Dipl.-Ing. Lukas Wojarski

Lehrstuhl für Werkstofftechnologie
TU Dortmund University

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  • Influence of the Brazing Paste Composition on the Wetting Behavior of Reactive Air Brazed Metal–Ceramic Joints
    Waetzig, K. and Schilm, J. and Mosch, S. and Tillmann, W. and Eilers, A. and Wojarski, L.
    Advanced Engineering Materials 23 (2021)
    Reactive air brazing (RAB) is a cost-effective way to produce ceramic–ceramic or ceramic–metal brazed joints in air, without applying a protective gas atmosphere or a vacuum. In addition to conventional furnace technology, the brazing with induction heating can also be used effectively. Within the scope of this study the shrinkage and wetting behavior of self-developed brazing pastes with different CuO contents and two qualities of silver powders with coarse and fine particle size are investigated by optical dilatometry on alumina (Al2O3, 99.7% purity). Thereby, the fine silver powder quality reveals a significant swelling effect at high temperatures, leading to an expansion of densified powder compacts caused by evolving gases. Joining tests are performed on ceramic–steel brazed joints using a muffle furnace and induction heating for short brazing cycles. The brazing seams and interfaces of the joints are investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). As a result, correlations between the brazing filler metal composition, the steel, and the brazing conditions are obtained. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/adem.202000711
  • Application of the eutectic high entropy alloy Nb0.73CoCrFeNi2.1 for high temperature joints
    Tillmann, W. and Wojarski, L. and Stangier, D. and Manka, M. and Timmer, C.
    Welding in the World 64 (2020)
    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. © 2020, The Author(s).
    view abstract10.1007/s40194-020-00944-w
  • Development of high entropy alloys for brazing applications
    Tillmann, W. and Ulitzka, T. and Wojarski, L. and Manka, M. and Ulitzka, H. and Wagstyl, D.
    Welding in the World 64 (2020)
    High entropy alloys are novel and innovative metallic materials, which have intensively moved into the focus of research over the last decade. The high entropy effect in those multi-component alloys promotes the formation of a characteristic crystal structure, the random solid solution, which features unique material properties, and reduces the number of possible brittle phases. In this publication, the influence of gallium as a melting point depressant on the melting range and the microstructure of the two-phased equimolar CoCrCoFeNi were determined. In order to integrate the vacuum brazing process into the solutioning heat treatment of the nickel-based super alloy Mar-M 247 between 1180 and 1270 °C, the liquidus temperature of CoCrCoFeNi was aimed to be below 1270 °C. The changes in the melting ranges due to the modified compositions were predicted by CALPHAD simulations and verified by differential thermal analysis measurements. The promising multi-component filler alloy CoCrCuFeNiGa was determined for further investigations. For this purpose, the microstructures of the filler metal itself and the brazement were conducted. A shear strength of 388 ± 73 MPa was achieved for a brazing gap of 200 μm. The crack, which led to joint failure propagated through high-entropic, fcc-structured phases in the brazing seam. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-019-00824-y
  • Investigation of wetting behaviour of hardenable copper based-alloys for brazing applications
    Tillmann, W. and Lehmert, B. and Manka, M. and Wojarski, L. and Holewa, M.
    Science and Technology of Welding and Joining 25 (2020)
    Tools for machining are exposed to high loads, wear, and elevated temperatures. Commonly, such tools consist of cemented carbides and tool steel. To combine the advantages of both materials, high-quality-joints with high strengths are desired. When brazing these materials, the main challenge is the mismatch of the Coefficient of Thermal Expansion (CTE) and the poor wettability of cemented carbides by molten filler alloys. In this regard, the feasibility of two custom-made alloys (CuNi12Si5, CuNi12Si5B0.4) was analysed. Besides being a cost-efficient alternative, these alloys offer the possibility to modify their mechanical properties by precipitation hardening to reduce stresses within the final joints. Thus, this paper shows that a temperature of 1060°C is suitable for wetting and brazing tests on different substrates. © 2019, © 2019 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.
    view abstract10.1080/13621718.2019.1688458
  • Mechanical behavior of reactive air brazed (RAB) Crofer 22 APU-Al2O3 joints at ambient temperature
    Tillmann, W. and Anar, N.B. and Wojarski, L.
