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Review

Advancements and Prospects in Electro-Sinter-Forging

by
Alessandro Fais
EPoS Technologies SA, Z.I. du Vivier 22, CH-1680 Villaz Saint Pierre, Switzerland
Metals 2022, 12(5), 748; https://doi.org/10.3390/met12050748
Submission received: 22 February 2022 / Revised: 18 April 2022 / Accepted: 24 April 2022 / Published: 27 April 2022
(This article belongs to the Special Issue Recent Advances in Field-Assisted Sintering Technologies)
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Abstract

:
A summary of the recent advancements, future prospects and open issues in the materials, methods and machines for the technology known as electro-sinter-forging is presented here. After a background introduction, the key characteristics of the procedure are explained. Metal systems that have been processed based on iron, copper and aluminium are discussed as single elements, and as alloys and composites. Intermetallic materials such as gamma titanium aluminide, Nd2Fe14B and Bi2Te3 are finally presented before discussing the experimental evidence of the atomic diffusion mechanisms involved, and a critical assessment of the limitations of the technique is performed.

1. Introduction

Field-assisted sintering techniques (commonly named FAST) have been progressively gaining ground and interest as alternative, faster and more versatile technologies than press and sinter [1,2]. The concept behind these techniques is to replace the classical irradiative and convective flux of energy with joule heat and/or conduction from an electrically heated conductive die. A typical FAST machine, such as the spark plasma sintering (SPS) machines or the more industrial direct hot pressing (DHP) or resistance sintering (RS) machines, can be used to consolidate metals, ceramics and composites thanks to a wide range of maximum temperatures, an ample choice of holding times and temperature rise rates, a controlled atmosphere and versatile dies, typically made of graphite, which can be employed up to extremely high temperatures. The drawback of these machines is the limited productivity compared to the combination of one or more powder presses and ovens, combined with issues of carbon contamination from the dies [3,4,5] along with their degradation. The cycle time in an SPS/DHP system requires at least 10–15 min of temperature rise and holding time, while cooling can be carried out separately and can be enhanced by water cooling. An emerging, alternative sub-category of the FAST is the single-pulse electro-discharge sintering (EDS) or electro-discharge consolidation (EDC) [6,7]. The difference is that EDS/EDC use a single electric power pulse which may last a few micro-seconds (for EDS) up to one second (as in the high-energy high-rate technique [8,9,10]) to consolidate the material, since it acts through direct joule heating of the powders, while SPS/DHP use oscillating waves or trains of waves [11]. In single-pulse techniques, the values of the current densities are several orders of magnitude higher than SPS/DHP, but there is no temperature control and no holding time since the pulse is generated through freely oscillating power circuits (analogous to an RLC circuit with a switch) [12]. We can think of EDS/EDC as the equivalent of a single-blow drop hammer forging compared to screw presses: high speed and power versus lower power and time control. First resistance sintering and more recently spark plasma sintering have already entered the factories of manufacturing industries, while EDS/EDC have been, for now, only explored and used in academic research labs throughout the world (USA [13], USSR/Russia [14], UK [15], India [16], Korea [17], Germany [18] and Italy [19]) because of the un-reliability of the discharge apparatuses and the use of ceramic dies needed to guarantee the flow of current through the powders. However, progress is happening, and novel, single-pulse electro-discharge sintering techniques, such as capacitor discharge sintering [12] and electro-sinter-forging [20], are emerging thanks to advancements in the discharge circuits and in the die technologies [21]. What is presented in this review are the latest advancements in electro-sinter-forging or eForging, a technique that is gaining ground and attracting industrial interests like no other EDS/EDC in the past, developed by the author from previous experience on capacitor discharge sintering [12,19,22]. After a brief description of the process, a review of published and unpublished data of materials that have been consolidated will be provided and evidence of the prevailing diffusion mechanism will be discussed.

