Alloy casting performance concept: Casting performance refers to the ability of an alloy to be cast and produce high-quality castings.
Alloy casting performance indicators: Fillability (fluidity), shrinkage, oxidizability, gas segregation and absorption, etc.
The quality of alloy casting performance significantly impacts the casting process, casting quality and casting structure design.
Therefore, when choosing materials for castings, preference should be given to materials with good casting performance, ensuring operational performance.
However, in actual production, to ensure operational performance, alloys with worse casting performance are often used.
In these cases, more attention should be paid to the design of the casting structure and suitable casting process conditions should be provided to produce high-quality castings. Therefore, a comprehensive understanding of the casting performance of an alloy is necessary.
I. Alloy Casting Performance – Alloy Filling Capacity
01 Alloy filling capacity definition
Definition: The ability of the casting alloy to fill the mold and produce a casting with correct dimensions and clear contours is called the filling capacity of the casting alloy.
The molten alloy filling process is the first step in the formation of the casting. This step involves a series of physical and chemical changes, such as the flow of the molten alloy and the heat exchange between it and the mold, along with the crystallization of the alloy.
Therefore, fillability not only depends on the fluidity of the alloy itself, but is also influenced by external conditions such as mold properties, casting conditions and casting structure.
02 Impact on casting quality
Impact on casting quality: If the filling capacity of the cast alloy is strong, it will be easier to obtain complex and thin-walled castings. This results in fewer defects such as unclear contours, insufficient pouring and cold closing.
It also facilitates the rise and expulsion of gases and non-metallic inclusions in the molten metal, reducing defects such as pores and slag inclusions. Furthermore, it can increase the feeding capacity, thus reducing the tendency to shrinkage and porosity.
03 Factors affecting the filling capacity of alloys and technological countermeasures
(1) League Fluidity
Definition:
Fluidity refers to the ability of the molten alloy to flow. It is an inherent property of an alloy, depending on the type of alloy, crystallization characteristics and other physical properties (for example, the lower the viscosity and the higher the heat capacity, the lower the thermal conductivity and the higher the latent heat of crystallization, and the lower the surface tension, the better the fluidity).
Measurement method:
To compare the fluidity of different alloys, the standard spiral sample casting method is often used. The length of the fluidity sample obtained under the same mold (generally using a sand mold) and casting conditions (such as the same casting temperature or the same superheat temperature) can represent the fluidity of the tested alloy.
Among common cast alloys, gray cast iron and silicon brass have the best fluidity, while cast steel has the worst. For the same alloy, fluidity samples can also be used to study the impact of various casting process factors on its fillability.
The length of the flow sample obtained is the product of the time and the flow velocity of the molten metal from the beginning of the pour to the end of the flow. Therefore, any factors that affect these two factors will impact fluidity (or fillability).
The chemical composition of the alloy determines its crystallization characteristics, and the crystallization characteristics dominate the impact on fluidity. Alloys with eutectic components (such as iron-carbon alloys with a carbon mass fraction of 4.3%) solidify at a constant temperature, the inner surface of the solidification layer is relatively smooth, and the resistance to flow for the subsequent molten metal is small.
In addition, the solidification temperature of the eutectic component alloy is low, which makes it easier to obtain a higher degree of superheat, so the fluidity is good. In addition to eutectic alloys and pure metals, other component alloys solidify within a certain temperature range, and there is a two-phase zone of liquid and solid in the casting section.
The first dendritic crystals formed create a greater resistance to flow for the subsequent molten metal, so that fluidity decreases. The more the alloy composition deviates from the eutectic component, the greater the solidification temperature range and the worse the fluidity. Therefore, alloys close to eutectic composition are often used as casting materials.
(2) Mold Properties
① Mold heat storage coefficient represents the mold's ability to absorb and store heat from molten metal.
The higher the thermal conductivity, specific heat capacity and density of the mold material, the stronger its heat storage capacity, the stronger the quenching ability of the molten metal, the shorter the time for the molten metal to maintain the flow and the worse the filling capacity will be.
