An indirect additive manufacturing method combining a 3D printing of polymers with a centrifugal casting is outlined for manufacturing 3D octet truss metals (Al and Cu alloys) having a unit cell length of 5 mm with a wall thickness of 0.5 mm.
One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM). In spite of its potential processing capability, the direct AM method has several disadvantages such as high cost, poor surface finish of final products, limitation in material selection, high thermal stress, and anisotropic properties of parts. We propose a cost-effective method to manufacture 3D lattice metals. The objective of this study is to provide a detailed protocol on fabrication of 3D lattice metals having a complex shape and a thin wall thickness; e.g., octet truss made of Al and Cu alloys having a unit cell length of 5 mm and a cell wall thickness of 0.5 mm. An overall experimental procedure is divided into eight sections: (a) 3D printing of sacrificial patterns (b) melt-out of support materials (c) removal of residue of support materials (d) pattern assembly (e) investment (f) burn-out of sacrificial patterns (g) centrifugal casting (h) post-processing for final products. The suggested indirect AM technique provides the potential to manufacture ultra-lightweight lattice metals; e.g., lattice structures with Al alloys. It appears that the process parameters should be properly controlled depending on materials and lattice geometry, observing the final products of octet truss metals by the indirect AM technique.
Cellular metals are the metals made up of an interconnected network of solid struts or plates and have complex micro-architectures with voids1. Examples include both i) randomly structured stochastic foams and ii) periodically ordered two-dimensional (2D) honeycombs and three-dimensional (3D) lattice truss structures. They have received attention due to their high specific stiffness and strength1-3and high specific resilience4-5, excellent energy absorption for impact loading6, acoustic insulation7, possible design of heat dissipaters and heat exchangers8. Especially, periodically ordered lattice structures have the potential to engineer the properties superior with a capability to control the internal porous network geometry.
Due to their complex internal porous network geometry, it is difficult to manufacture cellular metals using the conventional subtractive machining. As such, researchers have started looking for alternative methods to fabricate cellular metals: forming gas in liquid metal or mixing metal powder with blowing agents were explored for manufacturing stochastic metal forms9. Due to the lack of control over cell topology, it is hard to tailor mechanical properties. Alternatively, manufacturing methods for periodically ordered cellular metals were explored: stamping thin sheets of metal into a corrugated shape followed by joining them to create periodic structures10, bonding slotted metal sheets11, extrusion12, weaving and blazing metal filaments to fabricate textiles13. Even though these manufacturing methods offer repeatable patterns, the patterns are still limited in the planar direction. In an effort to generate 3D pattern repetition, researchers started using additive manufacturing (AM); e.g., Selective Laser Melting (SLM)14, Electron Beam Melting (EBM)15, and Direct-Metal Laser Sintering (DMLS)16. Despite their capability to fabricate 3D ordered complex lattice geometries, there still exist some limitations: difficulty using metals with high thermal conductivity and high optical reflectivity17, high thermal residual stress18, poor surface finish with the 'balling' phenomenon during laser or electron melting 19, anisotropic properties20-21 of parts caused by a combined effect of the layered manufacturing, anisotropic formation of grains, powder size, power and scanning speed of laser or electron beam15, high energy consumption, etc.
Combining polymer based AM with metal casting may provide an alternative method to manufacture lattice metals. One may call this "indirect AM". Indirect AM may provide a solution to overcome the technical problems of direct AM of metals mentioned above. Several efforts have been made to manufacture lattice metals using indirect AM combining 3D printing of polymers with gravity based casting22-25; e.g., an investment casting combined with fused deposition modeling (FDM) to fabricate a lattice alloy22-25 or sand casting combined with a sand powder based AM23. The gravity based casting appears to remain a technical challenge to overcome – misrun and porosity caused by sudden solidification of molten metals when they meet network structures with sharp corners of lattice structural molds25-26. Relatively large surface area of lattice structural molds also appears to contribute to sudden cooling, resulting in premature solidification25-26.
