Principles of superplastic diffusion bonding

Abstract

Superplastic diffusion bonding is now established as an important technique for solid state bonding. There are two types of superplastic diffusion bonding using either transformation or isothermal superplasticity. In this overview, the principles of these two types are summarised and the applications are discussed, including superplastic forming with concurrent diffusion bonding (the SPF-DB process), pressure welding, and the use of superplastic diffusion bonding in sintering.

MST/835     

Effect of surface finish and sheet thickness on isostatic diffusion bonding of superplastic Ti-6Al-4V

Abstract

The aim of the present work was to determine the process conditions of pressure, temperature, and time required to produce high quality isostatic diffusion bonds in Ti-6Al-4Vfor two surface finishes. In addition, the conditions to produce a sound bond between two sheets or plates of Ti-6Al-4V having different thicknesses were investigated. An assessment of the quality of bonds produced was made on the basis of metallographic examination, lap shear strength measurements, and SEM examination of the shear fracture surfaces. Sound bonds were produced in the as received and pickled 1.6 mm thick sheet in under 1 h with an applied pressure of 2.1 MPa at both 940 and 920°C and for the P60 ground surface finish at 940°C, but the time increased to 1.5 h at 920°C for the coarser surface finish. Variations in sheet thickness from 1.6 to 10 mm were found to have no effect on the bonding time. These results were compared with the predictions of a model of isostatic diffusion bonding.     

Development of computer simulation of industrial superplastic sheet forming

A new technique estimating progressive shape changes for three-dimensional shells based on the generalized cross-sections method is introduced. The formulation of mathematical simulation of technological conditions of forming of three-dimensional shells in conditions of superplasticity is reviewed. An original algorithm of a solution finite element method with usage of orthogonal sections results. The veracity of a receivable solution is analyzed. Experimental tests are realized and a procedure to correlate the idealized forecasts with experimental data is constructed. The results and designed algorithms allow essentially to increase performance and operationability at mining new and retrofit of existing manufacturing processes of gaseous forming in conditions of superplasticity.     

Deformation mechanism maps for superplastic materials

(i) sufficient experimental results are now available for the construction of deformation maps for two superplastic materials, Zn-22% Al and Pb-62% Sn, and (ii) when plotted in the form of grain size versus stress, the maps for these two alloys provide strong evidence that superplasticity only occurs at grain sizes sufficiently small that a stable subgrain structure is not formed during deformation.     

Synergic model of the superplastic deformation of materials

A study is made of a synergic model of superplastic deformation of ultrafine-grained materials at high temperatures as a manifestation of collective modes of motion and self-organization in a system of stimulated grain-boundary slip. The evolution of grain-boundary slip is accompanied by the formation of a wavefront which separates two steady states of the system and leads to the appearance of the experimentally observed “running neck” at the surface of the sample. The synergic model is used to explain the scaling effect of superplasticity. Pis’ma Zh. Tekh. Fiz. 24, 43–47 (July 12, 1998)     

Ti_3Al超塑性扩散连接的理论计算

研究了Ti3 Al超塑性扩散连接的物理模型和数学模型 ,对Ti3 Al超塑性扩散连接的工艺过程进行了理论计算 ,探讨了工艺参数对Ti3 Al超塑性扩散连接质量的影响 .     

Clinching for sheet materials

Abstract

Latest developments in the clinching of sheet materials are reviewed in this article. Important issues are discussed, such as tool design, process parameters and joinability of some new lightweight sheet materials. Hybrid and modified clinching processes are introduced to a general reader. Several unaddressed issues in the clinching of sheet materials are identified.     

The development of cavity growth maps for superplastic materials

Superplastic alloys usually deform to very large extents but excessive cavitation can lead to premature cavitation failure in these materials. Several mechanisms can contribute to the growth of cavities during superplastic deformation although, generally, there is only one mechanism that controls cavity growth. Cavity growth mechanisms of relevance to superplastic materials are analysed in detail and the possible transitions in rate-controlling cavity growth mechanisms are considered. The contribution of lattice diffusion to the diffusional growth of cavities is included in the overall analysis of cavity growth. Cavity growth maps are constructed to show the dominant cavity growth mechanisms under different experimental conditions. Equations are developed to predict the appropriate transitions in cavity growth mechanisms with increasing cavity radii. Finally, it is demonstrated that the predictions of the cavity growth maps are consistent with the experimental results in several superplastic materials.     