For example, metal mold casting is more likely to produce defects such as insufficient pouring and cold sealing than sand mold casting.
② Preheating the mold temperature can reduce the temperature difference between the mold and the molten metal, reduce the intensity of heat exchange, and thus improve the filling capacity of the molten metal.
For example, when casting aluminum alloy castings with a metal mold, increasing the mold temperature from 340°C to 520°C increases the length of the spiral specimen from 525mm to 950mm under the same casting temperature (760° W). Therefore, mold preheating is one of the necessary process measures in metal mold casting.
③ The gas in the mold has a certain ability to emit gas, which can form a gas film between the molten metal and the mold, reducing flow resistance and making filling easier. But if the gas emission is too large and the mold exhaust is not smooth, the back pressure of the gas generated in the mold cavity will make it difficult for the molten metal to flow.
Therefore, to improve the permeability of mold sand (core), it is necessary and often applied to open ventilation holes in the mold.
(3) Leak conditions
① Pouring temperature
The pouring temperature has a decisive impact on the filling capacity of the molten metal. Increasing the pouring temperature decreases the viscosity of the alloy and prolongs the time it remains fluid, thus increasing the filling capacity; conversely, the filling capacity will decrease.
For thin-walled castings or alloys with low fluidity, increasing the pouring temperature to improve filling capacity is often used and relatively convenient in production.
However, as the casting temperature increases, the gas absorption and oxidation of the alloy becomes serious, the total shrinkage increases, and defects such as blisters, shrinkage holes and sand adhesion occur easily, and the crystalline structure of the part molten becomes coarse.
Therefore, in principle, the pouring temperature should be reduced as much as possible while ensuring sufficient fluidity.
② Filling pressure
The greater the pressure on the molten metal in the flow direction, the greater the flow rate and the better the filling capacity. Therefore, methods such as increasing the height of the sprue or applying artificial pressure (such as pressure casting, low pressure casting, etc.) are often used to improve the filling capacity of cast alloys.
(4) Casting Structure
When the wall thickness of the casting is too small, the wall thickness changes drastically or there is a larger horizontal surface, making it difficult to fill the alloy liquid. Therefore, when designing the casting structure, the wall
the thickness of the casting must be greater than the minimum allowable value; some castings need to design flow channels; and ribs should be placed on large, flat surfaces. This not only facilitates the smooth filling of alloy liquid, but also prevents the occurrence of sand inclusion defects.
II. Alloy Casting Performance – Alloy Segregation
Segregation
This term refers to the uneven distribution of chemical composition in castings. Segregation can make the properties of castings uneven and, in severe cases, can lead to defective products.
Segregation can be divided into two categories: Microsegregation and Macrosegregation.
Microsegregation:
Intragranular segregation (also known as dendritic segregation) – This is the phenomenon where different parts of the same grain have varying chemical compositions. For alloys that form solid solutions, only under conditions of very slow cooling can atoms diffuse enough to obtain chemically homogeneous grains during the crystallization process.
Under real casting conditions, the solidification rate of the alloy is faster and the atoms do not have enough time to completely diffuse. As a result, grains that grow in a dendritic manner inevitably have uneven chemical compositions.
To eliminate intragranular segregation, the casting can be reheated to an elevated temperature and held for a long period of time to allow sufficient atomic diffusion. This heat treatment method is known as diffusion annealing.
Macro-segregation:
Density Segregation (formerly known as gravity segregation) – This is the phenomenon where the top and bottom parts of the casting have irregular chemical compositions. When the densities of alloying elements differ significantly, elements with lower density tend to accumulate at the top after complete solidification of the casting, while elements with higher density tend to accumulate at the bottom.
To prevent density segregation, the molten metal must be stirred well or cooled rapidly during casting to prevent separation of elements with different densities.
There are many types of macrosegregation, including positive segregation, negative segregation, V-shaped segregation, and band segregation, in addition to density segregation.
III. Alloy Casting Performance – Alloy Gas Absorption
Gas absorption of alloys – This term refers to the property of alloys to absorb gases during melting and casting.
