Avoiding Defects in Investment Casting
Manufacturing processes for investment casting include a series of operations that expose the green casting to heat and pressure.
These factors, coupled with the properties of the alloy powders and the design of the final casting, make certain defects inevitable.
While there is no doubt that the investment casting process benefits a wide range of industries, all may not be perfect.
Critical industries such as aerospace casting services and automotive investment casting services need proper ways of identifying casting defects and causes.
To enjoy the full investment casting benefits, we are going to learn more about avoiding defects in the investment casting process.
Contents
Shrinkage
Shrinkage is a defect associated with insufficient heat when the casting is poured, which results in a cavity in the casting.
When the casting cools down, the molten metal shrinks, leaving an empty space in the mold. This defect can be avoided by making sure the pouring temperature is high enough.
Visually, you can identify shrinkage defects as a bubble or a hollow area in the casting. On the X-ray, these areas appear as a dark object. In many cases, the defect is so large that the casting cannot be repaired.
The best way to avoid shrinkage defects is to control the pouring temperature in the investment casting process.
Make sure the investment casting molten metal is at a temperature of 1,150° Fahrenheit or higher. You can also make use of a good-quality temperature gauge to measure the pouring temperature.
Aluminum casting defects
When used for castings, aluminum alloys may suffer from a number of defects. These defects may include porosity, cold shut, Gas-Porosity and inclusion.
– Porosity: This defect occurs when air or gases are trapped inside the casting. Porosity can be caused by oxidizing sand, high pouring temperature, dirty equipment, etc. To reduce the chances of getting a porous casting, use clean sand, keep the pouring temperature low, and use a vacuum pump to remove gases from the mold.
– Cold shut: This defect occurs when the aluminum alloy freezes inside the mold. This happens when the casting temperature is too low. To avoid this defect, keep the casting temperature as high as possible.
Cold shut defect in casting
Cold shut is an aluminum casting defect that occurs when the cool down rate of the casting is too slow.
It can also be caused by a sudden change in the cooling rate, but this is less common.
When the cooling rate is too slow, the molten aluminum contracts and squeezes the molten slag out of the mold.
Eventually, it will come into contact with the water inside the mold and freeze. The surface of the mold freezes before the aluminum does, which causes the surface of the aluminum to become brittle.
If the surface of the mold breaks before the aluminum cools, the aluminum will contract, creating a crack.
To avoid cold shut defects, make sure to cool the mold slowly. If possible, use a water-cooled mold to cool the casting more quickly. If you’re using a water-cooled mold, make sure the water is clean and doesn’t have any impurities in it.
Sand casting defects
Sand casting defects are often the result of poor design, but they can also be the result of impurities in the sand. They can negatively impact the strength and surface finish of the casting.
Gas Porosity: This defect occurs when air and gases inside the mold combine with molten aluminum to form bubbles. To avoid this defect, make sure the mold is dry, and the molten aluminum doesn’t come into contact with water.
Inclusions: This defect occurs when impurities are present inside the sand casting. To avoid this defect, make sure the sand used in the mold is clean and dry, and use a clean ladle to pour aluminum into the mold.
Die casting defects
Die casting defects include uneven cooling, which creates an unbalanced casting, and wrinkling, which occurs when the molten metal is trapped in the die.
Both defects negatively affect the surface finish of the castings.
Porosity in investment casting
Porosity is an opening in the casting caused by trapped gases that are released during the solidification process.
This defect negatively affects the strength and integrity of the castings.
Preventing defects using ultrasonic flaw detectors
Ultrasonic flaw detectors are designed to scan your investment casting parts and components to find defects such as inclusions, voids, and cracks.
By scanning the parts before and after the casting process, you can better control for potential defects that could appear during the process.
The standard investment casting process may expose the parts to up to 7,000 psi of pressure.
This can create internal flaws that are difficult to detect using standard inspection techniques. The ultrasonic flaw detectors can detect internal flaws that would otherwise go unnoticed.
How a flaw detection works
A flaw detection is used for part inspection and quality control. It is specifically designed to find internal flaws in your parts, which are very difficult to find by visual inspection. The powerful combination of high-frequency sound waves and electrical current penetrates the surface of the part to look inside.
This allows the flaw detector to identify defects such as porosity, inclusions, cracks, and voids.
The data from the flaw detection is fed into the CAM system to generate a soundness map.
Choose reputable investment casting manufacturer and suppliers in China
Because investment casting is such a mature process, you have a wide range of options in terms of manufacturer and supplier. It is important to conduct due diligence with each company you consider to ensure you have a positive experience.
Some key factors to keep in mind when selecting a manufacturer include their equipment, staff experience, and service capabilities.
Make sure the company you choose has the proper equipment for your part, including the right size and configuration of the molds and the proper type of furnace.
Conclusion
Investment casting is a tried-and-true method of creating durable, complex parts. It is almost entirely immune to the effects of heat and pressure, making it ideal for intricate parts that might be damaged by traditional machining methods.
This article has covered the basics of how to avoid defects in the investment casting process.
There are ways to manage these risks in your operation through a combination of material selection and risk mitigation techniques.
Check out our website for more investment casting defects and remedies pdf.