1 Introduction Rapid Prototyping (RP) is an advanced manufacturing technology that has been developed since 1987. It integrates the latest achievements in modern CNC technology, CAD/CAM technology, laser processing technology and materials science. The technology uses CAD software to design the 3D solid model of the part, and then according to the specific process requirements, the model is layered and sliced ​​according to a certain thickness, which is discretized into a series of 2D layers, and then the data is analyzed for the 2D layer information. Processing and adding processing parameters, generating a numerical control code input molding machine, controlling the movement sequence of the molding machine to complete the molding manufacturing of each layer until the prototype or part conforming to the CAD model is processed. It can automatically and quickly convert design ideas into prototypes or direct manufacturing parts with certain structure and function, so that product design can be quickly evaluated and modified to respond to market demand. It does not require a large number of tooling molds required by traditional methods. It saves costs, shortens the processing cycle, and achieves the goal of high efficiency, low consumption, and intelligence.
Rapid prototyping technology was originally used to make casting models, and later developed to manufacture prototype parts, mainly for visual inspection of models or parts. The key is to require accurate shape and low mechanical properties. Liquid photosensitive resin, wax, paper and other alternative materials. In recent years, rapid development technology has developed rapidly and has begun to be applied in the preparation of metal materials and ceramic materials. The main goal is to quickly manufacture dense metal parts that meet the performance requirements.
2 Research Status of Metal Powder Laser Rapid Prototyping Technology Metal powder sintering molding technology is a hot research field in the world. It can automatically and quickly produce complex metal parts or models from 3D CAD models. The manufacturing methods mainly include selective laser. Sintering (SLS) and laser cladding are two techniques.
2.1 Selective Laser Sintering (SLS)
(1) SLS principle Selective laser sintering is a method in which a solid powder [2, 3] is selectively layered by a laser, and a solidified layer formed by sintering is laminated to form a desired shape. Firstly, the part model is generated by CAD, and processed by layered slicing software to obtain the information parameters of each cross-section shape, as the trajectory of the laser beam for two-dimensional scanning. The beam emitted by the laser is under the control of the computer, according to the geometric shape. The coordinate data of the layer section selectively scans the material powder layer, and the powder is sintered together at the position of the laser irradiation. After one layer of sintering is completed, the powder is powdered for the next layer of scanning and sintering, and the new layer and the previous layer are The layers are naturally sintered together to ultimately create a three-dimensional shaped part.
(2) Characteristics of SLS Compared with other traditional manufacturing methods, the outstanding advantages of selective laser sintering technology are:
1 With high flexibility, it is convenient and quick to produce complex-shaped parts that are difficult to achieve by conventional machining methods under the control of a computer, such as parts with complex concave and convex portions and hollow parts.
2 The technology is highly integrated, integrating computer technology, numerical control technology, laser technology and material technology.
3 The production cycle is short. The technology is only a few hours to tens of hours from the CAD design to the completion of the parts, which is especially suitable for the development of new products.
(3) Research status Selective laser sintering technology has been widely used, especially in the direct production of metal parts and metal molds. DTM Company of the United States successfully produced steel-copper alloy injection molds using the SLS2000 system. The Hannover Laser Center in Germany used Nd:YAG pulsed lasers and optical scanning systems to test alloy materials of different sizes such as nickel, copper, aluminum and bronze. Domestically, the Foundry Center of North University and the Special Processing Laboratory of Nanjing University of Aeronautics and Astronautics have carried out basic research on selective laser sintering technology. At present, the special processing laboratory of Nanjing University of Aeronautics and Astronautics has completed a single-layer sintering test in powder ratio and laser. Good results were obtained in the selection of sintering parameters. On this basis, they conducted an initial attempt to multi-layer sintering, and two-dimensional solid parts with simple shapes were sintered. North China University has developed variable length line scanning SLS RPT based on selective laser sintering, which is the world's first novel rapid prototyping method.
Generally, the SLS RPT is to form a CO2 laser output beam through a focusing lens to form a spot with a very high energy density and a small size on the working surface. This spot sinters the powder material laid on the table, and the variable length line scan SLS RPT The laser beam is passed through the cylindrical lens to form an elongated wire harness having the same energy density as the above-mentioned spot on the table, and has the advantages of large molding size and high molding efficiency. Moreover, the selective laser rapid sintering technology for metal powder has only carried out a series of researches in North China University, and has studied the post-treatment of molded parts such as copper infiltration and high-temperature sintering, and achieved remarkable results.
