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Review

Three-Dimensional Printing of Metallic Parts by Means of Fused Filament Fabrication (FFF)

by
Irene Buj-Corral
1,*,
Felip Fenollosa-Artés
1,2 and
Joaquim Minguella-Canela
1,2
1
Departament of Mechanical Engineering (DEM), Barcelona School of Industrial Engineering (ETSEIB), Universitat Politècnica de Catalunya (UPC), Av. Diagonal 647, 08028 Barcelona, Spain
2
Fundació Privada Centre CIM, C/Llorens i Artigas, 12, 08028 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Metals 2024, 14(11), 1291; https://doi.org/10.3390/met14111291
Submission received: 30 September 2024 / Revised: 28 October 2024 / Accepted: 11 November 2024 / Published: 14 November 2024
(This article belongs to the Section Additive Manufacturing)
Browse Figures
Versions Notes

Abstract

:
Obtaining metallic parts via Additive Manufacturing can yield several advantages over using other traditional manufacturing methods such as machining. Material extrusion (MEX) can handle complex shapes with porous structures and, at the present time, much low-end and desktop equipment is available. In the present work, different industrial and medical applications of metallic Fused Filament Fabrication (FFF) parts are presented. First, an overview of the process, equipment, and of the metal-filled filaments currently available is provided. Then, the properties of parts and different applications are shown. For example, metal-filled filaments with a low metal content that can be used to obtain plastic parts with metallic appearance (with either steel, copper, or bronze), and filaments with a high metallic content allow obtaining metallic parts with high mechanical strength after a sintering operation. The present contribution aims to be an up-to-date panorama for current industrial and medical results and lessons learnt from the application of FFF to obtain metallic parts.

1. Introduction

According to ISO/ASTM 52900 [1], Additive Manufacturing (AM) technologies can be classified into seven main groups: Binder Jetting (BJT), Directed Energy Deposition (DED), material extrusion (MEX), Material Jetting (MJT), Powder Bed Fusion (PBF), Sheet lamination (SHL), and Vat Photopolymerization (VPP). The seven groups can be joined according to three categories, namely (i) sintering processes, (ii) melting processes, and (iii) other processes. The category ‘sintering processes’ encompasses manufacturing technologies based on PBF as well as on DED. Fused Filament Fabrication (FFF), also referred as Fused Deposition Modeling (FDM), belongs to the category ‘melting processes’, and, in particular, to the subcategory of MEX.
Material extrusion (MEX) is an AM technology that bases its construction functioning on laying threads of molten material, layer by layer, in the space coordinates where it is required by the part [2]. The molten material is, in the general case of FFF, obtained in an extruding head from the raw plastic filament of circular section. The temperature of such an extruding head can be set to certain temperature levels depending on the material to be printed. Also, the standard MEX machines have a flat bed that can, in many cases, be set at a certain temperature level (cold or hot, depending on the part to be printed). Some machines are caged in a closed frame and can also incorporate temperature control and or humidity control of the entire construction and materials [3].
The general taxonomy classification of the most relevant Metal AM (MAM) technologies in the present study is presented in Table 1.
Among the seven categories defined in the ASTM 52900 standard [1], four of them are directly related to metals: Powder Bed Fusion (PBF), in which metallic powder is sintered; Directed Energy Deposition (DED), in which either metallic powder or wire are used as raw material; material extrusion, in which the material (for example, in the form of molten filament) is extruded through a nozzle; and Binder Jetting, in which metallic powder is joined with the help of a binder. Another AM process in which metals are processed is Moldjet by TRITONE.
The temperatures at which the MEX technology can extrude materials generally discard pure metals to be 3D-printed standalone [5]. But indeed, FFF/FDM is a highly versatile AM process, in which many different materials can be processed. Even in the simplest case, which would be having a single extruding head, the machines are capable of loading and processing filaments made up of many different composite material compositions, including metal–polymer compounds [6].
Concerning metal 3D printing, this opens a window for achieving compound parts [7] based on metal particles with some polymer acting as a binder. Also, in the broader MEX case, it is possible to habilitate several extrusion heads, and to process materials coming in different formats than filaments (such as pellets or slurries) [8]. Finally, it is possible to apply post-processing operations to the 3D-printed parts, which would enhance the infiltration of metals in the processed materials (see Figure 1a,b).
In this context, the industrial and medical sectors are very interested both in the intrinsic advantages of AM (customization, directly seamless CAD-to-part, ability to perform “impossible” geometries optimized to their function) [9,10] and in the fact that the application of MEX technology to metals allows both significant cost savings and making 3D printing more accessible. On the one hand, a genuine cost-saving aspect comes from the circumstance that it is not necessary to bring the metal material to the melting point, which would in many cases require a high-energy laser or electron beams [11]. Using MEX processes, this step can be completed in a much more competitive manner in an oven, once the green part is formed (see Figure 1c,d).
On the other hand, FFF/FDM is one of the most commonly modelled AM processes [12] and has matured as a market standard that needs no introduction [13]. Being already present in the technical offices of many industries, as well as in many hospitals, to reproduce prototypes, visualize geometries, materialize spare parts, etc., it is still somehow limited to polymeric parts, using thermoplastic filaments (e.g., Polylactic Acid (PLA) and acrylonitrile butadiene styrene (ABS)).
If metallic powder is added to the polymeric filament, this technology competes directly with the traditional technologies such as Metal Injection Molding (MIM), in which a feedstock with metallic powder and a binder is injected, and machining, offering design freedom without the manufacturing constraints of the a.m. technologies, but keeping similar mechanical properties. Although it is still under development, it has some industrial and medical applications yet.
In industry, for instance, an end-user brake pedal has been obtained from AISI 316L stainless steel [14]. In the medical sector, for example, antimicrobial thermoplastic nanocomposites can be 3D-printed to personalize medical devices [15]. For example, when mixed with copper, they have been used to manufacture finger prostheses [16]. In another application, nanocomposite scaffolds were prepared with poly(lactic-co-glycolic acid) (PLGA) and copper/zinc-based zeolitic imidazolate frameworks to be used in bone repair [17]. In addition, maxillofacial implant prototypes were developed from titanium alloy [18]. For titanium implants, Table 2 provides a comparison among the FFF, EBM, and MIM processes.
Thus, metal FFF/FDM is a promising process in both industrial and medical applications, although it is still under development. In this sense, this work provides a global view of the current advances in metal FFF/FDM process, from the description of available feedstock materials to different machines and post-processing operations. In addition, different properties of the 3D-printed parts such as dimensional accuracy, surface finish, or biocompatibility are addressed.
The organization of this paper is as follows: results are presented in Section 2, Section 3, Section 4, Section 5 and Section 6, while Section 7 corresponds to the conclusions of this paper. In Section 2, the process for obtaining 3D-printed parts with metallic content with extrusion processes is studied. From that point, this paper focuses on metal FFF/FDM processes. In Section 3, the different types of feedstock materials used are presented. In Section 4, different FFF/FDM 3D printing machines for metals that are available in the market are shown. Section 5 summarizes the post-processing operations that are commonly available. In Section 6, the properties of the 3D-printed parts are studied.

