Is there a significant difference in 3D printing accuracy among different metal materials?

1. Material properties: the physical and chemical characteristics that affect the possibility for accuracy
The precision of 3D printing with metal materials is mostly due to the interplay between the material's thermodynamic behavior during the melting and solidification process and the process management. The changes in the properties of the following four common materials directly affect how accurate their prints are:
Alloy of titanium (like TC4/Ti-6Al-4V)
Titanium alloys are strong, light, and resistant to rust, however they can only be printed accurately because of two main reasons:
High thermal shrinkage rate: TC4 has a linear expansion coefficient of 8.6 × 10 ⁻⁶/℃, which means that it can readily cause residual stress when it cools quickly, which can cause parts to warp and change shape. For instance, if you don't utilize hot isostatic pressing (HIP) after printing a mold for an aircraft engine blade, the dimensions can be off by as much as ± 0.3mm. After HIP processing, the dimensions can be off by as little as ± 0.05mm.
Low laser absorption rate: Titanium alloy may reflect up to 60% of laser light, so it needs a high energy density (typically > 100W/mm²) to melt evenly. However, too much energy might create splashing and change the roughness of the surface. You can lower the surface roughness from Ra25 μ m to Ra10 μ m by changing the scanning technique (such using checkerboard scanning).
316L stainless steel is one example.
Stainless steel has a large process window and is not very sensitive to heat cracks, which makes it better for printing:
Stable wide melt pool: 316L melts at 1375 °C, and a steady melt pool may be made with a laser power of 50 to 200W and a dimensional accuracy of ± 0.05mm. A company that makes medical devices used SLM technology to print bone plates with an aperture tolerance of ± 0.02mm, which met the requirements for putting together orthopedic implants.
Uniformity in the organization: 316L's austenitic nature makes it less likely to separate during the printing process. When used with solid solution treatment (insulation at 1050 °C for 1 hour and water quenching), interlayer bonding faults can be fixed, which increases fatigue life by 30%.
Aluminum alloy, such AlSi10Mg
The biggest problem with printing aluminum alloy is that it has a high thermal conductivity and is prone to cracking when it becomes hot.
Cracks form when something cools quickly: AlSi10Mg has a thermal conductivity of 150W/(m · K), and the melt pool can cool down at a rate of 10 ⁶ ℃/s, which makes it simple for hot fractures to form at grain boundaries. Adding 0.5% Sc element can make the grain size smaller than 1 μm, which lowers the crack rate from 15% to 0.5%.
Effect of surface oxide film: The aluminum surface is likely to generate a thick oxide film (Al ₂ O3), which makes the powder not flow well and means that printing needs to be done with vacuum inert gas protection. After improving the gas circulation system, the surface roughness of a new energy vehicle battery pack bracket went from Ra50 μm to Ra15 μm.
Nickel-based high-temperature alloys, such Inconel 718
The challenge of high-temperature alloys is controlling the microstructure at very high temperatures:
Columnar crystal growth tendency: During printing, Inconel 718 tends to generate columnar crystals that develop in the direction of construction. This makes the material anisotropic. Changing the scanning speed (600–1000 mm/s) and layer thickness (30–50 μm) may make the grain size go from 500 μm to 100 μm, which makes the tensile strength go up by 15%.
Sensitivity to microcracks: The γ 'phase (Ni ∝ (Al, Ti)) is likely to form uneven deposits when it cools quickly, which can cause microcracks. More than 90% of microcracks may be removed by using graded heat treatment (720 °C insulation for 8 hours, followed by air cooling and 620 °C insulation for 8 hours).
2. Adaptability of the process: Choosing a path for precise implementation
The precision of metal 3D printing depends on both the material and how well the process type matches. The following four common processes have quite different levels of accuracy:
Laser Selective Melting (SLM) Precision benefit: The laser spot diameter of SLM can be as tiny as 50 μm, the layer thickness can be between 20 and 60 μm, the dimensional precision can be as high as ± 0.05mm, and the surface roughness Ra can be as low as 10 μm. An aviation company used SLM to print turbine blades, making sure that the blade profile tolerance was within ± 0.03mm, which is what aviation engines need to be able to be put together.
Material restrictions: To make materials with high reflectivity (like copper) absorb more, you need to use a green laser (532nm) or a blue laser (450nm). However, the cost of the equipment goes up by 30% to 50%.
Electron Beam Melting (EBM) Accuracy features: EBM works in a vacuum with a lot of energy density in the electron beam (up to 10 ⁴ W/mm ²), which makes it good for printing materials with high melting points, including titanium alloys. A certain orthopedic implant manufacturer used EBM to print hip joint cups. The surface roughness was Ra ≤ 8 μm, there was no oxide layer, and the cups were more biocompatible than those made with traditional methods.
Thermal stress control: EBM can warm parts to 700 degrees Celsius, which can cut down on residual stress and warping by 80%.
Directed Energy Deposition (DED) has a nozzle diameter of 0.8 to 2 mm, a layer thickness of 0.5 to 2 mm, a dimensional precision of ± 0.5 mm, and a surface roughness of Ra20 to 100 μ m. A certain aviation engine company used DED to fix a turbine disk. The repair layer and the substrate are bonded together with a metallurgical strength of 400MPa, which meets the service requirements.
Advantage of efficiency: DED has a sedimentation rate of 200 cm³/h, which is more than 10 times that of SLM. This makes it good for fixing or preforming huge pieces.
Spray Adhesive (BJ)
Potential for precision: BJ has a dimensional accuracy of ± 0.1mm and a surface roughness of Ra20-60 μm. However, it needs to be degreased (400-600 °C) and sintered (1200-1300 °C) after treatment, which causes it to shrink by 15% to 20%. A certain car firm employs BJ printed mold inserts, and after they are finished, the size stays stable within ± 0.05mm, which is what is needed for mass manufacturing.
Cost advantage: BJ's single piece costs 60% to 70% less than SLM, which makes it good for situations where medium precision and big scale are needed.
3. Typical instance: Practical application verification of accuracy discrepancies
Titanium alloy blades for aircraft engines Printing using SLM
One aviation company employed SLM technology to make TC4 titanium alloy blades. By improving scanning techniques (like spiral scanning) and support structures (like lattice support), the blade profile tolerance went from ± 0.1 mm to ± 0.03 mm, and the surface roughness went from Ra25 μ m to Ra8 μ m. This made the engine 2% more efficient.
316L stainless steel is used for medical device implantation. SLM printing
An orthopedic company used SLM to print 316L stainless steel bone plates. The aperture tolerance was set at ± 0.02mm, and after electrolytic polishing, the surface roughness Ra < 0.8 μ m met the ISO 13485 medical standard, cutting the time it took for the bone to integrate by 30%.
Battery pack for a new energy vehicle: AlSi10Mg Printed with SLM
A new energy vehicle firm employs SLM printed battery pack brackets to change the grain size by adding 0.5% Sc element, which lowers the hot fracture rate from 15% to 0.5%. The bracket's strength goes up by 25% and its weight goes down by 30% when it is heat-treated with T6 (530 °C solid solution + 170 °C aging).