How Will 3D Printed Carbide Be in the Future?

How Will 3D Printed Carbide Be in the Future?

• Admin
• 2024/12/15

Due to its excellent wear resistance and corrosion resistance, cemented carbide is widely used in many fields.3D printing, also known as“additive manufacturing”, is an emerging advanced molding technology which has been applied in aerospace, bio-materials and other fields. However, in the field of refractory metals and their carbides, such as cemented carbide, it is still in its infancy. Some companies and institutes are working on or paying attention to the development of 3D printed cemented carbide, but so far, there is still no substantial progress. It means that there is no 3D printing method can comprehensively solve all the problems such as poor surface, microscopic porosity and low strength etc. of cemented carbide printing products. In this paper, the characteristics and also problems of 3D printing methods for cemented carbide, such as SLS / SLM, FFF, BJP and DLP etc. in domestic and foreign countries in recent years has been organized, discussed and analyzed, and the development of 3D printing technology for cemented carbide is prospected.

Cemented carbide is a multiphase composite material with one or more refractory metal carbides (such as WC) as the matrix and transition group metals (usually Fe, Co, Ni, etc.) as the binder. The content of the binder phase of the general cemented carbide (the content is the mass fraction unless otherwise stated) is below 30%, and the WC grain size is from a few hundred nanometers to a few microns. Cemented carbide has been widely used and occupies an important position in the fields of metal cutting, petroleum drilling, mining engineering, etc. due to its high hardness and wear resistance, as well as excellent compressive and corrosion resistance, and is known as ” Industrial teeth”.

3D printing, also known as “additive manufacturing”, is considered a manufacturing technology that has shaken the world in the 21st century. This technology is based on the principle of discrete-stacking, using 3D mapping software to design or draw a target 3D solid model, through a layered software drive module, according to the 2D data of each layer to control the laser, plasma, electron beam and arc to act on the powder 、Liquid or silk material, processing the thin layer of the shape and size required by the software, and accumulating layer by layer to form a solid model manufacturing technology. Compared with the traditional “reduced material manufacturing”, 3D printing is very significant in terms of material savings. For example, compared with traditional casting, the material savings can usually exceed 60% (see Figure 1).

Since the first SLA250 printing equipment was launched by 3D Systems in the mid-1980s, 3D printing, as an emerging advanced molding technology, has been applied in aerospace, biomaterials and other fields, and gradually The variety of materials expands its applications.

Fig.1 Material utilization comparison between casting and 3D printing

The high hardness of cemented carbide makes it difficult to adjust the shape and size after sintering, and it also requires a lot of expense. Therefore, the main reasons for 3D printing cemented carbide to attract industry attention are:

• Manufacture of hard alloy parts that cannot be prepared by traditional molding, extrusion, etc.;
• Save raw material costs and avoid processing. Complex tools, cutting tools, and parts with internal cavity structures are inevitable for subsequent processing. These types of cemented carbide products are important areas for 3D printing.

At present, the application of 3D printing on metal materials such as Ni-based alloys, Ti and Ti alloys and tool steels has matured, and industrial applications in aerospace, biomaterials and other fields have begun. However, refractory metals and their carbides, such as cemented carbide, are still in their infancy.

Although there are a small amount of reports and information, it is only a few possible method speculations and a small amount of literature and patents, and these have yet to be verified by experiments. Therefore, it can be speculated that some enterprises and scientific research institutions are devoting or paying attention to the research and development of 3D printed cemented carbide, but so far, there has been no substantial progress-that is, there is no technology that can comprehensively solve the surface finish of cemented carbide printed products Poor, high micro-porosity and low strength.

In recent years, SLM-Selecting Laser Melt-ing/ SLS-Selecting Laser Sintering, FFF-fused-filament fabrication, and BJP-Binder Jet Printing) and other methods are used by researchers to try 3D printing of cemented carbide. The results show that these methods do have their own advantages, but without exception, there are major problems that hinder their continued development.

This article sorts out and analyzes the research situation of these methods, and prospects the development of the 3D printing cemented carbide industry.

