Metal Powder - the Raw Material of Future Production BY GÜNTER BUSCH* SYNOPSIS Alongside Mobile Internet, Cloud Computing, Robotics, Energy Storage and Autonomous Vehicles, Additive Manufacturing is one of the driving technologies in the near future. The potential economic impact in 2025 will be at the level of 400 billion USD. Additive manufacturing allows new designs and significantly reduces manufacturing costs. With the advantage of fast and economical prototyping the main applications are seen in the aircraft and automotive industries, and for production of medical implants, jewellery and special tools. The raw material used for these applications is metallic powder. Spherical powder shapes are beneficial for additive manufacturing processes, such as Metal Injection Moulding (MIM) or 3D-Printing. Vacuum Induction Melting in a crucible combined with atomization via an Argon high pressure nozzle allows consistent production of spherical powder particles having a narrow scatter band in terms of diameter, which isin the level of 15 60 µm. Nickelbase and Super-Alloy powders can be produced using this technology without any risk of oxidation and/or pick up of impurities. Reactive Titanium and Zirconium alloyed powders are produced using non-contact melting to avoid any kind of contamination. This paper considers the manufacturing of high quality metal powders, even from very reactive materials, by using vacuum technologies. Keywords: 3D Printing, Atomization, Metal Powder, Vacuum Metallurgy, Nickel Base Alloy, Titanium Alloy *Head of South East Asian Operation, ALD Vacuum Technologies GmbH, Hanau, Germany.
Metal Powder - the Raw Material of Future Production Introduction Alongside mobile internet, cloud computing, Robotics, Energy Storage and Autonomous driving, Additive Manufacturing is one of the driving technologies in the near future. The term Additive manufacturing stands for making parts based on a 3D model with layer-by-layer manufacturing technologies. The potential economic impact is huge and is forecast to be at a level of 20 billion USD by 2020 [1]. Press & Sinter Special Metal 60% 94% 6% 50% 40% 30% 20% 10% 0% Extrusion HIP MIM 3D Print Fig.: 1 Global Market for Powder Metallurgy in 2016. [2] At the present time additive manufacturing technologies, mainly by using 3D Printing techniques, make up 2-3% of the entire powder market, but there is a huge demand for more components to be produced via additive manufacturing. Applications for Powder Metallurgy Beside Additive Manufacturing, the Micro Metal Injection Moulding Process also requires supply of high quality metal powders. Typical application for these technologies are found in Automotive Industry (valves, transmission, impeller for turbo charger) Medical Application (medical implants) Aircraft Industry (structural parts, engine parts, turbine plates) Chemical Industry (valves, catalysts) Jewellery The high flexibility of powder metallurgical processes allows parts starting from less than one gram up to nearly 30 tons.
A B C D Fig.: 2 Some examples of powder metallurgical applications A: artificial hip / B: impeller / C: turbo charger / D: turbine plate The key advantages of Additive Manufacturing are Absolute freedom of design This walls & shapes, which are impossible to produce by casting Very complex shapes, inner cavities Customized designs and flexibility in design changes Multiple pieces built as one No tooling is needed No machine set-up Short production time Another advantage is the impact of the reduction in energy to produce a part. Figure 3 shows the relative power consumption for the production of a part with different production methods. Powder metallurgy requires only 40% of the energy that is used for machining. Fig.: 3 Relative energy consumption to produce a part [3] As shown in Fig. 4 the raw material utilisation is with ~75%, which is the highest compared to other parts production methods like casting, extrusion, forging or machining
100% 80% 60% 40% 20% 0% Powder Metallurgy Casting Forging Machining Fig.: 4 Raw Material Utilisation at various production methods [4] Material for Powder Metallurgical Parts In principle, all alloys could be used in powder metallurgy but the typical alloys used are Stainless Steel such as S316, 17-4PH High Speed Steel, Hot and Cold Working Steel Nickel Base Alloy such as Inconel s (625, 718) and CoCr, F75, etc. Titanium Alloys: Ti6Al4, CPT Aluminium Alloys: AlSi10Mg, etc. Precious Metals like Gold, Silver, Platinum For any given alloy, the morphology of the powder determines final product quality. Therefore metal powder shall have Spherical shape to ensure a good flow ability Homogenous particle sizes distribution Controlled chemical composition Low concentration of un-desired elements like Nitrogen, Oxygen and Hydrogen Methods for Powder Production There are several technologies and processes to produce metallic powder. These are Mechanical milling, grinding the metal or melting and atomization Physical like evaporation and condensation, or atomisation in an electric arc Chemical like reduction, or the carbonyl process Electrolytic bath by collecting the powder at the cathode The most efficient methods of production are the mechanical techniques, which are milling & grinding, or melting and atomization. The technology of milling and grinding will cause irregular shapes as shown in Fig.: 5, Fig.: 6 shows the spherical form of metal powder after melting and atomisation
Fig.: 5. Typically unregularly powder particle shape after grinding Fig.: 6 typical spherical powder particle shape after atomization The spherical form of the powder particles is clearly beneficial for process of Micro Metal Injection Moulding and Additive Manufacturing. To produce spherical powders the alloy must be melted and atomized. The atomisation could take place in water or in an inert gas stream. Water has the advantage of rapid heat transfer, but there is a big risk of Hydrogen and Oxygen pick up during the atomization. For higher alloyed materials, precious metals and alloys of reactive materials, such as Titanium or Zirconium, vacuum melting and gas atomization by using Argon gas are the most preferable techniques for the production of powder. System to Produce Metallic Powder Device for charging under vacuum Vacuum Melting Chamber with pre heated Tundish Atomization Nozzle Powder Tower for droplet solidification Cyclone to separate inert Gas and Powder Powder Collection Box Fig.: 7 Typical Systems for the Production of Metal Powder
