Do metal printing materials require vacuum storage?

一, Material properties: the material has to deal with both oxidation and moisture absorption.
There are two main things that the storage environment for metal printing materials needs to do: keep oxidation processes from happening and regulate the amount of moisture in the air. These two things directly affect how stable and adaptable the material is chemically.
1. The harmful effects of oxidation reactions
When metal powder comes into contact with air, an oxide layer forms on the surface very quickly. For example, titanium alloy powder combines with oxygen at normal temperature to make TiO ₂. At high temperatures, as above 650 °C in the electron beam melting process, the oxidation rate goes up by a lot. There are three main difficulties that can happen when there is an oxide layer:
Less fluidity: The aggregation of oxidised particles makes the powder's particle size distribution wider, which makes it harder to disperse the powder evenly. A 3D printing business did an experiment that demonstrated that after 72 hours of exposure to air, the variation of the titanium alloy powder layer thickness went from ± 10 μ m to ± 35 μ m. This made the surface of printed objects 40% rougher.
More melting flaws: The oxide layer breaks down when it is hit by a laser or electron beam, which generates gas and pore flaws. When printing aviation engine blades, titanium oxide powder is 2.3 times more porous than powder maintained in a vacuum, and its fatigue life is cut by 60%.
Degradation of material performance: The oxide layer affects the way the metal crystals are arranged, which makes the metal less strong. The tensile strength of 316L stainless steel powder dropped from 680MPa to 520MPa after oxidation, while the elongation rate dropped from 45% to 28%.
2. The possibility of moisture absorption in chains
Metal powders are very good at soaking up moisture from the air because they have a porous structure. For instance, aluminum-based alloys can be held in a place with 60% relative humidity for 24 hours, and the moisture content can reach 0.15%, which is far higher than the process safety standard of 0.05%. Three significant dangers come from moisture absorption:
Possibility of spontaneous combustion: When wet powder rubs against something or is exposed to static charge, it can catch fire on its own. In 2023, there was an explosion of titanium alloy powder in a 3D printing business in Germany. The investigation found that the accident powder had a moisture percentage of 0.22%, which is much higher than what is safe.
Not a stable process: The laser makes the water evaporate right away, which makes the molten pool splash. A certain automotive parts supplier's records show that when moisture-absorbing powder was used for printing, the number of splash defects went up from 0.3% to 2.7%, and the scrap rate went up by 8 times.
The hydrogen embrittlement phenomenon: When water breaks down, it releases hydrogen atoms that get into the metal lattice and make it less robust. When printing aviation fasteners, the fracture toughness of the moisture-absorbing powder parts went down from 45MPa · m ¹/² to 28MPa · m ¹/², which is not strong enough to pass airworthiness tests.
二,Process risk: full control of the chain from storage to printing
The way metal printing supplies are stored has a direct impact on how safe and stable the next steps will be. By creating a confined space, hoover storage technology controls three key risks:
1. Safety control that won't let an explosion happen
Metal powder is a type of flammable dust, and its lower explosive limit (LEL) is closely related to the size of the particles and the amount of oxygen in the air. For instance, titanium alloy powder has a lower LEL of only 20g/m³ than aluminium powder, which has a higher LEL of 60g/m³. The following ways that hoover storage lowers the risk of explosion:
Fill a vacuum with nitrogen or argon gas to keep the oxygen level below 0.5%. This is called inert gas protection. A military company did an experiment that demonstrated that when the oxygen level dropped from 21% to 0.5%, the explosion pressure of titanium alloy powder dropped from 0.8 MPa to 0.02 MPa, and the safety factor went up by 40 times.
Static elimination design: The inside of the vacuum storage box is covered in anti-static material that has a resistance of less than 10 cm. This keeps static sparks from happening when the powder rubs against the walls.
Temperature control: The vacuum atmosphere slows down thermal convection, and when combined with a semiconductor refrigeration system, it keeps the storage temperature between 15 and 25 degrees Celsius. This stops high temperatures from speeding up oxidation reactions.
2. The stability of the process parameters
Metal printing is quite sensitive to how well the powder works, and vacuum storage can keep the following important indicators:
Distribution of particle size: The vacuum atmosphere keeps the D50 particle size steady and stops powder from clumping together. A medical implant business says that the standard deviation of particle size for powders maintained in a vacuum has gone down from 0.8 μm to 0.3 μm. The standard variation of porosity for printed stents has gone down from 1.2% to 0.5%.
Loose density: The range of fluctuation for the loose density of vacuum-stored powder has been cut in half, from ± 0.05g/cm ³ to ± 0.01g/cm ³. This makes sure that the powder thickness is the same throughout.
Liquidity: When the humidity in the air is kept below 5% RH, the angle of repose of vacuum-stored powders goes from 45 ° to 32 °, which makes them 40% more fluid.
三, Industry application: strong demand in the high-end manufacturing sector
In high-end areas including aerospace, medical implants, and nuclear energy, vacuum storage has become the conventional way to store metal printing ingredients.
1. The field of aerospace
When printing fuel nozzles for GE LEAP engines, cobalt chromium alloy powder kept in a vacuum is employed to make three big advances:
Longer lifespan: The nozzle for vacuum storage powder printing has gone through 10,000 thermal cycle testing and will last three times longer than regular castings.
Improved efficiency: porosity went from 0.8% to 0.2%, and fuel efficiency went up by 15%.
Cost savings: The powder usage rate went from 75% to 92%, and the cost of a single piece went down by 28%.
2. The topic of medical implants
In the therapeutic use of 3D printed porous titanium alloy interbody fusion devices, vacuum storage technology fixes two big problems:
Biocompatibility: The vacuum-stored powder has less than 0.05% impurities (O, N, H), which meets the ISO 13782 standard.
The effect on bone formation: The rate of bone growth over three months went up by 60% compared to powder that wasn't held in a vacuum, and the survival rate over five years went up from 85% to 97%.
3. The field of nuclear energy
When making flow channel plates for nuclear reactors, tantalum powder kept in a vacuum does the following: It resists corrosion: In a high-temperature steam environment at 560 °C, the corrosion rate drops from 0.02 mm/year to 0.005 mm/year.
Neutron absorption performance: The amount of boron stays steady at 1.2 wt%, which is safe for nuclear use.