What are the requirements of 3D printing for metal raw material powder ?
Compared with the traditional subtractive manufacturing method, 3D printing almost does not cause the waste of metal materials, and this "additive manufacturing" direct forming feature makes the product equipment problems in the production process greatly reduced. Metal powder materials are the raw materials for metal 3D printing. The basic properties of the powder have a great relationship with the quality of the final molded product. The requirements of metal 3D printing for powders are mainly in the aspects of chemical composition, particle shape, particle size and particle size distribution, fluidity, and recycling.
The main chemical components of the raw materials include metal elements and impurities. The main metal elements commonly used are Fe, Ti, Ni, Al, Cu, Co, Cr, and precious metals Ag, Au, etc. Impurities include Si, Mn, C, S, P, O in reduced iron, other impurities mixed from raw materials and powder production, water and other gases adsorbed on the powder surface.
During the molding process, impurities may react with the substrate, change the properties of the substrate, and negatively affect product quality. The presence of dopants will also cause the powder to melt unevenly, easily causing internal defects in the product. When the oxygen content of the powder is high, the metal powder not only easily oxidizes and forms an oxide film, but also causes spheroidization, which affects the density and quality of the product.
Therefore, it is necessary to strictly control the impurities and doping of the raw material powder to ensure the quality of the product. Therefore, the metal powder for 3D printing needs to use a metal powder material with a higher purity.
Particle shape, powder size and particle size distribution
1. Shape requirements. Common particle shapes are spherical, nearly spherical, flake, needle and other irregular shapes. Irregular particles have a larger surface area, which is beneficial to increase sintering drive. However, the powder particles with high sphericity have good fluidity, and the powder is spread evenly, which is conducive to improving the density and uniformity of the product. Therefore, powder particles for 3D printing are generally required to be spherical or nearly spherical.
2. Powder particle size and particle size distribution. Studies have shown that powders are melted and sintered by directly absorbing energy from laser or electron beam scanning. Smaller particles have a larger surface area and more direct energy absorption, which is easier to heat up and more conducive to sintering. In addition, the particle size of the powder is small, the gap between the particles is small, the bulk density is high, and the density of the parts after forming is high, which is conducive to improving the strength and surface quality of the product. However, if the particle size of the powder is too small, the powder is prone to adhesion and agglomeration, resulting in a decrease in the fluidity of the powder, which affects the powder transportation and spreading the powder evenly.
Therefore, the fine powder and coarse powder should be mixed in a certain proportion, and the appropriate particle size and particle size distribution should be selected to achieve the desired forming effect.
Process performance requirements of powder
The process performance of the powder mainly includes loose packing density, tap density, fluidity and recycling performance.
1. The bulk density is the density when the powder is naturally stacked, and the tap density is the density after vibration. The powder with good sphericity and wide particle size distribution has high bulk density and low porosity, and the formed parts have high density and good forming quality.
2. Liquidity. The fluidity of the powder directly affects the uniformity of powder spreading or the stability of powder feeding. Poor fluidity of the powder can easily cause uneven thickness of the powder layer and uneven metal melting in the scanning area, resulting in uneven internal structure of the product and affecting the forming quality. The high-fluidity powder is easy to fluidize, deposit evenly, and has a high powder utilization rate, which is conducive to improving the dimensional accuracy of the 3D printed molded parts and the uniform and dense surface.
3. Cycle performance. After the 3D printing process is completed, the unmelted powder remaining in the powder bed can still be used after being recovered by screening. However, under a long-term high-temperature environment, the powder in the powder bed will have certain performance changes.