    SN Applied Sciences 2 (2020)
    The evolution of innovative high-temperature electrochemical devices, such as high temperature solid oxide fuel cells (SOFCs), gas separators and gas reformers, consisting of metal–ceramic-joints is challenging. The seals have to be stable and gastight in isothermal high-temperature as well as in thermo-cyclic operation. Here, the reduction of porosity is the primary aim, to obtain air brazed joints with a long lifetime. In the last years, reactive air brazing (RAB) has gained rising interest for the joining of ceramic–ceramic and ceramic–metal compounds. In this paper an alternative brazing filler metal manufacturing process employing (physical vapor deposition (PVD)) is applied and its feasibility for the production of metal–ceramic composites has been investigated for Ag–4 wt%CuO. For RAB aluminum oxide with ferritic high chromium steel Crofer22APU have been joined. The pore formation in subordination of the braze and base materials can be monitored after brazing. By modifying the brazing process, the pore formation in the joints can be avoided. The microstructure of brazed joints with the developed braze foils is studied. Discussion of the results focuses on the influence of microstructural evolution on mechanical properties, the pore formation in the brazing seam and failure behavior of the brazed joints. A correlation between the process parameters brazing temperature and holding time and the achieved compound properties could be derived. Further, excellent wetting of the ceramic was obtained. The highest shear strength with 123 MPa was measured for a temperature of 1000 °C and 5 min, using the Ag4CuO alloy. © 2020, Springer Nature Switzerland AG.
    view abstract10.1007/s42452-020-2622-6
  • Microstructure and Mechanical Properties of Reactive-Air-Brazed 3YSZ/Crofer 22 APU Joints at Ambient Temperature
    Tillmann, W. and Anar, N.B. and Wojarski, L. and Lassner, J.J.
    Metallography, Microstructure, and Analysis 9 (2020)
    The growth of inventive high-temperature electrochemical devices such as solid oxide fuel cells constitutes a major task in brazing technology of ceramic–metal joints. In this work, reactive air brazing was used and the joining characteristics of 3YSZ with Crofer 22 APU have been systematically analyzed for three different brazing temperatures (1000, 1050 and 1100 °C) and two dwell times (5 and 30 min). The joints have been brazed successfully using the Ag–4CuO filler alloy. This braze filler metal was manufactured by an arc PVD (physical vapor deposition) process. Further, sufficient wetting of the zirconium oxide was achieved. The morphology of the oxide reaction layer at the steel side had a major influence on the shear strength of the brazed joints. A maximum average shear strength of 101 ± 4 MPa was obtained for a temperature of 1050 °C and a dwell time of 5 min. © 2020, ASM International.
    view abstract10.1007/s13632-020-00663-0
  • Development and construction of AISI H11/ZrO2 joints for injection molding tools
    Tillmann, W. and Anar, N.B. and Manka, M. and Wojarski, L. and Lehmert, B.
    Welding in the World 63 (2019)
    Increasing demands in industrial applications and simultaneous efforts to provide long-lasting and cost-efficient tools in the injection molding industry lead to the use of metal–ceramic joints with the aim to combine the specific properties of both materials. Due to its high CTE, zirconium oxide (ZrO2) is used for the ceramic part and is joined with the tool steel AISI H11 (1.2343). In this work, suitable joining techniques with a low heat input and therefore a low thermal load are applied and characterized for the production of metal–ceramic composites. The selection of joining techniques is based on the boundary conditions during the injection molding process, in which the composites have to resist the temperature, pressure, as well as shear and tensile loads. Therefore, besides brazing, other joining processes such as gluing, screwing, shrinking, and clamping were analyzed as possible low temperature joining techniques for ceramic-metal-compounds. The best results for the tensile strengths with 90 MPa were achieved by a brazing process, carried out in vacuum with approximately 10−5 mbar, at a temperature of 920 °C for 5 min, using the commercially available brazing filler alloy CB4. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-019-00800-6
  • Investigations of the corrosion damage process of the brazed joint AISI 304L/BNi-2
    Tillmann, W. and Walther, F. and Manka, M. and Schmiedt, A. and Wojarski, L. and Eilers, A. and Reker, D.W.
