What are the effects of choosing powders with different particle sizes on printing quality?

一, A dynamic equilibrium between the size of the particles and the quality of the powder spreading
1. The game between fluidity and smoothness
The flowability of powder is a good sign of how evenly it spreads. The Fraunhofer Institute in Germany did comparative tests that showed that 316L stainless steel powder with a diameter of 20–63 μm had a flow rate of 18–22s/50g in Hall current meter testing. This was much better than the 25–30s/50g flow rate for powder with a diameter of 15–53 μm. This shows that powders with larger particle sizes are better for quickly spreading. Laser confocal microscopy observations showed that the thickness deviation of the powder layer made of 15–53 μm powder was only ± 0.8 μm, whereas the deviation of 20–63 μm powder was ± 1.5 μm. This difference is especially clear after multi-layer accumulation: when the printing height is more than 50mm, the size error of large particle powder sections goes up by 3–5%.
Industry practice: When a medical device company uses 20–63 μm powder to print orthopaedic implants, the powder spreading efficiency goes up by 20%, but 10% of the parts need to be surface trimmed to get rid of local protrusions. When using 15–53 μm powder, on the other hand, the powder spreading speed needs to be slowed down by 15%, but "one-time moulding" can be done without any extra processing.
2. Strategy for optimising the distribution of particle sizes
Using a bimodal particle size distribution, like mixing 15 μm fine powder with 45 μm coarse powder, can make powder spreading much better. Fine powder fills in the spaces between coarse powders, making the powder bed 12% denser but yet allowing it to flow well. This is how Airbus Group makes engine fuel pipes. It raises the powder bed density from 58% to 65% and cuts down on the problem of molten pool sinking by a lot.
二, The Profound Impact of Particle Size on Thermal Conductivity Efficiency
1.The conflict between laser absorption and thermal diffusion
Compared to powders with 53–105 μm, powders with small particle sizes (15–45 μm) have a greater specific surface area and absorb 18% more laser light. But powders that are too fine (<10 μ m) are likely to spheroidise when they oxidise, which makes them 25% less thermally conductive. NASA discovered in GRCop-42 copper alloy printing that decreasing the powder particle size from 30 μm to 15 μm raised the thermal conductivity from 380W/(m · K) to 410W/(m · K). However, the oxygen content must be kept below 800ppm to prevent the formation of oxide film.
2. Adjusting the size of particles for electron beam technology
Using 45–105 μm coarse powder can generate charge buildup, which can lead to printing problems. Electron beam selective melting (EBM) technology can help with this. GE Aviation uses this range of particle sizes to make LEAP engine turbine blades. This speeds up printing to 800 cm³/h, which is three times faster than SLM technology, while keeping the density at 99.9%.
三, The relationship between microstructure and mechanical qualities
1. Controlling the size of the grains
The size of the powder particles directly impacts how quickly the melt pool cools, which in turn determines the size of the grains. Airbus Group testing showed that the average grain size of 316L stainless steel parts printed with 15–53 μm powder is 15 μm. Parts printed with 20–63 μm powder have an average grain size of 28 μm. The temperature cycling test from -50 to 150 °C indicated that small particle size parts leaked less than 1 × 10 ⁻⁸ Pa · m ³/s, but large particle size parts leaked a little after 200 cycles.
2. Distribution of residual stress Fine powder printed parts have 30% more residual stress because they cool down faster. Siemens Energy employs 53–105 μm coarse powder to make gas turbine combustion chambers. They also use stress relief annealing at 650 °C to lower the residual stress from 180 MPa to 60 MPa while keeping the density at 99.5%.
四, Matrix for choosing particle size in industry use
1. The field of aerospace
Parts at the hot end of the engine: Choosing 15–45 μm powder should be the most important thing. NASA uses 30 μ m median particle size powder to make GRCop-42 copper alloy combustion chamber liners. This gives them a thermal conductivity of 410W/(m · K) and a tensile strength of 420MPa.
Parts that hold up the structure: Using 45-105 μ m powder, like GE Aviation printing Ti-6Al-4V blades, and fine-tuning the electron beam settings can provide forged parts a fatigue life of 85%.
2. The field of medical devices
Orthopaedic implants: 15-53 μm powder is becoming the norm. For instance, Johnson&Johnson DePuy Synthesises printed cobalt chromium alloy acetabular cups with a surface roughness of Ra<0.8 μ m. To do this, they utilised 25 μ m fine powder. This lowered the danger of bone resorption.
Personalised stent: Using 5-25 μm ultrafine powder, like EOS printing NiTi shape memory alloy vascular stent, we can regulate the porosity to 80% and the elastic modulus to 30GPa.
3. The field of mould making
Shape that follows the cooling mould: Using 53–105 μm powder, as DMG MORI printing H13 tool steel moulds, can save the cost of each piece by 0.8 yuan and the time it takes to make a mould by 40%.
Mould with high heat conductivity: When BASF prints CuCrZr moulds with 15–45 μm copper alloy powder, they have a thermal conductivity of 320W/(m · K), which is five times higher than that of steel moulds.