Binder Jet metal Additive Manufacturing: Process chain considerations when moving towards series production

Binder Jetting is an Additive Manufacturing process that promises some interesting benefits, technologically as well as economically. Despite originally being developed in the early 1990s [1], other powder-based Additive Manufacturing technologies have gained higher visibility in recent years, in particular Powder Bed Fusion processes employing laser (LB-PBF) or electron beam (EB-PBF) energy sources. These processes are also more widely distributed in the industrial environment.

When comparing Powder Bed Fusion processes with Binder Jetting, there are several basic advantages for the Binder Jetting process. As it makes use of a liquid binder instead of high-energy electro-magnetic radiation, there is no melting of the material, which can lead to internal stresses and distortion. Therefore, no support structures are required, a wider selection of materials is possible – including non-weldable materials – and the building of closed layers above loose powder areas, and thus the stacking of parts, is enabled. As the surrounding powder does not stick to the part as with a melt, the undersides of part features show the same surface qualities as the upsides. Therefore, parts can be designed differently and design features that would not be achievable by Powder Bed Fusion processes can be realised (Fig. 1). The main drawbacks are to be seen in the sintering step; one factor is the non-isotropic shrinkage, the other the potential for distortion during sintering due to the weight of the part and friction with the sintering setters (Fig. 2), which can limit the maximum part size.

Aside from technological aspects arguing for or against the use of either AM process, Binder Jetting is especially interesting for companies that already make use of sintering, and therefore have experience with the process step. The process is well established, and the material properties of parts made by Binder Jetting are close to conventional sintered parts, such as those produced by Metal Injection Moulding. Thus, prototypes and small series parts show comparable properties to production series parts.

Still, when considering implementing a technology such as Binder Jetting, which is unproven for volume production, the decision not only comprises technological factors, but also everything that surrounds the technology. Awareness of the complete process chain is, therefore, essential as the basis for decision-making. This is the motivation for this article; to broaden the discussion of the Binder Jetting process beyond the core part building process. Independent of machines or machine manufacturers, certain points have to be addressed in the process chain and an understanding of interdependent process factors has to be created.

Fig. 3 shows a general description of all relevant process steps for the Binder Jetting process of metals, based on commercially available materials. Underlying all process steps is the control of the process, which can take on various forms, depending on each process step itself. In the following, all process steps will be considered and discussed, including possible challenges, important impacts of specific factors on the overall process and control aspects. No claim is made that descriptions are complete, and further factors may be missing. Nonetheless, the intent of this article is to raise awareness of the whole process chain and to stimulate discussions on all process areas.

Material considerations in Binder Jetting

Materials of relevance to the complete Binder Jetting process chain may include more than just the materials associated with the build process, namely metal powder and binder. They may further comprise materials for cleaning or post-processing – for example mechanical machining, grinding, shot blasting, etc. After sourcing, materials have to be stored properly, according to safety guidelines and in a way that excludes property changes. Such changes may be of a chemical nature, for example in the case of reactive chemicals, or a separation of powders that would destroy a homogeneous or statistical mixture of differently sized powder particles, affecting final part quality and reproducibility.

With regard to their respective effects on the processability, handling and final quality of the parts, powder and binder have the biggest influence. The powder characteristics determine several factors that have a direct effect on important part properties. For example, particle size, particle size distribution and particle morphology determine characteristics such as apparent density, tap density and powder flowability. These in turn influence powder bed density [2] and possible anisotropies, which are directly responsible for the sintering behaviour, including achievable density [3, 4], also influenced by the intrinsic material properties, and shrinkage behaviour [5], though this may also depend on the part orientation in the building chamber. The binder is also crucial for further processing steps and their robustness, as it gives strength to the green parts and thus their handling capabilities. Additionally, the binder can influence achievable sintering densities due to its burnout properties and associated residuals that may interact with the solid material during thermal treatment. There is also the interaction of binder and powder to be considered. The wettability of the powder with the binder and the binder viscosity, which influences droplet size and capillary effects, also have an impact on the accuracy of the build, for example in the sharpness of edges.

