[Technology] Formation of shrinkage shrinkage and shrinkage of castings

During the solidification process of a casting, due to the liquid shrinkage and solidification shrinkage of the alloy, holes often appear at the final solidification of the casting, which is called shrinkage cavity. The large and concentrated holes are called concentrated shrinkage holes, or abbreviated as shrink holes; the small and scattered holes are called scattered shrink holes, or abbreviated as shrinkage holes. The shape of the shrinkage hole is irregular, the surface is not smooth, and the developed dendrite tip can be seen, so it can be distinguished from the blowhole.

The presence of any form of shrinkage cavity in the casting results in a significant reduction in the mechanical properties of the casting due to the fact that they reduce the effective area of ​​the force, as well as stress concentration at the shrinkage cavity. Due to the presence of shrinkage holes, the hermeticity and physical and chemical properties of the castings are also reduced. Therefore, shrinkage is one of the important defects of castings and must be eliminated

1. Shrinkage hole and shrinkage cavity In the solidification process of the casting, if the liquid shrinkage and solidification shrinkage are not timely compensated, holes will be formed in the corresponding parts, that is, the formation of shrinkage holes or shrinkage.

1. The formation process and conditions of shrinkage cavity The volume of the shrinkage cavity is large, and most of them are concentrated in the upper part of the casting and the part that is finally solidified. Taking the cylindrical casting as an example, the forming process of the shrinkage hole is analyzed. Assuming that the poured metal is solidified at a fixed temperature, or that the crystallization temperature range is very narrow, the casting solidifies layer by layer from the surface to the inside. Due to the heat absorption of the mold, the temperature of the liquid metal drops, and liquid shrinkage occurs, but it will be obtained from the casting system. In addition, the cavity is always filled with liquid metal during this time. When the temperature of the casting surface drops to the solidification temperature, a hard shell is solidified on the surface of the casting and tightly surrounds the internal liquid metal. The inner gate is frozen at this time

Upon further cooling, the liquid metal in the hard shell undergoes liquid shrinkage due to a decrease in temperature, and the liquid surface is decreased in addition to the solidification and contraction upon the formation of a hard shell. At the same time, the solid-state hard shell also reduces the appearance of the casting due to the reduced temperature. If the volumetric shrinkage due to liquid shrinkage and solidification shrinkage is equal to the volume reduction caused by the shrinking of the shell size, the solidified shell remains in intimate contact with the inner liquid metal and no shrinkage cavity is produced. However, since the liquid shrinkage and solidification shrinkage of the alloy exceed the solid shrinkage of the hard shell, the liquid will be separated from the top surface of the hard shell.

Successively, the hard shell continues to thicken, the liquid level will continue to decline, after the metal is completely solidified, an inverted conical shrinkage hole is formed in the upper part of the casting. The volume of the entire casting shrinks as the temperature drops to room temperature, and the absolute volume of the shrinkage hole decreases, but its value does not change much. If the top of the casting is equipped with a riser, the shrink hole will be moved into the riser

The shrinkage cavity often occurs in the thick part of the casting or the last solidification part of the upper part, often with an inverted cone shape, and the inner surface is rough. The formation process of the shrinkage cavity is shown in Figure 3-5. After the liquid alloy is filled with the mold cavity, as shown in Figure 3-5Q,

Due to the heat absorption of the mold, the temperature of the liquid alloy decreases, and the metal near the surface of the cavity solidifies into a shell. At this time, the inner runner is solidified, and the shrinkage of the molten metal in the shell is hindered by the outer shell, so that the shrinkage cannot be obtained. The surface begins to fall, as shown in Figure 3-5. The temperature continues to drop, the shell is thickened, and the remaining liquid inside due to the liquid shrinkage and the shrinkage of the supplemental solidified layer causes the volume to shrink and the liquid level to continue to drop, as shown in FIGS. 3-5 Sand. This process continued until the end of solidification, forming a shrinkage cavity in the upper part of the casting, as shown in Figure 3-50 temperature continues to fall to room temperature, due to solid shrinkage to make the outer shell outline size slightly reduced, as shown in Figure 3-5. Alloys of pure metals and eutectic components tend to form concentrated shrinkage pores

In the case that the liquid alloy contains little gas, when the liquid metal is detached from the top surface of the hard shell, a vacuum is formed on the liquid surface, and the thin shell above may be recessed in the shrinkage direction under atmospheric pressure. Therefore, the shrinkage hole should include two parts, the outer shrinkage cavity and the inner shrinkage cavity. If the hardness of the hard shell on the top surface of the casting is very large, the shrinkage cavity may not appear.

