Analysis on the Development Trend of Train Casting Technology

Section 1 Product Technology Development Status

Chinese train castings generally adopt the investment casting method. China began to apply investment casting to industrial production in the 1950s and 1960s. Since then, this advanced casting technology has been greatly developed and has been widely used in manufacturing industries such as aviation, automobiles, machine tools, ships, internal combustion engines, gas turbines, telecommunication equipment, weapons, medical equipment, and knives. Manufacture of fine art.

In general, academic research in the field of foundry in China is not backward, ad many research results are at the internationally advanced level, but few of them are transformed into actual productivity. The high level of domestic foundry production technology is limited to a few backbone enterprises. The overall technical level of the industry is backward, the quality of castings is low, the consumption of materials and energy is high, the economic benefits are poor, the working conditions are harsh, and the pollution is serious. The specific manifestation is that the mold is still processed by hand or simple machinery; the socialization, specialization, and commercialization of the production and supply of casting raw and auxiliary materials are far apart, and the variety and quality are far from meeting the needs of new technology and new technology development; The production level and quality of alloy materials are low; production management is backward; process design is mostly based on personal experience, and computer technology is rarely used; basic conditions such as casting technology and equipment are poor; the proportion of manual operations in the production process is high, and the technical quality of on-site workers is low; 1. The foundry of the Internal Combustion Engine Group adopts advanced molding core-making technology. Most foundry companies still use shock molding machines or even manual molding. The core making is mainly based on binder sand such as tung oil, synthetic resin, and clay. Most investment foundries mainly use sodium silicate shells; low-pressure casting can only produce non-ferrous or cast iron small and medium-sized pieces, and cannot produce steel castings; stable production with EPC technology is limited to castings such as exhaust pipes and shells, and the productivity Below 30 models/hour, the dimensional accuracy and surface roughness of castings are low; although a relatively complete casting industry-standard system has been established, most enterprises passively implement the standards, and the enterprise standards are mostly lower than GB (national standard) and ISO (international standard), Some companies have a scrap rate as high as 30%; quality and market awareness is not strong, and only a few specialized foundry companies have passed ISO9000 certification. The quality management research combined with the characteristics of foundry enterprises is very weak.

In recent years, some advanced smelting equipment has been developed and promoted, which has improved the temperature and comprehensive quality of molten metal. For example, the externally heated hot blast cupola has begun to be used, but the number is small, and only 1% of the casting coke is used. Some casting non-ferrous alloy factories still use backward smelting technologies such as fuel oil and coke crucible furnaces. The cupola-electric furnace duplex process is only applied in a small number of mass production lines. A small number of large and medium-sized electric arc furnaces adopt ultra-high power (600-700kVA/t) technology.

Started to introduce AOD, VOD, and other refining equipment and technologies to improve the inherent quality of high-grade alloy cast steel. The ultra-low-carbon high-strength martensitic stainless steel used in important projects adopts refining technology to improve the purity of molten steel and improve its performance. 0Cr16Ni5Mo, Cr13Ni5Mo cast martensitic stainless steel on the basis of maintaining the original toughness, the yield ratio is increased from 0.70-0.75 to 0.85-0.90, the strength is increased by 30%-60%, and the hardness is increased by 20%-50%.

Widely used domestic rich rare earth resources, such as rare earth magnesium treated nodular cast iron used in automobiles, diesel engines, and other products; rare earth medium carbon low alloy cast steel, rare earth heat-resistant steel used in machinery and metallurgical equipment; initially formed a domestic series of inoculants, nebulizers, and vernacularizes, which promote the improvement of the quality of iron castings.

High-strength, high-elastic-modulus gray cast iron is used for machine tool castings. The application of high-strength thin-walled gray iron castings makes the hardness difference between the cylinder body and cylinder head castings with the thinnest wall thickness of 4-6mm less than HB30, and the structure is uniform and dense. Gray cast iron surface laser strengthening technology is used for production. Application of artificial intelligence technology in performance prediction of gray cast iron. Vermicular graphite cast iron has been used in automobile exhaust pipes and high-horsepower diesel engine cylinder heads, and the service life of automobile exhaust pipes has been increased by 4 to 5 times. Vanadium-titanium wear-resistant cast iron is used on machine tool guide rails, cylinder liners, and piston rings, and the service life is increased by 1-2 times. High, medium, and low chromium wear-resistant cast iron is used on grinding balls, liners, impurity pumps, and bimetallic composite rolls to improve service life. Applying filtration technology to the production line of high-strength thin-walled castings such as cylinder blocks and cylinder heads reduces slag inclusions and pore defects and improves the internal quality of castings.