2. Processes for Obtaining Metallic Parts via Material Extrusion

The usual equipment configuration of MEX processes can be further categorized into plunger-based systems, screw-based systems, and filament-based systems [7]. In the plunger-based systems, cartridges are used, with either metal or ceramic powder. The screw-based systems usually employ pellets as feedstock and are used when the materials are too rigid to be spooled. The machines using filament are ram extruders, in which the filament acts as a ram that pushes the molten material.
Indeed, the development of metal-filled filaments with different metal percentages in their composition leads to not full metallic parts, but rather to parts with some metal content [22]. The specific percentage of metals in the final compound will as a consequence lead to different material properties of the final parts obtained. However, if high mechanical strength is required, high metal percentage is employed, the parts will undergo thermal treatments to be completed, and the binder can be almost completely removed during the heat treatment, leading to full metallic parts.

2.1. FDMS Process Overview

The application of FFF/FDM to metals is sometimes referred as to Fused Deposition Modeling and Sintering (FMDS) or Metal Fused Filament Fabrication (MF3). In the general case, the metal particles can be embedded in a polymeric binder, to be extruded from a single wire feedstock, forming a green part from which the non-metallic component will be removed at a later stage [23]. In these cases, the part obtained after the debinding post-processing operation is referred as a brown part. Printing gaps in green parts are usually difficult to eliminate, resulting in parts with poorer characteristics than via other processing methods [24]. Also, the sintering operations required have undesired effects on the part, such as shrinkage and deformation.
Some successful examples in the literature correspond to filament stock prepared as a mixture of stainless steel 316 L powder with a water-soluble binder (Embemould K83), with powder loading between 50 and 63% in volume [25], which yield stable and homogeneous samples. The polymer matrix can also accommodate filaments of Ti-6Al-4V, for example, in two powder particle size distributions, fine (D50 = 13 μm) and coarse (D50 = 30 μm), yielding filaments with 59 vol.% (87 wt.%) capable of achieving printable compounds [26]. Also, some commercially available materials blend PLA plus bronze (78 wt.% or 36 vol.%) or PLA plus iron (46 wt.% or 12 vol.%), although these combinations are claimed to be fine only for simulation of bronze or iron but not recommended for parts that require handling mechanical loads [27]. ABS is also employed in metal-filled filaments [22]. As a general trend, the tensile strength of the material decreases with the metallic content (in non-sintered parts), but its thermal conductivity increases. For this reason, they allow manufacturing electric components [28].
Another usual combination to achieve metallic materials when using other materials as binders is to pair the metals with ceramics, such as stainless-steel SS 17-4 PH powder mixed with ECG2 binders [26] or AISI 630 martensitic with Zirconia–Yttria [29]. In recent years, direct extrusion of metallic filament has been in the focus of many research groups, as it would be the straightforward route to achieve superior physical properties [30]. However, direct extrusion of metallic filaments usually requires specific coatings and more resistant designs of the extruder heads, as it is more demanding than MEX applied to polymers or other compounds. One common practice, in particular for the manufacturing of equipment for the conformation of abrasive material, is the use of tungsten carbide coatings, which empowers the filament forming heads with superior wear resistance [31].