In this study, we propose an alternative indirect AM that may overcome the misrun during manufacturing of lattice metals – centrifugal casting to a lattice mold cavity made by a 3D printed lattice sacrificial polymer pattern. We use a digital light processing (DLP) based 3D printing method to build a lattice structural sacrificial pattern followed by centrifugal casting of Al and Cu alloys. The objective of this study is to provide a detailed protocol on fabrication of 3D lattice metals having a complex shape and a thin wall thickness. The main contribution of this process is to provide an opportunity to extend the selection of materials with low manufacturing cost for manufacturing lattice metals.
1. Planning of Experiment
2. Fabrication of Sacrificial Pattern
3. Fabrication of Mold
4. Post Processing for Final Products of Octet Truss Metals
Using the indirect additive manufacturing described in the protocol section, Al and Cu alloys were used for manufacturing octet truss metals, as shown in Figure 1. The whole casting procedure is summarized in Figure 2. The procedure consists of eight sections: (a) sacrificial pattern printing (b) melting-out of support material (c) removal of residue of support material (d) pattern assembly (e) investment (f) burn-out of sacrificial patterns (g) centrifugal casting, and (h) post-processing. The investment mixing process was performed in order to make sure that there were no lumps in the investment-water mixture, as shown in Figure 3. The burn-out process was carried out for 6 hr to melt out the sacrificial pattern as shown in Figure 4, followed by the centrifugal casting process (Figure 2G and Figure 5). Figure 6 shows the final products of octet truss metals with Al and Cu alloys. It shows that the molten Al alloy fully fills the entire lattice mold cavity without misrun. On the other hand, the molten Cu alloy appears to have a casting defect with premature solidification at the early stage of injection of molten metal at the inlet.
Figure 1. A Schematic of Octet Truss Structure with a Sprue System. Figure 1 shows a schematic of a sacrificial pattern of octet truss structure with a sprue system used in this study. The sprue system consists of a sheet of a 1 mm thickness, a 25 mm width, and a pillar having 10 mm in height and 6 mm in diameter. The sprue system can be modified using CAD software, if needed, for design of better fluidity of liquid metal. Please click here to view a larger version of this figure.
Figure 2. An Overview of the Indirect AM with Centrifugal Casting Procedure: (A) pattern printing (B) melt-out of support material (C) removal of residue of support material (D) pattern assembly (E) investment (F) burn-out of sacrificial pattern (G) centrifugal casting, and (H) post processing. This figure shows the whole procedure on fabricating octet truss metals using indirect AM with centrifugal casting. Please click here to view a larger version of this figure.
Figure 3. Work Schedule on Preparation of the Plaster Mold. Figure 3 shows the preparation of the plaster mold and the procedure to harden it in the flask. Please click here to view a larger version of this figure.
Figure 4. Burn-out Schedule of Sacrificial Pattern Inside the Plaster Mold. Figure 4 shows the burn-out process of the sacrificial pattern inside the hardened mixture. Please click here to view a larger version of this figure.
Figure 5. A Schematic of a Centrifugal Casting Machine. The centrifugal casting machine consists of eight components: main shaft, base, casting arm, weights, flask cradles, flask cradles holding arms, crucible holder, and crucible holding arms. The casing arm is balanced with moving weights along the casting arm. Please click here to view a larger version of this figure.
Figure 6. Final Product of Octet Truss Metals: Octet truss structures with (A) Al and (B) Cu alloys. It shows a fairly good degree of completion on the octet truss Al alloy. On the other hand, a poor degree of completion is observed with the octet truss Cu alloy. Please click here to view a larger version of this figure.
Property | Value [unit] |
Density at 20 °C | 1,019 [kg/m^3] |
Thermal conductivity at 20 °C | 0.47 [W/(m*K)] |
Thermal expansion coefficient at 20 °C | 7.22E-6 [/°C] |
Roughness | 2.72E-6 [m] |
Table 1. Properties of the Investment Powder. This table shows the physical properties of the investment powder used in this study.
