3D printing technology belongs to rapid prototyping technology. Different from traditional subtractive manufacturing, 3D printing technology is called additive manufacturing technology. The manufacturing of traditional parts generally requires tools and molds, and it is difficult to process parts with complex shapes and uneven surfaces. 3D printing technology uses modern means such as computers, lasers and CNC to create a 3D model file of the part to be processed in the computer. After the model is built, it is imported into the slicing software to set the processing parameters, such as processing speed, layer height, etc. After the settings are completed, it is imported into the 3D printer. The printer picks up the processing parameters and realizes the processing of the object by printing the material layer by layer. The materials used in ordinary 3D printing technology are generally resins, PLA, ABS plastics, etc., while the materials used in metal 3D printing technology are metals or alloy materials. According to the different metal 3D printing processes, it can be roughly divided into selective laser sintering technology (SLS), selective laser melting technology (SLM), electron beam selective melting technology (EBSM), laser near-net shaping technology (LENS), direct metal laser sintering technology (DMLS) and other new technologies. Metal 3D printing technology has been widely used in many fields such as precision manufacturing, aerospace, and medical equipment because of its ability to process parts of any shape.
With the development of society and the continuous advancement of science and technology, metal 3D printing technology has quickly occupied an important position in the metal manufacturing industry with its high material utilization rate, short manufacturing cycle, and high flexibility. Metal 3D printing technology can print some small, complex, and high-precision metal parts, so this technology plays a key role in improving the quality and efficiency of the entire industrial production, improving the current status of metal parts manufacturing, providing more possibilities in the process of manufacturing metal parts, and promoting the development of the metal manufacturing industry.
1 Application of metal 3D printing technology
At present, the mainstream metal 3D printing technologies used directly in the market for manufacturing metal parts are: selective laser sintering (SLS), selective laser melting (SLM), direct metal laser sintering (DMLS), laser near net shaping (LENS), and electron beam selective melting (EBSM).
1.1 Selective laser sintering (SLS) technology
Selective laser sintering (SLS) technology is the earliest metal 3D printing technology. The metallurgical mechanism used is the liquid phase sintering mechanism. The material used is a mixed powder of high melting point metal and low melting point metal or polymer material. During the melting process, the low melting point metal or polymer material powder melts, while the high melting point metal powder does not melt and retains its solid phase core as a structural metal. The melted material acts as a bonding metal and generates a liquid phase during the melting process to cover, wet and bond the solid metal to achieve sintering densification. The entire process device consists of two parts: a powder cylinder and a molding cylinder. During operation, the powder cylinder on the left rises one layer, and then the powder roller spreads a layer of powder evenly in the molding cylinder. The laser beam controlled by the computer scans the powder according to the sliced model, so that the metal powder reaches the melting point and sintering to complete a layer of the part. After completion, the molding cylinder drops one layer, and the powder roller will spread a uniform layer of powder in the molding cylinder again to sinter the next layer. This process is repeated to complete the production of the entire part.
Features of selective laser sintering: Advantages: (1) A variety of materials can be used. Including polymer materials, metal powders, ceramic powders, nylon powders, etc., with strong selectivity. (2) No support is required. Because the unsintered powder can support the generated suspended layer during the printing process. (3) High material utilization rate. No support is required during the printing process, and the material price is low. Disadvantages include: (1) Rough surface. The surface of the prototype manufactured by the SLS process is powdered and bonded, and is in the form of powder particles, so the surface quality is not high. (2) There is an odor during the process. This is because polymer materials or powder particles will emit an odor during sintering.
1.2 Selective Laser Melting (SLM) Technology
Selective laser melting (SLM) technology is developed on the basis of SLS. Its basic principle is similar to SLS. First, the computer 3D modeling software is used to build the model, then the slice software is used to adjust the parameters and obtain the data of each layer, and then the computer controls the laser beam to scan and melt layer by layer to form the layer by layer. It should be noted that in order to prevent the metal from reacting with other gases at high temperature, the SLM process needs to be carried out under inert gas. Unlike the SLS process, the SLM process requires the metal powder to be completely melted and then cooled to form, so a high-power density laser is required to scan the powder.
Features of selective laser melting: Advantages: (1) The powder is completely melted during the processing and no bonding material is required. Therefore, the precision and mechanical properties of the parts formed by the processing are better than those formed by SLS. (2) High density. The laser beam spot diameter is fine and the density is close to 100%, which is almost equal to metallurgy. (3) It can simply and directly manufacture metal parts with complex shapes. Disadvantages include: (1) Expensive equipment and complex operation. Professionals are required to operate. (2) Complex post-processing. The SLM process requires the addition of supports, and the molded parts need to be post-processed to remove the supports.
1.3 Electron beam selective melting (EBSM) technology
The two most important parts of the EBSM equipment include the electron gun and the vacuum chamber. The electron gun includes an anode, a cathode, a grid, a filament, a deflection coil, and a focusing coil. The vacuum chamber includes a powder spreader, a piston, and a powder storage box. The working principle is that the filament at the top of the electron gun (usually a tungsten filament) generates a large number of hot electrons on its surface under high temperature conditions and emits them through the cathode. There is a small hole at the top of the grid. The relative position with the cathode can control the amount of electron beam passing through. Under the acceleration of the anode, it obtains a very high kinetic energy, which can be accelerated to about half to one-third of the speed of light. The electron beam is focused by the focusing coil and then enters the deflection coil. The electron beam can be deflected by the deflection coil and the powder is selectively scanned under the control of the computer. The powder is placed in the powder storage box. During operation, a layer of powder is evenly spread on the powder bed by the powder spreader. The powder bed is preheated by a low-energy, low-scanning-speed electron beam to keep the temperature below the melting point of the metal powder. Then, a higher energy and scanning speed are used to melt the powder. When the electron beam collides with the metal powder, its kinetic energy is converted into heat energy to melt the metal powder. After completing a layer of scanning, the piston descends by one layer, and the powder spreader spreads powder again to preheat and melt the new powder layer. This process is repeated until the metal part is completely formed. It should be noted that the EBSM process needs to be carried out under vacuum conditions. After the part is made, the device needs to be moved into the post-processing equipment to remove the surrounding powder by blowing compressed gas to obtain the final print, and the remaining powder can be reused.