2.2. Other Material Extrusion Variants

2.2.1. FFF Using MIM Feedstock

As explained earlier, MIM is a relatively mature manufacturing process that can be used to manufacture small parts of many different materials [25], including metal matrix composites, such as Ti-SiC compounds. Therefore, some research efforts in the literature have tried to integrate the MIM feedstock processing of SS 316L to be applied to commercially available FFF/FDM technology [32]. In this case, the parts were extruded at nozzle temperatures between 180 °C and 200 °C, achieving theoretical densities ranging between 84% and 95%. One possible shortcoming of the application of MIM techniques may be the attention that has to be given to the different debinding operations that the green parts have to undergo to be finalized [33], although thermal processes will be required in FDMS as well. Also, the relatively big extrusion heads required for handling the feedstock could be inconvenient, although the size issues could be mitigated with specific machine architecture designs [34].

2.2.2. FFF Using Semi-Solid Metal

On another line, some previous studies in the literature also addressed MEX for metallic parts not by the use of metal powder fillers, but by the use of semi-solid metal (SSM) combined with the deposition process. This process variant reported to be capable of producing fully dense low-melting-temperature Sn-Pb-based parts, and to be minimally affected by shrinkage [35]. However, SSM parts commonly failed to withstand high service stresses due to a dependency of the rheological properties of the semi-solid metal, which in turn are dependent on the microstructure [36,37]. Therefore, the research efforts in this line refocused on the use of molding techniques or direct metal 3D printing.

2.2.3. FFF Impregnation Process

Another possible application of FFF/FDM to obtain metallic parts consists of the materialization of full polymeric parts (for example PLA) and the ulterior infiltration of metallic nanoparticles (such as of silver and iron metal salt) [38]. The use of such AgNPs fillers, or even chitosan-mediated silver nanoparticles (AgCH-NPs) is very relevant for the medical industry because they can activate antimicrobial, antifungal, and antiviral properties [39]. The process for obtaining parts with this approach requires a cleaning and an etching process of the PLA surfaces to be carried out after they are 3D-printed, which generate a porous area that is therefore occupied by the metal nanoparticles. These approaches, however, do not obtain highly dense metallic parts, but composite parts that can successfully be used, for example, as catalytical devices for oxidation processes. Still, with the FFF/FDM infiltration the cost levels can be minimized compared to other available process alternatives [40].

2.2.4. Direct Ink Writing (DIW)

Direct Ink Writing is a technique that uses low-melting-temperature alloys, such as water–metal powder suspensions, to form part shapes by extruding the material through specific nozzles, thus representing a favourable alternative to energy deposition techniques.
Some successful process materialisations of DIW processes (3D gel printing) encompass the incorporation of SS 316L into a gelation system based on HEMA (methaerylate-2-hydroxy ethyl) [41]. The so-called 3DGP process remains a promising method for producing complex shape parts. Anyhow, the extrusion of metal-based slurries, such as Ti6Al4V, has been demonstrated to produce porous scaffolds with regular and reproducible patterns meeting tissue engineering requirements [42]
Some shortcomings of this technology are the dependency of many inks to temperature, as well as poor dimensional accuracy and surface roughness in lateral walls.

2.2.5. Liquid Additive Manufacturing (LAM)

This process is similar to DIW, but it uses soft polymers. So far, it has mainly been used to print materials such as pure silicone. One of the main advantages of LAM is that it requires few fillers, only those directly related to the functional properties of the materials. It has some drawbacks; for example, it is required to remove unwanted lines, because the printing operation is continuous, the shapes to be obtained are limited, and it provides low dimensional accuracy and high layer thickness [43].

3. Feedstock Materials for Metallic Parts

As explained in Section 2, in the plunger-based systems, metal powder cartridges are used. The screw-based systems usually employ pellets as feedstock, and the ram extruders use filament. This section will focus on filament raw materials.
The use of metals in deposition processes dates back to the 1990s [44]. However, only in recent years have metal-filled filaments become widely available. In metal-filled filaments, the binder plastic material requires mechanical strength but also flexibility, and it must provide enough strength to green parts [7]. Since the filament needs to be melted during the extrusion process, important parameters of the plastic binder are melting temperature and degradation temperature [23].
Some filaments have a low metallic content, around 30 vol.%, and their main purpose is decorative. For example, Colorfabb offers three different kinds of metallic filaments, with either bronze, copper, or stainless-steel powder, whose commercial names are Bronzefill, Copperfill, and Steelfill [45,46,47]. Once the part is 3D-printed, its surface is expected to be sand-blaster-polished in order to provide a shiny appearance. In addition, it is recommended to used hardened steel nozzles instead of the conventional brass nozzles during the extrusion process.
But moving on from FDM to FDMS, other filaments with a higher metallic content are used to achieve final metallic parts, which, as has been explained, need to be subjected to thermal treatment after 3D printing. For instance, BASF provides different metal-filled filaments with their Ultrafuse® product range [48]. Ultrafuse®316L corresponds to stainless-steel parts to be manufactured in conventional 3D printers. Ultrafuse®17-4 PH is made with 17-4 stainless-steel when high hardness and high mechanical strength parts are required. Finally, as has been cited in previous section, an Ultrafuse® support layer is to be employed for printing interface material between a part and supports (both made of Ultrafuse®17-4 PH), in order to minimize distortion of the printed parts.
As for high-content metal-filled filaments, Cerejo et al. [49] manufactured AISI 316 stainless steel-filled filaments, with a particle size of d50 = 6.85 μm and with a metal content of 60 vol.%. Different components were added to the filament, and the most appropriate one contained SS 316L + M1 + TPE + P, where SS 316L is the stainless steel powder, M1 is the master material, made of a mixture of polyolefin waxes and >60 wt.% of polyoxymethylene (POM), TPE is a thermoplastic elastomer, and P is an external plasticizer. Guessasma and Belhabib [50] used a magnetic iron-reinforced PLA filament, with a maximum iron particle size of 0.250 mm and iron content < 45 wt.%, from Protopasta (Vancouver, BC, Canada). The 3D-printed parts showed a decrease in stiffness of around 39% with respect to the use of the PLA filament.
It is also possible to print parts made of two different fillers, for example, high-carbon iron and 316L stainless steel, in three different ways: mixed, coupled, and graded [51].