40 ml water to 100 g powder | |||||||
Flask diameter | Height | ||||||
5.08 cm | 6.35 cm | 7.62 cm | 8.89 cm | 10.16 cm | 12.7 cm | 15.24 cm | |
6.35 cm | 226.8 g 91 ml | 283.5 g 114 ml | 340.19 g 136 ml | 396.89 g 160 ml | 453.59 g 183 ml | 566.99 g 228 ml | |
(Top figure – Investment powder (g), Bottom figure – Water (ml)) | |||||||
Note: In this study, a flask with a height of 6.35 cm and a diameter of 6.35 cm is used. |
Table 2. Mixing Conditions of Investment Powder with Water for a Varying Flask Size: This table shows the mixing conditions of the investment powder and water for a varying flask size recommended by manufacturer. In this study, a flask with a height of 6.35 cm and a diameter of 6.35 cm is used.
(a) Chemical composition of Al alloy | |||
Material | Unit | ||
Aluminum alloy | Chemical composition | ||
Al | Cr | ||
Min./Max. | >99 | <0.05 | |
(b) Physical properties of Al alloy | |||
Property | Unit | ||
Liquidus temperature | 660 °C | ||
Solidus temperature | 660 °C | ||
Density | 2,340[kg/m^3] @850 °C | ||
Specific heat | 1,090 J/kg∙°C | ||
Thermal conductivity | 0.9428 [W/(cm*°C)] @850 °C | ||
Viscosity | 0.00087 [Pa∙s] @850 °C | ||
Surface tension coefficient | 900 [N/mm] |
Table 3. Chemical Composition and Physical Properties of Al Alloy30: (a) Chemical composition of Al alloy and (b) Physical properties of Al alloy.
(a) Chemical composition of Cu alloy | ||||||
Material | Unit | |||||
Copper alloy (Jewelry bronze) | Chemical Composition (%Max, unless shown as range of mean) | |||||
Cu | Si | Zn | Mg | Pb | ||
Min./Max. | 91.9 | 4 | 4 | 0.25 Max | 0.25 Max | |
(b) Physical properties of Cu alloy | ||||||
Property | Unit | |||||
Liquidus temperature | 1,035 °C | |||||
Solidus temperature | 1,005 °C | |||||
Density | 7,200 [kg/m^3] @1200 °C | |||||
Specific heat | 380 J/kg∙°C | |||||
Thermal conductivity | 1.44 [W/(cm*℃)] @1200 °C | |||||
Viscosity | 0.0038[Pa∙s] @1200 °C | |||||
Surface tension coefficient | 1,500 [N/mm] |
Table 4. Chemical Composition and Physical Properties of Cu Alloy30: (a) Chemical composition of Cu alloy and (b) Physical properties of Cu alloy.
For conventional metal casting, it is important to keep the molten metal's flow smooth and streamlined in 'laminar' in the mold cavity and rather avoid irregular and agitated flow generally observed in turbulent flow27. Accordingly, it is important to properly design the inlet of the sprue system associated with the rotating speed of a centrifugal arm to maintain the flow of molten metal inside the lattice mold cavity 'laminar'.
In this study, the critical steps of the protocol are the mixing of investment powder with water, the burn-out of sacrificial pattern, and the centrifugal casting process. The mixing process of investment powder with water is important because quality of the plaster mold is mainly controlled by this process, subsequently influencing the degree of completion of the final products of octet truss metals. For instance, if the investment-water mixture is too coarse, the surface roughness becomes high, resulting in poor surface finish of final products. The burn-out process of the sacrificial pattern is also essential to determine the quality of final products because residue of the sacrificial pattern causes a casting defect such as misrun or metal penetration if the sacrificial pattern is not completely burned out. The last key step is the centrifugal casting process. Proper setup for casting is required to fully fill liquid metal out in the investment mold cavity with a complex network shaped octet truss structure.