2. Characteristics of the Process

Electro-sinter-forging can be described as the superposition of a single mechanical pulse with a single electro-magnetic pulse that generates a direct electric current through powders previously loaded inside an insulating die [20]. As long as we have the superposition of the two pulses, no matter how long these pulses are, we are in the domain of electro-sinter-forging. If not (see, for example, the erroneous wording in [23,24,25]), we cannot really talk about electro-sinter-forging but more about resistance sintering. The effect of this superposition when acting on metal and intermetallic powders can be nearly full to full consolidation, even when the two synchronised pulses last, as in all the results described below, only 20–30 ms. The method is carried out in air as no advantage has been observed thus far in using either vacuum or argon as a protective media during the consolidation. A typical present day capacitor discharge-based machine will produce peak voltages on the plungers between 40 and 60 V and peak currents from 800 kA to 1.5 MA, with energies stored on the capacitors from 30 to 120 kJ. The axes are powdered by electrical linear actuators for versatility, speed and reactivity. The current machines have a peak force of 120 kN while the newest ones, currently under design, will be 240 kN or above.
All the fundamental metallurgical mechanisms involved during the process are quite complex as a complete understanding requires a full electro-magnetic-thermo-mechanical description of the process mediated by the powder distribution inside the die as well as the microstructure of the powders and their composition and density of defects [26]. At the microstructural level, the sum of the effects of mechanical stress amplification and the amplification of the current density on the pores causes the pores to collapse in a way similar to what Sheng et al. have observed [27]. In general, the higher the electromagnetic energy employed and/or the pressure, the higher the number of pores that collapse entirely. At the atomic level, the current belief is that plasticity is enhanced by the increase in current densities (a phenomenon that goes by the name of electro-plasticity [28,29,30,31]), or, in other words, the mobility of dislocations (only edge dislocations) is increased by the rapid rise in the electric field, and that short-range atomic diffusion is aided by the electric field and the intense thermal gradients present in the powders during the process. To fully uncover all the atomic phenomena and, most of all, their complete quantification, is and will continue to take time but, as we will see, new indications are arising from the extensive testing that is going on through the years on different materials and powders.

3. Materials

Through the ten years of existence of the technique, a large, although not yet comprehensive, range of materials and powders have been explored. The most emblematic or recent and yet unpublished will be described and discussed below, and the processing parameters of the most significant samples are provided in Table 1. As experimenting progresses rapidly and there are currently size limitations on the samples (maximum 250 mm2 of projected area to reach full density), we are exploring most of the different systems through simple basic tests: density, resistance to impact and micro- and macro-hardness testing.

4. Metals

4.1. Iron-Based

As the most important of the alloy systems employed in industry, iron has also been at the centre of inquisitive attention with ESF. After a first work on a ball-milled nanocrystalline AISI M2 [22] consolidated through CDS and some trials on pure iron powders and a commercial iron powder usually employed with the addition of graphite to produce PM steels [32], a few years ago we explored how iron-molybdenum-chromium pre-alloyed powders [36] behave when added with graphite, to produce a 100Cr6 equivalent for the bearings industry. Tool steels and stainless steels are also discussed as, for different reasons, they both constitute challenges for conventional press and sintering technologies.

4.2. 100Cr6-Like Composition

Bearing steels, although common and widespread, have never before been manufactured through powder metallurgy. On the one hand, full-density techniques, such as pressing, sintering and HIP, or hot-forging pre-sintered parts, were too costly to compete with cast and machining, and on the other hand, cheaper routes such as press and sintering could not guarantee the reliability and component life of the traditional cast, hot/warm deformation, machining and heat treatment route. Residual porosities which are unavoidable with a simple press and sinter method cause premature failure by decreasing fatigue resistance and have hence hindered the path to industrialisation for this high-carbon, high-chromium composition. ESF, on the contrary, has demonstrated through both metallurgical analysis and benchmarking on motor rigs a sensible improvement over traditional press and sinter. After developing and studying the right combination of commercial powders, we designed and produced a cam follower for a novel internal combustion engine, see Figure 1. The parts had theoretical density, net shape (with tolerances of ±8 µm on the inner and outer diameters) and hardness values between 58 and 62 HRC. All parts survived the extensive testing.

