An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

Transient Liquid phase bonding (TLP) has evolved as a viable method of joining difficult-to-weld superalloys with potential of producing joints with comparable mechanical properties to the base material. Although the high temperature properties of aerospace superalloys have been studied extensively, there is little information on the corrosion behaviour of these special class of materials that had been subjected to TLP bonding. In this work, electrochemical assessment of the corrosion behaviour of TLP bonded nickel-based superalloy was performed. Microstructural evaluation of the TLP bonded joint revealed the presence of a centreline eutectic when isothermal solidification was not completed and the corrosion resistance increased with a decrease in this eutectic width. The use of a composite interlayer produces TLP joints with smaller eutectic size and results in complete isothermal solidification in shorter processing time. Complete isothermal solidification, achieved with the composite interlayer, results in a uniform chromium distribution in the joint centre and produced a corrosion performance similar to the as-received cast base metal. It was found that aside from the mere presence of chromium, which is widely recognised as necessary for corrosion resistance, its uniform distribution within the joint region is imperative for achieving adequate corrosion resistance in TLP joints. / May 2016

Dissertation (Ph.D.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Yes / Diffusion brazing has gained much popularity as a technique capable of joining dissimilar lightweight metal alloys and has the potential for a wide range of applications in aerospace and transportation industries, where microstructural changes that will determine the mechanical and chemical properties of the final joint must be controlled. This study explores the effect of Al2O3 nanoparticles on the mechanical and microstructural properties of diffusion brazed magnesium (AZ31) and aluminium (Al-1100) joints. The results showed that the addition of Al2O3 nanoparticle to the electrodeposited Cu coating increased the volume of eutectic liquid formed at the interface which caused a change to the bonding mechanism and accelerated the bonding process. When the Cu/Al2O3 nanocomposite coatings were used as the interlayer, a maximum bond strength of 46 MPa was achieved after 2 min bonding time while samples bonded using pure-Cu interlayers achieved maximum strength after 10 min bonding time. Chemical analysis of the bond region confirmed that when short bonding times are used, the intermetallic compounds formed at the interface are limited to the compounds consumed in the eutectic reaction.

Oxide dispersion strengthened (ODS) alloys have been developed with unique mechanical properties. However, in order to achieve commercial application an appropriate joining process is necessary which minimizes disruption to the alloy microstructure. Transient liquid phase (TLP) bonding is a promising joining method, but previous work has shown that the segregation of dispersoids within the joint region results in bonds with poor mechanical strengths. This research work was undertaken to further explore particulate segregation at the joint region when TLP bonding and to develop bonding techniques to prevent it. A Ni-Cr-Fe-Si-B interlayer was used to bond an alloy MA 758. The effects of parent alloy grain size, bonding temperature, and external pressure on the TLP bonding process were investigated. Three melting stages were identified for the interlayer, and the bonding temperature was chosen so that the interlayer was in the semi-solid state during bonding. This novel bonding mechanism is described and applied to counteract the segregation of Y203 dispersoids. The grain size of the parent alloy does not alter the particulate segregation behaviour. It is concluded that a low bonding temperature with moderate pressure applied during bonding is preferable for producing bonds with less disruption to the microstructures of the parent alloy. Joint shear tests revealed that a near parent alloy strength can be achieved. This study also shed some light on choosing the right bonding parameters suitable for joining the complicated alloy systems. A Ni-P interlayer was also used to bond the ODS alloy. Microstructural examination indicated that a thin joint width and less disruption to the parent grain structure were achieved when bonding the alloy in the fine grain state. The time for isothermal solidification was found to be shorter when compared with bonds made with the parent alloy in the recrystallized state. All these observations were attributed to the greater diffusivity of P along the grain boundaries than that of the bulk material. A high Cr content within the parent alloy changes the mechanism of the bonding process. The diffusion of Cr into the liquid interlayer has the effect of raising the solidus temperature, which not only accelerates the isothermal solidification process, but also reduces the extent of parent alloy dissolution.

Dissertation (Ph.D.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references.