In summary, the basic reason for the occurrence of concentrated shrinkage in castings is that the liquid shrinkage and solidification shrinkage value of the alloy is greater than the solid shrinkage value, resulting in the condition of concentrated shrinkage cavity, that is, the casting is solidified layer by layer from floor to floor, not whole When the volume condenses at the same time, the shrinkage hole concentrates on the final solidification

To sum up, there are four main factors that affect the shrinkage hole: 1 Influence of the alloy itself: 1 The greater the liquid shrinkage, the greater the shrinkage volume; 2 The greater the solidification shrinkage rate, the greater the shrinkage volume; 3 Solid shrinkage The larger the shrinkage volume, the smaller the volume; 4 The greater the thermal conductivity, the uniform temperature field, the smaller the shrinkage volume

2 The effect of cooling conditions: 1 The external cooling intensity is increased, the edges are filled and the shrinkage is reduced, the shrinkage volume is reduced, but the shrinkage hole is concentrated; 2 the mold has small rigidity, displacement, and the shrinkage volume increases.

3 The influence of pouring process: 1 The higher the pouring temperature, the greater the liquid shrinkage, the greater the volume of the shrinkage hole; 2 The low pouring speed, the side pouring edge side feeding, the shrinkage hole volume decreases

(4) Influence of slab structure: The thickness of the slab increases, and after the surface shell is formed, the internal liquid metal increases and the total shrinkage increases.

2. Formation process and conditions of shrinkage shrinkage is often distributed in the axial area of ​​the casting wall, thick part, near the roots of the riser and the ingate. After the casting is cut, dense holes can be observed directly. Shrinkage has a great influence on the mechanical properties of castings. Due to its wide distribution and difficulty in feeding, it is one of the most dangerous defects in castings.

The basic cause of shrinkage formation is the same as the formation of shrinkage cavity because the liquid shrinkage and solidification shrinkage of the alloy is greater than the solid shrinkage. However, the basic condition for the formation of shrinkage is that the alloy has a wide crystallization temperature range, tends to be in a paste-like solidification manner, shrinkage and dispersion, or has a small temperature gradient in the cross-section of the casting in the shrinkage-reducing region, a wide solidification region, and an alloy liquid. Almost simultaneously solidifies, fine pores formed due to liquid shrinkage and solidification shrinkage are dispersed and no external alloy liquid is supplemented. The wider the solidification area of ​​the casting, the more likely it is to produce a shrinkage. Castings with uniform section thickness, such as plate or rod castings, are not readily replenished by the external alloying solution during the solidification stage and tend to produce shrinkage in the axial region, known as axial shrinkage.

The formation process of shrinkage is shown in Figure 3-6.

After the liquid alloy fills the cavity, due to the temperature drop, the crust is immediately crusted against the wall, and there is a wide liquid-solid two-phase coexistence zone in the interior, as shown in FIG. 3-6Q. The temperature continues to fall, the crust thickens, and the two-phase coexistence zone gradually pushes toward the center. The developed dendrites separate the central part of the alloy liquid into many independent small liquid zones, as shown in Figure 3-6. These separate small liquid areas eventually tend to solidify at the same time, resulting in shrinkage due to lack of liquid metal, as shown in Figure 3-6 (c).

Micro-contraction shrinkage often occurs in castings. Microscopic shrinkage occurs between the intergranular and the branches, and is difficult to distinguish from the fine precipitated pores, and often occurs simultaneously and can only be observed under the microscope. Microshrinkage is more or less present in various alloy castings. It reduces the mechanical properties of the casting, and has a greater influence on the impact toughness and elongation of the casting. It also reduces the hermeticity and physical and chemical properties of the casting. Castings are often not as defects. However, under special circumstances, if the casting is required to have high airtightness, high mechanical properties and physical and chemical properties, it must try to reduce and prevent the occurrence of microscopic shrinkage.

Second, shrinkage and shrinkage of mutual transformation For a certain composition of the alloy, casting temperature at a certain time, the alloy shrink volume meet the following relationship: total shrink volume = liquid shrinkage volume + solidification shrinkage volume = shrinkage volume + shrinkage volume two constant

However, the shrinkage and shrinkage volume can be transformed into each other, resulting in the transformation of the root cause is the change of the solidification mode: the paste solidification or solidification layer by layer. Table 3-2 shows the factors affecting the mutual transformation of shrinkage and shrinkage volume

Table 3-2 3. Prevention of shrinkage and shrinkage pathways The basic principle for preventing shrinkage and shrinkage in castings is to establish the correct casting process for the shrinkage and solidification characteristics of the alloy and to establish the casting in the solidification process. The conditions for good feeding are to convert the shrinkage to shrinkage as much as possible, and to make the shrinkage appear where the casting finally solidifies. In this way, a certain size of riser is placed where the casting finally solidifies, the shrinkage hole is concentrated in the riser, or the gate is directly filled in the final solidified place to obtain a sound casting.

The castings establish a good feeding condition during the solidification process, mainly by controlling the solidification direction of the casting so that it conforms to a sequential solidification principle or a simultaneous solidification principle. Practice has proved that as long as the casting can achieve solidification in order, although the alloy shrinkage is large, dense castings without shrinkage can be obtained.

