Welding is often used to join metal components during the fabrication of medical devices, instruments, and implants. Traditionally, the dominant welding technology has been gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding. However, GTAW can produce some undesirable results, particularly when welding thin metals or when shallow weld penetration is required (e.g., < 1 mm). In many cases, laser welding offers an attractive alternative that can be employed to cut production costs and improve results. This article compares and contrasts the capabilities of laser welding versus GTAW for medical device fabrication. It is intended to provide product development engineers some specific guidance on when and how to utilize the technology to save money and optimize results during production.
Laser Welding versus GTAW
In GTAW, two pieces of metal are first brought into contact or close physical proximity. A high voltage is established between a tungsten electrode and the workpieces, creating an arc that melts the materials. This melted material flows together and then subsequently resolidifies to form the weld seam. GTAW can use filler material to close any gap between the workpieces, or it can be performed autogenously (without filler). GTAW also uses argon shield gas to prevent oxidation of the electrode and the melted material.
In laser beam welding (LBW), a focused, high-intensity laser spot is used to melt the material (instead of an electrical arc). Just as with GTAW, filler material is sometimes used, and an inert shield gas is often present. However, there are actually two distinct regimes of LBW, namely, conduction and keyhole welding.
In keyhole welding, the laser is focused so as to achieve a very high power density (typically at least 1 megawatt/cm²) at the work piece. At the center of the focused beam (where the laser power density is usually highest), the metal actually vaporizes, opening up a blind hole (the keyhole) into the molten metal pool. This enables thicker materials to be pieced and makes keyhole mode welding most useful for deep penetration welds where high aspect ratios are desirable.
In conduction welding, lower laser intensity is used. This intensity isn’t sufficient to initiate the keyhole, and only surface melting occurs. Heat transfer into the bulk material then occurs by conduction. As a result, conduction mode welds are typically shallow (0.005 in. to 0.020 in. in depth) and are free from porosity or cracking. Conduction-mode LBW also leaves the surface cosmetically clean and free of weld splatter and doesn’t produce significant dimensional distortions into the part.
Reduced Heat Input
The primary advantage of LBW, especially in conduction mode, is that it produces substantially less heat input into the part than GTAW. And, this smaller heat affected zone (HAZ) proves particularly advantageous in the fabrication of medical devices, where device miniaturization is an overarching trend that dictates tighter process control requirements.
The smaller HAZ delivers several important benefits. First, it allows welding to be performed on an essentially finished part without producing significant cosmetic, functional, or dimensional changes to the part. Thus, any engraving, coating, or other surface treatments already on the part are not affected. Similarly, other delicate parts, such as springs, magnets, or plastic components (handles, etc.) aren’t damaged or altered during the welding process.
The smaller HAZ also eliminates the pre- and post-processing steps that must often accompany GTAW. Specifically, the more-pronounced thermal cycling of GTAW can require preheating as well as a post-welding annealing cycle to minimize or correct for thermally induced stress or warpage in a welded device.
GTAW can also produce discoloration, which may have to be removed using some form of post processing. This generally doesn’t occur with LBW. Furthermore, LBW produces a small, straight, and consistent weld seam that is cosmetically attractive. This largely eliminates the need to subsequently grind down the weld bead, which is, again, a common requirement for GTAW. The accompanying photos allow a visual comparison between GTAW and LBW for the types of welds often required on medical devices.
Above: Figure 1. A 0.125-in. GTAW fillet weld using ER630 filler metal on an implant holder showing some discoloration due to the applied heat. However, GTAW is used here because these parts tend to be thicker and therefore require a deeper penetration weld.
Above: Figure 2. A 0.200-in. GTAW fillet weld using ER630 filler metal on an impact stand showing substantial discoloration. Weld joint around a post can be difficult to produce, and welding often requires going through multiple rotations to prevent overheating and distorting the part. GTAW is used here because of part thickness.
Above: Figure 3. A 1-mm LBW fillet weld on a funnel using 308L filler metal. This is a smaller weld joint compared with what GTAW would produce, and there is essentially no heat affected zone, no discoloration, and no distortion. Parts like this are typically thin walled, which would be difficult to GTAW weld.
Above: Figure 4. This is an autogenous LBW square groove weld on 17-4 stainless steel material funnel. The weld has no discoloration, and little to no cleanup was required. Many parts can be welded sub-flush to eliminate the need for buffing after welding. Joints like this one are good for sealing parts together with the added benefit of not needing subsequent cleanup of material.
To summarize, the ability of LBW to weld a virtually finished part without introducing dimensional or cosmetic changes lowers production costs and increases design flexibility. Specifically, it eliminates pre- and post-processing steps, which all involve cost (and time), and it reduces scrap.
