Optimization of friction stir welding parameters for dissimilar aluminum alloys

Assembly consisting of cast and wrought aluminum alloys has wide spread application in defense and aero space industries. For the efficacious use of the transition joints, the weld should have adequate strength and formability. In the present investigation, A356 and 6061 aluminum alloys were friction stir welded under tool rotational speed of 1000-1400 rpm and traversing speed of 80-240 mm/min, keeping other parameters same. The variable process window is responsible for the change in total heat input and cooling rate during welding. Structural characterization of the bonded assemblies exhibits recovery-recrystallization in the stirring zone and breaking of coarse eutectic network of Al-Si. Dispersion of fine Si rich particles, refinement of 6061 grain size, low residual stress level and high defect density within weld nugget contribute towards the improvement in bond strength. Lower will be the tool rotational and traversing speed, more dominant will be the above phenomena. Therefore, the joint fabricated using lowest tool traversing and rotational speed, exhibits substantial improvement in bond strength (similar to 98% of that of 6061 alloy), which is also maximum with respect to others. (C) 2010 Elsevier Ltd. All rights reserved.

An elastic-viscoplastic constitutive model for the hot-forming of aluminum alloys

Under hot-forming conditions characterized by high homologous temperatures and strain-rates, metals usually exhibit rate-dependent inelastic behavior. An elastic-viscoplastic constitutive model is presented here to describe metal behavior during hot-forming. The model uses an isotropic internal variable to represent the resistance offered to plastic deformation by the microstructure. Evolution equations are developed for the inelastic strain and the deformation resistance based on experimental results. A methodology is presented for extracting model parameters from constant true strain-rate compression tests performed at different temperatures. Model parameters are determined for an Al-1Mn alloy and an Al-Mg-Si alloy, and the predictions of the model are shown to be in good agreement with the experimental data. (C) 2000 Kluwer Academic Publishers.

Nucleation and growth of Al2O3/metal composites by oxidation of aluminum alloys

The nucleation and growth mechanisms during high temperature oxidation of liquid Al-3% Mg and Al-3% Mg-3% Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk Al2O3/Al composite forms. A duplex oxide layer, MgO (upper) and MgAl2O4 (lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl2O4 (probably discontinuous), Al alloy, and finally the bulk Al2O3 composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of Al2O3 on the existing composite body. The roles of Mg and Si in the composite growth process are discussed.

Accumulative roll bonding of aluminum alloys 2219/5086 laminates: Microstructural evolution and tensile properties

The present study describes the course of microstructure evolution during accumulative roll bonding (ARB) of dissimilar aluminum alloys AA2219 and AA5086. The two alloys were sandwiched as alternate layers and rolled at 300 degrees C up to 8 passes with 50% height reduction per pass. A strong bonding between successive layers accompanied by substantial grain refinement (similar to 200-300 nm) is achieved after 8 passes of ARB. The processing schedule has successfully maintained the iso-strain condition up to 6 cycles between the two alloys. Afterwards, the fracture and fragmentation of AA5086 layers dominate the microstructure evolution. Mechanical properties of the 8 pass ARB processed material were evaluated in comparison to the two starting alloy sheets via room temperature tensile tests along the rolling direction. The strength of the 8 pass ARB processed material lies between that of the two starting alloys while the ductility decreases after ARB than that of the two constituent starting alloys. These differences in mechanical behavior have been attributed to the microstructural aspects of the individual layer and the fragmentation process. (C) 2011 Elsevier Ltd. All rights reserved.

Friction Stir-Welded Dissimilar Aluminum Alloys: Microstructure, Mechanical Properties, and Physical State

A356 and 6061 aluminum alloys were joined by friction stir welding at constant tool rotational rate with different tool-traversing speeds. Thermomechanical data of welding showed that increment in tool speed reduced the pseudo heat index and temperature at weld nugget (WN). On the other hand, volume of material within extrusion zone, strain rate, and Zenner Hollomon parameter were reduced with decrease in tool speed. Optical microstructure of WN exhibited nearly uniform dispersion of Si-rich particles, fine grain size of 6061 Al alloy, and disappearance of second phase within 6061 Al alloy. With enhancement in welding speed, matrix grain size became finer, yet size of Si-rich particles did not reduce incessantly. Size of Si-rich particles was governed by interaction time between tool and substrate. Mechanical property of WN was evaluated. It has been found that the maximum joint efficiency of 116% with respect to that of 6061 alloy was obtained at an intermediate tool-traversing speed, where matrix grain size was significantly fine and those of Si-rich particles were substantially small.

Restoration Mechanisms During the Friction Stir Processing of Aluminum Alloys

In the current study, correlation of microstructure evolution with bulk crystallographic texture formation during friction stir processing (FSP) of commercial aluminum alloys has been attempted. Electron back-scattered diffraction and X-ray diffraction techniques were employed for characterizing the nugget zone of optimum friction stir processed samples. Volume fraction of measured texture components revealed that the texture formation in aluminum alloys is similar irrespective of the alloy composition. Recrystallization behavior during FSP was more of a composition dependent phenomenon.