As can be seen in Figure 6, under the same processing conditions, e.g., the inlet velocity, the inlet temperature of molten metal, pre-heat temperature of mold, Cu alloy shows a premature solidification in a lattice mold cavity with a wall thickness of 0.5 mm (Figure 6 (b)). This is thought be caused by the high surface tension (1,500 N/mm) and high viscosity (0.00038 Pa·s) of Cu alloy through the micro-channel in the lattice mold cavity. On the other hand, it appears that a relatively low surface tension coefficient (900 N/mm) and viscosity (0.00087 Pa·s) of the molten Al alloy enables it to fully fill the octet truss lattice mold cavity. It is recommended to find an optimum inlet velocity of molten metal for casting 3D network lattice structures with a thin wall thickness in order to overcome the sudden change in flow-direction of molten metal and the effect of surface tension in the micro-channel of the lattice structural mold cavity.
The surface tension effect of molten metal is known to be dominant at a thin channel where this lattice cavity mold geometry in this study can be applied. It may be possible to manufacture a lattice Cu alloy with a thicker wall thickness and higher injection velocity of centrifugal casting as demonstrated in our previous work 28-29.
Direct AM methods such as SLM and EBM have been considered for possible manufacturing methods for lightweight 3D lattice metals. However, direct AM methods appear to be limited in selecting materials. For example, current EBM technology is limited to Ti-6Al-4V and Inconel31. In spite of its extensive use in aerospace and bio-implant applications, aluminum, for example, is not being produced using this technology. Theoretically, it could be possible to extend the direct AM to other powder metals through close control of process parameters. However, practically, the direct AM has been reported to have difficulty in fabricating parts with metal powders with high optical reflectance and high thermal conductivity, e.g., Al. Moreover, evaporation and possible explosion inside the build chamber have been issued for aluminum powder31.
The proposed indirect AM technique is significant because this enables to manufacture lattice structures with metals where direct AM methods experience difficulty, resulting in empowering overall manufacturing capability of 3D lattice metals with both direct and indirect AMs by expanding the range of selection of metals. In addition, centrifugal casting, a step in this study, is known to provide an isotropic property of metal parts due to relatively equal spread of molten metal into the mold cavity. This may resolve the current issue of direct AM on anisotropy caused by both layered manufacturing and anisotropic formation of grains20-21.
Exploring other metals for indirect AM together with studies on the effect of process parameters on lattice geometries will be left to our future work.
The authors have nothing to disclose.
This study received support from the Research Initiation Grant (RIG) of the Vice President for Research and Economic Development at the University of North Texas (UNT). The authors also thank KCIS Co. Ltd for partially supporting this study. The support from PACCAR Technology Institute at UNT to the success of this publication is greatly appreciated.
Motorized centrifugal casting machine | Rey Motorized Centrifugal Casting Machine, Rey Industries Inc. | Made in U.S.A. by Rey industries, Inc. Dallas, TX 75220 | |
Gypsum powder | Satin Cast 20, FindingKing Kerr | 7960 | Gypsum powder is used to make the investment mixture |
Ployjet 3D printer | Projet HD3500 Plus, 3D Systems | This polymer based 3D printer to print out sacrificial pattern for casting. | |
Cartridge materials – UV curable and castable acrylic plastic | VisiJet Procast, 3D Systems | This is castable material that is going to be burn out before casting | |
Cartridge materials- support material | VijiJet S300, 3D Systems | This is support material that is going to be removed before pattern assemble | |
