increase efficiency for automotive powertrains by improving energy storage, cutting size and preserving reliability.
Whether a driver intends to use an EV for a daily commute or only sporadically, they expect that battery to hold a charge and not lose too much energy in use. In other words, manufacturers aim to design batteries with greater range between charges and that, when they need to be charged, charge faster. A primary cause of battery energy loss is resistance. In a perfect world, there would be no resistance between battery and load, but the connection of a tab to a terminal implies a certain amount of loss.
This, of course, can be mitigated by using more conductive materials. With the standard stainless steel terminals of lithium ion batteries, for example, this presents an engineering opportunity: To reduce resistance, weld the stainless steel to a dissimilar and less resistant metal.
Challenges abound, however, when seeking to weld two different metals together. Different metals have different melting temperatures, different expansion coefficients and, sometimes, incompatible chemistry. For example, aluminum melts at 660 degrees C – fully 840 degrees C lower than stainless steel’s 1,500-degree C melting temperature. Welding these metals effectively is impossible with the contact process of resistance welding.
Dissimilar joining
Resistance welding has been a cornerstone technology for welding battery packs for many years. Typically, this technology has been used join nickel tab material to steel battery cans. It has proven less successful for welding other metals.
“Resistance welding does not work well with aluminum or copper because they are more conductive, so the weld head cannot create the requisite energy build up,” explains Mark Boyle, product manager, Amada Miyachi America Inc.
Thus, alternative technologies such as laser welding need to be considered for joining conductive materials. Even with laser technology, however, bonding dissimilar metals is not straightforward. The melting points of various metals vary greatly (for example, aluminum melts at 660 degrees C and copper at 1,080 degrees C), and the metallurgy of these different materials forms brittle intermetallic bonds when fusion welded together.
New laser sources provide the opportunity for such dissimilar metal joining, including continuous wave (CW) and pulsed fiber lasers. Amada Miyachi’s 500-W single-mode CW fiber laser micro welder, for example, concentrates the energy into a small spot to aid energy coupling into the material and resultant penetration. There is little change in penetration even with increasing scan speeds.
The benefit of this is two-fold: faster processing and minimized heat input. A closer look at the process shows that it is not like traditional fusion welding as there is no large mixture of the two metals. For example, a resulting weld between aluminum and cold rolled steel has no cracks and minor porosity, an impossible result in classic welding techniques because the intermetallic mix of aluminum and steel is naturally brittle.
This new technique shows promise for a variety of applications. In one case study, mechanical testing found peel strength to be good; after thermal soak and shock, 50 samples of the weld peeled within ±2 N. This narrow band of peel results shows reproducibility and reliability needed in production.
Amada Miyachi has also used a 70-W pulsed fiber laser to join thin dissimilar metals. These require more welds to create the same contact area in a weld zone and ensure appropriate weld strength at the material interface. (The pulsed laser delivers limited power and small spot size.) The system is programed to deliver a spiral weld pattern that groups small spot welds in close proximity in a geometry that creates good weld strength.
“The [resulting] weld profile closely resembles a multi-staking process,” Boyle says, “and does not show the characteristic form of conventional pulsed spot welds.” A cross-section of the weld shows the intermetallic zone is less than 10 microns in size. Even though the individual welds are small, the technique produces single-layer shear strengths of around 44 N and double-layer shear strengths of around 88 N. In addition to joining aluminum to steel, potential weld combinations include copper to steel, titanium to steel, copper to titanium and copper to aluminum.
Creating efficiencies
The ability to weld dissimilar metals creates a variety of efficiency advantages for EV batteries. Together, aluminum and copper, which are very conductive materials, provide ideal conditions to transfer the electrical current efficiently.
Of course, efficiency is important not only for the end user, but also on the production side. Developments in laser beam delivery combined with intelligent tooling have decreased process times.
As with any process, access to the weld area is key. Historically, this meant that the part was mounted on an XY stage and moved relative to a fixed laser beam.
Over the past several years, however, Amada Miyachi developed galvo scanning beam delivery as a quick and efficient way to guide the laser beam over the part. In a 2-D galvo scan head, two mirrors are adjusted to steer the laser beam in an XY plane. Because this system only moves small, lightweight mirrors, as opposed to the entire part, the motion can be extremely fast – process speeds can reach up to 3 m per sec. This can dramatically cut down process time.
A key challenge of this delivery system is properly tooling the parts so that each and every location of the weld is in close contact with the laser. “The fundamental tenet of welding is that you can’t weld air,” quips Boyle. As a secondary tooling concern, the laser beam must have access to the weld zone.
One solution to these challenges is to simply machine tapered holes to minimize openings but maximize laser access. But, this illustrates the type of tooling consideration required to integrate laser welding into a production process.
Despite clear benefits, there are still challenges to using laser welding in EV battery production. While resistance welding monitors play an important role in product development and quality control, the technology for similar devices to evaluate laser welds is still being researched.
“On the laser side, monitoring is in its infancy,” Boyle says. This developing technology, though, promises to push battery production efficiency for EVs even further when it becomes available. Therefore, advanced laser welding technology seems poised to continue to drive EV efficiencies in the short and long term.
