Additive manufacturing (AM) technology, or 3D printing, is the process of producing a three-dimensional component from a computer-aided design (CAD) model by adding material layer by layer. AM technology was originally developed for polymers in the 1980s. Metal powder bed fusion techniques, the first example of truly additive metal manufacturing, were developed in 1995.1 Throughout the ‘90s, the process was considered feasible only for prototyping, but further development has made AM viable for commercial applications. The metal powder bed fusion techniques, for example, have been commercially used since 2005.1 Although research into the use of aluminum for AM manufacturing has been ongoing over the past decade, interest in the material has grown significantly in recent years.
AM is able to provide very complex shapes or geometries that are not possible with other manufacturing processes, enabling high levels of customization. As a result, a number of industries, in particular automotive, have shown a considerable interest in expanding the commercial potential of this technology. Currently, five top automakers are pursuing aluminum and other metal AM applications for their vehicles, with projects ranging from prototyping to customizable components.2 In addition, aluminum manufacturers, such as Arconic, Kaiser, and Gränges, have all announced investments in research and partnerships to further investigate the potential for aluminum in AM.3-5
The global AM market reached $9.3 billion in 2018, a growth of 18% from the previous year,6 and is expected to continue to grow, moving towards high volume, more commercially accessible products. Aluminum will likely take part in this growth as the automotive and other industries continue to demand lighter weight, higher strength customizable components.
Given the exponential growth of aluminum in AM, it was essential for the industry to develop a designation system in order to standardize the alloys and give manufacturers confidence in the aluminum parts they are implementing. Therefore, the Aluminum Association is introducing the new Aluminum Alloy Designations and Chemical Composition Limits for Additive Manufacturing (AM) and Powder Metallurgy (PM) Feedstock and Products, otherwise known as the Purple Sheets, which will be published later this year. The Purple Sheets will allow innovation to continue while providing a baseline for the growth of AM.
The new Purple Sheets cover the full range of additive processes, including powders and their products and wire and rod additive products. The designation system is consistent with the rest of the Rainbow Sheets produced by the Aluminum Association, especially International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys (better known as the Teal Sheets) and Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot (better known as the Pink Sheets). Although there are distinctions, it is logical to use the Teal Sheets as a partial base for the new AM system, since AM produces similar quality products to wrought aluminum. In terms of how alloys are designated, the Purple Sheets use a system similar to the Pink Sheets, which indicate whether an alloy is used to produce a casting or an ingot. This consistency is key in effectively integrating the Purple Sheets into widespread use by manufacturers and specifiers.
Powder and Product Alloys
Using five numbers, a decimal, and up to two letters, the Purple Sheets are able to distinguish important characteristics of powder and product alloys used in AM. The system shows the primary alloying element and which alloy family the product is in, as well as any modifications made to the original alloy. Further, the product’s form, either powder or a product from feedstock, as well as the original powder composition are specified through the system. Lastly, if the alloy is not a powder but a finished product, the system denotes which particular AM process was used in its creation. The simplicity of this standardization system is crucial in an industry that changes day by day.
The basic form of an alloy designation number in the Purple Sheets for powders and their products is wZxx.yvS (Figure 1). The w is a number from 1 to 8 representing the alloy family used, aligned with the wrought alloy families. Z is a letter designation, where A represents the original alloy, and B-Z represents the various modifications from the original alloy. In the third and fourth place, xx represents the minimum Al content for Group 1 or is an arbitrary distinguisher for Groups 2-8. The fifth place y indicates the product form, with a 5 for a powder and a 6 for a product made from powder feedstock. The v in sixth place shows the original powder composition with an 0, or a variation of this composition with digits 1 to 9. S is the suffix letter indicating the process used to make the product. Currently, processes used are L for laser powder bed fusion, E for electron beam powder bed fusion, D for laser direct powder deposit, S for sintered products, and C for cold spray products. An example of this designation would be 3B98.61S, which would be the composition of a sintered product made with 3B98.51 powder. Here, 3B98 indicates the second modification of the original powder alloy 3A98.50 because B is the second letter in the alphabet. 0.61 indicates the product is made from the first powder variation.
Wire and Rod
Wire and rod alloys are wrought products, which will continue to be registered in the Teal Sheets. However, AM products made from wire and rod feedstock have a unique classification system within the Purple Sheets. The basic form is xxxxy.7S (Figure 2). The xxxx represents the wrought designation for the wire or rod as found in the Teal Sheets. The y is optional. When used, it shows a variation of the original wrought alloy. The .7 is similar to the .6 used for products of powder alloys, as it shows a product made from wire or rod feedstock. The suffix letter, S, similarly shows the given process used to make the product, in which P stands for plasma direct deposit, E for electron beam deposit, D for a laser direct deposit, and S for a solid state deposit.
