By Stan Ream
Edison Welding Institute
Editor's Note: As stated in the February My View column, ILS is taking a one-on-one approach to raising the awareness of the benefits, technical and economical, of laser welding. To start this campaign rolling, I have asked Stan Ream, a laser welding expert, to answer the question of why the limited acceptance of laser welding by industry. Stan is uniquely positioned to comment on this subject by education, background, and years of experience in this field. ILS welcomes your comments on this piece.
When manufacturing engineers think about lasers, there's more than a 10 to 1 chance that they're thinking about laser drilling, cutting, or marking. Actually, that ratio is probably much higher now, considering the relatively explosive growth in marking lasers in recent years. Still, no matter how we make the comparison, the number of laser welding applications in the world today falls way behind the number of these other applications. There are many reasons for this, and understanding the reasons may help open the doors to new opportunities in laser welding.
The first, most obvious, reason for this apparently low laser welding utilization is that many more things in the manufacturing world require material removal (cutting, drilling, machining, etc.) than materials joining. Among the millions of manufactured component parts only a small fraction of them require an actual welding operation. Most component parts are bolted, screwed, pressed, riveted, molded, adhesively bonded, interlocked, or otherwise assembled into a completed product.
For the remaining parts that do require welding the laser must compete with more entrenched, well-known processes. And, as long as we're being honest, we must add that laser welding will almost certainly require a more expensive equipment choice, at least from the standpoint of initial capital cost. Why would anyone choose a more expensive solution? There are at least three answers to this question: 1) the welding requirements cannot be met any other way, 2) the net manufacturing cost of laser welding is actually lower per part than other joining processes, or 3) a visionary takes a leap of faith or simply seeks a "cool" factor. Let's consider each of these scenarios.
There may be very few materials joining challenges that only have one solution, but they can be noteworthy. One classic example is an implantable medical device (e.g. pacemaker, defibrillator, pain suppressor, etc.) (see FIGURE 1). These devices contain batteries and sensitive electronics that must be hermetically encapsulated in a rugged, light-weight, biocompatible (usually titanium) case. The small and sensitive internal electronics rule out electrically based welding processes (TIG, plasma, electron beam), and adhesives do not offer the longevity that the titanium case material affords. Laser welding of these titanium cases provides a smooth, low heat input, hermetically acceptable, and reliably producible weld joint. Making this application even easier for laser welding to tackle is the fact that the parts being welded are quite expensive themselves. Thus, this application is a clear win-win situation for laser welding, and essentially all these devices are laser welded.
FIGURE 1. Laser welding of titanium pacemaker cases provides a smooth, low heat input, hermetically acceptable, and reliably producible weld joint. (photo courtesy: Miyachi Unitek)
On the other end of the manufacturing spectrum are high volume parts with a low threshold for added manufacturing costs. The industry that typifies this condition (especially lately) is the auto industry. Here every added cost falls under considerable scrutiny. Cost justifications in this environment are complex. But before the cost questions are even considered, the candidate laser weld must be shown to perform as well as or better than the competing welding technology. Providing proof of this is often a complicated, time-consuming, and sometimes expensive process. And lately, as auto companies deplete their ranks of experienced welding application engineers, just defining the welding challenges can be problematic.
Nevertheless, despite high process development hurdles, there are dozens of laser welding applications in modern automotive manufacturing. Further, it is reasonable to conclude that all of these automotive laser welding applications have been chosen because they represent the net lower cost solution. A very successful example of this net lower cost situation is illustrated in the tailored blank welding application, which migrated from Germany in the early 1990s. [Yes, for you purists, there was a much earlier, similar case in the U.S., but it didn't take hold at that time.] The laser blank welding application (see FIGURE 2) produces multi-gauge sheet metal blanks for many millions of automotive body-in-white and hang-on parts today. Laser welded blanks eliminate other components and operations, reduce vehicle weight, improve fit, and enhance crash-worthiness. The net result of all these benefits was able to be expressed in lower costs, when viewed collectively. Yet, despite its resultant broad and long-lasting success the initial application justification was a steep, up-hill battle to overcome the industry's reluctance to accept a higher priced, single component. Even today the auto industry tendency to look narrowly at component costs threatens to undermine this successful application.
