Casting is the process of melting metal into a liquid that meets certain requirements and pouring it into the mold. After cooling, solidification and cleaning treatment, the castings with predetermined shape, size and performance can be obtained. Casting is one of the basic processes of modern mechanical manufacturing industry.
Casting is a kind of metal hot working technology which has been mastered by human beings for about 6000 years. China has entered the heyday of bronze casting from about 1700 BC to 1000 BC, and its technology has reached a quite high level. The 875kg simuwangding of the Shang Dynasty, the zunpan of Marquis Yi of the Warring States period and the transparent mirror of the Western Han Dynasty are all representative products of ancient casting.
Most of the early castings were tools or utensils for agricultural production, religion, life and other aspects, with strong artistic color. At that time, the foundry technology developed in parallel with the ceramic technology, which was greatly influenced by the pottery.
In 513 B.C., China cast the firstin the world, Jin casting tripod, weighing about 270 kg. Iron castings were also produced in Europe around the eighth century A.D. The appearance of iron castings enlarges the application of castings. For example, in the 15th-17th century, Germany, France and other countries successively laid many cast iron pipes to provide drinking water for residents. After the industrial revolution in the 18th century, steam engine, textile machine, railway and other industries rose, casting entered a new period of service for large industry, casting technology began to have a great development.
In the 20th century, the development speed of casting is very fast. One of the important factors is the progress of product technology, which requires that all kinds of mechanical and physical properties of castings are better, while they still have good machining performance. The other reason is that the development of mechanical industry itself and other industries, such as chemical industry and instruments, has created favorable material conditions for the casting industry. For example, the development of detection means ensures the improvement and stability of casting quality, and provides conditions for the development of casting theory; the invention of electron microscope helps people go deep into the micro world of metal, explore the mystery of metal crystallization, study the theory of metal solidification, and guide casting production.
During this period, a large number of new cast metal materials with superior properties and rich varieties were developed, such as nodular cast iron, malleable cast iron that can be welded, ultra-low carbon stainless steel, aluminum copper, aluminum silicon, aluminum magnesium alloy, titanium base, nickel base alloy, etc., and a new process for inoculation treatment of gray cast iron was invented, making the adaptability of castings more extensive.
After the 1950s, new technologies such as wet sand high-pressure molding, chemical hardening sand molding and core making, negative pressure molding and other special casting and shot blasting cleaning appeared, which made the castings have high shape, dimensional accuracy and good surface finish. The working conditions and environmental sanitation of the casting workshop were greatly improved.
In the great progress ofsince the 20th century, the two new processes of inoculation treatment and chemical hardening sand molding of grey cast iron have special significance. These two inventions broke through the traditional method which lasted for thousands of years, opened up a new field for foundry technology, and had a significant impact on improving the competitiveness of castings.
Casting is generally classified according to the molding method, which is usually divided into ordinaryand special casting. Common sand casting includes wet sand, dry sand and chemical hardening sand. According to the different molding materials, special casting can be divided into two categories: one is to use natural mineral sand as the main molding materials, such as investment casting, shell casting, negative pressure casting, mud casting, solid casting, ceramic casting, etc.; the other is to use metal as the main molding materials, such as , centrifugal casting, continuous casting, pressure casting, low pressure casting, etc.
Thecan be divided into three basic parts: the preparation of casting metal, the preparation of casting mold and the treatment of casting. Cast metal refers to the metal material used for casting in casting production. It is an alloy composed of a metal element as the main component and other metal or non-metal elements. It is commonly known as cast alloy, mainly including cast iron, cast steel and cast non-ferrous alloy.
Metal smelting is not only a simple melting, but also a smelting process, so that the metal poured into the mold meets the expected requirements in terms of temperature, chemical composition and purity. Therefore, in the smelting process, it is necessary to carry out various inspection and tests for the purpose of quality control, and the liquid metal can be poured only after reaching various specified indexes. Sometimes, in order to meet higher requirements, the molten metal must be treated outside the furnace after being discharged, such as desulfurization, vacuum degassing, refining outside the furnace, inoculation or modification. The commonly used equipment for smelting metal include cupola, electric arc furnace, induction furnace, resistance furnace, reflection furnace, etc.
