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Data courtesy of GKN Metallurgy
Metal Additive Manufacturing — a world of possibilities
Metal Additive Manufacturing (MAM) is a disruptive manufacturing technology that offers a whole world of possibilities. This is as true in terms of offering new levels of design freedom, as it is about the sheer choice of metal materials that can be used with metal 3D printers.
Take titanium as an example. Where conventional processes can be costly, Metal Additive Manufacturing can be an attractive technology for the processing of this advanced material, and others like it, such as alloys which can only be manufactured under high cooling rates.
Metal Additive Manufacturing opens a world of new applications and opportunities — one great example is in the transportation sector, where it can enable lightweight engineering and design techniques, which are becoming more important in efforts not only to reduce, but also to redistribute a vehicle’s mass, in order to save energy during use, reduce manufacturing costs or improve the working performance.
Metal Additive Manufacturing also allows for the cost-effective production of customized products in the medical and orthodontic sectors, that are personalized and “tuned” to a patient’s individual needs — and this level of flexibility also points to the potential for use within the consumer goods market.
Will Metal Additive Manufacturing become a mainstream manufacturing technology?
Metal Additive Manufacturing is being adopted by companies focused on innovation and creating new value. However, there are still some challenges that must be overcome — including education around applications, Design for Additive Manufacturing (DfAM), industry standards, regulations, certifications and metal materials quality — to accelerate its widespread adoption.
How does metal 3D printing work? Some MAM technology basics
An important starting point is understanding the main Metal Additive Manufacturing technologies and processes and how they work, in order to define which is most suitable for a specific application.
The choice of the right process and machine for your application wholly depends on various factors, including specifications, budget, and product lifecycle.
The most common Metal Additive Manufacturing processes
- Powder Bed or Powder Bed Fusion process
- Binder Jetting
- Direct Metal Deposition (DMD) or Direct Energy Deposition (DED)
- Metal Extrusion or Material Extrusion
Powder Bed Fusion process
One of the most common metal 3D printing methods is the Powder Bed or Powder Bed Fusion process.
The various types of Powder Bed Fusion typically use heat or light energy, in the form of a laser or electron beam, to fuse or melt metal powder material together - and involve spreading the material over previous layers. There are different mechanisms to spread the material layer, such as a roller or blade, while a hopper or a reservoir positioned below the bed will add a fresh material supply. Once the powder layer has been distributed, the fusing or melting process is then repeated slice by slice, layer by layer, until all of the layers are fused or melted together. The 3D printed part is then removed from the powder bed and processed to create the final piece.
As the name suggests, Powder Bed processes use metal materials in powder format, and for metal additive manufacturing, these include metals such as stainless steel, titanium, aluminium, cobalt chrome and copper, to name a few.
There are different types of Powder Bed Fusion, each of which use a slightly different process to form the 3D printed part—the most common techniques include:
- Direct Metal Laser Sintering (DMLS) or Selective Laser Melting (SLM)
- Electron Beam Melting (EBM)
Direct Metal Laser Sintering (DMLS) or Selective Laser Melting (SLM)
Direct Metal Laser Sintering (DMLS) — also known as Selective Laser Melting (SLM), uses a laser beam to partially melt (in the case of DMLS) or fully melt (in the case of SLM) metal powder and form the material into a solid part. The solid mass is formed by heating and applying pressure to the metal material. Layers are added with a roller before a platform lowers the model to the next layer.
These processes are performed in a closed inert environment that is often pressurized to eliminate the possibility of oxygen contamination. The inert gas might change based on what will react with the material being processed, while the build plate can be heated to minimize cooling rates.
DMLS and SLM both require support structures that are usually created using the same material as the main part being 3D printed — this needs to be factored into the total cost of material required to produce the part.
Electron Beam Melting (EBM)
Unlike other methods, Electron Beam Melting (EBM) requires a vacuum build chamber or environment in order to fuse metals and alloys to create a variety of functional metal parts. As the name suggests, it uses a high-energy electron beam, and can only be used with conductive materials. EBM involves a number of process parameters, meaning it typically requires more expertise and time to perform to an optimal level. EBM also requires support structures.
Binder Jetting
With Binder Jetting, an adhesive liquid binding agent is dispensed into a thin layer of powder material to build the metal part layer by layer. Unlike SLM and DMLS, no support structures are required for the Binder Jetting printing process. The layers bind together to form a solid object that is initially in a ‘green state’ also known as a ‘green part’. Green parts are in an interim state, with low mechanical properties that are very often weak and brittle, because the metal powder material particles are held together by the binding agent.