4.3. Tool Steels

High-alloy, high-carbon steels for the use in tooling, high-wear and high-temperature applications are already produced by powder metallurgy through hot isostatic pressing [37]. This procedure, although long and costly, allows to manufacture a large number of bars and sheets which can be, after HIP, processed like cast materials. The final products though, at the end of the manufacturing cycles, are frequently small cutting tools, drilling tips and interchangeable inserts, so processing large bars and sheets, although more affordable for the consolidation step, increases the machining costs and the scraps involved. For this reason, ESF is an interesting alternative solution offering the advantages of a press and sinter system with the properties of a full-density material. In cooperation with tool producers, we developed a die for pre-forms of exchangeable drilling tips used to drill low-carbon steel parts in the automotive industry. We densified commercial powders of AISI H13 and AISI T15 both to full density. The AISI H13 was also mixed with TiC to improve hardness and wear resistance (more details on the H13 + TiC MMC below). Neither the M2 nor the H13 or T15 grades are normally produced through press and sinter because of their low compressibility and high mechanical resistance. Through ESF, all three systems could be sintered to theoretical density with energies between 1.8 and 2.2 kJ/g and final maximum pressures around 300 MPa.
The same AISI H13 commercial powder cited above was also mixed with progressively higher volume percentages of titanium carbide (TiC) to create a metal matrix composite. The values of hardness for the composites with 60%, 70% and 80% in volume of metal alloy are plotted in Figure 2, together with the theoretical values of hardness of a composite following iso-stress and iso-strain models (usually used for the Young’s modulus of MMCs) of hardness. The measured values show that electro-sinter-forged parts stand in the average of the two models, indicating a good consolidation and bonding of the ceramic particles. All the composites densified to theoretical or nearly theoretical density in the shape CS0 with an area of 188.55 mm2 and a height of 4.6 mm, shown in Figure 3 (left). The processing parameters are reported in Table 1. The shape was a preform to produce a metal drilling tip (Figure 3, right).

4.4. Stainless Steels

With stainless steels, as with aluminium- and titanium-based alloys, the critical factor is the quality of the surfaces of the powders. As the technique is fast and no protective or reducing atmosphere gives an advantage, if used, the higher the amount of surface oxides, the more difficult the consolidation (if any at all) and the more fragile the material. When using “fresh” powders or powders that have been properly treated to eliminate or reduce the oxide layer, consolidation is full, and the mechanical properties seem comparable to cast material. Stainless steel samples of 316L and 17-4PH powders have both been consolidated to theoretical densities. The samples produced from properly processed powders were extremely tough and resilient when subjected to multiple impact tests and the material reacted simply by deforming and adapting to the mechanical solicitation without fracturing. With untreated and “old” powders, we did observe and measure densification, even up to 95–98% of the theoretical density, but the material always exhibited a fragile behaviour and tended to fracture easily at the first blow when impacted. With heavily oxidised powders, no consolidation was possible as nearly any current could flow through the powders when subjected to the electric pulse.