As powder and binder have such a major influence, control aspects become very important. With regard to the binder, any changes in composition, for example due to chemical reactions or solvent evaporation, must be prevented prior to starting the build process. The same applies to powder and knowledge of powder characteristics. Therefore, analytical tools should be implemented to monitor material quality in order to assure reliable series production.

Material feeding

The feeding through of materials is one aspect of Binder Jetting that is seldom considered in research environments, but it is an important aspect when it comes to production. Usually, a Binder Jetting system is loaded manually with powder, and binder is filled into a reservoir. In the case of high production volumes, multi-machine operations, or when safety concerns regarding open powder handling come into play, a closed, automated material feeding system may be necessary. In such a case, the influence of these systems on material quality has to be considered. If, for example, the powder is stored in larger quantities and distributed by pneumatic feeding, this may affect the particle distribution due to de-mixing. Again, the monitoring of material properties becomes important.

The build process: planning and implementation

Design and placement

Having solved the question of the material feeding strategy, the build process has to be planned. This includes various stages, beginning with a review of the part design. Using CAD, parts must be designed along process specific guidelines, which vary from those of other AM technologies. Such guidelines should take all process limits and material properties into account. Process limits are set by all process steps, not only by the properties of the Binder Jetting machine (resolution, size of building chamber, etc.). The possible size of feasible channel structures are influenced by the powder characteristics and the strategy for depowdering. Distortion of part features may occur during sintering, which is closely linked to wall thicknesses, while automated handling may require sufficiently strong handling features. A further question may arise regarding the placement of the as-built parts in the sintering furnace: do the parts have a flat surface that can be placed directly on sintering trays or is a separate sintering support necessary to avoid distortion? Such a support would have to be designed and built, preferably in the same job. In order to be able to prepare thorough and reliable design guidelines, a complete understanding of the whole process is required.

Having designed the parts, they must be placed virtually in the build chamber, taking factors such as an anisotropic shrinkage into account. Depending on how the parts are placed, a direction-dependent size factor may have to be used to scale the parts.

Machine preparation

After placing all parts for the build job, physical preparation of the AM machine follows. This may vary considerably depending on the build and include specific steps aside from a sufficient material supply. The settings for the build job have to be chosen (for example, layer thickness, binder saturation, powder deposition settings, drying time, etc.), the powder bed prepared and possibly several system checks performed.

The final step before starting the build job is slicing, in which the virtual model of the build chamber is transformed into slices of the chosen thickness and combined with the build parameters.

Build step 1: Preparation of the powder layer

The Binder Jetting build process consists of three steps that are carried out in a repeating loop until the build job is finished. The first step is the deposition of the metal powder layer that includes the setting of the layer height, spreading of powder (either by a moving, vibrating container above the build chamber or by drawing powder from an adjacent powder reservoir), smoothing and compacting the layer by use of a doctor blade, roller or similar tool.

The method of powder spreading and the type of smoothing tool influence the characteristics of the powder layer [6]. In spreading the powder via a moving reservoir above the powder bed, the powder can be distributed evenly on the powder bed prior to smoothing.

Drawing the powder from an adjacent reservoir leads to a higher powder build-up at the beginning of the powder bed that slowly decreases over the distance to the end of the bed. The amount of powder build-up and the type of tool determine the pressure distribution in the powder and, with that, the compaction of the powder layer. Ideally, the compaction should be homogeneous in the powder bed to minimise anisotropy.

Build step 2: Binder deposition

The second step of the build is the deposition of binder into the powder bed. Position and amount of binder are set during build job preparation. The binder viscosity has to fit a window, given by the nozzle. Build accuracy is determined by the nozzle resolution and the droplet size, as well as the powder characteristics and the aforementioned interaction of powder and binder.

Build step 3: Drying

The third step of the build loop is a drying step, during which the binder solvent is evaporated to a certain point that prevents the following layer from shifting during powder smoothing. In addition, the powder bed is heated to a set temperature, which is crucial to guarantee uniform conditions for all layers and shorten the build time.

Between the binder deposition and the drying step, a setting time for the binder may be required in order to ensure sufficient time for the binder to soak into the powder layer and enable adhesion to the layer below.

