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The effect of casting solidification and shrinkage on casting defects

Issuing time:2021-01-07 15:45Author:momo

The alloy solidification temperature range and the casting temperature gradient will affect the solidification mode of the casting. The different chemical composition, pouring temperature and casting structure will affect the gradual shrinkage.

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Casting solidification method and influencing factors

The solidification method of the casting. (A) The solid phase and liquid phase onthe cross section of the alloy are clearly separated by a boundary line during the solidification process. This solidification method is called layer-by-layer solidification. Common alloys such as gray cast iron, low carbon steel,industrial pure copper, industrial pure aluminum, eutectic aluminum siliconalloy and some brass are all alloys that solidify layer by layer. (B) The alloy is mushy first and then solidifies during the solidification process. This solidification method is called mushy solidification. Nodular cast iron, high-carbon steel, tin bronzeand certain brasses are all alloys that solidify in a paste state. (C) The solidification of most alloys is between layer-by-layer solidification and paste solidification, which is called intermediate solidification. Medium carbon steel, high manganese steel, white cast iron, etc. have intermediate solidification methods.

The influencing factors of the solidification method: increasing the temperature gradient can transform the solidification method of the alloy to layer-by-layer solidification; conversely, the solidification method of the casting transforms to paste solidification.

Shrinkage of casting alloy

When the casting alloy is cooled from the liquid state to room temperature, the shrinkage of its volume and size is called shrinkage. It mainly includes three stages:

(A) Liquid shrinkage. The volume shrinkage of metal in liquid state due to temperature drop.

(B)Solidification shrinkage. The volume shrinkage of molten metal during the solidification stage. Liquid shrinkage and solidification shrinkage are the basic reasons for shrinkage and porosity in castings.

(C) Solidstate shrinkage. The volume shrinkage of a metal in a solid state due to adecrease in temperature. Solid state shrinkage has a great influence on the shape and dimensional accuracy of castings, and is the basic cause of casting stress, deformation and cracks.

Factors affecting alloy shrinkage

(A) The shrinkage of alloys with different compositions is generally different. Among the commonly used casting alloys, cast steel has the largest shrinkage and graycast iron has the smallest shrinkage.

(B) The higher the alloy pouring temperature, the greater the superheat and the greater the liquid shrinkage.

(C) When castings cool and shrink, because of their different shapes and sizes, the cooling speeds of each part are different, resulting in inconsistent shrinkage and mutual obstruction. In addition, the resistance of the mold and core to the shrinkage of the casting, so the actual shrinkage rate of the casting always less than its free shrinkage rate. The greater this resistance, the smaller the actual shrinkage of the casting.

The effect of shrinkage on casting quality

A.Shrinkage and porosity

Shrinkage cavity always appears in the upper part of the casting or the final solidified part, and its shape characteristics are: the inner surface is rough, the shape is irregular, and it is more close to an inverted cone. Shrinkage holes are usually hidden inside the casting, and sometimes they can only be exposed after cutting. The main reason for shrinkage cavity formation is liquid shrinkage and solidification shrinkage.

The macroscopic shrinkage porosity is mostly distributed in the final solidified part of the casting, while the microscopic shrinkage porosity is the tiny holes existing between the crystal grains. The main reason for the shrinkage porosityis liquid shrinkage and solidification shrinkage.

Measures to prevent shrinkage and porosity. (A) Adopt the principle of directional solidification. The so-called directional solidification is the process of gradually solidifying the casting from one part to another in a prescribed direction. Reasonable use of riser and chilled iron can cause directional solidification of castings and effectively eliminate shrinkage and porosity. (B) Reasonably determine the pouring position of the casting, the position of the inner runner and the pouring process. The choice of pouring position should follow the principle of directional solidification; the inner runner should be opened at the thick wall of the casting or close to the riser; the pouring temperature and pouring speed should be selected reasonably, and the pouring temperature and the pouring temperature should be reduced as much as possible without adding other defects pouring speed.

B.Casting stress,deformation and cracks

During the solidification of the casting and the subsequent cooling process, as the temperature continues to decrease, shrinkage continues to occur. If this shrinkage is hindered, stress will be generated in the casting, causing deformation or cracking. The occurrence of such defects will seriously affect the quality of the casting.

Measures to prevent and eliminate casting stress. (A) Adopting the principle of simultaneous solidification.Simultaneous solidification refers to the process measures such as setting the chilled iron and arranging the gate position to make the temperature difference of the casting as small as possible, and basically realize the solidification of all parts of the casting at the same time. (B) Increase the mold temperature.   (C) Improve the recession of the mold and core. (D) Perform stress relief annealing.

Deformationand prevention of castings.   The deformation of the casting includes the deformation that occursafter the solidification of the casting and the subsequent cutting deformation.There are several methods to prevent casting deformation:   (A) The reverse deformation method can be used to make a pre-deformation equal to the deformation of the casting but in the opposite direction to offset the deformation of the casting. This method is called the reverse deformation method. (B) Stress relief annealing should be performed before machining of stress relief annealing castings to stabilize the size of the castings and reduce the degree of machining deformation. (C) Setting process ribs in order to prevent the as-cast deformation of the castings, process ribs can be set at the places that are easy to deform.

Cracks and prevention of castings. (A) Classification and morphology of cracks in castings. Castingsgenerally have two cracking methods: hot cracking and cold cracking. When the linear shrinkage of the solid alloy is hindered, if the stress generated exceeds the strength of the alloy at this temperature, hot cracking will occur; while the cold cracking is caused by the casting stress exceeding the strength limit of the alloy when the casting is in an elastic state. Hot cracks generally occur and develop along the grain boundary, and their appearance is short and twisted, and the inner surface of the cracks is oxidized; cold cracks are often transcrystalline fractures, the cracks are small, and the shape is continuous straight or smooth, and the cracks are clean. , Sometimes it is slightly oxidized. (B) Prevention of casting cracks. In order to effectively prevent the occurrence of casting cracks, measures should be taken to reduce casting stressas much as possible. At the same time, during the metal smelting process, the amount of elements that may expand the solidification temperature range of the metal and sulfur and phosphorus content in steel should be strictly controlled.

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