4.5. Copper-Based

Following the importance of iron, copper-based alloys have been explored since the beginning of the adventure with CDS [19]. Successively, with ESF, the following copper-based powders have been consolidated to theoretical density: pure dendritic copper, ball-milled copper and many alloys of the Cu-Sn system, such as 96-4, 90-10, 85-15 and 88-10-2 with Zn at 2%. The most interesting results come from two 85-15 powders: a commercial powder dubbed “bronzo finissimo” and another commercial 450-mesh, also irregular powder, shown in Figure 4. Both were densified in flat 20 × 10 × 4 mm bars, milled to produce a miniaturised bone-shaped sample (see Figure 5), and stretched to fracture in a tensile test performed at Politecnico di Bari in Donato Sorgente’s group. The first of the two had a very interesting tensile curve (Figure 6), with a maximum tensile strength of 529 MPa and a maximum deformation of 5.2%, while the second had reduced tensile strength and plasticity, with 171 MPa of tensile strength and 0.3% of maximum deformation. The lower plasticity of ESFed samples from the second powder was also confirmed when, in another set of tests at Politecnico di Torino [33], the 450-mesh powder was densified in long 80 × 5 × 3 mm bars, pictured in Figure 7, and subsequently subjected to lamination trials to verify their plasticity and workability, which was quite scarce. Investigations on the reasons for this large difference in mechanical properties have led us to find a visible fraction of elementary, unalloyed copper in the 450-mesh powder and a large amount of ε intermetallic. After densification, these two phases remained as such inside the copper-tin matrix. On average, the powder composition was, indeed, 85-15, but the initial distribution of tin was inadequate, thereby reducing the overall mechanical properties of the consolidated material, which kept the initial distribution intact.

4.6. Aluminium-Based

Historically difficult to handle and consolidate, aluminium powders subjected to the electro-sinter-forging procedure have manifested issues similar to stainless steels. “Old” powders, although electrically conductive enough to allow for the current pulse to be consistent, always have a hard time reaching maximum density and optimal mechanical properties. Conversely, “new” powders, or powders that have been treated to remove oxides and properly handled and stored, produced highly dense and mechanically resistant components. The evaluation of the amount of surface oxides, and hence the quality of the powders, is quite subtle to verify without sophisticated equipment, so the determination of the shelf-life for each powder system is simply carried out experimentally by periodically checking the sinterability of the powders. AlSi10 and AlSiMg10 powders, previously consolidated to full density [34], have a surprising shelf-life, higher than twelve months, while commercially pure Al powders will not produce consistent results even after only one week after opening their package. We worked and consolidated pre-mixed 6061 and 7075 powders to full density, although the mechanical properties were far from ideal, whereas the family of Al-Si40, Al-Si50 and Al-Si60 never experienced problems, leading to believe that silicon is attracting a great part of the oxygen and acting as a sacrificial preferential oxidation site with respect to aluminium.
Pre-mixed aluminium powders of 6061 and 7075, supplied and analysed after consolidation by ECKA Granules/Kymera International, were also interesting from the metallurgical point of view (see Figure 8). These powders were made of a mix of high-alloyed powders and pure aluminium in order to have the same average composition as UNS A96061, in complete analogy with the bronze powders discussed previously. Once again, the high-alloyed powders remained as such (see, for example, in Figure 8, the bottom right area on the high enlargement image to the right), and no or very limited diffusion occurred. Analogously, the three-point bend tests of this alloy gave poor results of 296 MPa and elongation of 7% compared to 330 MPa and elongation of 21% in the traditional press and sintered T1 condition. We expect to see quite different results in fully pre-alloyed powders, and the tests are currently ongoing.
A last, quite an important example of what can be done with this technique with aluminium is the recent work on aluminium–titanium composites [38,39]. Full-density aluminium (AlSi10Mg) titanium (Cp-Ti) composites with two different volume percentages were densified to full density without any formation of intermetallics different from the ones present in the original powders, except in one condition: in the very small particles of aluminium of the titanium-rich composites sintered at the highest energy values. In this very peculiar, extreme condition, with energies beyond what is needed to reach full density, the concentration of electric currents in the highly electrically conductive aluminium probably increased the temperatures to values in which substitutional diffusion could occur.

