Hanadi G. Salem

Microstructure and retention of superplasticity of friction stir welded superplastic 2095 sheet

Abstract Friction stir welding (FSW) was used to join superplastic, rolled sheets of an AA 2095. Microstructure was examined by light optical and transmission electron microscopy. The superplastic behavior of welded sections and of the base metal were compared. Superplasticity was retained after FSW.

Fabrication of nanostructured W–Y2O3 materials by chemical methods

A novel method for the fabrication of highly uniform oxide dispersion-strengthened (ODS) materials made by chemical processing is presented. The powders are fabricated by a two-step route starting with a chemical synthesis at room temperature, producing nanocrystalline yttrium doped tungsten trioxide hydrate precursor powders. Thermogravimetric analysis with evolved gas analysis revealed the presence of ammonium nitrate in the precursors. The second step is the reduction of the precursor in a hydrogen atmosphere at 600 and 800 °C. The reduced powders, containing W-1.2%Y2O3, showed two types of tungsten particles, cube-shaped with a size less than 250 nm and finer particles (<50 nm) of both spherical and cubic shape. The powder was consolidated by spark plasma sintering at 1100 °C, producing a bulk material with a relative density of 88%. Characterization of the sintered materials by high resolution scanning electron microscopy revealed a uniform microstructure with tungsten grains of less than 300 nm and nanosized oxide particles uniformly dispersed at the tungsten grain boundaries, as well as inside the tungsten grains. Experimental determination of the elastic properties was conducted by nanoindentation tests and fracture toughness was studied by radial indentation cracking.

Bulk behavior of ball milled AA2124 nanostructured powders reinforced with TiC

In the current research work, a top-down approach was employed for the refinement of a micron scale AA2124 alloy powder 40 µm in average size using high-energy ball milling up to 60 hours. The produced nanopowders were investigated compared to the micron gas atomized powder both in the monolithic and the reinforced composite states. 1 µm powder of TiC with internal structure <100nm was used for the reinforcement of the 2124-Al matrices. Milling time of 36 hours produced a <100nm nanopowders with internal structure size <20 nm. The nanopowder monolithic consolidates exhibited compressive strength of 388MPa compared to 313MPa for micronpowder one. Addition of TiC nanostructured powder to the nanopowder consolidated matrix resulted in increase of 130% in compressive strength compared to that produced for the microscale one. Nanopowder of Alalloys produced by mechanical milling reinforced with 10wt% TiC is recommended for products suitable for high wear and erosion resistance applications. Peak aging increased the hardness and compressive strength of the as compacted micronpowder matrices by an average of 188% and 123%, while increased that of the nanopowder matrices by an average of 110% and 117%, respectively.

Effect of Hygrothermal Aging on the Fracture of Composite Overlays on Concrete

This research assessed the effects of elevated temperature and humidity exposures on the durability of fiber reinforced epoxy composite overlays on concrete substrates. Modified double cantilever beam samples were used to determine the Mode I strain energy release rate of the composite-to-concrete bond after exposure to temperatures of 23°, 60° or 100°C, humidity levels of 50 or 95% RH, and times up to 40 days. The only statistically significant degradation in toughness was observed after exposure to the 100°C, 95% RH environment. However, chemical changes in the matrix occurred during this extreme exposure condition that may not represent the in-service aging of composite-repaired concrete structures.

Mechanical and Tribological Properties of AA2124‐Graphene Self Lubricating Nanocomposite

In this paper AA2124/graphene self-lubricating nanocomposites with different graphene addition of 0.5,3 and 5%wt. were prepared using P/M technique. A combination of cold compaction and hot extrusion (H.E) at ~0.45Tm were employed for fabrication of the nanocomposites. Addition of graphene significantly increased the compressive strength and hardness of the composites, while poor results were obtained for the ductility at room temperature. The microstructures of the composites were studied using OM and SEM. The effects of graphene addition on the friction and wear performances of the nanocomposites at room temperature in air in sliding against plain AA 2124 were investigated for the HE conditions using a pin-on-disk tribometer. Results showed that the wear rates of AA21224 could be remarkably reduced when 3wt% graphene is added and when cold compaction followed by HE at relatively low temperature was employed.

Effect of equal channel angular extrusion on the microstructure and superplasticity of an Al-Li alloy

This research investigates the use of equal channel angular extrusion (ECAE) processing to produce a superplastic form of the aluminum alloy 2098. The starting material was a hot-rolled and precipitation-hardened plate with elongated grains of width 67–92 µm, and a composition in weight percent of 2.2% Li, 1.3% Cu, 0.73% Mg, 0.05% Zr, balance Al. Microstructural evolution was investigated with optical and transmission electron microscopy (TEM) and microhardness measurements after each step of a multipass ECAE process. ECAE produced a submicron grain structure with an average size of about 0.5 µm. The sub-grain microstructure size was a function of the magnitude of the input strain and the extrusion temperature. Misorientation angles of the developed submicron structure increase with increasing number of passes at warm working temperatures. Superplastic behavior of the ECAE-processed alloy was achieved. However, the low zirconium content of the 2098 alloy resulted in grain growth of the refined structure at the superplastic processing temperatures, placing a lower limit on the deformation rates that can be used.