Characteristics of electron beam selective melting: Advantages: (1) EBSM technology has a high preheating temperature under vacuum conditions, which can melt high-melting-point metals, reduce thermal stress concentration, and avoid bending and deformation of molded parts. (2) No support is required during the molding process. Unsintered powder is used as support, and after the production is completed, only the powder needs to be blown away. Disadvantages: (1) “Powder blowing” phenomenon. The powder spread on the powder bed by the powder spreader leaves the pre-laid position under the action of the electron beam. The reason for this is that the electron beam causes the powder with poor conductivity to carry static electricity, and the repulsive force of static electricity causes the powder to collapse. (2) “Spheroidization” phenomenon. It refers to the metal not being completely melted and forming a group of metal balls separated from each other. (3) The equipment needs to be completed under vacuum conditions, with high maintenance costs, and gamma rays will be generated during the electron beam deposition process, which may cause leakage and pollute the environment.
1.4 Laser Near Net Shape (LENS) Technology
This technology was first introduced by Sandia National Laboratory in the United States in the last century. This process combines laser cladding technology with selective laser sintering (SLS) technology. It uses a coaxial powder feeding method to form a molten pool with the laser. The powder in the molten pool melts and solidifies to achieve the production of parts.
Characteristics of laser near net shape: Advantages: (1) LENS technology uses rapid metal melting and solidification, and the parts obtained by molding have high density and good mechanical properties. (2) No mold is required, which saves costs and can realize the processing of heterogeneous materials. Disadvantages: (1) The surface quality of the molded parts is not high, the surface is rough, the thermal stress is large during the molding process, and cracks are easy to occur. (2) Protective gas is required during the molding process. At the same time, due to the use of titanium alloy powder, the cost is relatively high.
1.5 Direct Metal Laser Sintering (DMLS) Technology
DMLS technology is a branch of SLS technology. It began to take shape in the 1990s. DMLS technology directly uses metal powder for sintering. The difference from SLM technology is that SLM technology requires the metal powder to be completely melted, while DMLS only needs to achieve sintering.
Characteristics of direct metal laser sintering: Advantages: (1) Metal parts can be sintered directly (2) A variety of materials can be used. For example, stainless steel, cobalt-based, nickel-based, etc. (3) The workpiece formed by processing has a dense structure and high bonding strength. Disadvantages: (1) “Spheroidization” phenomenon. (2) Easy to sinter and deform, and the density is not high.
1.6 New Technologies
For example, electric arc additive manufacturing (WAAM), nanoparticle jet metal forming (NPJ) and ultrasonic consolidation (UAM), etc., these technologies have great room for development in the future.
2 Development prospects of metal 3D printing technology
2.1 Expansion of application fields
Today, metal 3D printing is no longer limited to the fields of mechanical mold processing and manufacturing, but can also be applied to other fields. It can be applied to the aerospace field. Metal 3D printing technology can be used to replace some damaged parts, thereby avoiding the high-cost replacement of the entire machine and extending its service life. It can also print key components of aircraft. For example, in November 2018, the metal 3D printed engine bracket developed by GE was approved for use in aircraft manufacturing[7]. It can be applied to the field of education and teaching. Metal 3D printing can be used as a teaching instrument to guide students to understand this technology. It can also print teaching models to guide students to understand the model more intuitively and improve the quality of teaching. It can be applied to the automotive field. In 2017, the brake caliper printed by Volkswagen passed professional tests and met the goals of minimum weight and highest strength. It can also be used for repairing automotive parts. In addition, it can also be used in the medical field. Titanium alloy is the most commonly used material for dental implants. The traditional manufacturing method is not only expensive, but also single in size and cannot be personalized. Now it can be directly used by scanning the patient’s mouth, establishing a dental implant model and then directly printing it using metal sintering technology, which greatly reduces the cost and steps of processing. There are also potential application areas such as making some home furnishings, toys and animation models.
2.2 Printer equipment and material specialization
Metal 3D printing technology is in its early stages, with few and imperfect printing equipment, and its development is at a bottleneck. If this situation needs to be improved, it is necessary to create cost-effective equipment and continue to expand the printing mechanism. For example, it is necessary to conduct in-depth research on metal 3D printing mechanisms such as parallel printing, multi-material printing, multi-nozzle printing, large-piece printing, and continuous printing, and apply them to product manufacturing based on this. The limitations of printing materials also restrict the development of metal 3D printing to a certain extent. In terms of printing materials, it should be possible to print different materials and print different materials for different places. For example, cobalt materials can be used in gas turbines; nickel materials can be used in combustion chambers; precious metals can be used in electronic device integration, as well as some refractory metal materials such as tungsten. New printing methods and printing of new metal materials will be the research hotspots and focuses in the future, with the goal of improving the quality and output of metal 3D printing to meet production in different scenarios and conditions.