The domestic horizontal continuous casting production line was put into the market, which can produce gray cast iron and nodular cast iron profiles of Φ30-250mm round and square, rectangular or special-shaped cross-sections of corresponding sizes. Compared with the sand mold, the performance is improved by 1-2 grades, the utilization rate of molten iron is increased to more than 95%, energy saving is 30%, material saving is 30-50%, and the qualified rate of blank processing is more than 95%.

The cast iron pipe industry has introduced 23 sets of centrifugal casting equipment for medium-sized ductile iron pipes below Φ1000mm, of which 20 sets are currently in normal production. In addition, the ductile iron pipe centrifugal casting equipment developed by China has produced Φ2600mm cast pipes, of which the diameter of cast pipes produced in normal batches has reached 2200mm, and the cast pipe centrifugal casting equipment developed by China has reached 3000mm.

The research on metal matrix composites has made progress, and research on short fiber, external particle reinforcement, and in-situ particle reinforcement has achieved results, but few industrial applications have been realized.

some key industries have adopted direct-reading spectrometers and thermal analyzers to effectively control the composition of molten metal in front of the furnace, and use ultrasonic and other detection methods to control the quality of castings.

Environmental law enforcement is increasingly strengthened, forcing the foundry industry to pay attention to environmental protection technology. Shenyang Foundry Research Institute and others have developed large row spacing double-layer air supply cupola and cupola dehumidification and air supply technology; China has initially established a foundry coke production base and formed a batch scale. A series of achievements have been made in foundry dust and poison control, sewage purification, and waste residue utilization, and a variety of foundry environmental protection equipment (such as dust removal hoods for vibrating shakeout machines, mobile vacuum cleaners, smoke and dust purification devices, sewage purification, and recycling systems, foundry used sand Dry and wet regeneration technology and equipment, foundry waste sand, slag and waste plastics making composite material technology and equipment, etc.).

Commercialized CAE software is already on the market. Some large and medium-sized foundry enterprises have begun to use computer technology in smelting to control the composition, temperature, and productivity of molten metal. Chengdu University of Science and Technology has developed an online control system for sand processing, Tsinghua University has developed computer-aided sand mold control system software, and Huazhong University of Science and Technology has successfully developed commercial casting CAE software.

With the rapid development of the Internet in the foundry industry, some foundry companies have active online e-commerce activities. For example, some foundry mold factories have realized the remote design and remote manufacturing.

the research on the foundry expert system started late, but it made rapid progress. It has successively launched the molding sand quality management expert system, casting defect analysis expert system, no-bake sand quality analysis expert system, die-casting process parameter design and defect diagnosis expert system, etc. Manipulators and robots have begun to be used in shakeouts, casting cleaning, die casting, and investment casting production.

Section 2 Product Process Features or Processes

The so-called investment casting process, simply put, is to use fusible materials (such as wax or plastic) to make a fusible model (referred to as investment model or model), which is coated with several layers of special refractory coatings, dried and hardened. After forming an integral shell, the mold is melted from the shell with steam or hot water, and then the shell is placed in a sandbox, and dry sand is filled around it for molding, and finally, the mold is put into a roasting furnace for high-temperature Roasting (if a high-strength shell is used, the molded shell can be directly roasted without molding ), after the mold or shell is roasted, molten metal is poured into it to obtain a casting.

The dimensional accuracy of investment castings is high, generally up to ct4-6 (ct10~13 for sand casting, ct5~7 for die casting). Of course, due to the complexity of the investment casting process, there are many factors that affect the dimensional accuracy of castings, such as mold The shrinkage of the material, the deformation of the investment mold, the change of the line amount of the shell during the heating and cooling process, the shrinkage rate of the alloy, and the deformation of the casting during the solidification process, etc., so although the dimensional accuracy of ordinary investment castings is high, its The consistency still needs to be improved (the dimensional consistency of castings using medium and high-temperature waxes should be improved a lot).

When pressing the investment mold, the pressure type with a high surface finish of the cavity is used, so the surface finish of the investment mold is relatively high. In addition, the mold shell is made of high temperature-resistant special binder and refractory paint prepared by refractory materials, which are hung on the investment mold, and the inner surface of the mold cavity that is in direct contact with the molten metal has a high finish. Therefore, the surface finish of investment castings is higher than that of ordinary castings, generally up to ra.1.6~3.2μm.

The biggest advantage of investment casting is that due to the high dimensional accuracy and surface finish of investment castings, machining work can be reduced, and only a small amount of machining allowance can be left on parts with higher requirements, and even some castings Only the grinding and polishing allowance is left, and it can be used without machining. It can be seen that the investment casting method can greatly save machine tool equipment and processing man-hours, and greatly save metal raw materials.

