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Solidification, risering and pouring of the casting

Introduction: 

In order to properly consider shrinkage and casting, it is necessary to understand alloy solidification. The solidification mechanisms of different alloys may vary greatly. Therefore, the methods for producing dense and defect free castings are also different. This article considers some basic factors that affect the solidification mode and discusses how these factors affect the design methods of risers and gates.

Solidification mechanisms

To properly consider shrinkage, it is first necessary to understand how the alloy solidifies. It is not enough to simply distinguish alloys such as bronze, steel, or aluminum by similar general names and consider them to be filled with similar methods of shrinkage. Because these alloys cover all solidification mechanisms. From a practical perspective, it is usually sufficient to classify the crystal range into two categories: "long" crystal range and "short" crystal range.

Solidification of Short Freezing Range Alloys:

When the short solidification range alloy is cooled in the sand mold, the part that reaches the liquidus first begins to solidify. This situation usually occurs at the junction of the casting with the highest heat transfer and the mold. The cooling of the mold wall generates a metal solidification layer around the liquid phase. Heat is further transferred through the solidified metal, causing the liquid phase to solidify and the solidified layer to thicken. The solid and liquid phases are separated by a clear boundary, and as further heat is transferred, the solid front steadily moves towards the center of the casting. The growth of the crystallization front is relatively short, corresponding to the crystallization starting at the vertex and ending at the bottom. Short solidification range alloys can be sequentially solidified even at relatively low temperature gradients.

Solidification of Long Freezing Range Alloys: 

For alloys with long crystalline ranges, sequential solidification is relatively difficult. Although a thin solidification layer may initially form at the mold wall, solidification cannot immediately proceed towards the hot core of the casting. On the contrary, corresponding to the liquid isothermal line, the solidification "nucleation wave" starts from the mold wall and proceeds inward. After a period of time, corresponding to the solid phase temperature line, the second wave of "termination crystallization wave" leaves the mold wall, followed by the "nucleation wave" pushing towards the center of the casting. When the nucleation wave passes through, solidification begins at every point of the casting until the final solidification wave arrives. In general, alloys with a long solidification range have three distinct regions during solidification: the fully liquid region at the hot center of the casting; The solidified metal area at the mold wall; The partially solidified area between the liquid and solid regions. Typical long solidification range alloys, such as thick section tin bronze alloys, have a wide solidification range and low cooling rate resulting in low temperature gradients, with both liquid and solid phases coexisting throughout the entire casting section.

Factors affecting solidification mechanism

There are many factors that affect the solidification mode of specific alloys. The solidification range of an alloy, measured by temperature, is not its true indicator. However, the time interval between the beginning and end of crystallization determines how the alloy solidifies. The spacing between liquid and solid phases is determined by the following factors:

Crystallization range of alloy:

As shown in its phase diagram, this is a fundamental characteristic of a specific alloy. The crystallization range refers to the temperature difference between the beginning and end of solidification. The wider the temperature interval at a fixed heat transfer rate, the longer the effective time for crystal growth, making shrinkage more difficult.

Thermal properties of molds:

The thermal conductivity of the model affects the heat transfer rate of the casting, which in turn affects the temperature gradient of the casting. The higher the thermal conductivity and heat capacity of the casting material, the greater the thermal conductivity rate of the casting, and the shorter the time interval between the liquid and solid phases. Therefore, the steeper the temperature gradient, the shorter the crystal growth, and the more favorable the construction of shrinkage. The thermal conductivity of sand molds is relatively low, resulting in low temperature gradients in castings, especially for thick casting sections. Molding materials such as chromite sand or zircon sand have higher thermal conductivity and heat capacity compared to silica sand, and help increase temperature gradients and improve casting density, especially for thin section castings.

Thermal conductivity of solidified alloys:

The high thermal conductivity of alloys such as copper or aluminum reduces the temperature gradient during casting solidification, allowing the temperature of the entire casting section to quickly become uniform. As a result, crystal growth becomes longer and shrinking becomes more difficult.

Solidification temperature:

The higher the solidification temperature of the alloy, the greater the heat transfer rate and temperature gradient of the casting section. Due to the high solidification temperature, crystal growth is inhibited and shrinkage becomes more effective.

Solidification modulus:

As the solidification modulus or solidification time increases, the temperature gradient of the casting section decreases. The growth of crystals and the increase in crystal width result in a gradual temperature gradient and an increase in internal shrinkage pores.

