Metal 3D Printing: DMLS, SLM, Materials & Tolerances

Metal additive manufacturing, mainly DMLS (direct metal laser sintering) and DMLM or SLM (selective laser melting), fuses metal powder layer by layer with a laser inside an inert chamber. Its value is geometric, not economic: it builds shapes that machining and casting cannot reach, such as internal cooling channels, lightweight lattices, and consolidated assemblies, usually in aluminum, stainless steel, or titanium.

DMLS, DMLM, and SLM are near-synonyms for laser-based metal powder-bed fusion. EOS pioneered the DMLS process, which sinters the powder, while SLM fully melts it, but the equipment, materials, and design rules are effectively the same, and the finished parts are interchangeable for most engineering work. Metal additive is an advanced, high-cost process, and the parts almost always need stress relief, support removal, and machining of critical faces, which is why it is reserved for geometry that earns its cost.

How metal additive works

A metal additive machine spreads a thin layer of metal powder, typically 20 to 80 micrometers thick, across a build plate inside a chamber filled with inert gas, usually argon or nitrogen. A high-power laser traces the cross-section of the part and fully melts or sinters the powder where the part exists, bonding it to the layer below and to the build plate. The plate lowers by one layer height, a recoater spreads fresh powder, and the next layer fuses.

The inert atmosphere is essential, because molten metal would oxidize or burn in air, and the powder itself is a fire and inhalation hazard that must be handled carefully. Because each layer is laser-fused at high heat, the part builds up large residual stresses, which is why a stress-relief heat treatment is standard before the part is removed from the plate. The build is also slow, since each thin layer is fused point by point across a hard metal bed, which is a major driver of cost.

Supports, anchors, and heat management

Unlike polymer powder-bed processes, metal additive needs metal supports. The supports anchor the part to the build plate to resist warping from thermal stress, hold up overhangs, and conduct heat away from the melt pool. They are removed mechanically after the build, which is labor-intensive, so parts are oriented to minimize support volume and to place supports on non-critical faces. Self-supporting angles, often about 45 degrees from vertical, reduce the support load, and large flat down-skin surfaces are avoided because they warp and need heavy supports.

Metal materials

The three common alloys

Three alloys dominate metal additive work, each chosen for a different job. Aluminum AlSi10Mg is the light, weldable, general-purpose choice, well suited to lightweight structures and housings, and it is commonly run on machines with the largest practical build envelopes. Stainless 316L adds corrosion resistance for harsh, chemical, or marine environments, with good strength and toughness. Titanium Ti-6Al-4V is the aerospace and medical choice, with a tensile strength in the 895 to 1105 MPa range and a density of 4.43 grams per cubic centimeter, about 60 percent of steel, which gives an exceptional strength-to-weight ratio.

A note on build volumes

A note on the build volumes in the table: build volume is a property of the machine and its configuration, not of the alloy itself, and the same machine family can run more than one alloy. The figures shown are typical build envelopes for common metal additive machines running each alloy, and they set the largest single part a given setup can produce, not a limit imposed by the material.

Titanium and Inconel: the machining challenge

Titanium also illustrates the challenge of metal additive. It machines slowly, at only about 20 to 30 percent of the rate of free-machining brass, because it work-hardens and resists cutting, and its low thermal conductivity, about 6.7 watts per meter-kelvin, means heat builds up at the tool instead of flowing into the chip, which demands carbide tooling and flood coolant when the printed blank is post-machined. The Inconel family extends the range to high-temperature service but is harder and costlier still, and is treated as a specialty material.

Tolerances and accuracy

As-built tolerance and surface

Dimensional tolerance runs about 0.3 percent of the part dimension with a floor near 0.3mm, which on a 100mm part is about 0.3mm, comparable to SLS but on a much harder material. As-built surface finish is comparatively rough, around Ra 5 to 10 micrometers after shot peening, because each fused layer leaves a textured bead. Post-machining brings a critical face to about Ra 1.6 micrometers and to a tight tolerance the as-built process cannot hold, which is why mating faces, bores, and threads are machined.

Treat the part as a near-net blank

The practical rule is to treat the as-built part as a near-net blank, not a finished part. Leave machining stock on every critical face, model bores undersize for a finish bore, and plan the datum scheme so the machinist has a clean reference. For example, a printed titanium bracket leaves 0.5mm of stock on the bolt-hole faces, which are then milling and bored to a true locating fit, while the rest of the geometry stays as-built.

Design rules for metal additive

Metal additive design is built around post-machining, supports, and thermal stress. Orient the part to minimize support volume and to place supports on non-critical faces, and keep self-supporting angles above about 45 degrees where possible to cut the support load. Minimum wall thickness is about 1.0mm for supported walls and 1.2mm for unsupported walls, minimum feature size is about 0.5mm, minimum pin diameter is 1.5mm, and clearance for an assembled fit is about 0.5mm.

Design for post-machining

Because the as-built surface and tolerance do not meet a precision fit, design every critical face to be machined. Leave stock on mating faces, model bores and threads undersize for a finish operation, and provide a clean, accessible datum for the machinist to reference. A part that cannot be held or referenced on a machine is hard to finish, so include flats, datums, and access for tooling in the design. e.g., a consolidated bracket that replaces a five-part weldment is printed near-net and then finish-machined on the two mating faces, arriving as one strong part instead of five welded ones.

