Customized wear-resistant composite components by additive manufacturing and hot isostatic pressing
Contact
Problem
The powder metallurgical processes of additive manufacturing (AM) and hot isostatic pressing (HIP) can be combined in a variety of ways. It is true that HIP is already used to improve mechanical properties such as fatigue strength of AM components. However, only a comprehensive process chain involving numerical simulation makes it possible to easily and flexibly manufacture high-performance components that would conventionally only be possible at very high cost.
Objectives
- Determination of scientifically and economically optimal parameter sets for additive processing of wear-resistant, high-carbon steel by Powder Bed Fusion - Electron Beam and Metal Binder Jetting
- Adaptation of existing FEM approaches for geometry optimization of additively manufactured capsules by determining material parameters
- Development of a HIP-suitable brazing technology for joining a powder filling tube to capsule materials with low weldability
- Elaboration of a suitable heat treatment route for tempering the composite components consisting of wear resistant and hot work tool steel
- Fabrication and qualification of an industrially relevant demonstrator
Procedure
IGF project 21074 BG demonstrated a way to manufacture complex-shaped, wear-resistant composite components close to final contour. For this purpose, a HIP capsule was built up from a wear-resistant steel, filled with a tough quenched and tempered steel powder and consolidated to full density. The compaction and deformation caused by HIP are taken into account by numerical simulation prior to AM manufacturing and added to the capsule geometry. After HIP, the component is almost in final dimensions and only needs to be reworked on functional surfaces.
In the previous project, the basic procedure was successfully demonstrated with a monolithic component made of a stainless, austenitic steel. However, it was not possible to process the highly carbide-containing wear-resistant steel FeCrV10 without cracks using the laser beam melting (PBF-LB) AM process.
In the now completed project, the first step was therefore to evaluate the two AM processes PBF-EB and BJT for their suitability for processing high-carbon steels. For this purpose, two different materials were selected: the locking-resistant hard material ferro-titanite in the specification Nikro128® (DEW brand) and the martensitic carbide-rich cold-work tool steel FeCrV10.Finally, a process was selected to build and characterize a complexly shaped, wear-resistant demonstrator component. All work as well as material selection and the determination of the demonstrator geometry were always carried out in close coordination with the PA.
Results
Metal Binder Jetting
Both materials can be processed by metal binder jetting. Overall, it can be seen that the achievable properties depend to a large extent on the properties of the powder and its processability.
The material Nikro 128® can be processed in the particle size distribution 0-20 µm by metal binder jetting and sintered to high density with simple sample geometries. However, for a robust and reproducible process, a powder with higher flowability is required to achieve higher green particle density and more consistent powder deposition. However, this requires deep intervention in the production of the powder material provided by the PbA.
When processing the FeCrV10, the powder bed oxidizes during standard curing in air due to the non-corrosion resistant matrix. When curing under argon, the corrosion can be reduced and the green parts can be de-powdered without defects. Due to the good flowability of the powder (Hausner factor of 1.21), the FeCrV10 can be processed at a higher application speed without a drop in powder bed and green part density. The imaging accuracy and green part strength is high, allowing printing and depowdering of complex structures.
In the sintering behavior of FeCrV10, the atmosphere has a great effect on the microstructure. When sintering in liquid phase under nitrogen, complete densification can be achieved with small size of carbides. At the same time, the original vanadium carbides of the material transform into carbonitrides, releasing carbon. This diffuses into the matrix. Sintering under high vacuum leads to complete densification of the material. In the EDX measurements, the matrix exhibits a homogeneous chemical composition. Compared to the sample sintered under nitrogen, the carbide size increases due to the higher sintering temperature. With these parameters, the material FeCrV10 can be processed excellently by MBJ.
Powder Bed Fusion by Electron Beam
Both the Nikro128 and the FeCr10V could be processed by Powder Bed Fusion by Electron Beam (PBF-EB). In the process, properties could be derived that are even higher than those of the conventional material.
The material Nikro 128® can be processed in particle size distribution using PBF-EB. A stable process requires a double exposure technique in which the individual layers are first precompressed with a defocused electron beam and then focused and melted with an increased amount of energy. The excess powder is characterized by the breakup of isolated particles during doctoring or under the action of the electron beam, resulting in lower flowability. Nevertheless, nearly dense specimens and simplified demonstrators can be produced.
The processability of the FeCr10V powder using PBF-EB can be rated as very good in terms of good flowability and high process stability. Recycling of the excess powder is possible without any problems. In addition, very fine-grained microstructures with a homogeneous distribution of the predominantly submicrometer-sized vanadium carbides can be produced (Figure 1). The hardness and toughness of FeCr10V can be specifically influenced by downstream heat treatment. The material also has outstanding wear resistance.
Figure 1: Microstructure of FeCr10V produced by (a) MBJ sintered at 1260 °C under high vacuum and (b) PBF-EB after heat treatment at 1020 °C, 1 h and 3 x 540 °C for 1 h each.