The gas absorption of alloys increases with temperature. Gases are much more soluble in the molten alloy than in the solid state. The greater the superheat of the alloy, the more gas it contains. The presence of gases in castings takes three forms: solid solution, composite and porosity.
(1) Porosity in castings
Based on the source of the gas in the alloy, porosity can be divided into three categories:
The. Exudation porosity
When the gases dissolved in the alloy liquid exude during the solidification process due to the decrease in gas solubility, and cannot be expelled in time, the porosity formed in the castings is called exudation porosity.
Exudation porosity is most common in aluminum alloys, with diameters generally less than 1 mm. It not only affects the mechanical properties of the alloy, but also seriously affects the tightness of the casting.
B. Invasive porosity
Invasive porosity refers to pores formed by gases gathered in the surface layer of the sand mold that invade the alloy liquid.
w. Reactive porosity
Reactive porosity refers to pores formed in castings by gases produced through chemical reactions between the molten alloy poured into the mold and moisture, rust, etc. in mold material, core supports, coolers or slag.
Reactive porosity comes in many types and shapes. For example, the pores created by chemical reactions between the alloy liquid and the sand mold interface are generally distributed 1-2 mm below the surface of the casting. After the surface is machined or cleaned, many small holes are exposed, which is why they are called underground pores.
Pores disturb the continuity of the alloy, reduce the effective load-bearing area and cause stress concentration around the pores, thereby reducing the mechanical properties of castings, especially impact toughness and fatigue resistance. Dispersed pores can also promote the formation of microporosity, reducing the tightness of the casting.
(2) Measures to Prevent Porosity
The. Reduce gas emission from molding sand (core sand) and increase mold exhaust capacity.
B. Control the temperature of the alloy liquid, reduce unnecessary overheating, and reduce the original gas content of the alloy liquid.
w. Apply pressure to solidify the alloy and prevent gas exudation. Changes in pressure directly affect gas exudation. For example, if liquid aluminum alloy is crystallized in a pressure chamber at 405-608 kPa (4-6 atmospheres), a poreless casting can be obtained.
d. During melting and casting, try to reduce the chance of contact of the alloy liquid with the gases. For example, apply a coating to protect the surface from the alloy liquid or use vacuum melting technology.
It is. Degas the alloy liquid. For example, introducing chlorine gas into the aluminum alloy liquid. When the undissolved chlorine gas bubbles rise, the hydrogen atoms dissolved in the aluminum alloy liquid continuously diffuse into the chlorine gas bubbles and are removed from the alloy liquid.
f. The surfaces of chillers, central supports, etc. they must not be rusty or oily and must be kept dry, etc.
4. Alloy Casting Properties – Solidification and Shrinkage of Alloys
01 Solidification and shrinkage of the alloy
(1) Definitions of Solidification and Shrinkage
Solidification is the process in which a substance passes from a liquid state to a solid state.
Shrinkage refers to the reduction in volume that occurs in castings during the solidification and cooling processes.
(2) Impact on casting quality
If solidification and shrinkage are not adequately controlled during the cooling process of the liquid metal poured into the mold, the casting may develop defects such as shrinkage cavities, shrinkage porosity, casting stresses, warping and cracking.
02 Casting Solidification Methods and Influence Factors
(1) Casting Solidification Methods
During solidification, there are normally three areas in the cross section of the casting: the solid phase area, the solidification area, and the liquid phase area. The coexistence of liquid and solid phases in the solidification area significantly influences the quality of the casting.
The “solidification method” of casting is categorized based on the breadth of this solidification area into the following three types:
① Layer-by-layer solidification
Pure metals or eutectic alloys solidify without a coexisting liquid and solid phase in the solidification area, as shown in Figure 2(a). Thus, a clear boundary (solidification front) separates the outer solid layer and the inner liquid layer in the cross section.
As the temperature drops, the solid layer thickens and the liquid layer thins until the solidification front reaches the center. This solidification method is called layer-by-layer solidification.
② Paste-type solidification
If the crystallization temperature range of an alloy is wide and the temperature distribution curve inside the casting is relatively flat, there will be no solid layer on the surface of the casting during a certain period of solidification.
