A.K. Mukhopadhyay

On the formation of faceted Al3Zr (β′) precipitates in Al–Li–Cu–Mg–Zr alloys

Abstract Trace additions of Zr to Al alloys inhibit recrystallization through the formation of spherical and coherent Al 3 Zr ( β ′) precipitates. Recently, observations have been made of faceted β ′ precipitates in several hot deformed Al alloys, although no systematic experimental study of either the causes of the formation of such precipitates or their orientation relationships with the Al matrix has so far been reported. A detailed examination of the orientation relationships shows that the cube-on-cube orientation relationship existing between spherical, coherent β ′ precipitates and the Al matrix does not hold good for the faceted β ′ particles and that the faceted β ′ particles are twin-related with the matrix. It is shown that the twin-related β ′ particles are not incoherent, but bound by large facets fully coherent with the matrix, and that such particles are associated with fairly significant coherency strains. A probable shape of the faceted β ′ is also described.

Influence of scandium on weldability of 7010 aluminium alloy

Abstract The commercial 7000 series aluminium alloys are based on medium strength Al–Zn–Mg and high strength Al–Zn–Mg–Cu systems. The medium strength alloys are weldable, whereas the high strength alloys are non-weldable. This is because the amount of copper present in these alloys gives rise to hot cracking during solidification of welds. As a result, the high strength Al–Zn–Mg– Cu base alloys are not used for applications where joining of components by welding is an essential step. In the present study, using a combination of qualitative Houldcroft test and quantitative Varestraint test, it is shown that a small addition of scandium to the commercial 7010 alloy reduces the hot cracking susceptibility during solidification of welds produced by the gas tungsten arc welding process. The improvement in weldability is found to be the result of the considerable grain refinement in the weld structure following the scandium addition. The results of microhardness and tensile tests are further described within the context of the present work to demonstrate that the 7010+Sc welds also exhibit a combination of improved strength and ductility.

Effects of different modes of hot cross-rolling in 7010 aluminum alloy: part II. mechanical properties anisotropy

The influence of microstructure and texture developed by different modes of hot cross-rolling on in-plane anisotropy (AIP) of yield strength, work hardening behavior, and anisotropy of Knoop hardness (KHN) yield locus has been investigated. The AIP and work hardening behavior are evaluated by tensile testing at 0 deg, 45 deg, and 90 deg to the rolling direction, while yield loci have been generated by directional KHN measurements. It has been observed that specimens especially in the peak-aged temper, in spite of having a strong, rotated Brass texture, show low AIP. The results are discussed on the basis of Schmid factor analyses in conjunction with microstructural features, namely grain morphology and precipitation effects. For the specimen having a single-component texture, the yield strength variation as a function of orientation can be rationalized by the Schmid factor analysis of a perfectly textured material behaving as a quasi-single crystal. The work hardening behavior is significantly affected by the presence of solute in the matrix and the state of precipitation rather than texture, while yield loci derived from KHN measurements reiterate the low anisotropy of the materials. Theoretic yield loci calculated from the texture data using the visco-plastic self-consistent model and Hill’s anisotropic equation are compared with that obtained experimentally.

A study on precipitation characteristics induced strength variation by nonlinear ultrasonic parameter

Nonlinear ultrasonic study has been carried out to characterize the variation in strength induced by precipitation characteristics. An age hardenable aluminum alloy has been taken as a model alloy for the present investigation. It is shown that the second order nonlinear ultrasonic (NLU) parameter scales with strength property that in turn depends on coherent to incoherent precipitation phase transition. The observed variations in NLU parameter has been explained by modifying an existing dislocation-coherent precipitate interaction model for harmonics generation in order to account for a weaker dislocation-semicoherent precipitate interaction. The model proposed can in general be applicable to all precipitation hardenable alloy systems undergoing coherent to incoherent precipitate phase transition.

Key Microstructural Features Responsible for Improved Stress Corrosion Cracking Resistance and Weldability in 7xxx Series Al Alloys Containing Micro / Trace Alloying Additions

The commercial 7xxx series Al alloys are based on medium strength Al-Zn-Mg and high strength Al-Zn-Mg-Cu systems. The medium strength alloys are weldable, whilst the high strength alloys are nonweldable. On the other hand, the Cu-free, weldable alloys suffer from poor SCC resistance. It is the purpose of this article to provide quantitative data and microstructural analysis to demonstrate that small additions of either Ag or Sc to Al-Zn-Mg and Al-Zn-Mg-Cu alloys bring about very significant improvement in SCC resistance and weldability, respectively. The improvement in SCC resistance of the Cu-bearing alloys due to over aging and retrogression and reaging (RRA) is further discussed in light of a similar improvement in the SCC resistance of these alloys, when peak aged, due to Ag and Sc additions.

