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How strong are metal parts produced by SLM 3D printing compared to traditional machining?
The short answer
At room temperature, as-built SLM parts land close to wrought: SLM 316L prints at about 570 MPa tensile against 485 to 515 MPa for annealed wrought bar, and SLM Ti6Al4V reaches about 1,200 MPa against roughly 950 MPa wrought. Yield strength is often higher and elongation lower. Fatigue is the gap: as-built surfaces sit at Ra 6.3 to 12.5 micrometres.
The comparison, alloy by alloy
The headline numbers are better than most engineers expect, and the reason is the process itself. Laser melting solidifies metal at cooling rates between roughly 10 to the third and 10 to the sixth kelvin per second, which is fast enough to produce a much finer grain and cellular structure than casting or even forging. Finer structure means higher yield strength. What it also means is residual stress locked into the part, a rougher surface than any machined equivalent, and anisotropy, because the properties in the build direction are not the same as the properties across it.
| Alloy | SLM as-built | SLM after heat treatment | Wrought or machined reference |
|---|---|---|---|
| 316L stainless | 570 MPa tensile, 470 MPa yield, 40 percent elongation | 570 MPa tensile, yield drops to about 380 MPa, 35 percent elongation | Annealed bar: 485 to 515 MPa tensile, 170 to 205 MPa yield |
| Ti6Al4V | 1,200 MPa tensile, 1,050 MPa yield, 8 percent elongation | Above 930 MPa tensile, above 860 MPa yield, above 10 percent elongation | Annealed wrought: about 950 MPa tensile, about 880 MPa yield |
| AlSi10Mg | 360 MPa tensile, 240 MPa yield, 6 percent elongation | Above 267 MPa tensile, above 200 MPa yield, about 10 percent elongation | Cast A360: about 320 MPa tensile |
| Inconel 718 | 960 MPa tensile, 600 MPa yield, 30 percent elongation | Annealed and aged: above 1,240 MPa tensile, above 940 MPa yield | Wrought, aged: about 1,100 MPa and up |
| Maraging steel | 1,100 MPa tensile, 1,000 MPa yield, 8 percent elongation | Aged: 1,950 MPa tensile, 1,900 MPa yield, 2 percent elongation | Wrought maraging, aged: 1,900 to 2,000 MPa |
Two rules fall out of the table. First, as-built prints match or beat the wrought reference on tensile strength for stainless, titanium and aluminium alloys, because of the fine solidification structure. Second, once you heat treat for ductility, the advantage narrows and in some alloys reverses: heat treated Ti6Al4V at above 930 MPa is below the 1,200 MPa of the as-built print, and 316L loses most of its yield advantage.
Why as-built yield can beat wrought, and what heat treatment gives back
As-built 316L commonly reports yield strength in the 440 to 470 MPa range against 170 to 205 MPa for annealed wrought bar. That is not a measurement error. The fine cellular subgrain structure produced by rapid solidification pins dislocation movement in much the same way that cold work would, and the residual stress in the part adds to the effect. It is also the reason the part can distort: the same residual stress that raises yield strength is released unevenly if a thin section is machined away or if the part is heated without support.
Stress relief therefore does two things at once. It removes the internal stresses that cause distortion during subsequent machining and heat treatment, and it gives back ductility. A typical profile is exactly what the table shows: tensile strength essentially unchanged, yield strength reduced toward the wrought value, elongation improved. For a part that has to bend rather than crack in service, that trade is usually the right one. The practical consequence for a drawing is that the mechanical property callout has to name the condition as well as the alloy, because as-built and heat treated are two different materials with the same name.
Fatigue is the real difference
Static strength is not what breaks most metal parts in service. Fatigue is, and it is where an untreated printed part is weakest. The mechanism is geometric rather than metallurgical: an as-built SLM surface has a roughness of roughly Ra 6.3 to 12.5 micrometres, and every peak of that surface is a stress concentration sitting exactly where the load is highest. A machined surface at Ra 0.8 to 1.6 micrometres removes those notches. Between the two, published comparisons consistently show as-built printed parts losing a substantial share of their fatigue life relative to polished wrought material, even where tensile strength is equal or better.
Three further factors move fatigue life up or down, and none of them is visible in a tensile test. Internal porosity, which acts as a crack initiation site, is reduced by hot isostatic pressing. Build orientation matters, because layers loaded in tension across the layer boundaries behave differently from layers loaded within the plane. And residual stress changes the mean stress the part sees. The practical conclusion is that fatigue life for a printed part has to be established by testing the actual geometry and surface condition, not read from a material datasheet.
What each post-processing step buys
Post-processing is not cosmetic housekeeping. Each step changes a specific failure mode, and the sequence matters.