Ancient Bronze Casting Grain | Rio Grande | 706051 | This true bronze grain contains no zinc. Highly fluid, it melts quickly, casts cleanly and provides a good balance between strength and durability. The warm, deep-bronze color has rich red undertones, and the alloy takes a good patina. Composition is 90% copper with an amount of tin; fits into the CDA#90700 category. This grain is sold in 1-lb. packages. |
Aluminum Round Wire, 1/8", 1-Lb. Spool, Dead Soft | Rio Grande | 134700 | Lightweight and strong, aluminum wire is an economical and versatile choice. Not as bright-white as silver, aluminum offers a warmer tone much like that of platinum. Solder ONLY with low-temp solders such as Stay-Brite; suitable for both pulse-arc and laser welding. This quality aluminum wire is packaged on 1-lb. spools. |
Computer aided design software (Pro-e) | This software can be replaced with the others such as Auto CAD, Catia, and so on. | ||
ProJet Finisher 1-A | 3D Systems | This machine is used to melt the support material. | |
160 Watt 2.5 Liters Digital Ultrasonic Cleaner with Timer Heater Rings Tools | Chicago, Electric, Power Tools | 85 oz. capacity, Five cleaning cycles: 90, 180, 280, 380 and 480 seconds, Clean with or without heat, Easy-to-read LED digital timer, Clear-view window | |
Fan | Honeywell Inc. | HT-800 | 120V A.C., 60Hz., 0.85A. TP |
Paraffin wax for wax sheet – Modeler's Pink Wax Sheet, 3" by 6", 24-Ga. | Rio Grande | 700075 | Sheet wax is flexible and can be cut or formed into any shape. It’s ideal for designing since you can draw or trace directly onto the sheet; choose green or pink depending on which will best show your designs. High manufacturing standards ensure exceptional consistency and significant price savings. Value is enhanced by larger package quantities at the same price as the smaller packages available elsewhere. Each 8-oz. package contains approximately 30 sheets. |
Paraffin wax for wax stick – Modeler's Medium Red Sprue Wax, 8-Ga | Rio Grande | 700741 | A pliable, softer sprue wax than the firm blue. Good for forming gates and sprues and burns out cleanly with no residue. |
Alcohol Lamp | Rio Grande | 700008 | Use this lamp to heat wax-working tools or as a flame polisher. The heavy glass reservoir has faceted sides to allow it to be tipped for angling the flame. A screw adjustment for the 7" x 3/16" wick controls the height of the flame. A safety cap snuffs the flame and prevents fuel evaporation. For the best flame, use methyl alcohol fuel. Replacement wicks available. Reservoir holds 5 oz. (150ml) of fuel. |
Wax carving tool set – Soft Grip Wax Carvers, Set of 10 | Rio Grande | 700329 | This boxed set offers the best in cutting and shaping technology. Each of these ten high-quality steel wax-carving tools features a 5/16" PVC covered handle that ensures a sure, comfortable grip through hours of work and all have sharp edges for shaping and fine detailing. Sharpen or custom-shape each tool to fit your needs. These tools provide exceptional tool strength and deliver excellent results. This set comes in a hinged, foam-lined wood box. |
Rubber Mixing Bowl, 1-1/2 Qt. | Rio Grande | 702131 | This highly-flexible vulcanized rubber bowl is easy to grip, will not be marred by a spatula and cleans with ease. |
Pyrex Beaker, 1,000ml | Rio Grande | 335040 | Ideal for holding and heating bath plating solutions, this genuine Pyrex glass beaker is sturdy and durable. |
Rio Premium Stainless Steel Flask, 2-1/2" dia. | Rio Grande | 70201514 | This solid, #304-quality stainless steel flask is corrosion-resistant, durable for a long service life and performs under extreme temperature without distortion. |
CAST/T Ceramic Casting Crucible, 450g | Rio Grande | 705047 | Made exclusively for the CAST/T centrifugal casting machine, this crucible is designed with an angled base that slides into the hinged bracket on top of the casting machine. This brings the crucible into perfect alignment with the center of the flask ring to ensure an error-free pour. |
MyWeigh iBalance 300 Digital Scale | Rio Grande | 116850 | This scale is used to measure the weight of the sacrificial and sprue system for metal which is going to be used for centrifugal casting. |
Rubber bottom – CAST/T Flask Ring Base | Rio Grande | 705025 | Specially made for the CAST/T centrifugal casting machine, this rubber base accommodates all Table King flask ring styles, creating a secure, airtight seal throughout the investment process. The center post fits either of the wax disc styles for complete versatility. |
