- Email: [email protected]
Raw materials for metal additive manufacturing
CHAPTER 4
Raw materials for metal additive manufacturing Abbreviations AM ASTM AWS DED EBAM MER PBF P/M PREP RPD SMD UAM WAAM
Additive manufacturing American Society for Testing Materials American Welding Society Direct energy deposition Electron beam additive manufacturing Materials and Electrochemical Research (Corporation) Powder bed fusion Powder metallurgy Plasma rotating electrode process Rapid plasma deposition Shaped metal deposition Ultrasonic additive manufacturing Wire arc additive manufacturing
4.1 Introduction While most additive manufacturing technologies use powder as the raw material, there are a few exceptions., Electron beam additive manufacturing (EBAM), rapid plasma deposition (RPD), shaped metal deposition (SMD), and wire arc additive manufacturing (WAAM) use metal wire, while ultrasonic additive manufacturing (UAM) use sheet metal and/or thin foils as feedstock. Since the majority of AM technologies, including all PBF and most DED technologies, use metal powder, more emphasis will be given to powder production and its requirements for AM usage. Most metal powders that are produced today are sold in the traditional press and sinter powder metallurgy market, metal injection molding (MIM) market, or coatings market. Based on data from 2013, the powder market share for AM was $32.6 million; only a small fraction (0.0047%) of the overall metal powder market [1]. However, given the exponential growth of AM technologies, it is expected that the AM market share will grow substantially in coming years.
Science, Technology and Applications of Metals in Additive Manufacturing DOI: https://doi.org/10.1016/B978-0-12-816634-5.00004-2
© 2019 Elsevier Inc. All rights reserved.
77
78
Science, Technology and Applications of Metals in Additive Manufacturing
4.2 Powder preparation techniques There are various techniques for producing metal powders that include traditional crushing and grinding or many other modern techniques such as plasma spheroidization [1,2]. Generally, free-flowing spherical powders are preferred for AM, however angular powders have also been successfully processed [3 5].
4.2.1 Regular spherical powder Spherical powders are the most preferred powder shape and form used in metal AM processes today. The most popular techniques to produce spherical shaped powders are listed below. Gas atomization (GA) is the most popular technique for producing spherical powders. Typically, it involves melting elemental feedstock in a closed furnace under an air or inert gas blanket, or under vacuum (Fig. 4.1). The molten metal is then fed through a delivery nozzle. In some atomization processes, high gas pressure is used to force the molten metal through the delivery nozzle. On exiting the nozzle, the liquid metal
Vacuum Induction Furnace
Tundish
Atomization Die Cyclone Separator
Cooling Tower
Powder Collection Canisters
Figure 4.1 Schematic of the gas atomization process for powder production. Courtesy: ATI Specialty Materials, Andrzej Wojcieszynski.
Raw materials for metal additive manufacturing
79
is subjected to a high velocity air, N2, He or Ar gas that impinges onto the flowing melt and breaks it up in to small droplets. Many atomizers today, however, rely on gravity instead of forced gas feed. Gas-atomized powders are mostly spherical, with some asymmetric particles and satellites present. A satellite is created when a smaller particle sticks to a larger one during solidification. Heat lot size range from 5 to 5000 kg and the powder size ranges from 0 to 500 µm. Yield within the 20 150 µm range varies from 10% to 50% of the total. Gas atomization is used for most metals, including steels, stainless steels, Ni alloys, Co alloys, Al alloys, Cu alloys, and Ti alloys. Plasma atomization involves feeding wire feedstock into a plasma torch that, with the aid of gases, atomizes the powder. The powder size ranges from 0 to 200 µm. Plasma atomization produces high-quality spherical metal powders, however, this is limited to alloys that can be formed into a wire feedstock, such as Ti alloys. The plasma rotating electrode process (PREP) involves a rotating feedstock in the form of a bar that is melted when it comes into contact with a plasma (Fig. 4.2). PREP powders are extremely spherical but yields are limited below 100 µm, so the price can be very high. They can be used for production of all metal alloys available in bar form, however, this is most economic for metals such as titanium, Inconel, etc. Plasma spheroidization uses high-energy plasma to transform agglomerated or irregular-shaped powders into spherical powders. Powder is gravity fed from the top and is sprayed through the plasma using various nozzle types depending on the specific powder characteristics. Individual powder particles melt completely while traveling and resolidify into spherical shapes. Plasma-treated powder is fully dense, spherical, and very clean. Surface contamination is significantly reduced through the vaporization of impurities. Plasma spheroidization can be used to spheroidize irregularshaped powders produced through crushing, grinding, water atomizing or sintering techniques. High melting temperature refractory metals such as Ta, W, Nb, and Mo are also good candidate materials for cost-effective processing. It should be noted that powder size distributions remain essentially unchanged during spheroidization.
4.2.2 Irregular-shaped powder The high cost of spherical metal powders is often a major obstacle to the commercialization of AM technologies as part of mainstream
80
Science, Technology and Applications of Metals in Additive Manufacturing
Vacuum inert gas
Drive belt
Long electrode Spindle Infeed Brush assembly Transferred arc plasma torch
Powder collection port
Figure 4.2 Schematic of the PREP process for spherical powder production. Courtesy: TIMET, Craig Anderson.