Instead, the solidification area where liquid and solid phases coexist extends across the entire cross section, as shown in Figure 1 (C). This solidification method is similar to cement solidification, initially pasty before solidifying, and is therefore called pasty solidification.
③ Intermediate Solidification
Most alloys solidify using a method between the above two, called intermediate solidification.
Relationship between Casting Solidification and Casting Defects:
Generally, layer-by-layer solidification facilitates alloy filling and shrinkage compensation, avoiding shrinkage cavities and porosity. Achieving dense structural castings can be challenging during pasty solidification.
(2) Main Factors Influencing Casting Solidification Methods
① Alloy crystallization temperature range
A smaller crystallization temperature range of an alloy results in a narrower solidification area and a tendency for layer-by-layer solidification. For example, during sand casting, low-carbon steel solidifies layer by layer, while high-carbon steel, with a wide crystallization temperature range, solidifies in a pasty manner.
②Temperature gradient of casting cross section
Given a specific crystallization temperature range of an alloy, the width of the solidification area depends on the temperature gradient of the cross-section of the casting, as shown in Figure 2 (T1 → T2). If the casting temperature gradient increases, the corresponding solidification area decreases.
The temperature gradient of a casting mainly depends on:
The. Alloy Properties: The lower the solidification temperature of an alloy, the greater its thermal conductivity, and the greater its latent heat of crystallization, the better its ability to equalize internal temperatures, resulting in a lower gradient of temperature (as with most aluminum alloys).
B. Mold heat retention capacity: A higher heat retention coefficient of the mold increases its rapid cooling capacity, leading to a greater casting temperature gradient.
w. Pouring temperature: A higher pouring temperature introduces more heat into the mold, reducing the temperature gradient of the casting.
d. Casting wall thickness: Thicker casting walls result in a smaller temperature gradient.
From the above discussion, it can be concluded that alloys that tend to solidify layer by layer (such as gray cast iron, aluminum-silicon alloys, etc.) are more suitable for casting and should be used whenever possible.
When alloys that tend to paste solidification (such as tin-bronze, aluminum-copper alloy, ductile iron, etc.) are to be used, appropriate process measures (e.g. metal mold casting) should be considered to reduce their area of solidification.
03 League shrinkage and its influencing factors
(1) Alloy Shrinkage Principle and Process
The structure of a liquid alloy consists of atomic clusters and “voids”. The atoms within the clusters are arranged in an orderly fashion, but the distance between the atoms is greater than in the solid state. When the liquid alloy is poured into the mold, the temperature continues to drop, the voids decrease, the atomic distances decrease, and the volume of the alloy liquid decreases.
As the alloy liquid solidifies, the voids disappear and the atomic distances decrease further. During the cooling process to room temperature after solidification, the atomic distances continue to decrease.
The shrinkage of an alloy from casting temperature to room temperature goes through the following three steps:
①Liquid shrinkage
This is the shrinkage of the alloy from the casting temperature to the start of solidification (liquidus line temperature) while the alloy is in the liquid state. This results in a drop in the liquid level within the mold cavity.
②Solidification shrinkage
This is the shrinkage of the alloy from the beginning of solidification to the end of solidification. Generally, solidification contraction still manifests mainly as a drop in liquid level.
③Solid state shrinkage
This is the shrinkage of the alloy from the end of solidification to room temperature, when the alloy is in the solid state. Shrinkage at this stage is characterized by a decrease in the linear dimensions of the casting.
Liquid and solidification shrinkage of an alloy are the main causes of shrinkage cavities and porosity in a casting, while solid-state shrinkage is the fundamental cause of stress, deformation and cracking of the casting, and directly affects dimensional accuracy. of the casting.
(2) Main factors influencing alloy shrinkage
①Chemical Composition of the Alloy
Different alloys have different shrinkage rates. Among commonly used alloys, cast steel has the highest shrinkage rate, while gray cast iron has the lowest. The reason why gray cast iron has a very small shrinkage rate is that most of the carbon contained in it exists in the form of graphite, which has a large specific volume. The volumetric expansion produced by graphite precipitation during the crystallization process compensates for part of the alloy's shrinkage.
