Understanding Porosity

The term porosity is used extensively by both casting suppliers and customers when talking about casting defects, but it does little to describe the actual problem. “Porosity is such an all-encompassing word,” said Vadim Pikhovich, materials engineer for specialty heavy truck manufacturer Oshkosh Corp., Oshkosh, Wis. “It can be shrink in the form of micropores, sponge-type voids, large macro-voids, inclusions. It all gets bundled up in one big word: porosity.”

Porosity is often used to describe any void or hole found in a casting. But the morphology of pores in terms of their size, shape, surface constituents, location and frequency are important in defining the specific defect, determining its cause and analyzing the extent to which it will adversely affect the casting’s properties or functionality. In many cases, the existence of porosity will not be detrimental to the casting’s function.

Understanding the various porosity-type defects also helps in casting design. While some defects can be fixed by the manufacturing process, others can be fixed through design changes or a combination of both. By knowing the casting factors that are likely to contribute to the different defects, you can work with the casting supplier to avoid potential problems,  relocate porosity-prone areas to nonstructural sections of the part and establish mutually acceptable levels of porosity present.

Gas-related porosity appears as voids with rounded edges.
Identify and Quantify

When a defect is found in a delivered casting, usually uncovered during machining, you should be able to tell the casting supplier three things—the type of defect, the location, and the quantity or frequency of its occurrence.

“If you talk to a lot of [metalcasting facilities], they cringe when the word porosity is used because it explains nothing as to type of defect,” Pikhovich said. “The person relaying material non-conformance condition to the supplier should have a more advanced knowledge of defects based on alloy and process. OEMs will greatly benefit if design and quality engineers identify the defect and understand what they are dealing with before talking to the caster. The casting may even have the potential for salvaging.”

Parts cast in steel, and, to a lesser extent, aluminum, may be weld-repaired rather than scrapped and recast.
Once the defect has been identified, Pikhovich recommends sending proper documentation of the defect, along with pictures and the casting, back to the supplier. During the discussion with the metalcaster, request that someone involved in the manufacturing process is present, such as materials engineer, casting design engineer, quality control personnel, pattern shop supervisor or plant manager. 

Depending on the defect, the part may require design changes to improve its castability and solidification or a change in the casting process, such as better melt control or a different method of gating and core venting.

“Whoever is in a quality assurance function or does evaluation of castings would be well served to understand what different porosity-type defects look like,” said Mike Gwyn, vice president of metals technology at the Advanced Technology Institute, Charleston, S.C. “Ultimately, you are trying to solve a problem. A lot can be learned by where the defect is found.”

An improperly vented core caused these blowhole defects on the surface.
Gas vs. Shrinkage

The casting voids most often referred to as porosity can be caused by gas formation, solidification shrinkage, or non-metallic compound formation, all while the metal is liquid.

Large gas-related voids caused by trapped mold or core gases in the molten metal are called blows or blowholes. They generally are large enough to look like bubbles with smooth interior surfaces and are always buoyant and will float near the top of the casting, as it was oriented in the mold.  Sometimes they can get caught on the bottom surface of a core lower in the mold, but these gas pockets are always “up” against something, if not the top of the mold.

Pinholes are caused by gas atoms dissolved in liquid metal that connect and become molecules. They stay small, but float to a top surface somewhere in the casting. 

“It’s important to visualize how the gas formation occurred, if it’s the kind that will float, and where you found it,” Gwyn said. “Knowing those clues really helps analyze the cause.”

Shrinkage-related casting voids are caused by sections of the casting that solidify later than the surrounding sections and do not have enough metal flow into the section for a complete fill. This is generally because the area is too hot for too long due to a thick section or a tight corner. Shrinkage porosity will have a dendritic (jagged) or linear appearance and can occur in either the cope or drag portion of the casting, usually below the surface. Shrinkage also can participate in the formation of hot tears, which appear as jagged “cracks” where solidification contraction also pulls on the casting.  Hot corners and pockets are a good place to find a hot tear with some solidification shrinkage underneath it.

According to Gwyn, hot tears often occur when a casting undergoes the predictable contraction caused by solidification, also known as patternmaker’s contraction. “To eliminate hot tears, you might decrease the mass of a section, help it cool off quicker, and/or reduce the mold constraint across the region where the hot tear is forming,” he said.

Shrinkage defects can be avoided through alterations in casting geometry, such as changing the location of thick and thin sections, and molding geometry, such as altering the placement or size of gating and risers.
“With shrinkage, the two parties—casting supplier and customer—would need to start talking to play with the shape,” Gwyn said. “The aim is to find a way to make the problem section less prone to staying hot for too long.”

Shrinkage defects tend to have jagged edges and are caused by improper casting solidification.

Sometimes machining a casting will uncover voids that look both jagged and smooth. During solidification, a vacuum can be created in local areas by the shrinkage process. Atoms such as hydrogen will migrate into that vacuum and become molecules of H2 gas. As more atoms move into the shrinkage cavity, the newly formed gas molecules begin to create partial pressure, and the still liquid metal nearby will get smoothed surfaces from the little bubble formations. The result is fine, jagged, dendritic shrinkage cavities with some smooth surfaces mixed in. This is called gas microporosity, which is common in the light metals, particularly aluminum.  It is generally found buried in heavier sections where solidification shrinkage has occurred. 
The best way to fight gas microporosity in castings is to first work on the melting process to reduce the parts per million of gas atoms and then work to eliminate solidification shrinkage, which creates the vacuum that turns the gas atoms into gas molecule bubbles.

