A subset of additive manufacturing, powder bed fusion (PBF) is a 3D-printing technology that utilizes a high-energy power source to fuse powder material in order to construct a three-dimensional object. The heat source is directly applied to specific areas of a powder bed, causing particles to coalesce. As one layer of material is formed, more powder is spread over, and the process starts again to build an object layer-by-layer. The built layers solidify upon cooling.
Several PBF techniques are used in the industry today, and they all share the same core principle of additive manufacturing, i.e., layer-by-layer production based on a digital 3D model. They also provide the common benefits of 3D printing, such as customization, cost-effectiveness, reduced assembly, and reduced material usage.
One vital advantage that polymer PBF processes, in particular, hold over other 3D printing processes is that of not requiring support structures. Since sections like overhangs are held in position by the unfused powder material surrounding them, there is no need for any support structure to hold such parts. This allows for more complexity in production as no support structures need to be removed upon completion.
These advantages have enabled PBF to be widely used in commercial applications, especially in highly demanding industries, such as the aerospace and medical industries. These industries offer opportunities to produce custom products with complex geometries in a small volume. Such opportunities are ideal for PBF processes as they can help produce high-value products with high levels of complexity that are not technically achievable with traditional manufacturing.
The powder bed fusion market has been growing quite rapidly in recent years and is expected to reach the multi-billion USD mark with a CAGR of 13.5% between 2018 and 2030, after hitting the USD 1.02 billion mark back in 2018. This growth follows the increasing demand for PBF technologies in major industries such as aerospace, medicine, automotive, and oil & gas.
Powder bed fusion (PBF): Step-by-step
There are five distinct steps to a PBF process, defined as follows:
- A layer of powder material (typically around 0.1 mm thick) is spread over the bed.
- A heat source (laser, electron beam) hits the powder and fuses the first layer of the model based on a predesigned digital 3D model.
- The powder bed is lowered, and a new layer of powder material is spread over the previous layer by means of a roller.
- More and more layers are fused and added over previous layers.
- The process continues this loop until the whole model is manufactured. The unfused powder is then removed during the post-processing stages.
Powder bed fusion (PBF) methods
There are several PBF technologies on the market today, and they differ based on the way they induce fusion and the material they are applied to.
The four main PBF methods are:
- Selective Laser Sintering (SLS)
- Selective Laser Melting (SLM)
- Electron Beam Melting (EBM)
- Multi-Jet Fusion (MJF)
What is Selective Laser Sintering (SLS)?
Selective laser sintering (SLS) is a powder-based AM method that applies a laser as a high-energy heat source onto a bed of powder material, atomically fusing targeted particles at a temperature slightly below their melting point (~85% of the melting point) – otherwise known as sintering, which is providing just enough energy for atoms of a certain particle to diffuse across the boundaries of another particle. This results in a specifically-shaped, high-density layer of fused powder that solidifies upon cooling. The process continues as more powder is spread across evenly to build further layers.
The SLS method can be applied to different types of powder material, including polymers and metals. It also requires no additional support structures to hold the part during manufacturing.
It is commonly used for rapid prototyping and fully-functional, end-use parts. Similar to other AM methods, it follows a computer-aided design (CAD) model that defines the shape and positioning of the layers. The quality or resolution of the end product is based on several parameters, including:
- Laser power
- Scan speed
- Scan spacing
- Nature of the powder
- Particle size
What is Selective Laser Melting (SLM)?
Selective laser melting (SLM) is another powder-based AM method that utilizes a high-energy laser as a heat source. Unlike SLS, the laser in the SLM method is used to heat metal powders above their melting point, so they completely melt and fuse in order to form near-net-shape objects with high relative density. It also follows the layer-by-layer manufacturing process following a CAD model, where every time a layer is completed, the bed moves down, and a new amount of powder is spread evenly to create the next layer.
SLM is exclusively used for powders of metal or metal alloys. These include:
- Steel and iron-based alloys (e.g. maraging steel 300)
- Nickel-based alloys (e.g. Inconel 625)
- Titanium-based alloys (e.g. Ti64-GS)
- Aluminium alloys (e.g. AlSi10Mg)
- Alumina
- Silicon carbide
- Yttria stabilized zirconia
Other names for SLM include laser powder bed fusion (LPBF) and direct metal laser melting (DMLM). It is also often used for rapid prototyping or end-use products, such as biomedical device components not technically feasible to produce with other conventional methods. For example, SLM enables the precise manufacture of complex porous scaffolds and parts with intended porosities. Other applications are found in the aerospace, automotive, construction, and jewellery industries.
What is Electron Beam Melting (EBM)?
Electron beam melting (EBM), also known as selective electron beam melting (SEBM), is yet another powder-based AM method. This one, however, uses a high-energy electron beam as a thermal energy source to fuse the powder. It is fairly comparable to SLM in its working principle, but it does have its specifics, with the heat source being a major difference.
