At present, the application and research of 3D printing technology are mainly concentrated in the field of processing and repairing of precision parts such as aerospace, electronics, medical treatment, etc., but the application in the field of large-scale casting and forgings mainly based on iron is still very few. The weight of large-scale casting and forging products, such as thermal power spindles, nuclear power heads, turbine blades, etc., is often several tens of hundreds of tons, and the current metal 3D printing technology averages a processing speed of 1kg / h and a material cost of thousands of yuan per kilogram. It is impossible to complete the manufacturing of large-scale casting and forging by adopting the overall 3D printing technology of large-scale casting and forging. However, this does not mean that 3D printing technology cannot be widely used in the field of large castings and forgings.
As long as it is properly applied, 3D printing technology can become an indispensable and important production process in the manufacture of large castings and forgings like casting, forging and welding, and play a role in the technological innovation, cost reduction and efficiency enhancement, and quality improvement of the large casting and forgings industry. Important role. This paper analyzes the future application of 3D printing technology in the field of large-scale casting and forgings mainly based on iron-based metals by referring to the research achievements made by other metal and non-metal 3D printing technologies in other fields. Provide reference for development.
1. Metal material 3D printing technology
3D printed titanium alloy large integral main load-bearing structural parts and other large parts metal materials can reach or approach the level of forgings. 3D printing technology repairs turbofan engine blade technology to achieve the performance of the area where 3D printing is combined with the part base to meet the requirements of the parts. Compared with traditional welding, metal 3D printing repair has the following characteristics:
(1) The heat affected zone is small and does not affect the stress distribution of the matrix structure;
(2) No heating and subsequent heat treatment;
(3) The 3D printing organization is tightly combined with the base metallurgy, and its performance is close to the original organization of the part;
(4) The organizational performance of the 3D printing area reaches the level of forgings;
(5) Automated control, less processing margin.
1.1 Local 3D printing of large forgings
Taking the main pipeline of AP1000 nuclear power as an example, the design requires the use of ultra-low carbon nitrogen-controlled stainless steel forging. The main difficulty of forging production is the formation of two nozzles, and the material utilization rate of forgings is below 15%. Inspired by 3D printing technology to repair turbofan engine blades, the formation of nuclear power main pipelines can be simplified to forging or extruding stainless steel tubes + 3D printing nozzle forming. This method can greatly reduce the production difficulty and cost of nuclear power main pipelines.
Large forgings such as nuclear power main pipes and multi-nozzle heads with local hard forming features can be formed by forging body + 3D printing local method. This method will greatly reduce the production difficulty and cost of forgings while ensuring the overall quality of the forgings And cycle. In addition, the idea of ​​partial 3D printing forming of large forgings can also be extended to large-scale parts 3D printing tailored welding technology. If the metal material performance of the 3D printing area and the area where the 3D printing area is combined with the substrate can meet the requirements of forgings, this indicates that in the future, 3D printed tailor-welded forgings can be used to replace the whole large forgings.
1.2 Defect repair of large forgings
Quality problems such as over-standard defects on the surface and inside of large forgings and insufficient machining allowances due to lack of materials may lead to the overall scrapping of the forgings, resulting in huge economic losses and waste of energy. Because the performance of the welding structure is lower than that of the forging, the forgings generally do not allow repair welding, but the emergence of metal 3D printing technology can change this situation. At present, some materials of metal 3D printing can reach the level of forgings. If the combination of 3D printing organization and forging base can meet the requirements of forgings, 3D printing technology can be used to locally repair the defect areas of large forgings, thereby improving the forging qualification rate. In the future, with the improvement of 3D printing technology, similar to large castings that allow repair welding, large forgings also allow 3D printing repair, which will be a revolutionary breakthrough in the production process of large forgings.
1.3 Online repair of large parts
Large-scale parts such as generator rotors, turbine blades, marine crankshafts and other large parts will have failure problems such as local cracks, wear and deformation. The traditional repair welding method needs to be processed through machining → preheating → welding → machining → heat treatment and other processes. repair. The repair work needs to be carried out on large professional equipment, but the performance of the repair area is lower than the original organization of the parts, which increases the maintenance cost and maintenance cycle, and the maintenance effect is not ideal. The use of a combination of robots and 3D printing technology to form a portable and removable metal 3D printing device can change this status quo and enable on-site repair of large parts or even online repair. The specific operation steps of the robot metal 3D printing technology to repair large parts are:
(1) Determine the repair plan and pre-treat the repair area;
(2) Three-dimensional reverse modeling of the repaired area using three-dimensional imaging technology;
(3) Convert the three-dimensional model of the repaired area into the moving path of the manipulator;
(4) Determine the 3D printing parameters, locate the manipulator, and repair it;
(5) Surface treatment and inspection of repair area.
1.4 Surface treatment of large parts
An important development direction of metal 3D printing technology is the 3D printing of multi-materials and gradual materials. The research results in this field can be applied to the surface treatment of large parts, especially those with complex structures or complex functions. Compared with the traditional metal surface treatment methods such as electroplating, thermal spraying and chemical vapor deposition, metal 3D printing technology is regarded as an extension of the laser coating process, which has a strong bonding layer, a large coating thickness and controllable, digital control, The advantages of a wide range of materials, especially in the local modification and strengthening of large parts are obvious.
1.5 Research focus of metal material 3D printing technology
According to the current development of metal 3D printing technology, the above research directions are technically feasible, and many technologies have been applied to small parts. In order to enable metal 3D printing technology to be applied in the field of large castings and forgings at an early date, key research should be carried out in the following aspects:
(1) Material aspect. Compared with titanium alloys and high-temperature alloys, iron-based alloys used in large castings and forgings are relatively simple. The key is the control of material costs.
(2) Equipment. At present, the working table size and bearing capacity of 3D printing equipment are limited, which cannot meet the local processing requirements of large castings and forgings. In the future, it is necessary to develop more flexible and convenient processing equipment.
(3) System aspect. 3D printing manufacturing and repairing require three-dimensional reverse seeking, modeling, two-dimensional path conversion and other operations, and large-scale castings and forgings are mostly produced in small batches of single parts, and defects occur randomly. Therefore, the corresponding software and operations need to be integrated in the future, thereby Increase productivity.
(4) Technology. Continuously optimize the metal 3D printing process parameters to better adapt to the special requirements of large castings and forgings, and prevent the occurrence of defects such as cracking, deformation and shedding.
(5) Performance. In order to make 3D printing technology permanently applied to large parts such as nuclear power, thermal power, and large pressure vessels, material-based experimental data such as persistent creep and low cycle fatigue need to be accumulated for a long time.
(6) Standard issues. Unlike small parts, safe production is the focus of repair and manufacturing of large castings and forgings. The lack of various production and quality standards is an important bottleneck for the development of 3D printing technology in this field. The formulation of standards also takes a long time.
2. Non-metallic material 3D printing technology
3D printing technology based on wax mold rapid prototyping and sand mold rapid prototyping has been widely used in the field of small precision castings and casting rapid prototyping, and there is a trend towards large-scale development. At present, models of more than 2m can be produced.
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