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Technology overview: Direct Metal Additive Manufacturing

Additive manufacturing (AM) also commonly known as 3D printing, enables the freeform fabrication of 3D structures directly from digital data generally in a layer by layer methodology. In recent years AMhas become a significant complimentary technology to the conventional subtractive and formative manufacturing processes. Due to the high level of digitization and automation involved in AM processes, the technology plays a significant role in the Industry 4.0 manufacturing ideology.
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According to ASTM, Additive Manufacturing is defined as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies”. Some of the most common synonyms used for Additive manufacturing include: 3D printing, Freeform fabrication, Additive Layer Manufacturing, Rapid prototyping, Rapid tooling etc.

Market trends such as a shorter product life cycle and customization in addition to sustainability and environmental concerns have led to an increased pressure on the manufacturing industry. Additive manufacturing with its unique features has enabled the manufacturing industry to attain these objectives by optimizing material utilization, reduction of conventional tooling costs, elimination of product inventory, enabling localized manufacturing, enhancing manufacture of complicated design and light weight structures etc.

Since its commercialization in the mid 1980’s, Additive Manufacturing has constantly evolved to attain higher efficiency in addition to enhancing its material and design capability. Additive Manufacturing is often used as an umbrella term for multiple processes classified on the basis of their processing methodology, material options and application capabilities.

The first concept of Additive Manufacturing can be traced back to a patent file by Pierre Ciraud in 1971 which described manufacturing of structures on a substrate plate using metal powder and solidifying it by means of a beam of energy: “The invention makes possible the manufacture of parts which can have extremely complex shapes, without the need for casting moulds”. However, the patent idea was never commercialized due to limitations in computer and laser technology. In 1979, another patent was filed by Ross Housholder which has huge similarities to the current laser-sintering system. The objective of the invention was “to provide a new and unique molding process for forming threedimensional articles in layers and which process may be controlled by modern technology such as computers”. It also stated “fusible particles are employed to form each layer which is then selectively fused by a laser beam to fuse an area in each layer which defines that portion of the article in the layers”. This idea was also not commercialized at the time due to the high cost of laser, but was later licensed by DTM Corporation.

The first Additive Manufacturing system technology to be commercially available was Stereolithography (SL) from 3D systems in 1987. SLA® (Stereolithography Apparatus) is used by 3D systems as the commercial trademark to its Stereolithogrphy technology. The process is fundamentally based on the principle of selectively curing thin layers of photocurable resin using a UV laser. Other SL systems were also commercialized in Japan and Europe during the late 1980’s and early 1990’s. In 1991, Fused Deposition modelling (FDM) which was based on the concept of thermoplastic material extrusion in filament form was introduced by Stratasys. Laser Sintering (LS) based on the selective sintering of polymer powder using a CO2 laser was first introduced by DTM in 1992 as SLS® Selective Laser Sintering. Following that, in 1994, Laser Sintering of polymers was also introduced by EOS in Europe. Other non-metal AM processes such as 3D printing from Z-Corp, Polyject from Objet, DLPPerfactory from Envisiontec were introduced in the following years. Since the expiration of patents of FDM based and DLP based processes, there has been an exponential increase in the system technology manufacturers in these 2 process categories. Laser Metal Deposition (LMD) was first introduced as an Additive Manufacturing technology by Precision Optical Manufacturing (POM) under the Direct Metal Deposition (DMD) name in 2001. The LMD process is fundamentally based on the laser cladding technology in which metal powder is selectively deposited on the substrate and a simultaneous melting takes place using a high powdered laser.

The first commercial AM system to successfully process metal powder was called Direct Metal Laser Sintering (DMLS). A powder concept for pressureless sintering with very low shrinkage had been invented in 1989 by Nyrhilä. This powder concept from Electrolux Rapid Development (ERD)was known as Direct Metal and was combined with the Laser Sintering technology from EOS and was commercialized in the DMLS machine system EOSINT M 250 in 1995.

The current technological stage of Laser Beam Melting (LBM) is the result of a German government funded project which was started in 1995. The technology was exclusively patented by Fraunhofer ILT and some of the major project partners included EOS, the Fraunhofer institutes ILT and IPT, Fockele&Schwarze. The LBM process enables the fabrication of fully dense 3D structures by selective melting of pre-alloyed metal powder in a powder bed using a high intensity solid state fibre laser. The process is also commercially referred to as DMLS® Direct Metal Laser Sintering by EOS, SLM® Selective Laser Melting by SLM Solutions and Realizer, LaserCusing® by Concept Laser.Other major system technology OEM’s for the technology include Renishaw, Trumpf, Sisma, 3D systems, GE Additive.