1 3D Printing Cemented Carbide Research Methods

1.1 SLM-Selecting Laser Melting / SLS-Selecting Laser Sintering

Both SLS and SLM are rapid prototyping technologies, and their working principles are basically the same. That is, the discrete stacking principle is used to slice the three-dimensional solid model of the part along the Z direction and generate an STL file that stores the solid cross-section information of the part. According to the heat effect, according to the cross-sectional information of the parts, the solid powder materials are layered and piled up to form the parts. However, the mechanical properties of SLS parts are poor, the density is low, and the process is complicated (after printing, subsequent heat treatment or hot isostatic pressing is required, and then the metal is infiltrated to ensure the density).

Therefore, in 1995, the Fraunholfer Institute in Germany first proposed an alternative SLM process. In 2002, the research team of Leuven University in Belgium tried SLS in the application of WC-Co cemented carbide, but did not give the specific components of the study. Only the energy absorption rate of the material was evaluated. Sanjay Kumar of York University in Canada used WC-17%Co as the raw material to prepare the cemented carbide by the SLS method, and after the printing, the product was sintered at 400~1 000 ℃.

The research shows that the wear resistance of the WC -17%Co cemented carbide treated after 400 ℃ is 6 times higher than before treatment. However, after treatment at different temperatures, W2C phase and η phase (W₃Co₃C, W₆Co₆C, W₁₀Co₃C₄) were found in the alloy.

The University of California in the United States prepared the WC-10%Co cemented carbide using the SLM method in 2007, and found that the alloy prepared by this method contains W₂C and η phases (Co₃W₃C) other than WC and Co (as shown in Figure 2). The main reason is that during the laser sintering process, WC partially melted and reacted with C and Co in the raw materials. The results of this study are the same as those of Sanjay Kumar.

The researchers also observed and analyzed the microstructure of the alloy and found that there is a clear boundary between the printed layer and the layer in the alloy (as shown in Figure 3), and each layer contains two areas of light and dark; SEM photos show In the bright and dark regions, the size difference of WC grains is obvious, the small particles are below 100 nm, and the large particles can reach submicron or even micron level. The author believes that the main reason is that the laser scanning speed is faster, and the WC grains are too late to grow into single crystals compared with traditional sintering, and mostly exist in polycrystalline forms.

Fig.3 SEM photographs of cemented carbide prepared by SLM: (a) printing layers; (b) light layer area in figure (a);(c) phase composition of the alloy; (d) large particle WC in bright zone

Chen-Wei Li and others from National Tsinghua University in Taiwan conducted SLM printing on WC-NiAlCoCrCuFe alloy to study the microstructure and physical and mechanical properties of the alloy after printing. The experiment indicates that the surface of the cemented carbide product after SLM printing is rough and of poor quality (as shown in Figure 4). After the laser scanning melting above 2 000 ℃, WC decomposes, and it is easy to cause decarburization of the product, resulting in the final coexistence of WC, W₂C, η phase and FCC phase. This research result is the same as the literature.

Fig.4 Surface quality of cemented carbide prepared bySLMFig.5 Structure between melting pool boundary and core

At the same time, after testing the hardness and toughness of different parts of the alloy, it is known that the hard alloy prepared by SLM, from the surface to the core, due to the existence of the melting pool, the structure and performance of the two layers of obvious differentiation, as shown The structural difference from left to right in 5 and the performance difference in FIG. 6. The hardness test of WC-10%Co cemented carbide found that there is a clear upward trend in hardness from the surface of the sample to the core (as shown in Figure 6), which proves that the uniformity of the sample is poor.

Fig.6 Hardness and toughness change from surfaceto core

In addition, in the study of SLM printing of WC-Co cemented carbides by Eckart Uhlmann and others at the Fraunholfer Research Institute in Germany, it was also found that there are multiple unevenness in the microstructure of the alloy, while in the “sintered layer”, The large and large pore size is a very prominent problem (as shown in Figure 7).

