Friction Stir Welding of AZ31B-H24 Magnesium Alloy in Lap Configuration

Friction stir welding (FSW) being an enabling solid-state joining technology can be suitably applied for the assembly of lightweight magnesium alloys. In this study, AZ31B-H24 Mg alloy sheets with a thickness of 2 mm were friction stir welded in lap configuration using two tool rotational rates of 1000 and 1500 rpm and two welding speeds of 10 and 20 mm/s. The joint quality was characterized in terms of the residual stresses, welding defects, microstructure, and texture. The mechanical properties including hardness, room and elevated temperature tensile and fatigue properties were also evaluated and correlated to the structure and defects. It was observed that the hardness decreased from the base metal (BM) to the stir zone (SZ) across the heat-affected zone (HAZ) and thermomechanically-affected zone (TMAZ). The lowest value of hardness appeared in the SZ. With increasing tool rotational rate or decreasing welding speed, the average hardness in the SZ decreased owing to increasing grain sizes, and a Hall-Petch-type relationship was established. The shear tensile behavior of the lap joints was evaluated at low (-40°C), room (25°C), and elevated (180°C) temperatures. The failure load was highest in the lower heat input condition that was obtained at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s at all the test temperatures, due to a smaller hooking height, larger effective sheet thickness, and lower tensile residual stress, as compared with other two welding conditions that were obtained at a higher tool rotational rate or lower welding speed. The lap joints usually fractured on the advancing side of the top sheet near the interface between the TMAZ and the SZ. Elevated temperature testing of the weld assembled at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s led to the failure along the sheet interface in a shear fracture mode due to the high integrity of the joint that exhibited large plastic deformation and increased total energy absorption. Fatigue fracture of the lap welds always occurred at the interface between the SZ and TMAZ on the advancing side where a larger hooking defect was present (in comparison with the retreating side). The welding parameters had a significant influence on the hook height and the subsequent fatigue life. A relatively “cold” weld, conducted at a rotational rate of 1000 rpm and welding speed of 20 mm/s, gave rise to almost complete elimination of the hooking defect, thus considerably (over two orders of magnitude) improving the fatigue life. Fatigue crack propagation was basically characterized by the formation of fatigue striations concomitantly with secondary cracks.

Friction Stir Welding of AZ31B-H24 Magnesium Alloy in Lap Configuration

Friction stir welding (FSW) being an enabling solid-state joining technology can be suitably applied for the assembly of lightweight magnesium alloys. In this study, AZ31B-H24 Mg alloy sheets with a thickness of 2 mm were friction stir welded in lap configuration using two tool rotational rates of 1000 and 1500 rpm and two welding speeds of 10 and 20 mm/s. The joint quality was characterized in terms of the residual stresses, welding defects, microstructure, and texture. The mechanical properties including hardness, room and elevated temperature tensile and fatigue properties were also evaluated and correlated to the structure and defects. It was observed that the hardness decreased from the base metal (BM) to the stir zone (SZ) across the heat-affected zone (HAZ) and thermomechanically-affected zone (TMAZ). The lowest value of hardness appeared in the SZ. With increasing tool rotational rate or decreasing welding speed, the average hardness in the SZ decreased owing to increasing grain sizes, and a Hall-Petch-type relationship was established. The shear tensile behavior of the lap joints was evaluated at low (-40°C), room (25°C), and elevated (180°C) temperatures. The failure load was highest in the lower heat input condition that was obtained at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s at all the test temperatures, due to a smaller hooking height, larger effective sheet thickness, and lower tensile residual stress, as compared with other two welding conditions that were obtained at a higher tool rotational rate or lower welding speed. The lap joints usually fractured on the advancing side of the top sheet near the interface between the TMAZ and the SZ. Elevated temperature testing of the weld assembled at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s led to the failure along the sheet interface in a shear fracture mode due to the high integrity of the joint that exhibited large plastic deformation and increased total energy absorption. Fatigue fracture of the lap welds always occurred at the interface between the SZ and TMAZ on the advancing side where a larger hooking defect was present (in comparison with the retreating side). The welding parameters had a significant influence on the hook height and the subsequent fatigue life. A relatively “cold” weld, conducted at a rotational rate of 1000 rpm and welding speed of 20 mm/s, gave rise to almost complete elimination of the hooking defect, thus considerably (over two orders of magnitude) improving the fatigue life. Fatigue crack propagation was basically characterized by the formation of fatigue striations concomitantly with secondary cracks.