4. Material Extrusion Machines for Metallic Parts

Fused Filament Fabrication (FFF) is one of the most employed AM techniques, because of its simplicity, the possibility to use low-cost machines, and the availability of a great variety of materials. In FFF machines, the filament is pulled by driving wheels through a liquefier and a nozzle. The force that is required depends on many factors: the motors need enough torque, there must be enough friction between the wheels and the filament, the filament must avoid shearing from the wheels, the filament should not suffer buckling, and the filament needs to be flexible enough to avoid breaking when it is spooled [7]. This is relatively easy to accomplish when a plastic filament is used, but the addition of a filling material such a metal or a ceramic can lead to breakage of the filament [52].
It has been said that the temperatures at which the FFF filament technology can extrude materials are generally low, and due to this fact pure metals are not 3D-printed by this technology. Nevertheless, recent developments [53] are pushing in this direction, overcoming two difficulties: having a tool head that can be named as a crucible to melt metals, and having the means to focus energy on the underlying deposition point in order to assure the bonding between the extruded material and the previous layers. So, in fact this results in a sort of hybridization between MEX and DED technologies. The first development of this technology is capable of processing aluminum filament [54], with a resolution that can be considered as satisfactory as that for standard polymeric parts.
Different material extrusion systems for metallic parts are presented next.

4.1. Metal Pack from Barcelona Three Dimensional Printers

One relevant manufacturer is a Barcelona-sited company, BCN3D (Barcelona Three Dimensional Printers, S.L., Gavà, Spain), a spin-off from the technological Centre CIM (Universitat Politècnica de Catalunya UPC BarcelonaTECH, Barcelona, Spain). BCN3D offers a metal FFF printing technology aimed to produce stainless steel parts that in fact is an adaptation of their polymeric filament 3D printers to scale up to composite metal filaments, named Metal Pack [55]. One key point is their partnership with Ultrafuse® metal filaments [48] from Forward AM (BASF 3D Printing Solutions GmbH, Heidelberg, Germany), which was presented at Formnext 2021 in the Frankfurt Messe, Germany. Ultrafuse® metal filaments have high levels of stainless steel in combination with polymeric binders, a key point to make 3D printing possible. Figure 2a shows a part while being printed, and Figure 2b corresponds to a processed part.
Some consequences of the addition and removal of the binder include the change in shape and size of the part before and after the sintering process. In the case of BCN3D Metal Pack technology, this shrinkage for regular dimensions is estimated to be 19% in size along the X and Y direction, and 24% in the Z direction. Therefore, a previous expansion of the CAD part must be introduced before undergoing the slicing process: multiplying the dimensions contained in the horizontal plane by a factor of 1.19 in the expected extrusion printing direction. As long as this is a straight recipe and every part is going to behave differently in a sintering process, the probabilities of dimensional non-compliance are higher than if the parts were CNC machined. The effect corresponds to distortion rather than a simple regular and predictable shrinkage: depending on the design, bending and fracture effects can happen, causing invalid parts. So, Computer Assisted Engineering (CAE) simulation software should be used to predict these effects and properly assist the CAD designer to generate a design which could avoid them.
Related to this, the need for advanced calculation tools ostensibly reduces the advantage of abandoning the direct printing of metal parts: most small and medium-sized industries do not have access to these tools, or the personnel prepared to use them. For these reasons, the manufacturers provide palliative tools to minimize these effects. In the case of BCN3D, the advice is to add to the design a lower plate of regular geometry (circular or rectangular, equalling the maximum surface dimensions on which the part to be printed would be inscribed) of at least 3 mm thickness in the case of parts with an irregular section. This plate will act as a stiffener, limiting distortion during sintering.
Another stiffening strategy is linked with the common need for supports: most parts, if the design is complex enough—the reason why 3D printing is used—end up needing support structures so that, when exceeding a certain angle of inclination with respect to the vertical direction (typically between 45° and 60°), the materials being deposited do not fall onto the platform. Then, the strategy is to make the supports from the same material as the workpiece, causing the printing of a compact block prone to better resist sintering distortion. But then, as long as removing compact metal support after sintering can become a demanding task—as it is for Powder-Based Fusion metal technologies—at the junction between the support and the piece, a ceramic material can be placed that acts as a separator. In the case of BCN3D, the Ultrafuse® Support Layer filament is proposed. When this interface material is printed between the support and the object, it creates a barrier that during sintering facilitates the separation of the support from the metal part and prevents deformation of overhangs (Figure 3). However, the need to print two different materials limits the range of FFF/FDM manufacturers, due to the fact that most of them own a single extruder. In the case of BCN3D, their printers have two independent extruders, which explains why BCN3D coined the acronym IDEX (Independent Dual Extruders) to name 3D printers with this capability to use two materials for one part.