Transient liquid phase (TLP) bonding has proven to be the preferred method for joining extremely difficult-to-weld advanced materials, including similar and dissimilar superalloys. In this work, an approach that combines experiments and theoretical simulations are used to investigate the effect of temperature gradient (TG) in a vacuum furnace on the temperature distribution in TLP bonded samples. When joining similar materials by this technique, the simulated results with experimental verifications show that, irrespective of where the samples are placed inside the vacuum furnace, a TG in the furnace can translate into a symmetric temperature distribution in bonded samples provided the diffusion direction is parallel to the source of heat emission. In addition, the effects of TLP bonding parameters on the joint microstructure were investigated during the joining of nickel-based IN738 and CMSX-4 single crystal (SX) superalloys. An increase in holding time and reduction in gap size reduces the width of eutectic product that forms within the joint region. It was also found that Liquid-state diffusion (LSD) can occur and have significant effects on the microstructure of dissimilar TLP bonded joints even though its influence is often ignored during TLP bonding. The occurrence of LSD produced single crystal joint when a SX and polycrystal substrate were bonded. This formation of a SX joint which cannot be exclusively produced by solid-state diffusion has not been previously reported in the literature. / February 2017

The problem of inadequate measurement techniques for quantifying the isothermal solidification process during transient liquid phase sintering (TLPS) in binary isomorphous systems such as Ni-Cu, and the resulting uncertainty regarding the solidification mechanism and its sensitivity to important process parameters, has been investigated. A unique combination of differential scanning calorimetry (DSC), neutron diffraction (ND), and metallographic techniques has enabled the quantitative characterization of important TLPS stages (i.e., solid-state sintering, melting and dissolution, isothermal solidification, and homogenization) as well as verifying the re-melt behaviour of post-sintered specimens and measuring variable melting point (VMP) properties. This has resulted in the advancement of the fundamental understanding of liquid formation and its removal mechanism during isothermal, or diffusional, solidification. The Ni-Cu system was chosen for experimentation due to its commercial relevance as a braze filler material and also because it is an ideal model system (due to its isomorphous character) that is not well understood on a quantitative or phenomenological basis. Samples consisted of elemental Ni and Cu powder mixtures of varying particle size and composition. In DSC experiments, the progress of isothermal solidification was determined by measuring the enthalpy of melting and solidification after isothermal hold periods of varying length and comparing these to the measured enthalpy of pure Cu. The low melting enthalpies measured for all Ni/Cu mixtures heated just past the Cu melting point (1090°C) indicate that solid-state sintering and interdiffusion during heat-up significantly suppress initial liquid formation and densification from the wetting liquid. For samples heated well past the Cu melting point (1140°C), Ni dissolution causes increased initial liquid fractions and densification. It was found that significantly more time was required for complete liquid removal at 1140°C vs. 1090°C. This is attributed to the observed increase in initial liquid fractions formed at higher processing temperatures due to the dissolution of Ni. This effectively counteracts the increased diffusivities at these temperatures, and thus more time is required to completely remove the increased liquid content. TLP mixtures sintered at 1140°C using three different particle sizes revealed that fine base metal Ni particles cause high degrees of solid-state interdiffusion during heat-up, small initial liquid fractions, and accelerated liquid removal rates due to high surface area/volume ratios. A diffusion-based analytical model was developed to account for these effects (i.e., particle size, temperature, solid-state sintering, and dissolution). Comparison with experimental DSC results reveals that this model can accurately predict liquid removal given accurate diffusivities. Metallographic analysis of post-sintered DSC specimens via SEM and EDS indicates that isothermal liquid solidification leaves behind Ni-rich cores surrounded by Cu-rich matrix regions having compositions given by the Ni-Cu phase diagram solidus (CS) at a selected isothermal processing temperature (TP). ND experiments were used to investigate the melting event and interdiffusion during the isothermal hold segment by analyzing the evolution of the {200} FCC peaks of Ni and Cu. ND patterns were collected in situ at 1 minute intervals during prolonged sintering cycles for larger powder specimens. The Cu melting event was characterized by an abrupt decrease in Cu peak intensity at 1085°C as well as a shift towards higher 2 angles corresponding to lower Cu contents. This shifted residual peak (hereafter referred to as the CS peak) originates from regions of the specimen having compositions near solidus at TP. Immediately following the melting event, the evolution of ND patterns shows that these CS peaks grow rapidly, indicating the isothermal growth of a Cu-rich phase. These in situ findings confirmed the metallographic and DSC data and indicated that isothermal solidification of the liquid phase proceeds via the growth of a solute-rich solid solution layer surrounding the Ni particles. This occurs by the transient progression of the solid/liquid interface at compositions given by the liquidus and solidus (CS/CL). During sintering, diffraction intensities gradually increased at intermediate 2 angles between previous Ni and Cu peaks. ND patterns gradually evolved from initially having a broad double-peak profile to a sharper single-peak profile due to increased Ni-Cu interdiffusion. The 2position and width of the post-sintered peaks indicated very homogeneous sintered alloys. Metallographic analysis of post-sintered specimens having undergone prolonged sintering and homogenization revealed extensive Kirkendall pore formation from unequal diffusivities (DCu > DNi). In this study, the unique combination of diffusion-based modelling as well as DSC, ND, and supporting metallographic analysis has enabled the identification of characteristic sintering behaviour, important process parameters, and processing windows for TLPS in Ni-Cu systems. Quantitative and in situ information of this nature is absent in the previous TLPS literature.

