Do all metal 3D printed parts have to undergo post-processing?

1. Technical principle: "Congenital defects" in additive manufacturing and a way to fix them in post-processing
The basic idea behind metal 3D printing is "layer by layer stacking." This means melting metal powder with a laser or electron beam to make a three-dimensional shape. Even though this procedure may make complicated geometric structures without any cost, it will always have the following technical problems:

Too much roughness on the surface: The solidification markings of the melt pool between the printed layers create a "step effect," which makes the surface roughness (Ra value) higher than it would be with traditional processing. For instance, titanium alloy 3D printed parts can have a surface roughness of Ra 10–20 μm, whereas aircraft engine blades need a surface roughness of Ra ≤ 0.8 μm.
Building up residual stress: If you heat and cool a part quickly, it can produce tensile stress inside the part, which can cause it to bend, distort, or crack. A case study of an aviation structural component project demonstrates that 3D printed titanium alloy parts that aren't treated can bend by up to 1.2 mm after milling, which is far more than the design tolerance.
Defects in the microstructure: Micro flaws like porosity and lack of fusion in the pieces are caused by the way powder metallurgy forming works. Research data indicates that the porosity of aluminium alloy components produced with the SLM technique can attain 0.5% to 2%, markedly diminishing fatigue life.
Post-processing technology uses physical, chemical, or thermodynamic methods to fix the problems listed in a specific way:

Mechanical processing: A five-axis CNC machine tool can make crucial surfaces smooth enough to be below Ra 0.4 μ m.
Heat treatment is the process of getting rid of residual stress and improving the grain structure. Annealing and solution treatment are two examples of this. For instance, following heat treatment, the residual stress of a turbocharger part in a certain car goes down by 75%, and the stability of the dimensions goes up by 30%.
Hot isostatic pressing (HIP): compressing pores in a setting with high pressure and temperature to get a material density near to 100%. In the aerospace industry, HIP treatment has been found to double or triple the fatigue life of parts.
2. Quality requirement: going from "usable" to "reliable"
The quality criteria for the places where metal 3D printed parts will be used are directly tied to the post-processing needs of such parts. Post-processing has become a necessary step in the following situations:

Field of high precision: Medical implants must fulfil the ISO 13485 medical standard, and the surface roughness must be kept to Ra 0.8 m to keep bacteria from sticking to them. To make sure there are no internal flaws, aircraft engine blades must be tested in a way that doesn't damage them, such CT scanning. The porosity must be less than 0.1%.
Dynamic load environment: Parts like gears, shafts, and other parts in cars need to be able to handle changing stress. Residual stress concentration can cut the fatigue life of untreated 3D printed parts by more than 50%.
To be corrosion-resistant, marine engineering parts must pass the salt spray test (like the ASTM B117 standard). Surface treatments like anodising and electroplating can provide a protective film that makes parts more than ten times more resistant to corrosion.
But not every situation needs a lot of post-processing. For instance,

Stage of prototype verification: To speed up the development cycle, precision machining might leave out non-critical elements that are used for functional testing. By quickly producing shell prototypes, a consumer electronics company cut the time it took to make changes to the design from 6 weeks to 2 weeks.
Parts that make up the inside: If embedded elements or surfaces that don't touch each other don't influence overall performance, the original surface can be kept. The fuel pipeline bracket on a certain aeroplane engine has been improved so that just 40% of the area needs to be processed again.
System for special materials: Because of their homogeneous microstructure, 3D printed parts made of some novel alloys, including high entropy alloys, can immediately meet performance criteria. Studies have demonstrated that CoCrFeNi alloy components with particular compositions can fulfil wear resistance criteria without necessitating heat treatment post-printing.
3, Cost and Efficiency: The "Economic Boundary" of Post Processing
When choosing post-processing, you have to find a balance between better performance and higher costs. For example, let's look at a project for a certain part of an airplane's structure:

Cost of direct printing: It takes 120 hours to print the parts, and the equipment and powder costs roughly 80000 yuan.
The cost of post-processing is 20,000 yuan for heat treatment, 30,000 yuan for CNC precision machining, and 10,000 yuan for testing, for a total of 60,000 yuan.
Comprehensive benefits: Post-processing has raised the qualification rate of parts from 65% to 92%, lowered the unit cost from 123,000 yuan to 109,000 yuan, and met the requirements for airworthiness certification.
In the industry, the "economic boundary" of post-processing depends on the following things:

Part complexity: The more complicated the structure, the harder and more expensive it is to process. For instance, to remove support from a turbine disc with an internal flow channel, it costs more than 50,000 yuan each piece to cut through the wire with electrical discharge.
Batch scale: while making things in small batches, the cost of post-processing is spread out a lot; while making things on a large scale, process optimisation can lower the cost per unit. A certain car company has lowered the price of one item from 150 yuan to 80 yuan by using typical post-processing methods.
Performance redundancy: If the design lets for some changes in performance, it can cut down on the number of steps needed after processing. For instance, a certain tool fixture cut the delivery time by 40% by changing the tolerance range and getting rid of the heat treatment step.