The principle of gold and silver ingot casting - the defects of ingots and their causes
Ingot defects, including internal and surface defects of the ingot. In view of the cast ingots generally using a combination of silver formwork, so far produced integrally flat die cast metal ingot multiple defects. Common defects in gold and silver ingots are:
First, the purity and internal defects of the ingot
The so-called internal defects are generally found in a method that cannot be inspected by cutting or cutting a spindle (gate) after casting. If the chemical composition does not meet the quality requirements, it must be detected during the test; the internal shrinkage (clear shrinkage), shrinkage, and porosity should be found only during rolling and drawing.
purity. In order to ensure that the chemical composition (metal purity) of the ingot meets the quality requirements and the defects that prevent the chemical composition from being inconsistent, in addition to the use of raw materials, fluxes and oxidizing agents that meet the quality requirements, it is also necessary to carefully operate and remove the raw materials as much as possible. Part of the impurities to improve the color of the metal.
Shrinkage. Casting is carried out using a combined vertical die casting method in which condensation of metal begins in the vertical form and from the side. Since the ingot is small, the metal injection speed is not large, the condensation speed is fast, and the like, the condensation line of the metal in the mold has a special curve as shown in FIG. However, since the condensation of the metal starts from the bottom and the side, the central cooling rate is slow, which is favorable for gas discharge. In the case where the injection speed is appropriate, it is easy to obtain a dense ingot in the ingot. However, if the speed of injecting metal in the later stage of casting is not gradually reduced, or if the metal is not added in time, it is easy to produce a large shrinkage hole. When the shrinkage hole is in the shape of a tube, it is also called a shrink tube.
Figure 1 Condensation line of the ingot in the mold
Shrinkage holes are divided into shrink holes and shrinkage. The shrinkage hole is usually located at the upper part of the ingot, that is, where the molten metal is finally condensed, which is slightly funnel-shaped. Its formation is mainly due to condensation (shrinkage) of the metal and discharge of gas upon condensation. Because the edge of the ingot is condensed first, the liquid level at the center is the fastest. If the added metal is not timely, a shrinkage hole will be formed in the center of the ingot. This shrinkage hole is called a shrink hole (Fig. 2). The size of the shrinkage hole depends mainly on the speed of metal injection, temperature, mold temperature and metal condensation rate. In order to prevent the formation of shrinkage holes, it is necessary to appropriately increase the injection temperature of the metal, and gradually slow down the metal injection speed in the later stage of casting, and timely fill in the metal (referred to as feeding) before the center of the ingot is not condensed. Such shrinkage holes can often be found from the spindle (especially when the spindle is cut). When the amount of added metal is too large, the metal fills the gate, and the metal that is replenished is rapidly fused with the metal that is close to the condensate near the gate and condensed into a shell. Blocking the discharge of gas and the replenishment of the metal creates a shrink hole (Fig. 3). Such shrinkage holes are often not found when cutting the ingot, and the hazard is mainly due to delamination when the ingot is used for calendering. An effective way to prevent shrinkage holes is to reduce the metal injection speed when casting to two-thirds or three-fifths of the height of the ingot, and then gradually slow down until the metal is injected near the cap and continue to decelerate. The metal is injected into the cap opening without interruption. This not only facilitates the discharge of gas when the metal condenses, but also allows the injected fine stream to continuously condense with the uncondensed metal to fill the shrinkage cavity.
Figure 2 shows the shrinkage hole
Figure 3 shrink hole
Shrink. Also known as loose. It is because during the condensation process of the metal, a part of the crystals that grow up are arranged vertically and horizontally in the ingot, and part of the uncrystallized residual liquid (mother liquid) is not separated from the crystal by the crystal. When the crystal is condensed, the volume is further reduced to make the crystals A void appears and a shrinkage is formed. Such shrinkage holes are usually concentrated in the center of the ingot, and the size is different. When it exists in a large amount, the metal is loose, which is called a shrinkage defect. The reason for the shrinkage is generally caused by the fact that the molten metal is not timely, the speed is too slow and uneven. The temperature of the molten metal in the injection mold is too low, and shrinkage defects can also be generated. The material produced by using such an ingot is cracked due to looseness of the structure, low strength, and mechanical force.
Inner air hole. It refers to the pores inside the ingot. Because of the strong inhalation of gold and silver, the gas from the charge, the furnace gas and the atmosphere, and the sublimation of the paint into the metal is not discharged. The inner vent is located in the upper part of the ingot (draft casting) and may be seen when the spindle is cut (Figure 4). To prevent the formation of internal pore defects, on the one hand, the temperature and mold temperature of metal injection can be appropriately increased, the casting speed can be correctly grasped, and the metal in the upper part of the ingot can be kept in a liquid state for a long time, so that the gas can freely escape. On the other hand, during the melting of the metal until the casting process, a reducing agent is added to the molten metal surface to deoxidize or cover the liquid surface to reduce and remove the gas dissolved in the metal.