manufacturing. For some materials, such as titanium, niobium, and tantalum these cost differentials can be extremely high and create a very large barrier for successful commercialization of metal AM. On the other hand, consistent powder flow, shape, size distribution, and packing density are essential to produce quality parts through metal AM. Such stringent quality requirements lead to only few amenable powder manufacturing processes. Therefore, there is significant interest in the production of low-cost, generally angular-shaped powder that can be used in AM either as-is or after conversion to a spherical morphology. Some of the most popular production processes are sponge powder production, water atomization, and crushing and grinding. Sponge powder production is used to produce various powders, including iron and titanium. It involves reduction of compounds of the respective metal, separating the by-product and crushing and grinding of the metal cake. For example, sponge iron is produced by reduction of iron oxide,
Raw materials for metal additive manufacturing
81
and sponge titanium is produced by removing chloride ions from titanium tetrachloride. Sponge powders are irregular in shape, have a non-uniform outer surface, and are very low cost. The low flowability of these powders, however, restricts their use in AM. Water atomization (WA) involves impinging a falling stream of molten metal with jets of water which immediately solidify the metal into granules ( . 1 mm) or powder (,1 mm). High-pressure water atomization has proven to be a viable, low-cost process to achieve fine particle size distributions for iron, stainless steel, and low-alloy metal powders. The ability to use prealloyed materials allows for the production of numerous alloy powders. Compared to less modern techniques, such as crushing and grinding, water atomization presents a cost-effective and efficient approach to producing metallic powders. However, the irregular shape of wateratomized powders restrict their flowability and limits their use in AM. Crushing and grinding is one of the oldest techniques for producing metal powders. Traditional crushing and grinding processes followed by screening and sieving are used to produce many metal and alloy powders. Brittle materials are more suitable for crushing and grinding, which is a low-cost process for powder production. However, size and shape control are difficult, resulting in low powder flowability and limited use in AM. Table 4.1 gives a summary of common metal powder production technologies.
4.2.3 Powder quality and requirements As noted earlier, the majority of AM technologies use metal powders as feedstock. Various powder manufacturing processes produce powders of different characteristics and some are more suitable for certain applications than others. While metal powder production has been well standardized over the years, its successful use in AM requires close control of the powder production process and proper handling of the powder. Details of the characterization techniques used to evaluate powders for additive manufacturing technologies can be found in Table 4.2 [1,7 9]. Details of powder quality and qualification criteria is addressed in Chapter 8. The key factors related to powder feedstock that influence the quality of AM parts include the following. 1. Powder morphology: Depending on the powder production process, powders can have various morphologies, including acicular, flake, granular, irregular, needle, nodular, platelet, plates, and spherical. The
Table 4.1 Various processes for metal powder production [1,6]. Process
Type of material
Characteristics
Suppliers
Gas atomization (GA)
Steels (300 Maraging steel, 4140, 4340, H13, P20, P21, A11), stainless steels (316L, 304L, 309L, 17-4PH, 15-5PH), Ni alloys (In625, In718, Hastelloy X, Hastelloy 276, Wasp alloy), Co alloys (Co-Cr-Mo, Stellite 6, Stellite 21), Ti alloy (CP Ti, Ti-6Al-4V), Al alloy (A356, Al10SiMg, 6061Al, Scalmalloy), Cu alloy (bronze, Cu-Ni, GRcop84, C18150) Ti alloys (CP Ti, Ti-6Al-4V), Ni alloys (In625, In718), stainless steels (17-4PH, 15-5PH), Cu alloy (Cu-Ni, GRcop84, C18150)
Mostly spherical shape, but, usually contains some long-elongated particles, often with satellite particles, some materials are prone to produce porosity, relatively low pricing, most popular for AM
Carpenter Technology, Praxair Surface Technology, ATI Specialty Materials, Sandvik, Oerlikon Metco, Kennametal Stellite, Hoganas, GKN Hoeganaes, Toyal, Valimet, Reading Alloys (Ametek), H.C. Starck
Nearly 100% spherical shape, usually does not contain any satellite particles, and no porosity, very high price, suitable for use in AM, but restricted use due to high pricing Spherical shape, less satellite particles, price higher than GA but lower than PREP, suitable for AM
Timet Corp, Phelly Material
Plasma rotary electrode processing (PREP)
Rotary atomization (RA)
Ni alloys (In718, In625), stainless steels (JBK75)
HMI Corp
Plasma atomization
Ti alloys
Water atomization (WA)
All metals, such as steels, stainless steels, Ni alloys, Cu alloys, etc.
Crushing and grinding
Various kinds of materials are possible, but more suitable for brittle materials, refractory metals such as W, Ta, Nb, Mo alloys and, carbides, and oxides Mostly used for Fe and Ti
Sponge powder production Plasma spheroidization
Popular for refractory alloys, Ta, Nb, Mo, W, also used for Ti, Al, and Ni alloys
Spherical shape, high quality, suitable for AM Irregular shape, lower purity, consistent powder flow is a challenge, not suitable for AM Irregular shape, angular and faceted, lowest price, challenge to consistent powder flow rate and consistent packing density, limited use in AM Irregular-shaped particles, poor flowability, not suitable for AM This technique converts irregular powders to spherical shape, suitable for AM, low throughput and expensive
Advanced Powders & Coatings (GE Additive), PyroGenesis, Tekna Hoganas, H.C. Starck
Oerlikon Metco, HC Starck, Praxair
Micron Metals, CNPC Powders, Phelly Material, Reading Alloys (Ametek) Tekna, H.C. Starck
84
Science, Technology and Applications of Metals in Additive Manufacturing
Table 4.2 Critical powder properties and respective techniques for characterization. Powder property
Characterization technique
Particle shape (morphology) Particle size and size distribution Particle porosity
Scanning electron microscopy (SEM), optical microscopy Sieve analysis, laser diffraction, optical microscopy
Powder flowability Powder density
Surface area Chemical composition
Optical microscopy of particle cross-section, computed tomography (CT) Hall flow, dynamic flow testing, angle of repose Apparent density—Hall flow, Freeman FT4, Carney flowmeter, Scott volumeter Tap density—tapped density tester BET surface area analysis Inductively coupled plasma—optical emission spectroscopy/mass spectroscopy (ICP-OES/MS), flame emission spectroscopy (FES), atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), X-ray powder diffraction (XRD), inert gas fusion
preferred powder morphology for AM is a spherical shape without satellites. Since the morphology of powder particles determines the packing density of the powder it plays a major role in powder bed fusion technologies. The packing density will eventually determine the layer thickness and shrinkage in PBF processes. Powder morphology also plays an important role in powder spreading during PBF processing. Powder jamming and inconsistent spreading can result from high friction between powder particles [10]. While powder morphology is less critical in directed energy deposition (DED), it is nevertheless important to maintain a consistent powder flow rate and therefore a spherical or near-spherical geometry is highly recommended. PREPpowders produce some of the most spherical shapes with extremely smooth surfaces and without satellites (Fig. 4.3). Gas atomization produces a majority of particles with a spherical shape (Fig. 4.4), but contains some elongated particles. These powders also typically have satellite particles attached to the larger particles. While a small amount of satellite particles do not have a significant effect on the build quality, excessive satellites can cause non-uniform packing in PBF systems or inconsistent powder flow in DED systems and result in undesirable product quality. However, their relatively lower cost compared to
Raw materials for metal additive manufacturing
85
Figure 4.3 SEM micrograph of Ti-6Al-2Sn-4Zr-2Mo alloy powder manufactured by PREP. Courtesy: TIMET, Craig Anderson.
