The goal of a foundry is to produce high-quality castings that meet or exceed the customers’ specifications, at a competitive cost. In some cases, those specifications call out maximum defect size and/or require that the part must hold pressurized fluid or gasses. When the size of a defect is smaller than the maximum allowed by the specifications, many foundries use vacuum impregnation to correct it — and yet the part still is not capable of holding fluids or gasses under pressure. Vacuum impregnation is a proven process that seals internal defects without impacting any other feature of the manufactured part.
Many times the question is asked “What is the maximum size of a porosity defect that vacuum impregnation can seal?” This is undoubtedly prompted by metalcasters attempting to seal a defect to the maximum specification. For example, the assumption is that if the maximum defect is 5 mm and vacuum impregnation can seal a 5-mm defect, then vacuum impregnation will be effective.
Unfortunately, this is an oversimplification of a much larger issue. If the goal is to produce castings that do not leak fluids under pressure, then determining the maximum size of a porosity defect that can be effectively sealed by vacuum impregnation is the wrong way to address the problem. The foundry’s challenge is not to seal individual pores; Instead, one needs to understand the leak path, which is created by a series of interconnected pores (see Image 1).
Understanding porosity — While some metalcasters understand porosity to be a defect, it occurs naturally and is found in most materials, including man-made formulations and in naturally occurring elements and alloys. In metalcastings, porosity typically is considered any void found in the solid structure. Casting porosity can be caused by gas formation or solidification while the metal is transforming from a liquid state to a solid state. This porosity may range in size from sub-micron dimensions to voids greater than 10 mm wide, depending on the casting.
Metalcasting porosity can affect a part’s structural integrity too, creating a potential failure point. Porosity also can prevent the part from being pressure tight. This will impact performance if the part is designed to hold gases or fluids.
Understanding leak paths — Generally, one pore will not cause a leak path. Instead, a leak path usually is created through a series of interconnected pores. For this reason, it is difficult to pinpoint a generic porosity range that vacuum impregnation can seal.
For example, a breach caused by a 5mm pore interconnected with a series of smaller pores will be easily sealed (see Image 2).
Conversely, the same 5-mm pore breaching a 5-mm wall will be difficult — if not impossible — to seal, as there is little casting material to which the sealant may adhere. A pore of that type has characteristics similar to surface porosity such that it will not be a candidate for sealing through vacuum impregnation. Large open pores that breach both casting walls sometimes are called “see through” porosity (see Image 3).
It is important to understand that all materials permit leakage over time. In order for vacuum impregnation to seal the leak effectively (and maximize the volume of acceptable cast parts) the manufacturer must define the parts’ performance requirements, and develop measurable and repeatable standards around those needs. Defining these standards is done through leak-rate testing. Then, vacuum impregnation is used to seal specific leak paths to achieve the pre-defined leak rate.
Understanding leak-rate testing — The purpose of leak rate testing is to confirm that the manufacturing process is performing to specification and making acceptable parts. Finding defective parts early in the manufacturing process will reduce field failures, minimize unforeseen costs, and improve customer satisfaction.
Inspecting castings requires quantitative, measurable values that define what is and what is not acceptable, given a part’s intended use. The fact is that even materials cast with careful processes will allow some leakage, given enough time to fail. Casting manufacturers develop and adhere to leak-rate standards. Such standards define the maximum tolerable leakage for a part, typically specified by cc/min at a specified pressure and time duration.
Most automotive components operate with liquid. However, air is primarily used in leak rate testing for the following reasons:
1. Air is compressible and has a lower viscosity than liquid. Air can travel through a leak approximately 100-400 times faster than a liquid.
2. Air has no surface tension. This allows it to escape a leak more easily than a liquid.
3. Overall, testing methods using air are faster to conduct than those using a liquid.
Understanding testing protocols — To save time and resources, most manufacturers use the industry-recognized leak rates that are available for many products. Figure 1 shows typical ranges for existing parts (see Figure 1).
At times, industry-recognized standards are not relevant to a part’s application. To establish new standards, those parts should be analyzed by following these steps:
1. Test a large sample of production parts with an air-leak tester at the same part working pressure and time.
2. Pressurize parts with water at the same working pressure. This testing will identify the approximate size of a hole (measured by air leak rate) that does not allow fluid to flow through. The part’s resistance to fluid flow is defined by the hole diameter, part length, hole surface finish, fluid viscosity, and surface tension.
3. Set the reject leak rate at a point close to but below the highest air-leak rate that does not allow fluid to leak.
4. The final leak-rate tolerance should be stated as “specified air-leak rate at specified test pressure and time duration.”
It is important to note that even after a casting is sealed and passes the leak-test criteria, the casting still may exhibit leaks under more aggressive test conditions. Recall that all materials will leak in varying degrees. If the test standards were established so that the casting would hold oil at 1 bar, it may still exhibit a leak if tested with air or helium at 1 bar. Gasses are thinner than liquids (e.g., sealant, oil, etc.) and will leak through a path that would not pass fluids.
Understanding the role of vacuum impregnation — Once a leak rate is defined, parts within that range can be sealed through vacuum impregnation. Parts outside of the leak rate parameters are typically scrapped.
Vacuum impregnation is a process that seals metalcasting porosity. Specifically, it seals the internal, interconnecting path of porosity, which breaches the casting wall. The process is not a surface treatment, so it does not seal open pores found on the casting surface. Nor is it intended to seal casting structural defects such as cracks or open knit lines (see Image 4).
The wide range of casting parameters creates a limitless array of shapes and sizes of potential porosities. Despite all those possibilities, vacuum impregnation can seal porosity of any size. And still, while vacuum impregnation can seal porosity of any size it is important to realize that the leak path is the critical characteristic to evaluate, and not pore size. A leak path is created through a series of interconnect pores, and not a single pore.
Instead of asking, "What is the maximum size of a porosity defect that vacuum impregnation can seal?”, an informed metalcaster should ask, “Can vacuum impregnation seal the leak path?”
Andy Marin is the marketing coordinator for Godfrey & Wing, a developer of vacuum impregnation technology. Contact him at [email protected]