Laser welding is one of the oldest applications of industrial laser materials processing. In the majority of early applications, laser welding produced welds with higher quality, greater productivity, and/or at lower cost than other joining processes even though the parts being welded had been originally designed based on a different joining process.
Much has changed since the early days of laser welding. The classes of lasers have changed. Laser sources now have higher power, different wavelengths, and a wider range of pulsing capability. Add to this new developments in beam delivery, machine control hardware and software, process sensors and a better understanding of the laser welding process.
As important, 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. The processes to achieve them require far less setup time and can be automated. The result is lower product cost. All of this has further expanded the capability of laser welding for new product designs especially in the last few years.
Fiber Lasers Spur New Product Designs
Previous generations of laser sources, including rod-based Nd:YAG (pulsed and continuous wave) and CO2 lasers, were limited in their welding application either 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 (CW) and quasi-continuous wave (QCW) fiber lasers with a near infrared wavelength and ability to control with high speed and high resolution. Fiber laser systems manufacturers have developed new hardware that make 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.
Two Classes Of Fiber Laser Welding For Different Product Design Needs
Autogenous Welding: Materials are joined without the addition of extra materials which requires the highest level of fixturing and joint preparation. Since 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 fit-up of the weld joint is a key to successful fiber laser 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 fit-up, 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 fit-up: By adding extra material, the joint becomes more tolerant to joint mismatch. Acceptable welds may be produced from joints with less than perfect fit-up.
- 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 which 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 which are otherwise metallurgically incompatible.
Addition of wire or powder to the weld joint creates extra control variables. There are product applications where differences in metal microstructure is considerable. So careful evaluation is needed before choosing the weld class. An example is 300 series stainless steels which require 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 addition of filler metal to control the microstructure of the weld joint. Specifically, welds of dissimilar metal 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.
Products Designed Using Fiber Laser Welding
There are successful applications of laser welding in many industries using different metals, component shapes, sizes, and volume. Some important industry examples are listed below.
The increased application for lithium batteries in electric cars and many electronic devices now utilize fiber laser welding 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 which are candidates for the new fiber laser welding processes include those shown below.
Overlap, butt and fillet-welded joints make the various connections within the battery. 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.
Since 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 Component 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 are 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 were: (1) consistently higher quality of the laser welds, (2) ease of changeover from one component configuration to another that reduced setup time, (3) automating the laser welding process using a four-axis CNC laser machine improved throughput while decreasing assembly costs.
Hermetic Welding of Electronic Packages
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. 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 both 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.
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 a critical design criteria. For this reason, designers 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 diameter filler wire is used in the automated process. Addition of the filler wire to a butt joint leads to a consistent crown on both 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 of Sheet Metal Components
The ability to create products using different metals and alloys greatly increases both 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 a 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 since the zinc coating can present serious problems with weld porosity. During the welding process, the energy that melts steel and stainless steel will vaporize the zinc at approximately 900⁰C, 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 will escape 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. As shown below, 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.
Fiber Laser Welding Solutions Are Everywhere With More Coming
Laser welding shouldn’t be considered ‘nontraditional’ given the many applications over many years. Now with fiber laser welding, new product applications are everywhere – in electronic packages, medical devices, the vehicles we drive, the aircraft in which we fly, in 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.
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