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Welding residual stresses are considered as significant factors for the reduction of fatigue life and load bearing of welded structures. In this article, characterization of a low-temperature stress-relief method called vibratory stress relief process was investigated. In this regard, welding of a butt joint of two steel plates was simulated and induced welding residual stresses were calculated. An experimental residual stress measurement was conducted to verify the numerical results. It was established that the simulated residual stresses are in good agreement with the measured results obtained from the hole drilling strain gauge measurements. In the following, the vibratory stress relief process of the welded joint was simulated using finite element method (FEM). It was observed that by controlling the process parameters, a notable reduction in the residual stress is achievable. Effects of magnitude and frequency of the cyclic force on the effectiveness of the stress relief process were studied. The results indicated that by increasing the applied load frequency up to 95% of the natural frequency, the longitudinal residual stress decreased more than 80%. Also, it has been shown that by increasing the force magnitude, the welding residual stresses were reduced drastically. [ABSTRACT FROM AUTHOR]

The primary disadvantage of friction stir spot welding is the keyhole reminds after the welding process which removing of this keyhole is considered in this study. In addition, alumina nanopowder is incorporated into the weld stir zone as a technique to increase the joint strength. Flat friction stir spot welds with and without alumina nanopowder are examined, and the influences of the main process parameters on the joint strength and weld microstructure were investigated. It was found that the most effective parameters on the joint strength were the normal plunge depth, dwell time, and tool rotational speed, respectively. Tool rotational speed of 1120 rpm, 5 s dwell time, and normal plunge depth of 2.75 were found as the optimum process parameters. In this case, the strength of the friction stir spot weld increased 23% due to adding alumina nanopowder. The hardness of the stir zone was also increased, somewhat. [ABSTRACT FROM AUTHOR]

In this study, a new approach for dissimilar friction stir welding of aluminum alloy and magnesium alloy was developed. In order to make metal matrix composite in the weld stir zone, silicon carbide nanoparticles were embedded into the weldment. The admixture pattern of the stir zone, weld microstructures, intermetallic phases, powder distribution, and mechanical properties of the welds were investigated in this paper. Due to using silicon carbide nanoparticles as reinforcement in friction stir welding, the weld microstructure was affected by pinning mechanism. Meanwhile, it was observed that the formation of brittle and hard intermetallic phases also affected the welded joint microstructure. The effects of friction stir welding process parameter such as tool rotational and traverse speeds were also examined. It was observed that increasing the tool rotational speed and decreasing the tool traverse speed improved the mixing of two alloys at the stir zone. By controlling the process parameters, a fine microstructure with an average grain size of 4.3 μm at the stir zone was obtained. [ABSTRACT FROM AUTHOR]

In this study, the effects of tool rotational and traverse speeds on the mechanical properties and the microstructure of the friction stir welded joints from Al 5086-H34 were studied. Insufficient heat generation and inadequate metal transportation at very low rotational speed and high turbulence in the plasticized metal at very high rotational speed emerged tunnel and worm hole defects into the weldment. It was discovered that as rotational and traverse speed increased, the average grain size of the weldment decreased due to more dynamic recrystallization and amplified stirring effect. An increase in the rotational speed increased the ultimate tensile strength and microhardness of the specimens. At the best welding condition, by employing the rotational and traverse speeds of 1250 rpm and 80 mm/min, respectively, 51 % enhancement in the elongation and 8 % increase in the microhardness of the welded samples were obtained. Moreover, the ultimate tensile strength of this welded joint reached to 85 % of the base metal. [ABSTRACT FROM AUTHOR]

This research considers an open shop scheduling problem with preventive maintenance. A specific level of reliability is assumed and a mathematical model is presented to schedule both production and maintenance tasks, simultaneously. Three different and conflicting objective functions containing machine availability, make-span, and total tardiness and earliness have been optimized in the proposed model. When there are more than two machines in the open shop problem, it is classified in the category of NP-hard problems. Consequently, classical approaches cannot reach to an optimal solution in a reasonable time. Thus, two well-known algorithms for multi-objective problems containing non-dominated sorting genetic algorithm-II (NSGA-II) and multi-objective particle swarm optimization (MOPSO) are developed to find the best near-optimal solutions. The surface response methodology (RSM) is applied to tune parameters of the developed algorithms. Then, the reliabilities of the presented algorithms are illustrated based on three evaluation metrics comprising the number of Pareto solutions, spacing, and diversity. Furthermore, the superiority of the proposed algorithms is shown through benchmarking approach. The algorithms may be used in other open shop problems because they are able to find the best and reliable near-optimal solutions in a reasonable processing time. [ABSTRACT FROM AUTHOR]