Transient liquid-phase bonding of alumina and metal matrix composite base materials

Transient liquid-phase (TLP) bonding of aluminium-based metal matrix composite (MMC) and Al2O3 ceramic materials has been investigated, particularly the relationship between particle segregation, copper interlayer thickness, holding time and joint shear strength properties. The long completion time and the slow rate of movement of the solid–liquid interface during MMC/Al2O3 bonding markedly increased the likelihood of forming a particle-segregated layer at the dissimilar joint interface. Preferential failure occurred through the particle-segregated layer in dissimilar joints produced using 20 and 30 μm thick copper foils and long holding times (≥20 min). When the particle-segregated layer was very thin (<10 μm), joint failure was determined by the residual stress distribution in the Al2O3/MMC joints, not by preferential fracture through the particle-segregated layer located at the bondline. Satisfactory shear strength properties were obtained when a thin (5 μm thick) copper foil was used during TLP bonding at 853 K. This revised version was published online in November 2006 with corrections to the Cover Date.     

TLP bonding of SiCp/2618Al composites using mixed Al–Ag–Cu system powders as interlayers

Mixed Al–Ag–Cu and Al–Ag–Cu–Ti powders were used as interlayers for transient liquid phase diffusion bonding (TLP bonding) of SiC particulate reinforced 2618 aluminum alloy matrix composite (SiCp/2618Al MMC). The results show that by using mixed Al–Ag–Cu powder with the eutectic composition as an interlayer, SiCp/2618Al MMC can be TLP bonded at 540 °C, however, the joining layer is porous. Adding a certain amount of titanium into the Al–Ag–Cu interlayer, the TLP bonding quality can be improved. The titanium added into the Al–Ag–Cu interlayer has an effect of shortening the solidification time of the joining layer, thus decreasing SiC particles from the parent materials entering into the joining layer. The joints bonded using Al–Ag–Cu–Ti interlayers have a maximum shear strength of 101 MPa when 2.1% titanium is added.     

Overview of transient liquid phase and partial transient liquid phase bonding

Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented. Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side. This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.     

Improvement of Joint Strength of SiCp/Al Metal Matrix Composite in Transient Liquid Phase Bonding Using Cu/Ni/Cu Film Interlayer

The compact oxide on the surface of SiCp/Al metal matrix composite （SiCp/Al MMC） greatly depends on the property of the joint. Inlaid sputtering target was applied to etch the oxide completely on the bonding surface of SiCp/Al MMC by plasma erosion. Cu/Ni/Cu film of 5μm in thickness was prepared by magnetron sputtering method on the clean bonding surface in the same vacuum chamber, which was acted as an interlayer in transient liquid phase （TLP） bonding process. Compared with the same thickness of single Cu foil and Ni foil interlayer, the shear strength of 200 MPa was obtained using Cu/Ni/Cu film interlayer during TLP bonding, which was 89.7% that of base metal. In addition, homogenization of the bonding region and no particle segregation in interfacial region were found by analysis of the joint microstructure. Scanning electron microscopy （SEM） was used to observe the micrograph of the joint interface. The result shows that a homogenous microstructure of joint was achieved, which is similar with that of based metal.     

Transient liquid phase bonding of HfC-based ceramics

Transient liquid phase (TLP) bonding enables joining at lower temperatures than traditional bonding techniques and preserves the potential for high-temperature applications, making it particularly attractive for joining ultra-high-temperature ceramics (UHTCs) such as carbides and borides. The feasibility of a TLP joint between “pure” carbides has been recently demonstrated. The present study examines the interactions that occur between undoped HfC or MoSi₂-doped HfC and a Ni/Nb/Ni multilayer interlayer during TLP bonding. Bonding is performed at 1400 °C for 30 min in a high-vacuum furnace. SEM–EDS characterization shows that the reaction layer formed at the interlayer/ceramic interface contains mixed carbides and depending upon the ceramic, Ni–Nb–Hf, or Ni–Nb–Hf–Si, or Ni–Nb–Si alloys. Nanoindentation tests traversing the reaction layer between the bulk ceramic and Nb foil midplane also show a clear transition zone across which the indentation modulus and hardness vary. Crack-free joints have been obtained with undoped HfC. The addition of 5 vol% MoSi₂ introduces small (<5 μm long) isolated cracks within the reaction layer, whereas with 15 vol% MoSi₂ added, cracking was pervasive within the reaction layer. When the reaction layer exceeds a critical thickness, as in the case of the bond obtained with HfC doped with 15 vol% MoSi₂, residual stresses become sufficiently large to cause extensive cracking and bond failure. The results suggest a need to characterize and balance the positive role of additives on sintering with the potentially deleterious role they may have on joining.     