Figure 4.4 SEM micrograph of 15-5 stainless steel alloy powder manufactured using gas atomization. Left: As atomized powder. Right: Powder after screening to a desired mesh size. Courtesy: ATI Specialty Materials, Andrzej Wojcieszynski.
PREP powders and the ease of manufacturing from elemental blends make gas-atomized powders the most popular choice for metal AM processes. 2. Size and size distribution: In additive manufacturing, powder particle size determines the minimum part layer thickness, as well as the minimum buildable feature sizes and surface finish on a part in the as-built condition. In addition, powder size distribution also plays a role in packing density for the powder bed fusion technologies. While laserbased powder bed fusion (PBF) technologies use 15 45 µm powders, electron beam-based (EBM) PBF uses 45 106 µm sized powders.
86
Science, Technology and Applications of Metals in Additive Manufacturing
In comparison, DED technologies use a 45 150 µm size range for powders. Powder size and consistent size distribution play a major role in determining powder packing density which is critical for PBF technologies. Powder size and size range are also critical for consistent powder spreading during PBF processes [10]. 3. Chemical composition: Chemical composition of the powder plays a major role as raw material chemistry determines the final part chemistry and properties. The ASTM F42 committee, responsible for developing AM standards, has published chemistry specifications for the two most commonly used Ti alloys, namely, Ti-6Al-4V and Ti-6Al-4VELI (extra low interstitial) alloys. It has also produced standards for powders for powder bed fusion [11,12], 316L stainless steel [13] and Inconels (In625 and In718) [14,15]. It is to be noted that these standards only require chemistry compliance for the end part and that they allow for chemistry variation in the powder form to adjust to the differences of various processes. It is worth noting that AM techniques involving electron beam melting operate in a vacuum environment and therefore lead to loss of elements, such as Al. Therefore, the raw feedstock, such as Ti-6Al-4V powder for electron beam-based processes often uses an additional amount of Al to compensate for the losses during processing. The raw material chemistry for Ti6Al-4V alloy for DED processes can be found in the AMS 4999A specification [16]. 4. Flow: Powder flow is a critical parameter for successful AM operations. Whether it is the recoater in PBF processes or the powder delivery system in DED processes, a consistent layer thickness depends on good and consistent powder flow. Powder morphology, size, size distribution, and surface characteristics, including surface contamination, can affect powder flow. For the purpose of obtaining consistent and repeatable powder flow, spherical powder shape without any satellites and surface contaminations is preferred for powders in AM. The presence of a large fraction of small particles, i.e., fines, is also known to inhibit consistent powder flow. 5. Powder density: Depending on powder chemistry, some gas-atomized powders may entrap atomizing gas and thus yield particles with entrapped porosity. Such porosities are usually absent in PREP manufactured powders (Fig. 4.5). Such porosities in the powder particles can result in porosities in the 3D-printed metal products.
Raw materials for metal additive manufacturing
87
Figure 4.5 (Left) PREP-manufactured titanium powder, (right) gas-atomized copper powder. Courtesy: DM3D Technology.
6. Powder handling and contamination: As discussed earlier, powder contamination can cause impurities in the final part and affect its properties. It is essential to exercise care while handling powder during transfer to powder hoppers or the build chamber in machines, during sieving, etc. 7. Environmental conditions: Humidity plays a major role in AM powders. Moisture on the surface of the powder particles can cause porosity formation during printing. The surface moisture also reduces the powder flowability due to capillary action. Therefore, it is essential to store the powder in dry places. If possible, powder drying prior to use is strongly recommended. Reactive powders, ex. titanium and aluminum alloys, should also be stored in an inert gas environment, like argon, to reduce surface oxide formation and to preserve powder quality. Such storage can also be motivated by safety concerns to prevent powder reactions and explosions.
4.3 Powder recycling and reuse Powder recycling is a very important factor in AM processes. All PBF processes use only a fraction of the powder that is placed on the build plate and therefore powder reuse is mandatory for economic operation of these processes. Excess powder from the chamber is reused after a resizing (resieving) process. In DED techniques, depending on the beam diameters, powder capture efficiency can vary from 50% to 90%. Excess powder can be collected from the machine table and reused after resieving. However, powder reuse has to be exercised with caution.
88
Science, Technology and Applications of Metals in Additive Manufacturing
Even though all titanium processing is carried out under an inert environment or vacuum atmosphere, the powders are exposed to thermal effects from multiple layer builds and can loses alloy elements. ASTM F2924-14 and 3001-14 do not limit powder usage to any specific number of builds, and it is recommended to follow specific equipment guidelines for this purpose. Tang et al. [17] carried out an extensive study to evaluate the effect of powder reuse during electron beam melting (EBM) of Ti6Al-4V alloys. Their results indicated that the powder morphology changes from smooth, spherical to rough, irregular after recycling about 21 times. This change in powder morphology causes appreciable change in powder tap density (from 2.96 to 2.88 g/mL), however the flowability improves with reuse (from 32.47 to 28.34 s/50 g). Enhanced powder flow after recycling is attributed to the absence of satellites and removal of moisture during repeated exposure and recycling. During this recycling process, powder chemistry was also affected, while the V content changed marginally, Al content in the powder changed from 6.47% in virgin powder to 6.35% after 21 recycles and, correspondingly, in the test samples from 6.14% to 5.93%. Loss of Al content is due to vaporization of Al in low partial pressure (2 3 1022 Pa of He). Powder recycling is also associated with an increase in oxygen content from 0.08% in virgin powder (0.07% in the test sample) to 0.19% in powder (0.18% in the test sample) after 21 recycles. Repeated powder handling and exposure to air during recycling cause additional oxygen pick up in the powder. As a result, the UTS increased from 920 to 1039 MPa, yield strength increased from 834 to 960 MPa and tensile elongation dropped from 16% to 15.5%. In other work using Realizer powder bed fusion equipment over 10 repeated runs the oxygen content of Ti-6Al-4V was found to increase from 0.16 to 0.18 wt.%. Jacob et al. [18] performed extensive work on recycling of nitrogenatomized 17-4 PH stainless steel powder during laser-based PBF processing. Figs. 4.6 and 4.7 show the morphology of virgin powder and after recycling 11 times. Fig. 4.8 shows particle sizes for D10, D50, and D90. The results indicate no appreciable change in the powder size and distribution. Hall flow tests revealed reduction in flowability after the first and second recycling, but flowability gradually improves during successive recycling rounds. The apparent density of powder increases and then levels off with multiple recycling rounds. The chemical composition of the powder did not change significantly and chemistry after the 11th build
Raw materials for metal additive manufacturing
89
Figure 4.6 SEM image of nitrogen-atomized virgin 17-4 PH stainless steel powder: (A) low and (B) high magnification [18].
