Powder Bed Fusion

Selective laser melting (SLM) is one of many proprietary names for a metal additive manufacturing (AM) technology that uses a bed of powder with a source of heat to create metal parts. Also known as direct metal laser sintering (DMLS), the ASTM standard term is powder bed fusion (PBF). 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.

History

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.

The ASTM International F42 standards committee has grouped selective laser melting into the category of "laser sintering", although this is an acknowledged misnomer because the process fully melts the metal into a solid homogeneous fully dense mass, unlike selective laser sintering (SLS) which is a true sintering process. Another name for selective laser melting is direct metal laser sintering (DMLS), a name deposited by the EOS brand, however misleading on the real process because the part is being melted during the production, not sintered, which means the part is fully dense.

A similar process is electron beam melting (EBM), which uses an electron beam as the energy source.

Process

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. Since the components are built layer by layer, it is possible to design complex freeform geometries, internal features and challenging internal passages that could not be produced using conventional manufacturing techniques such as casting or otherwise machined. SLM produces fully dense durable metal parts that work well as both functional prototypes or end-use production parts.

The process starts by slicing the 3D CAD file data into layers, usually from 20 to 100 micrometers thick, creating a 2D cross-section of each layer; this file format is the industry standard .stl file used on most layer-based 3D printing or stereolithography technologies. This file is then loaded into a file preparation software package that assigns parameters, values and physical supports that allow the file to be interpreted and built by different types of additive manufacturing machines.

With selective laser melting, thin layers of atomized metal powder are evenly distributed using a re-coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing platform that moves in the vertical (Z) axis. 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. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors and remains in focus along the layer utilising an F-Theta lens arrangement. The laser energy is intense and focused enough to permit full melting (fusion) of the particles to form a solid structure. The process is repeated layer after layer until the part is complete.

SLM machines predominantly uses a high-powered Yb-fiber optic laser with standard laser powers ranging from 100–1000 W. Inside the build chamber area, there is a material dispensing platform and a build platform along with a recoater system (blade or roller) used to evenly spread new powder across the build platform. Parts are built up additively layer by layer, typically using layers 30–60 micrometers thick.

FZU_3Dprinting_4.jpg, Print finished (excess powder cleaned)

Materials

Selective laser melting (SLM) machines can operate with a work space up to 1 m (39.37 in) in X, Y and Z. Some of the materials being used in this process can include Ni based super alloys, copper, aluminum, stainless steel, tool steel, cobalt chrome, titanium and tungsten. SLM is especially useful for producing tungsten parts because of the high melting point and high ductile-brittle transition temperature of this metal. In order for the material to be used in the process it must exist in atomized form (powder form). These powders are generally gas atomized prealloys, being the most economical process to obtain spherical powders on an industrial scale. Sphericity is desired because it guarantees a high flowability and packing density, which translates into fast and reproducible spreading of the powder layers. Highly spherical powders with a low level of internal porosity are produced by plasma atomization and powder spheroidization. To further optimize flowability, narrow particle size distributions with a low percentage of fine particles like 15 – 45 μm or 20 – 63 μm are typically employed. Currently available alloys used in the process include AISI 316L, AISI 304, C67, F53, H13, 17-4 PH and 15-5 stainless steel, maraging steel, cobalt chromium, inconel 625 and 718, copper-based alloys (CW510 Brass, Ecobrass, Bronze), aluminum AlSi10Mg, and titanium Ti6Al4V.
The mechanical properties of samples produced using selective laser melting differ from those manufactured using casting. AlSiMg samples produced using direct metal laser sintering exhibit a higher yield strength than those constructed of commercial as-cast A360.0 alloy by 43% when constructed along the xy-plane and 36% along the z-plane. While the yield strength of AlSiMg has been shown to increase in both the xy-plane and z-plane, the elongation at break decreases along the build direction. These improvement of the mechanical properties of the direct metal laser sintering samples has been attributed to a very fine microstructure.