Fig.7 Typical microstructure of WC-Co cemented carbide prepared by SLM:(a) low magnification image;(b) high magnification image of rectangle area in fig.7(a)

Up to now, the application and theoretical research of SLM method in 3D printing of cemented carbide is the most. Although other universities and scientific research institutions have different researches on the selection of raw materials in this respect, the results obtained are similar to the above research. . The current problems are mainly: uneven microstructure and more pores, rough surface, coexistence of four phases of WC, W₂C, η phase and FCC phase, and obvious performance difference from surface to core.The main reasons are summarized as follows: during the laser scanning process, rapid melting and solidification make the alloy unable to complete the liquid phase full flow and WC particle rearrangement; the short-term appearance of the liquid phase causes the WC dissolution and precipitation to grow up is restricted; high temperature effect The lower WC decomposes and partially reacts with Co to produce W₂C and decarburized phases.

All the problems mentioned above, as well as some warpage and cracks after printing, are obstacles that restrict the further application of SLM / SLS technology in the field of cemented carbide.

1.2 FFF-fused-filament fabrication/DIW-Direct ink writing+Gel-printing

The two methods of fuse manufacturing and direct writing + gel/ink printing are similar in principle, and the products are obtained by layer-by-layer extrusion. Another method is to use a combination of 3D printing negative mold and slurry injection technology. Strictly speaking, only 3D printing mold is used, not 3D printing cemented carbide.

The method of fuse manufacturing (FFF) has been reported in Chinese patents and the 2018 European Powder Metallurgy Conference. The main process of this method is:

Slurry Preparation

Spray Drying to Obtain Granular Mixture

Extrusion Into Silk Material

3D Printing Into Blank

Degreasing and Sintering

The printing process is visually called “squeezing toothpaste” layer by layer, and the main problems are:

• The surface quality of cemented carbide prepared by FFF is poor. The typical shape of the blank is shown in Figure 8, and there are still obvious external traces after sintering (Figure 9)
• There are many discontinuous parts in the structure of cemented carbide products formed by this method, and the pores are larger as shown in Figure 10.

Fig.8 Cemented carbide blank prepared by FFF: (a) low magnification image; (b) high magnification imageFig.9 Surface of cemented carbide blank prepared by FFF after sinteringFig.10 Profile structure of cemented carbide prepared by FFF after sintering

DIW is a 3D printing method using high-viscosity paste, and has been studied in the field of 3D printing ceramics. The direct writing plus gel/ink printing method is also reported in Chinese patents, but it still needs to be verified by experiments. The research team of the University of Science and Technology Beijing has prepared the WC -20%Co cemented carbide using the direct-printing gel/ink 3D printing method. The study found that the local performance of the cemented carbide prepared by this method can be close to the alloy prepared by the conventional powder metallurgy method, but there are still very outstanding problems: poor surface quality and large pores in the overall structure (as shown in Figure 11) .

Fig.11 WC-20%Co sample prepared by direct writing and gel / ink 3D printing: (a) 3D model; (b) the dried blank;(c) sintered sample; (d) profile image of the sample at low magnification; (e) microstructure of the dried blank

1.3 BJP-Binder Jet Printing

BJP is a 3D printing technology in which an organic binder (Binder) is sprayed onto a powder bed according to a certain image, layer by layer, and a three-dimensional part is printed and formed. The green body printed and formed by this technology is degreased and sintered to obtain the target product.

Patent search analysis shows that in the past 20 years (2000~2019), the total number of domestic and foreign patent applications for 3D printed cemented carbides has exceeded 50. Among them, Sweden Sandvik has applied for the largest number of 3D printed cemented carbide patents. Figure 12 shows Sandvik’s patents related to 3D printed cemented carbide in the past four years. It can be seen from the figure that Sandvik has applied for related patents since 2015. Up to now, 22 applications have been applied. In 2016, the largest number of applications was 8, with 8 applications. Most of the company’s patents (17 items) focus on the use of BJP printing to form cemented carbide. Among them, 2 items are cemented carbide powders required for the preparation of BJP, and 15 items are BJP printing processes such as powder particle size distribution of the powder bed.