Figure 4.7 SEM image of nitrogen-atomized 17-4 PH stainless steel powder after 11 build cycles in a laser-based PBF machine: (A) low and (B) high magnification [18].
was within ASTM-specified limits. The most importantly properties of printed materials using recycled powder were found to have variations in a single build, Fig. 4.8. Horizontal and vertical surface roughness, bulk density, UTS, YS, and elongation of the samples were not significantly affected by recycling of powders. These results indicate that recycled powders can produce parts with properties comparable to those made from virgin powders.
90
Science, Technology and Applications of Metals in Additive Manufacturing
55
Particle size, x cmin (µm)
50 45 40 35 30 25 20 P40, Build #1
P46, Build #2
P61, Build #5
P67, Build #6
P92, Build #11
P98, after Build #11
Powder sample # D10
D50
D90
Figure 4.8 Effect of repeated runs through a laser-based PBF machine on the powder sizes [18].
4.4 Wire precursors Two directed energy deposition processes that use metal wire as feedstock are wire arc additive manufacturing (WAAM) (also called shaped metal deposition) patented by Rolls Royce Corp. and jointly developed with Cranfield University and electron beam additive manufacturing (EBAM) commercialized by Sciaky Inc. These processes both use standard welding wires specified by AWS specifications and typical gauge sizes of about 1.2 mm (0.05v) for SMD/WAAM [19] and 0.9 4.0 mm for EBAM [20]. According to AMS 4999A [16], the wire feedstock for the titanium deposition process should be either wire conforming to AWS A5.16 ERTi-5 (Al content up to 7.5% is allowed, and a minimum of 1400 ppm oxygen).
References [1] J. Dawes, R. Bowerman, R. Trepleton, Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain, Johnson Matthey Technol. Rev. 59 (3) (2015) 243 256. Available from: https://doi.org/10.1595/205651315X688686. [2] F.H. (Sam) Froes, Titanium powder metallurgy: developments and opportunities in a sector poised for growth, Powder Metall. Rev. 2 (4) (2013) 29 43. Winter.
Raw materials for metal additive manufacturing
91
[3] B. Dutta, F.H. (Sam) Froes, Additive Manufacturing of Titanium Alloys; State of the Art, Challenges and Opportunities, Elsevier, 2016. [4] ,https://www.metalysis.com/#process. (accessed 13.02.19). [5] ,https://www.pm-review.com/metalysis-titanium-powder-used-to-3d-print-automotiveparts/. (accessed 13.02.19). [6] A. Popovich, V. Sufiiarov, Metal Powder Additive Manufacturing, Chapter 10, Intech Open Access Chapter 10, ,https://doi.org/10.5772/63337.. [7] A. Cooke, J. Slotwinski, NIST internal report on Properties of Metal Powders for Additive Manufacturing: A Review of the State of the Art of Metal Powder Property Testing, NISTIR 7873, July 2012, ,https://doi.org/10.6028/NIST. IR.7873.. [8] J.A. Slotwinski, E.J. Garboczi, P.E. Stutzman, C.F. Ferraris, S.S. Watson, M.A. Peltz, Characterization of Metal Powders Used for Additive Manufacturing, J. Res. Natl. Inst. Standards Technol. 119 (2014) 460 493. Available from: https://doi.org/ 10.6028/jres.119.018. [9] ASTM F3049-14, Standard Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes, ASTM International, West Conshohocken, PA, 2014. [10] W. Nan, M. Pasha, T. Bonakdar, A. Lopez, U. Zafar, S. Nadimi, M. Ghadiri, et al., Jamming during particle spreading in additive manufacturing, Powder Technol. 338 (2018) 253 262. [11] ASTM F2924-14, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [12] ASTM F3001-14, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium ELI (Extra Low Interstitial) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [13] ASTM F3184-16, Standard Specification for Additive Manufacturing Stainless Steel Alloy (UNS S31603) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2016. [14] ASTM F3056-14e1, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N06625) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [15] ASTM F3055-14a, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N07718) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [16] AMS 4999A, Titanium Alloy Direct Deposited Products 6Al - 4V Annealed, SAE Int., Sep 2011. [17] H.P. Tang, M. Qian, N. Liu, X.Z. Zhang, G.Y. Yang, J. Wang, Effect of Powder Reuse Times on Additive Manufacturing of Ti-6Al-4V by Selective Electron Beam Melting, JOM 67 (3) (2015) 555 563. Available from: https://doi.org/10.1007/ s11837-015-1300-4. [18] G. Jacob, C. Brown, A. Donmez, S. Watson, J. Slotwinski, Effects of powder recycling on stainless steel powder and built material properties in metal powder bed fusion processes, NIST Advanced Manufacturing Series 100-6, ,https://doi.org/ 10.6028/NIST.AMS.100-6.. [19] E. Brandl, B. Baufeld, C. Leyens, R. Gault, Additive manufactured Ti-6Al-4V using welding wire: comparison of laser and arc beam deposition and evaluation with respect to aerospace material specifications, Phys. Proc. 5 (2010) 595 606. [20] , http://www.sciaky.com/additive-manufacturing/wire-am-vs-powder-am . (accessed 28.07.15).
