Avoiding Porosity in Mining Engineering Castings
As one of the world class steel fabricators active within the industry today, ChinaSavvy, is also a producer of high quality mining engineering equipment. Our highly trained team understands that porosity is an unavoidable consequence of:
- The gasification of containments at molten metal temperatures,
- The shrinkage that takes place as the molten metal solidifies,
- As well as an unexpected and uncontrolled change in humidity or temperature.
While porosity in some metal castings do have its own set of advantages (like seen in self lubricating parts), porosity lowers the final part or components mechanical and physical properties.
In mining engineering, porosity of casted parts has the following disadvantages:
- Because of the small voids within the part, the part becomes less stable. This is because the density of the particles that are melted together, is not uniformed or balanced.
- Porosity causes weakness in the material used in the casting process.
- Infiltration is also more likely to occur in porous parts as the density of the part is nor balanced.
Porosity occurs in castings as they change from a liquid state to a solid state and is found in castings made from a number of materials, more so in magnesium die cast parts or aluminum die cast parts. Here, three types of porosity is identified:
This type of porosity results in either a surface or internal defect and is caused by the conditions of the flow of the molten metal or poor pressure.
This type of porosity is usually internal and is caused by various kinds of gassed trapped in the casting die. The source of gas porosity is trapped air, burned lubricant and steam.
This is the most common kind of porosity seen and is also the most difficult to control. Shrink porosity is caused by the shrinkage during the solidification process.
When it comes to metal castings, porosity effects the entire manufacturing supply chain and is present in three basic forms:
This type of porosity is commonly not a problem, unless it is uncovered by subsequent machining operations.
Blind porosity is often the cause of internal corrosion and leads to blow-out on painted surfaces and spotting-out on plated surfaces.
Through porosity is an ‘in-service’ problem as gas or liquids seep through the casting.
Prevention of Porosity
While gas porosity is commonly caused by overheating the metal to be casted, or exposing the molten metal to air, it can also be caused by sulfur dioxide from recycled metal, which is contaminated with investment.
Shrinkage porosity is the most common kind of porosity and it is caused in cases where molten metal is not able to feed the casting as the casting cools and solidifies.
Porosity can be prevented by:
Heating the Metal
In order to produce a good casting, the correct temperature is vital. If a metal is too cold, it will freeze in the mold before being able to completely fill it.
If a metal is too hot, shrinkage porosity will occur in the heavier sections or the sections adjacent to these heaver sections. Overheating a metal can also cause base metals, primarily zinc, to either evaporate or burn-off.
Here, it is vital to protect the metal from oxidizing as well as from absorbing unwanted gasses. Protecting the metal is achieved by using a protective gas such as Nitrogen or Argon, using a gas flame or by using flux.
It is important that the sprue size is as least as heavy as the heaviest section of the casting. Because molten metal shrinks in the solidification process, it is important that molten metal is allowed to fill in these gaps. If a flow of metal is not facilitated, voids will occur in the casting.
A sprue that is too thin, it will solidify before the casting does, stopping the flow of molten metal into the mold.
Also, by attaching the sprue to the heavier sections of the casting, heavier sections will still be fed after the thinner sections have solidified
Porosity sealing, also know as vacuum impregnation, involves the filling of porous substrate to make the part used in mining engineering applications, airtight.
- Under a reduced pressure, the within the pores are expanded.
- Next, pressure reversal (which entails adding pressure outside of the part) saturates pores with monomer.
- After this process is complete, the next step entails rinsing the excess monomer from the surface of part.
- Finally, the liquid monomer now inside the pores are polymerized into a solid seal.
Methods of Vacuum Impregnations:
Dry Vacuum / Pressure (DVP)
One of the most complex methods of impregnation, it is also the most effective. Requiring two separate tanks (one for storing the sealant and another for processing the part itself), this process is best suited for metal parts which have extremely small, microscopic pores, in many cases invisible to the naked eye.
The Steps of the DVP Process:
- First, the parts are loaded into the process basket and placed into the processing tank.
- Next, a vacuum is activated inside the tank in order to remove all the air. This removal of air includes the air captured inside the pores.
- A sealant is now transferred from the storage tank to the processing tank. The entire basket containing parts are submerged.
- Next, the vacuum is released and the desired amount of pressure, using compressed air, is added to the process tank. Pressure aids the sealant to enter the pores of the part.
- After an efficient amount of time has passed, the pressure is released. After the pressure has been released, the sealant is drained back into the sealant tank .
- The basket of parts are now removed from the processing tank and washed. Final processing is completed in accordance with the metal part material and the specifications of the part itself.
Wet Vacuum / Pressure (VP)
Requiring only one tank, which contains the required sealant, this impregnation process consists of the following steps:
- First, the parts to be impregnated are loaded into the basket and placed inside the tank containing the sealant.
- Once the parts have been placed inside the sealant tank, a vacuum is activated in order to remove the air, including the air trapped inside the pores.
- Next, the vacuum is released and pressure is added by making use of compressed air.
- Once pressure is released from the tank, the processing basket is removed from the sealant tank.
- Finally, the parts are washed and final processing takes place in accordance with the requirements of the part itself as well as the material it is made of.
Wet Vacuum (WV)
The quickest impregnation process used in the industry, it needs one tank (which holds the sealant) and has the following steps:
- Like in the other impregnation processes, parts are loaded into the processing basket. The basket is then placed into the tank containing the sealant.
- Next, vacuum is activated within the tank, responsible for removing all the air as well as the air within the pores.
- After the air has been removed, the vacuum is released and the tank is vented to atmospheric pressure.
- Parts are then left to soak in the sealant for a required amount of time.
- After soaking is complete, the basket is removed from the tank and the parts are washed. Finally processing is done in accordance to the material used to make the parts as well as the final requirements of the part.
Porosity in Castings
Permanent mold casted parts and die casted parts commonly contains internal porosity. Here, porosity is, in general, localized to the deepest cross-sections of the part and does not generally extend to the surfaces of parts casted in this manner.
The problem occurs when these parts undergo machining processes. Machining causes these internal porosity to be exposed. The part itself will also leak if it is pressurized.
Acrylic resins are commonly used in order to seal machined die casted parts that are required to hold hot fluids – these parts include transmission cases, coolant connectors and intake manifolds.
Porosity in Powder Metallurgy
Components manufactured using powder metallurgy processes are sealed before plating processes are performed in order to reduce internal corrosion. Because plating involves submerging the part in an acid solution, residual acid internal to the part can prevent an acceptable plated finish and promote corrosion.
Internal voids are sealed before plating by saturating porosity with monomer. After saturation, the monomer is washed off and is completely clear of the surface. This leaves the surface to be effectively plated, with interior voids being sealed dry.
Parts manufactured using powder metallurgy is also commonly impregnated in order to enhance its machinability. Because these types of parts are, in general, difficult to machine, with some compositions capable of ruining cutting tools, the sealing of pores can effectively improve the cutting tool’s life span.
Note that resin impregnation is more effective on these types of parts and manufacturers are capable of selectively applying these resins for near net pressed parts.
At ChinaSavvy we strive not only to be the best steel fabricators in China today, but also to deliver high quality, tight and porous free mining engineering castings. If you would like to learn more about our available services, please feel free to visit our Industrial page or Contact Us today.
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Further Suggested Reading:
- Materials used in Mining Engineering Castings and Fabrications
- Tolerances in Mining Engineering
- Advantages in Shipping from China