Laser Metal Deposition with Diode Lasers

Laser metal deposition (LMD) refers to the coating of parts by welding using additive welding materials. The LMD process can be used for repairs, or to create surfaces with specific functions. Applying coatings with materials more resistant to wear than the base material is called hard facing. Coating with materials to increase the chemical resistance of surfaces is also called plating.

Laser Metal Deposition

One of the most important industrial applications of diode lasers in the industrial sector is laser cladding. This is an established process for producing or restoring very high-quality laser coatings. Components refined by diode laser deposition welding are used in a wide variety of areas, from heavy industry through to vehicle production and agriculture. In laser metal deposition welding, the laser beam creates a melt pool on the workpiece surface, with the coating material fed into this and melted by the laser at the same time. The additional material, supplied as a wire or powder, produces a layer on a base material after it has been melted by the laser. The short reaction time causes but slight distortion, and the cooling takes place quickly. The result is a layer that is metallurgically bonded to the base material. It is more hard-wearing than coatings produced by thermal spraying and, unlike hard chromium plating, harmless to health.

Surface protection and construction of complex structures

Diode lasers are an ideal tool for every variant of deposition welding. Their fundamental advantages include high flexibility and short processing times, low distortion of treated workpieces as well as fine-grained coatings with excellent adhesion. The extremely hard-wearing surfaces require hardly any reworking. There are various reasons why the additional layers might be desired. Every coating, whether protecting heavily stressed surfaces of metal workpieces from wear and corrosion or repairing high-quality components, can be carried out very effectively with lasers, as can the generative production of complex structures.

Aim: corrosion resistance

Laser coating is very well-suited to preventing creep corrosion, as well as crevice corrosion, with the additional layers. For this purpose, stainless steels and nickel alloys are applied to low-alloy steels. When diode lasers are used as an energy source, the mixing of the materials is at a level of typically less than 5%. As a result, one layer of approx. 1 mm is sufficient for good corrosion protection, while alternative and conventional processes require two layers.

Aim: wear protection

Another motivation behind this procedure is to protect surfaces against wear. Here, laser cladding is in competition with thermal spraying, among other options. Since laser deposition welding creates a metallurgical bond between the base material and the additional layer, it achieves a much longer service life than the purely mechanical spraying process. The materials are often Ni-based alloys (In 625) with tungsten carbides. They can amount to up to 60 percent of the applied layer by weight.

Repairing components

In addition to initial coatings, repair welding is also carried out in the form of wire or laser powder deposition welding. After the old coating has been removed and the workpiece surface cleaned, extremely stable new coatings can be produced, involving the material that is applied being metallurgically bonded to the base material. Unlike for wear or corrosion protection, the same materials are usually applied to the base material to repair a worn surface, broken pieces or other instances of component damage. As long as the material in question is capable of being welded, there are practically no limits.

Generative manufacturing

The last major area of laser deposition welding is the generation of components, often referred to as additive manufacturing (AM) or 3D printing. Here, too, layers of identical materials are applied. As a result, mixing need not be considered separately. Layer by layer, components with complex structures can also be generated if the processing system is programmed appropriately. In addition to stainless steels, aluminum, titanium and superalloys are increasingly being used here, as they are used in turbines, fuselages and wings in aircraft construction.

Laser powder cladding

The laser beam connects the metal workpiece with the powder that is applied. Various steels, cast iron, copper, aluminum, nickel-based and cobalt-based alloys can be used as the base material. The layers are formed from iron-based alloys (low-alloy steels, tool steels, stainless steels), nickel-based alloys such as Inconel (625, 718, 738), cobalt-based alloys such as stellites, high-temperature alloys, aluminum alloys, titanium alloys and materials containing carbides, to offer additional wear protection. The decisive factor is the availability of the filler material in powder form, with a typical particle size of 40-120 µm, to enable the use of a coaxial powder nozzle. Laserline LDM and LDF diode lasers achieve excellent results when melting metal powder: excellent adhesion, high precision, virtually no porosity and limited cracking with a high degree of hardness and minimal deformation. In most cases, the surface that results from the mixing does not require any further machining. Conventional hard facing processes, such as plasma powder deposition welding, on the other hand, do not achieve a sufficiently long service life for many applications.

Laser metal deposition with wire

In this process, the laser beam melts the wire that is fed in and the material of the component to be coated. Wire with diameters of approx. 0.8 to 1.6 mm is used, which is led to the additive welding process by commercially available wire feeders. Today, it is estimated that 90% of the applications involve being coated with powder and 10% with wire. Areas of application for laser deposition welding with wire include the repair of components and the functionalization of surfaces. The process is particularly economical and clean, and reworking is reduced to a minimum.