In its early days of development, Additive Manufacturing was often referred to as Rapid Prototyping due to its primary application in the field of prototype manufacture. With developments in materials, design and processes, AM has been able to develop new applications in tooling and end product manufacturing in addition to the existing prototyping applications. A significant part of this evolution has been the development of direct metal AM processes.

With its ability to fabricate fully dense 3D structures directly from digital data, direct metal AM processes have demonstrated the capability to deliver mechanical characteristics in line with the requirements of the conventional manufacturing industry. Some of the most significant direct metal AM processes using metal powder include the process of Laser Beam Melting (LBM), Electron Beam Melting (EBM) and Laser Metal Deposition (LMD). In recent years there has also been the development of wire and binder jet based direct metal AM processes motivated by the market need to enhance productivity and reduce capital and material costs associated with existing powder based direct metal AM processes.

The Additive manufacturing process of Electron Beam Melting (EBM) is fundamentally based on the electron beam welding process and enables that fabrication of fully dense 3D structures by selective melting of metal powder in vacuum using a high intensity electron beam. It was first commercialized in 2001 by ARCAM AB from Sweden. The process primarily focuses on high melting temperatures alloys such as Titanium, Nickel and cobalt-based alloys. Due to its high productivity and comparatively lower post processing requirements, it is often marketed as a production level system technology for aerospace and medical implant applications.

In an Electron Beam Melting (EBM) machine system, a high intensity electron beam is generated by providing current to the tungsten filament in the electron gun which acts as anode. A combination of electromagnetic lenses and magnetic scanning coils enable the focussing and deflection of the electron beam to perform selective melting on the powder bed. Other components of the machine system include powder supply units, recoating blade, and the build platform as illustrated in figure 1.

Figure 1: Schematics of an EBM machine system. 1) Electron gun, 2) Focussing lens, 3) Deflection lens, 4) Powder supply chambers, 5) Recoater blade, 6) Fabricated part, 7) Build platform

The entire process takes place in vacuum and helium gas is used to control build cooling and provide thermal stability during the process. The focussing and beam deflection mechanism enables high scanning speeds and creation of multiple beam spots simultaneously on the powder bed. The platform is pre-heated to a temperature of about 770°C and each powder layer is pre-sintered. This helps in an effective management of thermal stresses and reduced the need for support structures and post-build heat treatment. The beam deflection mechanisms along with reduced post processing requirements result in high productivity from the process. Also, the vacuum created during the process reduced the possibility of oxide formation on the structures. The EBM process however suffers from the limitation of available material options,high surface roughness and its inability to process very fine details. Figure 2 illustrates a SEM surface image of a Ti6Al4V structure fabricated using Electron Beam Melting. An improvement on the illustrated surface would be expected after performing a shot blasting surface treatment on the fabricated structure.

Figure 2:SEM surface image of Ti–6Al–4V as-fabricated EBM cylindrical component

The materials processable are primarily high melting point alloys including Titanium, Nickel and Cobalt based alloys. Considering the high capital investment and its ability to achieve high productivity, the EBM process is considered most applicable for production environments in the Aerospace and Medical implants industry. Table 1 below indicates the mechanical strength obtained by EBM components for the specified material options.

MaterialYield Strength, Rp 0.2 (MPa)Tensile Strength, Rm (MPa)Elongation at break (%)Hardness (HRC)
     
Ti6Al4V950 MPa1020 MPa14 %33 HRC
IN 718822 MPa1060 MPa22 %-
IN 625410 MPa750 MPa44 %14 HRC
CoCr600 MPa1050 MPa20 %-

Table 1:Mechanical Characteristics of components fabricated using Electron Beam Melting process

Research conducted on EBM of Ti64 has reported a variation between the hardness of the bottom (HRC: 50) and top (HRC: 40) sections of components. Variations in the microstructure of the components can also be observed as illustrated in figure 3. It should however be noted that the observed hardness values of both sections are higher that specified for the ASTM standard (HRC: 37).

Figure 3: Variation in the microstructure of the top and bottom section of Ti6Al4V component fabricated using Electron Beam Melting

The most popular applications of EBM can be noted in the orthopaedic implant industry for the fabrication of customized knee implants (figure 4). In additional to orthopaedic implants, EBM components are also used for other customized implant manufacturing applications. Aerospace components using Ti and Nickel based alloys are also manufactured using Electron Beam Melting. However, due to the limitations of material options considered processable, EBM has not been able to successfully apply itself in industries such as tooling, automotive and general engineering.