5. Other Metals

Numerous other metal alloys have been consolidated, among which the ones worth mentioning are the nickel superalloy Incoloy 625 produced by Höganas Belgium SA. This powder, probably thanks to its extreme oxidation resistance, could not only consolidate to full density but produced a material with high toughness and mechanical resistance. NiTiNOL, to the consolidation of which we have already dedicated a couple of papers [35,40], can be produced in both the super-elastic version and the shape memory one, while on titanium and its alloys, the trials and analyses are just at the beginning. Gold [41] and silver alloys have both been tested and have shown excellent processability with ESF, whereby 24, 22 and 18 carat red, white and yellow 2N and 3N gold alloys have all been consolidated to full density, and the residual defects inside are sufficiently small or marginal to accommodate the needs of the jewellery and watchmaking industries. Cobalt- and copper-based diamond tools have also been extensively produced and previously discussed elsewhere [21,42]. To end this section dedicated to pure metals and alloys, we can mention success also in the consolidation of platinum, iridium and molybdenum, all being transition metals with increasing melting temperatures.

6. Intermetallic Materials

6.1. TiAl

An extensive and yet to be published work on a two-phase Ti48Al2Nb2Cr gamma aluminide made in cooperation with the Italian National Research Council (CNR, Strada delle Cacce 73, Torino) has yielded interesting results. First of all, the starting powders consisted in a pure machining swarf. The swarf was produced from the dry turning of electron-beam sintered parts used in the aerospace industry for turbine blades. We electro-sinter-forged the as-produced swarf and two ball-milled versions of the same. The powders are shown in Figure 9 and the densified material in Figure 10. The powders were ESFed with increasing energies from 2.2 to 4.2 kJ/g to densities up to 3.95 g/cc and peak hardness values of 820 ± 27 Hv0.2. A second, also interesting result, comes from the observation of the precipitation of AlTi3 intermetallic phases at intermediate to high energy levels, which were evident from both the XRD spectra and the distribution of the hardness values. Even more interestingly, this precipitation did not occur (or the phase was again resolubilised) at the highest levels of specific energy.

6.2. Nd2Fe14B

Being at the centre of large industrial interests for the increased electrification of the automotive industry and of most of the automation solutions, plenty of work has been carried out and is still ongoing on the Nd2Fe14B intermetallic system. Due to industrial secrecy, most of the works are still classified, but some important information can be divulged. Full consolidation of magnetically hard Nd2Fe14B-based materials and its metal composites [43] is possible. The composites are particularly interesting because they exploit the absence of long-range diffusion of this technique. Hard magnetic Nd2Fe14B flakes can be combined with zinc, aluminium, copper, silver, gold, titanium and even high melting point metals such as molybdenum without or with minimal degradation of the hard magnetic phase. The values of coercivities of the materials produced have reached, for now, 400 to 1400 kA/m, with residual fields from 250 to 850 mT.
Another trial has been performed with pure MQU powders. a first discharge was used to consolidate the material in a closed die, then, in an open die upset configuration, the material was deformed with the same capacitor discharge to induce mechanical anisotropy. The test was successful and the mechanical anisotropy translated in magnetic anisotropy allowing for the material to have a residual field as high as 1.1–1.2 T, in a way similar to what has already been shown by Gutfleisch [44,45] for conventional hot pressing but at a much higher speed and entirely in air.

6.3. Bi2Te3

Bismuth telluride is an intermetallic compound used for its thermoelectric properties to produce low-temperature thermal recovery modules and as a cooling device for special applications. The current prevailing manufacturing process is mainly through vacuum sintering, hot extrusion or spark plasma sintering [46,47]. High-density samples with values from 96% to 100% (6.6 to 6.781 g/cc) of density have been produced with electro-sinter-forging. The material also showed preferential alignment in the as-sintered state compared to the original powders.

