New lasers and materials break barriers enabling new coatings and often replacing conventional technologies
Using lasers for additive manufacturing technologies such as cladding and sintering is a relatively new technology, first introduced to industrial applications in the 1970s for cladding valves and valve seats. One of the earliest production applications was in the Japanese car industry, where cladding the valve seats of aluminum alloy engines improved their wear properties while keeping the manufacturing cost in check. The U.S. heavy equipment industry followed with a number of applications. By the late 1980s Pratt & Whitney and GE started using laser cladding for bladed disk (blisk) repair. Both equipment and powder development quickly pushed the new technology forward, to the point where it is a standard for many applications ranging from repair of automotive cutting and forming tools to adding wear surfaces for downhole oil drilling equipment. The materials added are wide ranging, with stellites widely used for mold repair, tungsten carbide containing powders for wear protection, and base metal powders to repair and generate parts.
The standard process in everybody’s mind when talking about cladding is a powder fusion welding process. Depending on the materials required and the applications, cladding might also use a wire rather than powder, or a brazing process rather than welding. At the Fraunhofer IWS in Dresden, a process for direct deposition of bronze bearing material on steel hydraulic pump cylinders was developed and is in industrial use (see FIGURE 1). Even with a cladding rate of 9.2 kg per hour the process yields consistent clad thickness and excellent metallurgical characteristics, thanks to a homogeneous rectangular beam and optimized nozzle configuration. However, most of the applications are powder welding processes, excelling over conventional processes like plasma transferred arc (PTA), mostly due to lower heat input and better control over the melt pool resulting in less part distortion and improved metallurgy results.
Cladding applications usually work as a one-step process. The laser is focused and moved over the part, while a powder feeder transports powder to a special nozzle aiming at the melting zone. The nozzle deposits the powder on the part, where it becomes immediately molten by laser heating. Many different nozzle geometries can be used, typically driven by the requirements of the application. Off-axis nozzles are an economical choice when the process is unidirectional. Newly developed off-axis nozzles allow for a small powder focus for high powder efficiency, and have a small geometry enabling access of hard-to-reach areas. If the cladding tracks have to follow 2D or 3D tracks, coaxial nozzles are the best solution. They ensure that the powder added to the melt pool is uniformly distributed, independent of the direction of the process and of the position of the cladding head. Special nozzles and cladding heads are available for applications such as ID cladding and wide track cladding.
Directional downhole drilling tools are exposed to constant wear as they come in contact with the surrounding soil. Because drilling uses the earth’s magnetic field for navigation, non-magnetic materials and processes must be used for the drill string. Tungsten carbide (WC) embedded in a nickel matrix is the material of choice for this cladding application. The layer acts much like asphalt does on roadways; a soft (nickel) matrix contains hard (WC) particles. It’s important for the wear properties of the overlay that the WC particles remain intact or suffer minimal dilution. The ability to tailor the process allows for lower heat input of laser cladding compared to conventional methods and thus enables thinner layers with better chemistry and less distortion. Several different parts in the oil strings are being laser clad: FIGURE 2 shows a Lasercarb coating being applied on a wear band, which protects the connections in a drill string. Stabilizers, positioning the oil string against the bore hole often have 3D wing-like shapes so that soil and drill fluids can pass. The “smart” components of the drill string holding the expensive sensoring devices, such as antennas, collars, and fluid displacers, are laser hardfaced. Laser technology is suitable for repair of tools and can expand the life span up to six times compared to other technologies.
Pulp and paper mills are another industry using the wear-resistant properties of tungsten carbide coatings at different stages of the process. At the beginning of the process during the shredding, the wood is reduced into small abrasive chips that are loaded with undesirable debris. The pulp is mixed with water and then, using high pressure, is pushed through a series of finer and finer screens purifying the paste. As one of the last steps, the paper paste is dried in a big auger screw called the dewatering screw. The paper paste is pushed against an outer sieve, keeping the paper in and pressing the water out. The pressure and the shape of the particles make this operation very abrasive. The ability to produce very exact coatings for the screens and sieves make the laser the most attractive and cost-effective process for this application. An example of a tungsten carbide coated screen for pulp and paper processing is shown in FIGURE 3.
Taphole drills for blast furnaces have specific requirements regarding heat resistance and wear resistance. During the smelting process, the tapholes are filled with refractory clay. When this process is completed, a taphole is drilled through the plug, releasing the liquid iron. One tapping can require two to three drill bits to complete, as the clay is abrasive and exceedingly hot as the hole gets closer to the liquid iron. With a well-tailored laser cladding process using a 4-kW diode laser, the drill bits can be improved so that only one drill is used per taphole. FeCrV15Ni7 is used as cladding powder. The resulting layer has fine grained precipitations of vanadium carbide giving the layer excellent wear properties against the abrasiveness of the clay, as well as heat resistant characteristics (see FIGURE 4).
Lasers for cladding
Traditionally, CO2 lasers have been used for cladding processes, partially because they were the highest power lasers during the time the first cladding applications were developed. Their wavelength and size however make them less and less attractive against the newer crop of lasers in the near infrared, such as Nd:YAG, disks, fibers, and direct diodes. Comparison studies show that the process efficiency of CO2 lasers is just little more than half of that of lasers that operate in the near infrared around 1 µm—translated into real life it means that a 5-kW CO2 laser can be replaced by a 3-kW diode laser.
A homogeneous beam profile, both for small and for big focus is another key consideration when choosing the laser. The Gaussian distribution that high-beam-quality lasers yield and hot spots that sometimes exist in badly aligned direct lasers can both be detrimental to the cladding process. The lasers should be used in the focus, with the right spot size, and a flat top beam. Disk and fiber lasers are used to produce very fine structures, i.e. for medical device applications, where feature size can be as small as few tens of micrometers.
For many cladding applications, diode lasers have replaced CO2 lasers as the industry workhorse. Their flat top beam profile permits tailoring the process to achieve highest quality uniform coating layers. Power levels up to 10 kW enable high deposition rates competing with conventional processes, and fiber diameters down to 300 µm allow ID cladding and turbine repair with structures in the sub-millimeter region. Using fiber-coupled diode lasers, robotic or CNC based motion systems can be used, depending on the application’s requirements. All major manufacturers of cladding nozzles have shown them to work with diode lasers, including ID cladding nozzles.
Despite being a relative newcomer, laser cladding is a mature technology, in use in many different industries around the world, and new lasers and materials break barriers enabling new coatings and often are replacing conventional technologies.
Silke Pflueger ([email protected]) is director marketing & sales for Laserline Inc., Santa Clara, CA (www.laserline-inc.com).