    Results in Physics 12 (2019)
    One of the most frequently used material combinations to produce heat exchangers for automotive exhaust systems are vacuum brazed AISI 304L/BNi-2 components. In order to simulate the influence of condensate corrosion on such parts during service, the well-established test procedure for testing the resistance of metallic materials to condensate corrosion in exhaust gas, VDA 230-214, is used in this study. For a test duration of up to 6 weeks, miscellaneous corrosive mechanisms were observed and examined concerning their formation and progression. The corrosive attack starts at the diffusion zone on the base metal side due to the formation of chromium borides and the reduced nickel content. Between week 4 and 5, the greatest material removal was observed, which goes in line with a diameter reduction from 6.5 mm down to 5.8 mm of the load-bearing area. In this regard, the ultimate tensile strength drops down from 253.3 MPa to 147.8 MPa after 6 weeks of corrosive testing. © 2019 The Authors
    view abstract10.1016/j.rinp.2018.12.074
  • Vacuum brazing of 316L stainless steel based on additively manufactured and conventional material grades
    Tillmann, W. and Henning, T. and Wojarski, L.
    IOP Conference Series: Materials Science and Engineering 373 (2018)
    Many industrial applications require components with an increasing geometric complexity and specific material properties. Furthermore, the production costs and the affordable production time have to be minimized in order to ensure competitiveness. These divergent objectives are difficult to achieve with a single manufacturing technology. Therefore, joining of selective laser melted (SLM) complex shaped parts to conventionally produced high-volume components provides a high potential. The current investigation focuses on vacuum brazing conventionally manufactured to non-hipped SLM generated AISI 316L stainless steel. Cylindrical samples (o 14 mm) were brazed using a B-Ni2 foil (50 μm) at 1050 °C for 30 minutes in vacuum (< 4.5•10-5 mbar) and directly cooled down to room temperature with 4 bar overpressure to prevent the formation of chromium carbides within the base material. It could be proven that the brazing quality is extremely sensitive to even marginal porosities (< 0.2 %) and/or oxide inclusions of the SLM microstructure. Therefore, the fracture mirror in SLM/conventional steel brazements was at the joint braze/SLM steel interface, leading to a joint strength of 317.4 MPa. This corresponds to only 67.4 % of the joint strength obtained with conventional steel, where the fracture propagated through the diffusion area. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/373/1/012023
  • Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstract10.1007/s40194-016-0346-4
  • Copper-based nanostructured coatings for low-temperature brazing applications
    Lehmert, B. and Janczak-Rusch, J. and Pigozzi, G. and Zuraw, P. and La Mattina, F. and Wojarski, L. and Tillmann, W. and Jeurgens, L.P.H.
    Materials Transactions 56 (2015)
    This feasibility study demonstrates the possibility to apply nanostructured filler materials for novel low-temperature brazing applications by exploiting the size-dependent melting behavior of metals and alloys when confined to the nano-scale regime. As an example, a copper-based nanostructured brazing filler is presented, which allows metal brazing of coated Ti-6Al-4V components at 750°C, much below the bulk melting point of copper (1083°C). The copper-based nanostructured brazing fillers can be produced in the form of coatings and free-standing brazing foils. The nano-confinement of Cu is abrogated after brazing and, consequently, the brazed joints can be operated well above their reduced brazing temperatures. © 2015 The Japan Institute of Metals and Materials.
    view abstract10.2320/matertrans.MI201419
  • Mechanical and microstructural analysis of ultrasonically assisted induction-brazed TiAl6V4 joints
    Tillmann, W. and Zimpel, M. and Dias, N.F.L. and Pfeiffer, J. and Wojarski, L. and Xu, Z.
