What is post-processing for metal 3D printing? Why is post-processing necessary?

一, What post-processing for metal 3D printing is and what its main goals are
Post-processing of metal 3D printing is a series of steps taken on printed parts after metal additive manufacturing is done. These steps include fixing, optimising, and processing the parts to get rid of manufacturing flaws, improve performance indicators, and meet the needs of certain applications. The main goals of it can be summed up as:

Quality improvement: get rid of problems such interlayer bonding flaws and rough surfaces that make parts less reliable.
Performance optimisation: Heat treatment, surface modification, and other treatments can improve important qualities of materials like strength, hardness, and resistance to corrosion.
Dimensional correction: Make up for thermal deformation and shrinkage that happen during printing to make sure the pieces satisfy the design tolerance requirements.
Functional integration: giving pieces more complicated overall performance by strengthening their structure or combining different materials.
For example, in the aerospace industry, metal 3D printing is used to make the general frame of a certain type of rocket engine fuel tank. Then, hot isostatic pressing (HIP) treatment is used to get rid of any internal pores. After that, CNC milling is used to shape the sealing surface, and anodising treatment is used to make it more resistant to corrosion. This series of procedures after processing makes the parts 30% stronger, 40% lighter, and able to seal in very harsh conditions.

二, The main technology system for post-processing
There are four technological modules in the post-processing of metal 3D printing: material removal, heat treatment, surface treatment, and structural strengthening. Each module is a part of a bigger solution that works for diverse situations.

1. Material removal: carving with accuracy from "rough" to "fine"
Metal 3D printed items often can't be used right away since they have leftover support structures and rough surfaces (Ra values can be as high as 10–20 μm). Material removal technique uses mechanical processing, laser cutting, or chemical corrosion to do the following:

To remove the support structure, use cryogen-assisted peeling or mechanical cutting tools to make sure that the support is completely removed without damaging the printed object. For instance, a printed section of a car wheel hub is frozen at a low temperature, which makes the supporting structure fragile and easier to peel off. This increases efficiency by 50%.
Finishing the surface: CNC milling, grinding, or polishing can make the surface roughness less than Ra0.8 μ m. A five-axis linkage machining centre was utilised to mirror polish the flow channel's surface after printing the blades of a certain aircraft engine. This cut down on airflow resistance by 15%.
Size correction: Use a coordinate measuring equipment to get feedback data and fix any changes in size that happen during printing by using mechanical processing. Micro milling technology keeps the dimensional accuracy of a printed medical device implant within ± 0.01mm, which is what is needed for surgical insertion.
2. Heat treatment: a game-changer in controlling microstructure performance
Heat treatment gets rid of residual stresses that build up during printing (up to 50% to 70% of the material's yield strength) and improves the material's grain structure by regulating the heating and cooling curve. Some common methods are:

Annealing treatment: Heat the portion below the temperature at which it can recrystallise and keep it warm to get rid of internal stress and make it more flexible. Vacuum annealing treatment cut residual stress by 80% and tripled the fatigue life of a titanium alloy orthopaedic implant after it was printed.
Solid solution and ageing treatment: For materials like nickel-based high-temperature alloys, solid solution treatment dissolves the strengthening phase, and then ageing treatment causes fine precipitates to form, which greatly increases high-temperature strength. After printing the turbine disc for a given aviation engine, solid solution and ageing treatment improved its creep resistance at 650 °C by 40%.
Hot isostatic pressing (HIP) uses both high temperature (typically 0.7–0.9 times the melting point of the material) and high pressure (100–200MPa) to get rid of internal pores and make the material denser. After printing a specified part of a satellite structure, HIP processing raised the density from 99.2% to 99.95% and the fatigue limit by 25%.
3. Surface Treatment: From "Functionalisation" to "Intelligentization" in Surface Engineering
By modifying the surface morphology or chemical composition of objects, surface treatment technology gives them specific features including resistance to corrosion, wear, and biocompatibility. Some common technologies are:

Sandblasting and polishing: Sandblasting uses fast-moving sand particles to hit the surface, making it evenly rough (Ra3.2–6.3 μ m) and helping the coating stick better. Polishing then makes the surface even smoother, below Ra0.4 μ m, to fulfil optical or sealing needs.
Electroplating and chemical plating are two ways to add layers of metal or alloy to the surface of items to make them more resistant to rust or better at conducting electricity. Nickel plating treatment cut the corrosion rate by 90% in a 3.5% NaCl solution after printing a specific maritime engineering part.
Laser cladding: A high-energy laser beam melts alloy powder and forms a coating that is 0.1–5mm thick on the surface of the item. This makes it far more resistant to wear. Laser cladding of a Stellite 6 alloy coating boosted the wear resistance of the gears of a given piece of mining equipment by five times after they were printed.
Micro arc oxidation: A ceramic oxide film is put on the surface of aluminium and magnesium alloys to make them more resistant to wear and corrosion. Micro arc oxidation treatment increased the corrosion resistance time to more than 1000 hours in the salt spray test after printing the bracket for a new energy vehicle battery pack.
4. Structural strengthening: changing the performance from "single material" to "composite structure"
By adding reinforcing phases or improving load transfer routes, structural reinforcement technology makes parts work better mechanically overall. Some common ways are:

Fibre reinforcement: Putting carbon or ceramic fibres into a metal matrix to make a composite material structure. After printing, a certain part of an airplane's structure was made stronger by adding short cut carbon fibres, which made it 30% stronger in terms of specific strength.
Designing materials with gradients: You can vary the qualities of the material by changing the powder mix or the printing parameters. The printed pieces of a nuclear power valve have a gradient structure made of nickel-based alloy stainless steel. This makes them 40% more resistant to fatigue in a thermal-mechanical coupling environment.
Design of lattice structures: Using topology optimisation to make lightweight lattice structures that are more than 50% lighter but yet strong. After printing a specific satellite bracket, it takes on a tetrahedral lattice structure, which makes it twice as stiff and 60% lighter.
三, The need for post-processing: a jump from "technical feasibility" to "engineering reliability"
The need for post-processing in metal 3D printing arises from the conflict between the fundamental attributes of additive manufacturing technology and the rigorous demands of engineering applications. Specifically, its necessity is seen in the following aspects:

1. Get rid of production flaws and make sure the product works as it should.
Thermal stress from quick heating and cooling, pores that form when the powder doesn't fully fuse, and weak interlayer bonding can all make the pieces less durable and more likely to break throughout the metal 3D printing process. For instance, the fatigue limit of nickel-based high-temperature alloy printed parts without HIP treatment may be less than 50% of that of forged parts; however, after annealing to remove residual stress, the fatigue life of the forged part can exceed 80%.

2. Meet performance goals and broaden the range of applications
The requirements for part performance varies greatly depending on the application. In the aerospace industry, parts need to work well in environments with high temperatures, high pressures, and high vibrations. In the medical device field, parts need to be biocompatible and resistant to corrosion from body fluids. The automotive industry is more concerned with making parts lighter and cheaper. Post-processing technology allows metal 3D-printed items to satisfy these specific needs by optimising them for those purposes. For instance, the combustion chamber of a certain type of aircraft engine can still be structurally sound at a high temperature of 1200 ℃ after being printed, thanks to heat treatment and coating treatment. After printing a custom titanium alloy orthopaedic implant, the surface roughness was reduced to Ra0.2 μ m through acid washing polishing treatment, which greatly improved the adhesion of bone cells.

3. Make the economy more efficient and encourage big uses
Metal 3D printing is cheaper for making complicated structures, but the expenses of the raw materials (such titanium alloy powder, which costs several hundred yuan per kilogramme), the depreciation of the equipment, and the energy used are still quite high. Post-processing technology lowers the total lifecycle cost by making better use of materials (for example, recovering more than 80% of powder), lowering the scrap rate (for example, lowering the defect rate through online detection and real-time correction), and extending the life of parts (for example, making parts more resistant to corrosion through surface treatment). For instance, a certain car wheel hub printing production line has cut the time it takes to produce one piece from 8 hours to 2 hours by adding an automated post-processing system. This has led to a total cost savings of 35%.