Raw materials for metal additive manufacturing Abbreviations AM ASTM AWS DED EBAM MER PBF P/M PREP RPD SMD UAM WAAM
Additive manufacturing American Society for Testing Materials American Welding Society Direct energy deposition Electron beam additive manufacturing Materials and Electrochemical Research (Corporation) Powder bed fusion Powder metallurgy Plasma rotating electrode process Rapid plasma deposition Shaped metal deposition Ultrasonic additive manufacturing Wire arc additive manufacturing
4.1 Introduction While most additive manufacturing technologies use powder as the raw material, there are a few exceptions., Electron beam additive manufacturing (EBAM), rapid plasma deposition (RPD), shaped metal deposition (SMD), and wire arc additive manufacturing (WAAM) use metal wire, while ultrasonic additive manufacturing (UAM) use sheet metal and/or thin foils as feedstock. Since the majority of AM technologies, including all PBF and most DED technologies, use metal powder, more emphasis will be given to powder production and its requirements for AM usage. Most metal powders that are produced today are sold in the traditional press and sinter powder metallurgy market, metal injection molding (MIM) market, or coatings market. Based on data from 2013, the powder market share for AM was $32.6 million; only a small fraction (0.0047%) of the overall metal powder market [1]. However, given the exponential growth of AM technologies, it is expected that the AM market share will grow substantially in coming years.
Science, Technology and Applications of Metals in Additive Manufacturing DOI: https://doi.org/10.1016/B978-0-12-816634-5.00004-2
© 2019 Elsevier Inc. All rights reserved.
77
78
Science, Technology and Applications of Metals in Additive Manufacturing
4.2 Powder preparation techniques There are various techniques for producing metal powders that include traditional crushing and grinding or many other modern techniques such as plasma spheroidization [1,2]. Generally, free-flowing spherical powders are preferred for AM, however angular powders have also been successfully processed [3 5].
4.2.1 Regular spherical powder Spherical powders are the most preferred powder shape and form used in metal AM processes today. The most popular techniques to produce spherical shaped powders are listed below. Gas atomization (GA) is the most popular technique for producing spherical powders. Typically, it involves melting elemental feedstock in a closed furnace under an air or inert gas blanket, or under vacuum (Fig. 4.1). The molten metal is then fed through a delivery nozzle. In some atomization processes, high gas pressure is used to force the molten metal through the delivery nozzle. On exiting the nozzle, the liquid metal
Vacuum Induction Furnace
Tundish
Atomization Die Cyclone Separator
Cooling Tower
Powder Collection Canisters
Figure 4.1 Schematic of the gas atomization process for powder production. Courtesy: ATI Specialty Materials, Andrzej Wojcieszynski.
Raw materials for metal additive manufacturing
79
is subjected to a high velocity air, N2, He or Ar gas that impinges onto the flowing melt and breaks it up in to small droplets. Many atomizers today, however, rely on gravity instead of forced gas feed. Gas-atomized powders are mostly spherical, with some asymmetric particles and satellites present. A satellite is created when a smaller particle sticks to a larger one during solidification. Heat lot size range from 5 to 5000 kg and the powder size ranges from 0 to 500 µm. Yield within the 20 150 µm range varies from 10% to 50% of the total. Gas atomization is used for most metals, including steels, stainless steels, Ni alloys, Co alloys, Al alloys, Cu alloys, and Ti alloys. Plasma atomization involves feeding wire feedstock into a plasma torch that, with the aid of gases, atomizes the powder. The powder size ranges from 0 to 200 µm. Plasma atomization produces high-quality spherical metal powders, however, this is limited to alloys that can be formed into a wire feedstock, such as Ti alloys. The plasma rotating electrode process (PREP) involves a rotating feedstock in the form of a bar that is melted when it comes into contact with a plasma (Fig. 4.2). PREP powders are extremely spherical but yields are limited below 100 µm, so the price can be very high. They can be used for production of all metal alloys available in bar form, however, this is most economic for metals such as titanium, Inconel, etc. Plasma spheroidization uses high-energy plasma to transform agglomerated or irregular-shaped powders into spherical powders. Powder is gravity fed from the top and is sprayed through the plasma using various nozzle types depending on the specific powder characteristics. Individual powder particles melt completely while traveling and resolidify into spherical shapes. Plasma-treated powder is fully dense, spherical, and very clean. Surface contamination is significantly reduced through the vaporization of impurities. Plasma spheroidization can be used to spheroidize irregularshaped powders produced through crushing, grinding, water atomizing or sintering techniques. High melting temperature refractory metals such as Ta, W, Nb, and Mo are also good candidate materials for cost-effective processing. It should be noted that powder size distributions remain essentially unchanged during spheroidization.
4.2.2 Irregular-shaped powder The high cost of spherical metal powders is often a major obstacle to the commercialization of AM technologies as part of mainstream
80
Science, Technology and Applications of Metals in Additive Manufacturing
Vacuum inert gas
Drive belt
Long electrode Spindle Infeed Brush assembly Transferred arc plasma torch
Powder collection port
Figure 4.2 Schematic of the PREP process for spherical powder production. Courtesy: TIMET, Craig Anderson.
manufacturing. For some materials, such as titanium, niobium, and tantalum these cost differentials can be extremely high and create a very large barrier for successful commercialization of metal AM. On the other hand, consistent powder flow, shape, size distribution, and packing density are essential to produce quality parts through metal AM. Such stringent quality requirements lead to only few amenable powder manufacturing processes. Therefore, there is significant interest in the production of low-cost, generally angular-shaped powder that can be used in AM either as-is or after conversion to a spherical morphology. Some of the most popular production processes are sponge powder production, water atomization, and crushing and grinding. Sponge powder production is used to produce various powders, including iron and titanium. It involves reduction of compounds of the respective metal, separating the by-product and crushing and grinding of the metal cake. For example, sponge iron is produced by reduction of iron oxide,
Raw materials for metal additive manufacturing
81
and sponge titanium is produced by removing chloride ions from titanium tetrachloride. Sponge powders are irregular in shape, have a non-uniform outer surface, and are very low cost. The low flowability of these powders, however, restricts their use in AM. Water atomization (WA) involves impinging a falling stream of molten metal with jets of water which immediately solidify the metal into granules ( . 1 mm) or powder (,1 mm). High-pressure water atomization has proven to be a viable, low-cost process to achieve fine particle size distributions for iron, stainless steel, and low-alloy metal powders. The ability to use prealloyed materials allows for the production of numerous alloy powders. Compared to less modern techniques, such as crushing and grinding, water atomization presents a cost-effective and efficient approach to producing metallic powders. However, the irregular shape of wateratomized powders restrict their flowability and limits their use in AM. Crushing and grinding is one of the oldest techniques for producing metal powders. Traditional crushing and grinding processes followed by screening and sieving are used to produce many metal and alloy powders. Brittle materials are more suitable for crushing and grinding, which is a low-cost process for powder production. However, size and shape control are difficult, resulting in low powder flowability and limited use in AM. Table 4.1 gives a summary of common metal powder production technologies.