2.2 Laser coating (clay) manufacturing technology (1) Principle of laser coating manufacturing technology Laser coating manufacturing technology, also known as near-shaped technology (LENS), is based on laser cladding technology and rapid prototyping technology. A new technology. First, the part model is generated by CAD, and processed by layered slicing software to obtain the information parameters of each cross-section shape as the trajectory parameters of the moving table. Under the control of the computer, the workbench moves according to the coordinate data of the cross-section of each layer of the geometric body, and the material is stacked layer by layer by laser coating method to form a three-dimensional solid part with a certain shape.
(2) Characteristics of laser coating manufacturing technology When the focusing laser is used, the molten pool is formed to be small, and the parts with precise appearance can be produced. Since the size of the sintering spot is similar to the effective diameter of the laser beam, the wall thickness of the component can be precisely adjusted, reducing the post-processing process. The laser proximity method improves the design flexibility. By changing the CAD model file, it is convenient and economical to modify the supplementary parts, and flexibly change the components of different parts of the parts, so that the parts have excellent comprehensive performance. The production cycle is greatly shortened and the efficiency is high. As long as the material to be processed has a low reflectance to the laser wavelength of the laser used, it can be processed by the LENS method. Laser coating manufacturing technology facilitates selection cladding and can be used to repair large metal parts. There is no need to make expensive tooling and the production cost is low.
(3) Research and application status of laser coating manufacturing technology The Hannover Laser Center in Germany has studied cobalt-based (stellite6) and nickel-based alloys (Inconel625), using a CO2 laser with a maximum power of 3kW and a computer numerical control. 3D machining system. This set of equipment is used to produce metal parts with vertical and inclined thin walls. Regardless of the material used, the structural examination of the coated portion revealed a fine structure with a part of the dendritic structure. Tests on materials have shown that the coated parts have a density of nearly 100% and tensile strength and breaking strength similar to conventional sheet metal.
The Sandia National Laboratory in the United States has been able to produce high-density metal parts of various materials using laser proximity manufacturing, including nickel alloy Inconel 718, 625, 690, stainless steel 304 and 316, H13 tool steel, tungsten, titanium and magnetic NdFeB. Optimized manufacturing rates, part density, grain structure and surface quality can be achieved by varying laser modes, laser power, deposition rate, number of axes, and metal transport. The Ti-6Al-4V parts produced by the laboratory have a compactness of 99.996%, and the elongation and strength are greatly improved compared with the conventional methods.
Abbott et al. used laser proximity technology to make titanium parts, reducing manufacturing time by 50% to 75%. The test was carried out in a 3 m × 3 m × 1.2 m processing chamber and a new system capable of providing a high powder flow rate powder feeder capable of processing large-sized parts under an argon-protected atmosphere with an oxygen pressure of less than 10 ppm. This is extremely critical for titanium that is highly reactive with oxygen and nitrogen. The mechanical properties and compositional analysis of commercial pure titanium, Ti-6Al-4V and Ti-5Al-2.5Sn show that laser rapid prototyping parts can meet the needs of use.
3 Development prospects and existing problems Laser rapid prototyping technology of metal powder integrates computer-aided design, laser cladding and rapid prototyping, and can quickly prepare complex shapes of different materials without any hard tooling or model. The multi-variety and small-volume parts have high density and fast solidification structure, which can meet the requirements of direct use, and have broad application prospects in the preparation of aerospace devices, aircraft engine parts and weapon parts. It is also possible to develop a smart preparation system for materials by changing the molding materials to obtain parts of different materials composed of different materials.
Since the early 1990s, the exploration of the method of direct rapid prototyping of metal parts has become a research hotspot of RP technology. With the advent of high-power lasers, it is possible to directly manufacture metal parts by rapid prototyping method. Technology companies are conducting research on rapid prototyping of metal parts. Exploring the rapid prototyping technology that directly prepares metal parts that meet the engineering conditions will help transform the rapid prototyping technology into rapid manufacturing technology and greatly expand its application. In addition, using the advantages of layer-by-layer manufacturing, it is also a new development direction to explore the manufacture of parts with functional gradients, excellent comprehensive performance and special complex structures. The combination of rapid prototyping technology and traditional manufacturing technology to form a rapid product development-manufacturing system is also an important trend.
At present, the forming precision of rapid prototyping technology is on the order of 0.01 mm, and the surface quality is still poor, which needs further improvement. The most important thing is that the strength and toughness of the molded parts can not fully meet the actual needs of the project. It is very urgent to improve the existing rapid prototyping process and equipment, improve the forming precision, strength and toughness of the parts, and reduce the operating cost of the equipment.
