Lehrstuhl für Werkstofftechnologie

Permanent URI for this collection

http://hdl.handle.net/2003/107

Browse

Now showing 1 - 20 of 38

Vacuum brazing of 18MAR300 nickel maraging steel joints based on additively manufactured and conventional material grades
(2023-07-17) Tillmann, Wolfgang; Henning, Tim; Wojarski, Lukas; Bültena, Julia
Laser powder bed fusion (LPBF) processes offer the best possible design options for the production of highly complex components with unique functionalities. Although the development of additive manufacturing processes is progressing impressively and build rates have already been significantly increased, the economic production of high-volume components is still particularly truncated. Comparatively expensive powders, machine-hour rates, and a limited construction space demonstrate a high demand for joining LPBF components to conventionally manufactured parts. Vacuum brazing is an excellent technology for manufacturing and simultaneous heat treatment of such hybrid components. The aim of this study is to investigate the brazeability and the joint properties of ultrahigh-strength nickel maraging steel 18MAR300 (1.2709, X3NiCoMoTi18-9-5) using BVAg-30 (Ag68Cu27Pd5) as brazing filler metal. For this purpose, the effect of nickel-plated surfaces on the wettability and the microstructure of the joint is studied for conv./conv.-, LPBF/conv.-, and LPBF/LPBF joints. Furthermore, the quasi-static and quasi-dynamic joint strength is evaluated. The results prove that nickel-plated surfaces are vital to achieve a sufficient process control. The coating assures a deoxidized and sealed surface which favors the formation of NiCu(Fe,Pd,Ag) phases within the braze metal thus enhancing the joint strength. In addition, LPBF/LPBF joint features have significantly higher tensile strength than conv./conv. joints (825–627 MPa).
Modification of 316L steel powders with bronze using high energy ball milling for use as a binder component in PBF-LB/M printing of diamond-metal matrix composites
(2023-10-17) Tillmann, Wolfgang; Ferreira, Manuel Pinho
For the processing of diamond-metal matrix composites, the powder bed fusion using a laser for metals (PBF-LB/M), represents a new promising method for the additive manufacturing of diamond tools for concrete and rock machining, even with more complicated geometries. Previous research activities show a strong tendency for cracking and delamination during the construction process of the samples. This behavior is caused by thermal residual stresses associated with the embedded diamonds. To control these negative effects on the process side, the volume energy density is reduced accordingly, which, however, led to increased pore formation. This publication deals with an approach on the material side to modify a 316L stainless steel base powder with an addition of 20 wt% bronze via a high energy ball milling (HEBM) process in such a way that a homogeneous solid solution phase is created. A significantly increasing of the melting interval and a decreasing of both solidus and liquidus temperature was observed, which can reduce pore formation in the PBF-LB/M-process. In addition, XRD-diffractometry and SEM/EDS-analysis showed that the homogeneous solid solution phase of this alloyed powder segregates again into Fe- and Cu-rich phases when heated up to the melting point.
Investigation of the applicability of Cu–Fe–Mn–Ni based high entropy and compositionally complex alloys as metal matrix composites for cobalt free hot-pressed diamond tools
(2023-08-29) Timmer, Christian; Tillmann, Wolfgang; Wojarski, Lukas; Ferreira, Manuel Pinho
Due to the rising demand and carcinogenic effect of cobalt, alternative metal matrixes need to be developed for hot-pressed diamond tools. Due to this reason High-Entropy alloys without cobalt were calculated via phase fraction diagrams. Three alloys of the Al–Cu–Fe–Mn–Ni system Al30Cu30Fe5Mn25Ni10, Al11.25Cu35Fe5Mn20Ni28.75 and Al5Cu20Fe25Mn25Ni25 were chosen due to their different crystal structures ranging from pure bcc, eutectic fcc-bcc to pure fcc crystal structure. Cr5Cu20Fe25Mn25Ni25 was chosen to verify the change of one element on the consolidation properties. The alloys were mechanically alloyed and hot-pressed at 800 °C for 3 min without and at 900 °C for 3 min with diamonds. Porosity increased with the fraction of bcc solid solution in the investigated alloys of the Al–Cu–Fe–Mn–Ni system. Samples consisting of Cr5Cu20Fe25Mn25Ni25 showed the lowest porosity, which was attributed to precipitation of a second copper-rich fcc solid solution around the remaining pores. At a process temperature of 800 °C and 3 min isothermal hold the samples featured a porosity of only 2.72%. Within the XRD patterns and SEM images of the hot-pressed samples with diamonds no graphitization or formation of carbides could be observed. Therefore, Cr5Cu20Fe25Mn25Ni25 was identified as a promising cobalt free metal-matrix candidate for diamond tools.
