Metal casting is a process of melting metal into liquid that meets certain requirements and pouring it into a mold. After cooling, solidification and cleaning, a casting with a predetermined shape, size and performance is obtained. The casting blank is almost formed, so it can be machined or processed in small quantities, which reduces costs and time to a certain extent. Casting is one of the basic processes in the modern machinery manufacturing industry.
Introduction History In ancient times, China, India, Babylon, Egypt, Greece and Rome had already cast weapons, sacrificial vessels, works of art and household appliances. Early castings were made of gold, silver, copper and their alloys. The Bronze Age was a glorious page in the history of human civilization; later, cast iron pieces appeared. China used cast iron pieces extensively as early as the Warring States Period, and the technical tradition has a long history. Metal casting metallurgy and casting are complementary processes, and the ancient Chinese term "metallurgy and casting" reflects this fact. Although the casting process has developed into a relatively independent branch of mechanical manufacturing technology in later generations, it is still inseparable from metallurgical technology.
① Cast iron has a long history, but its development is slow. The situation did not change until 1722, when Leomme invented the "cupola" and began to study the structure and fracture of cast iron with a microscope. In 1734, Svedenberg's "Deferro" was published, which gave a preliminary theoretical understanding of the cast iron process. Between 1765 and 1785, due to the emergence of steam engines, cast iron was used extensively in the machinery manufacturing industry since the 1760s. Cast iron rails were also used in the transportation sector. In 1788, a total of 60 kilometers of cast iron pipes were produced for the Paris Waterworks. The increasing demand for cast iron has promoted the technological progress and theoretical research of cast iron.
② Cast steel has a high melting point and complex composition control (such as deoxidation). It was not until 1740 that the crucible method of steelmaking was invented and the process began to develop.
③ Aluminum casting In 1886, the electrolytic production of aluminum by molten salt was invented. By the beginning of the 20th century, the casting industry of aluminum and its alloys had developed rapidly and ranked first in non-ferrous metal casting. However, the proportion of traditional copper alloy castings gradually decreased.
Metal casting methods have been developed to suit a variety of needs and to economically produce castings of complex shapes and various sizes.The casting properties of metals include: ① fluidity, the ability of liquid metal to fill the mold; ② shrinkage tendency, the volume shrinks greatly from liquid to solid, which is prone to defects such as shrinkage holes, and the linear size shrinks greatly, affecting the final dimensional accuracy of the casting; ③ thermal cracking tendency, some metals and alloys are prone to cracks and segregation tendencies during the casting process.
Classification
Metal casting types and molding methods are usually divided into:
① Ordinary sand casting, including three types: wet sand mold, dry sand mold and chemical hardening sand mold.
② Special casting, according to the molding material, can be divided into special casting with natural mineral sand and stone as the main molding material (such as investment casting, mud casting, foundry shell casting, negative pressure casting, solid casting, ceramic casting, etc.) and special casting with metal as the main casting material (such as metal mold casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc.).
Metal casting processes usually include:
① Preparation of molds (containers that turn liquid metal into solid castings). Molds can be divided into sand molds, metal molds, ceramic molds, clay molds, graphite molds, etc. according to the materials used. They can be divided into disposable molds, semi-permanent molds and permanent molds according to the number of uses. The quality of mold preparation is the main factor affecting the quality of castings;
② Melting and pouring of cast metals. Cast metals (cast alloys) mainly include cast iron, cast steel and cast non-ferrous alloys;
③ Casting processing and inspection. Casting processing includes removing foreign matter from the core and casting surface, removing pouring heads, grinding burrs and seams and other protrusions, as well as heat treatment, shaping, rust prevention and rough processing, etc.
Features and advantages The metal mold cools faster, the casting structure is denser, and can be strengthened by heat treatment. The mechanical properties are about 15% higher than those of sand casting.
Metal mold casting has stable casting quality, better surface roughness than sand casting, and low scrap rate.
Good working conditions, high productivity, and easy for workers to master.
Disadvantages The metal mold has a large thermal conductivity and poor filling capacity.
The metal mold itself has no air permeability. Appropriate measures must be taken to effectively exhaust.
The metal mold has no yield and is prone to cracks and deformation during solidification.
Application of Sand Casting Sand casting has a wide range of adaptability, and can be used for small and large pieces, simple and complex pieces, single pieces and large batches. Sand molds are more refractory than metal molds, so materials with higher melting points such as copper alloys and ferrous metals also use this process.
The molds used for sand casting are generally made of wood, commonly known as wooden molds. In order to improve the dimensional accuracy, aluminum alloy molds or resin molds with longer service life are often used. Although the price has increased, it is still much cheaper than the molds used for metal casting. In the production of small batches and large pieces, the price advantage is particularly prominent.
Metal mold casting When using metal mold casting, the following factors must be considered comprehensively: long manufacturing cycle and high cost, not suitable for single piece and small batch production; not suitable for casting complex shapes (especially inner cavity), thin walls and large castings (metal molds are limited by the size of mold materials and the capacity of cavity processing equipment and casting equipment, so metal molds are not suitable for the production of particularly large castings) The mold fee is more expensive than sand molds and cheaper than die casting.
Gravity casting is widely used in the production of various non-ferrous castings, but metal mold casting also has disadvantages such as low metal utilization, difficulty in pouring thin-walled complex castings, and lower density of castings than pressure casting.
In high-pressure casting, the air in the cavity is inevitably enclosed in the casting during the process of filling the cavity under high pressure and high speed, forming subcutaneous pores. Therefore, aluminum alloy die castings are not suitable for heat treatment, and zinc alloy die castings are not suitable for surface spraying (but can be painted). Otherwise, when the pores inside the casting are heated for the above treatment, they will expand due to heat and cause the casting to deform or bubble.
The mechanical cutting allowance of die castings should also be smaller, generally around 0.5mm, which can not only reduce the weight of the casting, reduce the amount of cutting to reduce costs, but also avoid penetrating the dense layer on the surface, exposing subcutaneous pores, and causing the workpiece to be scrapped.
Since die castings are loose inside, with poor plasticity and toughness, they are not suitable for manufacturing parts that bear impact loads. The wall thickness of the casting is uniform, and 3~4mm thin-walled castings are preferred. The maximum wall thickness should be less than 6~8mm to prevent defects such as shrinkage cavities. Avoid machining to prevent internal holes from being exposed.
Low-pressure casting molten metal fills the mold under pressure, which can improve the fluidity of the molten metal, and the casting has good formability, which is conducive to the formation of castings with clear contours and smooth surfaces, and is more beneficial for the formation of large thin-walled castings; the castings crystallize and solidify under pressure, and can be fully compensated for shrinkage, so the castings have dense tissue and high mechanical properties; the process yield of the molten metal is improved, and generally no riser is required, which greatly improves the yield of the molten metal, and the yield can generally reach 90%. Good working conditions, high production efficiency, and easy mechanization and automation are also outstanding advantages of low-pressure casting.
Low-pressure casting has a wide range of application for alloy grades, and can basically be used for various casting alloys. It is not only used for casting non-ferrous alloys, but also for cast iron and cast steel. Especially for nonferrous alloys that are easily oxidized, it shows its superior performance, that is, it can effectively prevent the formation of oxidation slag in the molten metal during the pouring process. Low-pressure casting has no special requirements for casting materials.
