Amol A. Gokhale

Weld microstructure refinement in a 1441 grade aluminium-lithium alloy

Clad 2 mm thick sheets of Russian 1441 grade Al-Li alloys were welded using a gas tungsten arc welding process (GTAW). Comparisons were made between the weld beads obtained under (i) continuous current (CC), (ii) pulsed current (PC), and (iii) arc oscillation (AO) conditions for their macro- and microstructural details. In the case of CC GTAW, sound welds could be produced only under a narrow range of welding parameters. Centre line cracks, which occurred in CC GTAW welds under certain conditions, were halted by switching to PC or AO conditions while the welding was in progress. Microstructural refinement was significant in the case of PC and AO GTA welding.

Mechanical behaviour of aluminium-lithium alloys

Aluminium-lithium alloys hold promise of providing a breakthrough response to the crying need for lightweight alloys for use as structurals in aerospace applications. Considerable worldwide research has gone into developing a range of these alloys over the last three decades. As a result, substantial understanding has been developed of the microstructure-based micromechanisms of strengthening, of fatigue and fracture as well as of anisotropy in mechanical properties. However, these alloys have not yet greatly displaced the conventionally used denser Al alloys on account of their poorer ductility, fracture toughness and low cycle fatigue resistance. This review aims to summarise the work pertaining to study of structure and mechanical properties with a view to indicate the directions that have been and can be pursued to overcome property limitations.

Microstructure and mechanical properties of RSP/M Al-Fe-V-Si and Al-Fe-Ce alloys

Two aluminium alloys with nominal compositions of Al-8Fe-4Ce and Al-8Fe-1V-2Si (all compositions in wt %) were rapidly solidified by ultrasonic gas atomization. The atomized powders with an average particle size (d50) of 30 μm were vacuum hot pressed and subsequently hot extruded. The P/M extrusion exhibited similar microstructure and elevated temperature tensile properties. The tensile and stress rupture samples of both the alloys exhibited ductile dimple failure. However, the Al-Fe-V-Si extrusion samples exhibited significantly better creep and stress rupture properties. The Al-Fe-Ce alloy was found to be more susceptible to cavitation at elevated temperatures which resulted in poor stress rupture properties.

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.

Gas tungsten arc welding studies on similar and dissimilar combinations of Al - Zn - Mg alloy RDE 40 and Al-Li alloy 1441

Abstract The present study is concerned with gas tungsten arc welding of two high strength aluminium alloys, namely, an Al-Zn-Mg alloy (RDE 40) and an Al-Li based alloy of Russian grade 1441. One of the critical requirements of these two alloys is that they should be weldable. In the present work, weldability aspects of these alloys were studied in terms of solidification cracking tendency, microstructure, tensile properties, and microhardness across the welds. These studies were extended to dissimilar welds between RDE 40 and 1441 produced via conventional gas tungsten arc (GTA) welding as well as pulsed current GTA welding. It was found that RDE 40 welds were less sensitive to solidification cracking and weld metal porosity compared with 1441 alloy. The superior weldability of RDE 40 was related to the equiaxed nature of the fusion zone and a lower sensitivity to moisture pickup. It was possible to produce RDE 40-1441 welds without defects. Pulsed current welding of RDE 40 to 1441 showed improved mechanical properties compared with conventional GTA welding, and these were related to the refinement of the fusion zone microstructure.

Microstructure and Precipitate Characteristics of Aluminum–Lithium Alloys

This chapter reviews the precipitation and precipitate phases that occur during heat treatments in multi-component Al-Li based alloys. It describes aspects related to nucleation, growth, morphology and orientation relationships of the strengthening precipitates δ’ and T1, the toughening precipitate S’ and the recrystallisation-inhibiting precipitate β’. Equilibrium precipitate phases such as T2, which are deleterious to the mechanical and corrosion properties of the alloys, are also described. It is shown that careful alloy chemistry control, two-step homogenization and controlled stretching prior to ageing can be employed to improve the volume fraction and distribution of the precipitate phases. All these processing aspects are necessary to achieve optimum combinations of properties for the alloys.