4.2.3 Powder quality and requirements As noted earlier, the majority of AM technologies use metal powders as feedstock. Various powder manufacturing processes produce powders of different characteristics and some are more suitable for certain applications than others. While metal powder production has been well standardized over the years, its successful use in AM requires close control of the powder production process and proper handling of the powder. Details of the characterization techniques used to evaluate powders for additive manufacturing technologies can be found in Table 4.2 [1,7 9]. Details of powder quality and qualification criteria is addressed in Chapter 8. The key factors related to powder feedstock that influence the quality of AM parts include the following. 1. Powder morphology: Depending on the powder production process, powders can have various morphologies, including acicular, flake, granular, irregular, needle, nodular, platelet, plates, and spherical. The
Table 4.1 Various processes for metal powder production [1,6]. Process
Type of material
Characteristics
Suppliers
Gas atomization (GA)
Steels (300 Maraging steel, 4140, 4340, H13, P20, P21, A11), stainless steels (316L, 304L, 309L, 17-4PH, 15-5PH), Ni alloys (In625, In718, Hastelloy X, Hastelloy 276, Wasp alloy), Co alloys (Co-Cr-Mo, Stellite 6, Stellite 21), Ti alloy (CP Ti, Ti-6Al-4V), Al alloy (A356, Al10SiMg, 6061Al, Scalmalloy), Cu alloy (bronze, Cu-Ni, GRcop84, C18150) Ti alloys (CP Ti, Ti-6Al-4V), Ni alloys (In625, In718), stainless steels (17-4PH, 15-5PH), Cu alloy (Cu-Ni, GRcop84, C18150)
Mostly spherical shape, but, usually contains some long-elongated particles, often with satellite particles, some materials are prone to produce porosity, relatively low pricing, most popular for AM
Carpenter Technology, Praxair Surface Technology, ATI Specialty Materials, Sandvik, Oerlikon Metco, Kennametal Stellite, Hoganas, GKN Hoeganaes, Toyal, Valimet, Reading Alloys (Ametek), H.C. Starck
Nearly 100% spherical shape, usually does not contain any satellite particles, and no porosity, very high price, suitable for use in AM, but restricted use due to high pricing Spherical shape, less satellite particles, price higher than GA but lower than PREP, suitable for AM
Timet Corp, Phelly Material
Plasma rotary electrode processing (PREP)
Rotary atomization (RA)
Ni alloys (In718, In625), stainless steels (JBK75)
HMI Corp
Plasma atomization
Ti alloys
Water atomization (WA)
All metals, such as steels, stainless steels, Ni alloys, Cu alloys, etc.
Crushing and grinding
Various kinds of materials are possible, but more suitable for brittle materials, refractory metals such as W, Ta, Nb, Mo alloys and, carbides, and oxides Mostly used for Fe and Ti
Sponge powder production Plasma spheroidization
Popular for refractory alloys, Ta, Nb, Mo, W, also used for Ti, Al, and Ni alloys
Spherical shape, high quality, suitable for AM Irregular shape, lower purity, consistent powder flow is a challenge, not suitable for AM Irregular shape, angular and faceted, lowest price, challenge to consistent powder flow rate and consistent packing density, limited use in AM Irregular-shaped particles, poor flowability, not suitable for AM This technique converts irregular powders to spherical shape, suitable for AM, low throughput and expensive
Advanced Powders & Coatings (GE Additive), PyroGenesis, Tekna Hoganas, H.C. Starck
Oerlikon Metco, HC Starck, Praxair
Micron Metals, CNPC Powders, Phelly Material, Reading Alloys (Ametek) Tekna, H.C. Starck
84
Science, Technology and Applications of Metals in Additive Manufacturing
Table 4.2 Critical powder properties and respective techniques for characterization. Powder property
Characterization technique
Particle shape (morphology) Particle size and size distribution Particle porosity
Scanning electron microscopy (SEM), optical microscopy Sieve analysis, laser diffraction, optical microscopy
Powder flowability Powder density
Surface area Chemical composition
Optical microscopy of particle cross-section, computed tomography (CT) Hall flow, dynamic flow testing, angle of repose Apparent density—Hall flow, Freeman FT4, Carney flowmeter, Scott volumeter Tap density—tapped density tester BET surface area analysis Inductively coupled plasma—optical emission spectroscopy/mass spectroscopy (ICP-OES/MS), flame emission spectroscopy (FES), atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), X-ray powder diffraction (XRD), inert gas fusion
preferred powder morphology for AM is a spherical shape without satellites. Since the morphology of powder particles determines the packing density of the powder it plays a major role in powder bed fusion technologies. The packing density will eventually determine the layer thickness and shrinkage in PBF processes. Powder morphology also plays an important role in powder spreading during PBF processing. Powder jamming and inconsistent spreading can result from high friction between powder particles [10]. While powder morphology is less critical in directed energy deposition (DED), it is nevertheless important to maintain a consistent powder flow rate and therefore a spherical or near-spherical geometry is highly recommended. PREPpowders produce some of the most spherical shapes with extremely smooth surfaces and without satellites (Fig. 4.3). Gas atomization produces a majority of particles with a spherical shape (Fig. 4.4), but contains some elongated particles. These powders also typically have satellite particles attached to the larger particles. While a small amount of satellite particles do not have a significant effect on the build quality, excessive satellites can cause non-uniform packing in PBF systems or inconsistent powder flow in DED systems and result in undesirable product quality. However, their relatively lower cost compared to
Raw materials for metal additive manufacturing
85
Figure 4.3 SEM micrograph of Ti-6Al-2Sn-4Zr-2Mo alloy powder manufactured by PREP. Courtesy: TIMET, Craig Anderson.