The next step in the Binder Jetting process is known as ‘Debinding’, which is where a certain amount of the binding agent is selectively removed from the green part before sintering can take place.
The final step to form the solid part is the sintering process, where the debound green part is sintered in a furnace or oven, to form a highly dense solid part. After this step, the final metal part is cooled and can be finished according to surface quality specifications.
How does HP Metal Jet technology work?
HP Metal Jet technology is one of the newest binder jetting metal 3D printing processes, and also does not require support structures during printing. Using HP Thermal Inkjet nozzles to precisely deliver HP Metal Jet binding agent to a powder metal bed, and industry-standard metal injection molding (MIM) metal powders, HP Metal Jet is a binder jet technology that allows for:
- Multiple parts produced at the same time, or large parts, with an effective build volume of 430 x 309 x 140 mm which meets MPIF standards for stainless steel with HP Metal Jet SS 316L and 17-4PH materials
- Parts to be arranged freely in multiple levels in the powder bed to optimize packing density, productivity and cost
- Elimination of build plate (required for other technologies such as Selective Laser Melting (SLM))
- 1200 x 1200 dpi addressability in a layer 35 to 140 microns thick
- Finished metal parts with isotropic properties, that can be difficult to achieve with other 3D printing processes
All about the HP Metal Jet process
The process of building a metal part with an HP Metal Jet 3D printer is described schematically below. A detachable build unit, containing the powder bed and powder supplies, is rolled into the HP Metal Jet printer for part production. Note that throughout the printing process, the powder bed may be heated – shown by the “Energy” element – to evaporate volatile select components of HP Binding Agent.
Schematic of HP Metal Jet 3D printing process
The key steps in the HP Metal Jet process are:
Spreading the powder
The build begins with a scanning recoater laying down a uniform, thin layer of metal powder across the working area. The recoater is refilled from supply bins of metal powder located at each end of the scan. This enables bi-directional recoating for increased productivity.
Applying print agents
HP printheads jet HP Metal Jet binding agent at precise locations onto the powder bed to define the geometry of single or multiple parts.
Evaporation
The liquid components of HP Metal Jet binding agent evaporate.
Retracting the bed, printing next layer
The powder bed is retracted to the thickness of the printed layer, and the process repeats until the build is completed.
Curing the bed
The powder bed with its printed parts is heated to complete the evaporation of liquid components from HP Metal Jet binding agent and to cure the polymers to achieve high strength in the green part(s).
Loose powder removal
The powder bed is now cooled, and metal parts can be removed. During this step, loose powder is removed from the surface of the parts, and remaining surplus powder can be processed and reused for economical consumables management.
Sintering
The green parts are now moved into a furnace. At sintering temperatures, atomic diffusion at the surfaces of the metal particles binds them in a matrix that can exceed 96% solid density (depending on the type of material used). The polymer from the HP Metal Jet binding agent decomposes.
Finishing
The parts may now undergo post-processing to meet dimensional and surface finish requirements.
HP’s Metal Jet whitepaper describes the magic behind this technology in more detail and our ‘What can you make with a 3D printer?' article showcases some examples of innovative applications enabled thanks to HP Metal Jet technology.
Direct Metal Deposition (DMD) or Direct Energy Deposition (DED)
The DMD process involves melting metal material — typically cobalt, titanium, or chrome — as it is deposited, in the form of powders or wire.
DMD consists of a multi-axis robotic arm with a nozzle that deposits metal powder or wire onto a surface. There are two key technologies in this category:
- Laser Engineered Net Shaping (LENS)
- Electron Beam Additive Manufacturing (EBAM)
The core difference between the two technologies is the heat source used to melt the material and form the solid part. LENS uses a laser head, while EBAM uses an electron beam. With DMD the nozzle is not fixed to a specific axis and can move in multiple directions. This means that the material can be deposited from any angle and is melted upon deposition.
Metal Extrusion
Similar to the Material Extrusion or Fused Deposition Modeling (FDM) and Fused Filament Fabrication (FFF) processes that use plastic materials, metal extrusion is a relatively new metal additive manufacturing method where a filament is heated, squeezed through a nozzle, and deposited onto a build plate to form an object layer by layer.
The filament is typically made up of metal particles embedded in a resin or thermoplastic. The nozzle moves in the x and y axes across the build platform, forming the part and the build platform lowers in preparation for the next layer.
Once fully formed, the part requires sintering in a furnace to remove any remaining plastic and sinter the metal particles together. Support structures are required for both the 3D printing and the sintering steps.
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