4 Conclusion As a high-tech manufacturing technology, rapid prototyping will surely develop strengths and avoid weaknesses on the basis of rapid development over the past decade, and continuously develop new molding processes, forming materials and intelligent related technologies. The existing processes will also be refined. High-precision, low-cost, standardized development, and should be able to directly produce semi-functional, functional parts.
Rapid prototyping technology was originally used to make casting models, and later developed to manufacture prototype parts, mainly for visual inspection of models or parts. The key is to require accurate shape and low mechanical properties. Liquid photosensitive resin, wax, paper and other alternative materials. In recent years, rapid development technology has developed rapidly and has begun to be applied in the preparation of metal materials and ceramic materials. The main goal is to quickly manufacture dense metal parts that meet the performance requirements.
2 Research Status of Metal Powder Laser Rapid Prototyping Technology Metal powder sintering molding technology is a hot research field in the world. It can automatically and quickly produce complex metal parts or models from 3D CAD models. The manufacturing methods mainly include selective laser. Sintering (SLS) and laser cladding are two techniques.
2.1 Selective Laser Sintering (SLS)
(1) SLS principle Selective laser sintering is a method in which a solid powder [2, 3] is selectively layered by a laser, and a solidified layer formed by sintering is laminated to form a desired shape. Firstly, the part model is generated by CAD, and processed by layered slicing software to obtain the information parameters of each cross-section shape, as the trajectory of the laser beam for two-dimensional scanning. The beam emitted by the laser is under the control of the computer, according to the geometric shape. The coordinate data of the layer section selectively scans the material powder layer, and the powder is sintered together at the position of the laser irradiation. After one layer of sintering is completed, the powder is powdered for the next layer of scanning and sintering, and the new layer and the previous layer are The layers are naturally sintered together to ultimately create a three-dimensional shaped part.
(2) Characteristics of SLS Compared with other traditional manufacturing methods, the outstanding advantages of selective laser sintering technology are:
1 With high flexibility, it is convenient and quick to produce complex-shaped parts that are difficult to achieve by conventional machining methods under the control of a computer, such as parts with complex concave and convex portions and hollow parts.
2 The technology is highly integrated, integrating computer technology, numerical control technology, laser technology and material technology.
3 The production cycle is short. The technology is only a few hours to tens of hours from the CAD design to the completion of the parts, which is especially suitable for the development of new products.
(3) Research status Selective laser sintering technology has been widely used, especially in the direct production of metal parts and metal molds. DTM Company of the United States successfully produced steel-copper alloy injection molds using the SLS2000 system. The Hannover Laser Center in Germany used Nd:YAG pulsed lasers and optical scanning systems to test alloy materials of different sizes such as nickel, copper, aluminum and bronze. Domestically, the Foundry Center of North University and the Special Processing Laboratory of Nanjing University of Aeronautics and Astronautics have carried out basic research on selective laser sintering technology. At present, the special processing laboratory of Nanjing University of Aeronautics and Astronautics has completed a single-layer sintering test in powder ratio and laser. Good results were obtained in the selection of sintering parameters. On this basis, they conducted an initial attempt to multi-layer sintering, and two-dimensional solid parts with simple shapes were sintered. North China University has developed variable length line scanning SLS RPT based on selective laser sintering, which is the world's first novel rapid prototyping method.
Generally, the SLS RPT is to form a CO2 laser output beam through a focusing lens to form a spot with a very high energy density and a small size on the working surface. This spot sinters the powder material laid on the table, and the variable length line scan SLS RPT The laser beam is passed through the cylindrical lens to form an elongated wire harness having the same energy density as the above-mentioned spot on the table, and has the advantages of large molding size and high molding efficiency. Moreover, the selective laser rapid sintering technology for metal powder has only carried out a series of researches in North China University, and has studied the post-treatment of molded parts such as copper infiltration and high-temperature sintering, and achieved remarkable results.
2.2 Laser coating (clay) manufacturing technology (1) Principle of laser coating manufacturing technology Laser coating manufacturing technology, also known as near-shaped technology (LENS), is based on laser cladding technology and rapid prototyping technology. A new technology. First, the part model is generated by CAD, and processed by layered slicing software to obtain the information parameters of each cross-section shape as the trajectory parameters of the moving table. Under the control of the computer, the workbench moves according to the coordinate data of the cross-section of each layer of the geometric body, and the material is stacked layer by layer by laser coating method to form a three-dimensional solid part with a certain shape.