A Study of the Behaviour of Double Oxide Films in Al Alloy Melts

The mechanical properties of Al castings are reduced by inclusions, particularly double oxide films, or bifilms, which are formed due to surface turbulence of the liquid metal during handling and/or pouring. These defects have been reported not only to decrease the tensile and fatigue properties of Al alloy castings, but also to increase their scatter. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. In order to follow changes in the composition of the internal atmosphere of a double oxide film defect within an Al melt, a series of analogue experiments were carried out to determine the changes in gas composition of an air bubble trapped in a melt of commercial purity Al, subjected to stirring. The bubble contents were analysed using a mass spectrometer to determine their change in composition with time. Also, the solid species inside the bubbles solidified in the melt were analysed. The results suggested that first oxygen and then nitrogen inside the bubble were consumed, with consumption rates of 2.5x10-6 and 1.3x10-6 mol m-2s-1, respectively. Also, hydrogen diffused into the bubble from the melt at an average rate of 3.4x10-7 mol m-2s-1, although the rate of H diffusion increased significantly after the consumption of most of the oxygen inside the bubble. Based upon these reaction rates the time required for a typical alumina bifilm to lose all its oxygen and nitrogen was determined to be just under 10 minutes.

Structural Evolution and Superplastic Formability of Friction Stir Welded AA 2095 Sheets

Friction stir welding was used to join superplastic AA 2095 sheets. The effect of welding rate on the grain size distribution and grain boundary misorientations in the stir zone was investigated. The superplastic behavior of the weld nugget parallel to the welding direction was also characterized at 495 °C and strain rates from 10−4s−1 to 10−2s−1. Increasing the welding rate during friction stir welding augmented the formation of a fine-equiaxed high-angle grain boundary structure within the stir zone. Increasing intensity of plastic straining during friction stir welding resulted in enhanced properties during subsequent superplastic formation. The maximum strain-to-failure was obtained for the weld made at a tool speed of 1000 rpm and a weld rate of 4.2 mm/s when tested at a superplastic forming strain rate of 10−3s−1.

Consolidation of High Performance AA6061 and AA6061-SiCp Composite Processed by High Pressure Torsion

Discs of monolithic AA6061 and AA6061 reinforced with SiCp were processed via combination of hot compaction of the mixed powders followed by high pressure torsion (HPT). HPT processing was investigated using incremental revolutions up to four, under pressures of 1 and 3 GPa. Structural evolution of the powders before and after HPT processing was investigated using scanning electron microscope (SEM). HPT processing of AA6061 discs produced a trimodel structure with micron-scale grains, subgrains and nanoscale substructure of 29, 1.9 μm, and 250 nm, respectively. Reinforcement with SiCp resulted in a refined structure with micron-scale grains, subgrains and nanoscale substructure of 25, 1.9 μm, and 184 nm respectively. The presence of SiCp at the triple junctions and along the grain boundaries enhanced the rate of strain hardening of the Al-matrices and significantly refined the grain size. More pronounced refinements of the grains, subgrains, and substructures were observed with increasing the HPT pressure up to 3 GPa.

Characterization of structures and properties of amorphous nanostructured SiC thin films deposited on AISI 304 stainless steel using pulsed laser deposition

Amorphous nanostructure silicon carbide (a-SiC) recently received great attention for its use as protective coating for metallic substrates due to its good mechanical properties and corrosion resistance. In this article, a-SiC thin films were deposited on AISI 304 stainless steel substrates at room temperature for deposition times of 4 and 6 h using pulsed laser deposition technique. The deposition process was stopped every 2 h then resumed for an hour. The effect of interval time during deposition process and substrate type on the properties of the produced films was extensively investigated. The morphological features of the deposited SiC films were investigated using field-emission scanning electron microscope and atomic force microscope. The film structures were determined by transmission electron microscopy, X-ray photoelectron spectroscopy and energy-dispersive X-ray. The mechanical and tribological properties, such as Young’s modulus, hardness and scratch resistance, were determined using nanoindenter. The results showed the formation of uniform monocrystalline nanostructured Si interface between two a-SiC layers after the 2 h of no-deposition time intervals. The formation of crystalline Si interface attributed to the effect of high kinetic energy of the incoming ablated particles deposited on the grown a-SiC layers. The a-SiC films were amorphous having nanostructure grains with dimensions ≤ 100 nm. All films showed smooth surfaces with fine cracks due to the presence of intrinsic stresses. The deposited films showed low mechanical properties due to their amorphous structures.