7. Discussion and Critical Assessment

It seems plausible, although not yet fully verified, that electro-sinter-forging can be used for the production of all transition metals and most metals and their composites. Doubts remain about metals with an extreme tendency to form very stable oxides, such as manganese or chromium, although with an adequate treatment for the powders before consolidation they are both likely to sinter as well. The prevailing mechanisms of action during the procedure seem to be short-range substitutional diffusion, otherwise there would not be any mechanical bonding between adjacent particles and long-range interstitial diffusion, as in the example of the 100Cr6-like powders starting from an iron alloy and graphite flakes that densified to full density to form a fully martensitic homogeneous structure. On the other hand, there are a growing number of examples that demonstrate the very limited or complete absence of long-range substitutional diffusion: the “bronzo finissimo” powder, the aluminium pre-mixes, the aluminium-titanium composites and the Nd-Fe-B metal matrix composites. Other materials, previously not mentioned, which are also proof of the absence of long-range substitutional diffusion, are the WC-Co, WC-Invar and TiC-Invar produced to create cemented carbides [48]. The WC or TiC grains did not dissolve into the metal and change shape in any of them, and in none of them did we find traces of carbides different from the ones present in the original powders (no formation of the η carbides that usually form in the WC-Fe system, for example).
No long-range substitutional diffusion means that most alloys need to be prepared before the consolidation through some kind of alloying process that must occur during the powder preparation. Electrically insulating materials are also, clearly, impossible to process. However, composites of electrically insulating or semiconducting powders with metals, on the other hand, are easier to process than in many traditional procedures, as the electric currents influence mainly the phases with the lowest electrical conductivity (the metals) whose plasticity and fluidity is helped and enhanced by the flow of currents. Metal–metal composites of most metals are readily feasible, as already shown for the aluminium–titanium system, allowing the creation of a potentially high number of new combinations of materials.
Finally, a quite significant aspect is the size of the parts that can be produced. The two leading parameters to define the size are the maximum pressure reached at the end of the mechanical pulse and the energy available. Typical maximum pressures, with the current machines, are around 300 to 350 MPa, whereas the specific energy input (SEI) needed to consolidate a part varies between 0.7 and 4 kJ/g. To increase the force, machines with larger actuators can be built and are currently under design. To reach a determined value of SEI on the die apparatus, the machines are loaded with SEI/η, where η is the energy efficiency. The value changes with the voltage loaded on the capacitor banks and usually ranges from 0.9 at low voltages/energies to 0.14 for the highest values of voltages/energy, presently around 90 kJ. In other words, for a 1 kg part made in a material that is electro-sinter-forged with 4 kJ/g of energy, we would need 28.6 MJ (with η = 0.14). These energy levels are currently not in the ranges supplied by conventional capacitor discharge machine producers, but could, at least in theory, be reached, and have been reached for completely different equipment, such as railguns [49] and z-pinch nuclear reactors [50]. Therefore, we should expect, until further improvements or industrial advancements, parts with a weight below 100 g and for some materials, even below 10 g.

8. Future Prospects and Open Issues

How big and what exactly the markets will be, as well as the number and type of applications of this technology, are hard to estimate. As the technique is new and, for now, intrinsically more costly, high added-value markets such as specialty and precious materials might be more appealing than high number, lower margin applications, such as the ones in automotive or generic steel parts for mechanics. On the other hand, the speed, the low energy consumed, the absence of a protective atmosphere, the possibility to readily use recycled materials (or directly swarf) and the reduction of the machines and the manpower and processing steps to produce parts in large series are all good indications that, down the line, most applications, even the cheapest ones, can be reached by this novel approach to manufacturing metal-based parts.
There are many main open issues, and they will likely be taken on in the next five to ten years, some of which are reported below:
  • How do the mechanical properties compare to traditional cast and machined parts?
  • What maximum and minimum sizes of parts will be processable with the future machines?
  • What level of complexity of the components will be available? Is multi-axis/multi-level electro-sinter-forging possible?
  • Can extremely high values of vacuum improve the mechanical properties?
  • Are there ways to rapidly de-oxidise powders before electro-sinter-forging or should special powders be produced and properly stocked?
  • Is it possible to induce reactions with interstitial gases to create novel ceramic or composite materials?