(2) Characteristics of laser coating manufacturing technology When the focusing laser is used, the molten pool is formed to be small, and the parts with precise appearance can be produced. Since the size of the sintering spot is similar to the effective diameter of the laser beam, the wall thickness of the component can be precisely adjusted, reducing the post-processing process. The laser proximity method improves the design flexibility. By changing the CAD model file, it is convenient and economical to modify the supplementary parts, and flexibly change the components of different parts of the parts, so that the parts have excellent comprehensive performance. The production cycle is greatly shortened and the efficiency is high. As long as the material to be processed has a low reflectance to the laser wavelength of the laser used, it can be processed by the LENS method. Laser coating manufacturing technology facilitates selection cladding and can be used to repair large metal parts. There is no need to make expensive tooling and the production cost is low.
(3) Research and application status of laser coating manufacturing technology The Hannover Laser Center in Germany has studied cobalt-based (stellite6) and nickel-based alloys (Inconel625), using a CO2 laser with a maximum power of 3kW and a computer numerical control. 3D machining system. This set of equipment is used to produce metal parts with vertical and inclined thin walls. Regardless of the material used, the structural examination of the coated portion revealed a fine structure with a part of the dendritic structure. Tests on materials have shown that the coated parts have a density of nearly 100% and tensile strength and breaking strength similar to conventional sheet metal.
The Sandia National Laboratory in the United States has been able to produce high-density metal parts of various materials using laser proximity manufacturing, including nickel alloy Inconel 718, 625, 690, stainless steel 304 and 316, H13 tool steel, tungsten, titanium and magnetic NdFeB. Optimized manufacturing rates, part density, grain structure and surface quality can be achieved by varying laser modes, laser power, deposition rate, number of axes, and metal transport. The Ti-6Al-4V parts produced by the laboratory have a compactness of 99.996%, and the elongation and strength are greatly improved compared with the conventional methods.
Abbott et al. used laser proximity technology to make titanium parts, reducing manufacturing time by 50% to 75%. The test was carried out in a 3 m × 3 m × 1.2 m processing chamber and a new system capable of providing a high powder flow rate powder feeder capable of processing large-sized parts under an argon-protected atmosphere with an oxygen pressure of less than 10 ppm. This is extremely critical for titanium that is highly reactive with oxygen and nitrogen. The mechanical properties and compositional analysis of commercial pure titanium, Ti-6Al-4V and Ti-5Al-2.5Sn show that laser rapid prototyping parts can meet the needs of use.
3 Development prospects and existing problems Laser rapid prototyping technology of metal powder integrates computer-aided design, laser cladding and rapid prototyping, and can quickly prepare complex shapes of different materials without any hard tooling or model. The multi-variety and small-volume parts have high density and fast solidification structure, which can meet the requirements of direct use, and have broad application prospects in the preparation of aerospace devices, aircraft engine parts and weapon parts. It is also possible to develop a smart preparation system for materials by changing the molding materials to obtain parts of different materials composed of different materials.
Since the early 1990s, the exploration of the method of direct rapid prototyping of metal parts has become a research hotspot of RP technology. With the advent of high-power lasers, it is possible to directly manufacture metal parts by rapid prototyping method. Technology companies are conducting research on rapid prototyping of metal parts. Exploring the rapid prototyping technology that directly prepares metal parts that meet the engineering conditions will help transform the rapid prototyping technology into rapid manufacturing technology and greatly expand its application. In addition, using the advantages of layer-by-layer manufacturing, it is also a new development direction to explore the manufacture of parts with functional gradients, excellent comprehensive performance and special complex structures. The combination of rapid prototyping technology and traditional manufacturing technology to form a rapid product development-manufacturing system is also an important trend.
At present, the forming precision of rapid prototyping technology is on the order of 0.01 mm, and the surface quality is still poor, which needs further improvement. The most important thing is that the strength and toughness of the molded parts can not fully meet the actual needs of the project. It is very urgent to improve the existing rapid prototyping process and equipment, improve the forming precision, strength and toughness of the parts, and reduce the operating cost of the equipment.
4 Conclusion As a high-tech manufacturing technology, rapid prototyping will surely develop strengths and avoid weaknesses on the basis of rapid development over the past decade, and continuously develop new molding processes, forming materials and intelligent related technologies. The existing processes will also be refined. High-precision, low-cost, standardized development, and should be able to directly produce semi-functional, functional parts.
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