Another common casting abnormality that can be confused for porosity is an inclusion. Inclusions are impurities such as slag from the molten metal, loose molding sand, dirt or oxygen that become trapped in the metal as it solidifies. Inclusions appear on the surface or below and are typically the result of poor metal handling or mold cleanliness. Changes to melting, pouring, or molding at the metalcasting facility can alleviate their occurrence.

Smart Specs

Understanding the types of porosity defects goes further to helping your company’s bottom line when the knowledge is applied before the part is even sourced to a casting supplier.

“It’s so easy to say ‘we can’t stand porosity of a certain size or any at all,’” Gwyn said. “But you would save on cost by allowing porosity in areas where it is okay.”

Porosity in a casting does not automatically mean the casting is defective, and requiring tighter porosity specifications in noncritical areas may call for extra steps or less economical methods at the metalcasting facility that can lead to higher overall part cost. Pouring time may need to be increased, which will add to the man hours per part. Additional gating might be added, which will lower overall casting yield. More extensive inspection of a finished casting also will add to the cost. Higher specifications also will result in higher scrap rates, which metalcasters will account for in price when quoting a job. Clearly defining the acceptable locations and levels of porosity will help keep casting costs down.

The American Society for Testing and Materials (ASTM) lists standards for porosity in castings. Although these standards tend to be broad and generic, they can be used as a starting point for discussion between the casting supplier and customer. The Minerals Management Service (MMS) also offers porosity guidelines developed for valves and fittings. The ASTM and MSS standards show different levels of porosity using comparator plates or photographs. Casting end-users can use them to determine which levels of porosity would be acceptable in their parts.

“If a company doesn’t have its own standards or specifications, they can look at the ASTM or MSS levels and use them as a basis,” said Malcom Blair, vice president of technology for the Steel Founders Society of America. “Also, if a machine shop is the point at which porosity is discovered, the standards give them a way of quickly identifying the levels of porosity.”

You can either rely on the standards laid out by ASTM and MMS, or use them as a basis for working out standards between you and your metalcaster. Gwyn suggests specifying the diameter and frequency of voids across a surface to the minimum acceptable standard for an application. He also advises setting different porosity standards for various sections of the part requiring differing levels of structural integrity.

“If (the presence of porosity) doesn’t matter, don’t make a point of rejecting it on principle,” Gwyn said. “Take the time to call all the standards out for each section of a casting. It’s a pain for engineering, but for the purchasing and quality assurance side, it’s well worth doing.”

Specifying blanket tight porosity standards can limit your supplier base or result in high quoted costs.

Plan Bs and Cs

A change in alloy may be the answer to persistent porosity issues. Certain alloys are better than others at flowing through a mold. Aluminum alloys 356 and A356, for example, exhibit better casting characteristics than A206, which is prone to hot tears. According to David Howell, senior casting specialist at AlumAlloy Co. Inc., Ontario, Calif., aluminum with silicon will be easier to cast than alloys with no silicon.

The casting supplier can identify which alloys will be easier to cast than others and may suggest these alloys over a specified alloy considered to be unfriendly to the casting process. The final alloy decision is yours, but an alloy that is more difficult to cast may result in a higher bid.

“If that alloy is what you need, don’t be surprised if the first couple of trial castings are defective,” Howell said. “But the metalcaster can use special methods to achieve the required soundness.”

Howell recommends asking the metalcaster to perform x-ray testing on at least a sampling of the first few production parts. X-ray testing will add to the part cost—sometimes doubling the price depending on casting quantity and size. However, Howell argues that x-raying at least a small percentage can help save time and money in the long run.

“Many times porosity is discovered at the machine shop,” Howell said. “Without testing, a metalcaster can’t prove a casting is good until it is machined, and then the cost of machining, as well as time, may be lost.” 
Liquid penetrant inspection is less expensive than x-ray testing and can be used to view porosity defects on the surface of a casting. However, sections thicker than 3 in. typically will require x-ray testing for the best chance of revealing porosity at the center.

If testing reveals unacceptable levels of porosity in the first sample of castings, the casting supplier can take note of where the porosity is occurring and re-gate to alleviate the problem, change pouring technique or add chills to the mold to encourage better solidification. The customer also may be shown the x-rays and asked to determine whether the level of porosity is  acceptable as-is or if changes can be made to the part’s design (such as the thickness of a section) to improve castability.

Ask Questions

After the part has been commissioned to a supplier, Pikhovich suggests garnering as much information about the manufacturing process as possible from the metalcasting facility.

“Once a casting is quoted and production is awarded to a particular metalcaster, it is not very often that I get a call back with a description of how the [casting facility] will be making the part,” Pikhovich said. “One explanation for it may be the assumption on the caster’s part that designers are not necessarily well-versed in specific details of melting, mold-making, rigging system design, etc. However, how a part is cast adds another layer of complexity that will greatly influence long-term quality, delivery and costs.”

Pikhovich suggests asking how many castings will be poured per mold and pattern details, such as parting line location and gating and risering. Some metalcasting facilities will be hesitant to share rigging information due to proprietary reasons, but knowing how parts are cast can help determine the causes and potential solutions to porosity defects.

Parting line location, for instance, tells you what part of the casting will be in the cope and the drag. The flow of metal into the mold and the propensity of mold and core gases to float upward and collect at the cope surface means that those surfaces need to be closely inspected. It’s best for the sections of the casting where structural integrity depends of the quality of the surface finish be located in the drag. It is also very important to understand the type and frequency of quality inspections performed at the metalcasting facility during the casting process as well as prior to castings being shipped to the customer.

“I love working with the [metalcasters] because there’s a wealth of information there,” Pikhovich said. “Don’t be afraid to talk to the people in the [casting facility] that understand how to match the design of the part with the alloy chemistry and its solidification behavior.” METAL