Direct energy deposition-DED technology, the laser generates a molten pool in the deposition area and moves at high speed. The material is directly fed into the high-temperature melting area in the form of powder or filament, and is deposited layer by layer after melting. This is called laser direct energy deposition additive manufacturing technology.
This technology can only form a blank, and then rely on CNC machining to achieve its net size. Among them, the powder-feeding LENS technology (a type of DED technology) has a wide range of functions in the field of aerospace parts repair at home and abroad.
Is there a possibility? Make the surface quality achieved by direct energy deposition technology higher, or even achieve the effect of coating? In response to this pain point, researchers at the Fraunhofer Institute of Laser Technology (ILT) have developed an additive manufacturing method for coating and repairing metal parts-EHLA ultra-high-speed laser material deposition technology.
The Fraunhofer Institute of Laser Technology believes that Ultra High Speed ​​Laser Material Deposition (EHLA) has the potential to replace current corrosion and wear protection methods such as hard chrome plating and thermal spraying.
Hard chrome plating is a traditional surface plating technique that has been used for more than 70 years. The chrome plating layer has high hardness, wear resistance and corrosion resistance, and can keep the surface bright for a long time. The process is relatively simple and the cost is low. For a long time, in addition to being used as a decorative coating, chrome plating is also widely used as a wear-resistant and corrosion-resistant coating for mechanical parts. Electroplating hard chromium plating technology is often used to repair damaged parts.
However, the hard chromium plating process will cause serious environmental problems. The chromic acid solution used in the chrome plating process will produce chromic acid mist and wastewater, and there are other disadvantages, such as: the hardness is lower than some ceramics and cermet materials, and the hardness It will also decrease as the temperature increases; there are micro cracks in the chrome plating layer, which inevitably produces penetrating cracks, which causes the corrosion medium to penetrate from the surface to the interface and corrode the substrate, causing rust or even flaking on the surface of the plating layer; It is also not conducive to the application of thick plating.
Thermal spraying refers to depositing fine and dispersed metallic or non-metallic coating materials in a molten or semi-melted state on the surface of a prepared substrate to form a spray coating layer. The coating material can be powder, ribbon, filament or rod. The thermal spray gun provides the necessary heat by the fuel gas, electric arc or plasma arc, heats the thermal spray material to a plastic or molten state, and then undergoes the acceleration of compressed air, so that the restricted particle beam impinges on the surface of the substrate. The particles impacted on the surface are deformed by stamping to form a laminated sheet, which adheres to the surface of the prepared substrate, and then cools and continuously accumulates, eventually forming a layered coating. The coating can achieve high temperature corrosion resistance, wear resistance, heat insulation, electromagnetic wave resistance and other functions due to different coating materials.
According to the Fraunhofer Laser Research Institute, the EHLA process is superior in efficiency and speed to existing protection methods for corrosion and wear-resistant coatings. Fraunhofer can use EHLA technology to deposit a thin layer of one tenth of a millimeter on large-area parts in a short period of time, save resources, and make the process economical.
Image: Inventor of the EHLA process, Thomas Schopphoven, GerhardMariaBackes and AndresGasser
EHLA melts metal powder by laser. The metal powder "drops" into the weld pool in the form of liquid metal instead of being bonded together in the form of semi-melted sintered particles. This is beneficial because it means that the layer is more uniform and requires less material.
With the EHLA process, Fraunhofer stated that the process has an improved effect on the current process for corrosion and wear protection. Since hard chromium plating consumes a lot of energy and has the disadvantages of adhesion and porosity, thermal spraying can be quite wasteful in terms of the materials used. In contrast, the coating produced by the EHLA method is non-porous, thereby improving adhesion and reducing the possibility of cracks and porosity. In addition, according to Fraunhofer, EHLA technology saves 90% of the material compared to thermal spraying.
Image: EHLA process
Interestingly, Fraunhofer suggested that the new EHLA process can be used to repair existing metal parts. It is reported that the University of Akron researched through additive manufacturing technology to repair metal parts. After collaborating with an aircraft maintenance company, the university ’s NCERCAMP developed a supersonic particle deposition (SPD) technology, which uses a high-pressure jet method to compress air to give metal particles in a supersonic jet sufficient energy to impact a solid surface, Achieve bonding with solid surfaces without heat-affected zones generated during welding or high-temperature thermal spraying. If FAA certification is obtained, the SPD technology of the University of Akron can be used to repair metal aircraft parts. Fraunhofer's EHLA process can also have such applications, and which process is more suitable? This site will maintain continuous attention. 
Editor in charge: Liu Yang
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