Application of 3D Printing Technology in Large Casting and Forging
The application and research of 3D printing technology are mainly concentrated in the fields of aerospace, electronics, medical and other precision parts processing and repair, while the application in the field of large-scale casting and forging mainly based on iron-based metals is still rare. Large castings and forgings, such as thermal power spindles, nuclear power heads, turbine blades and so on, can weigh hundreds of tons. However, it is impossible to complete the manufacture of large castings and forgings by using the overall 3D printing technology of large castings and forgings based on the average processing speed of 1 kg/h and material cost of thousands of yuan per kilogram in the current metal 3D printing technology. However, this does not mean that 3D printing technology can not be widely used in the field of large castings and forgings.
As long as it is applied properly, 3D printing technology can also become an indispensable and important production process in the manufacture of large-scale castings and forgings just like casting, forging and welding, and play an important role in the process of technological innovation, cost reduction, efficiency improvement and quality improvement in the large-scale castings and forgings industry. By referring to the current research results of metal and non-metal 3D printing technology in other fields, this paper analyzed the application direction of 3D printing technology in the field of large-scale casting and forging mainly based on iron-based metals in the future, so as to provide reference for the development of large-scale casting and forging industry.
1. 3D Printing Technology of Metal Materials
The properties of metal materials for large parts such as large integral main bearing structure parts of titanium alloy printed in 3D can reach or approach the level of forgings. The technology of 3D printing repairing the blade of turbofan engine achieves the performance of the area where the 3D printing is combined with the Parts'matrix, which meets 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 matrix structure.
(2) No need for heating and subsequent heat treatment;
(3) The 3D printing structure is compact and metallurgical bonded with the matrix, and its performance is close to the original structure of the part.
(4) The structure and properties of 3D printing area reach forging level.
(5) Automation control, less processing allowance.
1.1 Local 3D Printing Forming of Large Forging
Taking AP1000 nuclear power main pipeline as an example, the overall forging of ultra-low carbon nitrogen-controlled stainless steel is required. The main difficulty in forging production is the formation of two nozzles, and the material utilization ratio of forgings is less than 15%. Inspired by the repair of turbofan engine blades by 3D printing technology, the forming of nuclear power main pipe can be simplified to forging or extruding stainless steel pipe + 3D printing nozzle. This method can greatly reduce the production difficulty and cost of nuclear power main pipe.
Large forgings such as nuclear power main pipeline and multi-nozzle head, which are difficult to form locally, can be formed by the way of forging body+3D printing locally. This method will greatly reduce the production difficulty, cost and cycle of forgings under the condition of ensuring the overall quality of forgings. In addition, the idea of local 3D printing of large forgings can be extended to 3D printing tailor-welded technology of large forgings. If the properties of metal materials in 3D printing area and 3D printing area can meet the requirements of forging, it indicates that 3D printing tailor-welded forging can be used to replace the whole large forging in the future.
1.2 Repair of Defects in Large Forgings
Over-standard defects on the surface and inside of large forgings and inadequate processing allowance due to lack of materials may lead to the overall scrap of forgings, resulting in huge economic losses and energy waste. Because the performance of welding structure is lower than that of forging structure, repair welding is generally not allowed for forgings, but the appearance of metal 3D printing technology can change this situation. At present, some material properties of metal 3D printing have reached the level of forgings. If the combination of 3D printing structure and forging matrix can meet the requirements of forgings, 3D printing technology can be used to repair the defect areas of large forgings, thereby improving the qualified rate of forgings. In the future, with the improvement of 3D printing technology, large forgings will be allowed to be repaired by 3D printing, just as large castings are allowed to be repaired by repair welding. This will be a revolutionary breakthrough for the production process of large forgings.
1.3 Online Repair of Large Parts
Local cracks, wear and deformation will occur in large forgings such as generator rotor, turbine blade and marine crankshaft during service. The traditional repair welding method can only be repaired by machining preheating welding machine processing heat treatment and other processes. Repair work needs to be carried out on large-scale professional equipment, but the performance of the repair area is lower than the original structure of the parts, which increases the maintenance cost and maintenance cycle, and the maintenance effect is not satisfactory. The portable portable metal 3D printing equipment composed of robots and 3D printing technology can change this situation and realize on-site repair of large parts, even online repair. The concrete operation steps of the robot metal 3D printing technology for repairing large parts are as follows:
(1) Determine the repair plan and pretreat the repair area;
(2) Three-dimensional imaging technology is used to model the repaired area.
(3) The three-dimensional model of the repaired area is transformed 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 repaired 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 gradient materials. The research results in this field can be applied to the surface treatment of large parts, especially those with complex structures or functions. Compared with traditional metal surface treatment methods such as electroplating, thermal spraying and chemical vapor deposition, metal 3D printing technology is regarded as an extension of laser coating technology. It has many advantages, such as strong bonding layer, large and controllable coating thickness, digital control and wide material range, especially in the local modification and strengthening of large parts.Research Focus of 1.5 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 in small parts. In order to make the 3D printing technology of metal be applied in the field of large-scale casting and forging as soon as possible, the following aspects should be studied at present.
(1) Material aspect. Compared with titanium alloys and superalloys, iron-based alloys used in large castings and forgings are relatively simple, and the key is the control of material cost.
(2) Equipment. At present, the worktable size and load-bearing of 3D printing equipment are limited, which can not meet the local processing requirements of large castings and forgings. In the future, more flexible and convenient processing equipment needs to be developed.
(3) System aspect. 3D printing manufacturing and repair need three-dimensional reverse, modeling, two-dimensional path transformation and other operations, while large castings and forgings are mostly produced in small batches, and the defects appear randomly. Therefore, in the future, the corresponding software and operations need to be integrated to improve production efficiency.
(4) Technological aspects. Continuously optimize metal 3D printing process parameters to better adapt to the special requirements of large-scale casting and forging, prevent cracking, deformation and falling off and other defects.
(5) Performance. In order to make 3D printing technology be applied to large parts such as nuclear power, thermal power and large pressure vessel for a long time, the basic experimental data of materials such as creep and low cycle fatigue need to be accumulated for a long time.
(6) Standards. Unlike small parts, safety in production is the key point in repairing and manufacturing large castings and forgings. The lack of various production and quality standards is an important bottleneck in the development of 3D printing technology in this field, and the formulation of standards also takes a long time.
Reprinted from the Antarctic Bear, reprinted to convey more information.