A Parametric Study of Solute Redistribution DuringTransient Liquid Phase Diffusion Bonding Process

Imposing a temperature gradient when transient liquid phase (TLP) diffusion bonding is the basis of a new bonding method which has recently been developed in the Department of Materials Science and Metallurgy, University of Cambridge (UK Patent 9709167.2). The use of temperature gradient TLP diffusion bonding results in joints with excellent reliability and also shear strengths as high as those of the parent material (e.g. aluminium-based alloys and composites). Another feature of this new method is that diffusion of solute in the solid phase is not necessary for solidification to take place and therefore bonding times are significantly shorter than those in conventional TLP diffusion bonding (when there is no imposed temperature gradient across the liquid phase). A mathematical model is proposed in this paper which is capable of predicting bonding times and bond line displacements when TLP diffusion bonding under a temperature gradient. The bond line displacement and bonding time predicted by the model for an ideal Al-Cu binary system are compared with experimental results obtained using pure aluminium and copper. The theoretical values predicted by the model are in agreement with the experimental results.

Commercially pure iron (Fe) and stainless steel 321 (SS 321) were transient liquid phase (TLP) diffusion bonded using Cu and/or Au-12Ge interlayers. The joining pressures, temperatures and times were investigated. Au atoms appeared to diffuse back at the joining interface during slow cooling from the joining temperature. A faster cooling method prevented the back-diffusion of Au atoms from interior to the joining interface. Residual interlayer disappeared when faster cooling methods such as air cooling, water cooling or water quenching were applied. However, microcracks appeared in the joint centerline if water cooling or quenching was applied. Au-rich particles also appeared in the joint microstructure. No microcracks appeared for the samples cooled in air from the bonding temperature. The highest strength of the TLP diffusion bonded Fe reached almost same as the normalized Fe. The maximum tensile strengths (UTS) obtained for the bonded Fe were 291 ± 2 MPa using a Cu interlayer and 315 ± 4 MPa. The maximum strength found was 387 ± 4 MPa for TLP diffusion bonded steel using an Au-12Ge interlayer. The maximum strength for steel was obtained for the samples bonded at 1050 °C for 20 h in vacuum and cooled in air. The maximum strength found was 387 ± 4 MPa which is 70.4 % of the base alloy strength.

Plastic deformation was newly introduced in transient liquid phase (TLP) diffusion bonding of steel sandwich panels. The effect of plastic deformation on bonding strength was investigated through lab experiments. It was assumed that three factors, including newly generated metal surface area, deformation heat, and lattice distortion, contribute to the acceleration of interface atoms diffusion and increase of diffusion coefficients. A numerical model of isothermal solidification time was developed for TLP bonding process under plastic deformation and applied to carbon steel sandwich panels bonding with copper interlayer. A reasonable isothermal solidification time was obtained when an effective diffusion coefficient was used. Based on lab experiments, the effects of plastic deformation on interlayer film thickness and isothermal solidification time were studied through theoretical calculation with the new model. The evolution of interlayer film thickness indicates a good agreement between the calculation and experimental measurement. The results show that the isothermal solidification time is obviously reduced due to the effect of plastic deformation. Furthermore, a new steel sandwich cooling panel for heat exchanger was fabricated by TLP diffusion bonding under 13.1% plastic deformation. The test results suggest that a steel sandwich panel of inequidistant fin structure can provide enhanced heat transfer efficiency.

In order to promote the molten metal penetration into the carbon/carbon (C/C) composite, a sequence of annular gaps is designed and prepared on the C/C surface. These gaps are created on the C/C surface under preoxidation conditions at 600–660 °C. The oxidized C/C composite is joined to Ti3Al via transient liquid phase (TLP) diffusion bonding at 880 °C with AgCuNiLi foil as an intermediate layer. The study investigates the effects of preoxidation temperature on the microstructure and characteristics of C/C composite and C/C–Ti3Al joints. The results show that a sequence of annular gaps with appropriate size can be established between the carbon fiber and pyrolytic carbon layer, while carbon fiber is barely oxidized after oxidation at 630 °C for 30 min. During the bonding process, the molten metal seeps through the C/C gaps, and a permeation layer composed of TiC, Ag (s, s), and C/C composite can be formed. The production of a permeation layer can enhance the bonding strength of the C/C–Ti3Al joint, and the maximum shear strength (42.12 MPa) of the joint can be 22% greater than the initial C/C–Ti3Al joint.

Microstructure and mechanical properties of C/C composite| Ti3Al alloy joints prepared by transient liquid phase diffusion bonding using AgCuNiLi alloy as joining material

Stable bonding of W and ODS steel fabricated by TLP diffusion technology through inserting a novel composite interlayer Zr/Cu

Effect of micro-oxidation and Cu modification of C/C composite on microstructure and mechanical properties of C/C|Ti3Al joints

Transient liquid phase diffusion bonding under a temperature gradient: modelling of the interface morphology

Transient liquid phase (TLP) diffusion bonding of Al-6061 containing 15 vol pct alumina particles was carried out at 873 K (600 °C) using electrodeposited nanocomposite coatings as the interlayer. Joint formation was attributed to the solid-state diffusion of Ni into the Al-6061 alloy followed by eutectic formation and isothermal solidification of the joint region. An examination of the joint region using an electron probe microanalyzer (EPMA), transmission electron microscopy (TEM), wavelength-dispersive spectroscopy (WDS), and X-ray diffraction (XRD) showed the formation of intermetallic phases such as Al3Ni, Al9FeNi, and Ni3Si within the joint zone. The result indicated that the incorporation of 50 nm Al2O3 dispersions into the interlayer can be used to improve the joint significantly.