Microstructural developments in TLP bonds using thin interlayers based on Ni–B coatings

Oxide dispersion strengthened alloy MA 758 was transient liquid phase (TLP) bonded using thin interlayers based on Ni–B electrodeposited coatings and the microstructural developments across the joint region were studied. The bonding surfaces were electrodeposited with a coat thickness of 2–9 μm and microstructural features were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The homogeneity of the joint was assessed performing micro-hardness test. The results showed that the coating thickness as well as the amount of melting point depressants (boron) in the coatings had a significant effect on the microstructural developments within the joint region. TLP bonds made using a 2 μm thick coating interlayer produced a joint with no visible precipitate formation and parent metal dissolution, and the absence of precipitates was attributed to the lower volume concentration of boron in the 2 μm thick coating interlayer.     

Counteracting particulate segregation during transient liquid-phase bonding of MMC-MMC and Al2O3-MMC joints

The microstructure and mechanical properties of MMC-MMC and Al2O3-MMC joints (MMC is metal matrix composite) produced at a bonding temperature of 853 K using copper foils ranging in thickness from 10 to 30 μm were examined. The particle segregation tendency during transient liquid-phase (TLP) bonding of aluminium-based MMC material markedly increases when the aluminium-based composite material contains large number of small radius (less than 10 μm) reinforcing particles. Also, the particle segregation tendency is much greater in dissimilar Al2O3-MMC joining since the rate of solid-liquid interface movement is much slower and the time required for completing the isothermal solidification during TLP bonding is much longer. The particle segregation tendency during MMC-MMC and Al2O3-MMC bonding can be counteracted using a combination of a short (1 min) holding time at the bonding temperature (853 K) and subsequent post-weld heat treatment at 773 K for 4 h. This TLP-bonding-heat-treatment cycle removes the retained eutectic phase present at the joint centreline. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mg/Zn/Al瞬间液相过冷连接界面的组织与性能研究

采用瞬间液相过冷连接方法对AZ31镁合金/锌中间层/5083铝合金进行连接,利用SEM、XRD、拉伸实验机和微观硬度计对结合界面的微观组织、力学性能进行了表征。结果表明,以锌作中间层,采用瞬间液相过冷连接可以实现AZ31镁合金与5083铝合金的有效连接,接头的最高抗拉强度可以达到38.5MPa,随着低温扩散保温时间的延长,扩散层厚度随之增加,接头的抗拉强度也随之升高;接头的拉伸断口属于脆性断裂,结合界面形成了MgZn2和少量的Mg17Al12金属间化合物;结合界面的微观硬度最高达170。     

Microstructural evolution during transient liquid-phase bonding in a Ni-base superalloy/sapphire fiber composite

Transient-liquid-phase (TLP) bonding was used to fabricate a Haynes 230 Ni-base superalloy/sapphire fiber composite for high-temperature applications. Boron was used as a melting-point depressant for the Ni, to aid superalloy infiltration of the fibers. Preliminary study of the composite indicated an incomplete TLP bonding cycle. Therefore, microstructural and microchemical analyses were carried out to determine the TLP bonding mechanism. It was found that the TLP process did not occur under local thermodynamic equilibrium conditions at the solid/liquid interfaces, contrary to the primary assumption of conventional models, so a modified model for TLP bonding is proposed. The main differences between the proposed and the conventional models are: (a) the concentration of the melting-point depressant increases with time during isothermal solidification, (b) extensive boride segregation at grain boundaries and boride precipitation occurs within grains adjacent to the interlayer in the initial composite assembly, (c) because of the relatively high boron concentration in the interlayer, the TLP bonding cycle was incomplete, resulting in residual-liquid borides. To achieve ideal TLP bonding, four modifications are recommended: (i) use less boron, (ii) use finer sapphire fibers, (iii) create smaller initial grain sizes in the matrix and (iv) increase the homogenization time.     

Transient liquid phase bonding of Gamma Met PX: microstructure and mechanical properties

Abstract

An investigation of microstructural development and structure–property relationships of transient liquid phase (TLP) bonded current generation γ-TiAl alloy, known as Gamma Met PX, is presented. This joining technique employed a composite interlayer consisting of a non-melting constituent (TiAl alloy) and a liquid forming constituent (copper). The microstructures of the bonds, identified using light microscopy and scanning electron microscopy, are correlated with room temperature mechanical performance. These studies suggested that joints can retain properties similar (i.e. >90%) to that of the bulk material when employing a suitable composite interlayer, bonding conditions and post-bond heat treatment. Additionally, comparisons are drawn between wide gap TLP bonding of an earlier generation γ-TiAl alloy, Ti–48Al–2Cr–2Nb (at.-%), and wide gap TLP bonding of Gamma Met PX.     