Influence of welding techniques on microstructure and pitting corrosion behaviour of 1441 grade Al–Li alloy gas tungsten arc welds

Abstract The effects of changing the welding technique on the weld bead solidification structure and resistance to pitting corrosion of an aeronautical grade Al-Li based alloy have been studied. Russian alloy 1441, with a nominal composition Al-1.9Li-1.8Cu-1.0Mg-0.1Zr (wt-%) and in the form of sheet, was welded using constant current, pulsed current and arc oscillation gas tungsten arc welding techniques. Under the optimum conditions of welding, the solidification structure was coarse columnar in constant current welds, fine equiaxed in pulsed current welds and fine equiaxed intermixed with fine cellular grains in arc oscillation welds. The distribution of interdendritic (and intergranular) non-equilibrium solidification products became increasing discrete on changing from constant current to pulsed current and arc oscillation welding. Resitance to pitting corrosion was found to be greatest in welds having a discrete second phase distribution, i.e. those produced using the pulsed current and arc oscillation methods.

The effect of Mg addition on microstructure and tensile and stress rupture properties of a P/M Al-Fe-Ce alloy

Dispersion strengthened Aluminium alloys based on Al-TM (TM = transition metal) type systems are being developed for elevated temperature structural applications in aerospace at temperatures up to 623K as possible substitutes for the more expensive and refractory Ti alloys currently used. Alloy systems containing transition metals such as Fe, Ni, V, Cr, Zr, Mo and Ti with possible additions of Si and Ce (or misch metal) have been investigated. Al-Fe-Ce alloys belong to this class of dispersion strengthened aluminium alloys which are strengthened primarily by ternary Al-Fe-Ce precipitates. The matrix in this case is almost pure aluminium. Further strengthening of the alloy may therefore be possible by addition of elements such as Mg which impart solid solution strengthening. This should also result in enhancement in elevated temperature strength and stress rupture properties. It is reported that the addition of Mg to Al-Fe-Mm (Mm = misch metal) alloy does not result in formation of new phases in melt spun ribbons even after annealing at 773K thereby confirming that Mg is retained in solid solution. The aim of the present work is to investigate the effect of Mg addition on the microstructure and tensile and stress rupture properties of an Al-Fe-Ce alloy.

Processing and Stability of LM25/SiCP Composite Foams

A simple method of metal foam production is to introduce a blowing agent (e.g. TiH2) into an aluminium melt containing foam stabilisers such as oxides (usually Ca-based) and/or particles (e.g. SiC, Al2O3). In this work, Al/SiC composites (in-house and commercial Duralcan) [both of them with LM25 matrix (Al–7Si–0.3Mg)] containing particles of various sizes and contents were foamed at different temperatures using TiH2. Foamability is characterised through their expansion and collapse. It is observed that high expansions and good quality foams could be obtained upon manipulating SiC particle size and content. However, irrespective of particle size/vol.% combination, significant effect of foaming temperature is noticed on the fundamental stability of the liquid foam until solidification. Both cell size and foam density varied along the ingot height. The distribution of SiCP within the cell wall is random with no preferential segregation to gas/metal interface. The evolution of foam, and the role of SiC on foam stability are discussed based on macro and cell wall microstructural results.

Sequence of Precipitation of T2 and δ Phases during Ageing of Al-Li Alloy 8090C

The precipitation of two dominant equilibrium phases viz. T 2 (Al 6 CuLi 3 ) and δ (AlLi) in an Al-2.2Li-1.0Cu-0.7Mg-0.04Zr (wt.%) base 8090 C alloy has been studied. The sequence of precipitation of T 2 and 8 during ageing at 190°C was established. Further, the preferred nucleation sites for the equilibrium phases were identified for different ageing times. Faceted precipitates of β (Al 3 Zr) were shown to nucleate T 2 which, in turn, nucleated δ.