2.3 Powder Bed Processes

Powder Bed Fusion Laser Metal System Architecture

For metallic powder bed systems, the basic machine architecture consists of the following:

• Powder Distribution System
  • Powder Feed Tank or Hopper
  • Roller, Rake or Knife
• Heat Source and Control System
  • Laser with Optics
  • Electron Beam with Magnetic Coil
• Build Platform and Motion System
• Build Plate
• Gas System or Vacuum Chamber
• Computer / HMI

Figure 2.5 Basic architecture of a metal powder bed fusion machine

Recoating systems

The recoating system is a critical aspect of powder-based AM processes. It ensures that the layer is uniform and even, and it controls the thickness of the layer. Recoating, then, influences the structure, shape, and function of the part. Common recoating systems are described below.

• Roller - The roller system, offered in a few powder bed fusion laser metal systems, affords the ability for consistent powder spreading without risk of catching on previously melted features that may have heaved out of the powder during fabrication. In some systems the roller additionally provides a downforce that compacts the powder bed, which is useful in materials systems that have some degree of compressibility.
• Hard knife blade – While not as forgiving as the roller, this system is found in most powder bed fusion laser metal systems and electron beam melting systems and is made of a blade that spreads the powder evenly. This offers superior powder spreading consistency over the roller method. Blades can be made of several different materials from hardened steels, to erosion resistant cobalt alloys such as stellite to ceramics.
• Soft gasket wiper blade - Offered as an option on many powder bed fusion laser metal systems. This type of system acts as a hybrid approach to powder spreading by allowing for superior powder spreading, while at the same time, remaining flexible enough to accommodate the occasional feature heaving from the powder bed during fabrication.
• Dosing wheel systems – This system is designed to dose, or drop a measured amount of feedstock material over the bed as the recoating system traverses the print bed. Dosing is coordinated with the motion of the recoating mechanism so each area of the bed receives a uniform dose. Dosing wheel systems are often followed by a soft gasket-style wiper blade to ensure uniformity. As designed today, dosing wheel systems can require feedstocks with high flowability requirements than the other recoating system styles.

Energy sources

• Laser: The Yb-fiber laser is by far the most common laser found in commercial metal powder bed fusion machines. Depending on the relative scale of the machine, this laser can be specified from 100W to 1000W. Other lasers for specific applications, like green for printing copper are available.
• Electron Beam: The electron beam gun is used in powder bed electron beam machines. This gun is capable of over 3000W of electron energy for metallic fusion, and consists of a filament used to generate electrons, and a strong magnet used to accelerate them towards the build layer.

Material Feedstock

Typically, the powdered material used to date for both laser and EBM systems are different in terms of the powder particle size distribution range. Most Laser Powder Bed Fusion users specify finer PSDs, such as 15-45 microns, whereas EBM powder lots are typically specified at larger PSD ranges, such as 45-105 microns. This is because the EBM process runs at an elevated temperature, and fine powders have a higher driving force for sintering which results in powder cake that is difficult to exhume parts from.

Common materials used in metal powder bed fusion laser machines include Aluminum, Tool Steel, Cobalt Chrome, Copper, Nickel Alloys, Tungsten, Stainless Steel, and Titanium.

Common materials used in metal powder bed electron beam machines include Titanium, Cobalt Chrome, Niobium, Pure Copper, and Nickel Alloys.

Selective Laser Sintering – Polymer System Architecture

For Selective Laser Sintering – Polymer Systems, the basic machine architecture consists of the following:

• Powder Distribution System
  • Powder Feed Tank or Hopper
  • Roller or Hard Blade
• Heat Source and Control System
  • Laser with Optics
• Build Platform and Motion System
• Build Plate
• Gas System
• Computer / HMI

Binder Jet Technology System Architecture

ASTM defines binder jet technology as an additive manufacturing process in which liquid bonding catalyst is selectively deposited to join powder materials. Once joined, the parts are fired in a sintering oven to melt out of the bonding catalyst and form a dense part. For binder jet machines, which range from machine size from 100mm3 to 10m3. The basic machine architecture consists of the following:

• Powder Distribution System – Powder Feed and Leveling Roller
• Binder Printhead
• Build Platform and Motion System
• Build Plate
• Computer / HMI

Recoating system

• Roller - The roller system, typically used in direct manufacturing systems, affords the ability for consistent powder spreading from the feed bin.