Electrochemical removal of pyrite scale using green formulations

Pyrite scale formation is a critical problem in the hydrocarbon production industry; it affects the flow of hydrocarbon within the reservoir and the surface facilities. Treatments with inorganic acids, such as HCl, results in generation toxic hydrogen sulfide, high corrosion rates, and low dissolving power. In this work, the dissolution of pyrite scale is enhanced by the introduction of electrical current to aid the chemical dissolution. The electrolytes used in this study are chemical formulations mainly composed of diethylenetriamine-pentaacetic acid–potassium (DTPAK5) with potassium carbonate; diethylenetriamine pentaacetic acid sodium-based (DTPANa5), and l-glutamic acid-N, N-diacetic acid (GLDA). DTPA and GLDA have shown some ability to dissolve iron sulfide without generating hydrogen sulfide. The effect of these chemical formulations, disc rotational rate and current density on the electro-assisted dissolution of pyrite are investigated using Galvanostatic experiments at room temperature. The total iron dissolved of pyrite using the electrochemical process is more than 400 times higher than the chemical dissolution using the same chelating agent-based formulation and under the same conditions. The dissolution rate increased by 12-folds with the increase of current density from 5 to 50 mA/cm2. Acid and neutral formulations had better dissolution capacities than basic ones. In addition, doubling the rotational rate did not yield a significant increase in electro-assisted pyrite scale dissolution. XPS analysis confirmed the electrochemical dissolution is mainly due to oxidation of Fe2+ on pyrite surface lattice to Fe3+. The results obtained in this study suggest that electro-assisted dissolution is a promising technique for scale removal.

Visible and near-infrared spectro-interferometric analysis of the edge-on Be star o Aquarii

Aims. We present a detailed visible and near-infrared spectro-interferometric analysis of the Be-shell star o Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. Methods. We analyzed spectro-interferometric data in the Hα (VEGA) and Brγ (AMBER) lines using models of increasing complexity: simple geometric models, kinematic models, and radiative transfer models computed with the 3D non-LTE code HDUST. Results. We measured the stellar radius of o Aquarii in the visible with a precision of 8%: 4.0 ± 0.3 R⊙. We constrained the circumstellar disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The emitting disk sizes in the Hα and Brγ lines were found to be similar, at ~10–12 stellar diameters, which is uncommon since most results for Be stars indicate a larger extension in Hα than in Brγ. We found that the inclination angle i derived from Hα is significantly lower (~15°) than the one derived from Brγ: i ~ 61.2° and 75.9°, respectively. While the two lines originate from a similar region of the disk, the disk kinematics were found to be near to the Keplerian rotation (i.e., β = −0.5) in Brγ (β ~ −0.43), but not in Hα (β ~ −0.30). After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the circumstellar disk in both lines: a base disk surface density Σ0 = 0.12 g cm−2 and a radial density law exponent m = 3.0. The same kind of discrepancy, as with the kinematic model, is found in the determination of i using the BeAtlas grid. The stellar rotational rate was found to be very close (~96%) to the critical value. Despite being derived purely from the fit to interferometric data, our best-fit HDUST model provides a very reasonable match to non-interferometric observables of o Aquarii: the observed spectral energy distribution, Hα and Brγ line profiles, and polarimetric quantities. Finally, our analysis of multi-epoch Hα profiles and imaging polarimetry indicates that the disk structure has been (globally) stable for at least 20 yr. Conclusions. Looking at the visible continuum and Brγ emission line only, o Aquarii fits in the global scheme of Be stars and their circumstellar disk: a (nearly) Keplerian rotating disk well described by the viscous decretion disk (VDD) model. However, the data in the Hα line shows a substantially different picture that cannot fully be understood using the current generation of physical models of Be star disks. The Be star o Aquarii presents a stable disk (close to the steady-state), but, as in previous analyses, the measured m is lower than the standard value in the VDD model for the steady-state regime (m = 3.5). This suggests that some assumptions of this model should be reconsidered. Also, such long-term disk stability could be understood in terms of the high rotational rate that we measured for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.

Смещения предела устойчивости течения при модуляции скорости вращения

The possibilities of the control of stability limit location were experimentally investigated in spherical Couette flow. The inner sphere rotational rate is periodically varied about non-zero average value, the outer sphere is fixed. Instability in the form of travelling azimuthal waves, the same as at stationary rotation, is caused by increasing of average rotational rate. Flow velocity measurements were carried out by laser Doppler anemometer. It was shown, that when modulation frequency become close to eigenfrequency of linear mode, both flow destabilization and stabilization were possible depending on modulation amplitude.