Welding Dissimilar Alloys
Another key feature of LBW is that it is easier to weld dissimilar alloys of a single material together with it than by using GTAW. Specifically, this means welding together various alloys of stainless steel (e.g., 420 to 17-4 or 304, etc.), aluminum, or titanium, but it cannot weld together different metals, such as steel to aluminum or steel to titanium. The traditional challenge in GTAW with dissimilar alloys is that differences in hardness and chemical composition between the materials mean that the weld does not solidify uniformly. This can cause cracking or porosity in the weld seam.
With GTAW, welding dissimilar alloys is usually accomplished using filler materials or pre- or post-heating steps to control the differential thermal expansion and contraction experienced during welding. Again, these measures all have costs associated with them.
In contrast, LBW can usually produce crack-free welds in dissimilar alloys using simpler and therefore less costly to implement methods. One of these is controlling the beam shape, and therefore the precise spatial extent of the applied heat, during the welding process. Another useful approach is to control the temporal characteristics of the delivered laser pulse.
For example, when welding a highly reflective material, such as aluminum or copper, it’s typical to use a pulse shape that starts with a high peak power (to pierce the material and initiate the welding process) and then to drop the power down once melting occurs (so as not to couple in too much power). But, with materials prone to cracking, it might be more advantageous to start at a lower power and then subsequently ramp it up—essentially preheating the material with each pulse. This pulse-shaping capability isn’t universally available on laser welding equipment. But when it is an available capability, it provides a powerful method for optimizing weld quality in what might otherwise be difficult-to-join materials. Thus, LBW, especially with pulse shaping, provides the medical product designer with another degree of freedom in terms of utilizing multiple materials in a product.
Above: Figure 5. Some laser beam welding systems allow laser pulse shape to be adjusted in order to optimize weld results. This shows the selection screen for various pre-programmed pulse shapes in the Coherent Select Laser Welding Workstation.
Above: Figure 6. The Coherent Select Laser Welding Workstation control software allows customized pulse shapes to be defined.
While there are clear technical advantages to LBW, traditionally it has only been available in the form of fully automated solutions, which typically carried a high price tag. This made them appropriate only for higher-volume production. However, the introduction of manual, or partially automated, LBW systems over the past few years has now made this technology accessible and cost competitive with GTAW for the single-piece or low-quantity production environments often found in medical device manufacturing.
In these types of systems, the operator actually holds the workpiece by hand and manually moves it under the laser beam. This way, even complex contours can be easily laser welded, which otherwise would require a sophisticated and expensive motion control system. To ensure operator safety from laser radiation, these systems are configured with a laser-safe enclosure, so only the operator’s hands are in the area where laser exposure occurs. The operator then observes and controls the welding process by looking through a microscope.
Some LBW products also allow part fixturing and limited, motorized movement, if higher precision or some level of automation is required. For example, the part might be fixtured on a motorized mount and then moved under the laser beam using joystick control. Typically, workpiece motion and laser pulse repetition rate are synchronized so that results are consistent regardless of the speed at which the operator moves the workpiece. For example, a low pulse-repetition rate is used when the piece is moved slowly, and a high repetition rate is employed when the workpiece is moved fast. This ensures a constant overlap of the laser pulses, regardless of the motion speed, delivering a consistent weld seam independent of operator and specific operating conditions.
The most advanced products of this type combine this type of laser pulse synchronization with the ability to record the welding path (first “taught” to the system by manual input from an operator). This allows a weld path to be run without operator attention and to be repeated with high accuracy and repeatability in small batches.
This type of limited automation is much more difficult to implement with GTAW, because the process is inherently less repeatable than LBW. While this doesn’t change the type or characteristics of welds that can be produced, it does directly impact production costs. And, this is particularly true of medical device production, where lot sizes (at least between engineering revisions) are quite small—usually in the 20 to 30 range. Thus, it doesn’t pay to automate a production process under these circumstances unless that can be done easily.
Summary of Recommendations
Using a pulsed power laser for conduction-mode LBW generally offers better results for welding thin materials (<0.040 in.) or shallow penetration welds, and it is cost competitive with GTAW. Conversely, GTAW is still faster and therefore more economical for welding thicker materials that require filler metal. Determining the optimum welding approach for a specific task should always involve dialog with the fabricator, but for the engineer, the key takeaways to note are:
- LBW facilitates joining of dissimilar alloys.
- It’s possible to join different metals with LBW, but this is more difficult and requires more process development.
- LBW enables welding close to other heat-sensitive areas and components on a part.
- Some LBW systems support short-run automation, which can save money, depending upon batch size.
LBW can reduce or eliminate the need for ancillary processes and thus lower production costs as compared with other welding methods. Thus, it is well worth it for engineers to understand the characteristics of LBW so that they can take maximum advantage of its potential during the design process.
For more details, visit Coherent Inc. at Booth #2819 at MD&M West February 11-13, 2020.