Blade – Blade based systems are common in the sand and aggregate printing used in the foundry industry. These systems typically consist of a moving hopper containing the aggregate. The hopper deposits the aggregate on the powder bed, and connected hard recoater blade that ensures good layer uniformity. Vibration and/or rotary distribution systems are used to uniformly fill the moving hopper.

One of the biggest advantages to using binder jet technology is that no sacrificial supports need to be produced with the parts. The powder itself self supports part made from the process.

Energy source

The energy source used for binder jet printing is the ink jetting process itself. The ink jet deposition from the print head flows droplets of a catalyst material that acts like a glue to ‘bind’ the catalyst material to the raw material loaded in the powder bed.

Material Feedstock

As the binder jet process requires no heated physical transformation, a number of different materials are available for printing. The following list of materials are commercially available for binder jetting:

• Ceramics
  • Silicon Carbide
  • Alumina
  • Zirconia
  • Boron Carbide
  • Silica
  • Sand

• Metals
  • Titanium
  • Tool steel
  • Stainless steel
  • Inconel
  • Tungsten
  • Tungsten Carbide

Curing

In many binder jet processes, the printed part contains binder that is partially cured during the print process. This is known as the brown stage. In order to develop sufficient binder strength to be able to handle the parts, the entire build box is thermally cured in an oven at moderate temperature (between 100 and 300 C). After curing the parts are in the green stage, and can be removed from the build box.

De-powdering

De-powdering is the act of removing unfused powder from fused powder and is typically accomplished by vacuuming of the unbound material out of the print bed, followed by layerwise removal of the printed objects.

Debinding, Infiltration and Sintering

For direct manufacturing operations, after printing is complete, the parts are placed in a high temperature furnace for either the process of infiltration or sintering. Because the binder content is typically less that 5% by weight, a separate de-binding step is rarely used in BJP processes. Once parts are in the furnace, the binder is melted out burned or vaporized from the part early in the thermal cycle, and separated from the raw material powder, leaving voided areas. This is what is known as debinding. At this point in the process, the part exhibits between 50%-75% porosity, depending on the density of the powder bed, and initial binder content.

• Infiltration - In infiltration another material is introduced, such as bronze, that wicks into the part void areas, resulting in lower porosity. The infiltrated material has typical mechanical properties between the base material and the infiltrate so is used for prototyping, art pieces and consumer products like jewelry.
• Sintering - Alternatively, the part is placed in an even higher temperature furnace directly after de-powdering to go through a sintering process. During early stages in this process, the raw material powder is fused together, and as temperature increases begins the process of densification. Densification can be thought of as a process whereby the individual powder particle centers move towards each other, resulting in a shrinkage of the part. As the part densifies, the porosity present after printing curing and de-binding is eliminated, resulting in a solid piece of the printed material.

During these secondary processes, it is important to note that significant shrinkage of the part will occur. Shrinkage may be as much as 20%-30% by volume if sintering process is used, whereas if the infiltration process is used, shrinkage may only be 1- 3% of the part volume.

Hewlett Packard has commercialized a binder jet technology known as Multi Jet Fusion (MJF). MJF is similar to traditional binder jet processes, however, with two notable differences. 1) the materials used in MJF are nylon polymers instead of metal and ceramic 2) the parts are heated at each layer of fusion, therefore eliminating any post-processing step.

Like all binder jet processes, MJF affords the ability to rapidly produce a large quantity of plastic parts, nested in all three dimensions within the build chamber, without the need for sacrificial support structures. As such, this technology is meant to compete more with the injection molding industry, but with the notable difference of being able to print complex parts that may be difficult to injection mold.