Metal 3D printing

Metal 3D printing (also called metal additive manufacturing) is a group of additive processes that build metal parts layer by layer directly from 3D data. The result is a fully metallic component whose properties, after proper heat treatment, approach those of wrought or cast material.

The most widespread process is laser powder bed fusion (L-PBF), also known as SLM (selective laser melting) and DMLS (direct metal laser sintering). A layer of metal powder, typically 0.02 to 0.06 mm thick, is selectively melted by a laser in a protective argon or nitrogen atmosphere. The part is fused to a build plate, and surfaces inclined at less than about 45° to the horizontal need supports, which also conduct heat away and limit distortion from residual stress. Part density is typically above 99%, accuracy on the order of ±0.1 to 0.2 mm and as-built surface roughness usually Ra 5 to 15 µm.

Other variants include binder jetting, in which the powder is bonded layer by layer with a binder and the part is sintered in a furnace afterward, and printing with metal-filled filament, where the part is likewise sintered once the binder has been removed. These parts shrink significantly during sintering, and the shrinkage is compensated in the data. For large parts and repairs, directed energy deposition (DED) melts powder or wire with a laser, an electron beam or an electric arc (WAAM, wire arc additive manufacturing), producing a near-net-shape blank that is then machined.

Printable materials include stainless steels (316L, 17-4 PH), maraging steel 1.2709 (18Ni300) for tooling, the aluminum alloy AlSi10Mg, the titanium alloy Ti6Al4V, nickel superalloys such as Inconel 718, cobalt-chrome and copper alloys. Printing is usually followed by stress relief annealing, removal from the build plate, support removal, hot isostatic pressing (HIP) for demanding applications and machining of the functional surfaces.

Metal 3D printing pays off where a shape cannot be machined or cast at all, or only at very high cost: curved internal channels (conformal cooling in injection molds, hydraulic manifolds, heat exchangers), lightweight parts from topology optimization, lattice structures, or consolidating an assembly of many parts into one piece. Typical users are aerospace, motorsport, medical (titanium implants and instruments) and toolmaking.

It also makes sense for one-off and low-volume production of complex parts and for replacement parts for which no mold or casting pattern exists anymore. Simple prismatic parts, by contrast, are almost always cheaper by CNC machining, because the cost of printing grows with part volume and build height and drops only slightly with quantity.

The most common mistake is printing a part designed for machining or casting without changes. For printing, you must resolve build orientation, supports and access for removing them, minimum wall thicknesses, powder removal from internal channels and distortion from residual stress. Design fits, sealing faces and threads with a machining allowance, usually a few tenths of a millimeter, and expect the surfaces that rested on supports to be rougher.

Agree with the supplier on the specific material and its standard designation, the heat treatment and the required mechanical properties, the inspection method (dimensional measurement, test coupons printed together with the parts, CT scanning for internal defects where needed) and the material certificates to be delivered. For cyclically loaded components, keep in mind that an unmachined as-built surface significantly reduces fatigue strength.