Can heat treatment improve the strength of metal 3D printed parts?

1. The principles of heat treatment technology: changing the microstructure into macroscopic properties
By heating, insulating, cooling, and doing other things, heat treatment modifies the crystal structure, phase composition, and flaw distribution of metal materials, which improves their mechanical properties. The most important thing about heat treatment for metal 3D printed objects is:
Relief of residual stress
During 3D printing, materials go through quick cycles of melting and solidifying, which leaves residual stresses inside the components. These stresses can cause the parts to bend, break, or fail from fatigue. Annealing therapy is heating the parts above the temperature at which they can recrystallize and then slowly cooling them. This can make the grains recover and recrystallize, which lets out stress inside the parts. For instance, a rail transit company in Zhuzhou used vacuum annealing to prepare SLM-printed titanium alloy transmission parts. This cut residual stress to 30% of the original value and made the parts much more stable in size.
Refining and mixing grains
When 3D printed pieces cool too quickly, they might get coarse grains and an uneven orientation, which makes the material weaker. By adjusting the cooling rate, normalization treatment may make the grain size 50% to 70% smaller than it was before, and it can also stop component segregation. In the aerospace industry, for instance, normalizing treatment is often used to improve the grain structure of Inconel 718 high-temperature alloy parts, which raises their yield strength by 15% to 20%.
Precipitation during the strengthening phase
By manipulating temperature and time, aging treatment encourages solute atoms to come together to create distributed strengthening phases (such β phase and γ' phase), which makes the material much harder. The team at Monash University in Australia aged the Beta-C titanium alloy printed by SLM at 480 °C. The final tensile strength was 1611 MPa, which is a new record for the specific strength of 3D printed metals. The strengthening mechanism is the creation of high-density nano twin precipitates.
Complete control over mechanical performance
The process of quenching and tempering (quenching with high-temperature tempering) uses martensitic transformation and tempering softening to find a balance between high strength and good toughness in parts. After being quenched and tempered, the 17-4PH stainless steel produced by SLM has a tensile strength of 1035MPa, a yield strength of 860MPa, and an elongation of roughly 10%. This makes it strong enough for structural parts that need to hold a lot of weight.
2. Industrial Practice: New Uses for Heat Treatment in Important Areas
The field of aerospace
Aircraft engine blades, combustion chambers, and other parts need to be able to handle very high temperatures, pressures, and stresses. This means that the materials used to make them must be very strong. GE Aviation prints Inconel 718 fuel nozzles using SLM technology. They then employ hot isostatic pressing (HIP) and solution aging treatment to get rid of internal pores (density ≥ 99.9%), which makes the parts 40% stronger at 650 °C. It has worked well for making a lot of LEAP engines.
Field of medical devices
It is important for orthopedic implants to be strong and safe for the body. Johnson & Johnson Medical made Ti6Al4V hip joints using SLM and then used vacuum annealing to get rid of surface residual stress. Chemical polishing was also employed to smooth down the surface to Ra0.8 μm, which tripled the implant's fatigue life and met long-term clinical needs.
Field of rail transit
SLM is used by a company in Zhuzhou to print parts for titanium alloy transmissions. The tensile strength of the parts is 850MPa, the hardness is HRC35, the internal density is 99.8%, the dimensional accuracy is ± 0.03mm, and the service life of the parts is 50% longer than that of traditional casting parts. This is because the laser parameters (power 300W, scanning speed 1200mm/s) were optimized and the parts were treated with vacuum annealing.