Registering a new aluminum alloy for use in AM involves several steps (Figure 3). Producers can request to register an alloy using a short form found on the Aluminum Association website.7 After the form is submitted, it is reviewed by the director of Standards and Technology to determine accuracy, completeness, and any egregious errors. Next, it is shared with the sub-committee on Alloys and Temper Registrations (SCATR), where it is commented on and reviewed for 30 days. If there are changes or need for clarification, the requester is contacted for additional information. When all revisions are completed, it is then registered and subsequently included in the Purple Sheets. The entire process takes 30 days or more, depending on the changes required. Alloys registered with the Aluminum Association’s Purple Sheets designation can then be added to other American National Standards Institute (ANSI) standards developed by different standards development organizations, such as ASTM International and the American Society of Mechanical Engineers (ASME).
In the future, tempers of AM product alloys will be registered in the forthcoming Tempers for Aluminum and Aluminum Alloy Additive Manufacturing (AM) and Powder Metallurgy (PM) Products, more commonly known as the Orange Sheets. The Orange Sheets will allow for dependable tempered alloys, augmenting the trustworthiness of the alloys prior to tempering as referenced in the Purple Sheets.
HRL Laboratories, LLC in Malibu, CA, was the first company to register an alloy with the Purple Sheets designation system, with an additive alloy comparable to 7075. Typically, AM of metal powders involves the application of powder using a laser or other direct heat source to melt and solidify the layers. As a result, high-strength unweldable aluminum alloys, such as 7075, would develop severe hot cracking during the manufacturing process.
HRL solved this problem by inoculating the 7075 alloy powder with specially selected nanoparticles that serve as grain nucleation sites during solidification resulting in an equiaxed grain structure. The smaller equiaxed-grain microstructure resulted in better overall mechanical properties including strength, ductility, and toughness of the AM aluminum part (Figure 4).
“Companies have claimed that they can produce high strength alloys in the past, but we’re essentially the first to be able to produce a wrought equivalent aluminum alloy,” said Hunter Martin, lead scientist on the HRL team that created the alloy. “We mimicked 7075 because it’s a conventional alloy that people in the industry are comfortable with. They know how it behaves and understand its fatigue, corrosion, and so on. We just adapted the alloy so that it could be additively manufactured, enabling manufacturers to take advantage of all the design freedom provided by AM without having to rely on a brand new alloy that hasn’t really been tested in the field or a lower strength alloy that has typically been more amenable for the additive manufacturing process.”
As the company moved to commercialize their new alloy, they looked for a way to validate it, which led them to registering with the Aluminum Association. HRL has registered several alloys, including 7A77.50, 7A77.51, 7A75.50, and 7A75.51 for powder feedstock and 7A77.60L, 7A77.61L, 7A75.60L, and 7A75.61L for AM products made from aluminum powder.
“We wanted to make sure this was an industrialized and adoptable process and alloy,” said Martin. “By locking in the alloy through the registration process and having it externally verified, we’re assuring customers that the functional composition will remain the same whether they order the alloy today or 15 years from now. They can have confidence in the material and how it will perform.”
As a developing technology, the field of AM is growing chaotically and needs a reliable global standard. The Purple Sheets provide a simple alloy designation system for AM aluminum alloys that will help the innovative field of additive manufacturing grow through trustworthy products. Registered alloys enable customers and producers to have informed, productive conversations with a common ground. Having the status of being registered proves an alloy is both commercially available and actively used. In addition to HRL, Arconic and Additive Technologies have registered alloys through the new designation system, with more registrations underway.
“The Purple Sheets are a true game-changer for the aluminum industry,” said Jerome Fourmann, global technical director at Rio Tinto Aluminum and chairman of the Association’s Technical Committee on Product Standards. “For the first time ever, a materials industry has developed a designation system specific to additive manufacturing, opening tremendous growth potential through standardization.”
- Gao, Wei, et al., “The status, challenges, and future of additive manufacturing in engineering,” Computer-Aided Design, Vol. 69, 2015, pp. 65-89.
- Twentyman, Jessica, “Five automakers motoring ahead with 3D printing,” Internet of Business, May 15, 2018.
- “Arconic and Lockheed Martin to Collaborate on 3D Printing Materials Development,” Light Metal Age, July 16, 2018.
- “Kaiser Aluminum Acquires Additive Metal Manufacturer,” Light Metal Age, October 4, 2018, .
- “Gränges Signs MOU with Aurora Labs for Research on Aluminum Additive Manufacturing,” Light Metal Age, July 17, 2019.
- Sher, Davide, “The global additive manufacturing market 2018 is worth $9.3 billion,” 3D Printing Media Network, December 14, 2018.
- Alloy & Product Registration Process and Request Forms, The Aluminum Association.
The Aluminum Association is the industry’s leading voice in Washington, D.C. It provides global standards and industry statistics. Association designations and product standards information is used throughout all facets of aluminum commerce, as well as in other organizations’ codes and standards. Most industry product standards for aluminum mill products are published in Aluminum Standards and Data available in both customary and metric editions. For more information: www.aluminum.org/standards.