Automotive electronic laser welding

Automotive electronic laser welding

From the print edition: Nuburu’s blue-laser technology enhances speed, quality, flexibility in copper-joint welds.

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In 2017, more than 1 million electric vehicles were bought worldwide. According to market research firm McKinsey & Co., the market – led by China – will continue to grow at impressive rates. An electric vehicle (EV) is built around electrical components: motors, batteries, and electrical assemblies. But even traditional vehicles are becoming increasingly electronic with ubiquitous sensors and actuators. While those electronics bring great capabilities, they also present a new array of challenges. Specifically, each component and each element within those components must be joined with high mechanical and electrical fidelity. Traditional methods have drawbacks – poor quality, unnecessarily large joints, or slow processing speeds.

In late 2018, Nuburu introduced their second product, the 500W AO-500 industrial blue laser. Blue lasers weld copper joints with flexibility, speed, and quality, making them attractive for enhancing productivity for future automobile production.

Lasers have become indispensable in automotive manufacturing because of their flexible energy delivery and ability to easily integrate with automation. That’s why they are replacing resistance welding, ultrasonic welding, and other conventional processes. But traditional industrial lasers operate at infrared wavelengths that are poorly absorbed by yellow metals, primarily copper. Copper absorbs blue light much better, leading to advantages in welding performance. That physical advantage, combined with advancing design requirements for batteries and motors, makes blue the optimum choice to enhance the fabrication of these essential components.

 Blue physics, technology

Copper absorbs less than 5% of the infrared radiation delivered by traditional industrial lasers, and poor absorption leads to other specific processing problems. In copper welding, poor absorption means excess energy must be delivered to the melt pool to initiate welding. But the melt pool absorbs more infrared energy than the base metal, and the excess energy causes the metal to vaporize, forming bubbles in the melt pool that leave voids and spatter material from the joint.

Infrared (IR) welding systems have implemented workarounds, including complex irradiation patterns, to minimize defects. Techniques such as the spiral wobbling pattern minimize but do not eliminate spatter and voids. Marginal performance improvements come at a cost – increased process time and processing equipment. Infrared laser welding of copper and similar metals becomes a balancing act, delivering enough energy to initiate a weld, but not so much as to introduce excessive voids and spatter. This leads to a narrow process window, or in some cases no possible process window at all, and it limits yields.

Copper absorbs blue wavelengths more than 10x more efficiently than infrared, and the energy required to maintain a weld is essentially the same as the energy required to initiate. Figure 1 (page 23) displays a frame from a video, showing the uniformity of the melt pool and the lack of vaporization within the weld. This consistency allows process control and leads to rapid, high-quality copper welds free of voids and spatter.

The physical advantages of blue wavelengths are well known, but engineering a high-power blue laser has been a technical challenge. In 2017, Nuburu introduced the AO-150 high-power, direct-emission blue laser, and the AO-500 follows the same design path. AO series lasers combine the output of dozens of individual gallium nitride (GaN) diode lasers into a single beam and couples that beam into an optical fiber. In the AO-500, micro- and macro-optics combine the individual diode beams into a 400µm-core optical fiber. The output of the fiber is a highly symmetric 500W beam of 450nm light, offering unmatched brightness.

After wavelength and output power, brightness is the next most important parameter for materials processing applications. The energy density delivered to a workpiece determines the effectiveness of a process. For some applications, the output beam is conditioned with relay optics, but the optical efficiency is still limited by the initial beam parameter product: higher brightness means more effective energy delivery.

The wide process window and laser stability make blue-laser welding a highly deterministic process – able to produce void-free, spatter-free welds in three different welding modes: conduction, transition, and keyhole modes.

 Dense, compact, lightweight assemblies

The efficiency of electrical components generally depends on their density. The number of turns in a solenoid or a motor coil, for example, drives the coupling efficiency. The energy storage capacity of EV lithium-ion batteries increases with the surface area of the electrodes that are made from layers of thin foils. The more layers, the greater the energy storage capacity.

So, increasing EV system performance often means putting more copper components into smaller areas. For the welding process, that means voids and spatter have more severe effects. Also, more copper elements require more joints, increasing production time and cost. Higher density and smaller sizes mean there is no tolerance for poor joint quality. In small joints, it doesn’t take many voids to quickly degrade joint conductivity. Dense components also require a tight tolerance for spatter, as spattered particles can produce shorts between closely spaced elements. It’s difficult to minimize voids and spatter with IR welding, and almost impossible to eliminate them entirely. Manufacturers have used ultrasonic welding to join copper foils in batteries, but particulate contamination is nearly unavoidable here as well.

High absorption for blue wavelengths means efficient energy transfer from the incident laser light to heat in the absorbing material, leading to faster processing. A connector for an automotive light detection and ranging (LiDAR) system, for example, must join copper and silver-plated copper, presenting the dissimilar metals joining challenge. A 150W blue laser can produce a defect-free, 200µm-wide weld in a few tens of milliseconds. An IR laser presented with the same challenge would need to illuminate the workpiece in a wobbling spiral pattern to minimize voids and spatter, creating a 500µm-wide joint produced in 400ms or 500ms. And, the joint would still exhibit voids and spatter.

Ultrasonic welding requires physical contact of the material and ultrasonic welding tool, generating unwelcome particles, limiting the minimum thickness of the individual copper elements, and requiring frequent adjustment that shuts down production.

 Automotive applications value

High-brightness, high-power blue industrial lasers have already demonstrated significant qualitative and quantitative advantages compared to alternative copper processing techniques, translating directly into automotive applications. For example, some manufacturers are replacing coil-wound electric motors with bar windings that offer efficiency and cooling advantages. However, with multiple hairpin joints required for every motor, bar windings require more welding. Joining methods, such as mechanically twisting the hairpin elements, TIG welding, or welding with an infrared laser, have inherent disadvantages. Mechanical joining requires extra material to allow a margin to twist the elements, adding weight. The wide TIG torch delivers excess heat outside of the weld, introducing undesirable thermal damage. The image at the top right of the page shows a hairpin joint for a small electric motor produced with a blue laser. Weld dimensions illustrate the fine process control possible with blue.

EV production growth is expected to continue. Considering the simultaneous growth in sensor and actuator systems for automotive applications, it’s clear that rapid production of high-quality copper joints will be increasingly important for the industry. Blue laser technology offers performance advantages compared to traditional materials processing methods and has proven reliable in battery fabrication, e-mobility, and electronics packaging.

 Nuburu Inc.

http://www.nuburu.net

 About the authors: Jean-Michel Pelaprat is co-founder and chief marketing and strategy officer at Nuburu Inc., Matthew Finuf is applications group director, Robert Fritz is an applications engineer, and Mark Zediker is co-founder and CEO. They can be reached at jmp@nuburu.net.

From the April 2019 issue of Today's Motor Vehicles.