Additive manufacturing (AM) is used to fabricate parts from a variety of materials including metals or plastics, and the raw feedstock can be in the form of filament, powder, or wire. The resulting part may require post processing in an oven or furnace, or even machining with a CNC. Depending on the material and process, the final part can be used in applications as varied as toys, blow mold tooling, or as components in jet engines or nuclear reactors.
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For many applications, plastic parts can't perform the way that metals can. High temperatures and high strength applications, or parts that need to be electrically or thermally conductive, are much more likely to be produced using metal.
Conventional metal manufacturing methods like forging, stamping, rolling, and extrusion are well suited to producing uncomplicated, high quantity parts. But what if you need complex parts cost-effectively produced in low quantities? This is where AM shines ' stepping in to produce parts that conventional manufacturing can't.
But how do AM metal parts compare to traditional parts and what are the different ways that metal can be 3D printed? In this article we talk about three different metal additive manufacturing methods ' bound metal, laser powder bed fusion, and hybrid additive CNC ' and discuss the relative benefits and disadvantages.
The most common type of bound metal 3D printing uses Fused Filament Fabrication (FFF). FFF is how most people envision 3D printing ' a filament of plastic is fed into a heated nozzle mounted on a moveable platform and heated plastic is deposited onto a prepared bed where the part is built up, layer by layer.
Limitations of FFF come from the nature of the extrusion process ' not only in speed and the complexity of the design, but in the porosity and strength of the final part. The filament is generally extruded as a solid tube of material, so it has a circular cross-section. As these noodles are being laid down in layers, they lay on top of each other (Figure 1). This method will always leave little gaps in the structure, and these gaps can lead to failure modes if the part needs to be put under any kind of load or stress.
For plastic FFF, the material simply cools and fuses together. To make a bound metal part using FFF, a special filament with embedded metallic particles must be used (BASF or The Virtual Foundry are good examples of source material suppliers). These materials print just like plastic, but afterwards the non-metallic binder material is subjected to a series of post-processing steps to make the final all-metallic part (Figure 2). The filament binder that contains the metal is dissolved using a special chemical solvent or it is burnt away using thermal debinding. After debinding, a furnace/kiln/oven treatment subjects the part to an even higher temperature to sinter the metal particles together (Figure 3).
This process results in a part that is now entirely metal, although it still contains porosity that is associated with the FFF process. There are better methods for creating bound metal parts, such as binder jet 3D printing, which can reduce the porosity but still requires the debinding and sintering process afterwards.
Bound metal AM has the benefits of being relatively cheap and easy to produce. Metal parts can be made using simple printers, including the kinds that hobbyists might use for FFF. The result is a part that is denser, stronger, and stiffer than plastic.
Disadvantages of metal FFF include porosity and layer adhesion. While sintering densifies the part, most metal FFF parts only achieve 80-95% density of a conventionally manufactured solid metal component. For those parts that have stringent fracture/creep/fatigue/strength requirements, porosity can be a deal breaker.
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A more mature metal AM method is laser powder bed fusion (LPBF), a process that uses atomized metallic powders in a highly controlled system to produce parts that are more than 99% dense in most cases.
LPBF works by depositing a thin layer of powder from the powder supply hopper onto the powder bed, using a coater that functions much like a windshield wiper blade. A targeted dose of laser intensity melts the powder in a portion of that layer. The powder bed then moves down, the coater applies more powder, and the process continues until the entire part is created, layer by layer (Figure 4).
Because the metal powder is being melted instead of sintered, the density of the fabricated parts is very high, rivalling cast or forged parts in some cases. This density leads to improved strength and fatigue characteristics in AM metal parts, and in some exotic cases, AM parts actually have better properties than cast or wrought counterparts.
LPBF can also be used to make very complex structures, since the resolution of the feature size is related to the size of the laser that melts the powder. The layer size for LPBF can be on the order of 100 microns, compared to an FFF printer that is limited by nozzle size to a layer thickness of 250 to 500 microns. In addition, unused powder in the bed acts as support material for the part that's being fabricated, so very complex lattice structures can be built.
LPBF has reduced manufacturing times compared to conventional subtractive manufacturing, but also generally compares favorably with FFF as well, since there is no furnace time required afterwards for sintering.