The Sandvik patent (EP2016805772, CN201780023242.4) discloses preparation methods including:

• Spray dry WC, Co and PEG to prepare porous cemented carbide mixed powder and dense cemented carbide mixed powder.
• Preparation of cemented carbide mixed powder with a composition of 65% to 85%, preferably 65% ​​to 75%, and a median particle size (D50) of 10 to 35 μm, preferably 10 to 25 μm, more preferably 15 to 20 μm porous cemented carbide and 15% to 35%, preferably 25% to 35%, with a median particle size (D50) of 3 to 10 μm, preferably 4 to 10 μm, more preferably 4 to 8 μm Carbide.
• Inkjet printing forming.
• Sintering and densification.

The researchers of Global Tungsten Company in the United States used WC-12%Co alloy as the research object, and prepared the blank using the 3D printing technology of bonding phase spraying. After printing, the blank was heated at 200 ℃ to improve the strength of the blank, and then passed 1 435~1 485 After sintering at different temperatures of 1.80 MPa and adding a pressure of 1.85 MPa (5 min), the alloy cuboid samples were obtained by sintering again at a pressure of 1 485 ℃ + 1.85 MPa (30 min) (see Figure 13).

Fig.12 Sandvik’s tendency to apply for 3D printing carbide patentsFig.13 Appearance of BJP produced WC-12%Co cementedcarbide blank, after vacuum degreasing andafter sintering

Observation and inspection of the microstructure and properties of the alloy show that this method can obtain a WC-12%Co cemented carbide with a density of approximately 13.1~13.5 g/cm3 and a hardness range of 1 217~1 357 (HV30). The comprehensive index of hardness and fracture toughness of this alloy reaches the standard of WC-12%Co medium grain carbide (as shown in Figure 14~Figure 15). The microstructure shows that even after heat treatment at 1 485 ℃ and 30 min after printing, there are still many pores (as shown in Figure 16), and the local aggregation of large WC particles is obvious (as shown in Figure 17).

Fig.14 Hardness of WC-12%Co cemented carbide prepared by BJP and conventional powder metallurgy

A comparison of microscopic hardness testing revealed that there was a significant difference in hardness between the WC large particle aggregation area and other medium particle areas. In addition, since this document only uses simpler rectangular parallelepipeds as samples, its ability to mold complex shaped samples still needs further verification.

Fig. 15 Toughness of WC-12%Co cemented carbide prepared by BJP and conventional powder metallurgy

1.4 DLP-Digital Light Processing

This method was developed on the basis of Stereo Lithography Apparatus, SLA. SLA/DLP uses a beam of specific wavelength and intensity to focus on the surface of the photocurable material, irradiate and cure it, and then complete the work on one level. Then the elevator moves the height of one layer in the vertical direction to cure another level. In this way, layers are stacked to form a three-dimensional entity.

Figure 18 shows a schematic diagram of SLA and DLP. The difference between SLA and DLP is:

• SLA is a high-energy laser, and DLP is ultraviolet (UV).
• The irradiated area of SLA is a small spot (light spot), and the irradiated area of DLP is the projection of the light beam, that is, the printed irradiated area is the entire plane.
• The energy of SLA is high and the price is relatively expensive, while the energy of DLP is low and the price is relatively cheap.
• The printing efficiency of DLP is higher than that of SLA.

At present, SLA/DLP has been widely used in the research of 3D printing of resins, hydrogels and ceramic materials. However, the research of this technology for the 3D printing of cemented carbide has not been reported.

Fig.16 Metallographic photograph of WC-12%Co ce⁃mented carbide after pressure sinteringFig.17 SEM photograph of pressure sintered WC-12%Co cemented carbide

Based on the above research, Zigong Cemented Carbide Co., Ltd.-Central South University combines the characteristics of cemented carbide products with sol-gel, light curing and other methods, using salt solution as the raw material, supplemented by light curing method, using Low-cost printing equipment, precision-molded carbide blanks, and alloying after treatment can produce finished products with surface quality close to that of molded cemented carbide, and staged progress has been made. This method is called digital light processing (or digital light processing)-DLP (Digital Light Processing), which can simultaneously form multiple samples at room temperature, and does not require special conditions such as protective atmosphere or vacuum.

