Additive ManufacturingHeat TreatmentPost-Processing

A closer look at FAST/SPS spark plasma sintering technology

And its implications for sinter-based ceramic and metal 3D printing

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At the recent Ceramitec fair in Munich, ExOne (part of Desktop Metal) showed the first ever fully sintered and fully dense silicon carbide (SiC) parts made by binder jetting, without requiring any post-process infiltration. They were sintered using FAST/SPS, an acronym that stands for field-assisted sintering technology/spark plasma sintering. But what is this technology and what could it mean for sinter-based AM?

FAST/SPS is a cutting-edge spark plasma sintering technology pioneered by FCT Systeme, Germany, that revolutionizes the consolidation of diverse powders across ceramic and metallic domains, facilitating the creation of significantly enhanced or entirely innovative materials.

Over the past decade, an abundance of scientific literature and presentations have showcased the ability of spark plasma sintering to swiftly consolidate powders. This technology has been extensively explored across various material categories, encompassing metals, alloys, intermetallics, Borides, Carbides, Nitrides, Silicides, Oxides, composites, and specialized material systems.

More recently, starting in 2016, the French company Norimat commercialized its own FAST/SPS process, developed and optimized through 15 years of R&D, to combine 3D printing and spark plasma sintering. Applies to the ExOne binder jetting 3D printed parts, the Norimat process enables rapid single-step sintering of parts with complex geometries. Thus, geometrically complex parts, with outstanding performance characteristics are now possible, quickly.

However, as FCT Systeme pointed out, while these investigations have yielded promising outcomes, predominantly within the scientific realm, they have also revealed tantalizing prospects for transitioning to industrial-scale production. Industrial manufacturing demands tailored equipment capable of optimizing cost efficiency, a distinct challenge from scientific experimentation. The successful industrial implementation of FAST/SPS hinges upon the availability of meticulously tailored equipment, poised to meet the unique demands of large-scale production.

FAST/SPS for rapid consolidation of powders

Spark plasma sintering, as well as its state-of-the-art version, called FAST/SPS, are sintering techniques, which are deduced from conventional hot pressing. Hence FAST/SPS systems comprise a water-cooled vessel, a hydraulic pressing system, and a computer-aided process control using temperature and force measurement and control, as well as a system for vacuum and atmospheric control inside the vessel respectively. The big difference compared to a conventional hot press is the absence of a heating element as well as the conventional thermal insulation of the vessel. Instead, a special power supply system feeds high current into the water-cooled machine rams, which that way act as electrodes simultaneously, feeding the high current directly through the pressing tool and the containing powder compact.

Image credit: FCT Systeme

This special construction leads to a homogeneous volume heating of the pressing tool as well as the containing powder using Joule heat. Only small thermal gradients are generated even at high heating rates, whereas conventional sintering methods are hampered by thermal gradients, allowing only moderate heating rates and requiring long dwell time for subsequent (but mostly incomplete) homogenization. The advantage of FAST/SPS is visualized in Figure 2 below in terms of the temperature of the center TI compared with the edge of the powder compact TA during the sintering cycle.

Spark Plasma Sintering (SPS) can produce high-performance samples with higher material properties compared to conventional processes. However, as samples get larger it is much harder to manage the thermal gradients inside the part. This could result in several drawbacks that can discourage the SPS user. One is the melting of the material, especially with metals, where the sintering temperature is close to the melting point. Another is microstructure inhomogeneity, as the temperature strongly affects the densification and the grain size, and thus, the physical properties of the final part. Yes another is cosmetic, as the use of pigments in some materials is very sensitive to the temperature and can result in a non-uniform color of the final part.

Image credit: FCT Systeme

Several tools are now available to the SPS user to limit the thermal gradient inside its parts, even for large samples. Norimat showed two examples illustrating this based on the use of carbon fiber-reinforced plates and mold design optimization. Carbon fiber-reinforced plates are increasingly used in the SPS community thanks to their ability to limit the thermal gradient inside the sintered parts as well as reduce the global SPS power. In the case of a 100mm zirconia sample, the use of CFC plates between the punches and spacers in the mold can reduce the thermal gradient by about 75%. However, the resulting gradient of 51°C can still be too high for certain applications. In addition to CFC plates, users can also optimize the design of their mold. For the same 100mm zirconia sample, using a higher and slightly thinner mold will reduce the thermal gradient to only 12°C. With this level of temperature difference inside the zirconia, no density or microstructure discrepancy is expected. This makes SPS a valuable and reliable process to produce high-performance materials.

