Selective laser melting
PBF is a rapid prototyping, 3D printing, or additive manufacturing technique designed to use a high power-density laser to melt and fuse metallic powders together.
[2][3] Selective laser melting is one of many proprietary powder bed fusion technologies, started in 1995 at the Fraunhofer Institute ILT in Aachen, Germany.
A research project run by Wilhelm Meiners, Konrad Wissenbach, and Andres Gasser resulted in the so-called basic ILT SLM patent.
[6] Selective laser melting is able to process a variety of alloys, allowing prototypes to be functional hardware made out of the same material as production components.
This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 1000 parts per million.
Sphericity is desired because it guarantees a high flowability and packing density, which translates into fast and reproducible spreading of the powder layers.
It is not only the Print operation and orientation that provides a change in material properties, it is also the required post processing via Hot Isostatic Pressure (HIP) Heat Treat and shot peen that change mechanical properties to a level of noticeable difference in comparison to equiaxed cast or wrought materials.
[citation needed] It is critical to have a full overview of the material along with its processing from print to required post-print to be able to finalize the mechanical properties for design use.
Requests such as requiring a quick turnaround in manufacturing material or having specific applications that need complex geometries are common issues that occur in industry.
These defects can arise from not using a laser source with adequate power or scanning across the powdered surface too quickly, thereby melting the metal insufficiently and preventing a strong bonding environment for solidification.
[24] Ultimately, these thermal fluid dynamical phenomena generate unwanted inconsistencies within the printed material, and further research into mitigating these effects will continue to be necessary.
Pores are revealed to form during changes in laser scan velocity due to the rapid formation then collapse of deep keyhole depressions in the surface which traps inert shielding gas in the solidifying metal.
One such example is the development of secondary phase precipitates within the bulk structure due to the repetitive heating within solidified lower layers as the laser beam scans across the powder bed.
Depending on the composition of the precipitates, this effect can remove important elements from the bulk material or even embrittle the printed structure.
For the reasons above, the mechanical properties of alloys produced by SLM can deviate substantially from those conventionally manufactured counterparts in their as-built state.
[30][31][32] Inconel IN625, a precipitation-hardened nickel-chromium alloy, showed equal or even higher creep strength at elevated temperatures of 650 ̊C and 800 ̊C than wrought IN625.
With multiple alloying elements and high aluminum/titanium fraction, these materials, when consolidated through SLM form various secondary phases, which affects the processability and leading to weakness within the structure.
[34] The cellular structure is considered to be the main cause of the differences in deformation behavior, especially during the first creep stage, primarily because it limits the work-hardening capacity of the material.
Much of the pioneering work with selective laser melting technologies is on lightweight parts for aerospace[36] where traditional manufacturing constraints, such as tooling and physical access to surfaces for machining, restrict the design of components.
This is the case e.g. for spares/replacement parts for obsolete equipment and machines (e.g. vintage cars) or customizable products like implants designed for individual patients.
[40] On September 5, 2013 Elon Musk tweeted an image of SpaceX's regeneratively-cooled SuperDraco rocket engine chamber emerging from an EOS 3D metal printer, noting that it was composed of the Inconel superalloy.
Using Inconel, an alloy of nickel and iron, additively-manufactured by direct metal laser sintering, the engine operates at a chamber pressure of 6,900 kilopascals (1,000 psi) at a very high temperature.
"[46] The 3D printing process for the SuperDraco engine dramatically reduces lead-time compared to the traditional cast parts, and "has superior strength, ductility, and fracture resistance, with a lower variability in materials properties.
Additionally, certain types of nanoparticles with minimized lattice misfit, similar atomic packing along matched crystallographic planes and thermodynamic stability can be introduced into metal powder to serve as grain refinement nucleates to achieve crack-free, equiaxed, fine-grained microstructures.
[57] To truly take advantage of the recyclability, a cradle-to-cradle approach can be implemented to ensure that all steel parts are properly discarded of at their end-life through disassembly.
The exception to this is in research environments where the machine is not constantly used and use is more infrequent, in this case, the embodied energy from primary processing and manufacturing is dominant.
Transportation costs will vary on manufacturing plants and consumers but these values are often negligible (<1%) in comparison to other high impacting parts of the SLM lifecycle.
[61] Another example is the 1kg weight reduction through a hydraulic valve body which estimates a saving of 24,500L of jet fuel and 63 tons of CO2 emissions from a lightweight design and decreased material used compared to traditional manufacturing methods.
[59] SLM is often a more sustainable option due to decreased raw material use, less complex tool use, lightweight part potential, near-perfect final geometries, and on-demand manufacturing.
[62] The aspects of size, feature details and surface finish, as well as print through dimensional error[clarification needed] in the Z axis may be factors that should be considered prior to the use of the technology.
