Electron Beam Melting (EBM)
Electron Beam Selective Melting (EBSM) Principle
Similar to laser selective sintering and Selective Laser Melting processes, electron beam selective melting technology (EBSM) is a rapid manufacturing technology that uses high-energy and high-speed electron beams to selectively bombard metal powder, thereby melting and forming powder materials.
The process of EBSM technology is as follows：first, spread a layer of powder on the powder spreading plane; then, under computer control, the electron beam is selectively melted according to the information of the cross-sectional profile, and the metal powder is melted together, bonded with the formed part below, and piled up layer by layer until the entire part is completely melted; Finally, excess powder is removed to yield the desired three-dimensional product. The real-time scanning signal of the upper computer is transmitted to the deflection yoke after digital-to-analog conversion and power amplification, and the electron beam is deflected under the action of the magnetic field generated by the corresponding deflection voltage to achieve selective melting. After more than ten years of research, it is found that some process parameters such as electron beam current, focusing current, action time, powder thickness, accelerating voltage, and scanning mode are carried out in orthogonal experiments. The action time has the greatest influence on the forming.
Advantages of EBSM
Electron beam direct metal forming technology uses high-energy electron beams as the processing heat source. Scanning forming can be performed without mechanical inertia by manipulating the magnetic deflection coil, and the vacuum environment of the electron beam can also prevent metal powder from being oxidized during liquid phase sintering or melting. Compared with laser, electron beam has the advantages of high energy utilization rate, large action depth, high material absorption rate, stability and low operation and maintenance costs. The benefits of EBM technology include high forming efficiency, low part deformation, no need for metal support during the forming process, denser microstructure, and so on. The electron beam deflection and focus control is faster and more sensitive. The deflection of the laser necessitates the use of a vibrating mirror, and the vibrating mirror’s rotating speed is extremely fast when the laser scans at high speeds. When the laser power is increased, the galvanometer requires a more complex cooling system, and its weight increases significantly. As a result, when using higher power scanning, the laser’s scanning speed will be limited. When scanning a large forming range, changing the focal length of the laser is also difficult. The deflection and focusing of the electron beam are accomplished by magnetic field. The deflection and focusing length of the electron beam can be controlled quickly and sensitively by changing the intensity and direction of the electric signal. The electron beam deflection focusing system will not be disturbed by metal evaporation. When melting metal with lasers and electron beams, the metal vapor will diffuse throughout the forming space and coat the surface of any object in contact with a metal film. The deflection and focusing of electron beams are all done in a magnetic field, so they will not be affected by metal evaporation; optical devices such as laser galvanometers are easily polluted by evaporation.
Laser Metal Deposition (LMD)
Laser Metal Deposition (LMD) was first proposed by Sandia National Laboratory in the United States in the 1990s, and then developed successively in many parts of the world. Since many universities and institutions conduct research independently, this technology There are many names, although the names are not the same, but their principles are basically the same. During the molding process, the powder is gathered on the working plane through the nozzle, and the laser beam is also gathered to this point, and the powder and light action points are coincident, and the stacked cladding entity is obtained by moving through the worktable or nozzle.
LENS technology uses kilowatt-class lasers. Due to the large laser focus spot, generally more than 1mm, although metallurgically bonded dense metal entities can be obtained, their dimensional accuracy and surface finish are not very good, and further machining is required before use. Laser cladding is a complex physical and chemical metallurgical process, and the parameters of the cladding process have a great influence on the quality of the clad parts. The process parameters in laser cladding mainly include laser power, spot diameter, defocusing amount, powder feeding speed, scanning speed, molten pool temperature, etc., which have a great impact on the dilution rate, crack, surface roughness and compactness of cladding parts. At the same time, each parameter also affects each other, which is a very complicated process. Appropriate control methods must be adopted to control various influencing factors within the allowable range of cladding process.
Direct Metal Laser Sintering (DMLS)
There are usually two methods for SLS to manufacture metal parts, one is the indirect method, that is, SLS of polymer-coated metal powder; the other is the direct method, that is, Direct Metal Laser Sintering (DMLS).Since the research on direct laser sintering of metal powder was carried out at Chatofci University in Leuvne in 1991, direct sintering of metal powder to form three-dimensional parts by SLS process is one of the ultimate goals of rapid prototyping. Compared with indirect SLS technology, the main advantage of DMLS process is the elimination of expensive and time-consuming pre-treatment and post-treatment process steps.
Features of DMLS
As a branch of SLS technology, DMLS technology has basically the same principle. However, it is difficult to accurately form metal parts with complex shapes by DMLS technology. In the final analysis, it is mainly due to the “spheroidization” effect and sintering deformation of metal powder in DMLS. Spheroidization is a phenomenon in which the surface shape of the molten metal liquid transforms to a spherical surface under the interfacial tension between the liquid metal and the surrounding medium in order to make the system composed of the surface of the molten metal liquid and the surface of the surrounding medium with minimum free energy. Spheroidization will make the metal powder unable to solidify after melting to form a continuous and smooth molten pool, so the formed parts are loose and porous, resulting in molding failure. Due to the relatively high viscosity of single-component metal powder in the liquid phase sintering stage, the “spheroidization” effect is particularly serious, and the spherical diameter is often larger than the diameter of the powder particles, which leads to a large number of pores in the sintered parts. Therefore, the DMLS of single-component metal powder has obvious process defects, and often requires subsequent treatment, not the real sense of “direct sintering”.
In order to overcome the “spheroidization” phenomenon of single component metal powder DMLS and the resulting process defects such as sintering deformation and loose density, it can be generally achieved by using multi-component metal powders with different melting points or using pre-alloying powders. The multi-component metal powder system is generally composed of high melting point metals, low melting point metals and some added elements. The high melting point metal powder as the skeleton metal can retain its solid core in DMLS. The low-melting point metal powder is used as a binder metal, which is melted in DMLS to form a liquid phase, and the resulting liquid phase coats, wets and bonds the solid phase metal particles to achieve sintering densification.
As a leading company in China’s 3D printing service industry, JSADD 3D will not forget its original intention, increase investment, innovate and develop more technologies, and believe that it will bring new 3D printing experience to the public.
Contributor: Sammi