4.2. METAL X System from Markforged

METAL X system from Markforged Inc., Waltham, MA, USA [57] is similar to Metal Pack, with the difference that METAL X is not an adaptation kit for a printer of polymer filaments, but a printer which is dedicated exclusively to mixed metal–plastic filaments to obtain metallic parts. They have named this metal FFF technology ADAM (Atomic Diffusion Additive Manufacturing). In fact, its solution includes its own hardware for the debinding and sintering process, making it unnecessary to send the printed parts to external sintering service providers, but which then implies having its own human resources with metallurgical knowledge in order to carry out the control of the delicate process that will entail obtaining reliable parts.

4.3. Studio System from Desktop Metal

Another manufacturer that opened the way to FFF/FDM technology for metals is Desktop Metal Inc., Burlington, MA, US, with the solution Studio System™ [58]. The materials available to be utilized in this equipment are unique to their system, and are not filaments in a coil but in the form of bound rods. This allows the use of closed-cell infill, which provides strong parts with lower weight.

4.4. Other Manufacturers

Another manufacturer, Pollen AM, Ivry-sur-Seine, France, proposes relevant techniques to the field. Its first particularity is the use of pellets as a material format, so Metal Injection Molding (MIM) standard feedstock from most of the manufacturers can be used, this point being a competitive key aspect for technology users [59]. In fact, as the 3D printer then is prepared to manage pellets as a feedstock, it becomes a suitable solution and not only for metal application but also for ceramic parts.
As has been shown, many developments fill the field of Metal AM by FFF/FDM. The manufacturers have named particular denominations within this common field, trying to show off their advantages due to their own particularities: FFD (Fused Feedstock Deposition), FMP (Filament Metal Printing), BMD (Bound Metal Deposition), MIM (Metal Injection Molding), etc.

5. Post-Processing Operations

In general, metallic 3D-printed parts, also known as green parts, are subjected to debinding and subsequent sintering. Debinding is used to remove the plastic base of the parts after 3D printing. Four main types of debinding are known, namely, thermal, solvent, water, and catalytic debinding. A combination of different debinding operations is also possible [23,60]. Thermal debinding consists of a thermal cycle. In solvent debinding, a solvent such as heptane, hexane, or ketone is used. In water debinding, the polymeric base is removed by water. In catalytic debinding, a catalytic acid vapor is employed. Typical debinding processes with a solvent and subsequent thermal debinding comprise three steps: diffusion of a solvent, dissolution of the binder, and diffusion of the remaining binder towards the outside of the part [61]. After debinding, the parts are known as brown parts.
Sintering is applied after debinding. It usually consists of a thermal cycle at high temperatures in order to ensure diffusion between metal particles, so that the part will achieve its final strength [62]. In thermal debinding and sintering cycles, not only final temperatures but also debinding and sintering rates are important parameters.
The specific treatments that are carried out by different manufacturers are presented next.

5.1. Thermal Treatment for Metal Pack from Barcelona Three Dimensional Printers

In this case, catalytic + thermal debinding is used. Once the part has been printed, the binder content can be removed at a later stage, out of the 3D printer, by means of dedicated post-processing equipment, in a disaggregation process known as catalytic debinding [63]. Finally, the part then undergoes a sintering process at temperatures just below that of the melting temperature of the metal (approximately 85% of the melting temperature in °C), which causes the metal particles to fuse and compact.

5.2. Thermal Treatment for METAL X System from Markforged

In particular, the debinding process, also catalytic, uses cleaning fluids for metal parts common in the industry such as Opteon SF 79. It is a blend of proprietary fluids and trans-1,2-dichloroethylene (t-DCE) with azeotrope-like properties [64]. Typical debinding equipment then must heat and circulate this sort of fluid, which will be capable of infiltrating the part and dissolving the polymer binding material, removing it. It is understood then that very thin walls or massive parts are not going to facilitate this so, despite the claimed “design freedom”, certain design rules must be followed in order to facilitate proper debinding. But in any case, the remaining binder is going to be burnt away at the start of the sintering process, facilitated by the fact that in the debinding process a porosity has been induced in the part from the surface to inside, and the new channels created in the removal of binder are going to facilitate the way out for last of the binder in gas form.