FCT Systeme shows an additional advantage of FAST/SPS in Figure 3 further down: the heating power is not only distributed over the volume of the powder compact homogeneously in a macroscopic scale but it is also dissipated exactly at the locations where energy is required for the sintering process, namely at the contact points of the powder particles. This results in a favorable sintering behavior with less grain growth and suppressed powder decomposition. Depending on the type of powder, additional advantageous effects include electro-migration or microplasma generation.

A closer look at FAST/SPS spark plasma sintering technology and its implications for sinter-based ceramic and metal 3D printing
Comparison of FAST/SPS with conventional Hot Pressing. Image credit: FCT Systeme

FAST/SPS for additive manufacturing

In Norimat’s FAST/SPS process, a high-intensity pulsed current passes through a graphite tool, which makes it possible to reach high heating and cooling rates of over 100°C/min. This means that the FAST/SPS process can reach a high temperature in only a few minutes compared to several hours for other sintering techniques.

The main advantage of this powder metallurgy technology is the absence of organic binders during the process which can cause defects during sintering. The technology limits residual stress within the material. through the efficient control of the part’s thermal gradient during the sintering process.

This reduced processing cycle combined with the fast-heating rate results in a fine material microstructure and an increase in the mechanical properties of the sintered materials. Thanks to these assets, this unique powder sintering process is used to easily manufacture high-performance and innovative materials (composites, hard metals, metal alloys, refractory materials). In addition, the process prevents grain growth within the materials, which maintain a fine microstructure and can densify any materials between 200°C and 2400°C to reach a density of 99.5%, with material waste less than 1%. All the powder sintered in the process is densified, and there is virtually no powder waste or any other waste.

Norimat also developed the Engemini Field Assisted Sintering Technology, Spark Plasma Sintering (FAST/SPS) simulation tool. This model enables all FAST/SPS users to perform digital experimentation to determine the thermal and mechanical evolution of parts and graphite molds during the physical sintering process.

Requirements for industrial application of FAST/SPS

The industrial application of the FAST/SPS sintering method for the rapid consolidation of novel materials requires special features, which must be fulfilled by the equipment and are different from the requirements of scientific work to some extent.

In order to assure high throughput (amongst other things), the system must provide sufficient electrical output power. It is important, that the electric losses in the system are low to generate high heating power at the location, where it is needed. The actual value of the required power depends on the size and material of the powder compact and the pressing tool as well as on the intended heating rates and maximum temperatures.

Depending on the type of the powder, several different sintering mechanisms are possible. The type of heating current can influence some of them. Therefore a power supply with high flexibility is important to achieve optimum sintering results in terms of throughput and material quality. FAST/SPS systems are capable of generating a wide range of pulsed DC with computer-controlled, arbitrary pulse parameters to the point of pure DC.

The correct sintering temperature is the most important process parameter besides time and heating rate. Due to a special design FAST/SPS systems are measuring the temperature in the vicinity of the powder compact center, which gives a much more significant value than the measurement of the die temperature.

As pointed out before, due to the special construction of FAST/SPS systems, the pressing tool system, consisting of the two pressing punches, the die, and other auxiliary components, is the “heart” of the system, because it not only contains the powder compact but also acts as the “heater” (in interaction with the compact). Even though the temperature gradients in the system are significantly lower than for conventional sintering methods, e.g. hot pressing (see Figure 2), design optimization is advantageous anyhow, especially if the highest heating rates, minimized dwell time and optimum material quality are desired.

