Influence of Various Binder Jet Printers on the Additive Manufacturing of Hardmetals

Abstract

Binder Jetting (BJT), a powder-based additive manufacturing technology, has been shown to be a fast and reproducible green shaping process for many different metals. Due to its high productivity and versatility in material processing, BJT is gaining increasing importance in the manufacturing sector. It can also be used for the production of WC-Co hardmetals, a primary ceramic-based composite often used for tools and wear parts. Five different BJT printers from four different manufacturers were evaluated to assess their effectiveness and feasibility in producing hardmetals based on the same WC-12 wt.% Co starting powders. The analysis focused on comparing the properties of the resulting hardmetals, as well as evaluating the printing performance. The results show that all tested BJT printers are fundamentally well suited for producing green hardmetal parts, which can achieve full density after sintering. This work highlights the potential of BJT technology in hardmetal manufacturing for tool production.

1. Introduction

An increasing variety of printing technologies has emerged in the sector of additive manufacturing (AM). Besides widely used beam-based AM methods [1,2,3], numerous sinter-based technologies have also developed. All seven AM categories defined by ISO 529200:2021 vary in their technology but also show differences within each group from manufacturer to manufacturer. In the case of Binder Jetting (BJT), printers predominantly differ in their powder handling during recoating, the method of adding the organic binder, and the behaviour of the printing liquid during post-processing. How far these aspects influence the performance and properties of printed and sintered samples has not been studied so far. Thus, for the first time, this study evaluates several BJT printers using the same WC-Co starting material.

Previous research has confirmed that hardmetals, often referred to as cemented carbides, can be shaped via BJT, and that the resulting green parts can be sintered to achieve full density [4,5,6]. Most of these studies utilised BJT printers from ExOne (now Desktop Metal), predominantly the “Innovent” or “Innovent+” printer [7,8,9,10,11,12]. However, our findings indicated earlier that effective processing can also be achieved with machines from ZCorporation (now 3D Systems, Rock Hill, SC, USA) [13].

In recent times, the additive manufacturing market has witnessed an increase in the availability of BJT printers, which has the potential to facilitate the commercialisation of a diverse range of models in the field of additive manufacturing, also for hardmetals.

Table 1. Overview of selected manufactures of BJT printers and associated materials.

Company	Materials	Types
Voxeljet (Friedberg, Germany)	Sand, Polymer, Ceramic	VX-Series
DesktopMetal/ExOne(North Huntingdon, PA, USA)	Metal, Sand, Ceramic	X-, S-, P-Series, Shop system
Markforged (former Digitalmetal, Waltham, MA, USA)	Metal	PX100
Sinterjet (Istanbul, Turkey)	Metal	M60
Hewlett Packard (HP, Palo Alto, CA, USA)	Metal, Polymer, Gypsum	Metal Jet S100, Jet Fusion Serie
EASYMFG (Wuhan, China)	Sand, Metal, Polymer, Ceramic	S-,M-Serie
3DSystems (former ZCorporation, Rock Hill, SC, USA)	Gypsum	ColorJet-Serie/ZPrinter-Serie
MicroJet Technology (Hsinchu, Taiwan)	Gypsum	ComeTrue M10, T10
Triditive (Asturias, Spain)	-	AMCELLJet-Serie
FHZL (Foshan, China)	Sand	PCM-Series
Coastruction (Rotterdam, The Netherlands)	Sand	Idefix, Asterix, Obelix
Desamanera (Rovigo, Italy)	Sand, Minerals	Stampanti 3D
CONCR3DE (Rotterdam, The Netherlands)	Sand, Metal, Ceramic	Armadillo-Serie, Elephant Gray
CMET(Yokohama, Japan)	Sand	SCM-1800
xyzPrinting (Suzhou, China)	Gypsum	PartPro350 xBC
General Electric Aerospace (GE, Cincinnati, OH, USA)	Metal	-
Stratasys (Eden Prairie, MN, USA)	Gypsum	H350

presents a concise overview of select BJT printer manufacturers and their respective product offerings.

The roots of BJT technology can be traced back to pioneering research by Emanuel M. Sachs at MIT in Boston, USA [14,15]. One of the initial materials explored for printing was WC-Co-based hardmetals as early as 1997 [16]. However, the results at that time were only moderately successful, particularly for hardmetals. Following the expiration of the BJT patent in 2010 [17], only a handful of companies established themselves in the BJT sector, with the printing of metals dominating the market. Nevertheless, interest in processing hardmetals has increased in the last six years, both at research institutions and at large companies like Kennametal Inc. (Pittsburgh, PA, USA), CERATIZIT S.A. (Mamer, Luxembourg), Sandvik AB (Stockholm, Sweden), and Hyperion Materials & Technologies Inc. (Worthington, OH, USA) [4], which provide their expertise in Binder Jetting for hardmetals. In this study, five different BJT printers from four of the manufacturers mentioned in Table 1 were used to assess their ability to produce hardmetal green parts using the same starting materials and to achieve full density after sintering.