The electron beam in an EBM process is basically generated by a so-called electron gun. Under vacuum conditions, the electron gun extracts electrons from a filament (usually made of tungsten) with a voltage range of 30-60 kV and discharges them at approximately half the speed of light towards a layer of metal powder that has been predeposited on the build plate.
The electron beam is directed with the help of two magnetic fields: one acting as a magnetic lens to narrow down the beam’s diameter and the other to deflect the beam towards the desired target points on the powder bed.
The electron bombardment instantly converts the electrons’ high kinetic energy into thermal energy, which raises the powder’s temperature beyond its melting point and enables the selective fusion of particles in the powder bed. This continues until the part is constructed in a layer-by-layer fashion (similar to SLM). Here, high-vacuum conditions are crucial to circumvent issues like oxidation, contamination, and atmospheric interference.
EBM is capable of processing highly reactive metals and high-melting-point metals and can provide exotic mechanical properties as a result. The most commonly used materials in EBM are titanium and chromium-cobalt alloys. Titanium alloys, for example, are of particular interest due to their biocompatibility, lightweight, and high mechanical strength. That is why it is ideal for applications that include spacecraft and biomedical implants. Turbine blades and engine parts are some of EBM’s most common applications.
EBM has proven to have a much higher manufacturing speed than SLM but tends to have lower accuracy and finish quality.
What is Multi-Jet Fusion (MJF)?
Multi-jet fusion (MJF) is a rather unique type of powder-based AM method that differs quite significantly from SLS, SLM, and EBM. Its working principle is not based on a high-energy power source but instead on the use of fusing agents.
In MJF, a liquid fusing agent is injected at particular regions on the powder bed using an array of very fine inkjet nozzles. This agent penetrates into the interstitial spaces between the particles, which are then heated up in order to melt. The fusing agent’s main role is to facilitate the particles’ absorption of the heating infrared energy.
If high-resolution parts are required, so-called detailing agents are further jetted around the outlines, which help enhance the finish.
Furthermore, upon completing the MJF process, the part would still require treatment under ultraviolet light to complete the process of curing and sintering, where all the bonds induced by the fusing agent are strengthened.
The process steps can be defined as follows:
- A layer of powder material is spread over the bed.
- An inkjet head with multiple nozzles pans across the powder bed and diffuses both the fusion agent and the detailing agent.
- The powder bed is heated. The areas over which the fusing agent has been deposited melt together to form a shape, while the areas with the detailing agent remain as unfused powder.
- The powder bed is lowered, and a new layer of powder material is spread over the previous layer by means of a roller.
- More and more layers are fused and added over previous layers.
- The process continues this loop until the whole model is manufactured. The whole powder bed is then subjected to UV radiation for curing and sintering in order to strengthen the built part. The unused powder is then vacuumed up to be reused.
MJF is generally applied to polymer powder, most commonly polyamide nylon (PA11, PA12), polypropylene (PP), and thermoplastic polyurethane (TPU). Its manufactured parts tend to have relatively high density and low porosity (compared to SLS). The resulting surface quality is quite smooth, and the manufacturing speed is relatively high. It is still considered in its infancy compared to other powder-based 3D-printing methods, but it is quite suitable for rapid prototyping applications and small series of end-use parts.
SLS vs SLM vs EBM vs MJF Comparison Table
|
PBF Process |
SLS |
SLM |
EBM |
MJF |
|
Fusion source |
Laser |
Laser |
Electron beam |
Fusing agent |
|
Compatible Materials |
Polymers, metals |
Metals |
Metals (limited) |
Polymers (limited) |
|
Printing resolution |
20-150 μm |
50-150 μm |
~ 80 μm |
|
|
Pros |
No need for support structures |
No need for support structures |
No need for support structures |
No need for support structures |
|
High speed |
High accuracy |
High speed |
High speed (short lead time) |
|
|
Excellent layer adhesion |
Good functionality |
No oxidation and no residual stress |
Good surface quality, low porosity |
|
|
Great for post-printing |
Doesn’t need a lot of post-processing |
Preheating limits deformations |
Prints directly in color |
|
|
Recyclability of unused powder (recommended amount: 50% |
Recyclability of unused powder (recommended amount: 50% |
Minimum material waste |
Recyclability of unused powder (recommended amount: 50% |
|
|
Cons |
Poor structural integrity (brittle and porous material) |
High surface roughness and residual stress |
Relatively low resolution |
Limited material options |
|
Prone to shrinkage and warping |
Anisotropic properties |
Limited commercial options |
Slightly rough surface finish |
|
|
Requires a lot of post-process cleaning |
Requires a supply of inert gas |
Highly energy-intensive |
Only single-color printing is possible |
|
|
Produces relatively a lot of waste |
Accuracy requires a longer process time |
Relatively small size of manufactured part |
Difficulty in producing curved, hollow shapes |
|
|
Expensive |
Expensive |
Highly expensive |
Highly expensive |