1. Sequential solidification The so-called sequential solidification is the process of placing a riser in a thick part where a shrinkage cavity may appear on the casting, and solidifies the part of the casting away from the riser, as shown in Fig. 3-7.

Then it is solidified near the riser, and finally it is the solidification of the riser itself.

According to such a solidification sequence, the contraction of the solidified portion is firstly supplemented by the molten metal in the post-solidification portion; the shrinkage of the post-solidification portion is supplemented by the molten metal in the riser. The contraction of the various parts of the casting can thus be supplemented, and the shrinkage hole is transferred into the appetite, ie an incremental temperature gradient is established between the part of the casting remote from the riser or gate and the riser or gate. The riser is the excess part of the casting and is removed when the casting is cleaned. Casting solidifies in accordance with the principle of solidification in order to ensure that the shrinkage holes are concentrated in the riser and obtain a dense casting

Figure 3-7 It must be noted that for alloys with a wide range of crystallization temperatures, after the onset of crystallization, the developed dendritic skeleton fills the entire cross-section, making the feeding path of feeders seriously impeded, and it is difficult to avoid the occurrence of microscopic shrinkage. . Obviously, it is appropriate to choose castings with a near eutectic composition or a narrow crystallization temperature range to produce castings.

The layer-by-layer solidification refers to the formation of a hard shell on the surface of a casting at a certain section of the casting, which then gradually grows to the center of the casting and finally solidifies at the center of the casting. Therefore, sequential solidification and layer-by-layer solidification are two different concepts.

The advantage of sequential coagulation is that appetite feeding is good, shrinkage and shrinkage can be prevented, and the casting is dense. Therefore, for alloys with large solidification shrinkage and small crystallization temperature range, this principle is often used to ensure the quality of castings. The disadvantage of sequential solidification is that due to temperature differences in various parts of the casting, hot cracks are likely to occur during solidification, and stress and deformation of the casting can easily occur after solidification. The principle of sequential solidification requires a riser and subsidy, a low production rate, and a costly cutting of the riser

If the bottom pouring gating system is adopted, the temperature of the molten metal is the highest since the molten metal flow time at the bottom of the casting is high, and the temperature of the molten metal rising to the appetite is the lowest. Therefore, the longitudinal temperature profile formed is that the temperature is highest away from the riser, the temperature of the riser is the lowest, and the temperature difference is formed in the opposite direction.

2. Simultaneous solidification The principle of solidification at the same time is to adopt technological measures to ensure that there is no temperature difference between the various parts of the casting structure or the temperature difference is as small as possible, so that the various parts are solidified at the same time. Under simultaneous solidification conditions, the expansion angle ψ is equal to zero and there is no feeding channel

At the same time, the solidification principle has the following advantages: During solidification, castings are not prone to thermal cracking, and they are not likely to cause stress or deformation after solidification. Since there is no appetite or riser small, metal is saved, the process is simplified, and labor is reduced. The disadvantage is that shrinkage occurs in the center of the casting and the casting is not dense. Therefore, this principle is generally used in the following situations

1 Gray cast iron with high carbon-silicon content has less body shrinkage or even shrinkage, and the alloy itself is not susceptible to shrinkage and shrinkage.

2 Crystallization temperature range is large, easy to produce shrinkage of the alloy æž· tin green 4), the air tightness requirements are not high, the principle of simultaneous solidification can be used to simplify the process. In fact, it is very difficult for this type of alloy to eliminate shrinkage even with a riser.

(3) Castings with even wall thicknesses, especially uniform thin-walled ones, tend to solidify at the same time, eliminate difficulties in shrinkage, and should adopt the principle of simultaneous solidification.

When the 4-ball iron casting uses the graphitization expansion force to achieve its own feeding, it must adopt the principle of simultaneous solidification.

5 From the perspective of alloy properties, castings suitable for the principle of sequential solidification can be used. When thermal cracking and deformation become the main contradictions, simultaneous solidification can also be used.

5 From the perspective of alloy properties, castings suitable for the principle of sequential solidification can be used. When thermal cracking and deformation become the main contradictions, simultaneous solidification can also be used.

The above describes the two solidification principles and their scope of application. For a specific casting, according to the characteristics of the alloy, the structure of the casting and its technical requirements, as well as other defects that may appear, such as stress, deformation, cracks and other comprehensive considerations, to find the main contradiction and reasonable to determine which kind of coagulation in principle. It should be pointed out that although the two solidification methods are contrary to the solidification sequence, they can be combined on a specific casting. The structure of castings is generally more complicated. For example, if a single casting has a uniform wall thickness as a whole, there are hot spots in individual parts. Therefore, sequential solidification or simultaneous solidification cannot be used, but a composite solidification method is often used. Since it is solidified at the same time as a whole, the casting part is solidified sequentially, or vice versa