Figure 4.4 SEM micrograph of 15-5 stainless steel alloy powder manufactured using gas atomization. Left: As atomized powder. Right: Powder after screening to a desired mesh size. Courtesy: ATI Specialty Materials, Andrzej Wojcieszynski.
PREP powders and the ease of manufacturing from elemental blends make gas-atomized powders the most popular choice for metal AM processes. 2. Size and size distribution: In additive manufacturing, powder particle size determines the minimum part layer thickness, as well as the minimum buildable feature sizes and surface finish on a part in the as-built condition. In addition, powder size distribution also plays a role in packing density for the powder bed fusion technologies. While laserbased powder bed fusion (PBF) technologies use 15 45 µm powders, electron beam-based (EBM) PBF uses 45 106 µm sized powders.
86
Science, Technology and Applications of Metals in Additive Manufacturing
In comparison, DED technologies use a 45 150 µm size range for powders. Powder size and consistent size distribution play a major role in determining powder packing density which is critical for PBF technologies. Powder size and size range are also critical for consistent powder spreading during PBF processes [10]. 3. Chemical composition: Chemical composition of the powder plays a major role as raw material chemistry determines the final part chemistry and properties. The ASTM F42 committee, responsible for developing AM standards, has published chemistry specifications for the two most commonly used Ti alloys, namely, Ti-6Al-4V and Ti-6Al-4VELI (extra low interstitial) alloys. It has also produced standards for powders for powder bed fusion [11,12], 316L stainless steel [13] and Inconels (In625 and In718) [14,15]. It is to be noted that these standards only require chemistry compliance for the end part and that they allow for chemistry variation in the powder form to adjust to the differences of various processes. It is worth noting that AM techniques involving electron beam melting operate in a vacuum environment and therefore lead to loss of elements, such as Al. Therefore, the raw feedstock, such as Ti-6Al-4V powder for electron beam-based processes often uses an additional amount of Al to compensate for the losses during processing. The raw material chemistry for Ti6Al-4V alloy for DED processes can be found in the AMS 4999A specification [16]. 4. Flow: Powder flow is a critical parameter for successful AM operations. Whether it is the recoater in PBF processes or the powder delivery system in DED processes, a consistent layer thickness depends on good and consistent powder flow. Powder morphology, size, size distribution, and surface characteristics, including surface contamination, can affect powder flow. For the purpose of obtaining consistent and repeatable powder flow, spherical powder shape without any satellites and surface contaminations is preferred for powders in AM. The presence of a large fraction of small particles, i.e., fines, is also known to inhibit consistent powder flow. 5. Powder density: Depending on powder chemistry, some gas-atomized powders may entrap atomizing gas and thus yield particles with entrapped porosity. Such porosities are usually absent in PREP manufactured powders (Fig. 4.5). Such porosities in the powder particles can result in porosities in the 3D-printed metal products.
Raw materials for metal additive manufacturing
87
Figure 4.5 (Left) PREP-manufactured titanium powder, (right) gas-atomized copper powder. Courtesy: DM3D Technology.
6. Powder handling and contamination: As discussed earlier, powder contamination can cause impurities in the final part and affect its properties. It is essential to exercise care while handling powder during transfer to powder hoppers or the build chamber in machines, during sieving, etc. 7. Environmental conditions: Humidity plays a major role in AM powders. Moisture on the surface of the powder particles can cause porosity formation during printing. The surface moisture also reduces the powder flowability due to capillary action. Therefore, it is essential to store the powder in dry places. If possible, powder drying prior to use is strongly recommended. Reactive powders, ex. titanium and aluminum alloys, should also be stored in an inert gas environment, like argon, to reduce surface oxide formation and to preserve powder quality. Such storage can also be motivated by safety concerns to prevent powder reactions and explosions.
4.3 Powder recycling and reuse Powder recycling is a very important factor in AM processes. All PBF processes use only a fraction of the powder that is placed on the build plate and therefore powder reuse is mandatory for economic operation of these processes. Excess powder from the chamber is reused after a resizing (resieving) process. In DED techniques, depending on the beam diameters, powder capture efficiency can vary from 50% to 90%. Excess powder can be collected from the machine table and reused after resieving. However, powder reuse has to be exercised with caution.
88
Science, Technology and Applications of Metals in Additive Manufacturing
Even though all titanium processing is carried out under an inert environment or vacuum atmosphere, the powders are exposed to thermal effects from multiple layer builds and can loses alloy elements. ASTM F2924-14 and 3001-14 do not limit powder usage to any specific number of builds, and it is recommended to follow specific equipment guidelines for this purpose. Tang et al. [17] carried out an extensive study to evaluate the effect of powder reuse during electron beam melting (EBM) of Ti6Al-4V alloys. Their results indicated that the powder morphology changes from smooth, spherical to rough, irregular after recycling about 21 times. This change in powder morphology causes appreciable change in powder tap density (from 2.96 to 2.88 g/mL), however the flowability improves with reuse (from 32.47 to 28.34 s/50 g). Enhanced powder flow after recycling is attributed to the absence of satellites and removal of moisture during repeated exposure and recycling. During this recycling process, powder chemistry was also affected, while the V content changed marginally, Al content in the powder changed from 6.47% in virgin powder to 6.35% after 21 recycles and, correspondingly, in the test samples from 6.14% to 5.93%. Loss of Al content is due to vaporization of Al in low partial pressure (2 3 1022 Pa of He). Powder recycling is also associated with an increase in oxygen content from 0.08% in virgin powder (0.07% in the test sample) to 0.19% in powder (0.18% in the test sample) after 21 recycles. Repeated powder handling and exposure to air during recycling cause additional oxygen pick up in the powder. As a result, the UTS increased from 920 to 1039 MPa, yield strength increased from 834 to 960 MPa and tensile elongation dropped from 16% to 15.5%. In other work using Realizer powder bed fusion equipment over 10 repeated runs the oxygen content of Ti-6Al-4V was found to increase from 0.16 to 0.18 wt.%. Jacob et al. [18] performed extensive work on recycling of nitrogenatomized 17-4 PH stainless steel powder during laser-based PBF processing. Figs. 4.6 and 4.7 show the morphology of virgin powder and after recycling 11 times. Fig. 4.8 shows particle sizes for D10, D50, and D90. The results indicate no appreciable change in the powder size and distribution. Hall flow tests revealed reduction in flowability after the first and second recycling, but flowability gradually improves during successive recycling rounds. The apparent density of powder increases and then levels off with multiple recycling rounds. The chemical composition of the powder did not change significantly and chemistry after the 11th build
Raw materials for metal additive manufacturing
89
Figure 4.6 SEM image of nitrogen-atomized virgin 17-4 PH stainless steel powder: (A) low and (B) high magnification [18].
