Accumulation of porosity can become damaging to the functionality and quality of a part. Quickly and accurately identifying, locating, quantifying and documenting porosity defects is necessary to ensure all parts are within specified porosity tolerance.
Introduction to Porosity
Accumulation of porosity can become detrimental to the quality and functionality of any component. Traditionally, users would cut open parts to identify and measure internal porosity to determine the most feasible method of reducing or removing all porosity. Technologically advanced testing methods, such as industrial Computed Tomography (CT) allow users to identify, locate, measure and access the orientation of internal porosity in 3D, without destroying the part.
This has provided users with an outlet to gain accurate insight for defects within parts quickly, enabling users with information to identify ways of ensuring porosity is within tolerance.
What is Porosity?
Porosity within a part or component, is a measure of pore volume or void space. The measurement is a ratio of the volume of pores or voids, in comparison to the total volume, which can be identified as a percentage. Porosity is identified as a defect, which can adversely affect the quality and/or functionality of a part or component. Porosity is a term often miscommunicated as an inclusion. Even though both terms are classified as a defect, an inclusion is used to describe impurities or irregularities within a part that consists of foreign material, which can be the result of human error or poor handling for example. However, porosity is essentially a cavity or an empty space or hole within a part. Porosity can commonly be found across multiple industries within materials including metals, composites, polymers, ceramics, silicone and glass, to name a few.
How porosity is commonly formed
Porosity can be formed in several different types of materials and can be accumulated through various manufacturing methods. Some methods of manufacturing, such as porous media molding commonly used for polymers, attempt to develop parts with desired controlled porosity for different applications. Other manufacturing methods have tight tolerances and industry standards for the level of porosity accepted. Moreover, many materials are preferred by manufacturers, as they have better characteristics to avoid accumulation of porosity.For example, a cast part with aluminum A206 is prone to develop hot tears, creating defects, as oppose to a cast part made with aluminum A356. There are many different ways porosity can be developed on the surface of a part or internally, including the design intent of the part. The method of manufacturing can also play a significant role in accumulating porosity, in regards to the machine pressures for machined components, heating and cooling temperatures for cast parts, mold design for molded components or the sintering process for sintered metals, to name a few.
Die Casting: Porosity
There are various different forms of die casting porosity. The most commonly identified form of porosity in die casts include gas porosity and shrinkage porosity. Gas porosity commonly appears to have rounded edges caused by trapped mold in the metal, whereas shrinkage porosity appears to have jagged edges due to inadequate solidification of the casting.
Porosity in die casts can be developed subject to many different factors including, but not limited to, the type of material being used, machine speed and compression, mold design, cast geometry and design, or metal and mold temperatures. The focus of the caster is to identify the type of defect, the frequency of occurrence for that defect, the location of the defect and to quantify the size of the defect, subject to specific standards of porosity tolerance. Understanding and documenting porosity characteristics assists casting supplier and manufacturer in identifying adjustments or changes that can be made in design and machining to avoid defective output. Often times however, porosity formation in die cast applications can be mended, using welding applications.
Common methods of detecting porosity
The two most common methods of detecting porosity within a part include X-ray testing and destructive testing. With the application of digital radiography (x-ray testing), users are able to seek out internal location and frequency of occurrence of porosity. This allows users to confirm that the porosity formation is within tolerance, without damaging the part in its entirety. Destructive testing methods enable users to cut open the part to visually inspect internal porosity. This will automatically scrap the part and can often provide inaccurate or misleading assumptions about the defects.
For an extensive and accurate approach for conducting a failure analysis on parts to identify porosity, users also have access to industrial Computed Tomography (CT). Industrial CT can provide users with internal and external part data on porosity occurrence, location, orientation and measurements. The most effective feature for Industrial CT is the fact that it is completely nondestructive and provides results in 3D, giving users access to the depth of porosity. This can play an instrumental role in qualifying parts and re-adjusting design to ensure porosity is within tolerance.
Purpose of detecting porosity
Whether it is a composite material, aluminum or steel casting, or a plastic injection molded component, formation of large porosities can become detrimental to the quality and functionality of any part. If these defects are not identified and quantified, the overall cost of the part, allocation of scrap parts and cost of quality control process can increase exponentially. Identifying and quantifying porosity can assist users in understanding the need to adjust design or even change manufacturing process, to avoid accumulation of porosity in the future. However, having porosity within a part does not disqualify the part immediately. Manufacturers and industry leaders must develop standards according to their own specifications and applications to understand the minimum and maximum porosity tolerances, in order to qualify a part for specific applications. Users must define maximum level of porosity acceptable in certain areas, subject to how critical the areas are to the functionality of the component.
Industry Standards: Porosity
The Minerals Managements Service (MMS) provides standards of porosity for components such as valves and fittings. The American Society for Testing and Materials (ASTM) has identified standards for porosity in castings. These standards have a relatively general approach, which many companies rely on for manufacturing parts. These are also commonly referenced and used as a foundation in order to develop project specific tolerances for various applications.