Will post-processing affect the dimensional accuracy of metal 3D printed parts?

一, The link between methods of post-processing and how accurate the dimensions are
There are three main types of post-processing for metal 3D printing: mechanical processing, heat treatment, and surface treatment. The way that different techniques affect dimensional accuracy is very different.

1. Mechanical processing: a double-edged sword for making things more accurate
The most straightforward way to fix dimensional discrepancies is by mechanical processing, like CNC milling, grinding, and electrical discharge machining. For example, after printing a specific aircraft engine turbine disc, five-axis linkage milling reduced the roundness error from 0.1 mm to 0.02 mm and improved the surface roughness Ra from 6.3 μm to 1.6 μm. However, to improve machining accuracy, you need to be able to control the performance of the equipment and the process parameters very carefully. For example, if the cutting depth is too deep or the feed rate is too fast, the parts may become thermally deformed. If the tool wear is not replaced on time, it may cause machining errors. For instance, one company didn't change the grinding wheel in time, which caused ripples to appear on the surface of a batch of titanium alloy parts. The scrap rate ended up being 15%.

2. Heat Treatment: The Battle between Stress Release and Size Changes
Heat treatment, which includes annealing, quenching, and solution treatment, changes the material's microstructure to get rid of residual stress. However, it can also affect the size of the material. For example, after solid solution and ageing treatment, the average fracture life of IN718 high-temperature alloy at 650 °C went up to 173 hours. However, the vertical dimensional shrinkage rate was 0.3% and the horizontal dimensional shrinkage rate was 0.15%. This uneven shrinkage needs to be regulated by either design compensation (such leaving a 0.5mm machining margin) or process optimisation (like graded quenching). A business that makes medical implants improved the heat treatment process parameters so that the size of 3D printed porous titanium alloy prosthesis only varies by ± 0.05mm, which is within the clinical implant accuracy standards.

3. Surface treatment: finding a balance between changing the size of the micro and the macro.
The goal of surface treatment (such sandblasting, polishing, or chemical polishing) is to make the surface better, but it can also change the size by a few micrometres. For instance, a company that makes car parts utilises chemical polishing to 3D print aluminium alloy water-cooled sleeves. This makes the surface less rough, going from 12 μ m to 0.8 μ m, but it also makes the inner cavity diameter smaller by 0.02 mm since the material dissolves. The company uses a combination process of "mechanical polishing+chemical polishing" to balance surface quality and dimensional accuracy. First, mechanical polishing removes large defects, and then chemical polishing modifies the surface on a nanoscale level. This keeps the dimensional tolerance within ± 0.01mm.

二, Control of process parameters: the most important part of making sure precision
To make sure that the dimensions are correct, you need to be able to precisely control the parameters of the post-processing procedure. Electrochemical machining (ECM) can reach sub-micron accuracy by dissolving materials layer by layer through electrochemical processes. However, parameters such as electrode gap, electrolyte concentration, and pulse frequency must be tightly matched:

Electrode gap: A gap that is too tiny can easily cause a short circuit, and a gap that is too big can make machining less efficient. By keeping an eye on the electrode gap in real time (regulated at 10–50 μm), a certain company has increased the machining precision of 3D printed nickel-based alloy turbine blades to ± 0.005mm.
Electrolyte concentration: If the concentration is too high, it can speed up the dissolution of materials, which can cause size changes. If the concentration is too low, it can create uneven processing. A research team improved the electrolyte formula (keeping the NaCl concentration between 15% and 20%) so that the surface roughness of 3D printed titanium alloy components went from 3.2 μ m to 0.4 μ m, while keeping the size changes within ± 0.01mm.
Pulse frequency: High-frequency pulses can lessen the effects of processing heat, but they can also generate vibration. Low-frequency pulses, on the other hand, make processing less efficient. A certain company employs a 10kHz pulse frequency to process 3D printed stainless steel items. This makes sure that the manufacturing is efficient and that the dimensions are accurate to within ± 0.008mm.
三, Design a compensation strategy: a "forward-looking" arrangement for precise control
During the design phase, a certain amount of money needs to be set aside to make up for the effect of post-processing on size. For example, the design process for a given part of an aeroplane is as follows:

Initial design: Use the CAD model to figure out the part's geometric shape and identify the important dimensional tolerances (for example, ± 0.05mm).
Simulating the process: Use finite element analysis (FEA) to model the stress distribution and dimensional shrinkage that happens during heat treatment. You should expect a vertical shrinkage rate of 0.3% and a horizontal shrinkage rate of 0.15%.
Designing compensation: Raise the machining allowance by 0.5mm for important dimensions and the contour tolerance by 0.05mm for angular places (from 0.02mm to 0.05mm).
Verification after processing: After mechanical processing, the difference between the actual size of the part and the design size is kept to within ± 0.01mm, and the pass rate goes up to 98%.
Also, when it comes to the interior cavity construction, you need to think about the material removal rate and the amount of compensation. The inner diameter of a 3D printed water-cooled jacket, for instance, is meant to be 20mm, with a processing allowance of 0.1mm. After being sandblasted (with a material removal rate of 0.02mm/time) and mechanically polished (with a material removal rate of 0.05mm/time), the final inner diameter stabilises at 20.03mm, which is what fluid dynamics needs.

四, Industry Practice: A Model Case of Precision Control
One company in the aerospace industry makes rocket engine nozzles using the "SLM printing+hot isostatic pressing (HIP)+CNC machining" technique. HIP treatment gets rid of the internal pores (reducing porosity from 5% to 0.1%), and then CNC machining controls the diameter precision of the nozzle throat to within ± 0.005mm to suit the sealing needs in high-pressure and high-temperature settings.
In the medical field, an orthopaedic company created a 3D-printed titanium alloy hip joint prosthetic. The surface roughness was decreased to 0.2 μ m and internal tension was removed using the method of "annealing treatment + chemical polishing." The size variation was kept within ± 0.02mm, which greatly lowered the danger of bacterial adhesion and problems after surgery.
In the automobile industry, one company makes new energy vehicle battery cooling plates using the "3D printing water cooling sleeve + electrochemical machining" technology. Electrochemical machining lowers the surface roughness of the inner cavity from 6.3 μm to 0.4 μm while keeping the dimensional tolerance at ± 0.01mm. This makes cooling 15% more efficient.