Figure 4: Co-Cr-Mo implant fabricated by Electron Beam Melting

Fundamental research in the field of EBM focuses on development of new material systems along with better capability of process monitoring and simulation. On the application side, EBM research has focussed on process optimization, standardization and certification, Design for Additive Manufacturing and other associated topics. With the recent collaboration between GE Additive and ARCAM, it is expected that significant developments in the technology and its application would follow.

Laser Metal Deposition (LMD) process has primarily derived from the surface engineering process of laser cladding. A weld track on the substrate plate is formed by the deposition of metal powder by blowing it on the melt zone using an injection nozzle and its subsequent melting using a high intensity laser beam as illustrated in figure 5.

Figure 5: Schematic of Laser Metal Deposition setup

The process has the ability to work with most metallic alloys and also form multi-material components. Unlike other AM processes applying a 2 and 1/2 D processing methodology in a layer by layer fashion, the LMD process applies a real 3D fabrication strategy by depositing continuous track of material using conventional NC tool path. Further processing capabilities can be applied with a 5- or 6-axis CNC control systems. The application of LMD is less limited by build size as material deposition head and laser source can be integrated with a conventional CNC machine system or a robotic arm or even a customized axis motion system enabling it to achieve high deposition rates. The process is best applied for repair applications in the tooling, aerospace and power generation industry.

In a typical Laser Metal Deposition (LMD) machine system, the part is built by means ofmelting the surface of the substrate to form a melt pool and the simultaneously applying the metal powder via an injection nozzle. Argon or Helium is used as a shield gas to prevent oxidation of the melt pool. A Nd:YAG, diode or CO2 laser is used as an energy source and the metal powder is fed by a coaxial nozzle as shown in Figure 6. In order to form a fully dense 3D structure, adjacent weld tracks are overlapped with each other and underlying material is re-melted with each consequent material deposition.

Figure 6: Illustration of the interaction zone between Laser Beam and Metal powder in LBM process

The working principal of a Laser Metal Deposition system enables a high deposition rate and the fabrication of multi material components, but the fabricated structures have limited design flexibility in comparison to other AM processes. Intricate lattice structures, conformal cooling channels and complex overhangs may not be achievable using the LMD process.  It is therefore typically used for production and repairing and modification of tools and machine parts.

In recent years the Laser Beam Melting (LBM) process has evolved as one of the most versatile and industrially applicable direct metal Additive Manufacturing process. The LBM process fundamentally evolves from the additive manufacturing process of laser sintering of polymers and laser beam welding. In the Laser Beam melting process a high intensity laser beam selectively irradiates micro-sized metal powder in a layer by layer methodology to form fully dense 3D structures. Figure 7 illustrates the basic structure of a LBM machine system. The major components of a LBM machine system include a laser beam source, beam deflection and focussing optics, powder recoating mechanism, build platform and an inert gas recirculation mechanism. The term Laser Beam Melting (LBM) is often used as a generic process term, other industrially used synonyms include Direct Metal Laser Sintering (DMLS), Selective Laser Melting (SLM), LaserCusing, Direct Metal Laser Melting, etc. All these processes work fundamentally on the same principle, but may differ in their machine structure and operating methodologies.

Figure 7:Illustration of Laser Beam Machine system

Most LBM machine systems use a Ytterbium or Nd:YAG laser with powers ranging from 100 W to 1 KW. The optical design includes a set of defocusing lens, galvanometer scanner and F-theta lens to ensure minimum intensity variation on the powder bed. The recoating mechanism ensures the deposition of micro-sized metal powder in layers typically ranging from 20 – 100 µm. LBM process takes place in an over-pressured inert atmosphere typically of Nitrogen or Argon. The recirculating inert gas also ensures spatter removal during the irradiation process. Mechanical characteristics obtained from LBM processed structures are comparable and in some cases even better as compared to conventionally fabricated parts. This is a result of very high relative density and a consistent micro-structure achieved during the LBM process. Figure 8 indicates the cross-section of a sample fabricated from Aluminium alloy, EN AW 6061 using the LBM process.

Figure 8:Optical image for relative density analysis and microstructure for EN AW 6061

Most common material categories commercially available for Laser Beam Melting include Titanium alloys, Nickel super alloys, Aluminium cast alloys, Stainless steel and tool steel. Other materials offered in the industry for specific applications also include Precious metal such as Gold and Silver, Aluminium wrought alloys, high carbon tool steels etc. Constant research in development of new materials for Laser Beam Melting is being performed globally to increase the penetration of the technology in the manufacturing industry. One of the key challenges in this regard is the availability of metal powder suitable for processing in a LBM machine system. The granulometry and morphology of the metal powder is critical for determining its flowability. In addition it is essential that the powder manufacturing method ensures a minimum oxidation of the powder and maintains the chemical composition. It has been found that the atomization method using an inert gas is the most suitable method for the manufacture of metal powder suitable for ABM processing. However constant research is being performed to develop more economical and sustainable processes for powder manufacture. Figure 9 indicates the SEM image for atomized powders of LBM processing.

