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The effect of casting solidification and shrinkage on casting defects

Issuing time:2020-09-24 13:46Author:momo

The effect of casting solidification and shrinkage on casting defects

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.


Casting solidification ways and influencing factors

A.   There are three main ways of casting solidification

A) Layer-by-layer solidification. The solid phase and liquid phase on the cross section of the alloy are clearly separated by a boundary line during the solidification process. This solidification is called layer-by-layer solidification. Common alloys such as gray cast iron, low carbon steel, industrial pure copper, industrial pure aluminum,eutectic aluminum-silicon alloy and some brass are all alloys that solidify layer by layer.

B) Pasty solidification.   The alloy is first mushy and then solidifies during the solidification process. This solidification is called mushy solidification. Ductile iron, high-carbon steel, tin bronze, and certain brassare all alloys that solidify in a paste state.

C) Intermediate solidification. 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. are all alloys that intermediate solidification .

B.   Influencing factors of solidification ways

A) The influence of alloy solidification temperature range

The liquidus and solid phases of the alloy cross together, or the distance is small, the metal tends to solidify layer by layer; if the distance between the two phase lines is large, it tends to solidify in a paste state; such as between two phase lines The smaller the distance, the intermediate solidification mode tends to be.

B)The influence of the temperature gradient of the casting

Increasing the temperature gradientcan 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 along with room temperature, the shrinkage of its volume and size is called shrinkage. It mainly includes the following three stages:

A.   Liquid shrinkage. The volume shrinkage that occurs when metal is in a liquid state due to a decrease in temperature.

B.   Solidification shrinkage. The volume shrinkage of molten metal in the solidification stage.Liquid shrinkage and solidification shrinkage are the basic causes of shrinkage and porosity in castings.

C.   Solidstate shrinkage. The volume shrinkage of metal in solid state due to temperature decrease. 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. Alloys with different chemical compositions generally have different shrinkage rates. Among the commonly used casting alloys, cast steel has the largest shrinkage and gray cast iron has the smallest shrinkage.

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

C. Casting structure and mold conditions. When the casting cools and shrinks, due to the difference in shape and size, the cooling rate of each part is different, resulting in inconsistent shrinkage and mutual obstruction, plus the resistance of the mold and core tothe shrinkage of the casting. Therefore, the actual shrinkage rate of the castingis 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.   Shrink holes and shrinkage

A)The formation of shrink holes.

Shrink holes always appear 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 itis more close to an inverted cone. Shrink holes are usually hidden inside the casting, and sometimes they can only be exposed after cutting. The main reason for shrink holes formation is liquid shrinkage and solidification shrinkage.

B)The formation of shrinkage porosity.

Macroscopic shrinkage porosity is mostly distributed in the final solidified part of the casting, while microscopic shrinkage porosity is the tiny pores existing between the crystalgrains. The main reason for the formation of shrinkage porosity is also liquid shrinkage and solidification shrinkage.

C)Measures to prevent shrinkage and shrink holes.

a) Adopting the principle of directional solidification. The so-called directional solidification is the process of gradually solidifying the casting from one part to another in aprescribed 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 selection of the pouring position should obey 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 reasonably selected , Without increasing other defects, the pouring temperature and pouring speed should be reduced as much as possible.

A.   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.

A)   The generation of casting stress. According to the reasons, the casting stress can be divided into three types:

a)    Thermal stress. In the process of solidification and cooling of the casting, the stress caused by uneven shrinkage of different parts.

b)    Solid phase transformation stress. Due to the solid phase transformation, the stress caused by the unbalanced changes in the volume of each part of the casting.

c) Shrinkage stress. When the casting shrinks in the solid state, it is the stress generated by external forces such as the mold, core, riser, box stop, etc. The internal stress that exists indifferent parts of the casting after the casting is called the residual stress.

B)   Measures to prevent and eliminate casting stress.

a)    Adopt the principle of simultaneous solidification. Simultaneous solidification refers to the process measures such as setting the cold iron and arranging the gate position to minimize the temperature difference of the casting as much aspossible, and basically realize the solidification of all parts of the casting at the same time.

b)    Increasethe mold temperature.

c)    Improve the retreat of mold and core.

d)    Perform stress relief annealing.

C) Deformation and prevention of casting. Deformation of casting includes the deformation that occurs after the solidification of the casting and the subsequent cutting deformation. There are several ways to prevent casting deformation:

a)   Adopt the anti-deformation method. A pre-deformation equal to the deformation of the casting but opposite to the direction can be made on the pattern to offset the deformation of the casting. This method is called inverse deformation.

b)   Perform stress relief annealing. Before machining the casting, stress relief annealing should be carried out to stabilize the size of the casting and reduce the degree of machining deformation.

c) Set up process ribs. In order to prevent the as-cast deformation of the casting, process ribs can be provided at the positions that are easily deformed.

D) Casting cracks and prevention.

a) The classification and morphology of casting cracks. Castings generally have two cracking methods, hot cracking and cold cracking. When the linear shrinkage of the solid alloy is hindered, ifthe stress generated exceeds the strength of the alloy at this temperature, hot cracks will occur; while cold cracks are produced when the casting stress exceeds 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 as much as possible to reduce the casting stress; at the same time,during the metal smelting process, the amount of elements that may expand the solidification temperature and the sulfur and phosphorus content range of the metal should be strictly controlled.

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