Control aspects during a build may become complex. It is important to make sure that each layer is processed properly, as even one defective layer can make the complete part unusable. Regarding the build environment, measures against the contamination of equipment and personnel have to be taken, including safety aspects relating to the use of powder such as proper ventilation and atmosphere control, the use of explosion proof filters, etc.

Curing

The curing of the binder is a step that is optional depending on the employed binder system. It may be a completely separate process, in which binder components react to form a polymeric structure that provides mechanical strength. Alternatively, it may simply consist of a drying step, through which a previously dissolved polymer provides cohesion in the green part. Other concepts, as implemented in polymer Binder Jetting, make use of chemically reactive binder components such as acrylates and induce a reaction during the build process (for example by UV radiation), which leads to a polymerisation. From a production point of view, omitting a separate step is favourable, but such concepts are not yet established for the Binder Jetting of metals. When considering binder concepts, factors such as processing strategy (including necessary equipment), processing time, mechanical strength and burnout properties have to be taken into account.

Post-processing in the green state

The post-processing of green parts is a crucial step in the overall process. Independent of the employed AM system, this always includes debedding and depowdering of the green parts. Strategies for these steps have to be adjusted to the part geometry; simpler geometries may be easily handled and depowdered by tapping, brushing and/or compressed air blasting, but depending on geometric features such as channels or fine undercuts, and the powder’s flow characteristics, these measures may not suffice. Other approaches, such as controlled vibration or immersion and ultrasonic excitation, may be necessary. Of course, the success of a defect-free depowdering strongly depends on the green part strength (Fig. 6). Improved binder stability may therefore enable the use of more intense depowdering measures, making certain geometries realisable.

Furthermore, the removal of handling aids, especially in the case of automated part handling, may be done in the green state. In any case, the green parts have to be prepared for the following thermal treatment by placing them on sintering trays, with or without a separating agent, or in combination with geometry-specific shrinkage supports.

Control measures taken during post-processing directly correlate with final part quality. Also, treating green parts and freeing them from unbound powder has to be done in closed surroundings to prevent the uncontrolled release of powder. Excess powder and removed powder has to be further treated in order to be reusable; this includes powder recovery, possibly drying, sieving, verifying its original particle size distribution and refeeding.

Thermal processing

Debinding

Thermal processing can be divided into phases, with the first being thermal debinding. This is a thermal decomposition of the organic binder in a manner that does not damage the part and leaves behind as little residual material as possible. This requires detailed knowledge of the binder’s thermal behaviour in a specific atmosphere, especially regarding degradation behaviour, which is often quite complex and requires several holding steps and slow heating. The in-line monitoring of decomposition products in the furnace would be a sensible method for quality control.

Depending on the basic approach of the Binder Jetting process, thermal debinding is either followed by a sintering step to achieve high densities, usually directly after debinding in the same furnace run, or by pre-sintering to achieve parts with specific porosity that is filled by infiltration with a lower melting point  material (e.g., bronze) in a separate furnace run.

Sintering

Sintering in general is a heat treatment applied to a powder compact in order to impart strength and integrity by diffusion processes. The temperature used is below the melting point of the major constituent of the material. The two selectable parameters are the time-temperature profile and the atmosphere or process gas, both of which have a significant influence on the sintering process and must be chosen depending on the material and the processed powder.

Among other things, the atmosphere is responsible for heat conduction and removal of the decomposed binder and can interact with the material. Hydrogen, for example, has a reductive effect, which leads to a higher density in stainless and low-carbon steels, and changes to the chemical composition and mechanical properties of medium-carbon steels, which are decarburised. It is also possible to sinter under vacuum, so that no gas-filled pores can arise, which could expand again during a heat treatment after Hot Isostatic Pressing.

The time-temperature profile is usually divided into three sections: heating, holding and cooling. The heating process includes the debinding step and is limited in its speed by heat conduction and part size in order to ensure a homogeneous temperature distribution in the green/brown part, which in turn leads to a uniform start to densification. The start of densification depends directly on the particle size and particle morphology, which also influence the required holding temperature and holding time. For finer powders, lower temperatures and a shorter holding time can be selected compared to coarser powders in order to achieve similar densities. During cooling, it is important to avoid large temperature gradients that can cause residual stresses. Current work mostly focuses on sintering and, for specific materials, relative densities of more than 99.5% can be reached for 316L stainless steel (Fig. 7).

