What are the common difficulties in post-processing metal 3D printing?

1. Surface roughness control: going from "blank" to "finished product"
Because metal 3D printing builds things up in layers, the surface has a stepped texture with a roughness (Ra value) that is usually between 6 and 12 μm. This is substantially rougher than traditional machining, which has a roughness value of 0.8 to 1.6 μm. For example, the roughness of the inner wall of the cooling channel for aircraft engine blades must be kept below 3 μ m, or else it will greatly lower the effectiveness of heat transfer.
Problems with technology:
Residual support structure: The support structure that is applied during printing to keep things from changing shape can leave pits or bumps on the surface after it is taken off.
Powder adhesion: When powder particles don't completely melt, they stick to the surface, which is called "spheroidization."
Interlayer bonding traces: Small bumps can form where laser scanning paths cross.
Answer:
Chemical polishing: Using acidic or alkaline solutions to selectively dissolve the surface layer can make it smoother than 1 μm, but you need to be very careful about how long you leave it in the solution to avoid too much corrosion.
Sandblasting treatment: A uniform matte surface is created by hitting the surface with high-speed sand flow. This is good for intricate internal cavity designs, but it may also create new surface faults.
Electrolytic polishing: This method uses electrochemical principles to level the surface on a microscopic level. It can provide a mirror effect (Ra<0.1 μ m), but the equipment is expensive.
2. Fixing internal defects: the key to making things denser and better.
The inside of metal 3D printed parts usually has a porosity of 0.1% to 5%. These tiny flaws can cause cracks to form, which greatly shortens the parts' fatigue life. For instance, titanium alloy implants with porosity greater than 0.5% may fail to integrate with bone.
Problems with technology:
Pores: If the laser intensity is too low or the powder has too much oxygen, the molten pool can break up.
Not enough fusion: weak bonding between layers, which leads to micro layering.
Crack: A hot or cold crack that happens when residual stress builds up.
Answer:
Hot isostatic pressing (HIP): The material is put under a lot of pressure (100–200 MPa) and heat (900–1200 °C). This makes it change shape, shut internal pores, and raise its density to above 99.9%. For instance, HIP treatment has tripled the fatigue life of fuel injectors in LEAP engines made by GE Aviation.
Local infiltration: Vacuum impregnation method fills in important areas of metal-based composite materials, making it good for fixing structures with thin walls.
Laser remelting: Doing a second scan on areas with surface or interior defects can help improve the grain, but it could also add new thermal stresses.
3. Managing Residual Stress: Systems Engineering for Controlling Deformation
When metal is 3D printed, the thermal stress from quickly heating and cooling can approach 50% to 80% of the material's yield strength. This can cause parts to distort, break, or change shape. Residual stress can induce deformation of several millimeters in big frame constructions, which is considerably above what is acceptable.
Problems with technology:
Uneven stress distribution: Complex geometric shapes cause big changes in temperature gradients.
Substrate constraint effect: Stress builds up at the point where the component meets the substrate, which can easily cause interlayer delamination.
Multi-material printing challenge: The fact that various materials expand at different rates makes stress build up faster.
Answer:
Before printing, heat the substrate to between 200 and 500 degrees Celsius to lower the temperature differential. For instance, the Precision series machines from Yunyao Shenwei have a 500 °C substrate preheating feature that lowers the chance of cracking in printed parts made of titanium alloy.
Optimizing the scanning strategy: Use "island scanning" or "chessboard scanning" to spread out the heat input and keep it from getting too hot in one place.
Stress relief annealing: After printing is done, insulation treatment is done at 600–700 °C to get rid of more than 80% of the stress that is still there.
4. Guarantee of dimensional accuracy: a step forward from "approximate molding" to "net molding"
Metal 3D printing is normally accurate to within ± 0.1mm, but for pieces that need to be very precise, like clock gears, further machining is needed. But it's very hard to work with intricate interior cavity structures, such lattice structures, and standard milling or electrical discharge machining (EDM) could hurt the internal structure.
Problems with technology:
Shrinkage deformation: When metal cools down, it shrinks in volume, which causes the dimensions to change.
Interference from supporting structures: Residual support makes it harder to find the machining reference plane.
Thin-walled structures aren't stiff enough, therefore processing vibrations can easily break tools.
Answer:
Designing compensation: Set the shrinkage amount in the CAD model ahead of time (typically between 0.2% and 0.5%) and check the fix by printing it multiple times.
Five-axis linkage machining: DMG MORI's LASERTEC 65 3D equipment is an example of a multi-axis CNC machine tool that can do both printing and milling at the same time.
Electrochemical machining (ECM) is a method of removing materials without requiring mechanical cutting force. It is good for precision machining of thin-walled structures.
5. Compatibility with several materials: the problem with functionally graded materials
Metal 3D printing is slowly moving toward the direction of multi-material composite in order to meet the needs for lightweight, corrosion resistance, and conductivity. But the fact that different materials have varying melting points and thermal expansion coefficients means that the bond strength between them isn't strong enough, which can quickly lead to delamination or cracking.
Problems with technology:
Cross-contamination of powder: Residual powder in compartments for printing with multiple materials impairs the purity of the materials.
Process parameter conflict: varying materials need to be matched with varying laser power, scanning speed, and other settings.
The performance of the interface gets worse: brittle phases occur quickly where different materials meet.
Answer:
Modular powder supply system: For instance, the RESEARCH series equipment from Yunyao Shenwei has independent powder supply tanks that let you swap between different layers of material.
Preprocessing the interface: Use laser cleaning or plasma spraying to make the interface stick better.
Numerical simulation optimization: Use ANSYS or COMSOL software to model how the thermal and mechanical properties of different materials interact during the printing process. This will help you establish the right parameters.
6. Finding the right balance between cost and efficiency: The biggest problem with large-scale production
Metal 3D printing costs 30% to 70% of the entire cost of a product, and the processing time is long (typically 2–5 times the printing time), which makes it hard to use in mass production. For example, the traditional casting procedure for a car engine's cylinder block costs roughly 500 yuan per piece. The cost of 3D printing and post-processing, on the other hand, might be more than 3000 yuan.
Problems with technology:
High equipment costs: High-end five-axis machining centers cost more than 5 million yuan, while HIP equipment can cost up to 20 million yuan.
Process chain length: You need to do a number of steps in order, such as heating, cutting wire, removing support, polishing, and polishing again.
Low level of automation: Manual work is still needed for post-processing complex parts, which makes it less efficient.
Answer:
Smart integration of production lines: Use AGV carts to connect 3D printers, heat treatment furnaces, and machining centers so that the whole process runs automatically. For instance, Platinum Technology's BLT-S800 equipment offers built-in online detection and adaptive processing capabilities.
Additive manufacturing: To cut down on the number of stages that come after printing, synchronize partial machining during the printing process. Mazak's INTEGREX i-400AM machines can switch between laser cladding and milling.
Digital process planning: Using Siemens NX or Magics software to find the best machining path and cut down on idle time.