The influence of solidification mechanism on the distribution of shrinkage pores

The diverse crystallization modes of casting alloys result in different forms of shrinkage in castings and risers. Generally speaking, short crystalline range alloys appear as deep tubular shapes in the riser during most of the solidification intervals when supplementing castings. The internal pores within the casting appear in the form of small shrinkage cavities during the late stage of solidification. At this point, the parallel parts of the solidification front are in contact with each other, and the metal shrinkage is thus cut off: this is usually referred to as central shrinkage. Another type of shrinkage in short crystalline range alloys occurs at the hot center and isolated "thick and large sections" that have not been properly filled. For alloys with long crystalline ranges, their risers usually exhibit extremely small shrinkage tubes. Due to the "paste like" solidification mode, the liquid is only allowed to flow during a partial solidification time. There are fine and uniform shrinkage holes on the entire cross-section of the casting, concentrated in the slower cooling parts such as joints and the lower part of the riser. Under normal casting conditions, it is impossible to obtain completely dense castings in extreme long solidification range alloys such as tin or phosphor copper. Usually not exceeding 60% of the total liquid content. The solidification shrinkage of this type of metal is dispersed throughout the entire cross-section of the casting.

Feeding of Castings casting shrinkage

Shrinkage compensation of short crystalline range alloys:

It has long been recognized that the necessary condition for producing dense castings of short crystalline range alloys is that the metal solidifies in the mold from a position far away from the riser and gradually advances towards the final solidified riser. All liquids and solidified shrinkage cavities remain in the riser, while the casting is dense. This continuous solidification form, sometimes referred to as "sequential solidification", is defined as ensuring that the solidification front forms a rough V-shape in the longitudinal section, with the large head of the V pointing towards the riser. This theoretical solution may not always be achievable in the practical design of complex castings and the difficulty of establishing sufficient temperature gradients across the entire casting cross-section. Generally speaking, for effective shrinkage control of short crystalline range alloys, the riser must be set above the hot center of the casting. The riser must solidify later than the casting where the riser is located, and there must be sufficient metal compensation for the liquid and solid shrinkage of the alloy. We also need to consider the shrinkage range of specific alloys. The range of shrinkage can be defined because the riser can change the temperature gradient of the same casting section, promoting sequential solidification.

Shrinkage compensation of long crystalline range alloys:

The concept of sequential solidification is less related to alloys with long crystalline ranges. For this type of alloy, attempting to implement sequential solidification, especially on thick casting sections, usually yields the opposite effect in terms of density, only concentrating shrinkage in localized areas. Especially in alloys with a long crystalline range based on copper. In this type of alloy, the high thermal conductivity of the alloy increases the difficulty of shrinkage compensation. The high thermal conductivity in liquids helps maintain a uniform thermal gradient within solidified castings. The high specific heat and latent heat of this type of alloy also exacerbate this situation. Usually, the goal of supplementing such alloys is not to completely remove shrinkage cavities, but to ensure that they are distributed as evenly as possible on the cross-section of the casting. A practical example is a bronze liner containing lead. This piece is usually cast without a riser, so the temperature gradient should be kept as consistent as possible. For the riser, it is best to only compensate for overheating and partial solidification shrinkage to avoid excessively prolonging the solidification time. These alloys basically have no shrinkage range, so they cannot be highly dense under normal casting conditions.

The gating system of Castings castings

The main function of the pouring system is to transfer clean and slag free molten steel from the ladle to the mold cavity, without generating secondary oxidation and suction during this process. To some extent, all alloys are affected by slag formation, with some alloys such as aluminum and copper being particularly susceptible. The most important thing in pouring is to introduce metal into the mold without turbulence, at the lowest possible speed, and maintaining an appropriate filling rate. The most suitable filling rate for a specific alloy cannot be considered a fixed value, but depends on many factors such as casting weight, cross-sectional thickness, and casting shape. Excessive flow velocity increases the possibility of turbulence, jet flow, and oxidation at the front end of the metal, which can result in a decrease in mechanical performance or even scrap. The necessary high filling rate and low flow rate are contradictory, especially for alloys that are prone to slag formation. Often leads to an oversized gating system, far exceeding the typical area based gating system ratio. This alloy is ideally poured through a "pressure free" pouring system that removes the top surface of the runner. This ensures that the runner can flow fully at all times. The distance between the sprue pit and the first inner sprue should be as large as possible to allow time for the slag to float out and be captured by the top surface of the runner. The sprue should be arranged at or near the bottom of the casting as much as possible to minimize turbulence inside the mold cavity.

