What are the advantages of HIP processing in post-processing of metal 3D printing?

1. Getting rid of internal flaws: going from "porosity" to "zero defects"
Non-equilibrium solidification produced by quick cooling can create tiny holes during metal 3D printing. On the other hand, taking apart support structures or not completely melting powder might cause macroscopic shrinkage. These flaws might cause cracks to start, which greatly shortens the parts' fatigue life. HIP technology fixes defects by using the following methods:
Closing pores and combining metals
When metal materials are heated to a high temperature (typically 0.5 to 0.8 times the melting point of the material) and put under a lot of pressure (100 to 200 MPa), they become very pliable. Gas pressure causes the metal around the pores to change shape, making contact with each other and forming metallurgical bonds. This makes the pore volume diminish until it is gone. For instance, after HIP treatment, the porosity of IN718 high-temperature alloy manufactured using the SLM technique went from 0.8% to 0.02%, which made it 99.99% dense, which is what the aerospace industry needs to make sure materials are reliable.
Healing of microcracks
Thermal stress in metal 3D printing can cause microcracks to occur. The high-temperature annealing action of HIP treatment gets rid of residual stress, and the high-pressure environment makes the fracture tip bend plastically, which closes the crack and makes a stable grain boundary structure. Experimental data indicates that HIP treatment can diminish the crack density of 316L stainless steel by 90% and enhance the fracture toughness by 30%.
Refining the grains and making the microstructure more uniform
The high-temperature process of HIP is the same as annealing treatment, which can get rid of the undercooled structure or metastable phase that forms when SLM cools quickly. After HIP treatment, for instance, the coarse columnar crystals of the Ti6Al4V alloy change into fine equiaxed crystals, and the grain size goes from 50 μm to 10 μm. This makes the material much more flexible and resistant to fatigue.
2. Making mechanical performance better: finding the right balance between strength and toughness
HIP processing has two effects on the mechanical characteristics of metal 3D printed parts:
Strength and plasticity go better together.
The strength of the material may go down a little (typically by 5% to 15%) after HIP treatment, but its plasticity indicators, like elongation, go up a lot. For instance, following HIP treatment, the tensile strength of AlSi10Mg aluminum alloy produced by SLM technique went down from 420MPa to 380MPa, but the elongation went up from 8% to 15%, which is good for lightweight structural parts in cars.
A significant improvement in fatigue resistance performance
The main reason for fatigue crack growth is internal flaws. By getting rid of pores and microcracks, HIP treatment greatly increases the fatigue life of parts. For example, the high-temperature fatigue life of IN718 alloy treated with HIP at 650 ℃ and 690MPa has gone from 50 hours without treatment to 173 hours. This meets the life requirements of GE aircraft engines for essential parts.
Anisotropic removal
The interlayer bonding qualities of metal 3D printing could cause the mechanical properties to be different in different directions. The material works the same way in all directions when it is treated with HIP, which uses 360° of uniform pressure. For instance, the difference in radial and axial friction coefficients between silicon nitride ceramic balls treated with HIP is less than 5%, which is far better than standard sintering methods.
3. Broadening the scope of applications: Going from "Available" to "Reliable"
HIP processing helps with the technical side of using metal 3D printing technology on a wide scale in fields where it is in great demand.
Aerospace sector
Turbine blades, combustion chambers, and other parts of an aircraft engine must be able to work in situations with high temperatures, high pressures, and high stress. HIP treatment can get rid of thermal stress cracks that happen when the SLM process cools down too quickly, and it can also make materials better at high-temperature creep. Rolls Royce, for instance, uses HIP-treated nickel-based high-temperature alloy turbine discs that raise the operating temperature from 1200 to 1400 degrees Celsius and the thrust-to-weight ratio by 20%.
Field of medical implants
Orthopedic implants need to be strong and safe for the body. HIP treatment can get rid of alpha phase segregation in Ti6Al4V alloy, lower the chance of metal ions leaking out, and make the material last longer under stress. Clinical evidence indicates that the failure rate of hip implants subjected to HIP has diminished from 3% to 0.5% after a decade.
The energy and shipping industries
Parts like nuclear reactor pressure vessels and deep-sea sensor enclosures need to be able to handle very harsh conditions. The HIP-treated zirconia ceramic can handle high pressure of 110MPa in the deep sea, and the silicon carbide-coated fuel element can stay stable at high temperatures of 1200 ℃. These materials are very important for the fourth generation of nuclear power technology.