Why is heat treatment necessary after metal 3D printing?

1. Getting rid of residual stress is the key to stopping deformation and cracking.
During the process of 3D printing metal, the material goes through quick cycles of heating and cooling, which leaves residual tensions in the products. For instance, in the laser powder bed melting (LPBF) process, the melt pool cools down quickly, which might put stress on the surrounding metal that hasn't melted yet. The part may bend, crack, or go beyond the size limit if the stress is too high for the material. For example, titanium alloy aircraft engine blades have thin walls. If they are not heat-treated after printing, residual tension might cause them to break unexpectedly during processing or usage, which would be very dangerous.
Stress reduction annealing and other heat treatment methods can get rid of residual stress very well. During the annealing process, the pieces are heated to a temperature below the recrystallization point (typically 50% to 70% of the melting point of the material), kept there for a set amount of time, and then progressively cooled. At this time, the material's internal dislocations are rearranged, the grains recover and recrystallize, and the stress is discharged. For instance, a specific kind of turbine disk was annealed at 650 °C for 4 hours, which brought down the residual stress from 320MPa to 80MPa and the deformation by 90%. This made sure that the machining would be accurate.
2. Improving microstructure: making materials work better overall
The quick solidification of metal 3D printing can cause rough microstructure and compositional segregation, which can damage the parts' performance. For instance, LPBF-printed 316L stainless steel may have rough columnar crystals, and its fatigue resistance is 40% lower than that of forged steel. Heat treatment can make things work better by controlling the microstructure:
Grain refinement: During annealing, the process of recrystallization can make the grains smaller. For instance, annealing aluminum alloy printed parts at 350 °C for 2 hours reduces the grain size from 100 μm to 20 μm and increases the yield strength by 15%.
Control of phase change: When you quench and temper steel, you can make a dual-phase structure comprising martensite and residual austenite. For instance, the hardness of mold steel printed components goes up to 58HRC after being quenched at 1050 °C and tempered at 200 °C. The wear resistance is three times higher than that of parts that have not been treated.
Getting rid of flaws: The synergistic action of high temperature (typically 0.7–0.9 times the melting point of the material) and high pressure (100–200MPa) in hot isostatic pressing (HIP) treatment can close internal holes and microcracks in parts. After HIP treatment, the density of high-temperature alloy parts for a certain aviation engine went up from 99.2% to 99.99%, and the parts lasted 5 times longer.
3. Meet high-strength standards to improve mechanical performance.
The mechanical qualities of metal 3D printed items are frequently not as good as those made using traditional methods. However, heat treatment can make them much stronger, harder, and tougher.
Quenching treatment makes a martensitic structure by cooling it quickly, which makes it much harder. For instance, the tensile strength of printed parts made of nickel-based high-temperature alloy went from 850 MPa to 1200 MPa after quenching at 1120 °C.
Better toughness: Tempering can get rid of quenching stress and make things tougher. For instance, after quenching and tempering at 550 °C, the impact toughness of a printed section of a car transmission shaft went from 15J/cm² to 35J/cm², which met the safety standards for collisions.
Maximizing fatigue performance: Heat treatment can greatly extend the life of a material by controlling its microstructure and residual stress. For example, twofold annealing (700 °C for 2 hours and 500 °C for 4 hours) raised the fatigue limit of titanium alloy orthopedic implants from 450 MPa to 600 MPa, which is enough to support the body's long-term weight.
4. Make sure the dimensions stay stable: match the standards for precise assembly.
After printing, metal 3D printed pieces may change size because of residual stress release or changes in the microstructure. This can make it harder to put them together correctly. Heat treatment can make dimensional stability much better by stabilizing the microstructure and getting rid of tension.
Lower deformation: Annealing treatment can lower the difference in thermal expansion coefficient between parts and lower machining deformation. For instance, after annealing, the diameter deviation of a printed part for a complex flow channel heat exchanger went down from ± 0.15mm to ± 0.05mm, which met the standards for sealing fluids.
Stability over time: Artificial aging and other aging treatments can get rid of supersaturated solid solutions in materials and stop them from changing size too much over time. For instance, the rate of size change for aluminum alloy printed parts dropped from 0.3% per year to 0.05% per year after being aged at 170 °C for 8 hours. This met the long-term service needs of aerospace.
5. Meeting unique performance needs: broadening the range of uses
Heat treatment can also provide metal 3D printed items specific qualities, which makes them useful in more places:
Improved resistance to corrosion: Solid solution treatment can dissolve the second phase in the material, which makes it less likely to corrode through electrochemical means. For instance, after being treated with a solution at 1050 °C, the pitting potential of 316L stainless steel printed components went from 320mV to 450mV, which is good for use in maritime conditions.
Controlling magnetic characteristics: Heat treatment can change the grain orientation and residual stress of soft magnetic materials to make their magnetic properties better. For instance, after being heated to 750 °C, the magnetic permeability of a given part of a solenoid valve goes up by 20%, and the amount of energy it uses goes down by 15%.
Improving biocompatibility: Medical implants need to be heated to get rid of surface contaminants and make a passivation film. After acid washing and annealing at 500 °C, for example, the surface roughness Ra of titanium alloy orthopedic implants went from 3.2 μm to 0.8 μm, and the cell adhesion rate went up by 40%.