2. Materials and Methods

The material chosen for this study was a WC-12 wt.% Co grade from Oerlikon, designated as WOKA3110FC (Oerlikon, Pfäffikon, Switzerland). Its particle size is specified as −25 + 10 µm. Due to the temporal discrepancy between the tests, different batches were used. All batches were characterised through their granule size distribution and an evaluation of their apparent material characteristics. While the studied powder is commercially utilised for thermal spraying, it possesses an internal structure with open porosity. However, the spherical shape makes it particularly well suited for powder-based additive manufacturing techniques like Binder Jetting.

According to Oerlikon’s data sheet, the WC grain size within the WC-12 Co particles is categorised as fine, with a WC grain size ranging from 0.8 to 1.3 µm. A comprehensive characterisation of the powder granules was performed, including measurements of apparent density in accordance with ISO 3923, tap density per ISO 3953, granule size distribution with a disperser pressure of 0.1 and 3.5 bar (MASTERSIZER 2000, Malvern Panalytical GmbH, Kassel, Germany), and imaging using a Field Emission Scanning Electron Microscope (FESEM) (LEO 982, Carl Zeiss SMT AG, Oberkochen, Germany). The powder was utilised to print cubes on the printers listed in

Table 2. Used BJT printers with selected printing parameters.

BJT Printer (Manufacturer)	Used Layer Thickness	Recoat System	Organic|Addition Place|Solvent
Z510 (ZCorporation)	100 µm	Roller	Alginate|feedstock|water
VX200 (Voxeljet)	100 µm	Blade	Alginate|feedstock|water
Metal Jet|Alpha (HP)	50 µm	Roller	Latex-emulsion|printing liquid|n.a.
Innovent (ExOne)	100 µm	Roller	BA005|printing liquid|water
Innovent+ (ExOne)	50 µm	Roller	BA005|printing liquid|water

. These cubes, each with an edge length of 10 mm, were printed with layer thicknesses between 50 µm and 100 µm for enhanced comparability. The green densities, excluding organic material, were calculated based on the shrinkage of sintered samples and their theoretical density, and are presented as approximate values. Printed cubes underwent debinding in a hydrogen atmosphere, followed by vacuum sintering and sinter/HIPing at 100 bar of Ar, reaching temperatures of up to 1500 °C. The resulting sintered samples were analysed for their density (ISO 3369), magnetic properties, porosity (ISO 4499-4), hardness (DIN EN ISO 14705), and microstructural characteristics.

3. Results and Discussion

The following section presents the results of the characterisation of the starting material, extending to the microstructure of sintered structures.

3.1. Starting Material

In order to guarantee the comparability of the printing tests, the granule size distribution of different batches is established and illustrated in Figure 1.

The granule-size distributions illustrate that the batches follow a similar curve and exhibit comparable granule distributions. Furthermore, the measurements conducted with varying pressures (0.1 bar to 3.5 bar) did not yield any notable alterations, which suggests that the granules possess an exceptional degree of stability because of the used pre-sintering process. It can be postulated that the observed consistency is not attributable to mechanical influences inherent to the printing procedure. Characteristic distribution values, given in

Table 3. Comparison of the granule-size distribution of the different batches from WOKA3110FC.

Delivery Date	d10 [µm]	d50 [µm]	d90 [µm]	d97 [µm]
01/17	12.5	20.3	32.1	38.0
05/18	11.8	20.2	32.9	39.2
10/18	11.9	19.6	31.5	37.6
04/22	11.2	19.6	32.9	39.4
07/24	14.1	21.6	32.5	38.3

, show the d₁₀, d₅₀, d₉₀, and d₉₇ values of measured distributions.

The agglomerated and sintered powder from Oerlikon exhibits an apparent density of 4.6 to 4.9 g/cm³, which corresponds to a theoretical density (TD) of 32.3 to 34.3%. In contrast, the tap density is measured in the range of ~5.7 g/cm³, which equals a theoretical density of 39.6%. Figure 2 presents scanning electron micrographs illustrating both surfaces and cross-sections of the used granules.

