F. Molleda

Special features of the formation of the diffusion bonded joints between copper and aluminium

A metallographic study of diffusion bonds between aluminium and copper has been made in order to further understanding of the mechanism of bond formation for joints between dissimilar metals that form intermediate phases or intermetallic compounds. A three-stage mechanistic model based upon sintering principles has been proposed to explain this kind of diffusion joint.

Solid-state transformations during diffusion bonding of copper to iron

Solid-state bonding between dissimilar metals, produced at elevated temperatures with the application of a bonding pressure, causes structural changes in the microstructure of the zones nearest to the bond interface. These metallurgical transformations, produced by interdiffusion in the vicinity of the bond, decide the final properties of the joint. In the present paper, such diffusional transformations have been investigated for diffusion-bonded joints of Armco iron and copper with different oxygen contents (ETPC and OFLPC). The formation of iron oxide (wustite) has been observed in the ETPC-Armco iron joints. This oxide did not appear in OFLPC-Armco iron diffusion-bonded joints. This suggests that iron oxide forms by reaction of iron with oxygen dissolved in the ETPC base metal. The formation of copper particles in the iron base matrix, near the bond interface, has been observed. This may be due to two different processes: the solid-state precipitation of copper into iron and the eutectoid reaction (γ →ε +α) at bonding temperatures above 900° C.

Diffusion bonding of grey cast iron to ARMCO iron and a carbon steel

The microstructure transformations produced during the diffusion bonding of grey cast iron to pure iron (ARMCO iron) and to a hypoeutectic steel (0.55% C) have been studied. The indirect determination of the carbon concentration profiles has produced a diffusion equation that relates the microstructure of the bond interface to the bonding temperature and time. A new tensile test specimen is described; this specimen has a variable circular section which allows the determination of true tensile strength of dissimilar diffusion bonds. Metallographic and fractographic studies have shown that the optimum bonding conditions for both types of joint are a bonding temperature at 980° C, for 5 min at a bonding pressure of 4.5 MPa.

Modeling of grain growth during arc welding of high strength low alloy steels

Abstract The addition of microalloying elements to a low carbon steel, followed by a thermomechanical treatment, gives rise to a high strength tough material which is also weldable. To predict a priori the microstructure after a welding thermal cycle, a numerical model of the grain growth process has been developed following that proposed by Aashby and Easterling. For the high strength low alloy steel studied, certain unknown kinetic constants must be determined by experiments using a welding simulator. The microstructural diagram obtained has been compared with the structures of real welds. The correlation was quite good except of the grain sizes closest to the fusion zone, but this can be explained by nonequiaxial growth of grains. Scanning electron microscopy together with energy dispersive spectroscopy analysis has been used to characterize the microstructure and in particular to study the morphology of nonmetallic includions, to determine the presence of shape controlling elements.

Tensile strength of Armco iron-ETP copper diffusion bonds

Etude de la reaction chimique (precipitation du fer) lors des assemblages par diffusion. Caracterisation de la microstructure par microscopie electronique et par fractographie

Precipitation of iron carbides in ferrite during diffusion bonding

Abstract Specimens of a steel (0.35% C) and a pearlitic cast iron (C.E. = 4.3%) were diffusion bonded to Armco iron by heat treatment at 880°C for times ranging from 10 to 30 min. The ferrite phase of Armco iron became enriched with carbon from the steel, or from the cast iron, and eventually part of it was transformed to austenite when the carbon content was sufficiently high. At room temperature, two well-defined zones were delineated. One was adjacent to the junction with a ferrite-pearlite structure, indicating that diffusion of carbon was sufficient to form austenite. The other, next to the first, was formed by large ferrite grains containing grain boundary and intragranular cementite precipitates of varying morphology (spherical, acicular, or dendritic).