5.3. Thermal Treatment for Studio System from Desktop Metal

Due to the fact that raw material is in the form of rods, catalytic debinding is avoided in this case, and therefore the only subsequent step after printing is taking the parts to the furnace, which is also facilitated in this solution.
The furnace first heats parts to remove the binder, typically between 200 and 550 °C. During this step, thermal debinding is performed by two possible methods, depending on the binder composition: degradation, where the binder reacts with the gas used in the furnace, being itself converted to gas that can be extracted, or evaporation, simply being evaporated as heat allows the binder to be converted directly into gas without a previous chemical reaction. In any case, most defects that can be detected after the final step of sintering are caused by an improper debinding process (being this catalytic as explained before or thermal as in this present case). So, expertise and control tools must be focused on this key point.

6. Properties of the 3D-Printed Metallic Parts

6.1. Dimensions and Surface Finish

One of the main future challenges of the extrusion processes is the improvement of the processing quality of the parts [65].
The dimensional accuracy of indirect FFF/FDM metal manufacturing via 3D printing of metal and binder composites plus post-processing treatments to remove the binder, such as washing and sintering in the case of SS 17-4 PH, is reported to reach ISO IT13 grades [66]. Also, in the general case, it has been reported that the dimensional surface deviations in FFF/FDM can be formulated as a function of the layer thickness and deposition angle selected in the process parameters [67]. Reportedly, some of the most common causes of errors in material extrusion processes are thermal warping and shrinkage [68]. In copper-filled PLA parts, Buj-Corral and Sivatte-Adroer [69] reported dimensional error values up to 5.8% in the X direction, and lower dimensional error values up to 3.5% in the Z direction. In steel-filled PLA parts, dimensional relative errors between 1% and 9% were reported in prismatic and bone-shaped specimens [70]. Figure 4 shows the dimensional measurement of a steel-filled dogbone specimen with a Mitutoyo micrometer.
As for shrinkage, Quarto et al. [71] sintered previously 3D-printed parts in AISI 316L stainless steel. They found shrinkage values of 16% both in the X and in the Y directions, with variable shrinkage values in the Z direction. For the same material, Kurose et al. reported shrinkage between 15% and 17% [72]. Tosto et al. found higher shrinkage values, around 25% in the Z direction [73].
However, debinding and sintering processes lead to distortion of the parts. In this sense, Shaikh et al. [74] compared resulting shrinkage when using an ABS filament or a Ti-6Al-4V-filled ABS filament. They found that the unfilled polymer showed higher shrinkage values and also higher warpage than the metal-filled polymer. The separation of the components of the filament can also cause distortion of the final parts [75].
Regarding surface finish, Boschetto et al. [76] found different profile shapes, from pseudo-triangular to a Gaussian distribution of peak heights, in steel 316L parts. In lateral walls and with 0° deposition angle, they measured Ra values up to 6.58 µm in printed parts, with a corresponding value of 4.51 µm in sintered parts. Regarding steel-filled PLA parts with a low metal content, Buj-Corral et al. [77] measured roughness on the upper surface of parts that were printed with a linear structure. They found Ra values up to 25.36 µm.

6.2. Internal Structure

The performance of AM parts is highly affected by the inner structure and infill patterns contained in the part [76]. This reveals why many efforts have been made to materialize almost fully dense parts, as well as with the highest percentage of metallic composition possible.
Concerning the obtention of composite-based material extrusion parts, several experiences achieved results matching the properties of parts obtained via MIM in terms of sintering density, shrinkage ratio, and microstructure [78]. In the case of FFF/FDM, some manufacturers and some research studies using image processing though cross-section cuts claim part densities of SS17-4 PH above 95% after the final sintering [79]. Nevertheless, some studies using hydrostatic balance techniques lower the expectations to values of approximately 90% density [66]. As for density, Quarto et al. [71] found that the combination of a linear pattern, print speed of 20 mm/s, and layer height of 0.1 mm provided the highest density in AISI 316L 3D-printed parts.
Also, the nature of layered manufacturing has an effect in the final parts obtained, which has to be considered. Concerning SS 316L, metallographic characterization in terms of porosity and grain size shows significant effects of the variable part orientation, while the contribution of extrusion velocity and layer height is regarded as less important [80].