Table 1 Shrinkage rates of different alloys
| Alloy type | Mass fraction of carbon | Pouring temperature /℃ |
Liquid shrinkage | Coagulation Shrinkage | Solid state shrinkage | Total volume shrinkage |
| Cast carbon steel | 0.35% | 1610 | 1.6% | 3% | 7.8% | 12.46% |
| White cast iron | 3.00% | 1400 | 2.4% | 4.2% | 5.4~6.3% | 12-12.9% |
| Gray cast iron | 3.50% | 1400 | 3.5% | 0.1% | 3.3~4.2% | 6.9~7.8% |
②Leaking temperature
The higher the pouring temperature, the greater the net shrinkage of the alloy.
③Mold conditions and casting structure
The actual shrinkage of a casting is different from the free shrinkage of an alloy. It is hampered by the mold and the core; and because the casting has a complex structure and uneven wall thickness, the mutual restrictions of various parts during cooling also make shrinkage difficult.
V. Solidification and shrinkage of alloys – Porosity and shrinkage in castings
Porosity and shrinkage are defined as the holes that form in the final solidified part of a casting if the liquid shrinkage and solidification shrinkage of the alloy are not compensated by the liquid alloy. Larger, concentrated voids are called porosity, while small, dispersed voids are called shrinkage.
The damage – Porosity and shrinkage reduce the effective load-bearing area of the casting, causing stress concentration and thus reducing mechanical properties. For parts that require watertightness, porosity and shrinkage can cause leaks and seriously affect their watertightness. Therefore, porosity and shrinkage are among the main casting defects.
01 Formation of Porosity and Shrinkage
① The process of porosity formation
When the liquid alloy is poured into a cylindrical mold, the temperature of the liquid alloy gradually decreases due to the cooling effect of the mold. Its net contraction continues, but when the sprue does not solidify, the mold cavity is always filled (see Figure 3 (a)).
As the temperature drops, the surface of the casting first solidifies into a hard shell, simultaneously closing the sprue (see Figure 3(b)). Upon further cooling, the liquid metal within the shell continues to shrink, compensating for the solidification contraction that occurred when the shell was formed.
As the liquid shrinkage and solidification shrinkage are much greater than the solid shrinkage of the shell, the liquid level drops and detaches from the top of the shell (see Figure 3 (c)). This continues, with the shell thickening and the liquid level dropping.
After the metal is completely solidified, a cone-shaped porosity forms at the top of the casting (see Figure 3(d)). When the casting continues to cool to room temperature, its volume decreases slightly, reducing the porosity volume (see Figure 3 (e)). If a riser is installed on top of the casting, the porosity will move into the riser.
② Porosity locations
It usually appears in the last solidified area of the casting, such as the top or center of the casting, near the sprue, or where the casting wall is thickest.
③ Shrink formation
This is caused by insufficient shrinkage compensation in the last solidified area of the casting, or because the alloy solidifies in a pasty state and the small liquid areas separated by dendritic crystals do not receive shrinkage compensation.
Retraction is divided into macroretraction and microretraction. Macroshrinkage is small holes visible to the naked eye or under a magnifying glass, often distributed along the central axis of the casting or below the porosity (Figure 4). Micro-shrinkage is small holes distributed between the grains, visible only under a microscope.
This type of shrinkage is more widespread, sometimes covering the entire section. Microshrinkage is difficult to avoid completely and is generally not treated as a defect in general castings. For castings with high requirements for tightness, mechanical properties, physical properties or chemical properties, efforts must be made to reduce them.
Different casting alloys have different tendencies to form porosity and shrinkage. Layered solidification alloys (pure metals, eutectic alloys or alloys with a narrow crystallization temperature range) have a high tendency to porosity and a low tendency to shrinkage.
Pasty solidification alloys, although less prone to porosity, are very prone to shrinkage. Because some process measurements can control the solidification mode of the casting, porosity and shrinkage can be mutually converted within a certain range.