The Main Printing Steps Are Divided Into:

Ink configuration

Light curing 3D

Print

Degreasing and sintering (including carbonization)

After printing and sintering, the blank of the cemented carbide sample without bonding phase is shown in Figure 19; the XRD result of the sample after sintering is shown in Figure 20, which proves that the sample contains only the WC phase.

Fig.18 3D printing schematic of SLA (a) and DLP (b)Fig.19 Printed and sintered cemented carbide samples (low⁃er right corner)

2 3D Printing Cemented Carbide Industry and Technology Development Prospects

Since the emergence of 3D printing technology in the 1990s, from the beginning, printing of polymer materials has gradually focused on metal powder.

In early 2012, US President Barack Obama established 3D printing technology as one of eleven important development technologies, and established the National 3D Printing Research Institute of the United States in collaboration with scientific research institutions, universities and enterprises.

In 2014, Germany proposed the “Industry 4.0” development plan, which will inevitably cause disruptive changes and innovations in the industrial field, and 3D printing technology will be a powerful thrust for the development of industrial intelligence. In June 2015, the European Union’s “3D Printing Standardization Support Action (SASAM)” project released a 3D printing standardization roadmap. The roadmap is intended to serve as a model template for European standards to regulate the position and direction of 3D printing technology in development strategies.

In 2020, the annual sales revenue of China’s 3D printing industry will exceed 20 billion yuan (as shown in Figure 21), with an average annual growth rate of over 30%.

It is foreseeable that in the future, China’s 3D printing technology will be more rapidly standardized and scaled, and will eventually become intelligent through the combination of advanced technologies such as CNC technology, big data, cloud computing, Internet of Things, intelligent materials, robotics, etc. An integral part of the manufacturing platform terminal. As one of the top ten materials listed in the “Most Promising 3D Printing Material Technology” in the future, 3D printed cemented carbide will receive more and more attention, more research results, and even technical breakthroughs. . It provides a strong guarantee for the production of complex and difficult-to-form cemented carbide parts, and may partially replace cemented carbide deep processing and become an important means of producing carbide parts.

According to the characteristics of cemented carbide products, the research of its 3D printing methods is mainly divided into two directions, namely “direct printing” and “indirect printing”. “Direct printing” means to get the final product in one printing, such as the SLS/SLM introduced in the article; while “indirect printing” is to get the blank first, and then obtain the alloy through subsequent heat treatment. Such as FFF, BJP and DLP in the text. Considering the prospect of industrial application of cemented carbide, the characteristics of several main 3D printing cemented carbide methods are shown in Table 1.

Printing methodManufacturing costSurface qualityManufacturing efficiencyBatch production

SLS/SLM	High	LOW	LOW	Hard
FFF	LOW	LOW	LOW	Easy
BJP	High	LOW	LOW	Hard
DLP	LOW	High	High	Easy

Table 1 Main methods and characteristics of 3D printing of cemented carbideFig.20 XRD results of sintered cemented carbide samplesFig.21 Scale of China’s 3D printing market

It can be seen from Table 1 that although methods such as SLS/SLM, FFF, and BJP started earlier and have a longer development time, there are different problems in terms of manufacturing cost, surface quality, and printing efficiency. The DLP method has low preparation cost, high surface quality and high printing efficiency, and mass production is easy to achieve. Whether for research institutions or production companies, the DLP method has potential for development.

So far, due to objective reasons such as high melting point of raw materials, insufficient WC grain growth and insufficient rearrangement, “direct printing” cemented carbides have been difficult to obtain cemented carbides with structures and properties comparable to traditional powder metallurgy methods; at the same time, The high requirements of WC powder, Co powder and other physical properties and uniformity also greatly restrict the development of this type of technology.

It is precisely because of the bottleneck encountered during the development of “direct printing” that the emergence of the “indirect printing” method combines the advantages of 3D printing and traditional methods of preparing cemented carbide, especially the introduction of the DLP method, which greatly improves the 3D Printing the surface quality and printing efficiency of cemented carbide, and how to further improve the density and performance of cemented carbide prepared by such methods is an important direction for the future development of 3D printed cemented carbide.

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