6.3. Mechanical Properties

Concerning full-polymeric FFF-produced parts, the average tensile strength values reported are 28.5 MPa for ABS and 56.6 MPa for PLA, and the average elastic modulus reported rank is 1807 MPa for ABS and 3368 MPa for PLA [81]. In this context, the incorporation of additives in the filament can yield significant increases in the maximum yield strengths reported. Concerning PLA, the incorporation of aramid additive (8.6 vol.% or 9.5 wt.%) increased the maximum yield strength to 203 MPa [82], and the incorporation of glass fibre (31.5 vol.% or 47.8 wt.%) helped to achieve an experimental maximum tensile strength of 268 MPa [83]. Regarding stainless-steel-filled filaments, maximum tensile strength values of 443.9 MPa and 497.40 MPa were reported for AISI 316L and 17-4 PH, respectively.
When utilizing direct filament fabricated parts, however, the incorporation of metal compounds in other matrices usually causes the mechanical properties to be reduced. For example, usual values of maximum yield strength for ABS can rank between 30 MPa and 48 MPa, while viable combinations that incorporate metallic components such as Cu (30 vol.%) or Fe (10 vol.%) can lead to maximum yield strengths of 26.5 MPa and 43.4 MPa, respectively [28]. Also, with it being common for PE to reach a maximum yield strength of 43 MPa, PE incorporating Cu (75 vol.%) leads to a maximum yield strength of 19.41 MPa [84]. These values are also significantly lower than the maximum yield strengths of pure Fe (between 400 and 500 MPa) and of that of pure Cu (which can reach 210 MPa). This indicates that the rationality of incorporating such metallic compounds is usually related to some other properties that can be achieved in the compounds and/or in the manufacturing process, for example, to electrical conductivity in the case of Cu, or even to enhance an improvement in the flowability of the polymer melt [85].
Concerning processes that include a deep debinding post-processing (several steps of sintering operations), the outcomes can be significantly higher. For example, using gel printing technology based on SS 316L reportedly can serve to achieve green samples with a bending strength of 16.1 MPa and surface roughness of 3.5 μm. Then, the sintered samples can achieve a surface roughness of 3.8 μm and tensile strength of 488 MPa [41]. This information is relatively aligned with other studies that reported a ultimate tensile strength of 453 MPa [72].
Also, it is important to note that the ultimate tensile strength can be maximized if the layer direction is arranged in the same direction that the tensile stresses are loaded, with the building orientation having a significant effect on tensile properties [86]. So, for example, SS 17-4 PH samples delivered an ultimate strength of 880 MPa in green, improving to 1140 MPa after solution and aging treatment. The same rationality applies for the common factors of study in FFF/FDM, namely, extrusion temperature, flow rate multiplier, and layer thickness [87].

6.4. Electrical Properties

Copper- and iron-filled acrylonitrile butadiene styrene (ABS) filaments have been employed to manufacture heat sinks [28], which can be used either for heat dissipation in medical implants or in electrical insulation. Copper-filled polyethylene filaments can also be used to obtain electromagnetic structures, for example, for shielding [84]. Other filaments containing copper, aluminium, brass, iron, or nickel have been found to be electrically conductive [88].

6.5. Biocompatibility

Cell growth has been tested in low-content metallic-filled PLA filaments, for biomedical applications [77]. Three different materials metals tested. Figure 5 shows the 3D-printed samples in bronze, copper, and stainless-steel, respectively.
While copper and its alloys are known to have antibacterial properties [89], stainless steel provided promising results [77], with the growth of mesenchymal cells on the disks’ surface.
In a similar line, Abudula et al. [90] investigated the use of 3D-printed metal and metal/oxide thermoplastic nanocomposites with antimicrobial properties. They concluded that mainly in vitro studies have been carried out in the past, while in vivo studies could bring to light the influence of inflammation on the results [91].

7. Conclusions

This article summarizes the current applications of metal-filled filaments found in the literature. It is also focused on several developers and manufacturers who offer solutions of interest to industry and medical sectors for obtaining metal parts from filaments. In many cases, composite filaments that can be used in printers are dedicated to materializing polymeric parts.
The main conclusions of the paper are as follows:
  • Both low- and high-metallic-content filaments are available, with a polymeric matrix. The former is mainly used for decorative purposes. The latter complies with specific requirements regarding electrical conductivity, mechanical strength, etc.
  • As for the 3D printing machines, some of them are conventional ones with special metal kits, while other are directly designed to print metallic parts. Thermal treatment is often required, often comprising debinding and subsequent sintering operations.
  • As a result, in the next years it can be foreseen that new developments linking FFF/FDM and direct metal 3D printing are going to bloom, if costs can be kept within a reasonable limit, due to the intrinsic advantages of eliminating intermediate steps (debinding and sintering) such as those linked to compound part mixing metals that have been explained.

Author Contributions

Conceptualization, I.B.-C.; methodology, I.B.-C., F.F.-A. and J.M.-C.; validation, I.B.-C.; formal analysis, I.B.-C., F.F.-A. and J.M.-C.; investigation, I.B.-C., F.F.-A. and J.M.-C.; resources, I.B.-C.; data curation, I.B.-C., F.F.-A. and J.M.-C.; writing—original draft preparation, I.B.-C., F.F.-A. and J.M.-C.; writing—review and editing, I.B.-C., F.F.-A. and J.M.-C.; visualization, I.B.-C., F.F.-A. and J.M.-C.; supervision, I.B.-C.; project administration, I.B.-C.; funding acquisition, I.B.-C. All authors have read and agreed to the published version of the manuscript.