Microstructure of Ni-10Co-8Cr-4W-13Zr alloy and its bonding behaviour for single-crystal nickel-base superalloy

In the later stages of solidification of zirconium-containing superalloys, the concentration of zirconium in the interdendritic melts is above 10 wt%. The dendrites formed in the early stage of solidification may be considered to be joined by the interdendritic zirconium-rich melt. Based on the composition of the zirconium-rich melt, an Ni-10Co-8Cr-4W-13Zr (wt %) alloy was selected as an interlayer alloy for brazing and transient liquid-phase (TLP) bonding of single crystal superalloys. All the elements in the interlayer alloy are beneficial to the single-crystal superalloys. A bonding microstructure which is very similar to that of the base alloy was obtained by means of the TLP process using this interlayer alloy. In the present work, the microstructural characteristics of the interlayer alloy and the phase relationships in the bond during brazing and TLP processes were investigated.     

Transient liquid-phase bonding of ferritic oxide dispersion strengthened superalloy MA957 using a conventional nickel braze and a novel iron-base foil

It has been shown that an iron foil based on the Fe-B-Si system is a suitable material for use as a high-temperature interlayer for transient liquid-phase (TLP) bonding of ferritic oxide dispersion strengthened alloys. TLP bonding produced ferritic joints, free from intermetallic precipitates and identical in composition to that of the parent metal. In contrast, however, TLP bonding using the nickel-base foil, Bni1a, resulted in an austenitic bond region stabilized by the high nickel concentration. Furthermore, the retention of the melting point depressants, particularly silicon, at the centre of the joints resulted in the bond solidifying with the formation of silicide-boride precipitates both at the bond centre and at the braze/parent metal interface. High-temperature heat treatments failed to remove the -Fe phase and the precipitates, and their presence detrimentally affected the mechanical properties of the joint. The formation of intermetallic precipitates at the braze centre has been attributed to the initial high concentration of chromium present in the Bni1a brazing foil. Preliminary mechanical tests showed that bond strengths of joints made using the iron-base foil were superior to those obtained using the commercial nickel braze. When the iron-base foil was used, bond strengths both at room temperature and at the service temperature (700 °C) were near parent metal strengths. However, at room temperature, failure was observed to occur away from the bond interface, whilst at elevated temperatures, joints failed along the bond interface; this was attributed to the effects of melt-back phenomenon characteristic in TLP bonding.     

Diffusion bonding of zirconia to silicon nitride using nickel interlayers

The possibilities of diffusion bonding of zirconia to silicon nitride using a nickel interlayer were studied by carrying out bonding experiments under various processing conditions. The process parameters considered were temperature, bonding pressure and interlayer thickness. The optimal process conditions were determined by evaluating the mechanical strength using shear strength testing. It was found that the bonding is optimal in the temperature range 1000–1100°C. The bond strength appears to be independent of the bonding pressure and interlayer thickness if threshold values are exceeded (bonding pressure >14 MPa, interlayer thickness >0.2 mm). At the Si3N4 Ni interface, Si3N4 decomposes, forming a solid solution of silicon in nickel. At the ZrO2–Ni interface, no reaction was observed. © 1998 Kluwer Academic Publishers     

Transient liquid phase bonding process using liquid phase sintered alloy as an interlayer material

An attempt was made of using a liquid phase sintered alloy, which will be a liquid phase coexisting with solid particles at the bonding temperature, as an interlayer for bonding metals. With an aim of revealing the fundamental features of this modified TLP bonding, investigated were the kinetics concerned with the isothermal solidification process and the growth of solid particles in Fe-4.5wt%P and Fe-1.16wt%B interlayers for bonding pure iron. The movement of the bond interface was linearly dependent on t ^1/2 with higher slope than expected in the normal TLP bonding. The higher slope is attributed to the contribution of the solid particles distributed in the interlayer. The solid particles have shown no growth. However, when pure Fe particles are allowed to coexist with the liquid of equilibrium composition, they grows very rapidly. Discussion was made on the growth kinetics of the pure Fe particles.     

