abstract
Casting is a kind of metal hot working technology that human beings mastered earlier, and it has a history of about 6000 years. China entered the heyday of bronze casting from about 1700 BC to 1000 BC, and its technology reached a fairly high level. Casting refers to the processing method of melting solid metal into liquid and pouring it into a mold with a specific shape, waiting for its solidification. Metals to be cast are: copper, iron, aluminum, tin, lead, etc. The materials of ordinary molds include raw sand, clay, water glass, resin and other auxiliary materials. Special casting molds include: investment casting, lost foam casting, metal mold casting, ceramic mold casting, etc. (Raw sand includes quartz sand, magnesia, zircon sand, chromite sand, forsterite sand, kyanite sand, graphite sand, iron sand, etc. )
early stage
Simuwu Fang Ding of Shang Dynasty in China, weighing 875kg, Ceng Houyi Zunpan of Warring States and transparent mirror of Western Han Dynasty are all representative products of ancient casting. Most of the early castings were tools or utensils used in agricultural production, religion and life, with strong artistic color. At that time, casting technology and pottery technology developed simultaneously and were greatly influenced by pottery.
develop
In 5 13 BC, China cast the earliest recorded iron castings in the world-Ding cast by Jin State, weighing about 270kg. Europe also began to produce iron castings around the eighth century. The appearance of iron castings has expanded the application scope of castings. For example, in the 15 ~ 17 century, Germany, France and other countries laid many cast iron pipes to supply drinking water for residents. /kloc-after the industrial revolution in the 0/8th century, industries such as steam engines, textile machines and railways rose, and castings entered a new era of serving large industries, and casting technology began to develop greatly.
Modern Times
In the 20th century, casting developed very rapidly. One of the important factors is the progress of product technology, which requires castings to have better mechanical and physical properties and good machinability. Another reason is that the development of machinery industry itself and other industries such as chemical industry and instruments has created favorable material conditions for the foundry 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 microscopic world of metal, explore the mystery of metal crystallization, study the theory of metal solidification and guide casting production.
Foundry definition
(GB/t 56 1 1- 1998)
Casting-melting metal, making a mold, pouring the molten metal into the mold, and solidifying to obtain a metal part blank with a certain shape, size and performance.
Casting is the process of melting metal into liquid that meets certain requirements and pouring it into the mold. After cooling, solidification and cleaning, castings with predetermined shapes, sizes and properties are obtained. Because the billet is almost shaped, it can achieve the purpose of no processing or a small amount of processing, which reduces the cost and production time to some extent. Casting is one of the basic processes in modern device manufacturing industry.
Foundry classification
There are two main categories: sand casting and special casting.
1 sand used for ordinary sand mold casting is also called sand mold casting and sand turning, including three types: wet sand mold, dry sand mold and chemically hardened sand mold, but not all sand can be used for casting. The advantage is that the cost is lower, because the sand used in the mold can be reused; The disadvantage is that the mold is time-consuming, and the mold itself cannot be reused, and the finished product can only be obtained after it is destroyed.
1. 1 sand mold (core) casting method: wet sand mold, resin self-hardening sand mold, sodium silicate sand mold, dry type and surface dry type, full mold casting and negative pressure molding.
1.2 manufacturing method of sand core: it is selected according to the size, shape, production batch and specific production conditions of sand core. In production, it can be generally divided into manual core making and machine core making.
Special casting can be divided into two types according to molding materials: special casting with natural ore as the main molding material (such as investment casting, mud casting, shell casting, negative pressure casting, full mold casting, ceramic mold casting, etc. ) and special casting with metal as the main mold material (such as metal mold casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc. ).
2. 1 permanent mold casting method
Make molds with metals with higher melting point than raw materials. Among them, it is subdivided into gravity casting, low pressure casting and high pressure casting.
Limited by the melting point of the mold, the metal that can be cast is also limited.
2.2 dewaxing casting method
This method can be external film casting method and solid casting method.
First, copy the articles to be cast with wax, then immerse them in a jar containing ceramics (or silica sol) to dry, so that the wax copy is covered with a ceramic outer film, and repeat the above steps until the outer film is enough to support the casting process (about 1/4 inches to 1/8 inches), and then melt the wax in the mold and take it out of the mold. After that, the mold needs to be heated several times to enhance the hardness before casting.
This method has good accuracy and can be used for casting high melting point metals (such as titanium). However, because ceramics are expensive, they need to be heated many times, and the production is complicated and the cost is quite expensive.
Molding process
1. Gravity casting: sand casting and hard mold casting. The molten metal is injected into the cavity by the gravity of the metal itself.
2. Pressure casting: low pressure casting, high pressure casting. The molten metal is instantly pressed into the casting cavity by extra pressure.
The casting process usually includes
(1) Prepare the mold (container for making liquid metal into solid castings). According to the materials used, molds can be divided into sand mold, metal mold, ceramic mold, clay mold, graphite mold and so on. According to the times of use, it can be divided into disposable molds, semi-permanent molds and permanent molds. Mold quality is the main factor affecting the quality of castings;
(2) Melting and pouring of casting metals, casting metals (casting alloys) mainly include all kinds of cast iron, cast steel and casting nonferrous metals and alloys;
(3) Casting processing and inspection. Casting treatment includes removing foreign bodies from the core and the surface of the casting, cutting off the riser, shoveling off burrs, overlapping seams and other protrusions, as well as heat treatment, shaping, rust prevention and rough machining.
The casting process can be divided into three basic parts, namely, casting metal preparation, mold preparation and casting treatment. Casting metal refers to the metal material used to pour castings in casting production. It is an alloy with a metallic element as the main component and other metallic or nonmetallic elements. Traditionally called cast alloy, it mainly includes cast iron, cast steel and cast nonferrous alloy.
Metal melting is not only a simple melting, but also a melting process, which makes the metal poured into the mold meet the expected requirements in temperature, chemical composition and purity. Therefore, in the process of smelting, all kinds of inspections and tests aimed at quality control should be carried out, and liquid metal can only be poured after reaching various specified indexes. Sometimes, in order to meet higher requirements, molten metal must be treated outside the furnace, such as desulfurization, vacuum degassing, refining outside the furnace, inoculation or deterioration. Commonly used metal smelting equipment includes cupola, electric arc furnace, induction furnace, resistance furnace and reverberatory furnace.
Advantages: 1, can produce parts with complex shapes, especially blanks with complex cavities;
2, wide adaptability, all metal materials commonly used in industry can be cast, ranging from a few grams to hundreds of tons;
3. Raw materials are widely available and cheap, such as scrap steel, waste parts and fragments.
4. The shape and size of the casting are very close to the parts, which reduces the cutting amount and belongs to non-cutting processing;
5. It is widely used. 40%~70% of agricultural machinery and 70%~80% of machine tools are castings.
Disadvantages: 1, mechanical properties are not as good as forgings, such as coarse structure and many defects;
2. Sand casting, single piece, small batch production, and the labor intensity of workers is high;
3. The quality of castings is unstable, there are many processes, and the influencing factors are complex, which is easy to produce many defects.
Casting defects have an important influence on casting quality. Therefore, we should start with the quality of castings and combine the formation and prevention of main defects of castings to lay a good foundation for the selection of casting alloys and casting methods.