The Pouring Bush: 

Except for the smallest castings, it is recommended to use appropriate sprue cups. The sprue cup should be designed to allow the caster to quickly fill the sprue and maintain a constant pressure during the pouring process. Offset design and coordination with dams can achieve this goal. Set the shape of the sprue cup to be rectangular, so that the upward circulation during pouring helps to remove slag. The outlet of the sprue cup should be arc-shaped and match the sprue. In the pouring cup, it is usually used to remove the plug. Before removing the plug, the caster can completely fill the pouring cup and have time for the slag to float up. It is not recommended to pour directly into the sprue or use a conical liner to flow directly into the sprue during pouring, as it not only causes air and slag to be generated and brought into the pouring system, but also generates excessive turbulence in the high-speed metal flow inside the pouring system.

The Sproue: 

The sprue controls the filling rate of the casting, making it the only and most important part of the pouring system. During production, the sprue should gradually become smaller, with a smaller control area at the bottom, and other pouring system components determined by the sprue outlet area. There are many formulas and useful charts to determine the slope of a sprue. Starting from the control area, providing a 5-degree taper is sufficient. When the height of the sprue exceeds 300mm, it is recommended to increase the diameter (or side length) of the cross-section by 50%. The cross-section of a sprue can be circular, square, or rectangular. There is evidence to recommend using rectangles as they have a tendency to reduce the formation of vortices and suction. If there are no other reasons, squares and rectangles are easier to manufacture than circular cone sections.

The Sproue Base: 

Because the flow velocity reaches its maximum at the outlet of the sprue, it is important to buffer the liquid flow and change it from vertical to horizontal with minimal turbulence. The recommended size for the sprue socket is: a diameter of 2-3 times the diameter of the sprue outlet, and a depth of 2 times the depth of the runner.

Runner and Gates:

As mentioned earlier, ideally, castings should be cast under a pressure free pouring system (in the transverse runner of the lower mold and the inner runner of the upper mold). The area of the runner should be 2-4 times the area of the sprue pit, and the total area of the inner runner should be at least equal to or greater than 2 times the area of the runner. This is to ensure the required filling rate at the lowest possible speed. Alloys that are particularly sensitive to slag require larger runners and runners to ensure the lowest flow rate. The ideal cross section of the sprue is rectangular, with a width to depth ratio of 2:1. The wide upper surface is designed to maximize the capture of slag and inclusions by the runner. When there are multiple sprues in the runner, to ensure uniform flow rate for each sprue, the area of the runner decreases with the area of each sprue it passes through. Setting up a slag collection bag at the end of the runner to collect the severely oxidized metal that was filled earliest is also a good method. The position of the sprue entering the mold cavity should be as low as possible to avoid turbulence caused by liquid falling. Like the runner, the cross-section of the sprue should be rectangular instead of square to avoid the formation of "hot spots" and subsequent shrinkage at the contact with the casting. The accurate ratio of width to thickness is determined by the solidification time of the casting. According to experience, the thickness of the sprue should be less than one-third of the thickness of the casting at the connection with the casting.

Filtration:

Metal filters have been widely used for several years. There are numerous forms, ranging from simple filters and woven fabrics to different types of ceramic blocks. There are two main types of ceramic filters: one is extruded, with straight and parallel holes; Second, foam ceramics, which is composed of ceramic materials with foam structure, have no specific pore direction. The benefits of using ceramic filter blocks are well documented. It is very effective in removing slag and inclusions, and can also improve mechanical properties. The foam ceramic filter has obvious advantages over the extruded filter. When the metal passes through, the initial metal flow is not separated. This reduces the possibility of secondary oxidation at the outlet of the filter. The ability of foam ceramic filter, or the total amount of metal to pass before blocking, changes with the tendency of slag formation of the cast alloy. The ability of the filter is also affected by the upstream process of the pouring system of the filter (similar to the previous process). For example, if the metal reaching the filter contains severe slag, the ability of the filter will sharply decrease. Filters should not be used hastily and need to be used in conjunction with a sound pouring system to achieve useful results.

Conclusion 

Casting alloys cover all solidification mechanisms and casting sensitivities, many of which are not ideal process designs. Therefore, only by thoroughly understanding the thermodynamics and fluid properties of alloys with problems in mind can the continuous successful results expected by foundry workers be achieved.