Microstructural and tribo-mechanical properties of arc-sprayed CoCr-based coatings
(2022-07-12) Hagen, Leif; Paulus, Michael; Tillmann, Wolfgang
Due to their superior wear and oxidation resistance, Stellite™ coatings are widely used in industrial applications, where the coatings are exposed to high temperature. Common processes for applying Stellite™ coatings include the high-velocity oxy-fuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite™ welding consumables or similar welding consumables in the form of cored wires (CoCr base without industrial property rights) are commercially available, there are hardly any studies on arc-sprayed Stellite™ coatings available in the literature. In this study, the microstructural characteristics of arc-sprayed deposits were investigated, which were produced using a CoCr-based cored wire with addition of 4.5 wt.% tungsten. The produced deposits were examined in its as-sprayed state as well as after exposed to elevated temperatures. The microstructure was scrutinized by means of electron microscopy, energy-dispersive x-ray spectroscopy, as well as x-ray diffraction analyses using synchrotron radiation. Tribo-mechanical tests were conducted in order to assess the performance of the arc-sprayed coating. The findings were discussed and compared to those obtained from conventional CoCr-based coatings. It was found that the arc-sprayed CoCr-based coating is predominantly composed of Co-rich, Cr-rich lamellae or lamellae comprising a Co(Cr)-rich solid solution interspersed with various oxides between the individual lamellae. Solid solution hardening serves as dominant strengthening mechanism, while precipitation hardening effects are hardly evident. With regard to the oxidation behaviour, the as-sprayed coating mainly contains CoCr2O4 as well as traces of Co3O4. For heating above 550 °C, coating surface additionally consists of Fe2O3 and Co3O4. In dry sliding experiments, the arc-sprayed CoCr-based coating shows a decreased wear resistance compared to CoCr-based coatings processed by HVOF and PTA, whereas the coefficient of friction (COF) sliding against alumina was similar to the COF observed for the HVOF-sprayed CoCr-based coating, but lower than the COF obtained for the CoCr-based hardfacing alloy deposited by PTA.
Characterization of the microstructure and thermomechanical properties of invar 36 coatings deposited by HVOF and cold gas processes
(2022-08-31) Tillmann, W.; Khalil, O.; Baumann, I.
The effect of impact velocity and temperature of invar particles deposited by high-velocity oxygen fuel (HVOF) and cold spray processes on the microstructure and oxidation content of invar coatings is not fully understood. Additionally, the effect of coating thickness on the coefficient of thermal expansion (CTE) of the coated material and the influence of cold working on the coating hardness are also insufficiently investigated. In the present study, invar coatings were deposited at temperatures close to and below the melting point of invar particles to maintain low CTE. It was found that particle impact temperature and velocity strongly affect pore formation and cohesiveness but slightly affect the hardness of invar coatings. Higher particle impact velocities with impact temperatures close to the invar’s melting point enhance highly the cohesiveness of HVOF-invar coatings. Furthermore, invar coatings stabilize the CTE of the coated material up to a temperature of 227 °C. An increment in the coating’s thickness of 150 µm leads to reducing the CTE of the coated material (Al) in the in-plane direction by 7.65%. Applying cold working using 200 kN compression increases the hardness of the treated coatings by 6% while machine hammer peening (MHP) has a slight effect.