Sintering equipment with the proper monitoring of temperature development and distribution is rather expensive, but a crucial part of the process chain. The quality of processing materials, especially the gases, is also of great importance. Furnace installation requires considerable effort, as demands on operational safety are high.

Post-processing in the sintered state

Mechanically, sintered parts are considerably more stable than green parts. Therefore, the removal of handling aids that would be too difficult to remove in green state may instead be carried out after sintering. Further treatments may include Hot Isostatic Pressing, calibrating via coining, or machining processes such as grinding, turning, milling and polishing in order to achieve the desired part finish.

Further control aspects

Several control aspects have been mentioned for specific process steps, such as special analytics and in-line monitoring for quality control purposes, but also controlling the necessary process environment with regard to safety.

In a production environment, further controls may need to be implemented. First of all, incoming goods inspection of all consumable materials is indispensable to ensure reproducibility. This requires different processes for each material,for example chemical analysis or powder characterisation. Other quality control measures may include SOPs (standard operation procedures) for all manually performed tasks, documentation and data storage. In order to prevent production downtimes, spare parts have to be in stock and the logistics of materials, parts and consecutive process steps have to be managed. This also includes the reuse of materials and waste management.

Conclusion

When considering the implementation of a relatively new series production process, many factors have to be taken into account. Aside from existing challenges regarding Binder Jetting technology, there are many more aspects surrounding the build process that could have an even greater impact on series production within the process chain. It is hoped that with this article, awareness of these aspects is raised, as discussions about the technology are usually limited to the advantages, disadvantages, possibilities and challenges of the build process. There is no doubt that this technology will be applied in the future, but we also see the need to shed light on all process aspects in order to prevent unnecessary setbacks in the implementation of Binder Jetting. The sooner more people are aware of challenges in the entire process chain, the sooner solutions for specific challenges will be developed.

References

[1] Sachs, Emanuel M.; Haggerty, John S.; Cima, Michael J.; Williams, Paul A. (1993): Three-Dimensional Printing Techniques. Angemeldet durch Massachusetts Institute of Technology. Anmeldenr: 07/447677. Veröffentlichungsnr: US5204055.

[2] Bai, Yun; Wagner, Grady; Williams, Christopher B. (Hg.) (2015): Effect of Bimodal Powder Mixture on Powder Packing Density and Sintered Density in Binder Jetting of Metals. 2015 Annual International Solid Freeform Fabrication Symposium.

[3] Verlee, B.; Dormal, T.; Lecomte-Beckers, J. (2012): Density and porosity control of sintered 316L stainless steel parts produced by Additive Manufacturing. In: Powder Metallurgy 55 (4), S. 260–267. DOI: 10.1179/0032589912Z.00000000082.

[4] German, R. M. (1992): Prediction of sintered density for bimodal powder mixtures. In: MTA 23 (5), S. 1455–1465. DOI: 10.1007/BF02647329.

[5] Sicre-Artalejo, J.; Petzoldt, F.; Campos, M.; Torralba, J. M. (2008): High-density inconel 718: Three-dimensional printing coupled with hot isostatic pressing. In: The International Journal of Powder Metallurgy 44 (No.1), S. 35–43.

[6] Cao, Shu; Qiu, Yang; Wei, Xing-Fang; Zhang, Hong-Hai (2015): Experimental and theoretical investigation on ultra-thin powder layering in three dimensional printing (3DP) by a novel double-smoothing mechanism. In: Journal of Materials Processing Technology 220, S. 231–242. DOI: 10.1016/j.jmatprotec.2015.01.016.

Authors

Sebastian Boris Hein, Claus Aumund-Kopp and Bastian Barthel
Fraunhofer IFAM
Wiener Str. 12,
28359 Bremen
Germany
Email: [email protected]
www.ifam.fraunhofer.de