Scanning electron micrographs (Figure 2) at two different magnifications demonstrate that the granules show a highly rounded morphology and a consistent internal structure, which contribute to their excellent flowability and elevated apparent density. An examination of the cross-sections indicates that the granules contain a certain degree of internal porosity. Figure 3 shows the surface of granules at higher magnifications. Here, WC grains are distinctly visible at the surface. Based on cross-sectional analysis, it can be seen that the WC grain size ranges from 1 to 2 µm (fine WC grain).

3.2. Debindering and Sintering of Printed Green Samples

Printed cubes of identical size were produced using the five distinct Binder Jetting printers. The debinding process is a crucial step in the production process, as the granules have to support themselves for several hundred degrees while the organic binder is driven out and the granules sinter together. After sintering at a temperature of 1500 °C, a comprehensive analysis of the produced hardmetal cubes was conducted to evaluate the influence of specific technologies on physical characteristics, including shrinkage, microstructure, and WC grain growth.

Cubes produced using the Z510 BJT printer from ZCorporation (now 3DSystems) displayed a relatively low apparent density. This phenomenon can be attributed to the technology employed, whereby the organic binder is incorporated as a solid additive prior to printing, as opposed to being printed directly into the powder bed. The binding of granules occurs via a selective reaction between the previously added organic binder and the mainly water-based printing fluid. Notwithstanding the mentioned limitations, the printer exhibits a high level of operational efficiency, capable of producing 10 mm high samples with a layer thickness of 100 µm in a mere 20 min. Figure 4 demonstrates that a dense hardmetal microstructure can be formed from the Z510-printed green part. At higher magnifications, the distribution of WC (dark grey) and Co (light grey) is shown, indicating a homogeneous phase distribution without visible undesired free carbon or eta phase. However, abnormal WC grain growth was observed, with some WC grains reaching sizes of up to 20 µm.

The Voxeljet VX200 BJT printer employs a comparable binder concept to the Z510, utilising a water-based printing fluid system. The printed cubes demonstrated stability throughout the debinding process. Figure 5 indicates no notable differences in phase composition, grain growth, or homogeneity in comparison to the Z510 results.

The HP BJT printers diverge from the previously referenced models in that they use a latex emulsion as the organic binder. The layer generation process is analogous to that of the Z510, yet offers enhanced precision, resulting in a longer build time of 8 h and 40 min for a 13 mm high cube with a 50 µm layer thickness. However, the BJT process parameters were designed with the objective of producing samples with consistent dimensional stability, rather than optimising the process for speed. Figure 6 shows that the microstructure of the sintered cube at 1500 °C did not exhibit significant differences from that of earlier samples. The latex binder was effectively removed during debinding, as no free carbon was detected within the WC-Co microstructure.

ExOne’s Innovent BJT printer also utilises a directly printed organic binder based on ethylene glycol, which results in the generation of stable samples during the debinding process. Processing with the Innovent printer resulted in the formation of a dense, two-phase structure with a uniform distribution and the discernible now-known abnormal grain growth, as illustrated in Figure 7.

The Innovent+ BJT printer (from ExOne), which is more recent in its version, features an advanced compaction technology (ACT) for powder recoating. This utilises a double roller system for consistent layer application. It should be noted that the organic binder system remains unchanged from the previous Innovent printer. The microstructure in Figure 8 resembles those previously discussed but shows more pronounced abnormal grain growth. This is potentially due to cross-contamination from steel powder previously used on the printer.

Table 4. Properties of sintered samples at 1500 °C manufactured with different BJT printers.

Properties	ZCorp	Voxeljet	HP	Exone (Innovent)	ExOne (Innovent+)
Density [g/cm3|%TD]	14.2|99.0	14.3|99.1	14.3|99.3	14.2|98.9	14.3|99.3
Mag. Saturation [µTm3/kg |%TmS]	22.6|93.7	22.3|92.4	23.7|98.2	22.7|94.1	23.3|96.6
Hc [kA/m]	7.1	7.8	6.6	6.6	5.6
Hardness [HV10]	1092 ± 6	1149 ± 18	1133 ± 5	1120 ± 11	1016 ± 11
Lin. shrinkage X|Y|Z [%]	28|29|29	23|24|30	20|21|22	28|30|31	28|29|29

