Today’s laser welding
- Category: Laser Welding
- Created: 2017-05-31
The latest fiber laser welding techniques for 3-D parts open up new product design possibilities
by Terry VanderWert, president, Prima Power Laserdyne
Laser welding is one of the oldest applications of industrial laser materials processing.
In the majority of early applications, lasers produced higher quality welds and allowed for greater productivity. Often times, it was at a lower cost compared to other joining processes, even though the parts being welded were originally designed based on a different joining process.
Much has changed, however, since those early days of laser welding. For starters, the classes of lasers have changed. Laser sources now have higher power, different wavelengths and a wider range of pulsing capability. Add to this the new developments in beam delivery, machine control hardware and software, process sensors and a better understanding of the laser welding process.
Laser welding involves focusing a laser beam onto the surface of the materials being joined. A key benefit is the small region affected by the process resulting from the small size of the focused laser beam.
Design engineers have learned the capability of laser welding and are designing products that take advantage of laser welding’s unique benefits. These include low heat input, narrow fusion and heat-affected zones, and excellent mechanical properties in materials previously difficult to weld using processes that yield greater heat input to the part.
The results are welds that are stronger and cosmetically more attractive. And laser welding requires far less setup time and can be automated, lowering product cost.
All of this has further expanded the capability of laser welding for new product designs, especially in the last few years.
Inspiring product design
Previous generations of laser sources, including rod-based Nd:YAG (pulsed and continuous wave) and CO2 lasers, were limited in their welding applications because of their wavelength, low average power or limited response to high-speed controls.
All of this changed with the advent of high-power continuous wave and quasi-continuous wave fiber lasers with a near infrared wavelength and ability to control with high speed and high resolution. Fiber laser system manufacturers, thus, have developed new hardware that makes new product applications possible.
Another development is the multipurpose fiber laser system. Today’s laser machines are more flexible than those of the past. It is now common for these systems to perform multiple laser operations, including cutting, welding, drilling and marking, using one machine on a single part or family of parts.
Because process control is more precise, the range of process parameters is wider. The additional control provides even greater capability to laser weld dissimilar materials.
Autogenous welding: Materials are joined without the addition of extra materials, which require the highest level of fixturing and joint preparation. Because no material is added, it is necessary for the materials to be welded to remain in intimate contact during the welding process. Any significant separation of the materials can result in an unacceptable weld profile or complete failure of the welded joint.
Fixturing to ensure consistent fitup of the weld joint is a key to successful autogenous welding. An important benefit is welded joints with exceptional cosmetic appearance. In some cases, these welds are almost perfectly blended with the surrounding material. Depending on the fixturing and joint fitup, some welds may have small amounts of concavity (which may not be acceptable for product designs that require fatigue properties similar to those of the base material) or convexity.
Additive welding: Material is added to the weld joint usually in the form of metallic wire or powder. Three reasons for adding material to the weld are:
- Joint fitup: By adding extra material, the joint becomes more tolerant to joint mismatch. Acceptable welds may be produced from joints with less-than-perfect fitup.
- Weld geometry: Addition of filler metal is used to control the shape and size of the weld. Maintaining a crown (convex surface of the weld) creates a reinforcement that is important for joints requiring mechanical strength and fatigue life in the overall product’s design performance.
- Dissimilar metals: Filler metal is added to facilitate welding of dissimilar metals and alloys that are otherwise metallurgically incompatible.
Addition of wire or powder to the weld joint creates extra control variables. There are product applications where differences in the metal microstructure are considerable.
Therefore, careful evaluation is needed before choosing the weld class. An example is 300 series stainless steel that requires lower heat input to reduce distortion of weld joints. This makes fiber laser welding the process of choice for welding thin metals, such as stainless steel.
In other applications, the welding process requires the addition of filler metal to control the microstructure of the welded joint. Specifically, welds of dissimilar metals or alloy combinations that are prone to cracking due to formation of brittle intermetallic compounds can be made weldable. This is accomplished by adding an alloy that produces a weld metal composition having better mechanical properties.
There are successful applications of fiber laser welding in many industries using different metals, component shapes, sizes and volume. Industry examples follow.
The increased application for lithium batteries in electric cars and many electronic devices means fiber laser welding is used in the product design. Components carrying electric current produced from copper or aluminum alloys join terminals using fiber laser welding to connect a series of cells in the battery.
Aluminum alloys, typically 3000 series, and pure copper are laser welded to create electrical contact to positive and negative battery terminals. The full range of materials and material combinations used in batteries that are candidates for the new fiber laser welding processes include those shown in Figure 1.