An investigation of the influence of integration of steel heat treatment and brazing process on the microstructure and performance of vacuum-brazed cemented carbide/steel joints
(2022-02-28) Tillmann, W.; Ulitzka, T.; Dahl, L.; Wojarski, L.; Ulitzka, H.
Cemented carbides are commonly brazed to transformation hardening tool steels without taking a proper and adequate steel heat treatment into account. This publication shows the limits and possibilities of integrating a steel heat treatment, including a quenching process, into a vacuum brazing process. Therefore, copper-based filler metals are selected to ensure the steel component’s high and homogenous hardness and supply a high joint quality. In this context, the aimed steel hardness was chosen in the range between 400 and 440 HV1 based on industrial experiences. This specific hardness range for the steel component was set to avoid wear of machining tools in subsequent machining steps if the steel hardness is too high and to prevent wear and deformation of the tool itself in case of a steel hardness too low. When using the transformation hardening tool steel 1.2344, the obtained shear strength values did not exceed a threshold of 20 MPa which can be attributed to the required N2-quenching from brazing respectively solution annealing temperature. However, the steel components featured a hardness of 527.1 HV1 for the specimens brazed with pure copper at 1100 °C and 494.0 HV1 for those brazed with a CuGeNi filler metal at 1040 °C. This publication also shows an alternative route to manufacture long-lasting tools with a cemented carbide/steel joint by applying the difficult to wet and not well researched, but for many other reasons very suitable precipitation hardening maraging steel. Especially, the comparable low coefficient of thermal expansion (CTE) and the capability of the lath martensite to compensate large amounts of externally imposed stresses during the austenite-to-martensite transformation as well as the cooling rate independent of the hardening mechanism of the maraging steel and a pre-applied nickel coating including the corresponding diffusion processes are responsible for a sound joint with a shear strength > 300 MPa. Moreover, the subsequent tempering process at 580 °C for 3 h provides the maraging steel joining partner with a hardness of 426.6 ± 6.0 HV1.
Statistical comparison of processing different powder feedstock in an HVOF thermal spray process
(2022-04-12) Tillmann, Wolfgang; Kuhnt, Sonja; Baumann, Ingor Theodor; Kalka, Arkadius; Becker-Emden, Eva-Christina; Brinkhoff, Alexander
Cermet coatings such as WC-Co and Cr3C2-NiCr are frequently applied by means of thermal spray processes to protect highly stressed surfaces against wear. The investigation of the respective spray materials and their coating properties and in-flight particle properties are often carried out in separate experiments. In this study, the coating characteristics (hardness, deposition rate, porosity, thickness) and in-flight particle properties (particle velocity and temperature) of three different WC-based powders and a Cr3C2-NiCr powder processed by means of an HVOF process are investigated as a function of some key process parameters such as kerosene flow rate, lambda, spray distance and feeder disc velocity. These parameters were varied within a design of experiments, whilst all other parameters were fixed. Both the design of experiments plan and the settings of the fixed parameters were defined identically. The in-flight particle properties and coating characteristics are statistically modeled as a function of the process parameters and their influences are compared. A well-selected, limited number of experimental runs using statistical design of experiment (DoE) enable this comparison. The deployed statistical models are generalized linear models with Gamma-distributed responses. The models show that particle velocity and particle temperature mainly depend on kerosene flow rate and spray distance. However, in the case of particle temperature, the model coefficients for Cr3C2-NiCr and WC powders have different signs, reflecting different qualitative behavior.
