P. V. Venkitakrishnan

Microstructure and Mechanical Properties of Friction Stir Lap Welded Aluminum Alloy AA2014

F riction stir lap welds were produced in 3 mm thick Alclad sheets of Al alloy 2014-T4 using two difierent tools (with triangular and threaded taper cylindrical pins). The efiects of tool geometry on weld microstructure, lap-shear performance and failure mode were investigated. The pin proflle was found to signiflcantly in∞uence the hook geometry, which in turn strongly in∞uenced the joint strength and the failure mode. Welds produced in alloy 2014-T4 Alclad sheets by using triangular and threaded taper cylindrical tools exhibited an average lap-shear failure load of 16.5 and 19.5 kN, respectively, while the average failure load for standard riveted joints was only 3.4 kN. Welds produced in alloy 2014-T6 Alclad sheets and in alloy 2014-T4 bare sheets (i:e:, no Alclad) were comparatively evaluated with those produced in alloy 2014-T4 Alclad sheets. While the welds made (with threaded taper cylindrical tool) in T6 and T4 conditions showed very similar lap-shear failure loads, the joint e‐ciency of the welds made in T6 condition (43%) was considerably lower (because of the higher base material strength) than those made in T4 condition (51%). The Alclad layers were found to present no special problems in friction stir lap welding. Welds made with triangular tool in alloy 2014-T4 Alclad and bare sheets showed very similar lap-shear failure loads. The present work provides some useful insights into the use of friction stir welding for joining Al alloys in lap conflguration.

Microstructures and Mechanical Properties of Friction Stir Spot Welded Aluminum Alloy AA2014

Friction stir spot welding (FSSW) is a relatively recent development, which can provide a superior alternative to resistance spot welding and riveting for fabrication of aluminum sheet metal structures. In the current work, FSSW experiments were conducted in 3-mm thick sheets of aluminum alloy 2014 in T4 and T6 conditions, with and without Alclad layers. The effects of tool geometry and welding process parameters on joint formation were investigated. A good correlation between process parameters, bond width, hook height, joint strength, and fracture mode was observed. The presence of Alclad layers and the base metal temper condition were found to have no major effect on joint formation and joint strength. Friction stir spot welds produced under optimum conditions were found to be superior to riveted joints in lap-shear and cross-tension tests. The prospects of FSSW in aluminum sheet metal fabrication are discussed.

Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion and nano-mechanical properties of the coating were examined by electrochemical impedance spectroscopy and nano-indentation tests and compared with the conventional electroplated cadmium and IVD aluminum coatings. Electrochemical impedance spectroscopy was performed by immersing the coated specimens in 3.5% NaCl solution, and the impedance behavior was recorded as a function of immersion time. The mechanical properties (hardness and elastic modulus) were obtained from each indentation as a function of the penetration depth across the coating cross section. The adhesion resistance of the coatings was evaluated by scratch tests on the coated surface using nano-indentation method. The results show that the gradient Al-Mo coating exhibits better corrosion resistance than the other coatings in view of the better microstructure. The impedance results were modeled using appropriate electrical equivalent circuits for all the coated systems. The uniform, smooth and dense Al-Mo coating obtained by magnetron sputtering exhibits good adhesion with the steel substrate as per scratch test method. The poor corrosion resistance of the later coatings was shown to be due to the defects/cracks as well as the lesser adhesion of the coatings with steel. The hardness and elastic modulus of the Al-Mo coating are found to be high when compared to the other coatings.

Why Do Fasteners Fail? An Insight in to the Modes and Mechanisms of Fastener Failures

Abstract In spite of the experiences in the design, material selection, fabrication and storage, fastener failures are reported frequently. While each failure leaves the user with some wisdom, analysing a failure is time consuming and affects the project schedules. In the aerospace scenario, it is all the more important to analyse the causes of fastener failures as a single failure can sometimes be catastrophic and mission critical. In the present paper, an attempt has been made to address why fasteners fail with a view to provide an insight into the modes and mechanisms. Finally, an attempt is made to bring out various modes and mechanisms of fastener failures. Four typical case studies on fastener failures are presented to highlight them.

Friction Stir Welding of Austenitic Stainless Steel to an Aluminum-Copper Alloy

Friction stir welding of austenitic stainless steel AISI 321 to an Al-Cu alloy AA 2219-T87 (3 mm thick sheets) was investigated for a specific aerospace application. Welding experiments were carried out using WC-Co tools with different pin profiles. The effects of process parameters, including tool positioning were studied. After careful process optimization, welds with a tensile strength of close to 250 MPa were successfully produced. The welds showed a very rugged stainless steel/aluminum interface as well as some fragments of stainless steel in the stir zone, confirming complete removal of the oxide film on the stainless steel faying surface due to the wearing action of the stirring aluminum. A very thin intermetallic layer (consisting of FeAl, Fe3Al and AlCrFe2 phases) was observed in the SS/Al interface.