Techniques for closing the capsules
HIP capsules require a filling tube for filling and evacuation, which is removed again during finishing. In contrast to conventional HIP capsules made from low-alloy or austenitic steels with good weldability, the material FeCrV10 tends to crack in TIG welding. Therefore, three different joining methods for filler tubes were tested and compared with respect to their gas tightness and compressive strength at HIP temperature and formation of brittle phases. These joining processes included (1) conventional TIG welding, (2) a conventional brazing process in a vacuum furnace with the commercial brazing material BNi-5, and (3) a self-developed inductive brazing process with a TLP (Transient Liquid Phase) brazing material. While brazing with the commercial brazing material in the vacuum furnace can be classified as expensive due to the long process time, inductive brazing could be performed similarly fast as TIG welding within 30 minutes. The solder was applied as a paste directly into a groove provided for the solder in the additively manufactured capsule, so that the filler tube only had to be "plugged on" before soldering. Of 5 capsules brazed in each case, all capsules were gas-tight after furnace brazing and TIG welding but only 2 capsules after induction brazing. Overheating was problematic due to the difficulty of measuring temperature using a pyrometer.
All three capsule variants were successfully filled with powder, evacuated, sealed and densified by HIP (1150 °C, 100 MPa, 240 min). Figure 2 shows exemplary micrographs. Although intermetallic phases were formed in the area of the brazing zones, no detrimental properties can be identified. In addition, vanadium carbides are also present in the self-developed brazing material and it can be assumed that the high wear resistance of the base material is maintained. EBSD investigations of the FeCrV10 capsule material showed a two-phase microstructure of martensite and vanadium monocarbide with individual chromium carbides. The hardness of the material from the welding tests is 705±2 HV10, while the material from the brazing tests has a higher hardness of 804±2 HV10. Future investigations must prove whether TLP brazing is also suitable for HIP capsules, which are, for example, composed of complex additively manufactured areas and simple conventionally manufactured parts.
Figure 2: Test capsules for the evaluation of joining processes after HIP compaction. a) TIG welding with AWS A5.28 ER80S-A1 filler metal, b) TLP brazing in a vacuum furnace with AWS A5.8 BNi-5 solder, c) Inductive TLP brazing with AISI A11+3.5wt% Si
Demonstrator components
Dies for powder pressing can be produced from FeCr10V using PBF-EB, whereby the number of internal support structures and the effort for powder removal can be minimized by a suitable orientation of the die in the build space (see Figure 3a).
Figure 3: a) Die for powder pressing manufactured using PBF-EB, b) Demonstrator components from the PBF-EB and MBJ after the combined process chain of additive manufacturing and hot isostatic pressing.
Starting from the PBF-EB of thin-walled gas-tight structures with wall thicknesses down to less than 500 µm, it is also possible to successfully print precast extruder screw demonstrators (Figure 3b). The 4th capsule in Figure 3 originates from the MBJ process, which is compacted to a stable shape by sintering in a ceramic powder bed. After TIG welding of the filling tube, filling with 56NiCrMoV7 powder and successful evacuation, the capsules are sealed gas-tight by forging the filling tube, filled and compacted into near-net-shape components via hot isostatic pressing at 1150 °C followed by rapid quenching (see Figure 3b). The deviation between the shrinkage predicted from the HIP simulation and the real demonstrator part is only 2%.
Project partners
We would like to thank the Fachverband Pulvermetallurgie e.V. (FPM) for its support during the application and funding phase, and all the companies on the project-accompanying committee: Additive Works, GmbH, Ampower GmbH & Co. KG, Barradas GmbH, Bleistahl Produktion-GmbH & Co. KG, Bodycote Specialist Technologies Deutschland GmbH, Coperion GmbH, Cremer Thermoprozessanlagen GmbH, Deutsche Edelstahlwerke Speciality Steel GmbH & Co, FIT AG, Höganäs Germany GmbH, Günther-Köhler-Institut für Fügetechnik und Werkstoffprüfung GmbH, PVA Löt- und Werkstofftechnik GmbH, Quintus Technologies AB
Funding notice
The IGF project 21074 BG of the Forschungsvereinigung Forschungsgesellschaft Stahlverformung e.V. (Research Association for Steel Forming) was funded by the German Federal Ministry of Economics and Climate Protection via the AiF within the framework of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The long version of the final report can be requested from FSV, Goldene Pforte 1, 58093 Hagen, Germany.
Documentation
In the course of the project, the current research results were published by the research institutions in the following scientific papers:
- S. Herzog, S. Fries, M. Mirz, A. Kaletsch, C. Broeckmann, Vom Pulver zum additiv hergestellten Bauteil, Teil 1: Perspektiven durch heiß-isostatisches Pressen, Werkstoffe 1/2021
- B. Barthel, M. Mirz, F. Petzoldt, Process Development of Grade A11 High Carbon Tool Steel for Metal Binder Jetting, In Proceedings of World PM 2022, Lyon 12. Oktober 2022 .
- A Comparative Study on Different Joining Techniques For PM-HIP Evacuation Tubes. WorldPM 2022, Lyon 12. Oktober 2022.
- M. Franke-Jurisch, M. Mirz, T. Wenz, A. Kirchner, B. Klöden, T. Weißgärber, PBF-EB of Fe-Cr-V Alloy for Wear Applications Materials 2022, 15(5), 1679; https://doi.org/10.3390/ma15051679