Figure 4.7 SEM image of nitrogen-atomized 17-4 PH stainless steel powder after 11 build cycles in a laser-based PBF machine: (A) low and (B) high magnification [18].
was within ASTM-specified limits. The most importantly properties of printed materials using recycled powder were found to have variations in a single build, Fig. 4.8. Horizontal and vertical surface roughness, bulk density, UTS, YS, and elongation of the samples were not significantly affected by recycling of powders. These results indicate that recycled powders can produce parts with properties comparable to those made from virgin powders.
90
Science, Technology and Applications of Metals in Additive Manufacturing
55
Particle size, x cmin (µm)
50 45 40 35 30 25 20 P40, Build #1
P46, Build #2
P61, Build #5
P67, Build #6
P92, Build #11
P98, after Build #11
Powder sample # D10
D50
D90
Figure 4.8 Effect of repeated runs through a laser-based PBF machine on the powder sizes [18].
4.4 Wire precursors Two directed energy deposition processes that use metal wire as feedstock are wire arc additive manufacturing (WAAM) (also called shaped metal deposition) patented by Rolls Royce Corp. and jointly developed with Cranfield University and electron beam additive manufacturing (EBAM) commercialized by Sciaky Inc. These processes both use standard welding wires specified by AWS specifications and typical gauge sizes of about 1.2 mm (0.05v) for SMD/WAAM [19] and 0.9 4.0 mm for EBAM [20]. According to AMS 4999A [16], the wire feedstock for the titanium deposition process should be either wire conforming to AWS A5.16 ERTi-5 (Al content up to 7.5% is allowed, and a minimum of 1400 ppm oxygen).
References [1] J. Dawes, R. Bowerman, R. Trepleton, Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain, Johnson Matthey Technol. Rev. 59 (3) (2015) 243 256. Available from: https://doi.org/10.1595/205651315X688686. [2] F.H. (Sam) Froes, Titanium powder metallurgy: developments and opportunities in a sector poised for growth, Powder Metall. Rev. 2 (4) (2013) 29 43. Winter.
Raw materials for metal additive manufacturing
91
[3] B. Dutta, F.H. (Sam) Froes, Additive Manufacturing of Titanium Alloys; State of the Art, Challenges and Opportunities, Elsevier, 2016. [4] ,https://www.metalysis.com/#process. (accessed 13.02.19). [5] ,https://www.pm-review.com/metalysis-titanium-powder-used-to-3d-print-automotiveparts/. (accessed 13.02.19). [6] A. Popovich, V. Sufiiarov, Metal Powder Additive Manufacturing, Chapter 10, Intech Open Access Chapter 10, ,https://doi.org/10.5772/63337.. [7] A. Cooke, J. Slotwinski, NIST internal report on Properties of Metal Powders for Additive Manufacturing: A Review of the State of the Art of Metal Powder Property Testing, NISTIR 7873, July 2012, ,https://doi.org/10.6028/NIST. IR.7873.. [8] J.A. Slotwinski, E.J. Garboczi, P.E. Stutzman, C.F. Ferraris, S.S. Watson, M.A. Peltz, Characterization of Metal Powders Used for Additive Manufacturing, J. Res. Natl. Inst. Standards Technol. 119 (2014) 460 493. Available from: https://doi.org/ 10.6028/jres.119.018. [9] ASTM F3049-14, Standard Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes, ASTM International, West Conshohocken, PA, 2014. [10] W. Nan, M. Pasha, T. Bonakdar, A. Lopez, U. Zafar, S. Nadimi, M. Ghadiri, et al., Jamming during particle spreading in additive manufacturing, Powder Technol. 338 (2018) 253 262. [11] ASTM F2924-14, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [12] ASTM F3001-14, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium ELI (Extra Low Interstitial) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [13] ASTM F3184-16, Standard Specification for Additive Manufacturing Stainless Steel Alloy (UNS S31603) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2016. [14] ASTM F3056-14e1, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N06625) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [15] ASTM F3055-14a, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N07718) with Powder Bed Fusion, ASTM International, West Conshohocken, PA, 2014. [16] AMS 4999A, Titanium Alloy Direct Deposited Products 6Al - 4V Annealed, SAE Int., Sep 2011. [17] H.P. Tang, M. Qian, N. Liu, X.Z. Zhang, G.Y. Yang, J. Wang, Effect of Powder Reuse Times on Additive Manufacturing of Ti-6Al-4V by Selective Electron Beam Melting, JOM 67 (3) (2015) 555 563. Available from: https://doi.org/10.1007/ s11837-015-1300-4. [18] G. Jacob, C. Brown, A. Donmez, S. Watson, J. Slotwinski, Effects of powder recycling on stainless steel powder and built material properties in metal powder bed fusion processes, NIST Advanced Manufacturing Series 100-6, ,https://doi.org/ 10.6028/NIST.AMS.100-6.. [19] E. Brandl, B. Baufeld, C. Leyens, R. Gault, Additive manufactured Ti-6Al-4V using welding wire: comparison of laser and arc beam deposition and evaluation with respect to aerospace material specifications, Phys. Proc. 5 (2010) 595 606. [20] , http://www.sciaky.com/additive-manufacturing/wire-am-vs-powder-am . (accessed 28.07.15).