Fabrication of zirconia-alumina functionally gradient material by superplastic diffusion bonding

Functionally gradient ZrO₂-Al₂O₃ material was fabricated by superplastic diffusion bonding in the present study. Conditions included a bonding temperature of 1550 °C, a time of 30 min, and strains of 17, 33 and 50%, Complete bonding was obtained under all of these bonding conditions. The apparent bending strength of functionally gradient material fabricated by superplastic diffusion bonding was 1860 MPa at the strain of 50%.     

Modelling superplastic deformation of materials with a nonequiaxed microstructure

The superplastic deformation behaviour of a two-dimensional nonequiaxed microstructure is investigated on the basis of the grain-rolling mechanism proposed by Paidar and Takeuchi for an equiaxed microstructure. Analysis shows that not only the deformation geometry but also the dynamics of grain boundary dislocation activity will be altered if grains are elongated rather than equiaxed. Constitutive equations thus derived indicate that the grain aspect ratio can impose a remarkable influence on the superplastic stress-strain rate relationship and such an influence can be quite different if the orientation of the applied stress lies over an angle with respect to the longer axis of the grains. The complexity of modelling the superplastic deformation of engineering materials is also discussed.     

Localized shear in sheet materials

Procedures and results of numerical simulations and experimental investigations of sheet materials under localized plastic deformation in shear are presented. Under dynamic loading these materials are considered to be viscoelastoplastic solids. Localized deformation and stress-strain distributions resulting from plastic shear are examined as a one-dimensional problem with account for nonlinear viscosity, damage, and temperature effects. Mild steel and high-strength aluminum alloy strips were tested under multiple impact loading with a small strain increment per impact (which corresponds to isothermal loading with sufficient accuracy). The experimental relations of localized shear deformation under such loading allow one to make the conclusion of a prevailing stress concentration effect on the kinetics of localized shear at the edge of the strip. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 53–63, September–October, 1999.     

Non-destructive testing and acoustic microscopy of diffusion bonds

Despite successful applications of diffusion bonding, there is a lack of good quality assessment methods. A survey of published results shows non-destructive testing techniques (NDT) developed for fusion welds to be ineffective in indentifying the much smaller defects found in diffusion bonds. An investigation into the possibilities of using electrical resistance measurements for NDT shows the requirement of equipment with a very high stability and sensitivity. Finally the use of very high frequency ultrasonic sound in an acoustic microscope is considered as a method of defect investigations. This acoustic microscopy proved to be a highly encouraging new line of investigation for the examination of diffusion bonds.     

Impact strength of En8 steel diffusion bonds

The use of diffusion bonding for the joining of steel is not widespread due partly to the large number of alternative welding processes and partly to a lack of information on mechanical properties of the resultant bonds. In this paper, data are provided on both tensile and impact properties of diffusion bonds in a plain carbon steel containing 0.4 % C. Several hypotheses are proposed for the observed low impact properties and one of these, the presence of a planar layer of ferrite at the bond line, is discounted.     

Finite element analysis of the superplastic forming of thick sheet using the incremental flow formulation

The paper discusses the finite element analysis of the superplastic forming of thick sheet components. The incremental formulation proposed is based on a geometrical approximation of the flow type of constitutive equations that describe the behaviour of the alloy during forming. The spatial discretization is achieved using eight-noded finite elements. An algorithm capable of predicting the correct forming pressure is also presented in a form consistent with the incremental flow formulation. Some experimental validation of these techniques will be shown together with a number of more realistic applications which will illustrate the generality of these techniques and their ability to simulate the forming of complex components. Most of the material in this section is standard but has been included for the purpose of completeness and to introduce the reader to the notation used in the paper. © 1997 John Wiley & Sons, Ltd.     

Molecular dynamics study of adhesion strength and diffusion at interfaces between interconnect materials and underlay materials

A molecular-dynamics technique for determining the adhesion strength and analyzing diffusion at interfaces between different materials has been developed. The adhesion strength is determined by calculating the adhesive fracture energy defined as the difference between the total potential energy of the material-connected state and that of the material-separated state. This technique is used to determine the adhesion strength and analyze diffusion at the interfaces between Cu films and high-melting-point materials that are used as underlay materials for Cu interconnects in ULSIs. The adhesive fracture energy shows that the adhesion strength of the Cu/Ru and Cu/Ir interfaces is much higher than that of the Cu/W, Cu/TiN, and Cu/Ta interfaces. Because the diffusion of Cu atoms at the Cu/Ru and Cu/Ir interfaces is suppressed, the surface smoothness of Cu films on Ru and Ir is much better than that on W, TiN, and Ta. It is also found that adhesion strength and smoothness increase with decreasing lattice mismatch between Cu and the underlay material. These results are confirmed by SEM (scanning electron microscope) and scratch testing. Received 27 October 1999     

Key factors of stretch-flangeability of sheet materials

Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.