An Engineer’s Guide: Metal Powders for 3D Printing.

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Metal powders play a crucial role in the world of metal additive manufacturing (AM). As the technology continues to expand, the metal powder market is projected to grow significantly in the coming years. In 2021, the global metal powder market was valued at $6.75 billion, with expectations to reach $10.79 billion by 2029. While the COVID-19 pandemic impacted demand in various industries, including metal additive manufacturing, the market is expected to rebound as the technology proves beneficial in addressing supply chain issues.

To understand the importance of metal powders in metal AM, it is essential to delve into their characteristics and considerations for selection, storage, and usage. The use of metal powders to create solid objects dates back centuries, with the Incas and ancient Egyptians using powdered metals for jewelry and valuable artifacts. However, the mass production of metal powders gained momentum with the introduction of incandescent light bulbs and tungsten wires in the 19th century.

There are several methods for producing metal powders, with the most common being the sponge iron process and atomization through water or gas. The sponge iron process involves mixing magnetite ore with coke and lime, which is then reduced to create an iron “cake” and slag. The cake is subsequently crushed and annealed to produce powder. These particles are highly porous, hence the term “sponge” iron.

Atomization, on the other hand, utilizes a molten metal stream forced through a nozzle. Turbulence is introduced into the flow through the supply of inert gas or high-speed atomized water streams. Gas atomization produces spherical powders, enhancing flowability and making them desirable for metal additive manufacturing. Meanwhile, water atomized powders have more irregular shapes, but recent efforts have been made to improve their morphology for better compatibility with AM processes. Gas atomized powders also boast higher purity levels, thanks to the use of inert gases during processing, reducing the risk of contamination.

It is worth noting that impurities in water atomized powders can be removed through post-processing techniques, such as sieving and washing. However, when low oxygen content is critical, gas atomized powders are generally the preferred option. It is important to mention that gas atomized powders are more expensive to produce due to the complexity of the processing involved. In contrast, water atomized powders offer a more cost-effective alternative, making them attractive for less demanding applications or cost-sensitive projects.

Metal powders used in metal AM share some similarities with those used in powder compaction processes like die pressing, powder forging, hot isostatic pressing (HIP), or metal injection molding (MIM). However, there are notable differences in terms of powder size distribution, flowability requirements, melting characteristics, and specific chemistries based on the AM process.

When selecting, storing, and using metal powders for metal AM, it is essential to consider these variations to ensure optimal performance and desired outcomes. Factors such as particle size, flowability, and purity levels play a crucial role in achieving successful metal 3D printing.

In conclusion, metal powders are a vital component of metal additive manufacturing. As the technology continues to advance, the metal powder market is expected to grow significantly. Understanding the differences between metal powders used in additive manufacturing and other processes, as well as their characteristics and considerations, is crucial for successful implementation. Whether it’s gas atomized powders with their higher purity levels or water atomized powders for cost-sensitive projects, selecting the right metal powder is key to leveraging the full potential of metal 3D printing.

Metal powders play a crucial role in the field of additive manufacturing (AM). Different AM processes require different types of metal powders, and choosing the right powder is essential for achieving high-quality, functional parts.

One of the main distinctions in metal powders for AM is their shape and size. Metals powders used in metal injection molding (MIM) are generally finer and not necessarily spherical in shape. On the other hand, metal powders for AM processes like laser powder bed fusion (L-PBF) or direct metal laser sintering (DMLS) are preferred to have a spherical morphology, although there have been advancements in processing non-spherical powders like flakes or water atomized powder through AM.

The flowability of the powder is crucial in certain AM processes. In powder bed fusion (PBF) processes, where the powder is spread using a blade, the spherical shape of the powder particles prevents them from sticking. In contrast, processes like directed energy deposition (DED) or MIM are more tolerant of randomly shaped particles. These differences in flowability and spreadability influence the selection of metal powders for specific AM processes.

Chemical composition is another important factor to consider when selecting metal powders for AM. The chemistry of the powder should align with the end application requirements. For critical applications like aerospace and medical, conformance to the chemical composition is crucial, including the control of oxygen and nitrogen content.

Powder cleanliness, uniformity, and consistency are also essential considerations. The powder should be free from contaminants and surface oxides, have a consistent particle size distribution to ensure uniform melting and minimize porosity in the final product, and exhibit batch-to-batch consistency to avoid variations in the printed components.

Powder flowability and spreadability are influenced by the factors mentioned above. Flowability is important for top-fed or gravity-fed machines, while spreadability ensures uniform powder layers. It is necessary to distinguish between flowability and spreadability as they are distinct features.

Cost is always a significant factor in manufacturing processes. For metal powders used in AM, striking a balance between material performance and cost is challenging, especially for cost-sensitive applications. The powder should not only have good atomization properties but also be processed at high enough speeds to make a profitable business case.

In summary, selecting the right metal powders for AM applications requires considering various factors. These include powder shape and size, flowability and spreadability, chemical composition, cleanliness, uniformity, consistency, and cost. By carefully evaluating these factors, manufacturers can optimize the AM process and achieve high-quality, functional parts.

Title: Overcoming Challenges in 3D Printing with Refractory Metals

Introduction:

In recent years, additive manufacturing, also known as 3D printing, has revolutionized the manufacturing industry. With the ability to create complex geometries and reduce material waste, 3D printing has become a game-changer in various sectors. However, when it comes to printing refractory metals such as tungsten and molybdenum, unique challenges arise that require innovative solutions.

Challenges and Solutions:

Dr. Ryan Dehoff, a researcher at Oak Ridge National Laboratory (ORNL), explains that one of the key obstacles in printing refractory metals is the difficulty in achieving fusion between layers. Refractory metals have high melting points, making it challenging to provide enough energy to melt the material adequately. As a result, cracking during solidification or cooling becomes a major issue.

In response to this challenge, ORNL has invested significant resources to enhance the printing process for refractory metals. The development of new techniques and technologies aimed at overcoming fusion challenges is crucial. By determining optimal energy densities and refining the printing process for tungsten, molybdenum, and other refractories, the ORNL team is progressing towards successful fabrication.

Another factor affecting the printing of refractory metals is the aggressive aging of the powder. Compared to DTM metals like titanium and certain steels, refractory metals require higher energy densities. This leads to larger heat-affected zones (HAZ) within the non-melted areas. These zones experience material property changes due to exposure to high temperatures.

Dr. Suresh Saharan, a materials scientist, mentions that the slow speed of the printing process is another challenge. Refractory metals have exceptionally high melting points, making the process time-consuming. This, in turn, renders the parts produced through these processes non-commercially viable in many instances.

Applications and Future Directions:

Despite these challenges, refractory metals, particularly tungsten carbide, find extensive use in powder metallurgy applications where cooling rates can be accurately controlled in a furnace. Cutting tool inserts are a prime example of these applications. By controlling the cooling rate, it is possible to produce high-quality parts with desired characteristics.

As research and development continue, scientists and engineers are collaborating to find more efficient ways to print refractory metals. By addressing crucial issues such as fusion defects, cracking, and slow processing speeds, they aim to make these processes commercially viable across a wider range of applications.

Conclusion:

The quest to overcome challenges in 3D printing with refractory metals is an ongoing endeavor. Researchers, scientists, and engineers are continuously pushing the boundaries to develop innovative solutions. Through these efforts, the commercial viability of printed refractory metal parts can be significantly improved, opening up new possibilities in industries that rely on these exceptional materials. As progress continues, the manufacturing landscape is poised for further transformative advancements in additive manufacturing technology.

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