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The following people, institutions and companies are acknowledged for their active support in the production of the results and the advancements described: Paolo Scardi from the University of Trento, Maria Giulia Faga and Giovanna Gautier from CNR, Davide Martini from SKF Italia now in C&U Europe, Marco Actis Grande, Silvia Spriano, Federico Gobber, Ilaria Forno and Robert Bidulsky from Politecnico di Torino, Donato Sorgente from Politecnico di Bari, Matteo Zanon and Armin Müller from Kymera International, Andreas Zeller, Serena Bonetti and Serena DiNunzio from EPoS srl, Gabriele Brean for his work in EPoS srl and now in EPoS Technologies SA, and finally, Nicolas Corsi, Christophe Fragnière and all the people at the Nivalis Group.

Conflicts of Interest

The author is founder and CEO at EPoS Technologies SA.

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Metals 12 00748 g001 550
Figure 1. eForged 100Cr6 cam follower prototypes with internal diameter of 9.25 mm, external diameter of 15.45 mm and thickness of 4.5 mm, as produced from ESF. Processing parameters: initial pressure 10 MPa, maximum pressure 200 MPa and SEI = 2.08 kJ/g.
Figure 1. eForged 100Cr6 cam follower prototypes with internal diameter of 9.25 mm, external diameter of 15.45 mm and thickness of 4.5 mm, as produced from ESF. Processing parameters: initial pressure 10 MPa, maximum pressure 200 MPa and SEI = 2.08 kJ/g.
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Metals 12 00748 g002 550
Figure 2. Hardness versus % volume of AISI H13 in an AISI H13 + TiC electro-sinter-forged metal matrix composite (open circles) plotted with iso-strain, iso-stress and point-by-point average models of hardness.
Figure 2. Hardness versus % volume of AISI H13 in an AISI H13 + TiC electro-sinter-forged metal matrix composite (open circles) plotted with iso-strain, iso-stress and point-by-point average models of hardness.
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Metals 12 00748 g003 550
Figure 3. Preform CS0 (left) and drilling tip made from the preform in an AISI H13 + TiC composite.
Figure 3. Preform CS0 (left) and drilling tip made from the preform in an AISI H13 + TiC composite.
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Metals 12 00748 g004 550
Figure 4. 1000× scanning electron microscope image of 85-15 powders: “finissimo” bronze (left) and 25 GR 85-15 450-mesh bronze (right).
Figure 4. 1000× scanning electron microscope image of 85-15 powders: “finissimo” bronze (left) and 25 GR 85-15 450-mesh bronze (right).
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Metals 12 00748 g005 550
Figure 5. Micro-tensile specimens for traction on ESFed materials, measurements are in millimetres.
Figure 5. Micro-tensile specimens for traction on ESFed materials, measurements are in millimetres.
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Metals 12 00748 g006 550
Figure 6. True stress and true strain curve for (A) ESFed 85-15 Cu-Sn from a pre-alloyed “bronzo finissimo” powder and (B) from an 85-15 irregular powder made from a master alloy with copper additions.
Figure 6. True stress and true strain curve for (A) ESFed 85-15 Cu-Sn from a pre-alloyed “bronzo finissimo” powder and (B) from an 85-15 irregular powder made from a master alloy with copper additions.
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Figure 7. ESFed 85-15 Cu-Sn as-produced 80 × 5 × 3 mm bars. Processing parameters: initial pressure 50 MPa, maximum pressure 215 MPa, SEI = 1 kJ/g.
Figure 7. ESFed 85-15 Cu-Sn as-produced 80 × 5 × 3 mm bars. Processing parameters: initial pressure 50 MPa, maximum pressure 215 MPa, SEI = 1 kJ/g.
