Diode Lasers for Laser Cladding: status quo – quo vadis
Fiber coupled high power diode lasers have established themselves in the industry as the ideal tool for most cladding applications during the last years. Their compact and robust design as well as their high power density offer a high flexibility in conjunction with lowest cost. The high wall-plug efficiency of diode lasers also allows the conservation resources and therefore not only providing economical but also ecological advantages for material processing.
Laser Cladding Basics
Cladding is a versatile welding process, which is suitable for modifying the surface of a component locally or completely in order to be able to generate functional surfaces with tailored properties. Some examples for cladding applications are the deposition of corrosion and wear resistant layers or to generate local non-magnetic regions on components.
One of the main advantages of laser cladding versus conventional methods like TIG or PTA welding is the low heat input into the base material. This results in less distortion of the component. The high cooling rate produces a very desirable fine-grained microstructure. The overlay is fully metallurgical bonded to the base material with much lower dilution compared to conventional welding methods, so a single coating layer is in most cases sufficient to achieve full transformation between the two materials. Thickness tolerances and surface quality is very good and depending on the application requires minimal or no rework.
To repair more complex three-dimensional components, powder cladding is preferred due to the easier handling. The following two figures show the principle of cladding with powder: the deposited material is fed into the melt pool either coaxially (Figure 1) or laterally (Figure 2). An inert gas is typically used to transport and inject the powder particles to the process.
Typical process parameters for laser cladding are a particle size of the powder material between 20–200 µm, a laser powder density of around 104–105 W/cm2 and an interaction time between powder particles and the laser beam of 0.1 s. (Dr. Sc. Ing Torims, 2013).
The laser types used for cladding depend on the application. CO2-laser, disc and fiber lasers and most commonly diode lasers are used in industrial environments.
In the 70’s CO2-lasers were first used to successfully clad with powder. But extremely high initial and running cost as well as their susceptibility to collect dust on the delivery optics, especially when using powdered metal, was a big disadvantage. The high running costs are mainly caused by their low wall-plug efficiency of about 10% and the cost for the consumed material (e. g. gases). Even at identical laser beam power, CO2-lasers have a two times lower process efficiency compared to diode lasers. The reason is the lower absorption of their far- infrared wavelength of 10,6 mm by the metal surface. Much of the delivered energy gets reflected and is lost – only about 11% of the energy gets absorbed. CO2-lasers and their beam delivery systems via copper mirrors also require frequent maintenance and cleaning, causing down time and production loss. Their beam cannot be delivered via fiber optic cables which also makes them less flexible.
The near-infrared wavelength of solid state lasers like Nd: YAG, fiber or disc lasers has an absorption of about 35%, which is similar to diode lasers. However lamp-pumped Nd: YAG lasers have been almost completely replaced by diode lasers in continuous wave (cw) applications due to their 10 times better wall plug efficiency and otherwise nearly identical beam properties. The high beam quality of fiber or disc lasers makes them suitable for remote welding (welding with a very long working distance) as well as for welding applications, which require an extremely narrow weld bead, but they are considered less robust for many applications. Fiber coupled diode laser on the other hand with their round focus and a "top-hat" (steep edges and homogeneous) power distribution are the ideal tool for laser cladding (Stilles, Dr. Himmer, & Prof.Dr.Beyer, 2005). They are more tolerant to back reflections and less sensitive to dirt and vibrations compared to a monoblock design of direct diode lasers, since the laser source is located outside the work area, possibly even in a different room. Another advantage regarding direct diode lasers is that the tool center point (TCP) of fiber coupled diode lasers does not have to be re-calibrated after maintenance. A variety of focus geometries, like circles, rectangles, rings, lines and others can be easily generated via optical modules as well.
Repair and Refurbishment
Due to the increased cost- and environmental consciousness of today’s companies the repair and refurbishment sector of production technology is experiencing a massive boom. The demand for more sustainable manufacturing methods is steadily increasing and laser cladding is in many cases a suitable way to achieve this goal.
To repair large components, multi-kilowatt lasers are most suitable. Modern high-power diode lasers with wall-plug efficiency of up to 50% and output powers reaching 25kW allow depositing wide track layers to large surface areas at high speeds. For example weld track widths of 12 mm at a thickness of 1.5 mm with 2 m/min velocity and a powder utilization of more than 95% are realized in an industrial environment (Fraunhofer IWS Dresden, 2014). A typical laser cladding process with 6 kW laser power is able to deposit about 4–5 kg/h of powder-formed Inconel 625 onto mild steel.