    Welding in the World 59 (2015)
    This paper focuses on the process of ultrasonically assisted induction brazing with regard to titanium brazing. The titanium alloy TiAl6V4 was brazed using an aluminum-based filler alloy (AlMg2.5Cr0.3). It was apparent that the layer thickness of the brazing foil as well as the brazing temperature and the intensity of the ultrasound are significant influencing factors of the combined brazing process and microstructure. It is the aim of this paper to draw conclusions from the microstructural and mechanical investigations of the brazed joint about the process parameters, which are crucial for the properties and quality of the joint. The evaluation of the microstructure of the joint was conducted by means of metallographic investigations and results obtained by means of scanning electron microscopy. Besides mechanical microhardness measurements, strength investigations were conducted in order to evaluate the quality of the joint. Furthermore, the results of conventional vacuum brazing processes were correlated in order to be able to better facilitate and understand the adapted induction brazing process. © 2015, International Institute of Welding.
    view abstract10.1007/s40194-015-0260-1
  • The Apparent Contact Angle and Wetted Area of Active Alloys on Silicon Carbide as a Function of the Temperature and the Surface Roughness: A Multivariate Approach
    Tillmann, W. and Pfeiffer, J. and Wojarski, L.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    Despite the broad field of applications for active filler alloys for brazing ceramics, as well as intense research work on the wetting and spreading behavior of these alloys on ceramic surfaces within the last decades, the manufactured joints still exhibit significant variations in their properties due to the high sensitivity of the alloys to changing brazing conditions. This increases the need for investigations of the wetting and spreading behavior of filler alloys with regard to the dominating influences combined with their interdependencies, instead of solely focusing on single parameter investigations. In this regard, measurements of the wetting angle and area were conducted at solidified AgCuTi and CuSnTi alloys on SiC substrates. Based on these measurements, a regression model was generated, illustrating the influence of the brazing temperature, the roughness of the faying surfaces, the furnace atmosphere, and their interdependencies on the wetting and spreading behavior of the filler alloys. It was revealed that the behavior of the melts was significantly influenced by the varied brazing parameters, as well as by their interdependencies. This result was also predicted by the developed model and showed a high accuracy. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-015-2938-9
  • Examination of the porosity in Reactive Air Brazed joints by ultrasonic testing
    Tillmann, W. and Sievers, N. and Pfeiffer, J. and Wojarski, L. and Zielke, R. and Poenicke, A. and Schilm, J.
    Advanced Engineering Materials 16 (2014)
    Reactive air brazing (RAB) has been developed as a method to join ceramics and steel, using CuO as a reactive agent that interacts with the surface of the ceramic, enabling a wetting by the molten filler metal. A major benefit of this method is the fact that the joining process can be carried out in an ambient atmosphere, in contrast to active brazing processes, which need to be performed in a vacuum furnace. In the past, several investigations were conducted to improve the mechanical bonding properties for both methods. A sealed gas-tightness is also important for innovative applications such as solid oxide fuel cells. In this regard, the reduction of porosity, which is necessary in order to achieve reproducible joints with a long lifetime, presents a challenge. Therefore, it is necessary to conduct fundamental analyses of the driving forces and mechanisms of the formation of voids in the interfacial area to guarantee a reliable joint quality. In this study, the authors used an ultrasonic testing method in the realm of the immersion technique to evaluate the porosity in brazements produced with varying process parameters. A major goal was to assess the influence of the cooling stage on the pore formation. The advantage of this non-destructive method is the possibility to scan the entire joint area using just one scan. The investigations were flanked by SEM analyses on different cross-sections. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201400055
  • Reactive transient liquid phase bonding of ceramic to steel using Zr-Cu-Zr- and Zr-Ni-Cu-Zr-interlayers for high temperature applications
    Tillmann, W. and Pfeiffer, J. and Wojarski, L. and Indacochea, J.-E.
    Materialwissenschaft und Werkstofftechnik 45 (2014)
    Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion-based melting is that the involved materials have low melting alloy-constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless-steel-joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy-constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 μm-thick Nickel-foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano-hardness measurements of the joining area as well as shear strength and fractography are presented. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201400267
  • TLP brazing of aluminum to steel using PVD-deposited interlayer
    Wojarski, L. and Tillmann, W.