FIGURE 2. The laser blank welding application produces multi-gauge sheet metal blanks for many millions of automotive body-in-white and hang-on parts today.
In contrast to the above process justification situations there are occasional instances of visionary implementation. Unquestionably the most expansive (and expensive) example of visionary laser welding implementation is the body-in-white laser welding application at Volkswagen (see FIGURE 3). Based on developments starting in the early 1990s VW made a massive commitment to build laser-based, body-in-white assembly plants around the world. Today they operate hundreds of YAG and disk lasers in this capacity. For those familiar with automotive body-in-white assembly you know that displacing resistance spot welding in most of the auto industry is essentially heresy. And yet it happened at VW, driven largely by upper-most management's vision that a laser-based, body-in-white assembly plant could be smaller, more flexible, affordable, and produce higher performance vehicles. The full extent to which this vision has been realized is certainly a VW secret, but it is clear that the endeavor has stimulated others in the automotive industry to take risks and advance their own manufacturing capabilities.
FIGURE 3. Unquestionably the most expansive (and expensive) example of visionary laser welding implementation is the body-in-white laser welding application at Volkswagen. (photo courtesy: Volkswagen).
There are hundreds of other successful laser welding stories, large and small, high volume and one-of-a-kind, well-known and secret. It's clear that laser welding is alive and well in the manufacturing world and that we are all benefiting from its use in a host of products. So, let's go back to our starting question, "Why so little laser welding?" The simple answer is that laser welding is hard to do. OK, there we've said it. But now, as we examine the reasons that make laser welding hard to do, please keep in mind that there are many hundreds of laser welding applications out there, generating many millions of dollars of revenue for those willing to do the hard work.
The first set of challenges in laser welding revolves around basic mechanical engineering issues. These mechanical challenges are simply those associated with the presentation of the weld joint to the laser welding process (or visa versa). Unlike laser drilling, cutting, marking, or heat treating, which require only that the beam be presented to one surface, laser welding requires that two, correctly prepared surfaces be presented to each other and that the laser process be accurately aligned at this location. These "simple," joint-related issues can be summarized as follows.
--Joint Preparation (edge quality, cleanliness, surface condition, etc.)
--Joint Alignment (fit-up, gap, mismatch, restraint against distortion, etc)
--Joint Positioning (relative position of the laser focal spot to the weld joint)
These three mechanical issues almost certainly account for the majority of the challenges facing potential users of laser welding, but they are not unique to laser welding. The uniqueness of these challenges in the laser industry is that the dimensions are much smaller, and the speeds are often much higher than they are in other processes. Still, they are mechanical challenges, not rocket science, and they can be managed. Rocket science or not, the most common cause of a failed production laser weld is a missed weld joint.
The next group of challenges is metallurgical. How will the metals to be welded respond to the high heating and cooling rates that are common in laser welding? Obviously, this response depends on the specific metal(s), but laser welding has been successfully applied to most of the metal alloys in welding production today. Mild steels and stainless steels are generally laser-friendly, as long as carbon content is below about 0.25% and the grade of steel is not a "free-machining" type (e.g. high sulfur). Higher carbon steels can also be welded with pre-heat, post-heat, and/or nickel filler wire. Aluminum laser welding is a far trickier proposition for reasons that are too numerous to cover here, but it can be done, and has been done. Titanium laser welding (in addition to the implantable device example above) is increasingly being developed for application to other high-value-added components in the aerospace and defense industries. Each of these important metals has its own set of challenges, but the technical and economic advantages of laser welding can and have been successfully exploited for all of them.
So, if laser welding is hard and expensive to implement, what kind of future does it have? Actually, quite a good one! Industrial lasers have clearly emerged from the clouds of unreliable complexity that haunted them in earlier years, and their costs are down. Available laser power, beam quality, and efficiencies are dramatically higher. An increasing percentage of today's young engineers embrace advanced manufacturing technologies, and they are eager to apply them. Finally, manufacturing competitiveness (especially in the U.S.) will increasingly be driven by the level of applied automation, and laser welding is fundamentally based on strong automation. With a little ingenuity and hard work the goals of better, faster, cheaper can actually be accomplished.