Different casting methods have different mold preparation contents. For example, the most widely used sand casting, mold preparation includes the preparation of molding materials and molding core. In sand casting, all kinds of raw materials used for molding and core making, such as casting sand, molding sand binder and other auxiliary materials, as well as molding sand, core sand and coating prepared from them, are collectively called molding material preparation tasks. According to the requirements of casting and the properties of metal, the task is to select appropriate raw sand, binder and auxiliary materials, and then mix them into a certain proportion Performance molding sand and core sand. The commonly used sand mixing equipment includes wheel mixer, counter current mixer and blade groove mixer. The latter is specially designed for mixing chemical self hardening sand, which is continuously mixed with high speed.
Molding and core making is based on the requirements of casting technology and the determination of molding methods and preparation of molding materials. The accuracy of castings and the economic effect of the whole production process mainly depend on this process. In many modern foundry workshops, molding and core making are mechanized or automated. The commonly used sand molding and core making equipment includes high, medium and low pressure molding machine, sanding machine, non box injection molding machine, core shooting machine, cold and hot core box machine, etc.
After the casting is taken out from the casting mold cooled by pouring, there are gates, risers and metal burrs. The casting of sand casting also adheres to sand, so it must go through the cleaning process. The equipment for this kind of work includes shot blasting machine, gate riser cutting machine, etc. Sand casting cleaning is a process with poor working conditions, so when choosing the molding method, we should try to create convenient conditions for sand cleaning. Some castings need to be treated after casting due to special requirements, such as heat treatment, reshaping, antirust treatment, rough machining, etc.
Casting is a relatively economic method of blank forming, which can show its economy for the parts with complex shape. Such as the cylinder block and cylinder head of automobile engine, ship propeller and exquisite works of art. Some parts that are difficult to cut, such as nickel base alloy parts of gas turbine, cannot be formed without casting.
In addition, the size and weight ofare widely used, and the metal types are almost unlimited; while the parts have general mechanical properties, they also have comprehensive properties such as wear resistance, corrosion resistance, shock absorption, etc., which other metal forming methods such as forging, rolling, welding, punching, etc. cannot do. Therefore, in the machine manufacturing industry, the number and tonnage of rough parts produced by casting method is still the largest.
The materials often used in foundry production include various metals, coke, wood, plastics, gas and liquid fuels, molding materials, etc. The required equipment includes various furnaces for smelting metal, various sand mixers for sand mixing, various molding machines and core making machines for molding and core making, sand dropping machines and shot blasting machines for cleaning castings, etc. There are also machines and equipment for special casting as well as many transportation and material handling equipment.
Casting production has different characteristics from other processes, such as wide adaptability, more materials and equipment, and environmental pollution. Foundry production will produce dust, harmful gas and noise pollution to the environment, which is more serious than other mechanical manufacturing processes, and measures need to be taken to control.
The development trend of casting products requires better comprehensive properties, higher precision, less allowance and cleaner surface. In addition, the demand for energy conservation and the demand of society for the restoration of the natural environment are also growing. In order to meet these requirements, new cast alloys will be developed, and new smelting processes and equipment will appear accordingly.
At the same time, the degree of mechanization and automation of foundry production is increasing, and it will develop to flexible production, so as to expand the adaptability to different batches and varieties of production. New technologies for saving energy and raw materials will be given priority, and new processes and equipment with little or no pollution will be given priority. The quality control technology will have a new development in the detection, nondestructive testing and stress measurement of each process.
With the development of electronic technology and testing methods, foundry workers will further explore the theory of metal solidification and sand compaction, so as to study the effective ways to improve the performance and internal quality of castings. The application of robot and electronic computer in the field of foundry production and management will be more and more extensive.