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.

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 primary goals of the research in this dissertation are to perform a systematic study to identify and understand the fundamental cause of prolonged processing time during transient liquid phase bonding of difficult-to-bond single crystal Ni-base materials, and use the acquired knowledge to develop an effective way to reduce the isothermal solidification time without sacrificing the single crystalline nature of the base materials. To achieve these objectives, a multi-scale numerical modeling approach, that involves the use of a 2-D fully implicit moving-mesh Finite Element method and a Cellular Automata method, was developed to theoretically investigate the cause of long isothermal solidification times and determine a viable way to minimize the problem. Subsequently, the predictions of the theoretical models are experimentally validated. Contrary to previous suggestions, numerical calculations and experimental verifications have shown that enhanced intergranular diffusivity has a negligible effect on solidification time in cast superalloys and that another important factor must be responsible. In addition, it was found that the concept of competition between solute diffusivity and solubility as predicted by standard analytical TLP bonding models and reported in the literature as a possible cause of long solidification times is not suitable to explain salient experimental observations. In contrast, however, this study shows that the problem of long solidification times, which anomalously increase with temperature is fundamentally caused by departure from diffusion controlled parabolic migration of the liquid-solid interface with holding time during bonding due to a significant reduction in the solute concentration gradient in the base material. Theoretical analyses showed it is possible to minimize the solidification time and prevent formation of stray-grains in joints between single crystal substrates by using a composite powder mixture of brazing alloy and base alloy as the interlayer material, which prior to the present work has been reported to be unsuitable. This was experimentally verified and the use of the composite powder mixture as interlayer material to reduce the solidification time and avoid stray-grain formation during TLP bonding of single crystal superalloys has been reported for the first time in this research.

The primary goals of the research in this dissertation are to perform a systematic study to identify and understand the fundamental cause of prolonged processing time during transient liquid phase bonding of difficult-to-bond single crystal Ni-base materials, and use the acquired knowledge to develop an effective way to reduce the isothermal solidification time without sacrificing the single crystalline nature of the base materials. To achieve these objectives, a multi-scale numerical modeling approach, that involves the use of a 2-D fully implicit moving-mesh Finite Element method and a Cellular Automata method, was developed to theoretically investigate the cause of long isothermal solidification times and determine a viable way to minimize the problem. Subsequently, the predictions of the theoretical models are experimentally validated. Contrary to previous suggestions, numerical calculations and experimental verifications have shown that enhanced intergranular diffusivity has a negligible effect on solidification time in cast superalloys and that another important factor must be responsible. In addition, it was found that the concept of competition between solute diffusivity and solubility as predicted by standard analytical TLP bonding models and reported in the literature as a possible cause of long solidification times is not suitable to explain salient experimental observations. In contrast, however, this study shows that the problem of long solidification times, which anomalously increase with temperature is fundamentally caused by departure from diffusion controlled parabolic migration of the liquid-solid interface with holding time during bonding due to a significant reduction in the solute concentration gradient in the base material. Theoretical analyses showed it is possible to minimize the solidification time and prevent formation of stray-grains in joints between single crystal substrates by using a composite powder mixture of brazing alloy and base alloy as the interlayer material, which prior to the present work has been reported to be unsuitable. This was experimentally verified and the use of the composite powder mixture as interlayer material to reduce the solidification time and avoid stray-grain formation during TLP bonding of single crystal superalloys has been reported for the first time in this research.

