Why does post-processing determine the final quality of metal 3D printed parts?

1. Fixing metallurgical defects: from "loose and porous" to "dense and flawless"
Metal 3D printing has intrinsic flaws at the tiny level because it melts and solidifies quickly.

Porosity problem: In the SLM (Selective Laser Melting) process, micropores can form inside the part if the powder doesn't fully fuse or gas is trapped. This can happen when the porosity is between 0.5% and 1%. These pores will be the first place where fatigue cracks occur, which will make the parts much less challenging to break. For instance, the average fracture life of the turbine blades of a given aircraft engine is only 13 hours at 650 °C and 690 MPa if they are not post-treated. However, after hot isostatic pressing (HIP) treatment, the fracture life improved to 131 hours, which met the design requirements.
Controlling residual stress: Uneven cooling of the material during printing can cause residual stress to build up inside the part, which can cause it to bend, break, or uniform fail to fit together. As an example, the residual stress in Ti6Al4V titanium alloy might be higher than the yield strength at the corners. If not annealed, the "edge explosion" phenomena is likely to occur during subsequent machining; annealing at 800-900 °C significantly enhances processing stability.
Important steps in post-processing:

Hot isostatic pressing (HIP): When materials are heated to very high temperatures (typically between 900 and 1200 °C) and very high pressures (100 and 200 MPa), their internal holes close, which makes them almost twice as dense.
Annealing for stress relief: Residual tensions are discharged to make the dimensions more stable by slowly heating and cooling the holding cycles. For instance, after being heated to 300 °C, the residual stress in AlSi10Mg aluminium alloy parts is cut by 80%, and the deformation is kept to within 0.1 mm.
2. Improving Performance: From "Anisotropy" to "Omnidirectional Balance"
Metal 3D printing's interlayer stacking qualities cause its mechanical properties to be different in different directions (anisotropy). Post-processing can balance performance by controlling the tissue:

Grain refinement: The material's flexibility and toughness can be lowered by the coarse columnar crystals that develop when the substance cools quickly after printing. Grain refining and the precipitation of strengthening phases can be sped up by solution treatment (like 1080 °C solution treatment of Inconel 718 high-temperature alloy) and ageing treatment (ageing at 550 °C for 8 hours). This can raise tensile strength to over 1300 MPa.
Hardness enhancement: The process of quenching quickly cools the material, creating a martensitic structure that makes the surface much harder. After quenching at 1050 °C, the hardness of 316L stainless steel parts goes from 180HV to 350HV, and the parts are three times more resistant to wear.
Important steps in post-processing:

Heat treatment package: A custom "annealing+solution+aging" method for each material, like "800 °C annealing+550 °C ageing" for Ti6Al4V, that can improve both strength and toughness at the same time.
By chemically heating the surface of the part, such as nitriding and carburising, a hard coating is produced that makes it more resistant to wear. After nitriding treatment, the surface hardness of gear parts can exceed 600HV, and the parts can last five times longer.
3, Controlling dimensional accuracy: from "extensive moulding" to "precision assembly"
Most metal 3D printed products have a dimensional accuracy of ± 0.1mm at first, which makes it hard to meet the needs of precision assembly. However, machining can make post-processing accurate to within a micrometre.

Correcting critical dimensions: CNC machining must be used to manage tolerances in the positions that include sealing, connecting, and moving pairs. For instance, the mating surface of a hydraulic valve body part needs to be machined on five axes to improve the dimensional precision from ± 0.05mm to ± 0.01mm.
Taking off the support structure: The support structure that was added during printing will leave behind certain marks that need to be carefully erased with electrolytic machining or laser cutting so that the main structure doesn't get damaged.
Important ways to process after:

Precision machining: employing ultra-precision grinding, electrical discharge machining (EDM), and other methods to get accuracy levels of IT5 to IT7.
Finding and fixing things online: A coordinate measurement machine (CMM) gives real-time data on dimensional discrepancies so that processing parameters can be changed and batch production can be consistent.
4. Better surface quality: from "rough layering" to "mirror-like smoothness"
The surface roughness (Ra) of metal 3D printing is usually between 8 and 12 μ m, which is substantially higher than the 0.8 to 3.2 μ m of traditional machining. Post-processing can make the surface smooth using both physical and chemical methods:

Driven by functional requirements: In the sector of medical devices, surface roughness must be kept below Ra<0.8 μ m to keep germs from sticking; in the field of optics, surface roughness must be below Ra<0.1 μ m to meet the need for transmittance.
Corrosion protection: Rough surfaces can make corrosive substances get in faster, thus they need to be polished or electroplated to make a thick protective coating. For instance, following electrolytic polishing, maritime engineering parts can now resist salt spray corrosion for 500 hours instead of just 24 hours.
Important post-processing methods:

Mechanical polishing: The Ra value is brought down to less than 0.4 μ m by applying methods like sand belt grinding and magneto rheological polishing.
Chemical plating or electroplating adds metal layers like nickel and chromium to the surface of objects to make them look better and protect them from rust. For instance, after chemical nickel plating, the shine of a certain car adornment is more than 90%.