Funding

The APC were covered with own funds. The author Joaquim Minguella-Canela is a Serra Húnter Fellow (lecturer).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

The authors would like to thank the Catalan Government for the quality accreditation given to their research group TECNOFAB (2021 SGR 01034). TECNOFAB belongs to the group DIGIFACT, which is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. They also thank Lídia Aisa-Puiggardeu for her help with the experimental tests.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Material extrusion equipment: (a) mono-extruder open-frame equipment, (b) multiple-extruder closed-frame equipment, (c) equipment for metal compound AM, and (d) oven post-processing of metal parts. Source: Laboratory of Manufacturing Technologies of ETSEIB and Fundació Privada Centre CIM.
Figure 1. Material extrusion equipment: (a) mono-extruder open-frame equipment, (b) multiple-extruder closed-frame equipment, (c) equipment for metal compound AM, and (d) oven post-processing of metal parts. Source: Laboratory of Manufacturing Technologies of ETSEIB and Fundació Privada Centre CIM.
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Figure 2. Metallic material extrusion part: (a) while being printed and (b) after being processed via debinding and sintering. Reprinted with permission from Ref. [56]. Diseño y Fabricación de Maquinaria para Alimentación S.L. (DIFMAC ROURE), Santa Perpètua de Mogoda, Spain, is the owner of the parts.
Figure 2. Metallic material extrusion part: (a) while being printed and (b) after being processed via debinding and sintering. Reprinted with permission from Ref. [56]. Diseño y Fabricación de Maquinaria para Alimentación S.L. (DIFMAC ROURE), Santa Perpètua de Mogoda, Spain, is the owner of the parts.
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Figure 3. Stiffening strategy for FDMS parts to avoid deformation: (a) printed block of part and supports with interface material and (b) final part after sintering. Reprinted with permission from Ref. [56]. Diseño y Fabricación de Maquinaria para Alimentación S.L. (DIFMAC ROURE), Santa Perpètua de Mogoda, Spain, is the owner of the parts.
Figure 3. Stiffening strategy for FDMS parts to avoid deformation: (a) printed block of part and supports with interface material and (b) final part after sintering. Reprinted with permission from Ref. [56]. Diseño y Fabricación de Maquinaria para Alimentación S.L. (DIFMAC ROURE), Santa Perpètua de Mogoda, Spain, is the owner of the parts.
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Figure 4. Dimensional measurement of a steel-filled PLA dogbone specimen with a Mitutoyo micrometer.
Figure 4. Dimensional measurement of a steel-filled PLA dogbone specimen with a Mitutoyo micrometer.
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Figure 5. Three-dimensional-printed disks in (a) bronze, (b) copper, and (c) stainless-steel. Reprinted from Ref. [77].
Figure 5. Three-dimensional-printed disks in (a) bronze, (b) copper, and (c) stainless-steel. Reprinted from Ref. [77].
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Table 1. General taxonomy classification of the most relevant Metal Additive Manufacturing (MAM) technologies. Adapted from Refs. [1,4].
Table 1. General taxonomy classification of the most relevant Metal Additive Manufacturing (MAM) technologies. Adapted from Refs. [1,4].
TechnologyProcess
Powder Bed Fusion (PBF)Selective laser melting (SLM)
Electron beam melting (EBM)
Directed Energy Deposition (DED)Laser metal deposition (LMD) (powder-based)
Electron Beam Additive Manufacturing (EBAM) (wire-based)
Wire Arc Additive Manufacturing (WAAM) (wire-based)
Material Extrusion (MEX)Fused Filament Fabrication (FFF)
Direct Ink Writing (DIW)
Liquid Additive Manufacturing (LAM)
Binder Jetting (BJT)Nanoparticle Jetting (NPJ)
Single-pass jetting (SPJ)
Supersonic deposition (SD)
OtherMoldjet by TRITONE
Table 2. Comparison of the FFF, EBM, and MIM technologies for titanium alloy implants. Adapted from Refs. [19,20,21].
Table 2. Comparison of the FFF, EBM, and MIM technologies for titanium alloy implants. Adapted from Refs. [19,20,21].
ProcessAdvantagesDrawbacks
FFFEasy processLow dimensional accuracy
Relatively cheap processPoor surface finish
Low developing timeDistortion during sintering operations
Support structures required
EBMHigh speedLow dimensional accuracy
High versatilityLimited build volume
Ultimate tensile strength above 1100 MPaRough surfaces
No support structures required
MIMHigh dimensional accuracyNeed of expensive Ti powder
Manufacture of complex partsLimited to small parts
Ultimate tensile strength above 700 MPaMechanical strength depends on oxygen content
High production speedHigh developing time
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Buj-Corral, I.; Fenollosa-Artés, F.; Minguella-Canela, J. Three-Dimensional Printing of Metallic Parts by Means of Fused Filament Fabrication (FFF). Metals 2024, 14, 1291. https://doi.org/10.3390/met14111291

AMA Style

Buj-Corral I, Fenollosa-Artés F, Minguella-Canela J. Three-Dimensional Printing of Metallic Parts by Means of Fused Filament Fabrication (FFF). Metals. 2024; 14(11):1291. https://doi.org/10.3390/met14111291

Chicago/Turabian Style

Buj-Corral, Irene, Felip Fenollosa-Artés, and Joaquim Minguella-Canela. 2024. "Three-Dimensional Printing of Metallic Parts by Means of Fused Filament Fabrication (FFF)" Metals 14, no. 11: 1291. https://doi.org/10.3390/met14111291

APA Style

Buj-Corral, I., Fenollosa-Artés, F., & Minguella-Canela, J. (2024). Three-Dimensional Printing of Metallic Parts by Means of Fused Filament Fabrication (FFF). Metals, 14(11), 1291. https://doi.org/10.3390/met14111291

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