Showing papers by "Amol A. Gokhale published in 2014"

Microstructure and Precipitate Characteristics of Aluminum–Lithium Alloys

Abstract: 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.

Effect of SiC particle content on foaming and mechanical properties of remelted and diluted A356/SiC composite

Abstract: A commercial Al–Si–Mg composite was diluted to different SiC (mean size: 8 µm) contents, and was foamed using TiH2 by melt based technique at a fixed foaming temperature. Foams with high expansions were achieved and uniform cell morphology is obtained at and below 10 vol% SiC. Despite good expansions, certain amount of foam collapse occurs for 5 vol% SiC composite. This work brings out that presence of SiC alone can lead to good foaming but these foams are unstable prior to solidification. This instability can be correlated to in situ oxide content. Both compression and impact (perforation) tests show improved properties with the increase in foam density. Compressive strength is not influenced by SiC content, and energy absorption too seems to be insensitive to the rate of loading within six orders of magnitude.

Mechanical Working of Aluminum–Lithium Alloys

Abstract: Mechanical working of Al–Li alloys is primarily concerned with aerospace alloy rolled products (sheet and plate), extrusions, and to a lesser extent forgings. These products are fabricated by hot working with intermittent and final heat treatments. This thermomechanical processing (TMP) can be rather complex for the modern 3rd generation Al-Li alloys, but is necessary to obtain optimum combinations of properties. This Chapter is in two parts. Part 1 discusses the ‘workability’ of metals and alloys and the hot deformation characteristics of Al–Li alloys, leading to the concept of Process Maps. A comprehensive Process Map for a binary Al–Li alloy illustrates the usefulness of these Maps for defining temperature–strain rate regions for safe and unsafe hot working, recrystallization and recovery, and superplastic behaviour Part 2 provides some general considerations about processing Al–Li alloy products, followed by a review and discussion of the currently available information for 3rd generation alloys. It is concluded that their complex TMP schedules may make it difficult to obtain optimum combinations of properties for thicker products.

Open-Cell Metal–SiC Composite Foams made by Electrolytic Codeposition on Polyurethane Substrates

Abstract: In this research, open cell Ni–SiC and Cu–SiC composite foams are produced by electrolytic codeposition on the polyurethane foam substrates using conventional metal plating electrolytes with suspended SiC nanoparticles. The method ensures the production of MMC open-cell foams; content of SiC inclusions in the metal matrix of up to 10 wt.% can be controlled by the SiC powder concentration in the electrolytes. Effects of the foam pore size (surface area) and the electrolyte composition on the content of SiC inclusions and the density gradient across the foam are studied.

Welding Aspects of Aluminum–Lithium Alloys

Abstract: The application of aluminium-lithium alloys over a wide range of engineering technologies will require development of both effective methods for joining these materials and through understanding of their welding metallurgy. This chapter covers the pertinent literature regarding the weld metal porosity, susceptibility to cracking during welding, eqiaxed zone formation and associated fusion boundary cracking, mechanical properties and corrosion behaviour of welds. Microstructural modification is especially attractive for alloys with pronounced hot cracking susceptibility. Aluminum–lithium alloys are one such class of materials. Since the hot cracking tendency is known to be a function of weld metal composition, several crack resistant filler materials such as AA 2319, AA 4043 and AA 5356 are in common use. While primary approach to the problem is thus to modify weld metal chemistry, a secondary solution is to reduce the coarseness of the solidification structure. Of the various techniques available for modifying the structure, pulsed current, magnetic arc oscillation techniques of gas tungsten arc welding and inoculation using grain refining additions offers the greatest promise for practical applications. Improving weldability of these alloys through modification of fusion zone microstructure are covered in this chapter. Lastly, solid state welding processes such as friction and friction stir welding of Al-Li alloys are briefly discussed.

Melting and Casting of Aluminum–Lithium Alloys

Abstract: Information on the technology of melting and casting aluminium lithium alloys is rarely published. Therefore, this chapter presents important findings from many research investigations on various aspects of melting, casting and cast structure development of Al-Li alloys. Presence of reactive element such as Li in the molten alloy requires special melt protection techniques, containment materials, alloying practices and casting equipment. Problem of grain refinement, linked to the presence of Zr in most commercial compositions, has also been discussed. Finally, it is surmised that the lower Li contents present in the third generation alloys compared to their older counterparts is expected to ease difficulties in their melting and casting.