02 Prevention of Shrinkage and Porosity Cavities
① Implementing “Directional Solidification”
To avoid shrinkage cavities and porosity, the casting must solidify according to the principle of “directional solidification”. This principle refers to the use of various technical measures to establish an increasing temperature gradient from the part of the casting furthest from the gate to the gate itself.
Solidification begins at the farthest part of the gate, gradually progressing towards the gate in order, with the gate itself being the last to solidify. This process facilitates effective solidification shrinkage by moving the shrinkage cavities into the gate and resulting in denser castings.
Therefore, the gate must be placed in the thickest and highest part of the casting, with a sufficiently large size. When possible, the sprue should be located in the gate, allowing molten metal to flow through the gate first.
At the same time, coolers can be placed in some particularly thick parts of the casting (as shown in Figure 5) to accelerate cooling and maximize the effect of gate solidification contraction.
A disadvantage of directional solidification is significant temperature differences in the casting, causing substantial thermal stress and potential warping or cracking of the casting.
Furthermore, the addition of a gate increases metal consumption and cleaning costs. Directional solidification is typically used for alloys with high shrinkage rates and narrow solidification temperature ranges (such as cast steel, malleable cast iron, and brass), as well as castings with significant differences in wall thickness and high airtightness requirements. .
② Pressure compensation
This involves placing the mold in a pressure chamber. After casting, the pressure chamber is quickly closed so that the casting solidifies under pressure, eliminating porosity and shrinkage cavities. This method is also known as “pressure cooker casting”.
③ Using impregnation technology to prevent leakage due to shrinkage cavities and porosity
This involves infiltrating a gel-like impregnating agent into the cavities of the casting, then hardening the impregnating agent and integrating it with the walls of the casting cavities to achieve watertightness.
Determination of shrinkage cavity and porosity locations
To avoid shrinkage cavities and porosity, it is essential to accurately assess their location in the casting during the development of the casting process, so that the necessary technical measures can be taken. The locations of shrinkage cavities and porosity are generally determined using the isothermal line method or the inscribed circle method.
① Isothermal Line Method
This method involves connecting points on the casting that reach the solidification temperature simultaneously to form isothermal lines based on the heat dissipation conditions of various parts of the casting. This is done layer by layer until the isothermal lines in the narrowest cross section touch.
In this way, the last solidified part of the casting can be determined, that is, the location of the shrinkage cavities and porosity. Figure 6 (a) shows the position of the shrinkage cavity determined by the isothermal line method, and Figure 6 (b) shows the actual position of the shrinkage cavity in the casting, which are basically consistent.
② Enrolled Circle Method
This method is often used to determine the location of shrinkage cavities in intersecting walls in the casting, as shown in Figure 7(a). In the part with the largest diameter of the inscribed circle (referred to as the “hot spot”), where more metal accumulates, solidification is generally the last to occur, easily leading to shrinkage cavities and porosity (Figure 7(b)).
SAW. Solidification and Contraction of the Alloy – Casting Stress, Deformation and Cracking
1. Classification and Formation of Casting Internal Stress
Definition :
The stress caused by the impeded contraction in the solid state of a casting is called casting stress. Casting stress can be divided into three types:
Mechanical Stress:
This type of tension is temporary, resulting from the mechanical impediment of retraction of the casting. Once the mechanical obstruction is eliminated, the stress disappears. The cause of mechanical obstacle includes high temperature resistance of molding sand (core), poor collapsibility, and obstruction by sand box strips and core impressions.
Thermal stress:
This internal stress, known as thermal stress, is generated due to varying cooling speeds of different parts of the casting, causing inconsistent contraction within the same period, and there are restrictions between these parts. This thermal stress remains even after the casting has cooled to room temperature, which is why it is also called residual stress.
Phase change stress:
Volume changes caused by phase changes in the alloy under elastic conditions can create phase change stresses. If different parts of the casting cool at different rates, phase changes do not occur simultaneously, leading to this stress.