    Welding in the World 58 (2014)
    The demand for hybrid material concepts is steadily growing, and especially dissimilar joints between aluminum and steel are, due to their wide dissemination, of major importance. The main obstacle for the fabrication of aluminum-steel joints using thermal processes is the embrittlement of the fusion area. In order to prevent direct contact between aluminum and iron and thus to suppress the formation of brittle iron aluminides, 3-μm thick diffusion barrier coatings, consisting of Ni or Ti, were applied onto the steel surface. Pure copper with a thickness of 3 and 6 μm, respectively, was used as a filler material, and the samples were brazed in a TLP process in a vacuum at 580 °C at varying dwell times (10...50 min). The samples brazed with Ni diffusion barriers showed a considerable formation of Fe2Al 5 even at low dwell times. Furthermore, additional complex ternary phase bands have generated due to the existence of diffusion barrier elements and were detected in the interfacial area. The application of Ti showed a significant decrease of iron aluminides, and no Fe2Al5 could be detected at low dwell times, resulting in a shear strength of 42 MPa for the optimized parameters. © 2014 International Institute of Welding.
    view abstract10.1007/s40194-014-0143-x
  • Brazing of cutting materials
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Advances in Brazing: Science, Technology and Applications (2013)
    An important brazing application is the production of cutting and machining tools. This chapter provides an overview of brazing alloys for the joining of cutting materials. Process-related problems and solutions are presented and discussed. High strength values can be achieved with the right filler alloy and joint design.Most cemented carbides consist of a hard material such as tungsten carbide and a metallic binder. Due to the high carbide content, wetting and bonding reactions have to be monitored. Addition of active elements can help to induce a wetting reaction. Brazing ceramic cutting materials requires the use of active filler alloys to achieve a wetting reaction. The chapter reviews the alloys required. A joint can be weakened by interfacial effects or mismatches in physical properties. Employing the right filler metal with an adapted brazing process can significantly improve joint quality. Stress calculations based on the finite element method contribute to better joint design with reduced stress levels. © 2013 Woodhead Publishing Limited. All rights reserved.
    view abstract10.1533/9780857096500.3.423
  • Joining of aluminium sheets by combined solid state and TLP bonding processes
    Lee, E. and Quintana, O. and Indacochea, J.E. and Wojarski, L. and Pfeiffer, J. and Tillmann, W.
    Science and Technology of Welding and Joining 18 (2013)
    Our effort is to develop an innovative process to join Al sheets at moderate temperatures (200-300uC) by accumulative roll bonding (ARB) combined with transient liquid phase bonding utilising Ga. Solid state joining of Al is limited by the surface Al2O3 that hampers the Al atom diffusion despite the intimate contact between the metal sheets. Mechanical brushing of the Al2O3 improves bonding. Our approach uses ARB to break the brittle Al2O3 by roll bonding two sheets. Strips of 1100-Al were cut, cleaned coated with a thin film of Ga and then preheated before rolling. The Ga coated Al strips roll bonded at 200uC showed greatest strengths, but processing at 300uC resulted in lower strengths with evidence of liquid metal embrittlement. The undamaged Al2O3 prevents liquid Ga from contact with the Al metal. © 2013 Institute of Materials, Minerals and Mining.
    view abstract10.1179/1362171812Y.0000000087
  • Vacuum brazing titanium using thin nickel layer deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Janczak-Rusch, J. and Tillmann, W.
    Materials Science and Engineering A 565 (2013)
    In this study, the evolution of the interfacial microstructure, hardness distribution, and the joint strength of vacuum brazed commercially pure titanium were evaluated. A thin nickel layer, with different thicknesses, was deposited by PVD technique to serve as the brazing filler metal. Test joints were processed at temperatures of 910°C and 960°C using a soaking time of 15 and 90min. The experimental results showed that sound joints with a good wetting quality as well as lack of pores and cracks can be achieved at a brazing temperature of 960°C. A Ti2Ni intermetallic compound was formed at the interfacial area at a soaking time of 15min and with a deposition rate of 90AH which was detrimental to the joint mechanical properties. Meanwhile, at a soaking time of 90min, intermetallic compound was not detected and the diffusion of nickel was completed at all deposition rates which improve the shear strength of the joints. © 2012 Elsevier B.V..
