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What is Powder Bed Fusion? Process Definition and Advantages

   

Powder bed fusion (PBF) is an additive manufacturing process and works on the same basic principle in that parts are formed through adding material rather than subtracting it through conventional forming operations such as milling. The PBF process begins with the creation of a 3D CAD model, which is numerically 'sliced' into several discrete layers. For each layer, a heat source scan path is calculated which defines both the boundary contour and some form of fill sequence, often a raster pattern since the heat source is typically an energy beam (e.g. a laser).

powder-bed-fusion-diagram

Each layer is then sequentially bonded on top of each other. PBF processes spread powdered material over the previously joined layer, ready for processing of the next layer hence the manufacturing is discrete rather than continuous (though each layer is fully consolidated to adjacent layers). A hopper supplies the powdered material which is then spread uniformly over the powder bed build platform area via a roller or blade. The optimal thickness of each layer of spread powder is dependent on the processing conditions and material used, but values of 25 to 100µm are common.

What are the Different Types of Powder Bed Fusion?

There are several variants of PBF, which are designated by the heat source used and the type of material joined. The two dominant types are laser beam (PBF-LB) and electron beam (PBF-EB) and have trademarked technologies under each. Each variant offers advantages and disadvantages, so suitability should be weighed on an application by application basis. Examples of are listed below:

Selective Laser Sintering (SLS)

SLS is a trademarked term and is PBF-LB. The process typically sinters powdered polymer materials such as nylon and polyetherketoneketone (PEKK).

Selective Laser Melting (SLM)

SLM is a trademarked term and comparable to SLS in that a laser is used to provide heat (therefore falling under PBF-LB), however the laser fully melts the powder rather than sintering it. The process is applied to metal powders such as: aluminium alloys; titanium and its alloys; and stainless steel. More exotic metals (e.g., tungsten) can be processed but tend to be more application based. An inert atmosphere (typically argon) is included in the build chamber to prevent oxidation and/or nitriding of the consolidated material.

Direct Metal Laser Sintering (DMLS)

DMLS is a trademark of EOS GmbH, a German additive manufacturing company and is similar in operation to SLM. Despite the term ’sintering’ being used, full melting is achieved.  TWI have the distinction of being the first UK entity to have a certified process for manufacturing via this technology.

Electron Beam Melting (EBM)

EBM is a comparable process to SLM, replacing the laser with an electron gun (hence a PBF-EB process). Owing to the use of an electron beam, the build chamber utilises a vacuum instead of an inert atmosphere, though a small amount of inert gas (typically helium) is used to allow better process control.

Post Processing

Post processing of PBF parts is commonly required to better enable them in their intended applications: this is particularly true for metals and alloys. This might occur for the following reasons:

  • Improve mechanical properties (via heat treatment)
  • Reduce residual stress (via heat treatment)
  • Improve surface finish (via chemical or laser polishing, and/or abrasive grit blasting)

What are the Advantages of Powder Bed Fusion?

Power bed fusion advantages include:

  • Reduced material wastage and cost (superior buy-to-fly ratio)
  • Improved production development times
  • Enablement of rapid prototyping and low volume production
  • Capable of building functionally graded parts
  • Fully customised parts on a batch by batch basis, eliminating fixed designs
  • Good resolution when compared to other additive manufacturing processes
  • Efficient recycling of un-melted powder
  • Ability to join many material grades, including ceramics, glass, plastics, metals and alloys
  • Elimination of the need for machining fixtures

What are the Applications of PBF?

PBF processes are used across a wide range of industrial sectors for numerous applications. For example, the process is implemented by the medical sector for making customised orthopaedic components, such as titanium alloy cranial or acetabular implants.

From an aerospace perspective, PBF processes are finding much interest and use for military and commercial aircraft. Examples of this include the PBF manufactured fuel nozzle on General Electric’s GE9X engine, which is used on Boeing 777 aircraft. The GE9X is the largest turbo-fan engine produced and the additively manufactured nozzle is five times more durable than previous versions [1]; the Boeing 777, with its two GE9X engines, includes 300 additively manufactured parts [2].

Swedish automotive manufacturer Koenigsegg has implemented PBF techniques throughout the manufacturing process of its latest hypercar, the ‘One:1’, from using rapid prototyping to ensure various details of the car looked and worked as envisioned, to implementing it to manufacture metal parts for production vehicles. Production parts include turbocharger housings, exhaust components, air ducts and interior mirrors [3]. Additive manufacturing processes enabled a reduction in material waste and costs for Koenigsegg. For lower production runs (as is common with high-end cars), building complex parts by additive manufacturing is cheaper, quicker and more efficient than building the necessary tooling for production of certain complex parts, which is common in the automotive industry [3].

How Can TWI Help?

TWI has a long history of working with its Members, and on collaborative projects, across a range of industry sectors, to assist in additive manufacturing: this is supported by employees who have considerable collective experience working in several areas of utilising the various technologies in wide ranging applications.

Please contact us to learn more:

contactus@twi.co.uk

For more information please email:


contactus@twi.co.uk