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Figure 8. Optical micrographs of eForged 6061 aluminium sample made from a pre-mix of a master alloy and pure aluminium. Cross-sections in the direction transverse to the pressing direction, at lower (left) and higher (right) magnifications. Areas of high-alloying elements (master alloy) visible in the right image, on the lower right corner and sparse throughout the sample in the left image.
Figure 8. Optical micrographs of eForged 6061 aluminium sample made from a pre-mix of a master alloy and pure aluminium. Cross-sections in the direction transverse to the pressing direction, at lower (left) and higher (right) magnifications. Areas of high-alloying elements (master alloy) visible in the right image, on the lower right corner and sparse throughout the sample in the left image.
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Metals 12 00748 g009 550
Figure 9. SEM low-magnification 100× as-received gamma aluminides from swarf (left), lightly milled (centre) and heavily milled powders (right).
Figure 9. SEM low-magnification 100× as-received gamma aluminides from swarf (left), lightly milled (centre) and heavily milled powders (right).
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Figure 10. SEM low-magnification 300× electro-sinter-forged gamma aluminides from swarf (left), lightly milled (centre) and heavily milled powders (right).
Figure 10. SEM low-magnification 300× electro-sinter-forged gamma aluminides from swarf (left), lightly milled (centre) and heavily milled powders (right).
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Table
Table 1. List of significant materials and their shape along with electro-sinter-forging processing parameters. ED and ID are internal diameter and external diameter. Shape CS0 is described as pressing area, A, and height, h, and is discussed in Section 4.3.
Table 1. List of significant materials and their shape along with electro-sinter-forging processing parameters. ED and ID are internal diameter and external diameter. Shape CS0 is described as pressing area, A, and height, h, and is discussed in Section 4.3.
MaterialSize/Dimensions (mm)Density (%)Starting PowderInitial Pressure (MPa)Maximum Pressure (MPa)SEI (kJ/g)References
Iron alloy20 × 10 × 499%Pre-alloyed Astaloy CrL501152.2[32]
0.2% carbon steel∅10 × 4100%Iron + graphite flakes803002.2Present work
AISI H1320 × 10 × 4100%Pre-alloyed602702.2Present work
AISI 316L∅10 × 3100%Pre-alloyed1502003.5Present work
17-4PH∅10 × 3100%Pre-alloyed2002402.1Present work
100Cr6 cam followerED 15.4 ID 9.3 h 4.5100%Astaloy CrM + Graphite202003Present work
H13 + 20% vol TiCA = 188.55 mm2 h = 4.6 mm, CS0100%Pre-alloyed + TiC, simply mixed304001.12Present work
H13 + 30% vol TiCA = 188.55 mm2 h = 4.6 mm, CS098%Pre-alloyed + TiC, simply mixed1602301.35Present work
H13 + 40% vol TiCA = 188.55 mm2 h = 4.6 mm, CS098%Pre-alloyed + TiC, simply mixed1603001.5Present work
85-15 Cu-Sn irregular∅10 × 6100%Pre-mix of master alloy + Cu801200.75Present work
85-15 Cu-Sn irregular20 × 10 × 4100%Pre-mix of master alloy + Cu201401.16Present work
85-15 Cu-Sn irregular80 × 5 × 3100%Pre-mix of master alloy + Cu502151Present work, used in [33]
85-15 Cu-Sn “bronzo finissimo”20 × 10 × 4100%Pre-alloyed201001.28Present work
AlSi10 < 63 µm∅10 × 4100%Pre-alloyed772604.7[34]
AlSi10Mg0.4 < 63 µm∅10 × 4100%Pre-alloyed402404.4[34]
Al 606120 × 10 × 499%Pre-mix of master alloy + Al202401.7Present work
Al 707520 × 10 × 4100%Pre-mix of master alloy + Al202102.9Present work
Incoloy 625∅10 × 4100%Pre-alloyed2002302.4Present work
NiTiNOL∅10 × 4100%Pre-alloyed1553502.6[35]
Bi2-Te3∅10 × 10100%Pre-alloyed1002440.22Present work
TiAl∅10 × 4100%Pre-alloyed (swarf) 1203203Present work
MQU NdFeB∅10 × 3.399.5%Flakes, Pre-alloyed1002502.7Present work
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Fais, A. Advancements and Prospects in Electro-Sinter-Forging. Metals 2022, 12, 748. https://doi.org/10.3390/met12050748

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