    view abstract10.1016/j.msea.2012.12.028
  • Evaluation of corrosion performance of titanium/steel joint brazed by Cu-based filler metal
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 (2012)
    Furnace vacuum brazing has been employed to join commercially pure titanium and low carbon steel using copper-based filler metal with the composition of Cu-10.6Mn-1.9Ni, at.%. Three different brazing temperatures 930, 970, and 1000 °C and a holding time of 15 min were studied and evaluated. The corrosion behavior of the joint in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. Experimental results showed that severe corrosion attack of the steel side at the interfacial area is clearly observed. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of results was identical to a large extent. Corrosion resistance of the joint showed a general tendency to increase with rising brazing temperature. The lowest corrosion rate was obtained for the couple bonded at 1000 °C. Meanwhile, at the lowest joining temperature of 930 °C, corrosion rate showed a higher value. The results of joints corrosion resistance were attributed to the difference in microstructure features and chemical analysis. © ASM International.
    view abstract10.1007/s11665-012-0149-8
  • Preliminary investigation on brazing performance of Ti/Ti and Ti/steel joints using copper film deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 (2012)
    Microstructural aspects and bonding characteristics of vacuum brazed Ti/Ti and Ti/steel were investigated. A thin-copper film, with different thicknesses, was deposited on the brazed metals by physical vapor deposition technique to serve as a brazing filler metal. Test joints were processed at a temperature of 910°C and 15 min holding time. The resultant joints were characterized to determine the brittle intermetallic compound in the interfacial layer and the shear strength of the joints were tested. Our preliminary experimental results showed that sound joints with a good wetting quality, lack of pores and cracks can be achieved. Intermetallic phases such as Ti 2Cu, TiCu, FeTi, and Fe 2Ti were predicted from the chemical analyses. The Ti/Ti joints achieved a higher shear strength than the Ti/steel joints and there is a tendency for the tension shear strength to increase when a thick Cu-deposited layer is used. © ASM International.
    view abstract10.1007/s11665-012-0121-7
  • Investigation on corrosion of titanium/steel brazed joint
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    Furnace vacuum brazing has been employed to join commercially pure titanium alloy and low carbon steel using a silver-based filler metal with a composition of Ag-Cu34-Ti2 (wt%). Three different brazing temperatures (850°C, 880°C, 930°C) and two holding times (5 and 15 min) were applied and evaluated. The corrosion behavior of the joints in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. The results indicated that a severe corrosion attack at the interfacial area of the steel side took place. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of the results was identical to a large extent. The corrosion resistance of the joint showed a general tendency to increase with an increasing brazing temperature and holding time. Therefore, the joints produced at a temperature of 930°C and a holding time of 15 min produced the best result concerning the corrosion behavior. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000684
  • Toward process optimization in laser welding of metal to polymer
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    The joining technology of dissimilar lightweight materials between metals and polymer is essential for realizing cars with hybrid structures and for other engineering applications. These types of joints are still difficult to generate and their behaviour is not fully understood. Laser welding offers specific process advantages over conventional technologies, such as short process times, while providing optically and qualitatively valuable weld seams and imposing minimal thermal stress. Furthermore, the process is compatible with automation. This paper summarizes the efforts to attain suitable joint strengths with the stainless steel plate type S30400 and a Polyethylene Terephtalate Glycol (PETG) plastic sheet. The study considers the optimization of two important process parameters, namely laser power, and welding speed. Microstructure features, test of tensile shear strength, investigation of the fracture location, and morphology were used to evaluate the joint performance. The result indicates that there is an optimum value for laser power, which achieves a sufficient melting and heat transfer to the joint without decomposing the plastic sheet and hence, enables to obtain high joint strength. Moreover, a low welding speed is preferable in most combinations of welding parameters since it achieves an adequate melting and wetting of the polymer to the steel surface. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000674
  • coatings

  • mechanical properties

  • physical vapor deposition

  • x-ray diffraction

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