Disadvantages for LPBF are that the powder must be atomized, which can be expensive to source and requires stringent safety precautions (ventilation, laser safety, powder flammability and reactivity). The machinery required is also expensive and while the parts are denser than bound metal methods, they are still porous; components that need to have superior creep and fatigue resistance will likely suffer failure.
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Laser wire direct energy deposition (DED) is an attractive alternative to metal AM powder methods. Instead of relying on powder ' which requires processing and presents considerable safety issues ' laser wire DED uses standard welding wire spools. The spools of wire are fed into the printer similarly to an FFF machine, but instead of simply being extruded from a nozzle at the point of assembly, the wire is laser welded to the previously deposited layer.
There are multiple benefits to a wire fed DED process. Because the wire is being laser welded layer by layer, the final part has a much higher density than either FFF or LPBF and, in most cases, the part has strength and porosity indistinguishable from a forged or cast part.
The wire feedstock is the same as conventional welding wire and can be sourced quickly and easily from traditional vendors without the cost and wait associated with powder materials. The storage of welding wire is straightforward, and there are no specialized requirements for handling the materials above standard protective and safety precautions.
A disadvantage of this process is that the resolution is relatively coarse ' as it is set by the size of the weld bead ' which is generally on the order of the wire radius. As a result, the final part may need to be turned or milled to achieve the appropriate shape and surface finish.
To solve this issue of course parts, at Multiscale Systems we incorporate our laser wire DED method directly into a 5-axis vertical CNC mill. With a hybrid additive CNC we build the part from the ground up, with machining steps occurring as needed during the additive process. This technique allows for large travel distances on the print head so that we can build larger parts.
Turning, drilling, and finishing can all be done with the same machine and complex geometric features can be created that would not be possible using additive or subtractive manufacturing methods on their own. The best part of hybrid additive CNC is the ability to use more than one material. Our dual-wire machine allows for a quick swap between wires so that we can create metamaterial composites from a variety of alloys.
Most commonly, implants are fixed in place with the help of bone cement or metal screws. However, the major drawback to bone cement is fragmentation which can result in foreign body response to the released debris; this mode of implant fixation usually leads to periprosthetic osteolysis, early aseptic loosening, and failure of the implant [48]. For quicker healing after implantation, osseointegration is critical. Osseointegration, meaning, bone tissue ingrowth into the implant surface. The correlation between the morphology and size of the porous surface and the strength of fixation with the surrounding tissue has also been determined [49]. Many studies have concluded that open porosity improves implant wettability and aids bodily fluids flow, thus improving osseointegration when >100 μm pore size is present [50]. The porosity is either homogeneously or non-homogeneously distributed based on the processing method. Discussed are several fabrication methods for producing open-cell porous implants or coatings.
Sintering is one of the oldest and most evolved techniques in powder metallurgy for producing density-controlled materials. The basic concept of the process is to prepare powder (metal/ceramics) by compacting and binding powdered raw material and providing thermal energy for the compacted powder for densification and grain growth. In this process, the powder particles bond at high temperatures with minor changes to the initial powder particle shape. Binders hold the powder particles together, providing enough area for mass transport during solid-state diffusion. Due to this technique's highly evolved nature, many studies on porous orthopedic implant structures have employed sintering or modified versions of sintering for preparing porous metal structures.
The porosity of the sintered structures can be controlled by tailoring the shape and size of the metal powder, the compaction pressure, and the temperature and time of sintering. It was observed that the compaction pressure and the sintering temperature significantly impact the microstructural and mechanical properties of the porous sintered metals. In general, it was observed that sintered Ti compacts at K, K, and K at no applied pressure; as the temperature increased, the porosity decreased. However, the porosity remained more significant than 30% for each temperature, and almost 100% was observed to be open porosity. Moreover, the effect of different applied pressures at different sintering temperatures has also been studied, and it was observed that the porosity decreases considerably ('19%). However, most of the porosity still was found to be open porosity. The scanning electron microscope (SEM) images of the microstructure of Ti compacts sintered at K, K, and K with an applied pressure of 10 MPa can be seen in figures (a)'(c), respectively [51].