Additionally, industry pressure has added more superalloy powders to the available processing including AM108.  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.  Based on research done at the Tokyo Metropolitan University, it is shown that creep rupture and ductility are typically lower for additive printed Ni based superalloys compared to wrought or cast material. The directionality of print is a major influencing factor along with grain size. Additionally, wear properties are typically better as seen with the studies done on additive Inconel 718 due to surface condition; the study also demonstrated the laser power's influence on density and microstructure. Material Density that is generated during the laser processing parameters can further influence crack behavior such that crack reopening post HIP process is reduced when density is increased. 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.

Overview and benefits

SLM is a fast developing process that is being implemented in both research and industry. This advancement is very important to both material science and the industry because it can not only create custom properties but it can reduce material usage and give more degrees of freedom with designs that manufacturing techniques can't achieve. Selective laser melting is very useful as a full-time materials and process engineer. 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. Having SLM would really improve the process of not only getting parts created and sold, but making sure the properties align with whatever is needed out in the field. Current challenges that occur with SLM are having a limit in processable materials, having undeveloped process settings and metallurgical defects such as cracking and porosity. The future challenges are being unable to create fully dense parts due to the processing of aluminum alloys. Aluminum powders are lightweight, have high reflectivity, high thermal conductivity, and low laser absorptivity in the range of wavelengths of the fiber lasers which are used in SLM.

These challenges can be improved with doing more research in how the materials interact when being fused together.

Defect formation

Despite the large successes that SLM has provided to additive manufacturing, the process of melting a powdered medium with a concentrated laser yields various microstructural defects through numerous mechanisms that can detrimentally affect the overall functionality and strength of the manufactured part. Although there are many defects that have been researched, we will review some of the major defects that may arise from SLM in this section.

Two of the most common mechanical defects include lack of fusion (LOF) or cracking within solidified regions. LOF involves the entrapment of gas within the structure rather than a cohesive solid. 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. Cracking is another mechanical defect in which low thermal conductivity and high thermal expansion coefficients generate sufficiently high amounts of internal stresses to break bonds within the material, especially along grain boundaries where dislocations are present.

Additionally, although SLM solidifies a structure from molten metal, the thermal fluid dynamics of the system often produces inhomogeneous compositions or unintended porosity which can cumulatively affect the overall strength and fatigue life of a printed structure. For example, the directed laser beam can induce convection currents upon direct impact in a narrow "keyhole" zone or throughout the semi-molten metal that can impact the material’s overall composition. Similarly, it is found that during solidification, dendritic microstructures progress along temperature gradients at different speeds, thus producing different segregation profiles within the material. 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.

Pore formation is a very important defect when samples are printed using SLM. 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. Another possible reason for pore formation is the so-called balling effect which was frequently obtained in case of austenitic stainless steels. Poor surface wettability and low energy inputs might lead to break-up of the melt track to minimize energy. Consequently, several spherical melting spots form, leaving pores after solidification.

Lastly, secondary effects that arise from the laser beam can unintentionally affect the structure’s properties. 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. Not only that, in powder beds containing oxides, the power of the laser and produced convection currents can vaporize and "splatter" oxides at other locations. These oxides accumulate and have a non-wetting behavior, thereby producing a slag that not only removes the beneficial nature of oxide within the composition but also provides a mechanistically favorable microenvironment for material cracking.

Mechanical properties

High temperature gradients are presented during selective laser melting (SLM) processes, which causes non-equilibrium conditions at the solid/liquid interface, thereby leading to rapid solidification as the melt pool undergoes a phase transformation from liquid to solid. As a consequence, a wide range of effects might take place like the formation of non-equilibrium phases and changes in the microstructure.

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. A central characteristic of SLM-manufactured alloys is large anisotropy in mechanical properties . While the grain structure in cast metals is typically characterized by roughly uniform, isotropic grains, alloys manufactured using SLM exhibit substantial elongation of grains in the build direction. The anisotropy in grain structure is associated with anisotropy in the distribution of defects, the direction of crack propagation, and ultimately the mechanical properties.

On the other hand, because of the special thermo-kinetic features associated with SLM, there are many novel microstructural architectures unique to this process . As a new processing technique, SLM can produce a unique microstructure that is difficult to achieve using conventional techniques.

Nickel-based superalloys

Enhancements in creep resistance, ultimate tensile strength and toughness

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