Characterization of S (Al2CuMg) phase particles present in as-cast and annealed Al–Cu–Mg(–Li)–Ag alloys

Abstract Scanning electron microscopy in combination with electron probe micro analysis have been used to evaluate the influence of small additions (i.e. 0.5 wt%) of Ag on the composition of S (Al 2 CuMg) phase particles present in as-cast and annealed Al–4wt%Cu–0.5wt%Mg and Al–5wt%Cu–1.2wt%Li–0.5wt%Mg alloys. It is shown that the S phase particles present in these alloys contain Ag, and that the Ag content varies from 1.6 to 5.3 wt%. Also, the Mg contents of the S phase particles present in as-cast Al–Cu–Mg alloys are increased by Ag additions, although such an effect appears to be counteracted by the presence of Li in the alloy. It is further revealed that the stability of the S phase particles during elevated temperature annealing is influenced by Li additions, as well as by relatively higher Mg contents of the alloy, and that these results have implications for the practical homogenizing annealing treatments.

Effects of different modes of hot cross-rolling in 7010 aluminum alloy: part I. evolution of microstructure and texture

The current study describes the evolution of microstructure and texture in an Al-Zn-Mg-Cu-Zr-based 7010 aluminum alloy during different modes of hot cross-rolling. Processing of materials involves three different types of cross-rolling. The development of texture in the one-step cross-rolled specimen can be described by a typical β-fiber having the maximum intensity near Copper (Cu) component. However, for the multi-step cross-rolled specimens, the as-rolled texture is mainly characterized by a strong rotated-Brass (Bs) component and a very weak rotated-cube component. Subsequent heat treatment leads to sharpening of the major texture component (i.e., rotated-Bs). Furthermore, the main texture components in all the specimens appear to be significantly rotated in a complex manner away from their ideal positions because of non-symmetric deformations in the two rolling directions. Detailed microstructural study indicates that dynamic recovery is the dominant restoration mechanism operating during the hot rolling. During subsequent heat treatment, static recovery dominates, while a combination of particle-stimulated nucleation (PSN) and strain-induced grain boundary migration (SIBM) causes partial recrystallization of the grain structure. The aforementioned restoration mechanisms play an important role in the development of texture components. The textural development in the current study could be attributed to the combined effects of (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component after each successive pass, (b) recrystallization resistance of Bs-oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids.

Selection and Design Principles of Wrought Aluminium Alloys for Structural Applications

This article discusses the fundamental principles associated with the selection of aluminium alloys for specific purposes, alloy design & heat treatment procedures and development of key microstructures responsible for obtaining desired properties in selected wrought Al alloys for aerospace and defence applications. Influence of micro/trace additions of suitable alloying elements on the microstructure and properties of high strength 7xxx series Al alloys is further highlighted.

On the microstructure developed in as-cast and homogenized 7010 aluminium alloy containing scandium

Abstract Grain refinement in the as-cast microstructure and the nature of the Sc-bearing primary and secondary phase particles present in as-cast and homogenized Al–Zn–Mg–Cu–Zr base 7010 alloys, containing Sc, have been examined by a combination of light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscopy (TEM). It is shown that addition of 0.25 wt.% Sc to the base alloy 7010 gives rise to the desired grain refinement in the as-cast microstructure. Further additions of Sc (investigated up to 0.5 wt.%) reduce the as-cast grain size further, although increased amounts of Sc additions give rise to an increased density of Sc-bearing coarse primary particles that always have an L12 crystal structure, dissolve Zr, and survive the homogenization treatment. The compositions of the Sc-bearing primary phase particles vary depending on their morphologies, i. e. cuboidal and irregular shape. The cuboidal shape of the Sc-bearing primary parti...

Extrusion Processing of High-Strength Al Alloy 7055

The effects of a combination of extrusion processing parameters and aging schedules on the microstructure and mechanical properties of the 7055 Al alloy were investigated. A safe extrusion processing zone is determined through a limit diagram constructed over the experimental initial billet temperature ranging from 380° to 420°C, extrusion ratio from 10:1 to 40:1, and the ram speed ranging from 1 to 15 mm s−1. Microstructural characterization of as-extruded, solution-treated, and artificially-aged materials was carried out using polarized light microscopy (for grain structure) and transmission electron microscopy (for precipitate morphology). A combination of hardness and tensile tests was used to evaluate mechanical properties. It is shown that in 7055 Al alloy, the optimization of alloy composition, extrusion processing parameters, and peak aging treatment results in reproducible tensile properties of 0.2% P.S. = 725 MPa, UTS = 750 MPa, and % elongation = 12.9. In order to improve the stress-corrosion resistance of peak aged material, retrogression and reaging (RRA) temper was established. A strength-electrical conductivity relationship has been established for the RRA temper between 36% and 37% International Annealed Copper Standard (IACS) electrical conductivity to enable selection of suitable combination of properties.