A closer look at FAST/SPS spark plasma sintering technology and its implications for sinter-based ceramic and metal 3D printing
Image credit: FCT Systeme

A helpful tool for design optimization is the numerical simulation (finite element method “FEM”) of the heating behavior, taking into account the temperature-dependent thermal and electrical properties of the applied tool materials as well as the powder compact. Figure 5 above shows the temperature distribution in a pressing tool system containing two powder compact circular disks of 200 mm diameter after heating to 1500°C within 12 min and 5 min dwell time. With the standard tool design (left) the remaining temperature difference in the compact amounts to 160 K, which can be reduced to 60 K by design optimization (right).

The benefit of optimized pressing tool systems is superior material quality and homogeneity, e.g. reflected by an even distribution of high hardness values across the diameter of a 200 mm circular disk compared with the standard pressing tool situation. Furthermore, the highest heating rates made possible that way are an essential condition for the realization of nanostructured materials, which are often impossible to sinter by conventional methods due to significantly longer sintering cycles.

The so-called “Hybrid Heating” is a combination of the FAST/SPS method with one or several additional heating systems, which act usually from the outside of the pressing tool systems, as illustrated in Figure 7. Thus the thermal gradients of FAST/SPS, which are directed from the interior to the exterior typically can be compensated by the inversely directed gradients of the additional heating system. The superposition of the gradients (left side) results in an extensive minimization of these gradients (right side). This allows further enhancement of the heating rates at simultaneously optimized homogeneity with all the advantages pointed out before.

A closer look at FAST/SPS spark plasma sintering technology and its implications for sinter-based ceramic and metal 3D printing
Image credit: FCT Systeme

A practical example showing the positive effect of hybrid heating can be found in Figure 9, which compares the sintering behavior of rectangular plates made of binderless tungsten carbide (size 150 x 175 mm). The light grey curves show the densification/densification by use of FAST/SPS, whereas the dark grey curves show the enhanced sintering behavior by use of hybrid heating.

The production capacity of an industrial FAST/SPS system is not only governed by the maximum possible heating rate and a minimized dwell time, but also by a fast cooling facility, which allows early discharge of the completed pressing tool. This is realized by an additional cooling chamber, separated from the actual sintering chamber by a gas/vacuum-proof, gate and equipped with special fast cooling rams (see figure 7). An automatically working handling system shifts the hot pressing tool system from the sintering chamber to the cooling chamber. After the automatic closing of the gate, the sintering chamber is ready for charging the next sintering cycle during the cooling of the previous pressing tool.

To realize a cost-efficient industrial application of FAST/SPS sintering systems, automation is an essential prerequisite. An important step is the semi-continuous operation mode mentioned above in conjunction with the fast cooling system. Due to a combination of robots and manipulators a fully automatic operation can be realized. The image at the top of this article shows a 250-ton hybrid FAST/SPS production system equipped with two ABB industrial robots for charging and discharging.

A 250ton hybrid FAST/SPS production system, optionally equipped with two ABB industrial robots for charging and discharging

High-throughput FAST/SPS industrial application

One of the first industrial applications of the FAST/SPS sintering technology is the manufacturing of plate-like, large-area articles, e.g. sputtering targets for the coating of goods with a wide range of functional surface layers. The hybrid FAST/SPS systems seen in the image above are ideally adapted for the high throughput manufacturing of such parts.

For the mass production of small parts (5 to 25 mm), FCT Systeme developed a special series of FAST/SPS systems called “FAST2” (FAST square = fast FAST), based on state-of-the-art powder pressing technology combined with the FAST/SPS sintering method, realizing rapid and fully automatic operation including powder handling, filling of the integrated pressing tool and discharge of the readily sintered parts. The throughput of such systems can be as high as six pieces per minute, depending on the sintering characteristics of the actual material as well as the size of the parts.

Besides all the possibilities mentioned above to realize an industrial production with high-cost efficiency, there is one other method: The use of multiple pressing tools for single or manifold powder compacts can provide several parts by one sintering cycle, enhancing the effective throughput of the FAST/SPS system significantly.

It has been shown, that the promising results of FAST/SPS can be transferred to a cost-efficient industrial production if the equipment meets the respective prerequisites. Current developments are related to the industrial production of more complex geometries, as well as further optimization of quality and costs.

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