Additional investigations produced porous functionally graded Ti using the sintering method by stacking layers of powders with varying particle sizes and the volume fraction of the additive (silicon), i.e. the low volume fraction of additive with finer powder (20% with 45 μm) to the high-volume fraction of additive with coarser powder (45% with 200 μm). The cross-sectional microstructure of a functionally graded porous Ti structure prepared in this manner is shown in figure (d) [45].
Another alternative form of porous surface characteristics to implant the structure's dense core via sintering is metal fibers instead of metal powder. This method has been investigated for both stainless steel and Ti fibers, and the procedure used to make such porous coatings is similar to the powder metal sintering process [53'56]. The metal fibers are laid complying with the form of the implant structure, compacted, and then sintered at high temperatures. The solid-state diffusion process forms a fully interconnected porous coating at each point of contact of the fibers [57]. However, the main drawback of this process is that compacting the metal fibers to the form of the implant structure is challenging and time-consuming. Moreover, the interfacial bond between the dense implant core and the fiber mesh coating depends on the complexity of the contours of the implant structure [53]. Even though the sintering method of building porous structures is relatively mature, several limitations exist. A most relevant limitation is that particle oxidation could inhibit the proper bonding of the particles because it is a high-temperature operation. Further, solid-state diffusion bonding of particles usually results in the neck formation comprised of brittle phases and might result in lower mechanical toughness and fatigue resistance. Moreover, the pore size and morphology are usually irregular and largely dependent on the particle size and shape. However, these limitations could be substantially improved upon by using appropriate sintering techniques [13].
Many modified sintering techniques have been investigated to produce porous metallic structures with improved porosity and controlled pore morphology, including the space holder, the spark plasma sintering (SPS), and the replication methods. In the space holder method, several investigations have used carbamide particles as the space holder in preparing porous Ti and porous Ti6Al4V alloys. Carbamide has been chosen in the space holder method because of its ideal spherical particle geometry and chemical properties, such as ease of removal before sintering [58, 59]. A properly sieved and sized mixture of Ti or Ti6Al4V with carbamide was weighed and compacted under pressure. The compaction was then heat treated so that the carbamide particles dissociated at lower temperatures ('193 °C), and the dissociated by-products were expelled by either using a vacuum furnace or by the continued flow of argon. Thereby, the heat treatment cycle was typically followed where the compact was first heated up to 100 °C, and then slower heating rates were used up to 500 °C to ensure enough time for most of the carbamide particles to dissociate, and the consequent by-products were expelled. Following this, much faster heating rates could be used up to the sintering temperature at which the compaction was held for a considerable time, depending on the size of the compaction being sintered [58]. In this method, the size, shape of the pores, and porosity could be controlled primarily by controlling the volume fraction and the shape of the space holder particles. Other parameters that determine the porosity and the pore morphology are the compaction pressure and the holding time during sintering. While the compaction pressure applied to prepare the pre-sintered compact varied between investigations, the dependence on the mean porosity and mechanical properties (such as yield strength) and the compaction pressure seemed similar, i.e. as the compaction pressure increased, the mean porosity decreased. Also, it was observed among several investigations that the porosities that could be produced by this method ranged between 55% and 75%. It was observed that most of the pores were considered to possess consecutive and open cell morphology, with the sizes of the majority of these pores (<700 μm) being less than the largest space holder particle size ('700 μm). The pores with larger sizes were observed to be the consequence of pore coalescence. Furthermore, the pore walls' thickness is believed to significantly impact the porous structure's mechanical strength. Thereby, it has been observed that, while the pore wall thickness of the structure produced by this method is in the same range as any other powder metallurgy process (i.e. '100'200 μm; compared with Berger's report), interconnected angular-shaped micropores were observed along the pore walls suggesting that the sintering process was incomplete because of low diffusivity. While these micropores were considered beneficial in improving the interconnectivity of the pores, they might significantly deteriorate the structure's mechanical properties. Different space holders or compaction techniques investigated other variants of this process [60, 61].
