Why Metal Additive Manufacturing?

Traditional Metal Manufacturing relies on melting large amounts of metal and pouring that liquid metal into a shape (casting) or forcing it into a shape (forging). Typically, these take on the general shape of a part (engine block, wrench, etc.) or are in the form of metal bars and blocks (such as gold blocks).

Subsequent manufacturing steps occur to shape those cast or forged parts into thinner sheets, smaller diameters, punched out pieces, or to cut off surfaces of metal to create the final, precise shapes required (forged wheels, laptop bodies, screws, etc.).

Traditional processes are great for creating many of the parts we see around us; however, they struggle to create complicated geometries and oftentimes a lot of material is "wasted" (turned into scrap) in later manufacturing steps (milling, punching, turning, etc.).

Traditional processes sometimes rely on metal powders (such as tungsten metal powder) to create the molds needed for casting, but primarily, metal powders are useful in Additive Manufacturing.

 

A typical plastic 3D printer utilizes FDM (Fused Deposition Modeling). In that process, a plastic wire is pulled into a heating element, which melts the plastic and forces it out of the nozzle. However, metals typically have quite high melting points, so this is not easily possible.

Instead, metal powder is typically used to create these Additively Manufactured metal components. There exist several different technologies or methods for creating parts out of metal powder, but they typically involve:                                                                                                             

• Pressing powder together,
• Melting powder into a part using a laser or electron beam, or
• Gluing powder together and using a furnace to sinter the powder into a solid part

Likewise, there are several different technologies utilizing these three approaches:

• Binder Jetting
• Powder Bed Fusion (Laser or Electron-Beam)
• Direct Energy Deposition
• Lithography Metal Manufacturing
• Metal Injection Molding
• Press and Sintering

 

A few of these powder metallurgy processes are termed "Additive Manufacturing" because they "build up" a part rather than "subtracting off" as is typical in traditional manufacturing.

The Metal AM lab under Dr. Fang has 4 different additive manufacturing technologies described below.

"Gluing and Sintering"

Binder Jet Printing works to "glue" or bind particles together, cure the parts so the binder "glue" can hold the part together, take the cured parts out of the powder box, and sinter the particles together at high temperatures.

A thin sheet of powder, "powder layer," is sprayed with binder in the same way that paper is sprayed with Ink in a conventional paper printer. After that layer has been sprayed with binder, the build plate moves down, and a new, thin layer of powder is laid on top. This process of laying powder, spraying, and moving the build plate down is repeated for up to thousands of layers to cover the full height of a part.

After printing, all of the powder "entire build volume" is placed in a furnace up to around 500°C. This allows the binder to fully cure/polymerize to give the glue enough strength to hold the powder together under small amounts of stress.

After curing, the parts are held together by the binder, but are still fragile. Much like how dinosaur bones are dug up, the parts are delicately brushed out of the powder bed. Any extra powder on the surface of the parts is also brushed off. These are referred to as "green parts" now that the binder is fully cured.

The last step requires the cured parts to become fully solidified metal. The parts at this point can be imagined to be like plastic balls in a ball pit, simply being held together by glue. The plastic balls might be held together by glue, but there is still a lot of empty space between the balls. The metal part has to shrink together to close almost all of the empty space between the small spherical powder particles. This is referred to as "shrinkage" and usually a part might have to shrink enough to lose 30-40% of its "empty" volume between particles! This step is referred to as "sintering" and usually requires one temperature that can evaporate the binder before a second temperature is reached in order to sinter or partially fuse the metal powder together. This high temperature is usually a few hundred degrees below the melting point of the metal.

The lab also employs its HSPT method during sintering to achieve wrought-like microstructure in its BinderJet components.

"Laser Melting"

Laser Powder Bed Fusion Printing (LPBF) uses a high-power laser to melt small regions of metal powder together. A thin layer of metal powder (30-60 microns) is spread across the top surface of the powder-bed. Subsequently, a laser melts together metal powder. After that layer has been fused, the entire powder bed is moved down 30-60 microns to allow the next layer of powder to be spread, and the process repeats. In the same way that a FDM printer nozzle  (a typical plastic printer) will follow a set of lines to create a plastic layer, the laser will follow a series of small straight lines to melt together the desired cross-section. 

"Spray and Melt Powder Simultaneously"

Direct Energy Deposition works by spraying power at the part and melting in air right about the surface that is being "built up". Typically, powder flows through a cone, which causes all the powder to spray toward a single point. For L-DED (Laser Direct Energy Deposition), a laser points straight down right through the point where the powder comes together, which causes all the powder to melt "in air." Luckily, this is very well controlled, so the melted metal falls straight down onto the previously-built part, allowing for the process to be slowly built up, layer by layer.

"Waxy Metal Curing"

Lithography Metal Printing works by utilizing a mixture of waxy resin and metal powder. At room temperature, this waxy block (kind of like butter) is solid; however, by applying a moderately high temperature (~60°C), the block is melted. Thus, the wax-powder block is moved up, a hot blade cuts off the top layer of wax-powder and melts it, and this is then spread across the top layer of the build, where cooling fans resolidify the wax.

Now, how does this build a part? The wax is cured by UV light. Thus, once the top layer has resolidified, a high-precision UV projector cures the top layer of the part. This repeats layer by layer to produce your part.  When the print is finished, a "cured" part will be floating within the solidified wax, kind of like ancient bugs sitting within tree sap. The cured wax won't melt, but the other wax will, so you can simply heat up the block to (~60°C) to melt off any wax and powder that are not part of your part. The part then has to be sintered to ensure that the metal powder will bond and create a fully dense part.

This process is actually quite similar to Binder Jet Printing in that both rely on a binder (glue) to hold the powder together before it is eventually sintered into a dense, metal part.