Casting defects are defects in castings that occur in the casting process. There are various defect phenomena such as cracks in castings, rough surfaces, and casting nests with porosities inside. Casting defects are classified into several types, each with different causes. In addition, since there are several pre-casting processes in the casting process, and even defects in the previous process lead to casting defects, there are complex cases where casting defects are not limited to one, and there are actually many complex causes.
Typical defects fall into six broad categories. This time, we will look at the causes and countermeasures of "porosity".
What is a Porosity?
A porosity is a phenomenon in which a hollow occurs inside a cast, and it is one of the typical casting defects in which the hollow appears on the surface layer.
Why do Porosity Occur and What are the Intermeasures?
Porosity can be caused by various causes, such as too high, too low, or gas. In doing so, it is necessary to review how to install the sprue, the casting conditions, especially the temperature.
By the way, basically, have you used the same bullion more than three times? Once cast, metals often have a different composition due to heat. Some metals are not recommended for reuse. Of course, there are alloys that can be used again by adding trace elements or adding new materials. I hear that in some parts, such as construction steel, the metal composition and characteristics that are expected to be not added when manufacturing products may not be obtained.
In the case of precious metals, of course, it is assumed that the feeder-head etc. will be reused. * Virgin 2/3: Recycle 1/3 is used for virgin metal and return materials with a melting amount of 10 at a time. In the case of precious metals, 1/2:1/2 will be a practical ratio because bullion is expensive. Even the biggest concession is 1/3:2/3. It is not recommended to use the bullion once cast more than three times, even if it is recycled bullion with some new materials. On the other hand, it is not threatening, but in some cases, new bullion may be prone to porosity because of the properties of the alloy itself, which prioritizes alloy strength and cost. In the case of precious metals, there are no industrial standards such as JIS standards. Only "purity" is guaranteed in the bullion to be purchased. There is no guarantee of castability except for alloys that are castable.
Professionally, you should also check the solid or liquid phase lines in the alloy's equilirium diagram if possible.
* Virgin Metal: Refers to refined metal.
Here are four main causes and countermeasures.
1 Gas Porosity
Because of the large number of bodies in which the bullion is deposited and the smooth replacement of the body and molten metal during casting, porosities are generated because the body remains inside the cast. It may occur alone or as a porous porosity. The gas porosity of this porous porosity is called a blowhole. However, the cause of gas porosity is not only due to bullion, but also to how to make casting molds, burn-out them, and attach sprue.
It is forcibly removed during the gas and dissolution of the bullion before casting. In some cases, the dissolution method can also be improved. As a method of degassing bullion, the bullion is dissolved once, poured into a light mold, etc. to make a rod-shaped ingot, which is crushed with a hammer or roller. This reduces the content of the gas. In addition, when the bullion is dissolved, a de-acid material tailored to each metal is poured into the molten metal. Depending on the type of deoxidant, if you put too much in it, the fluidity of the bullion at the time of dissolution may change, or the toughness after solidification may be affected and the brittleness may be high, so if you are worried, it is also one way to make a mother alloy. In the casting of metals containing a lot of gas such as stainless steel, we review the mounting place and thickness of the sprue, the shape of the wax tree, etc., improve the replacement of molten metal and gas, and devise so that the molten metal is close to the laminar flow. In some cases, it may be required to provide vents for degassing in the wax tree or product part.
Also check the burn-out curve of the casting mold to ensure that there is no residue in the mold. Since it may be due to the permeability of the casting mold, depending on the type of investment powder, check the mixing ratio and calcination temperature gradient of water, binder, and investment.
2 Gas Porosity generated from a crucible
Gas porosity can be generated from gas remaining in the mold to bullion or from a crucible point depending on the material.
In the case of platinum, degassing may be formed by re-dissolving in the crucible. Since the platinum dissolved in the casting machine is solidified in the crucible and the gas is inherent in the bullion with a bulge in the center, it is strongly recommended to dissolve the bullion (Pt) once before casting to check for gas content.
原因3 Engulfing Porosity
It is a type of gas porosity, and the turbulence generated at the time of casting inhibits the replacement of the molten metal that flows in with the gas inherent in the casting mold, and it remains so that the gas is involved in the vortex of the molten metal.
It is a casting nest that is mainly seen in centrifugal casting, and if it is a ring, it has a feature that occurs continuously vertically from the shoulder of the ring to the arm as shown on the right.
In the case of centrifugal casting, while the casting capacity is high, the flow of molten metal tends to be turbulent during metal pouring and does not flow in the stratum, so adjust the rotation speed of the centrifugal casting or change the mounting point of the sprue.
4 Porous Porosity (Dendrite)
A porosity that is generated by completing solidification in a dendrite state that occurs when molten metal solidifies due to a lack of molten metal temperature. It occurs throughout the surface layer of the cast, but there are many cases where it is difficult to correct the porosity because it is not under been found how much cavity is inside.
Increase mold casting temperature or molten metal casting temperature. It is recommended to try it in units of 50 ° C. Depending on how it looks, it may be seen as a blowhole. Since the coping method changes, please observe well and determine whether it is a gas porosity or a porous porosity.
原因5 shrinkage Porosity
When the cast solidifies, directional solidification cannot be secured, and the supply of molten metal commensurate with the solidification shrinkage of the metal is inhibited and occurs.
It generally occurs in the thick part of the cast product. Metals with high melting points and metals with high shrinkage have a high risk of shrinkage porosity, and for example, in jewelry and platinum alloys, it occurs remarkably in platinum with a large shrinkage ruthenium. This is a porosity commonly found in pressure casting.
In addition, "hot spots" that occur in the center and corner of intersections where hot water crosses (such as T-shaped and X-shaped) cause a delay in solidification and cause shrinkage porosity. In sand casting, the mold before casting may be made of the same wedge-shaped metal as the cast bullion, which may facilitate the cooling of molten metal in the hot spot area. This is called cold gold.
In any case, with directional solidification in mind, check the condition of the weir, such as how to install the sprue and the thickness, and supply molten metal to compensate for the shrinkage of the metal.
Summary of Porosity
We have introduced the causes and countermeasures, but we will summarize what to do when the casting nest occurs.
- Check the calcination temperature gradient
- Check the bullion for gas content
- Adjust the speed of the centrifugal casting machine
- Test casting temperature in 50°C increments
- Adjust the way the sprue is installed and the thickness to supply molten metal to compensate for the shrinkage of the metal
There is not always one cause. Please review all the items again and see how they work.