Another variant of the sintering technique is the SPS method. SPS or field assisted sintering technique (FAST) is a process similar to hot pressing where the heat required to sinter the powder particles in a compact is provided by joule heating due to the current flowing within the compact [62]. The general working principle of SPS, as indicated by figure (a), is that the sintering powder is compacted within a graphite die and DC voltage pulses pass through the die, and the compact (in the case of conductive sintering powder) produces joule heating, with heating rates up to °C·min'1, cooling rates of up to 400 °C·min'1 and maximum temperature of °C, sintering conditions could be facilitated by appropriately controlling the pulse voltages and durations [63]. Furthermore, this process could be controlled by controlling the die's measured temperature, power, or current. The rapid heating and cooling rates make this one of the fastest sintering processes, thereby ensuring limited grain growth in the sintered metal [64].
The powder mixture is produced similarly to the space holder technique to produce porous metal structures using the SPS technique. The metal or alloy powder is mixed with varying volume fractions of space holder constituents such as NaCl or NH4HCO3. Then, the powder mixture is cold compacted under pressure to produce green compacts, sintered using SPS in a specially designed graphite die, as shown in figure (b) [65]. In the case of the study, where NaCl was used as an additive, post-sintering dissolution of NaCl in deionized water produced porous Ti6Al4V structures [66]. And in the case of the study where NH4HCO3 was used as an additive, it dissociates into NH3, H2O, and CO2 and is expelled during the sintering process and thus produces porous Ti structures [65]. In both studies, the x-ray diffraction (XRD) analysis of the porous structure showed negligible impurity content indicating that the space holders used were eliminated during the process. Also, it was observed that the majority phase appears to be α-Ti in the post-sintered structure, and the grains developed in the sintered porous structure are mostly fine. This indicates that the SPS process offers extremely fast heating and cooling rates.
The porosity, pore size, and morphology of the porous structures produced by the SPS method showed similarity with the previously discussed space holder method. In general, two types of pores were observed: Macro pores caused due to the dissociation of the space holder, and the micropores, within the pore walls of the macro pores, caused because of the incomplete sintering between adjacent metal particles (as shown in figures (c) and (e) in both the studies). Both studies also observed that as the sintering temperature was increased or by heat treatment post-sintering, the pore walls became thicker, and the porosity reduced (as shown in figures (d) and (f) in both studies). Also, both studies produced 40%'70% porosities; most pores were well interconnected.
Finally, the replication method's third variant of the sintering method is discussed here. This process is typically three-step and has traditionally been used to prepare porous ceramics [13]. However, some investigations have used this method to produce porous Ti and Ti alloys. This method immersed polyurethane foams in a slurry and then rapidly dried for the metal powder to positively maintain the polyurethane foam replica. The slurry comprises 70% by weight Ti6Al4V and 20% by weight H2O and ammonia solution, where the ammonia solution has been added to improve the flow properties of the slurry. After repeating this process multiple times till the polyurethane foam struts were coated entirely with the Ti6Al4V powder, the polyurethane foam and binder were removed thermally, and the remaining Ti6Al4V powder arrangement was sintered, forming an open-cell reticulated Ti6Al4V foam [67]. This three-step process's schematic is shown in figure (a) [13].
As expected, in this process, the flowability of the slurry, controlled via particle size distribution, binder chemistry, the pH of the slurry, air bubble quantity, and the solid'liquid ratio, determines the quality of foam produced by this process [68]. By this process, Ti alloy foams of 88% primarily open porosity have been attained, and the pore morphology could be observed in the SEM image shown in figure (b) [67]. The pores formed are chiefly found to be of three types: the primary porosity, comprising of smaller pores on the strut surface; secondary porosity, medium-sized pores formed at the core of the hollow strut formed by previously occupied polyurethane foam; and the tertiary porosity, larger open pores between the struts. A subsequent study observed that a second coating of the powder slurry and second sintering on a previously sintered foam enhanced the density and mechanical properties of the Ti or Ti alloy foams [69].
Even though sintering is a very mature metallurgical process, up to 80% of porosities could be achieved with slight modification. However, the process has inherent limitations. Sintered porous metallic structures are susceptible to brittle fracture and low fatigue resistance. Since, in this process, the metal particles bond via a solid-state diffusion process, the neck formed between metal particles after sintering is usually quite brittle. Also, the sintering process usually produces non-homogeneous pore distribution and pore morphology.
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