Knowledge

 

To correctly consider feeding and gating, it is necessary to understand alloy solidification. The solidification mechanisms of different alloys can vary significantly. Therefore, the methods for producing dense and defect-free castings also differ. This article considers some basic factors that influence the solidification pattern and discusses how these factors affect the design methods of risers and gates.

 

Solidification Mechanisms

To correctly consider feeding, it is first necessary to understand how alloys solidify. It is not sufficient to merely distinguish alloys by similar general names such as bronze, steel, or aluminum and assume that they are fed in a similar manner. This is because these alloys cover the entire range of solidification mechanisms. From a practical perspective, it is usually sufficient to classify them into two broad categories based on the crystallization range: "long" and "short" crystallization range alloys.

 

Solidification of Short Freezing Range Alloys

When short freezing range alloys are cooled in a sand mold, the parts that first reach the liquidus line begin to solidify. This typically occurs at the interface between the casting and the mold where heat transfer is the greatest. The cooling of the mold wall causes a solidification layer of metal to form around the liquid. Heat is further transferred through the solidifying metal, and the liquid begins to solidify, and the solidification layer begins to thicken. The solid and liquid phases are separated by a clear boundary. As more heat is transferred, the solidification front steadily moves towards the center of the casting. The crystallization front grows relatively short, corresponding to the start of crystallization at the top and the end of crystallization at the bottom. Short freezing range alloys can solidify sequentially even at relatively low temperature gradients.

 

Solidification of Long Freezing Range Alloys

For long freezing range alloys, sequential solidification is more difficult. Although a thin solidification layer may initially form at the mold wall, solidification does not immediately proceed towards the hot center of the casting. Instead, a "nucleation wave" of solidification corresponding to the liquidus isosurface begins at the mold wall and moves inward. After some time, a second wave, the "termination crystallization wave," corresponding to the solidus isosurface, leaves the mold wall and follows the nucleation wave towards the center of the casting. As the nucleation wave passes, solidification begins at every point in the casting until the final termination wave arrives. Generally, long freezing range alloys solidify with three distinct regions: a completely liquid region in the hot center of the casting; a solidified metal region at the mold wall; and a partially solidified region between the liquid and solid regions. For typical long freezing range alloys, such as thick-section tin bronze alloys, the wide freezing range and low cooling rate result in a low temperature gradient, with liquid and solid phases coexisting throughout the cross-section of the casting.

 

Factors Affecting Solidification Mechanisms

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

Alloy's crystallization range:

As shown in its phase diagram, this is a fundamental characteristic of a specific alloy. The crystallization range is the temperature difference between the start and end of solidification. At a fixed heat transfer rate, the wider the temperature interval, the longer the effective time for crystallization growth, and thus feeding becomes more difficult.

 

Thermal properties of the mold:

The thermal conductivity of the mold affects the heat transfer rate of the casting and, consequently, the temperature gradient of the casting. The higher the thermal conductivity and heat capacity of the mold material, the greater the heat transfer 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 it is for constructing feeding. The thermal conductivity of sand molds is relatively low, resulting in a low temperature gradient in the castings, especially for thick and large cross-sections. Molding materials like chromite sand or zircon sand have higher thermal conductivity and heat capacity compared to silica sand, which helps increase the temperature gradient and improve the density of the castings, particularly for thin cross-sections.

 

Thermal conductivity of solidifying alloys:

Alloys such as copper-based or aluminum-based have high thermal conductivity, which reduces the temperature gradient during casting solidification, causing the temperature across the entire cross-section of the casting to quickly equalize. As a result, crystal growth becomes longer and feeding becomes more difficult.

 

Solidification temperature:

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

 

Solidification modulus:

An increase in the solidification modulus or solidification time reduces the temperature gradient across the cross-section of the casting. Crystal growth and crystallization width increase, leading to a flatter temperature gradient and an increase in internal shrinkage cavities.

 

The influence of solidification mechanism on shrinkage cavity distribution

The wide range of crystallization patterns of casting alloys leads to different shrinkage cavity forms in the castings and risers. Generally, alloys with a short crystallization range show deep tubular shrinkage cavities in the risers during most of the solidification interval. Internal porosity in the castings appears as small shrinkage cavities in the late stage of solidification. At this time, the solidification front parallel parts come into contact, and metal feeding is thus cut off: this is usually called central shrinkage cavity. Another type of shrinkage cavity in short crystallization range alloys occurs in the hot center and isolated "thick and large cross-sections" that are not properly fed.

 

For alloys with a long crystallization range, the risers usually show very small shrinkage tubes. Due to the "slurry" solidification mode, only liquid can flow during part of the solidification time. Fine and uniform shrinkage cavities exist throughout the cross-section of the casting, concentrated in the slower-cooling parts such as joints and the lower part of the riser. In general casting conditions, for extremely long solidification range alloys like tin or phosphor bronze, it is impossible to obtain completely dense castings. Usually, no more than 60% of the liquid volume can be solidified. The shrinkage cavities of this type of metal are distributed throughout the cross-section of the casting.

 

Feeding of Castings​​​​​​​

Feeding of short-range solidification alloys:

It has long been recognized that the necessary condition for producing dense castings of short-range solidification alloys is that the metal solidifies in the mold away from the riser and progresses towards the last solidifying riser. All liquid and solidification shrinkage cavities remain in the riser, while the casting is dense. This continuous solidification form, sometimes called "sequential solidification", is defined as ensuring that the solidification front forms a roughly V-shaped profile in the longitudinal section, with the wide end of the V pointing towards the riser. However, this theoretical scheme is not always achievable in the design of complex castings and in establishing sufficient temperature gradients across the entire casting section.

 

Generally speaking, for effective feeding of short-range solidification alloys, the riser must be placed above the thermal center of the casting. The riser must solidify later than the casting section where it is located and have sufficient metal to compensate for the liquid and solid shrinkage of the alloy.

 

The feeding range of a specific alloy should also be considered. The feeding range can be defined because the riser can change the temperature gradient of the same casting section and promote sequential solidification.

 

Feeding of long-range solidification alloys:

The concept of sequential solidification is less relevant to long-range solidification alloys. For such alloys, attempting to implement sequential solidification, especially in thick casting sections, often results in the opposite effect in terms of density, merely concentrating shrinkage cavities in local areas. This is particularly true for long-range solidification alloys based on copper. In these alloys, the high thermal conductivity of the alloy increases the difficulty of feeding. The high thermal conductivity in the liquid phase helps maintain a uniform thermal gradient within the solidifying casting. The high specific heat and latent heat of these alloys also exacerbate this situation.

 

Typically, the goal for feeding such alloys is not to completely eliminate shrinkage cavities but to ensure that they are as evenly distributed as possible across the casting section. A practical example is leaded bronze bushings. These are usually cast without risers, so the temperature gradient should be as uniform as possible.

 

It is best for risers to feed only the thermal and partial solidification shrinkage to avoid excessively prolonging the solidification time.

 

These alloys basically have no feeding range, so it is impossible to achieve high density under normal casting conditions.

 

Conclusion

 

Casting alloys encompass all solidification mechanisms and pouring sensitivities, many of which are less than ideal in terms of process design. Therefore, only by thoroughly understanding the thermodynamics and fluid properties of alloys with a problem-solving mindset can casting workers achieve the continuous successful results they desire.

 

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