In Figure 3, the process of repairing a cylindrical shaft is illustrated. Step 1 show the worn shaft. After depositing powder layers with a coaxial nozzle (step 2 and 3) the end of the shaft gets machined to the desired surface quality. The full functionality of the components has been restored with a relatively small use of energy and materials.
Laser cladding allows repairing or rebuilding worn and expensive components in an economical and ecological manner. Compared to conventional repair methods, a significant amount of material, machining, time and energy can be saved. In particular it is a preferred repair and refurbishment method in the field of mining, oil & gas, petrochemical industry, power generation, transportation (trains and ships) as well as heavy forging equipment.
Furthermore it is possible to clad the inside of hollow cylindrical components with special cladding heads (s. Figure 4). These Internal Diameter (ID) -cladding heads and optics are in most cases customized for the customer and the respective application. Typical applications are wear and corrosion protection on the inside of components as well as their repair.
Corrosion and Wear Protection
To decrease the wear and corrosion of metal components several processes can be used. Thermal coating, like Flame, HVOF, or plasma spraying is well suited for thin layers on large surface areas with low heat input. The results are relatively porous and only mechanically bonded to the substrate surface, so they tend to crack or peel off easier during local impact load. Conventional cladding methods like MIG or TIG welding are capable of producing layers with full metallurgical bonding. The main disadvantage is the very high heat input into the base material, which may result in distortion and heat affection of the components. In Table 1 the main properties of both processes in comparison to laser cadding are listed.
Laser cladding has the advantage to form a full metallurgical bonding between the layer and base material with low heat input and a small heat affect zone (HAZ). Compared to PTA welding, laser cladding enables a better utilization of the powder material and less formation of pores while achieving better corrosion resistance (Fu, Shi, & Shi, 2006).
Suitable additive materials for laser cladding are all weldable alloys. Commonly used materials are Nickel, Cobalt or Iron based alloys which can be combined with carbide particles for wear protection applications.
Besides from the protection of drilling equipment for the oil & gas industry from mechanical wear or corrosion, laser cladding also allows to generate nonmagnetic areas on components to enable the use of certain sensors to measure environmental conditions while drilling which otherwise would be disturbed by magnetic materials. Image 5 shows the cladding process of down-hole drilling equipment to improve its abrasive wear-resistance. This process is also referred to as "hard facing".
Today there are a number of different technologies for generative production of components. Laser cladding – in this context also called laser metal deposition (LMD) – is recently gaining on importance. It allows generating shapes and structures in a single production step with almost no material loss, post machining and tool wear (near-net-shape manufacturing).
A very interesting and promising approach in this area is to integrate the laser source into conventional machine tools. This combination of additive and subtractive tools achieves a new level of manufacturing. One example is the combination of a laser with a 5-axis milling machine. The integrated diode laser is depositing the powdered material layer by layer, generating a solid, fully dense metal part. The following milling operations produce surfaces in end-part quality in areas necessary without changing the setup. This flexible switch between laser and mill also allows to machine areas which would be impossible to reach on the final component. Designs with undercuts, internal geometry and overhanging without support structure are feasible with this technique. The production of completely new parts and new designs are possible. All weldable metals which are available in powder form can be utilized, for example steel, nickel and cobalt alloys as well as titanium, bronze or brass (Kroh, 2013). Compared to other additive manufacturing methods, for example powder bed, this process is up to 10 times faster. The turbine housing, shown in figure 6 is one example of this combination of additive & subtractive manufacturing.
Conclusions and Outlook
Laser cladding is a versatile process, suitable for manufacturing as well as repair. It can easily be automated, improving the productivity of this already economical process even further. A driving factor is the increasing power of the laser beam sources – fiber coupled diode lasers today deliver up to 25 kW optical power on the work piece.
Improvements in the laser cladding process are not only driven by higher laser power and better efficiency but also by a higher degree of integration with other manufacturing methods and their handling systems. Such integrated and intelligent machine tools enable the manufacturing of components with radically new designs and functionality.