What is the difference between post-processing of metal 3D printing and traditional machining?

一, Technical principle: The difference between additive and subtractive manufacturing processes
Metal 3D printing is a type of additive manufacturing (AM) that builds three-dimensional objects by stacking layers of metal powders (like titanium alloys and stainless steel) or wires and melting and solidifying them with heat sources like lasers and electron beams. This procedure doesn't need a mould and may turn a digital model into a real thing right away. However, the surface of the moulded part is rough (Ra6.3–12.5 μm), and there may be residual stress or micro pore defects inside that need to be fixed for better performance by post-processing.

Traditional machining is based on the idea of subtractive production. It starts with a full metal billet and uses cutting tools like turning, milling, and drilling to remove extra material until the desired shape is reached. It has the benefits of excellent moulding precision (up to IT5 level) and good surface smoothness (Ra0.1-0.4 μ m), but it is hard to work with complex shapes (such internal hollowing and uneven surfaces), and the material utilisation rate is low (only 50% to 70%).

Main Differences:

Material state: After 3D printing, the powder metallurgical qualities (such porosity) need to be worked on, and machining needs to fix the deformation problems that happen when cutting stress is applied.
Design freedom: 3D printing can do "unsupported printing" of complicated structures, and post-processing just needs to make sure that the local performance is as good as possible; Machining is limited by how easy it is to get cutting tools, and complicated structures need to be taken apart and put back together after machining.
二, Flow of the process: Path separation from "fixing problems" to "making things more accurate"
1. Post-processing of metal 3D printing: Multi-link collaborative optimisation
There are usually four main phases in the post-processing of metal 3D printed parts:

Cleaning and removing support: Get rid of the support structure that was made during printing and the leftover powder on the surface using ultrasonic cleaning. For instance, after printing aircraft engine blades, the titanium alloy support needs to be chemically etched away so that it doesn't have to be removed mechanically, which could harm the surface.
Heat treatment is the process of getting rid of leftover stress and making materials better by using methods like annealing and quenching. For instance, a supplier of automotive parts used T6 thermal treatment on a 3D-printed aluminium alloy bracket, which made it stronger and lighter by raising its tensile strength from 320MPa to 380MPa.
With mechanical precision machining, you can CNC mill or grind essential dimensions (such sealing surfaces and mating surfaces) to within ± 0.01 mm. For instance, a business that makes medical implants used a five-axis linkage machining centre to lower the surface roughness of 3D printed titanium alloy acetabular cups from Ra3.2 μ m to Ra0.8 μ m, which met standards for implant biocompatibility.
Surface treatment: Use methods like sandblasting, electroplating, anodising, and others to make the surface more resistant to corrosion. One maritime engineering company uses micro arc oxidation technology to make a thick oxide film on the surface of 3D printed aluminium alloy valves. This makes them five times more resistant to corrosion in seawater.
2. Traditional machining post-processing: improving both accuracy and usefulness at the same time
The major goals of post-processing traditional machining are to improve accuracy and functionality. The method is also quite simple:

Deburring and chamfering: Use manual or automated tools to get rid of burrs that are left over during cutting. This will keep the assembly from being damaged.
Surface strengthening: Making the surface harder by using methods like rolling and shot blasting. For instance, a company that makes gears used shot peening to raise the compressive stress on the surface of machined gears by 30% and double their fatigue life.
Functional coating is the process of giving parts certain qualities through methods like chemical plating and electroplating. For instance, a company that makes electronic connectors used a chemical nickel plating method to make a 0.5 μm thick nickel layer on machined copper alloy terminals, which made welding far more reliable.
Main comparison:

Process complexity: 3D printing post-processing needs collaboration between multiple links, and the heat treatment parameters need to be changed based on the properties of the material. The level of standardisation for post-processing procedures for machining is high, but complex structures may need multiple clamps.
Cost structure: The cost of post-processing 3D printing is a large part of the total cost (up to 40%), mostly because of the cost of heat treatment equipment and precision machining. The cost of post-processing in machining is low (about 10%–15%), but the cost of tool wear is high in large-scale production.
三, Application scenario: moving from "high value-added customisation" to "large-scale standardisation" in the same domain
1. Post-processing of metal 3D printing: concentrating on elevated thresholds and valuable scenarios.
Aerospace: A certain aerospace company uses 3D printing to make engine combustion chambers. To get a material density of 99.9%, they get rid of internal pores by treating them with hot isostatic pressing (HIP). This makes the parts reliable in high-pressure and high-temperature environments.
Medical implants: A particular orthopaedic company uses 3D-printed titanium alloy porous structure artificial bone, improves pore connection through electrolytic polishing treatment, encourages bone cell proliferation, and raises the clinical success rate to 98%.
Complex mould: A certain supplier of automobile interiors has cut the injection moulding cycle by 40% and raised the product yield to 99.5% by 3D printing conformal cooling channel moulds and using EDM (electric discharge machining) to improve the mould cavity.
2. Traditional machining post-processing: leading large-scale and standardised production
Automotive engine: A certain vehicle firm makes important parts like cylinder blocks and crankshafts by machining them and then treating them with carburising and quenching to make the surface harder. This makes the engine last for 200,000 kilometres.
Consumer electronics: A company that makes mobile phones employs CNC machining on aluminium alloy frames and anodising treatment to give them a colourful look. They can make more than 5 million units a month.
General Machinery: A certain valve company makes high-precision ball valves by machining them and then adding strong chrome plating to make them more resistant to wear. These valves last for more than 10 years.
Trends in the Market:

Integrated manufacturing: More and more businesses are using a mix of "3D printing+machining." For instance, one provider of aviation parts makes near-net formed blanks using 3D printing and then uses machining to get the final precision. This increases the use of materials to 85% and cuts production cycles by 60%.
Smart upgrade: AI algorithms are being added to 3D printing post-processing to improve process parameters. For instance, one company employed machine learning models to guess how heat treatment would change the shape of something, which raised the processing qualification rate from 82% to 95%.