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Characterization of Dissimilar Ti-6Al-4V/IN718 TLPB Joints using Pure Copper as Interlayer

Kasaei, Ali | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56733 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid
  7. Abstract:
  8. Successfully bonding nickel-base alloys to titanium-base alloys has proven to be one of the most challenging modern joints. Traditional welding methods for dissimilar titanium/nickel joints often lead to cracking in the fusion and heat-affected(HAZ)zones; due to the formation of brittle intermetallic compounds with low fracture toughnesses. Additionally, the significant difference in thermal expansion coefficients between titanium and nickel introduces significant thermal stress during bonding process, exacerbating the current issue. Creating successful dissimilar Ti/Ni joints requires overcoming the formation of continuous brittle intermetallic compounds (IMCs). Brazing stands out as a prominent alternative method, offering the possibility of good bonds regarding materials with a lesser degree of weldability. Unlike conventional fusion welding, brazing employs a filler alloy with melting depressant elements (MPD) that have a lower melting temperature than the base materials. This prevents complete melting and direct mixing of the base metals liquids. A relatively new and appealing method in brazing is the use of transient liquid phase bonding (TLPB). This approach enables the production of joints with shear strength equivalent to the base materials. In this research, the mechanism of melt formation was initially evaluated through the study of interfacial reactions and the identification of joint microstructures during the heating and initial stages of the bonding heat cycle. Assembled joint samples were investigated in both commercially pure (Ti/Cu/Ti and Ti/Cu/Ni) and alloyed (Ti-6Al-4V/Cu/IN718) states at two diffrent bonding temperatures(900℃ and 1050℃). Subsequently, the effect of holding time at these temperatures on the microstructural sequence and phase stability within the 15-75 minute range was examined. The analysis of microstructure and phase transformations resulting from bonding time and temperature was conducted using optical microscopy (OM), field emission electron microscopy (FESEM), and energy diffraction spectroscopy (EDS). Furthermore, the influence of phase and microstructural characteristics on joint mechanical properties was assessed through tensile-shear and hardness tests. In order to gain a better understanding of the produced phases, an analysis of the thermodynamic and kinetic behavior of the heat-affected zone (HAZ) of the IN718 base metal was performed using Thermo-Calc software and the Dictera module. In order to gain a better understanding of the produced phases, the thermodynamic and kinetic behavior of the IN718 heat-affected zone (HAZ) was modeled and analyzed using Thermo-Calc software and the Dictera module, followed by a comparison with existing microstructural data. The study revealed that melt formation occurred through interreactions at the titanium/copper interface during heating and the initial bonding stages. Eutectoid microstructures were observed in titanium profiles of joints, both in pure and alloyed joints, due to the eutectoid decomposition of β-Ti during cooling. In commercially pure titanium joints, those produced at 900°C displayed significantly lower shear strength due to the stability of 〖Ti〗_2 Cu and TiCu intermetallic compounds. By increasing holding time to up to 75 minutes, at both temperatures, there was an improvement in the maximum joint shear strength. In the Ti/Cu/Ti system, the highest strength was achieved in samples produced at1050°C and 75 minutes due to the homogenization of copper concentration in the bonding region. The maximum shear strength measured at 900°C and 1050°C was 84 MPa and 307 MPa, respectively. In the case of dissimilar systems like Ti/Cu/Ni and Ti-6Al-4V/Cu/IN718, joints produced at 900°C exhibited significantly higher shear strength. When investigating dissimilar joints between titanium and nickel in the pure state, crack growth was predominantly observed in the nickel-based half-joint and propagated through continuous 〖Ti〗_2 Ni intermetallic phase
  9. Keywords:
  10. Transient Liquid Phase Boneling (TLPB) ; Intermetallic Compounds ; Dissimilar Joint ; Inconel 718 Nickel Based Superalloy ; Pure Copper ; Thermodynamic Modeling ; Copper Powder Interlayer

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