Overlap, butt and fillet-welded joints make the various connections within the battery. Laser welding of tab material to negative and positive terminals creates the pack’s electrical contact. The final cell-assembly welding step, seam sealing of the aluminum cans, creates a barrier for the internal electrolyte.
Because the battery is expected to operate reliably for 10 or more years, these laser welds are consistently high quality. The bottom line: With the correct fiber laser welding equipment and process, laser welding is proven to consistently produce high-quality welds in 3000 series aluminum alloys that have connections within dissimilar metal joints.
Precision machined welding
Seals used in ships and chemical refineries and for pharmaceutical manufacturing were originally TIG welded. Because of their use in sensitive environments, these components are precision machined and ground from high-temperature and chemical-resistant nickel-based alloy material. Lot sizes are usually small and the number of setups is many.
The assembly of these components has been improved using fiber laser welding. Justification to replace the earlier robotic arc welding process with fiber laser welding using a four-axis cartesian coordinate machine tool includes consistently higher quality of the laser welds, ease of changeover from one component configuration to another reducing setup time, and improved throughput and decreased assembly costs by automating the laser welding process using a four-axis CNC laser machine.
Hermetic sealing electronics in medical devices, such as pacemakers and other electronic products has made fiber laser welding the process of choice for applications requiring the highest reliability (see Figure 2). A recent advance in the hermetic welding process has addressed concerns about laser welding and the end point of the weld, a critical location point in completing the hermetic seal.
Previous laser welding techniques resulted in a depression at the end point when the laser beam is turned off, even when ramping down the laser power. Advanced control of the laser beam eliminates the depression in thin and deep-penetration welds. The result is consistent geometry and lack of porosity at the end point with improved cosmetic appearance and more reliable hermeticity.
Fiber laser welding high-temperature, nickel-based aerospace components ensures a consistent and robust joint. Welds that meet the geometry and quality requirements for aerospace applications are readily produced.
Fiber laser welding nickel and titanium-based aerospace alloys requires control of the weld geometry and weld microstructure, including minimizing porosity and controlling grain size. In many aerospace applications, the fatigue properties of the weld are critical design criteria. For this reason, design engineers nearly always specify that the weld surfaces be convex, or slightly crowned, to create a reinforcement of the weld.
To achieve this, a 1.2-mm-dia. filler wire is used in the automated process. Addition of the filler wire to a butt joint leads to a consistent crown on the top and bottom weld bead. The selection of the alloy of the wire also contributes to the weld’s mechanical properties by ensuring a sound microstructure of the weld.
Dissimilar metal welding
The ability to create products using different metals and alloys greatly increases design and production flexibility. Optimizing properties, such as corrosion, wear and heat resistance, of the finished product while managing its cost, is a common motivation for dissimilar metal welding.
Joining stainless steel and zinc-coated (galvanized) steel is one example. Because of their excellent corrosion resistance, both 304 stainless steel and zinc-coated carbon steel have found widespread use in applications as diverse as kitchen appliances and aeronautical components.
The process presents some special challenges, particularly because the zinc coating can present serious problems with weld porosity. During the welding process, the energy that melts steel and stainless steel vaporizes the zinc at approximately 900 degrees C, which is significantly lower than the melting point of the stainless steel.
The low boiling (vaporization) point of zinc causes a vapor to form during the keyhole welding process. In seeking to escape the molten metal, the zinc vapor may become trapped in the solidifying weld pool resulting in excessive weld porosity. In some cases, the zinc vapor escapes as the metal is solidifying, creating blowholes or roughness of the weld surface.
With proper joint design and selection of laser process parameters, cosmetic and mechanically sound welds are readily produced. The top and bottom surfaces of an overlap weld of 0.6-mm-thick 304 stainless steel and 0.5-mm-thick zinc-coated steel exhibit no cracking, porosity or blowholes (see Figure 3).
Solutions in the pipeline
Given the many applications over the years, laser welding should no longer be considered nontraditional. With fiber laser welding, new product applications are everywhere – in electronic packages, medical devices, automobiles, aircraft, and process equipment and sensors. The list is almost endless. Most of the earlier limitations of laser welding no longer exist or are easily overcome.
While fiber laser welding may at first be intimidating, it has been repeatedly demonstrated to enable new product designs with significant improvements in cost, quality and performance. Laser system suppliers, with available applications engineering staff, now provide turnkey solutions. That includes not only the machine but also fixturing and easily learned processing techniques.