Figure 9: SEM image of metal powder for Stainless steel 316 L and Titanium alloy Ti6Al4V

Direct Metal additive Manufacturing has evolved into a significant complimentary manufacturing technology for industrial applications. With its ability to fabricate fully dense structure directly from digital data, it is being considered as a key manufacturing process in the Industry 4.0 ideology. Direct metal Additive Manufacturing is envisioned to play a key role in defining the future of manufacturing industry. The presented article aims to provide a technical overview on some of the most significant direct metal Am processes. In following article more in depth knowledge about the applications, challenges and future of Direct Metal Additive Manufacturing would be provided.

References:

  • DMLS – DEVELOPMENT HISTORY ANDSTATE OF THE ART, M. Shellabear, O. Nyrhilä; Presented at LANE 2004 conference, Erlangen, Germany, Sept. 21-24, 2004
  • Electron Beam Melting, LE Murr and SM Gaytan, The University of Texas at El Paso, El Paso, TX, USA; Comprehensive Materials Processing, Volume 10
  • Selective Laser Sintering/Melting, S Kumar, University of Michigan – Shanghai Jiao Tong University Joint Institute, Shanghai, China; Comprehensive Materials Processing, Volume 10
  • Laser Powder Deposition, R Vilar, UniversidadeTecnica de Lisboa, Lisboa, Portugal; Comprehensive Materials Processing, Volume 10
  • Additive manufacturing of metals, Dirk Herzog, Vanessa Seyda, Eric Wycisk, ClausEmmelmann, ActaMaterialia (2016) 1-22
  • Standard Terminology forAdditive Manufacturing Technologies, ASTM standard F2792 − 12a
  • Design for Additive Manufacturing: Trends, opportunities, considerations,and constraints, Mary Kathryn Thompson, Giovanni Moroni, Tom Vaneker, Georges Fadel, R. Ian Campbell, Ian Gibson, Alain Bernard, Joachim Schulz, Patricia Graf,Bhrigu Ahuja, Filomeno Martina, CIRP Annals – Manufacturing Technology 65 (2016) 737 – 760
  • A ROUND ROBIN STUDY FOR LASER BEAM MELTING IN METAL POWDER BED:COMPARING MECHANICAL CHARACTERISTICS WITH SYSTEM TECHNOLOGY VARIATION, B. Ahuja, A. Schaub, D. Junker, M. Karg, F. Tenner,R. Plettke, M. Merklein& M. Schmidt; South African Journal of Industrial Engineering August 2016 Vol 27(2), pp 30-42.
  • Arcam EAB system, Ti6Al4V Titanium alloy, http://www.arcam.com/wp-content/uploads/Arcam-Ti6Al4V-Titanium-Alloy.pdf
  • Arcam EAB system, ASTM F75 CoCr, http://www.arcam.com/wp-content/uploads/arc-024-cocr-mtrl-ds-v2.pdf
  • ADDITIVE MANUFACTURING OF INCONEL 718 USING ELECTRON BEAM MELTING: PROCESSING, POST-PROCESSING, & MECHANICAL PROPERTIES, DOCTOR OF PHILOSOPHYDissertation by WILLIAM JAMES SAMES, Submitted to the Office of Graduate and Professional Studies of Texas A&M University
  • Fabrication and Characterization of High Strength Al-Cu alloysProcessed Using Laser Beam Melting in Metal Powder Bed, Bhrigu Ahuja, Michael Karg, Konstantin Yu. Nagulin, Michael Schmidt, 8th International Conference on Laser Assisted Net Shape Engineering LANE 2014
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About the author

Bhrigu Ahuja

Bhrigu Ahuja

Bhrigu Ahuja had his first experience with Additive Manufacturing technology in 2005 during his Masters education in the UK. After finishing his education, he started his professional career with Materialise NV where he worked for 5 years at various locations across the globe. In 2012, he moved to Germany to work as a Scientific Researcher at the Lehrstuhl für Photonische Technologien, Friedrich-Alexander-Universität Erlangen-Nürnberg. There he worked on the Additive Manufacturing process of Laser Beam Melting in Metal powder bed specializing his research on material, design and process dynamics of the technology.

Bhrigu Ahuja is currently pursuing a PhD under the guidance of Prof. Michael Schmidt on the topic of Laser Beam Melting. Since 2016, Bhrigu has been working as a senior technology specialist at the Advanced Manufacturing Consultancy Group focussing on Additive Manufacturing technologies.