- Support removal and stress relief. Supports are cut away and the part is thermally relieved, usually in a vacuum furnace. This is the step that stops the part moving later.
- Hot isostatic pressing, where fatigue or sealing matters. Combined heat and pressure close internal porosity and raise density, which improves fatigue behaviour and makes a pressure-tight part possible. It does not fix a rough surface.
- Machining of functional faces. Sealing faces, bearing bores, threads, spigots and locating features are machined after printing rather than printed to size, because that is the only way to hold the tolerance and the surface finish together on the feature that has to seal or locate.
- Peening or polishing for fatigue. Shot peening puts the surface into compression, and vibratory polishing removes the peaks. Both improve fatigue life substantially, and neither restores as much as machining the surface to a controlled finish.
Where SLM beats machining outright
There are geometries where the comparison is not close, because machining cannot produce the feature at all. Conformal cooling channels that follow the contour of a mould insert instead of being drilled in straight lines. Lattice cores that carry a bending load with a fraction of the mass of a solid section. Internal flow passages with no line of sight to the outside. Multi-part assemblies consolidated into a single printed body, which removes joints, fasteners, leak paths and the tolerance stack between them.
In these cases the system-level answer can favour printing even when the material is marginally weaker. A lattice-cored bracket that carries the same load at a fraction of the mass is a better bracket than a solid machined one, and a mould insert with conformal cooling can cut cycle time enough to pay for its own printing. The comparison to make is between finished parts in service, not between material datasheets.
Where machining still wins, and the rules that decide the outcome
For most parts that fit inside the build envelope and do not need internal geometry, machining a wrought billet remains the better answer, for reasons that are structural rather than sentimental.
- Isotropy and certified stock. A wrought bar comes with a heat number, a mill certificate and properties that are the same in every direction. A printed part has directional properties and needs its own qualification route.
- Sealing, wearing and sliding surfaces. Faces that seal, bores that carry a bearing, threads that are loaded and fits that slide need a machined surface, whether or not the blank was printed.
- Large, flat, fine surfaces. A face that has to be flat over a long length and fine in finish is cheaper and more reliable to machine from solid than to print and then machine.
- Volume economics and size. At production volume, casting or forging plus machining wins on unit cost, and the build envelope limits both the size and the number of parts per build.
- Design rules are not optional. Minimum wall thickness is roughly 0.8 to 1.0 mm in aluminium and stainless steel and 1.0 to 1.5 mm in titanium, wall height to thickness should stay under about 8 to 1, and thin unsupported walls risk incomplete fusion and porosity. Orientation, support placement and powder reuse all change the result, and reused powder has to be controlled rather than simply topped up.
- Fatigue-critical parts need a programme. If fatigue governs the design and the geometry cannot be machined afterwards, the right answer is usually a qualification programme with test coupons in the actual orientation and surface condition, not a datasheet comparison.
Choosing between them in one pass
Five questions separate the two routes quickly. Does the part need an internal channel, a lattice or a consolidated assembly that machining cannot reach? If yes, print it and machine the functional faces afterwards. Is fatigue the governing failure mode? If yes, budget for machining, peening or HIP and test the finished geometry. Are the critical features sealing, wearing or locating surfaces? If yes, they will be machined regardless, so compare the two routes on the same finished part. Is annual volume high enough for tooling to amortise? If yes, casting, forging or stamping plus machining will beat printing on unit cost. Does the part need a mill certificate for its material lot? If yes, that requirement points to wrought stock and therefore to machining.
In practice the two processes are complements rather than competitors, and the strongest answer for a hard part is often both: print the geometry that cannot be machined, then machine the surfaces that have to be precise. See SLM metal 3D printing for the additive route, CNC machining for the subtractive one and surface finishing for the post-processing steps that decide fatigue life.
Scope and sources. Mechanical property values were compiled in 2026 from the direct metal laser sintering material data published by Protolabs for 316L, AlSi10Mg, Ti6Al4V, Inconel 718 and maraging steel in the as-built and heat treated conditions, and from a survey of as-printed SLM alloy properties covering the same alloys plus 17-4 PH and CoCrMo, together with the cooling-rate and anisotropy discussion. Surface roughness ranges, minimum wall thickness, aspect ratio guidance, density and the 20 to 40 percent post-processing cost add-on come from an SLM process guide. Wrought reference values for annealed 316L, Ti6Al4V and cast A360 are the standard published minimums for those product forms. These are typical published ranges from general-purpose machines and parameter sets; they are not a specification for any particular build, because properties depend on the machine, the parameters, the orientation, the powder lot and the post-processing route. Confirm properties by testing coupons in the same orientation and condition as the production part. Nothing on this page states or implies a certification held by any supplier, and no fatigue life should be assumed from the numbers above without testing the finished geometry.
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