The advantages of diode lasers vs Nd: YAG lasers are making them the increasingly popular choice for multistation operation
In the automotive industry, laser joining is proving itself as a production process that can stand up to the high demands of reproducibility and minimal heat-affected zones and the special aesthetic requirements made on the seams. In addition to Nd:YAG lasers, which have already established themselves in this field, the use of diode lasers is now becoming increasingly common.
The principal reasons for this are the financial benefits in combination with the high beam quality and the reliability of modern diode laser systems as seen, for example, in brazing and heat conduction welding in the mass production applications of the automotive industry.
A comparison of concepts
In relation to an Nd:YAG laser of the same output power, the diode laser requires only one-fifth of the space. Initially this may seem to be an insignificant difference. However, it has led to a fundamentally different machine concept, particularly for multiple station operation, as the diode laser is on rollers, is mobile, and therefore can be used wherever required.
Typically, Nd:YAG lasers are centrally located in a laser pool, from where they can be employed for various processing stations with the help of long laser light cables (in some cases >50 m) and complex beam switching systems. Both redundant lasers and lengths of laser light cable are required and are linked together by control systems to ensure that, should a component fail, laser power is still provided to all tasks and laser safety is maintained at all times. Tubing containing numerous fibers is laid from the laser pool to each of the processing stations and fibers have to be disconnected at the processing head every time the principal laser is replaced by a back-up laser. However, dust pollution is often particularly high in the processing cell, which increases the risk of contamination of the fiber ends. This may prevent the fibers from working after reconnection. The costs of long laser light cables and of laying them-initially and each time they have to be exchanged-are considerable. The maximum distance/number of possible stations is restricted by the maximum length of fiber available (see Figure 1).
The production process is optimized for this concept and often has to be modified to obtain the shortest possible distance from the lasers to the processing stations. It is often necessary to place Nd:YAG lasers on a platform above the processing stations to save space and reduce the length of the fibers. This, too, entails high expenditure in initial investment and subsequent maintenance costs.
Moreover, any subsequent addition of lasers or other processing stations becomes problematic. If lasers were required in different parts of the building, more such pools would have to be set up, each with its own back-up laser system.
In contrast, the compactness of the diode laser makes a fundamentally different and far simpler approach possible, which is based on the use of individual mobile lasers on rollers. This means that if required elsewhere, the diode laser can be moved to replace another laser at short notice by simply disconnecting the fibers on the laser end (as opposed to disconnecting fibers in the processing cell). This machine concept uses shorter and therefore less-expensive laser light cables (typical length 10-15 m) and none of the fibers need to be laid twice. In addition the beam switches used for multiple station operation are less complex and more reliable, so that a super-ordinate control system with which all lasers and fibers are monitored is not required.
The individual lasers can be separated spatially and used flexibly, as a single mobile laser system is enough to compensate should a laser fail to function, even if the production facilities extend over several buildings or over different parts of a building. Moreover, neither the number nor the distance to the individual stations is limited by the maximum length of fiber, as is otherwise the case. The manufacturing process that is optimized for production does not need to be modified for the laser system redundancy plan (see Figure 2).
Considering the economic advantages of diode lasers in terms of investment and operating costs, it may also be worthwhile investing in additional beam sources for multiple station operation, forgoing beam switches altogether. Here, too, one mobile back-up laser is sufficient for all stations. The individual stations can now operate fully independently of each other with up to 100 percent beam time and are no longer linked to each other by the cycle time of the beam switches (see Figure 3).
Investments costs for all three concepts are summarized in Figure 4. They can also be applied to multiple station operation with six or more processing cells. A comparison of the costs is shown in Figure 5. It should be noted that the complexity of a system based on Nd:YAG lasers can quickly become risky for production and should, for this reason, be avoided. By comparison, the concepts that use diode lasers are not only beneficial financially, but also provide a greater degree of flexibility and offer the potential to expand.
The lifetime of the new generation of diode lasers now attains more than 20,000-30,000 laser hours, which is equivalent to approximately five to seven years in three-shift production. By comparison, lamp-pumped Nd:YAG lasers need lamp replacements every 1000 hours or less, the consequences of which include production downtime and high operating costs.
Stack management, as normally used in production with modern fiber-coupled diode lasers, allows the diode laser to continue operating even if one of the diode lasers in a stack should unexpectedly fail. The defective component can then be replaced in the course of a standard planned service. In connection with their long lifetime, the uptime of the diode laser for production is far above 99.5 percent, which often means that replacement systems are not required.
The fibers that are used for kilowatt diode lasers are not only shorter than those used for Nd:YAG lasers but also generally have a greater diameter (from 1 to 1.5 mm in comparison with 600 µm for Nd:YAG lasers in the same application). This reduces the thermal stress on the fiber ends, as the same power is now distributed over an area that is three to six times in size. As a result the fibers are less sensitive to contamination and have an increased lifetime.
In a financial comparison, diode lasers have a significant cost advantage of more than 30 percent in comparison with Nd:YAG lasers. Furthermore, the direct diode laser is 50 percent more cost effective in comparison with conventional lasers.
Even if Nd:YAG lasers are already used in production and have been paid for in full, an investment in new diode laser technology may be economically viable. This is a result of the costs of energy, lamps, and production downtime with the lamp-pumped Nd:YAG lasers.
Successful mass production applications
The processing demands on the laser beam source for laser brazing of zinc-coated automobile sheet metals can be met equally by both types of laser (see Table). Bus interfaces such as Profibus DP or Interbus-S, 24-hour teleservice, as well as optical components used for Nd:YAG lasers also have been available for diode lasers for many years. For these reasons, an increasing number of users opt for diode lasers, especially in association with multiple station operation. Several automotive manufacturers worldwide now use diode lasers for brazing.
Besides the economic advantages for brazing with diode lasers, process improvements can be achieved on a technical level: A direct comparison of a brazing seam on automotive parts showed that the diode laser produced a particularly calm melt pool and stable brazing process because of higher temporal and spatial stability and because of a homogeneous beam profile with approximate top hat intensity distribution. The result: a smoother surface of the brazing seam.
The advantages of compact, mobile laser systems for production units in multiple station operation are not restricted to applications in the automotive industry. Diode lasers have already conquered a wealth of other applications. They have been used successfully for many years in laser plastics welding, heat conduction welding of metals and in hardening and coating/cladding applications. Rather than seeking to obtain the best possible beam quality, customers are now looking into the economic and technical benefits of diode lasers and then choosing a beam quality that is adequate for the task.
Andre Eltze is technical sales manager Europe at Laserline GmbH. Contact him at [email protected] or visit www.laserline.de.