How do heat treatment parameters affect the final part performance?

1. Temperature parameter: microstructure reconstruction driven by phase transition
Control of solid solution temperature and phase composition
The temperature of the solution treatment has a direct effect on how well alloying elements dissolve in the metal matrix. The strengthening core of 3D printed 17-4PH stainless steel is the copper particles that fall out of its martensitic matrix. When the temperature of the solid solution is kept between 1040 and 1080 °C, the copper element entirely dissolves in the austenite matrix, creating a supersaturated solid solution. If the temperature is below 1000 °C, leftover copper particles will not strengthen the material enough. If the temperature goes above 1100 °C, the material will become less robust because the grains will get bigger. According to research from the Institute of Metals at the Chinese Academy of Sciences, HIP treatment at 950 °C can help the α 'martensite change into an α+β two-phase structure in the Ti6Al4V alloy. This increases the elongation to 13.15% while keeping the yield strength at 909.5MPa.
Improving the temperature and precipitation behavior of aging
Time treatment makes things stronger by managing the size and spread of the second phase particles. The Shanghai University of Technology team aged the CuCrZr alloy made by SLM at 500 °C for 1 hour. This raised its tensile strength from 460 MPa to 585 MPa and its conductivity from 31% IACS to 64% IACS. The strengthening mechanism is based on the fact that Cr atoms come out of the copper matrix during the aging process. This creates nanoscale CrxZry particles that stop dislocation migration through the Orowan strengthening mechanism. As the aging temperature goes up to 550 °C, the precipitated phases are coarser, which makes the material weaker. However, the material becomes more ductile, with a 20% increase in ductility due to a drop in dislocation slip resistance.
2. Time parameter: The balance between fixing bugs and performance
Time to hold and effectiveness of pore closure
The holding duration has a direct impact on how well the pores are repaired after HIP treatment. Research on the HIP process of Ti6Al4V alloy shows that at 920 ℃/140MPa, a 2-hour treatment can reduce the porosity from 0.8% to 0.02% and achieve a density of 99.99%; If the holding time is extended to 4 hours, the porosity further decreases to 0.005%, but the grain size increases from 10 μ m to 15 μ m, resulting in an 8% decrease in yield strength. This means that while holding time can make things denser, it can also induce strange grain growth. So, a balance needs to be found between fixing defects and keeping performance up.
Kinetics of phase change and insulation time
The solid solution treatment's insulating time should make sure that all of the alloy elements are completely dissolved. For 3D-printed IN718 high-temperature alloy, keeping it at 1080 °C for 1 hour can totally dissolve the Nb element in the γ matrix. If the insulation period is cut down to 30 minutes, the strengthening phase of γ '' can't fully precipitate, which makes high-temperature creep performance drop by 40%. The length of time the material is insulated during aging treatment impacts the size of the phases that form. For instance, after aging at 720 °C for 8 hours, the γ '' phase in 718 alloy is 50nm in size, which is the best for strengthening. After 16 hours of aging, the precipitated phase grew to 100nm, which made the strength drop by 15%.
3. Cooling rate: refining the organization and controlling the residual stress
The rate of quenching and the production of martensite
The rate at which the metal cools during quenching influences what phase transition products it will have. For 3D printed H13 tool steel, the oil quenching cooling rate of 50 °C/s may make Flat noodles martensite with a hardness of 52HRC. If you cool it down with air (5 °C/s), the bainite structure will form and the hardness will drop to 40HRC. Even though rapid quenching can make things harder, it can also cause them to crack. To find the right balance between hardness and residual stress, graded quenching is needed (for example, first cooling to 600 °C and then oil cooling).
A slow cooling rate and stress relief
The sluggish cooling rate during annealing treatment has an effect on how residual stress is released. It took 2 hours to cool the AlSi10Mg aluminum alloy for 3D printing from 300 °C to room temperature at a rate of 5 °C/min. This reduced the residual stress by 70%. If the cooling rate is raised to 20 °C/min, the residual stress will only drop by 30%. Slow cooling helps dislocation rearrangement and grain boundary migration, which relieves stress. However, a cooling rate that is too slow can cause grains to become coarser, therefore appropriate material optimization settings are needed.
4. Multi-parameter collaborative optimization: from "trial and error" to "precise control"
Digital twin technology drives parameter prediction
Siemens and Boeing worked together to make a digital twin platform that can show how the temperature field, stress field, and microstructure of 3D printed Ti6Al4V alloy change during HIP treatment. The system can figure out the best HIP method (such 920 °C/140MPa/2h) by taking into account things like starting porosity and grain size. This can make the parts last three times longer and cut the number of testing in half.
Parameter inversion with the help of machine learning
GE Aviation employs machine learning techniques to look at 100,000 sets of heat treatment data and create a "temperature time cooling rate performance" mapping model. This model can figure out process settings that will work for certain performance needs. When IN718 alloy needs to keep a creep life of 1000 hours at 650 °C, for instance, the system suggests a process scheme of 1080 °C/1h solid solution+720 °C/8h aging. The measured creep life is 1200 hours.
5. Case study of an industry: going from the lab to the factory
The field of aerospace
To improve the 3D printing of nickel-based high-temperature alloy turbine discs, Rolls Royce adopts HIP treatment. Parts treated with HIP have a creep life of 173 hours at a high temperature of 1400 °C, which is more than the 50 hours that was needed for essential components of GE9X engines.
Field of Medical Implants
After 950 °C for 4 hours of HIP treatment on the 3D printed Ti6Al4V hip joint implant by Johnson&Johnson, its maximum fatigue strength reached 550MPa (107 cycles), which is the same as the forged annealed condition. Simultaneously, the surface roughness Ra<0.01 μm satisfied the biocompatibility criteria.