Another advantage of the investment casting method is that it can cast complex castings of various alloys, especially superalloy castings. For example, the blades of jet engines, whose streamlined outline and inner cavity for cooling, can hardly be formed by mechanical processing technology. The investment casting process can not only achieve mass production, but also ensure the consistency of the castings, and avoid the stress concentration of the remaining knife lines after machining.

Analysis of the future development trend of domestic and foreign technologies

Investment casting production has many advantages, but it also has the disadvantages of many procedures, complicated processes, long production cycles, and many factors affecting the quality of castings, which restricts the application and development of precision casting to a certain extent. With the rapid development of computer technology, the application of computer technology in precision casting, from the structural design of precision castings, and process formulation to pressure design and manufacturing, wax mold molding, shell manufacturing, core manufacturing, etc. production has brought about a huge change.

1. Application of computer technology numerical simulation technology in investment casting structure design and process formulation

Investment castings are developing towards lighter, thinner, and more refined parts. In recent years, net-shape or near-net-shape casting has been proposed to give full play to the advantages of investment casting and meet the needs of modern industries for high-quality parts. This requires a more reasonable structure of investment castings and a more optimized process plan, which puts forward higher and higher requirements for investment casting technology.

The traditional precision casting production process includes the following 5 steps:T

1) The casting user issues the design blueprint to the foundry;

2) The foundry makes a budget and puts forward suggestions for improving the design from the perspective of facilitating production and reducing costs;

3) Foundry design casting process equipment;

4) The foundry issues tooling drawings to the mold workshop or modeling workshop;

5) Pouring castings, casting inspection.

In the process of casting structure design, profiling design, wax injection process parameter formulation, pouring system, etc., traditional production mainly relies on the actual work experience of engineers and technicians, lacking a scientific theoretical basis. Especially for complex and important parts, it is often necessary to repeatedly modify the casting structure, molding, or casting process plan in production to meet the final technical requirements. Visualization technology combined with classical heat transfer, flow, and solidification theory, by simulating casting filling, solidification, and cooling, analyzing flow field, temperature field, and stress field in the precision casting process, predicting casting structure and many casting defects such as cold shut, shrinkage cavity, thermal cracking, and deformation, etc. Therefore, through concurrent engineering, computer technology can be used to simulate the structural manufacturability and casting process of castings, which provides an effective basis for technicians to design a more reasonable casting structure and determine a reasonable process plan, thereby avoiding the traditional experience of relying on on experience. The blindness of design and process formulation can shorten the production preparation cycle and save trial production costs.

Schematic diagram of the numerical simulation process

2. Application of rapid prototyping technology in molding and investment manufacturing

The emergence of rapid prototype manufacturing technology has greatly shortened the manufacturing cycle of pressing and investment molds. The so-called rapid prototyping is to first form the three-dimensional CAD data file of the investment casting on the computer, cut it into many thin slices along the height direction, and then manufacture and assemble in order to finally form a three-dimensional shape of the product.

1) Manufacture the profile with the rapid prototyping method

According to the molding method, the rapid prototyping method can be divided into two types: one is to use the rapid prototyping method to make a resin or wax master model (prototype) and then use it to reproduce the epoxy resin Or silicone rubber molding. This method can meet the requirements of small-batch precision casting production. For example, a metal layer about 2 mm thick is sprayed on the surface of the plastic master mold made by the SLA method, and the back part is filled with epoxy resin to make a metal-epoxy resin composite molding, which can meet the production of hundreds of precision castings in batches.

Another method is to make resin molding directly by SLA, SLS, and other methods according to the geometric model of the profiling block produced by the CAD system. The SLS method is used to manufacture the profile by changing the processing object from resin powder to steel powder with a thin layer of thermosetting resin on the surface. After laser sintering, it is bonded into a profile, and then the product is fired to burn off the resin. In infiltration, you can get a profile similar to the performance of the metal.

2) Rapid prototyping method to manufacture investment molds

Rapid prototyping methods – SLA, SLS, FDM, and LOM – can all be used for the rapid manufacturing of investment patterns. Wax patterns made by the SLS method and FDM method can be directly used as investment patterns for precision casting production; paper products produced by the LOM method need to be sprayed with polyurethane on the outer surface before they can be used as “investment patterns” for shell making. Or directly coat the paper product with a ceramic shell, and then burn the paper mold. The SLA method is to use a new type of resin to produce a resin pattern, pour out the uncured resin to form a hollow pattern, after hardening, seal the resin outlet with wax, and then install a wax casting system to make the shell.