Structure and tribo-mechanical properties of MoSx:N:Mo thin films synthesized by reactive dcMS/HiPIMS
(2021-12-14) Tillmann, Wolfgang; Wittig, Alexandra; Stangier, Dominic; Thomann, Carl Arne; Debus, Jörg; Aurich, Daniel; Brümmer, Andreas
Modifying MoS2 thin films by additional elements shows great potential in order to adjust the property profile and to meet the increasing requirements regarding high wear resistance and low friction properties of industrial components. Within that context, MoSx:N:Mo thin films were deposited by a reactive hybrid dcMS/HiPIMS process. By systematically increasing the Mo target cathode power, an investigation of the structural and the mechanical properties was conducted to understand the evolution of the tribological behavior. A low Mo target cathode power of 1 kW is related to the formation of the preferential (002) MoS2 basal-plane and thus a low friction with µ = 0.2. With an increasing amount of Mo, the film loses its solid lubricant MoS2 properties and a nitride constitution of the thin film is developing due to the formation of crystalline Mo and MoN phases. Related to this transformation, the hardness and elastic modulus are increased, but the adhesion and the tribological properties are impaired. The film loses its plasticity and the generated film material is directly removed from the contact area during the sliding contact.
Qualification of the low-pressure cold gas spraying for the additive manufacturing of copper-nickel-diamond grinding wheels
(2021-12-05) Tillmann, Wolfgang; Zajaczkowski, Jonas; Baumann, Ingor; Kipp, Monika; Biermann, Dirk
Grinding wheels are usually manufactured by powder metallurgical processes, i.e., by molding and sintering. Since this requires the production of special molds and the sintering is typically carried out in a continuous furnace, this process is time-consuming and cost-intensive. Therefore, it is only worthwhile for medium and large batches. Another influencing factor of the powder metallurgical process route is the high thermal load during the sintering process. Due to their high thermal sensitivity, superabrasives such as diamond or cubic boron nitride are very difficult to process in this way. In this study, a novel and innovative approach is presented, in which superabrasive grinding wheels are manufactured by thermal spraying. For this purpose, flat samples as well as grinding wheel bodies were coated by low-pressure (LP) cold gas spraying with a blend of a commercial Cu-Al2O3 cold gas spraying powder and nickel-coated diamonds. The coatings were examined metallographically in terms of their composition. A well-embedded superabrasive content of 12 % was achieved. After the spraying process, the grinding wheels were conditioned and tested for the grinding application of cemented carbides and the topographies of both the grinding wheel and the cemented carbide were evaluated. Surface qualities of the ground surface that are comparable to those of other finishing processes were reached. This novel process route offers great flexibility in the combination of binder and hard material as well as a cost-effective single-part and small-batch production.
Influence of direct splat-affecting parameters on the splat-type distribution, porosity, and density of segmentation Cracks in Plasma-Sprayed YSZ Coatings
(2021-03-18) Tillmann, Wolfgang; Khalil, Omar; Baumann, Ingor
The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks.
Internal diameter coating by warm spraying of fine WC-12Co powders (− 10 + 2 µm) with very short spray distances up to 10 mm
(2021-04-23) Baumann, Ingor; Tillmann, Wolfgang; Schaak, Christopher Sven; Schmidt, Katharina; Zajaczkowski, Jonas; Schmidtmann, Gunnar; Götz, Matthäus; Luo, Weifeng
The internal diameter (ID) coating by means of thermal spraying is currently experiencing growing interest in science and industry. In contrast to the well-established plasma- and arc-based spray techniques, there is a lack of knowledge concerning kinetic processes such as HVOF, HVAF and warm spray (WS). A major challenge represents the necessity of short spray distances and the compact design of novel ID spray guns with reduced combustion power. Conventional WC-Co powders (− 45 + 15 µm) are not able to achieve a sufficient heat and momentum transfer. The use of fine powders < 15 µm offers an approach to overcome this drawback as they feature a larger surface-to-volume ratio and a lower mass. However, the processing of fine powders requires suitable spray equipment and a sensitive parameter adjustment. In this study, warm spraying of fine WC-12Co powders (− 10 + 2 µm) with a novel ID spray gun (HVOF + N2) “ID RED” (Thermico Engineering GmbH, Germany) was investigated. First, the flame profile as well as the in-flight behavior of the particles along the spray jet (spray distances SD = 10-80 mm) was analyzed at different nitrogen flows NF = 15-115 L/min to find suitable spray parameter intervals. Subsequently, planar steel samples were coated with SD = 10-50 mm and constant NF = 90 L/min. Analyses regarding the microstructure, the mechanical properties and the phase evolution of the coatings were performed. The aim was to study spraying with the novel ID gun and to scrutinize shortest feasible spray distances. Finally, steel tubes (internal diameter of 81.6 mm and a wall thickness of 10.0 mm) were coated with SD = 20 mm and NF = 90 L/min to investigate in how far the results can be transferred to ID parts. Correlations between the particle behavior, the microstructure and the coating properties were made.