An investigation of transient liquid phase (TLP) diffusion bonding of a nickel-base directionally solidified superalloy, DZ22, was presented. Two interlayer alloys developed for DZ22 were employed. They were: Z2P, a powder alloy and Z2F, an amorphous foil. The results show that the microstructures and compositions of the joints bonded with Z2F amorphous foil were obvious more homogenous than the joints bonded with Z2P powder alloy. And the joints bonded with Z2F possessed a better stress-rupture property. The final obtained joint stress-rupture properties reached 90% of that of the base metal.

Solute diffusion is an important process that determines the dendrite growth during solidification. The theoretical model generally simplifies the solute diffusion coefficient in liquid phase into a constant. Nevertheless, the composition of the boundary layer changes greatly in the solidification process, the diffusion coefficient will no longer be a constant and is dependent on concentration. In this paper, the quantitative phase field model is used to simulate the effect of concentration-dependent diffusion coefficient on dendrite growth in directional solidification. In the model, the concentration-dependent diffusion process is investigated by coupling the concentration-dependent diffusion coefficient in the liquid solute diffusion equation. A series of simulation results confirms that the concentration-dependent diffusion process has a significant effect on the dendrite growth. The results show that the increase of the coupling intensity of solute concentration will enhance the diffusion of solute in the mushy zone between primary dendrites to the dendrite tip, resulting in the increase of solute enrichment at the dendrite tip, thereby increasing the tip undercooling. The variation of diffusion coefficient in liquid phase has little effect on the tip radius of dendrite, and the simulation results are in good agreement with those from the theoretical model. Moreover, the amplitude of dendritic side branches decreases with the increase of solute diffusion coefficient. In the study of dendrite arrays, it is found that the concentration-dependent diffusion coefficient increases the primary spacing and reduce the tip position. The results of this study indicate that for a system with a concentration-dependent coefficient significantly, the effect of concentration-dependent diffusion on tip undercooling and side branches should be considered in the quantitative and experimental verification of the existing model.

The primary carbide in high carbon chromium bearing steels, which arises from solute segregation during non-equilibrium solidification, is one of the key factors affecting the mechanical properties and performance of the related components. In this work, the effects of carbide forming element diffusion, primary austenite grain size, and the cooling rate on solute segregation and carbide precipitation during the solidification of an Fe–C binary alloy were studied by the phase-field method coupled with a thermodynamic database. It was clarified that increasing the ratio of solute diffusivity in solid and liquid, refining the grain size of primary austenite to lower than a critical value, and increasing the cooling rate can reduce the solute segregation and precipitation of primary carbide at late solidification. Two characteristic parameters were introduced to quantitatively evaluate the solute segregation during solidification including the phase fraction threshold of primary austenite when the solute concentration in liquid reaches the eutectic composition, and the maximum segregation ratio. Both parameters can be well-correlated to the ratio of solute diffusivity in solid and liquid, the grain size of primary austenite, and the cooling rate, which provides potential ways to control the solute segregation and precipitation of primary carbide in bearing steels.

The influence of linear, parabolic and inverted parabolic temperature gradients on the onset of double-diffusive Rayleigh-Darcy convection is theoretically investigated. The composite system is constrained horizontally by adiabatic and free-free thermal boundaries, and appropriate interfacial boundary conditions are used to connect fluid-porous layers. The regular perturbation approach is used to determine the critical Rayleigh number expression for different temperature gradients. Graphs are used to investigate the significance of a variety of dimensionless characteristics. The couple stress parameter, couple stress viscosity ratio, solute Rayleigh number, and solute diffusivity ratio clearly have a stabilizing effect on the system, whereas the Darcy number and thermal diffusivity ratio destabilize it.

Realistic predictions of macrosegregation formation during casting of aluminium alloys requires an accurate modeling of solute microsegregation accounting for multicomponent phase diagrams and secondary phase formation. In the present work, the stand alone Alstruc model, a microsegregation model for industrial multicomponent aluminium alloys, is coupled with the continuum model ALSIM which calculates the macroscopic transport of mass, enthalpy, momentum, and solutes as well as stresses and deformation during solidification of aluminium. Alstruc deals with multicomponent alloys accounting for temperature dependent partition coefficients, liquidus slopes and the precipitation of secondary phases. The challenge associated with computation of microsegregation for multicomponent alloys is solved in Alstruc by approximating the phase diagram data by simple, analytical expressions which allows for a CPU-time efficient coupling with the macroscopic transport model. In the present work, the coupled model has been applied in a study of macrosegregation including thermal and solutal convection, solidification shrinkage and surface exudation on an industrial DC-cast billet.

Prediction of processing maps for transient liquid phase diffusion bonding of Cu/Sn/Cu joints in microelectronics packaging