The operating stages of a system to convert steel and other alloys to spherical powder shown in Fig. 7 could be described is as follows: Melting the steel or alloy under vacuum or controlled inert gas atmosphere in a graphite or ceramic crucible. Final alloying and/or refining are possible. Pouring the liquid metal into a pre-heated tundish The metal flows through orifice into the atomisation nozzle system and the liquid metal is dispersed by the kinetic energy of the high pressure inert gas jet during the atomisation step. Atomized metal droplets solidify while falling through the vertical powder tower, located directly underneath the atomisation nozzle The powder/gas mixture is transported via a conveying tube into a cyclone, where the powder is separated from the atomisation gas. The powder is collected in the powder collection box. For alloys of reactive metals such as Titanium of Zirconium contactless melting is recommended to avoid any contamination from the crucible material. Chemical Composition Basically, a system for production of powder uses a normal vacuum induction melting furnace. It is possible to melt down, alloy and refine the material just before the atomisation. Various stirring methods allow a homogenous alloy distribution. Provision of a late charging device allows the alloying of sensitive elements, just before pouring. The melting is done under vacuum or low partial pressure of an inert gas. To avoid any cross contamination the selection of the crucible material is significant in ensuring purity. Typically ceramic or graphite crucibles are used for these applications. For the atomisation of Titanium alloys and other reactive alloys a contactless melting method by using any air coil is applicable. The process of vacuum induction melting and the right selection of the crucible material avoid trace elements and other impurities in the powder. The powder of an alloy should have the same chemical composition as the origin alloy. Due to the high surface area powder tends to pick up undesirable gases like Hydrogen and Oxygen. Melting under vacuum and inert gas atmosphere like Argon allows reduces such pickup to an absolute minimum. Using the technology of vacuum melting and inert gas atomization the Oxygen contents significantly below 100 ppm are possible.
Particle Sizes The particle size distribution is a very important issue in powder metallurgy. Depending on the additive manufacturing technology and equipment to be used two main types of particles sizes are requested: Particle sizes between 10µm and 50µm for powder bed system, such as 3D printer Particle sizes between 50 and 150 µm for Electron Beam or selective Laser system A close-coupled nozzle system enables production of particle size distributions typically in the range of 30 µm < d50 < 90 µm for a variety of stainless steel, high-alloyed steels, and nickel base or cobalt-chromium alloys. In general, the resulting particle size distributions are strongly material dependant and processing of other materials with lower melting points can result in particle size distributions with d50 values less than 20 µm. Particle size distribution always follows the Gaussian distribution with a width / standard deviation of d84 / d50 = d50 / d16 in the range of 1.5 to 2.5. Fig.: 8 Particle size distribution and scanning electron microscope (SEM of) a typical stainless steel powder. The particle size is influenced by various parameters. The most important are Diameter of the metal flow through the atomisation nozzle (3 8 mm) Flow rate of the metal (typical flow rate between 0,7 and 2,5 l/min) Pressure of the atomisation gas in the nozzle (up to 40 bar) Temperature of the atomization gas Economical Production Modern systems for vacuum melting and gas atomization allow economical production of premium quality powder. These systems have a closing valve between the powder tower and the vacuum melt chamber. The advantages are shorter pump down time of
the vacuum melting chamber and a very clean atmosphere in the powder tower, as there is no contamination through fresh air meeting powder remaining in the tower Re-charging under vacuum also allow the tap to tap time to be significantly reduced and increases the productivity of the entire system. The design of the atomization nozzle is beneficial for Argon consumption and an Argon recycling system may be required for larger production facilities. The typical Argon consumption is between 1,7 and 2,3 m³/kg powder. The high efficiency of the system allows cost beneficial production of high quality powder. Conclusion Additive manufacturing and 3-D printing applications have greatly increased the demand for high-quality metal powders produced by inert gas atomization to be used to produce parts for a variety of demanding applications, such as in the aircraft and automotive industries as well as for medical applications such as implants. Modern systems combining material melting in vacuum or inert gas atmosphere with inert gas atomization are most suitable to meet the market requirements for highvolume and cost-efficient production of many different metal powders with defined and reproducible particle size, spherical morphology, excellent flow characteristics and minimal oxygen and nitrogen concentrations. Current trends for atomization systems are the development of modifications such as gas recirculation and hot gas atomization systems to further improve powder quality and lower production cost. References and Sources: [1] Mc Kinsey Global Institute [2] SMR Premium GmbH [3] European powder metallurgy association (EPMA) Additive Manufacturing Technology [4] EPMA Vision 2025, Future Development for the European PM Industry [5] State of the Art Equipment for Production of High Quality Spherical Metal Powder using inert Gas Atomization. Christian Lehnert, Bernd Sitzmann, Franz Pfahls, Henrik Fran and Michael Hohmann, ALD Vacuum Technologies Germany [6] M. Hohmann, G. Brooks, C. Spiegelhauer, Stahl und Eisen, 125 (2005), 4, 35-41.