3D Printing in Metal: The Heavier Side of 3D Printing
Metal 3D printing lies on the far opposite side of the 3D printing spectrum of the MakerBot and its brightly colored tchotchkes. All 3D printing metal processes are expensive in equipment, labor and energy expenditure. Because of this, they have been adopted primarily in industries that generate high cost products. These include medical devices, aerospace, automotive and even fine jewelry. The additional effort between plastic and metal printing is due to two key features: the melting temperature of the material, and how the material changes when heated and cooled. The graph below illustrates the intense temperatures required to melt metal versus the common 3D printed plastics. In addition to the higher total energy need to melt metal, heated metal reacts with the atmosphere around it and can be stressed by the rapid heating and cooling.
3D Metal Printing processes include variations of the partially melted gummi bears (sintering) and layer cake (LOM), as well as a few options unique to the material.
There are two version of sintering with metal: Selective Laser Sintering (SLS) and Direct Metal Laser Sintering (DMLS). In SLS, metal particles are intermixed with polymer powder. A scanning laser melts the polymer, binding the particles into a part. This part is them put into a 900-degree furnace, burning out the plastic and fusing the metal particles together. This leave a porous part, which is infiltrated with molten bronze to create a solid part. This final component is about 70% of the original metal. By contrast, DMLS uses a very high-power laser that directly fuses the metal particles together. This process produces parts that are 95% solid metal, without multiple operations.
Laser Metal Deposition is a variation of the sintering process. Rather than starting with a flat field of metal particles, a fine stream of powder is shot directly into the tiny molten pool of metal created by a high powered laser. Because this system does not require a powder bed, the part can be built on top of existing metal parts. This feature makes it an excellent process for repairing worn tooling or re-tipping of turbine blades, in addition to creating whole new parts.
In addition to laser-based systems, there are e-bream (electron beam) based systems for 3D printing metal parts. EBM (Electron Beam Melting) uses a high-power electron beam to trace a part in a bed of metal powder kept in a high temperature vacuum. While the careful atmospheric conditions add to the significant cost of this process, EBM can produce highly detailed parts out of very pure material.
If EBM is the advanced version of laser sintering, then EBAM (Electron Beam Additive Manufacturing) is the e-beam version of Laser Metal Deposition. Like LMD, the raw material is fed into a pool of molten metal directly in front of the energy source. EBAM tends to use wire feedstock rather than powder to produce each layer, and can produce parts nearly 20 feet in diameter. Additionally, two different metal wired can be fed simultaneously into the melt pool to create a point-by-point custom alloy, allowing for a “graded” material across a part.
While additive methods for creating metal parts lower expenses by eliminating waste material and tooling setups, much of this savings is devoured by the high energy and equipment costs. As such, when printing in metal, the value is almost purely in creating geometry or material combinations that can not be achieved with machining, molding or forging. If you can make a part another way, you should do so. If you can’t, 3D metal printing may be an option.