Figure 4 internal air holes
Second, the surface defects of the ingot
Common surface defects are:
Slag. An irregular granular carbon black is embedded on the surface of the ingot, and after it is removed, a slag hole appears. Slag inclusions are commonly found in flat silver ingots cast in vertical molds. The slag inclusion defect is mainly caused by the sublimation of the coating faster than the rising speed of the molten metal surface (Fig. 5), or the fact that the silver liquid is not vertically poured into the center of the ingot mold. In the case of flat mold casting, when the slag on the metal surface of the crucible is not cleaned, slag inclusion phenomenon may also occur.
Figure 5 Coating sublimation is faster than liquid surface rising speed
Sticky mold and spindle corner defects. The spindle angle sticking mold is caused by uneven coating of the coating or premature take-up (when the metal is not completely condensed, the mold is opened and the ingot is opened), and the spindle angle is stuck on the mold, resulting in an irregular defect of the spindle angle, affecting Surface Quality. The lower spindle angle is a round defect, mainly caused by the liquid temperature or mold temperature of the injected metal. When the metal injection speed is too slow and too slow in the late casting, and the injected metal has not been filled at the four corners of the upper part of the ingot, it will be condensed, and a round defect of the upper lobe angle will be generated.
Surface pores. There are three main reasons for the gas stagnation on the surface of the ingot. First, the paint is too thick. The large amount of gas produced by the thick coating combustion is too late to escape due to the high casting pressure. These fine bubbles gather and expand between the mold wall and the metal ingot surface, and rupture the ingot which has been semi-solidified to form a circular pore on the ingot surface. Second, the metal is not vertically injected into the mold core, but is flushed down along the mold wall, which will wash away the paint and make the local mold wall temperature too high. Or the gas generated during the burning of the paint and the gas in the metal are blocked by the impact pressure of the injected metal, and cannot escape freely, and partially remain between the wall of the superheated mold and the surface of the ingot, and are expanded by aggregation to form pores. Third, the metal temperature is low or the metal is not continuously injected into the mold, so that the metal splashes on the beads and adheres to the mold wall, and the injected metal cannot be melted into the condensed pellet. When the beads fall off, they leave a round hole in the ingot. Also, when casting, the silver particles fall into the adjacent mold, and a round hole in which the beads fall off also occurs.
Indentation. The mold wall is uneven or the slag is not cleaned on the mold wall, and a deep indentation is generated on the surface of the ingot.
wrinkle. Wrinkles appear on the surface of the ingot, mainly due to the low temperature of the metal injection or the slow injection rate due to condensation. When a tantalum mold is used to cast a gold ingot, since it contains impurities such as copper , these impurities are oxidized by air during casting, and a layer of wrinkles is formed on the ingot surface. Wrinkles are also formed when the thin residue of the metal level is not cleaned.
Shell-like appearance. This defect occurs on the corners of the ingot, on the ribs or in the thickness direction of the ingot. This is caused by a low metal injection temperature or a slow speed. This defect occurs when the sublimation speed of the paint is slower than the rising speed of the metal level.
bubble. A bubble is a bubble generated by an ingot surface. The surface of the foam is mostly a thin shell, which is a round hole filled with air. This is due to the large amount of gas escaping when the metal condenses. At this time, a thin layer of metal on the ingot surface has condensed, which blocks the gas from escaping into a bubble. The thin shell of the bubbling surface, some intact, and some have been swelled. This bubble defect is more common in the ingot surface of a flat die-cast gold ingot (no watering or wet paper).
Ingot bottom. This defect is seen in the casting of gold or silver ingots in flat or vertical molds, with more flat molds. The bottom of the ingot is a circular hole that appears at the bottom of the ingot. The small hole is rounded on the surface of the bottom of the ingot, slightly enlarged inward, resembling a bee hole, or rounded, or elliptical. The reason for this is that the gas in the metal is not discharged and remains at the bottom of the mold, and is thermally expanded and tries to rise to the liquid surface. As the gas accumulates and expands, the length of the bubble increases, and then the neck is broken and broken into two, part of which rises, and part of which remains in the bottom of the ingot to form a bee eye (Fig. 6). Many operators believe that this bee eye is caused by overheating of the mold, but it is not entirely true. In order to avoid the formation of the bottom of the spindle, the injection speed of the metal is slightly slowed down at the beginning of the casting, so that the molten metal covers the mold base slowly, so that the gas rushes out at the moment when the metal is to be in contact with the mold wall.
Figure 6: The formation process of the bottom of the ingot and the internal gas
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