Post-processing

A metal additive part goes through several steps after the build. It is stress-relieved, often while still on the build plate, to remove residual stresses. It is then cut from the plate, supports are removed mechanically, and any trapped powder is recovered. Critical faces, bores, and threads are post-machined to tolerance and finish. A hot isostatic pressing, or HIP, cycle may be used to close internal porosity for fatigue-critical parts, and secondary treatments such as shot peening, passivation, anodizing, or polishing bring the surface to its final condition. This chain of steps is a large part of why metal additive is slow and expensive.

Applications and use cases

Metal additive earns its place on geometry that no subtractive process can reach. Internal conformal cooling channels in injection molds and tooling improve cycle time and part quality in ways a drilled channel cannot match. Lightweight lattice structures reduce mass in aerospace brackets and structural nodes while keeping stiffness, taking advantage of the strength where it is needed and removing it where it is not. Consolidated assemblies replace a multi-part weldment or bolted assembly with one printed part, cutting weight, assembly labor, and failure points. Low-volume titanium and Inconel components for aerospace, medical implants, and demanding industrial use round out the common applications.

A worked metal additive design example

Consider an injection mold insert that must carry a conformal cooling channel to cut cycle time. The channel must follow the contour of the cavity, which a drilled straight hole cannot do, and the geometry points straight to metal additive.

Step 1: choose the material

First, choose the material. The insert sees thermal cycling and must hold a polished cavity surface, so a tool steel or stainless grade is chosen over aluminum, and the build is planned around the post-machining the cavity will need. The cooling channel is modeled into the part along the cavity contour, something only additive can produce in one piece.

Step 2: design for supports and post-machining

Second, design for supports and post-machining. The insert is oriented so the cavity face points up with minimal supports, and the mating flange faces that bolt to the mold base are left with 0.5mm of machining stock. The internal channel is kept self-supporting above 45 degrees where possible, and any closed sections get a path for powder removal.

Step 3: plan the post-processing

Third, plan the post-processing. The insert is stress-relieved, cut from the plate, and the supports removed. The cavity is finish-machined and polished to a molding surface, while the conformal channel stays as-built inside, doing its cooling job that no drilled channel could match. The result is a mold that runs cooler and faster than a conventionally drilled one, which is the case where metal additive pays back its cost.

Metal additive strengths and limitations

Strengths

Metal additive is strong where complexity and consolidation matter. It produces geometry no subtractive process can reach, including internal channels, lattices, and undercuts, and it consolidates assemblies into single strong parts. In titanium and Inconel it offers an exceptional strength-to-weight ratio and high-temperature performance, which is why it is entrenched in aerospace and medical work.

Limitations

Its limitations are cost, speed, and finish. It is the most expensive common 3D printing family, the build is slow, and the as-built surface is rough and needs machining on critical faces. It cannot match the tolerance of CNC machining without post-machining, and it is not economic for high volumes or for simple parts that subtract easily. The material range is also narrower than machining, focused on the alloys that weld and fuse well in a powder bed, so it is a specialist tool for complex geometry, not a general replacement for machining.

When metal additive is worth it, and when not

Metal additive is worth it when the geometry is complex enough that machining or casting cannot produce it, or when low volume makes tooling uneconomic, or when consolidation cuts weight and assembly cost enough to justify the unit price. It is the wrong choice for simple parts that a CNC can make, which will be cheaper, more accurate, and faster, and for high volumes where casting or machining wins on unit cost. It is also the wrong choice when the part needs a fine cosmetic or mating surface everywhere, because the as-built surface is rough and only the machined faces are precise.

Inspection and qualification take a larger share of metal additive cost than they do for polymer prints, because the parts are usually high-value and often load-bearing or safety-relevant. A first-article inspection checks every critical dimension against the datum scheme, and for fatigue-critical or structural parts a hot isostatic pressing, or HIP, cycle closes the internal pores that can act as crack-initiation sites under cyclic load. After HIP, a defect-detection step such as dye-penetrant inspection or X-ray computed tomography confirms there are no surface or internal cracks, inclusions, or lack-of-fusion voids. These steps are part of the process for a structural metal additive part, not an optional add-on, and they are part of why the process is reserved for geometry or performance that justifies the cost. They should be scoped into the requirement early, alongside the machining stock and the datum scheme, so the part can be built, stress-relieved, machined, and inspected without a rework loop.

Cost and sourcing context

Metal additive is the most expensive common 3D printing family, driven by powder cost, slow build rate, and extensive post-processing. Most custom-part manufacturers do not run metal additive in-house; it requires large capital investment, powder-handling expertise, inert-gas systems, and specialist knowledge, so it is often sourced from specialist partners rather than produced on a general shop floor. This page describes the technology and its design rules; it does not claim any specific production capability, and any metal additive requirement should be confirmed against the actual operation and partner network available.

File format and units

Provide an STL with the units stated, and mark critical faces and datums for post-machining, because metal additive parts are near-net blanks that get finished on a CNC. STL carries no units metadata, so a file without explicit units is read against the supplier default, and a millimeter-versus-inch mistake produces a part 25.4 times the intended scale, which on a metal blank is an expensive error. State the units in the file and the filename, and confirm them in the order.