WC decomposition phenomena in ID-HVOF-sprayed WC-CoCr coatings using fine powder feedstock
(2022-01-23) Tillmann, Wolfgang; Hagen, Leif; Baumann, Ingor; Paulus, Michael
Over the last few decades, the high velocity oxygen fuel (HVOF) spraying of WC-CoCr for internal diameter (ID) coating has attracted much interest for hard chrome replacement. Current demands for the ID coating of small cylindrical parts necessitates the use of specialized spray gun equipment and powder feedstocks with small particle size fractions. Due to the limited spray distance inside cylindrical parts with small IDs, the process control, spraying fine WC-CoCr powders, meets new challenges to avoid significant WC decomposition, which increases the risk of mechanical degradation. Within the scope of this study, ID-HVOF spraying using a fine-structured WC-CoCr (−15 + 5 μm) feedstock with a mean WC particle size of 400 nm is examined with respect to the WC decomposition phenomena using X-ray diffraction (XRD). Hence, a statistical design of experiments (DoE) is utilized to systematically analyze various spray parameter settings along with their interaction as part of the WC to W2C conversion.
The effect of argon as atomization gas on the microstructure, machine hammer peening post-treatment, and corrosion behavior of twin wire arc sprayed (TWAS) ZnAl4 coatings
(2021-12-27) Tillmann, Wolfgang; Abdulgader, Mohamed; Wirtz, Andreas; Milz, Michael P.; Biermann, Dirk; Walther, Frank
In the twin wire arc spraying (TWAS) process, it is common to use compressed air as atomizing gas. Nitrogen or argon also are used to reduce oxidation and improve coating performance. The heat required to melt the feedstock material depends on the electrical conductivity of the wires used and the ionization energy of both the feedstock material and atomization gas. In the case of ZnAl4, no phase changes were recorded in the obtained coatings by using either compressed air or argon as atomization gas. This fact has led to the assumption that the melting behavior of ZnAl4 with its low melting and evaporating temperature is different from materials with a higher melting point, such as Fe and Ni, which also explains the unexpected compressive residual stresses in the as-sprayed conditions. The heavier atomization gas, argon, led to slightly higher compressive stresses and oxide content. Compressed air as atomization gas led to lower porosity, decreased surface roughness, and better corrosion resistance. In the case of argon, Al precipitated in the form of small particles. The post-treatment machine hammer peening (MHP) has induced horizontal cracks in compressed air sprayed coatings. These cracks were mainly initiated in the oxidized Al phase.
