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Shortening Property and Shrinkage Pore Composition of Cast Alloys


Popularity: 22 release time: 2018-10-23

I. Shortening of Cast Alloys

1. The Fundamental Concept of Shortening

When the temperature of liquid alloys decreases and changes from liquid to solid, volume reduction usually occurs due to the gradual change of metal atoms from short-range to long-distance ordering, and the reduction and disappearance of holes. After solidification of liquid alloys, with the continuous decrease of temperature, the atomic spacing will be shortened and the volume will be reduced further. In the process of liquid, solidification and solid cooling, the volume of casting alloy decreases due to the decrease of temperature, which is called the shortening property of casting alloy.

Shortening is also the fundamental cause of many defects in castings, such as shrinkage, shrinkage, stress, deformation and cracking. It is one of the important casting functions of casting alloys. It has a great influence on castings (such as obtaining the required shape and size, fine and high quality castings).

The volume change of casting alloys from liquid to normal temperature is expressed by volume change, which is called volume reduction. The shortening of alloys in solid state can be expressed not only by volume change but also by length, which is called line shortening. Because shortening lines is more meaningful when planning and making looks. The linear shortening rate is generally 1/3 of the volume shortening rate. The alloy has three stages of shortening: liquid shortening stage, solidification shortening stage and solid shortening stage.

(1) Liquid shortening is the shortening of liquidus temperature T when liquid alloy is cooled from pouring temperature T to beginning condensation. Because the alloy is in liquid state, it is called liquid shortening, which is manifested by the decrease of liquid level in the cavity.

(2) Condensation shortening is called condensation shortening when an alloy with a certain temperature scale is changed from liquid to solid because the alloy is in a condensation state. The condenser shortening of this kind of alloy mainly includes two parts: temperature drop (which is related to the crystallization temperature scale) and condition change (volume change when the condition changes).

For a small amount of alloys and metals, the volume increases during solidification because of the negative condensate shortening rate. (Bi, Si, Bi-Si alloys and gray cast iron).

Liquid shortening and condensation shortening are the basic causes of shrinkage and porosity in castings.

(3) Solid state shortening is called solid state shortening when casting alloy is cooled from solid state temperature T to room temperature T chamber shortening, because the alloy is in solid state.

But in practice production, because the solid state shortening is often manifested by the reduction of the size of the castings, the line shortening rate is generally used to express it.

If the wire shortening of the alloy is not obstructed by the external conditions of the mould, it is called free shortening. Negative, for the blocked line shortening.

Line shortening of cast alloys not only has a direct impact on the dimensional accuracy of castings, but also is the fundamental cause of stress, crack and deformation in castings.

2. casting line shortening rate

The shortening rate of cast alloys mentioned above is only related to chemical composition, shortening coefficient, temperature change and volume change during phase change. In the process planning of castings, considering the shortening, the demand will expand the scale of appearance. There are the following links between the scale of appearance L mold and the scale of castings L. (?) = L-die-L castings/L-die * * * * * * * * * * * * * * * * * * * * * * *

The casting shortening rate of castings is not only related to the elements of the alloys used, but also to the characteristics of the casting process, the structure and shape of the castings and the amount of dissolved gas in the melting process of the alloys.

Shrinkage and porosity in castings

1. Basic concepts of shrinkage and porosity

In the process of cooling and condensation of castings, because of the shortening of liquid state and condensation of alloys, voids often appear in the locality where the final condensation of castings takes place. Large and relatively convergent holes are called shrinkage holes; fine and scattered holes are called shrinkage holes. The irregular shape and rough appearance of shrinkage holes can show the flourishing dendrite endings, so it can be significantly different from stomata.

If there are shrinkage holes and porosity in the casting, on the one hand, it will reduce the useful bearing area of the casting, on the other hand, it will cause stress convergence, and the mechanical function of the casting will be significantly reduced. The air tightness and physicochemical function of the castings are also reduced. Especially for pressure-resistant parts, the castings are scrapped because of leakage.

2 the composition of shrinkage holes

The basic reason for the formation of shrinkage porosity is the same as shrinkage porosity. The main reason is that the crystallization temperature of the alloy is wide, the dendrite is prosperous, the liquid alloy almost condenses together, and the fine and scattered voids formed by liquid and shortened condensation can not be supplemented by the external liquid metal.

There is a certain regularity between the tendency of shrinkage and porosity in castings and the composition of alloys. Directionally condensed alloys tend to have convergent shrinkage holes; pasty condensed alloys tend to have convergent shrinkage holes. The number of shrinkage holes and shrinkage holes can vary from one another, but their total volume remains unchanged.

3. Factors affecting shrinkage and shrinkage size and prevention measures

The larger the liquid shortening of casting alloys is, the greater the tendency of shrinkage pore formation is; the wider the crystallization temperature scale is, the larger the condensation shortening is, the greater the tendency of shrinkage pore formation is. All the technological methods that can promote alloy to reduce liquid state and condensation period (such as adjusting chemical composition, decreasing pouring temperature and speed, increasing quenching ability of castings, adding feeding ability in the condensation process of castings, and promoting graphitization during condensation of gray castings) are conducive to reducing shrinkage holes and shrinkage. The formation of pine.

In view of the characteristics of alloy shortening and solidification, the correct casting process is worked out to set up excellent feeding conditions in the process of solidification, to transform shrinkage porosity into shrinkage cavity as far as possible, and to make shrinkage cavity.