additive manufacturing
Additive manufacturing, popularly 3D printing, builds a part up layer by layer from a digital model, adding material only where the design says it should be. That is the opposite of machining, which starts with a block and carves material away, and of molding, which needs a shaped mold. Because it just stacks thin layers, a printer can make hollow, latticed, and internally complex shapes in one go that no drill or mold could produce, with no part-specific tooling at all.
There are many methods. Fused deposition (FDM) squeezes out a molten polymer filament like a hot glue gun, tracing each layer. Stereolithography (SLA) cures a liquid resin with light. Powder-bed fusion, such as selective laser melting, spreads a thin layer of metal powder and melts a cross-section with a laser or electron beam, then repeats. Metal AM is essentially welding a part into existence one tiny track at a time, which means the metal melts and freezes extremely fast, over and over.
That repeated rapid melting stamps a strong microstructural fingerprint, and this is where the honesty matters. Metal-printed parts tend to have columnar grains aligned along the build direction, so they are anisotropic and often weaker between layers than within a layer. They carry high residual stress from the steep heating and cooling, can contain porosity and lack-of-fusion defects, and have rough surfaces, so they usually need stress-relief, hot isostatic pressing, and machining before use. Additive manufacturing is powerful for complex, low-volume, and lightweighted parts, but qualifying a printed part is genuinely hard precisely because the process so strongly shapes the structure.
A titanium bracket printed by laser powder-bed fusion melts one thin track at a time, freezing into columnar grains along the build height; it comes out with residual stress and some porosity, so it is stress-relieved, hot isostatic pressed, and machined before flying.
Printing metal is welding a part into being track by track, so it inherits weld-like anisotropy, residual stress, and porosity.
Printed metal parts are usually anisotropic (weaker between layers) and carry residual stress and porosity from rapid solidification, so they typically need post-processing; a printed part is not automatically as good as a wrought one.