Heat treatment of binder jet printed 17–4 PH stainless steel for subsequent deposition of tribo-functional diamond-like carbon coatings
(2021-12-06) Tillmann, Wolfgang; Lopes Dias, Nelson Filipe; Stangier, Dominic; Schaak, Christopher; Höges, Simon
Diamond-like carbon (DLC) coatings deposited on additively manufactured steel greatly improve the tribological properties. However, a high substrate hardness is crucial to sustaining high mechanical loads in the tribological contact. Herein, the heat treatment of binder jet printed 17–4 PH enhances the hardness from 24 to 39 HRC. Binder jet printed 17–4 PH substrates are coated by DLC of the types hydrogen-free amorphous carbon (a-C) of ∼23 GPa and hydrogenated amorphous carbon (a-C:H) of ∼20 GPa. The influence of the heat treatment on the tribo-mechanical properties of the DLC coatings is investigated. 17–4 PH demonstrates high friction and wear against steel counterparts, but the wear rate is reduced from 693 ± 43 × 10–6 mm3/Nm to 492 ± 41 × 10-6 mm3/Nm by heat treating the steel. Both a–C and a–C:H are effective in reducing the friction and wear with wear rates below 0.3 × 10–6 mm3/Nm. The a–C and a–C:H coatings demonstrate lower plastic wear on heat treated 17–4 PH due to the higher substrate hardness. Consequently, the heat treatment is an essential process step to ensure maximum tribological functionality of the DLC coating on additively manufactured 17–4 PH steel.
Impact of tungsten incorporation on the tribomechanical behavior of AlCrWxSiN films at room and elevated temperature
(2021-08-27) Tillmann, Wolfgang; Feher, Alexander; Stangier, Dominic
AlCrWxSiN thin films (0 ≤ x ≤ 17.1 at.%) were synthesized by means of a hybrid magnetron sputtering process, merging direct current (DC) as well as tungsten high power impulse magnetron sputtering (HiPIMS) supplies. The influences of increasing the tungsten contents on the structural as well as the friction and wear behavior at room and high temperatures (500 °C) were elaborated. As a reference, a W61.4N38.6 system served to analyze synergetic effects on the oxidation behavior. Increased tungsten contents in AlCrWxSiN resulted in more distinctive (200)-, (202)-, and (311)- crystal orientations. A W/Cr ratio of ~1 could be correlated with a denser film growth, the highest hardness (24.3 ± 0.7 GPa), and a significantly decreased wear coefficient (<0.3 × 10−5 mm3/Nm). Tribological tests performed at room temperature revealed that the coefficient of friction decreased with higher tungsten contents to µ~0.35. In contrast, at elevated temperatures, the coefficient of friction increased with higher W concentrations due to spotty oxidations in the wear track, which resulted in a locally increased surface roughness. Finally, a phase transformation of the WN film to m-WO3 did not contribute to a friction reduction at 500 °C.
Effect of substrate pre-treatment on the low cycle fatigue performance of tungsten carbide-cobalt coated additive manufactured 316 L substrates
(2020-09-25) Tillmann, Wolfgang; Hagen, Leif; Garthe, Kai-Uwe; Hoyer, Kay-Peter; Schaper, Mirko
Numerous studies already identified that the fatigue strength of 316 L parts processed by laser beam melting (LBM) is distinctly affected by the surface integrity. Among others, surface defects as well as residual stresses are of crucial importance. Despite new findings in the field of surface engineering of laser beam melting (LBM) parts, the low cycle fatigue strength of thermally sprayed additively manufactured substrates has not been in the focus of research to date. This study aims at evaluating the effect of different pre-treatments onto 316 L substrates processed by laser beam melting (LBM) prior to the deposition of a high velocity oxy-fuel (HVOF) sprayed tungsten carbide-cobalt coating and their effect on the low cycle fatigue strength. Therefore, 316 L substrates were examined in their as-built state as well as after grit blasting with regards to the surface roughness, strain hardening effects, and residual stresses. To differentiate between topographical effects and residual stress related phenomena, stress-relieved 316 L substrates served as reference throughout the investigations. The tungsten carbide-cobalt coated and differently pre-treated 316 L substrates were mechanically tested under quasi-static and dynamic load conditions. Besides the low cycle fatigue strength, the fracture toughness as well as the fracture mechanism were identified based on fracture surface analysis.