3. DSPC method directly manufactures the shell

Direct shell manufacturing, also known as the DSPC method, is essentially different from all shell-making processes so far. It mainly consists of two parts: shell design (SDV) and shell manufacturing (SPU).

The SDV method is to convert the CAD model of the manufactured part into a digital part of the shell and display it on the screen. After determining the number of parts on each shell, the thickness of the shell, the shrinkage rate, and the casting system and other casting parameters, the computer will quickly display the geometric shape of the casting shell and simulate the casting process, and then transmit the relevant data to the SPU.

The SPU controls a piston that can move up and down accurately, and a material box is connected to the piston; the nozzle connected to the hopper containing fine ceramic powder first “sprays” a thin layer of fine ceramic powder evenly in the material box; in addition, the computer According to the SPU data, a spray printing head is controlled, from which the silica sol binder can be sprayed out. When the print head brushes the fine ceramic powder in the material box, the binder is “sprayed” according to the instruction. In this way, in the area with a binder, the refractory materials are stuck together to form a section of the shell, and then the piston moves downward, and the nozzle sprays out a layer of powder… This is carried out layer by layer, and finally, an integral shell is made. The unbonded refractory powder can support the bonding layer. After firing, the unbonded powder can be recovered and the molten metal can be poured. Its working principle is shown in Figure 2. The DSPC method saves investment casting from the process of making pressure molds, making wax molds, and coating and hanging, the process is greatly simplified, and because factors such as wax mold deformation do not need to be considered, near-net-shape parts can be produced. Factories utilizing this process can deliver investment castings within a week of receiving an order.

Direct Shell Manufacturing (DSPC)

4. Use the computer-controlled laser to make a ceramic core

Many precision castings need to make ceramic cores, especially complex and fine ceramic cores, such as hollow blades of turbine engines, etc. The computer can control the laser beam to precisely process various cores with different structures on the ceramic core according to the CAD data. , especially for cores that are difficult to make with traditional core-making processes, its advantages are even more evident.

5. Application prospect of concurrent engineering and integrated technology in the investment casting industry

With the continuous development and popularization of computer technology, the application of concurrent engineering and integrated technology in the investment casting industry will gradually become more and more extensive, which will become the future development trend of the investment casting industry.

1) Concurrent Engineering

Concurrent engineering is to establish an electronic data communication network closely connected between users of precision castings and casting factories so that concurrent product and process design can be carried out between users and foundries. The user sends the electronic model diagram of the precision casting to the foundry through this network, and the casting engineer can see the three-dimensional image of the produced part from the computer workstation, and after determining several sets of process plans, carry out the numerical simulation of the process plan on the computer, which can be It shows possible problems under different process conditions, such as hot cracking, shrinkage cavity, etc., and the casting engineer will quickly transfer the defective electronic model data files to users and designers in order to make improvements and obtain high-quality castings. Similarly, the process of pressing, investment, and shell manufacturing can also be realized in parallel, which can greatly shorten the research and development production cycle, reduce costs, and improve the market competitiveness of products.

2) Integrated technology

For a part not designed by a CAD system or to copy a certain sample, CT detection technology, numerical simulation, and rapid sample manufacturing integrated technology can be used.

CT technology, namely computer tomography, is an X-ray detection technology that can be used to obtain two-dimensional images of parts sections, and combine the two-dimensional images of each section to obtain the three-dimensional shape of the measured object. Using this technology, the CAD model data of the casting can be accurately obtained, combined with rapid sample manufacturing and numerical simulation, it can shorten the production preparation time and reduce the cost of manufacturing the shell. At the same time, the shape of the part measured by CT technology can be used to compare the size of the designed casting and the production casting; to detect the degree of conformity between the defect position of the actual casting and the designed casting and the numerical simulation prediction results.

5. The application of computers in the precision casting industry overcomes the shortcomings of the precision casting production process, making the precision casting production technology more flexible and adaptable, and more suitable for the modern industry’s requirements for fast, high-quality, and complex castings.

1) The application of computer technology numerical simulation technology in investment casting structure design and process formulation provides an effective basis for technicians to design a more reasonable casting structure and determine a reasonable process plan.

2) The application of rapid prototyping technology in profiling and investment mold manufacturing greatly shortens the cycle of profiling and investment mold manufacturing.

3) The DSPC method directly manufactures the shell, which saves the long cycle of traditional shell making and coating the shell layer by layer.

4) The use of computer-controlled lasers to make ceramic cores can produce complex ceramic cores.

5) With the continuous development and popularization of computer technology, the application of concurrent engineering and integrated technology in the investment casting industry will gradually become more and more extensive, which will become the future development trend of the investment casting industry.

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