Will heat treatment affect the dimensional accuracy of parts?

一, The main way that heat treatment changes the size of things
1. Letting go of residual stress and thermal stress
The metal 3D printing method quickly heats and cools the material, which causes internal lattice distortion and residual stress. To relieve stress through lattice relaxing, heat treatment is heating to a temperature below the recrystallization temperature (for example, keeping titanium alloy at 800 °C for 2 hours). But the unevenness of stress release could induce local deformation. For instance, if the design of the support structure for aircraft engine turbine blades is not good, removing the support after heat treatment may induce local stress concentration and blade edge warping, with a deformation of 0.1–0.3mm.
2. Change in volume due to a phase change
During heat treatment, materials can change phases (such martensitic transformation), which can cause their volume to grow or shrink. For example, nickel-based high-temperature alloys can change from austenite to martensite if the cooling rate is too fast after solution treatment (1080 °C for 1 hour). This can cause parts printed by laser powder bed melting (LPBF) to expand in volume and change in size. Experimental data indicates that the critical dimension error of components lacking a regulated cooling rate can attain ± 0.05mm post-heat treatment, beyond the permissible limits established by aviation standards.
3. Growth of grains and homogeneity of tissues
Controlling the heating rate and holding time during heat treatment can improve tissue uniformity and grain size. But if the grains grow unevenly, it could cause different amounts of shrinkage in different places. For instance, heating continuous carbon fiber reinforced composite (CCFRC) to 100 °C raises its matrix crystallinity from 17.42% to 22.76%. But if the fibers are not evenly spaced, it might generate a size difference of 0.02 to 0.05 mm, which would make it harder to put things together correctly.
二, A common example: heat treatment has two effects on dimensional accuracy
1. Aerospace: finding the right balance between high accuracy and high performance
Boeing makes airplane brackets utilizing LPBF technology, however they need to be heat-treated to raise their tensile strength to 520MPa. But following heat treatment, it becomes hard to keep the pieces' dimensions stable. To get precise control, do the following:
Pre-deformation compensation: Use reverse pre-deformation on the original model to get the compensated model closer to the optimal size after heat treatment. This will enhance printing accuracy by 66.2%.
Segmented heating and cooling: Using a phased heating approach (holding at 50 ℃ for 30 minutes) and delayed cooling (air cooling after furnace cooling to 200 ℃) to reduce thermal stress produced by temperature gradients, with deformation regulated within ± 0.03mm.
2. Medical implants: the combination of biocompatibility and dimensional correctness
When 3D printing titanium alloy acetabular cups, the surface micro porous structure (5–10 μm) must be very precise in terms of size. A particular company obtains precise control with the combined method of "stress relief annealing + acid etching":
Stress reduction annealing: Hold at 650 °C for 2 hours to get rid of any leftover printing stress and make size alterations less likely during the next acid etching.
Acid etching treatment: Use a mixture of hydrofluoric acid and nitric acid solution to etch for 10 minutes to make homogeneous micropores. This will prevent stress release from causing local corrosion. Size differences should be kept to within ± 0.02mm.
3. Industrial molds: Finding the right balance between cost and use
Through the "solid solution+aging" procedure for heat treatment, a specific company has made aluminum alloy molds harder to 120HB. However, they need to find a compromise between cost and accuracy:
The cost-effective method is to only sandblast (Ra value ≤ 3.2 μ m) the parts to fulfill the typical plastic molding needs. This cuts the cost of each piece by 40%, but the dimensions are not very stable.
High-performance solution: Increase CNC precision machining (Ra value ≤ 0.8 μ m), which is good for mold parts that need to be very shiny or clear. This will triple the processing time, but the dimensional accuracy will be ± 0.01mm.
三, Strategy for controlling dimensional accuracy: improving processes and coming up with new technologies
1. Matching process parameters: making the heat treatment and printing processes work together
The settings for the heat treatment must be the same as those for the 3D printing process, such as the temperature and the rate of cooling. For instance, if you print Inconel 718 alloy using LPBF and the printing layer is 0.05mm thick, you need to employ a 1150 °C solution treatment and a 720 °C aging treatment to minimize cracks and dimensional changes that happen when the material cools too quickly. Tests have shown that matching parameters can make parts last three times longer and make their dimensions more stable by 50%.
2. Smart thermal management system: keeps an eye on things in real time and makes changes as needed
Using infrared sensors and temperature feedback control, the intelligent thermal management system makes the thermal field more uniform. The AI system made by Platinum Technology can change the laser power and scanning speed in real time. This keeps the temperature from changing too much during the printing of titanium alloy brackets, keeping it within ± 5 °C. After heat treatment, the system also reduces the dimensional deviation from ± 0.05mm to ± 0.02mm.
3. New ways to heat treat things: local heating and the composite process
Local heat treatment: Induction heating or laser local heat treatment is used on large pieces to keep them from getting deformed when they are heated all over. Through local solution treatment, a specific aviation bracket has reached a tensile strength of 520MPa and dimensional stability better than ± 0.03mm.
Composite process: using heat treatment and hot isostatic pressing (HIP) together to get rid of internal flaws and improve the structure. The fatigue life of GE aviation engine turbine blades is three times longer after HIP treatment, and the dimensional deformation is kept below 0.05%.