compares the measured properties of sintered samples from all five BJT printers. The relative density of sintered samples approaches ~99% in all cases. Lower values are attributed to subjective Archimedes density measurements, which are influenced by the surface roughness of the BJT printed samples. The microstructures show a minimal residual ISO porosity of A02B00C00. The measured magnetic saturation indicates the alloying state of the metallic phase, with either carbon or tungsten. A lower value points to a higher amount of tungsten and a higher value to less tungsten but more carbon. When there is a supersaturation of carbon, unbound carbon precipitates. When there is a supersaturation of tungsten, however, it results in precipitating so-called eta-phase grains (complex carbide grains of tungsten and cobalt). Both precipitates are unwanted in any case. Based on experience, the two-phase region of WC-Co without precipitates is in the range of 75% to 95% of the theoretical magnetic saturation (TmS). For produced samples, the saturation is similar and points to a position within or next to the WC + Co phase window, but closer to the region of free carbon. Microscopic analysis confirmed that all samples remained within the two-phase region of WC and Co. A slight variance in magnetic saturation indicates that samples utilising the organic binders from the Z510 and VX200 printers exhibited the lowest carbon retention during debinding. The printers utilising printhead-applied binders displayed slightly higher magnetic saturation, which is likely due to the presence of a higher residual carbon content after debinding. This higher residual carbon can affect growth of WC grains, resulting in a lower coercive field strength which, in turn, is related to a lower hardness.

In order to evaluate the surface quality, the cross-sections of samples were subjected to further analysis. Figure 9 shows the shape and surface quality of sintered cubes, demonstrating varying degrees of edge sharpness and roughness. HP- and ExOne BJT-printed samples exhibited the sharpest contours, which can be attributed to their precise printing technology. Any observed cut-off edges in some cubes are the result of sample preparation and should not be considered defects.

Furthermore, shrinkage measurements indicate that samples produced with the HP BJT printer exhibited the lowest shrinkage, while those produced with the ExOne and ZCorporation printers displayed comparably higher degrees of shrinkage. Figure 10 provides a graphical illustration of shrinkage, demonstrating that samples from the HP and Voxeljet printers exhibited the least variation in height (Z-Direction), width (Y-Direction), and length (Z-Direction).

3.3. Optimisation of the Material System Regarding the Fineness of the Microstructure

In order to address the abnormal WC grain growth observed in all samples, further optimisation was conducted using the ExOne Innovent+ BJT printer. A solution of grain growth inhibitor (powder of Cr₃C₂) was prepared and mixed with the starting material for a period of two hours.

Table 5. Comparison of sintered samples with and without Cr₃C₂ addition, printed on an Innovent+ printer.

Properties	WOKA3110FC	WOKA3110FC + Cr3C2
Density [g/cm3|%TD]	14.3|99.3	14.1|98.6
Mag. Saturation [µTm3/kg|%TmS]	23.3|96.6	22.7|96.9
Hc [kA/m]	5.6	13.2
Hardness [HV10]	1016 ± 11	1303 ± 7
Porosity (ISO 4499-4)	A02B00C00	A02B00C00

presents a comparison of sintered samples with and without the addition of Cr₃C₂.

The hardness demonstrates an increase from 1020 HV10 to 1300 HV10, which correlates with the grain refinement of the microstructure in Figure 11.

The reduction in grain growth achieved with the Cr₃C₂ addition demonstrates that the composition and properties of the powder exert a significant influence on the resulting microstructure and material properties, more so than the specific printer models or manufacturers. The discrepancies observed in the green bodies were insufficient in significantly altering the properties of the sintered parts. However, the WC-Co material system is notably tolerant of such variations due to the liquid phase sintering process and particle rearrangements. The findings indicate that by optimising the starting material, the microstructural properties can be tailored to achieve material properties comparable to those of conventionally produced hardmetals with similar Co content and WC grain size.

4. Conclusions

The present study demonstrated the successful shaping of hardmetal green bodies using Binder Jetting. A number of BJT printers, each equipped with distinct coating systems, polymer binder types, binder contents, and curing strategies, were assessed using a single WC-12 wt.% Co hardmetal powder. It is noteworthy that all BJT printers evaluated were suitable for producing fully dense hardmetal parts following the sintering process. Furthermore, the debinding phase was found to be stable for all components.

The debinding and sintering of green bodies derived from the same starting material resulted in the production of dense hardmetal bodies with comparable properties, all within the two-phase region of WC + Co. By optimising the material system, properties comparable to those of conventionally produced hardmetals with equivalent Co contents or WC grain sizes were achieved. The measured hardness for a Cr3C2-doped WC-12 wt.% Co powder is 1300 HV10.

The shaping method employed affects the properties of the green parts, including shape accuracy, density, and strength. Conversely, the carbon balance, which is influenced by the technology-specific organic binder used, impacts the sintering behaviour. Ultimately, this research confirms that hardmetals with comparable properties can be produced using different BJT printing technologies.

It is anticipated that the continuous advancement of existing printers, the introduction of new models, and the optimisation of powders for BJT will result in notable enhancements in quality, printing speed, material compositions, and corresponding properties, as well as the use of lower sintering temperatures.