Investigation of joints from laser powder fusion processed and conventional material grades of 18MAR300 nickel maraging steel
(2021-03-01) Tillmann, Wolfgang; Wojarski, Lukas; Henning, Tim
Even though the buildup rate of laser powder bed fusion processes (LPBF) has steadily increased in recent years by using more and more powerful laser systems, the production of large-volume parts is still extremely cost-intensive. Joining of an additively manufactured complex part to a high-volume part made of conventional material is a promising technology to enhance economics. Today, constructors have to select the most economical joining process with respect to the individual field of application. The aim of this research was to investigate the hybrid joint properties of LBPF and conventionally casted 18MAR300 nickel maraging steel depending on the manufacturing process and the heat treatment condition. Therefore, the microstructure and the strength of the hybrid joints manufactured by LPBF or vacuum brazing were examined and compared to solid material and joints of similar material. It was found that the vacuum-brazed hybrid joints using a 50.8-μm-thick AuNi18 foil provide a high tensile strength of 904 MPa which is sufficient for a broad field of application. Furthermore, the additively manufactured hybrid samples offered with 1998 MPa a tensile strength more than twice as high but showed a considerable impact of buildup failures to the strength in general.
TiAlN-Beschichtungen mittels hochenergetischer Kathodenzerstaeubung
(2020-06-18) Tillmann, Wolfgang; Stangier, Dominic; Grisales, Diego
Hochenergetische Plasmen stellen nach aktuellen Erkenntnissen ein vielversprechendes tribologisches Eigenschaftsprofil für nitridische Hartstoffschichten und deren Anwendungen dar. Jedoch resultiert aus der gesteigerten Ionisationsrate ein hohes Niveau an Druckeigenspannungen in TiAlN‐Dünnschichten (σTiAlN ∼ –6000 MPa), welches die Haftung zwischen Substrat und Beschichtung negativ beeinflussen und zu Delamination und Schädigung führen kann. In diesem Zusammenhang belegen die durchgeführten Untersuchungen, dass durch eine hybride Prozessführung, die aus der synchronen Verwendung von Gleichstrom (DC)‐ und Hochleistungs‐Impuls‐Kathodenzerstäubung (HiPIMS) besteht, eine Reduktion des Eigenspannungszustands in der Schicht erfolgen kann. Dies ermöglicht die Vorteile beider Technologien zu vereinen und somit TiAlN‐Dünnschichten für ein erweitertes Einsatzspektrum als tribologische Beschichtung für wärmebehandelte Werkzeugstähle zu qualifizieren.
Adhesion of HVOF-sprayed WC-Co coatings on 316L substrates processed by SLM
(2020-08-07) Tillmann, Wolfgang; Hagen, Leif; Schaak, Christopher; Liß, J.; Schaper, M.; Hoyer, K.-P.; Aydinöz, M. E.; Garthe, K.-U.
Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect.
Application of the eutectic high entropy alloy Nb0.73CoCrFeNi2.1 for high temperature joints
(2020-07-06) Tillmann, Wolfgang; Wojarski, Lukas; Stangier, Dominic; Manka, Matthias; Timmer, Christian
The eutectic high entropy alloy Nb0.73CoCrFeNi2.1 was manufactured by means of arc smelting and the obtained ingots were cut into 300-μm-thick foils, which were used as filler alloys to braze Crofer 22 APU to Hf-metallized yttria-stabilized zirconia (3YSZ). The brazing process was conducted in a vacuum furnace at 1200 °C for 5 min at a vacuum of 4.3·10–4 mbar. In order to minimize the intense diffusion and erosion of the steel substrate, a heating and cooling rate of 50 K/min was applied. Sound joints without any pores or flaws were obtained. The microstructure of the joints consisted of an HfO2 reaction layer at the ceramic interface and the same eutectic structure consisting of a Laves phase and a solid solution that was already detected in the smelted foil. The average hardness of the microstructure in the joint seam amounted to 352 ± 17 HV0.01 and the joints reached strength values up to 97 ± 7 MPa while the fracture area was always located at the ceramic interface in the HfO2 layer. Comparable joints, with AgCuTi3 as filler metal, brazed at 920 °C, only reached a shear strength of ~ 52 ± 2 MPa.

Browse

Recent Submissions