Superalloys are nickel-based or cobalt-based alloys that are resistant to high temperatures, perform better, are corrosion and oxidation resistant, and last longer than other metals. Superalloy parts are traditionally produced from powdered metals (PM) using subtractive manufacturing techniques. These parts are most often used in heavy industry or in areas having a corrosive environment, as well as to produce critical components such as gas turbines and jet engines.
New techniques such as additive manufacturing (AM) have been developed within the last five years, and the use of powder metals to produce parts in the medical and dental fields, the automobile and aerospace industries, and the consumer goods market is increasing. The AM technology requires the same stringent material properties and cleanliness standards as traditional methods to ensure selection of the correct material (PM) used to “grow” the required parts.
Powder Metals and Cleanliness
Today’s powder metal technology produces more high-quality, functioning parts using a variety of robust powder metals materials than ever before. Because of the critical nature of the parts produced using this technology, consistency and reliability of the powder metal is of utmost importance. One factor in determining a powder metal’s performance is its cleanliness, which has been linked to its durability. Cleanliness is a function of the original powder atomization process and, in the case of additive manufacturing, the recycling of powder between printing runs.
The cleanliness of powder metal can be assessed in a variety of ways. For high end use, there can only be rare occurrences of the contaminants, and they need to be physically separated from the bulk of the powder particles in order to characterize them. Two of the more common methods of separating these particles include:
- Water Elutriation, which involves settling the particles downward in a column of water that is flowing upward. This method is time consuming and the settling depends on particle composition AND size, so it is difficult to get adequate separation.
- Acid Digestion, in which the nickel powder is dissolved out, leaving the residue. However, the acid attacks the defect particles as well, so it is not evident what was in the powder before treatment.
A Unique Method
A unique approach for separating the particles in powder metal is by using heavy liquid separation (HLS), a method that separates the particles based on density alone. Using a specialized sample preparation combined with computer-controlled scanning electron microscopy (CCSEM) for data acquisition (particle-by-particle image/spectra), this method solidly delivers the results needed to determine the cleanliness of both the virgin and recycled powder.
Heavy Liquid Separation (HLS) – How It’s Done
- Because the particles in powder metal are so rare, the first step is to separate the powder from the non-metallic low density particles. This is done by Heavy Liquid Separation, in which the powder sinks and the defect particles float.
- The second step is to evaluate the float particles to assign them a compositional type. The particles may be in the virgin powder or the recycled powder in the AM process. Further, the particles may be an artifact of sampling or non-deleterious; these can be identified and removed from consideration.
- The third step is to evaluate the particle size. The larger the particle, the more problematic it is.
- The fourth step is to report the results. Reports generally list the size of the particles in order from largest to smallest along with their descriptions.
For the most part, the end use of many powders is in parts that are subjected to very high temperature and a great deal of centrifugal force (high stress), an environment that can promote part failure and the consequences of failure could be dramatic. In other powder end use, such as additive manufacturing, cleanliness influences the quality of the part (orthopedic devices, dental appliances) and the efficiency of production. In either case, powder cleanliness, a factor that influences the material properties of the finished product, must always be confirmed.
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