Scotch® Colored Duct Tape, 1 7/8" x 20 Yd., Blue | OfficeMax | 22353766 | This scotch tape is used to make sure that the gypsum-water mixture fully covers the assembled sacrificial pattern inside the flask by allowing for extra material above the flask height |
Vacuum casting machine – V.I.C. 12 Tabletop Solid- and Perforated-Flask Casting Machine with The Rio Assistant, 110-Volt | Rio Grande | 70511814 | The V.I.C. 12 casting machine offers all the latest technical innovations for efficient, productive vacuum investing and casting. Designed to meet the demands of medium-sized casting operations, this machine includes a powerful 1/2hp, 5cfm vacuum pump for effective vacuuming and outstanding casting results. The V.I.C. 12 casts small or large flasks. Includes an adapter table that accepts standard solid flasks up to 5" x 7" high and is mounted on rubber feet for stability. |
Furnace for burn out sacrificial pattern -Rio Model 1000 Enameling Kiln with Nine Program Controller | Rio Grande | 703121 | The Rio enameling kiln features three pre-set firing temperatures for enamels and six that you can define. Use the exclusive Rio controller to set and maintain firing temperatures. Perfect for all types of enameling, including tall pieces. Includes ample space for firing and an easy-latch door that will not jar your enamels when opening and closing. Also suitable for metal clay, glass and ceramics. Galvanized steel case with high-temperature insulating firebrick keeps them cool. Element protected in recessed groove. Includes user instructions. |
Smith Complete Little Torch Acetylene and Oxygen System | Rio Grande | 500030 | Get everything you need to equip your shop for soldering and brazing. Use Little Torch systems for gold or silver soldering, brazing and casting applications. Complete every soldering and melting job with confidence and ease! This system accepts all Little Torch accessory tips for melting, brazing and large soldering jobs and is a staple for every jeweler. |
Heat-Resistant Safety Apron | Rio Grande | 750160 | The specially designed apron has an 800°F (427°C) temperature resistance. Its reflective finish repels hot metal splashes and helps insulate the wearer from heat. |
Radnor Heat-Resistant Gloves | Rio Grande | 350050 | These flexible, heat-resistant gloves are ideal for enameling projects, allowing you to grip even small tongs securely. Blue, shoulder-split leather gloves are made of tough cowhide and lined with cotton and foam, and have reinforced thumb wings. |
Platinum Soldering Glasses, #7 | Rio Grande | 113914 | Protect yourself and your employees when soldering platinum. Comfortable glasses feature adjustable earpieces and 52mm IR green polycarbonate #7 lenses. The #7 lens is approved by The Platinum Guild. |
Economy Light-Duty Flask Tongs | Rio Grande | 704026 | Constructed of bent steel, these tongs are designed to handle flasks 3-1/2" or less in diameter. The small-angle notches grip smaller flask sizes and the larger, rounded contour area securely holds larger flasks. |
Separating Screen Bucket | Rio Grande | 201360 | 15"-diameter, 11-1/2"-deep |
Sand blaster – Econoline – 101701CB-A – Free-Standing Cabinets Workspace Width (Inch): 60 Workspace Depth (Inch): 48 | MSC industrial supply Co. | 223818 | Ree-Standing Cabinets; Workspace Width (Inch): 60; Workspace Depth (Inch): 48; Workspace Height (Inch): 40; Air Requirement: 12 CFM @ 80 psi; Overall Cabinet Width (Inch): 65; Maximum Cabinet Depth (Inch): 86 |
Johnson's Baby Oil Shea & Cocoa Butter | Wal-Mart | 260074132 | This baby oil is used for removing the residue of the support material for the castable sacrificial pattern using Digital Ultrasonic Cleaner. |
German 4" Saw Frame and Saw Blade Kit | Rio Grande | 110112 | Quality, German-made frames are our most popular saw frames, and this frame includes a sampler pack of Rio German saw blades! The adjustable saw frame allows you to achieve the blade tension you want. Throat depth is 102mm (4"). Saw blades have rounded backs that make cutting curves and corners easy and are made from hardened, tempered steel. |