What is the main purpose of post-processing for metal 3D printing?

1. Getting rid of production flaws: making sure items fulfil quality criteria
Because metal 3D printing builds up layers, there will always be flaws on the surface and inside the object. If these problems aren't fixed, they will directly effect how the parts work and how long they last.

Fixing surface flaws
Printed parts often have problems on their surfaces, like layer lines, burrs, and leftover support. For instance, the surface roughness of printed aluminium alloy parts can be as high as Ra10–20 μm, which is substantially greater than the Ra1.6 μm of traditional processing. Surface treatment methods including sandblasting, polishing, and grinding can make the surface much smoother. For example, following sandblasting, the surface roughness of a certain aviation engine blade went from Ra15 μ m to Ra3.2 μ m. This got rid of surface microcracks and stopped fatigue fractures from happening because of stress concentration.
Getting rid of internal flaws
When metal is 3D printed, it can have internal flaws like pores and cracks if the powder doesn't fully fuse or the gas doesn't come out quickly enough. Hot isostatic pressing (HIP) technology uses a high-temperature and high-pressure environment (typically 1000–1200 °C and 100–200 MPa) to change the shape of materials, close internal pores, and raise the density from 98% to more than 99.9%. After a certain medical device company used HIP processing to 3D print titanium alloy hip joint prostheses, the fatigue life went from 10 ⁶ times to 10 ⁷ times, which met international standards.
2. Improve the performance of materials: make sure they can handle tough working circumstances
The microstructure of metal 3D printed parts is different from that of items made in the past, and post-processing is needed to make them work better overall.

Relieving residual stress
When metal 3D printing heats up and cools down quickly, it leaves behind residual stress. Residual stress can be 50% to 70% of the yield strength of stainless steel printed parts, which can easily lead them to bend or shatter. Holding something at 500–700 °C for 2–4 hours and then letting it cool slowly is an example of an annealing procedure that can lower residual stress by more than 80%. A certain company that makes car parts enhanced the life of its 3D-printed mould steel from 50,000 times to 200,000 times after annealing it. They also cut down on deformation by 90%.
Regulation of organisational structure
The columnar crystal structure of printed materials causes anisotropy, which makes performance less consistent. Quenching and tempering can make the grain size smaller and create a consistent martensitic structure. After quenching (cooling in water at 1050 °C) and tempering (cooling in air at 650 °C), the tensile strength of 316L stainless steel went from 680MPa to 920MPa, and the elongation went from 40% to 25%. However, the isotropy improved a lot, which is what aerospace structural components need.
Improving surface performance
Parts can be made more resistant to corrosion and wear by using surface treatments including electroplating, chemical plating, anodising, and others. A certain maritime engineering company uses the chemical nickel phosphorus plating technique to treat 3D-printed stainless steel valves. This makes them resistant to salt spray corrosion for 2000 hours instead of just 240 hours, which is what is needed for deep-sea operations.
3. Enhancing dimensional accuracy: meeting precise assembly standards
The dimensional accuracy of metal 3D printing is typically ± 0.1-0.5mm, which makes it hard to meet the needs of high-precision assembly. Post-processing uses methods like machining and wire cutting to make things fit perfectly.

Matching the processing of surfaces
For surfaces that need to fit with other parts, such holes, shafts, and planes, techniques like milling and grinding must be used to get the right design dimensions. A certain aerospace company employs a five-axis linkage machining centre to precisely process 3D-printed titanium alloy brackets. This keeps the mating surface's dimensional tolerance between ± 0.3mm and ± 0.02mm, making sure that the brackets fit perfectly with the engine system.
Fixing threads
Printing threads might have difficulties including missing tooth profiles and pitch errors. Thread milling or rolling can fix thread accuracy. A certain medical equipment company employs thread rolling technology to work with 3D printed stainless steel bone nails. The thread accuracy is at the 6g level (ISO standard), and the torque fluctuation range when paired with bone plates goes down from ± 15% to ± 5%.
4. Comply with industry standards: get certified
In industries like healthcare and aviation, there are high standards for the quality of components. Post-processing is an important step in attaining these standards.

healthcare field
The FDA says that implants must have a surface roughness of Ra < 0.8 μ m and not leave behind any hazardous substances. A company employed electrolytic polishing to 3D print cobalt chromium alloy joint prosthesis. This made the surface less rough (Ra0.4 μ m) and got rid of any leftover nickel ions on the surface. The prostheses also passed biocompatibility tests.
aviation industry
NASA requirements say that important structural parts must last at least 10 times longer than normal. A company used HIP+heat treatment composite technology to make a 3D-printed nickel-based high-temperature alloy turbine disc. The disc lasted for 1.2 × 10 ⁷ cycles and passed airworthiness testing.
5. Lowering overall costs: finding the right balance between performance and cost
Post-processing does add more steps, but in the end, it lowers costs by making parts work better and lowering the amount of scrap.

Cut down on the costs of rework
After using 3D printing and heat treatment technology, a specific company that makes moulds for cars raised the first-time pass rate of moulds from 60% to 95% and cut the cost of rework due to deformation and cracking by 70%.
Lengthen the life of the service
A particular energy business 3D printed stainless steel pump bodies using a surface nitriding treatment. This made them three times more resistant to wear, lengthened maintenance cycles from three months to twelve months, and cut annual maintenance expenditures by 60%.