The problem of inadequate measurement techniques for quantifying the isothermal solidification process during transient liquid phase sintering (TLPS) in binary isomorphous systems such as Ni-Cu, and the resulting uncertainty regarding the solidification mechanism and its sensitivity to important process parameters, has been investigated. A unique combination of differential scanning calorimetry (DSC), neutron diffraction (ND), and metallographic techniques has enabled the quantitative characterization of important TLPS stages (i.e., solid-state sintering, melting and dissolution, isothermal solidification, and homogenization) as well as verifying the re-melt behaviour of post-sintered specimens and measuring variable melting point (VMP) properties. This has resulted in the advancement of the fundamental understanding of liquid formation and its removal mechanism during isothermal, or diffusional, solidification. The Ni-Cu system was chosen for experimentation due to its commercial relevance as a braze filler material and also because it is an ideal model system (due to its isomorphous character) that is not well understood on a quantitative or phenomenological basis. Samples consisted of elemental Ni and Cu powder mixtures of varying particle size and composition. In DSC experiments, the progress of isothermal solidification was determined by measuring the enthalpy of melting and solidification after isothermal hold periods of varying length and comparing these to the measured enthalpy of pure Cu. The low melting enthalpies measured for all Ni/Cu mixtures heated just past the Cu melting point (1090°C) indicate that solid-state sintering and interdiffusion during heat-up significantly suppress initial liquid formation and densification from the wetting liquid. For samples heated well past the Cu melting point (1140°C), Ni dissolution causes increased initial liquid fractions and densification. It was found that significantly more time was required for complete liquid removal at 1140°C vs. 1090°C. This is attributed to the observed increase in initial liquid fractions formed at higher processing temperatures due to the dissolution of Ni. This effectively counteracts the increased diffusivities at these temperatures, and thus more time is required to completely remove the increased liquid content. TLP mixtures sintered at 1140°C using three different particle sizes revealed that fine base metal Ni particles cause high degrees of solid-state interdiffusion during heat-up, small initial liquid fractions, and accelerated liquid removal rates due to high surface area/volume ratios. A diffusion-based analytical model was developed to account for these effects (i.e., particle size, temperature, solid-state sintering, and dissolution). Comparison with experimental DSC results reveals that this model can accurately predict liquid removal given accurate diffusivities. Metallographic analysis of post-sintered DSC specimens via SEM and EDS indicates that isothermal liquid solidification leaves behind Ni-rich cores surrounded by Cu-rich matrix regions having compositions given by the Ni-Cu phase diagram solidus (CS) at a selected isothermal processing temperature (TP). ND experiments were used to investigate the melting event and interdiffusion during the isothermal hold segment by analyzing the evolution of the {200} FCC peaks of Ni and Cu. ND patterns were collected in situ at 1 minute intervals during prolonged sintering cycles for larger powder specimens. The Cu melting event was characterized by an abrupt decrease in Cu peak intensity at 1085°C as well as a shift towards higher 2 angles corresponding to lower Cu contents. This shifted residual peak (hereafter referred to as the CS peak) originates from regions of the specimen having compositions near solidus at TP. Immediately following the melting event, the evolution of ND patterns shows that these CS peaks grow rapidly, indicating the isothermal growth of a Cu-rich phase. These in situ findings confirmed the metallographic and DSC data and indicated that isothermal solidification of the liquid phase proceeds via the growth of a solute-rich solid solution layer surrounding the Ni particles. This occurs by the transient progression of the solid/liquid interface at compositions given by the liquidus and solidus (CS/CL). During sintering, diffraction intensities gradually increased at intermediate 2 angles between previous Ni and Cu peaks. ND patterns gradually evolved from initially having a broad double-peak profile to a sharper single-peak profile due to increased Ni-Cu interdiffusion. The 2position and width of the post-sintered peaks indicated very homogeneous sintered alloys. Metallographic analysis of post-sintered specimens having undergone prolonged sintering and homogenization revealed extensive Kirkendall pore formation from unequal diffusivities (DCu > DNi). In this study, the unique combination of diffusion-based modelling as well as DSC, ND, and supporting metallographic analysis has enabled the identification of characteristic sintering behaviour, important process parameters, and processing windows for TLPS in Ni-Cu systems. Quantitative and in situ information of this nature is absent in the previous TLPS literature.