L'axe de recherche suivi dans cette thèse comprend principalement. L'élaboration, les caractérisations structurale par rayons X (XRD), microstructurale par microscopies électroniques (SEM et EBSD), chimique (EDS et EPMA) et mécanique (essais de traction et dureté) de jonctions de pièces d'acier au carbone à travers le procédé appelé ‘Transient Liquid Phase Bonding' (TLPB), en utilisant comme matériel d'apport des rubans amorphes des systèmes Fe-B-Si et Fe-B, et des feuilles de Cu.Les jonctions TLPB ont été obtenus en chauffant les pièces à unir à une température de 1300ºC, qui est maintenue pendant 7 min, en même temps qu'on applique une pression de 5 MPa.Les résultats EBSD et SEM montrent que lorsque des rubans amorphe de Fe-B-Si sont utilisés comme matériel d'apport, on observe dans la zone de jonction des tubes une microstructure caractérisée par des grains de ferrite alors que dans la zone affectée par la chaleur (Heat Affected Zone, HAZ), on observe une microstructure ferritique-perlitique. Les grains de ferrite de la jonction ne sont généralement pas partagés avec ceux de la HAZ et sont clairement délimités par des bords de grains. Grâce aux profils de compositions obtenus par EDS et EPMA, on peut montrer que le jonction s'enrichit en Si et s'appauvrit en Mn. Cette microsegregation de Si et Mn produite par le procédé TLPB fait de la jonction une région de formation prématurée de ferrite au bord des grains de l'austénite de la HAZ. Après l'austénite de la HAZ se transforme au refroidissement pour former une structure ferritique/perlitique, qui contraste avec la jonction. Les propriétés mécaniques, montrent que la fracture se produit dans la HAZ loin de la jonction. Les mesures de dureté dans la jonction et la HAZ sont en accord avec les microstructures observées.Une étude complémentaire sur des régions avec une solidification isothermique incomplète montre que dans une première étape la phase primaire qui solidifie est pareille à celle du procédé TLPB et ensuite d'autre phases apparaissent. La phase métastable Fe23B6, a pu être détecté par une expérience de microdiffraction XRD (ID27, ESRF en Grenoble).Lorsque l'on utilise des rubans amorphes de Fe-B comme matériel d'apport, on ne distingue pas clairement les microstructure de la jonction de celle de la HAZ. Les grains de ferrite de la jonction sont partagés avec ceux de la HAZ et on peut visualiser une solidification épitaxiale dans la jonction à partir des grains de la HAZ. Les propriétés mécaniques, montrent que la résistance à la traction est d'au moins 88% de la valeur des pièces métalliques. Dans ce cas la rupture se produit à la jonction bien que les valeurs de dureté correspondent à celle attendus pour les microstructures présentes.Finalement , lorsque une feuille de Cu est utilisé comme matériaux d'apport on observe des microstructures similaires pour la jonction et la HAZ. Près de la surface on observe une porosité du à l'effet Kirkendall (le Cu de la jonction diffuse dans la pièce métallique plus rapidement que le Fe de celle-ci diffuse dans la jonction ce qui génère un flux de lacunes vers la jonction d'où sa porosité). Cet effet est moins marqué (moins de porosité) loin des bords car la pression au niveau de la jonction est plus grande. Ceci indique la haute sensibilité de l'effet Kirkendall avec la pression. Les propriétés mécaniques montrent que la résistance à la traction est d'au moins 85% de la valeur des pièces métalliques et la rupture se produit à la jonction. La rupture est lié à la présence de phases secondaires du à l'abondance de régions avec une solidification isothermique incomplète (ces régions cèdent sous tractions ce qui réduit l'aire efficace lors de l'essai entrainant la rupture par surcharge). Les mesures de dureté dans la jonction et la HAZ sont en accord avec les microstructures observées. / The main scientific activities carried out in this thesis includes: The structural characterization by X-Ray diffraction (XRD), microstructure analysis by electron microscopy (SEM and EBSD), chemical analysis (EDS and EPMA) and mechanical testing - tensile and hardness tests - of the joints of bonded carbon steel parts by means of the Transient Liquid Phase Bonding (TLPB) process, using as filler materials amorphous ribbons of Fe-B and Fe-Si-B systems, and Cu foils.The TLPB bonded joints were obtained by heating the assembly to a temperature of 1300ºC, which is maintained for 7 min, at the same time a pressure of 5 MPa is applied.The results obtained both by SEM and EBSD show that when amorphous Fe-Si-B ribbons are used as filler material, at the joint of the bonded parts a microstructure consisting of ferrite grains is observed, in contrast with ferritic-pearlitic microstructure at the heat affected zone (HAZ).The ferrite grains at the joint are not generally shared with those of the HAZ, and are clearly delimited by grain boundaries. The composition profiles obtained both by EDS and EPMA show that the joint is enriched in Si and is depleted in Mn. During cooling, this microsegregation of Mn and Si produced by the TLPB makes the joint a region where ferrite is formed prematurely at austenite grains boundaries of the HAZ. Afterwards, the austenite of the HAZ transforms to form a ferritic/pearlitic microstructure, which contrasts with that of the joint. The tensile tests of specimens from the bonded parts show that the fracture occurs in the HAZ, far from the junction. Hardness measurements both at the joint and at the HAZ are consistent with the observed microstructures.A complementary study at the joint was carried out where the isothermal solidification completion was not achieved. During cooling, at a first stage the phase which solidifies is the same than that during the TLPB process. Finally, the appearance of other phases takes place. The metastable phase Fe23B6 was detected by X-Ray microdiffraction (ID27, ESRF at Grenoble).When amorphous Fe-B ribbons are used as filler material, there is no clear distinction between the microstructure at the joint and at the HAZ. The ferrite grains at the joint are shared with those of the HAZ, and epitaxial solidification of these grains can be visualized from the grains of the HAZ.When tensile tested, the bonded parts attain at least 88% of the ultimate tensile strength (UTS) of the base metal. In this case, fracture occurred at the joint, although the values of hardness correspond to those expected for the observed microstructures.Finally, when Cu foils are used as filler material, the microstructure observed at the joint is similar to that of the HAZ. Close to the outer surface, porosity due to Kirkendall effect is observed (the Cu of the joint diffuses into the base metal faster than the Fe into the joint, which generates a flow of vacancies towards the joint, thus developing porosity). This effect is less pronounced (less porosity) away from the outer surface where the pressure at the joint is larger. This indicates the high sensitivity of the Kirkendall effect with pressure. The tensile test shows that the joint attains at least 85% of the UTS of the base metal, and that it fails at the joint. The latter is related to the abundance of secondary phases due to an incomplete isothermal solidification (these areas - with lower strength compared with the base metal - fail before under traction, which reduces the effective area during the test, resulting in an overload failure). Hardness measurements at the joint and at the HAZ are consistent with the observed microstructres.