Casting stress is the algebraic sum of thermal stress, mechanical stress and phase change stress. Depending on the situation, these three tensions can overlap or counterbalance each other. The presence of stresses in the casting can have a series of adverse effects, such as causing deformations and cracks in the casting, reducing load capacity and affecting machining accuracy.
2. Ways to reduce and eliminate stress in the cast
① Technological aspects:
The. The casting must be solidified according to the principle of “simultaneous solidification”. To achieve this, the gate system must be placed in the thin-walled area and cooled in the thick-walled area. This ensures that the temperature difference between different parts of the casting is minimized and simultaneous solidification occurs, thus reducing thermal stress to the lowest level. It should be noted that the central area of the casting often has inadequate shrinkage porosity and compaction at this time.
B. By increasing the collapsibility of the mold and core, removing sand and packing the box as early as possible to eliminate mechanical obstacles, and slowly cooling the casting in a heat preservation well can also reduce the stress of the casting.
② Structural design:
Strive for a simple structure with uniform wall thickness and gradual transitions from thin walls to thick walls in order to reduce temperature differences and allow each part to shrink more freely.
③ Thermal stress in castings can be eliminated using methods such as natural aging and artificial aging.
3. Deformations and Cracks
① Deformation:
Stress castings are in an unstable state and spontaneously reduce stress through deformation to reach a stable state. It is evident that only when the elastically stretched parts contract and the elastically compressed parts extend can the stress in the casting be reduced or potentially eliminated.
The deformation direction of T-shaped castings is shown by the dotted line in Figure 9 (a). This is because after cooling the T-shaped casting, the thick wall is under tension and the thin wall is under compression, similar to two springs of different lengths (Figure 9(b)). The shorter spring at the top is stretched and the longer spring below is compressed to maintain the same length (Figure 9(c)).
However, this combination of springs is unstable and seeks to restore the original state of equilibrium. Therefore, a bending deformation similar to the situation above appears (Figure 9 (d)).
Danger, Countermeasures:
The fundamental measure to avoid deformation of the casting is to reduce the internal stresses of the casting. For example, during the design phase, strive to obtain a uniform wall thickness of the casting. When establishing casting processes, try to cool all parts of the casting simultaneously and increase the collapsibility of the molding sand (core).
In pattern manufacturing, the reverse deformation method can be used, that is, the pattern is made in advance in a shape opposite to the deformation of the casting to compensate for the deformation of the casting. The machine tool base shown in Figure 10 exhibits bending deformation due to the thick rails and thin sidewalls after casting. If the pattern is made with the opposite curvature represented by the line of double ends, the rails will be straight after casting.
It should be noted that after the deformation of the casting, it can often only reduce, but not completely eliminate, the stress of the casting. After machining, the stress imbalance in the part causes further deformation, affecting machining accuracy. Therefore, for important castings, stress relief annealing must be performed before machining.
② Cracks:
When the casting stress exceeds the strength limit of the material at that time, cracks may occur in the casting.
Cracks can be divided into hot cracks and cold cracks.
Hot Cracks:
They are formed at high temperatures and are one of the most common casting defects in the production of steel castings, forgeable cast iron blanks, and some light alloy castings. Its characteristics are: the crack shape is tortuous and irregular, the crack surface appears oxidized (the crack surface of steel casting appears almost black, while aluminum alloy is dark gray), and the crack passes along the edges of the grains. Hot cracks often appear in the last solidified parts of castings or on the surface where stress concentrations are likely to occur.
Cold Cracks:
These are formed at low temperatures. Alloys with low plasticity, high brittleness and low thermal conductivity, such as white cast iron, high carbon steel and some alloy steels, are prone to cold cracking. Its characteristics are: the shape of the crack is a continuous straight line or a smooth curve, often passing through the grains. The surface of the crack is clean, with a metallic luster or light oxidation color. Cold cracks frequently occur in stressed parts of the casting, especially in areas of stress concentration such as internal sharp corners, shrinkage cavities, and near non-metallic inclusions.
Danger, Countermeasures:
Factors that reduce casting stress or decrease alloy brittleness (such as reducing sulfur and phosphorus content in steel) have a positive effect on crack prevention.
