Laser welding basics

A close-up view of a CO2 laser weld on copper

Automotive manufacturers, Tier 1 auto suppliers, and other high-volume manufacturers have been implementing laser welding successfully for years, but it is much less prevalent in small and medium-size businesses.

Beneficial to both part design and lean manufacturing efforts, laser welding (Figure 1) generates a fraction of the heat input of other welding methods, requires access to only one side of the part, and produces a narrow bead. Because no electrodes and usually no filler material are needed, repeatability with laser welding is very good. It is such a stable process that a high level of automation can be incorporated.

Though part fixturing for laser welding can be quite involved, the process of laser welding is actually very simple, and it can be performed on the most basic of machines, including an everyday laser cutting system.

Laser Welding Basics

Both laser cutting and welding apply a highly focused laser beam to a workpiece, and either the workpiece or the laser beam moves to create the geometry. The laser beam melts a small portion of the workpiece as it moves.

The difference between laser cutting and welding is the focal head and gas delivery method used (Figure 2). In cutting, molten material is ejected from the workpiece by a focused assist-gas jet applied from the top. In welding, the weld bead is protected from oxygen contamination with a gentle and unfocused cover gas applied from the side.

Passing a laser cutting machine over a workpiece with the assist gas set very low results in a weld bead rather than a cut. Changing the assist gas to helium or argon ( or other material-appropriate inert gas) produces a shiny weld rather than a porous, contaminated one. Finally, changing the gas application (now called cover or shield gas) so that it flows gently across the top of the part, rather than being projected down on the part, will make a cosmetically appealing weld.

Laser Welding Heads

The three types of welding heads used for most laser welding applications are a transmissive head, a parabolic head, and a remote or scanning head. A transmissive head having a removable focal lens often is used for simple, low-power applications. These heads are easy to use and configure with different focal length lenses. On the downside, a focal lens is more susceptible to damage from weld spatter and is harder to cool efficiently because it can be cooled only from the sides.

A parabolic welding head (Figure 3) is a better choice for higher-power welding because the mirror is less susceptible to damage and can be readily cleaned with a mirror polishing cream. Also, because the back of the mirror can be directly cooled with water, it is much less likely to be damaged by thermal stress or change its focal characteristics with varying laser power.

A remote welding head, or scanning head, is a sophisticated choice for high-speed, high-volume welding applications that have more difficult geometry and fixturing needs, such as automotive body panels. Because a remote welding head can position the focal point of the laser beam anywhere within its working range simply by angling its internal mirrors, it is fast and often best-suited for complex parts.

A remote welding head does have limitations. Because it requires very long focal length optics, typically 20 to 40 in., the beam will not focus as tightly, which reduces the laser's intensity and, therefore, the speed and penetration for any given laser power. For that reason, a remote welding application has more critical requirements in terms of laser power and beam quality.

Laser Welding Basics of Focal Length and Mode Quality

The factors that determine the cross section (depth versus width) of the weld joint are the laser's beam quality and the characteristics of the focusing optics.

A high-quality, or bright, laser beam provides a small, intense focus spot and a deep, narrow weld at any given laser power and processing speed. Conversely, a lower-quality beam provides a wider weld bead with more shallow penetration. (See Figure 4.)

The same relationship exists for the focusing optics: a short-focal-length optic provides a small spot for deeper and narrow welds.

At first thought it might seem that a deep weld joint is always desirable, but that's not the case. For thin parts where full penetration is easy to achieve, it may be preferable to have a wider weld joint. Also, a wider focal spot and longer depth of field can be more forgiving with regards to fixturing and alignment. When you are working with a tool that is only a few thousandths of an inch in diameter, part fit-up and tool alignment are critical.

Laser Types

Each type of laser used in welding applications has positives and negatives and different cost structures. Some applications can be performed by any laser; some are so complex or specialized that only one type of laser is appropriate. The most common types today are CO₂, Nd:YAG, fiber, and disk lasers.

The oldest tool on the bench, CO₂ lasers generate a laser beam by applying electrical energy to an enclosed mixture of gases, which stimulates the CO₂ molecule to give off photons of light, or heat. They offer high beam quality at a good dollar-per-watt ratio, can exceed 99 percent uptime, and are relatively eye-safe.

Nd:YAG lasers are solid-state lasers that pump a high-intensity light source (flash lamps or diodes) into a specially coated crystal rod, which causes the rod to generate a laser beam. Their beam quality is not as good as a CO₂'s; however, they have few if any moving parts and can be transmitted through a flexible fiber optic.

Disk lasers, a variant of the Nd:YAG, pump diode light into a disk or wafer rather than a rod, which produces a better beam quality than a typical Nd:YAG rod laser.

Fiber lasers are a new type of solid-state laser that generates the laser beam by pumping diode light directly into a fiber optic, causing a laser beam to be generated right inside the fiber optic. As with a disk laser, fiber lasers offer excellent beam quality, fiber-optic capability, and no moving parts. Their modular design adds easy scalability.

While each of these lasers has its pros and cons, all are modern and reliable manufacturing tools.

Laser Welding Basics Costs

Something to consider before purchasing any piece of equipment is how much it costs to operate. For a specific application, you would perform a cost-per-part calculation; what follows is a more general cost determination.

Calculating the costs of laser welding at 100 percent green-light time would be very straightforward, but not realistic, given that even a highly automated system has some unproductive load and unload time, and of course a manually loaded or simple automated system has more. In the following examples, costs are calculated at 70 percent beam-on time.

A 2.5-kW laser with chiller costs $3.48 per hour to run at 70 percent beam-on time, inclusive of all electricity, gas, and long-term maintenance costs. Welding consumables for this type of laser include shield gas and focal lenses and mirrors. Using helium as a cover gas typically costs $2 to $2.50 per hour, and the external optics should not cost more than $0.25 per hour for an estimated total cost of $5.98 per hour.

A 5-kW laser with chiller costs $6.42 per hour to run at 70 percent beam-on time; the consumables are similar to the preceding example, so the estimated cost is $8.92 per hour. Calculating the cost for an inch of weld yields such a small number that the formula has been adjusted in Table 1 to show the cost for 1,000 linear inches of weld.

The data listed under "Hourly Operating Costs" shows that the return on investment for a more powerful laser is actually negative for thinner material and gets better as you move to thicker materials. But if you include the costs for capital equipment, floor space, overhead, and labor—that data to the right—things get more interesting.