Transient Liquid Phase Bonding of Ni—base Single Crystal Superalloy

The Ni-base single crystal superalloy was bonded by the transient liquid phase(TLP) bonding,using a Ni-base flexible metal cloth as an insert alloy.TLP bonding of superalloy was carried out at 1473-1523K for 0.5-24h in vacuum.The [001] orientation of each test specimen was aligned perpendicular to the joint interface.The bonded region was observed by optical microscopy, and the microstructural and compositional analyses across the bonded interlayer were performed by using a scanning electron microscopy(SEM) .The electron back scattering diffraction(EBSD) method was applied to determine the crystallographic orientation.The resultsindicated that the chemical homogeneity across the bonded region can be achieved,and γˊphase both in the bonded interlayer and in the superalloy substrate is almost identical,while the bonded interlayer had almost matched the crystallographic orientation of the bonded substrates.     

Liquid phase bonding of aluminium and aluminium/Nicalon composite using interlayers of Cu-Ag alloy

Abstract

The liquid phase bonding in air of unreinforced and fibre reinforced aluminium using interlayers of Cu-Ag alloy has been investigated. Bond strengths were measured using a simple shear jig and the associated microstructures characterised by electron microscopy and electron probe microanalysis. A shear strength of 65 MN m^-2 was achieved when bonding unreinforced aluminium at 510°C using a 50 μm thick alloy interlayer, and a pressure of 10 MPa for 30 min; the bonded region covered ~85% of the area of the joint. With reinforced aluminium, a bonding pressure of 20 MPa was required to achieve sufficient contact and a similar area of bond; after application of pressure for 30 min at 510°C, a shear strength of 54 MN m^-2 was developed at the joint. The diffusional behaviour and interphase reactions which occurred during the process are discussed and a mechanism for bonding proposed.     

Partial transient liquid phase diffusion bonding of Zircaloy-4 to stabilized austenitic stainless steel 321

An innovative method was applied for bonding Zircaloy-4 to stabilized austenitic stainless steel 321 using an active titanium interlayer. Specimens were joined by a partial transient liquid phase diffusion bonding method in a vacuum furnace at different temperatures under 1 MPa dynamic pressure of contact. The influence of different bonding temperatures on the microstructure, microindentation hardness, joint strength and interlayer thickness has been studied. The diffusion of Fe, Cr, Ni and Zr has been investigated by scanning electron microscopy and energy dispersive spectroscopy elemental analyses. Results showed that control of the heating and cooling rate and 20 min soaking at 1223 K produces a perfect joint. However, solid-state diffusion of the melting point depressant elements into the joint metal causes the solid/liquid interface to advance until the joint is solidified. The tensile strength of all the bonded specimens was found around 480–670 MPa. Energy dispersive spectroscopy studies indicated that the melting occurred along the interface of the bonded specimens as a result of the transfer of atoms between the interlayer and the matrix during bonding. This technique provides a reliable method of bonding zirconium alloy to stainless steel.     

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

Abstract

Transient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are generally larger than those produced when the alloy is bonded in the recrystallised condition. The differences in microstructural developments across the joint are discussed in terms of stored strain energy of the parent metal grains.     

Chemical reaction assisted transient liquid phase bonding of alumina in combination with cold isostatic pressing

Abstract

A new method of transient liquid phase (TLP) bonding of alumina specimens has been developed using a mixture of aluminium powder and silica powder as insert materials. A chemical reaction of aluminium with silica occurs in the inter layer to produce alumina and silicon. Some of the specimens were subjected to cold isostatic pressing (cipping) before bonding to improve the bonding strength. Specimens with an interlayer of powder mixture were joined for Al/SiO₂ ratios of 1 : 0.84 and 1 : 0.42, but did not join for an interlayer with a theoretical ratio of 1 : 1.67. When specimens were subjected to cipping before bonding, bonds were far stronger than bonds without cipping in a temperature range from room temperature to elevated temperatures above the melting point of aluminium. In the mechanical test (bending test), fracture occurs at the boundary between the alumina matrix and the interlayer at room temperature, and in the interlayer at temperatures above the melting point of aluminium.     

Transient liquid phase bonding of Sn–Bi solder with added Cu particles

The aim of this study was to apply the transient liquid phase (TLP) bonding technique to low-temperature Sn–Bi-based solders to enable their use in high-temperature applications. The microstructure of the eutectic Sn–Bi solders with and without added Cu particles was investigated with the solders sandwiched between two Cu substrates. The flux of the Cu atoms successfully consumed the Sn phase and resulted in the formation of Sn–Cu intermetallic compounds and a Bi-rich phase in the solder joint. This caused the melting point of the solder joint to increase from 139 to 201 °C. The results of this study show the potential of using low-temperature solders in high-temperature applications. This study also provides new insight into the advantages of using particles in the TLP bonding process.