QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY
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PRINT ISSN: 0288-4771

QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol. 38 (2020), No. 4

  • A Simple Repair Method of Fatigue Cracks Using Stop-holes Reinforced with Wedge Members

    pp. 335-350

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    DOI:10.2207/qjjws.38.335

    A simple method of repairing fatigue cracks using stop-holes reinforced with wedge members, that was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. Specifically, slope-type wedge members have been adopted, and an adaptive mechanism of the wedge member has been devised using a pulley and a wire-type displacement meter. Fatigue tests were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of simple and adaptive wedge members. In addition, an anti-fatigue smart paste, which consists of fine alumina particles and silicone grease, was applied to the periphery of the drill hole, and its effects on automatic restraint and visual detection of crack growth were investigated. FE analyses using contact elements were also carried out for a comparative study. As a result, it was found that the strain range on the specimen side is reduced to 44.0% and the fatigue life is prolonged by 12.4 times by application of the adaptive wedge member as compared with the case of the conventional stop-hole. As for the anti-fatigue smart paste, it was found that the fatigue life is further prolonged by about three times that in the case of the adaptive wedge member and the paste has an evident function to support visual detection of cracks.
  • Suppression of root flaw in friction stir welded 6061-T6 aluminum alloy using double spiral tool

    pp. 351-354

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    DOI:10.2207/qjjws.38.351

    Friction Stir Welding (FSW) has been attracting attention for its usefulness such as relatively easy joining compared to conventional arc welding, and its application to industries is being expanded, for example thick aluminum plates for infra-structures.FSW is known to yield defects called root flaws depending on welding conditions. In order to suppress the defects, it is necessary to optimally control the plastic flow and to stir the materials to be welded cooperatively. The present study tried to make a sound joint of 6061-T6 aluminum alloy sheets with 3mm in thickness using a newly designed tool called “Double Spiral Tool” which can increase the volume of the stir zone around the tool. Two types of tools were used: a normal type with M4 to M3 screws (called single spiral tool) and a “double spiral” type with the same pitch and twice amount of lead length. The welding tests revealed that the root flaw was completely disappeared with the double spiral tool under the welding condition of 900rpm-50mm/min, while it still remained in the joint produced by the single spiral tool. Thus, the present study successfully proved that the double spiral tool is effective to suppress the root flaw in FSW joints of 6061 aluminum alloy.
  • Numerical simulation of weld pool formation in submerged arc welding process

    pp. 355-362

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    DOI:10.2207/qjjws.38.355

    Submerged arc welding process which is applied to large constructions is a high heat input and high deposition rate process. To achieve a higher productive and higher quality process, the phenomena in the process have to be understood and controlled. However, the molten metal and the molten slag are covered by a flux during the process, and cannot be observed directly. In this study, a numerical simulation model of the weld pool formation in the submerged arc welding process is constructed to visualize the phenomena. In this model, both the molten metal and molten slag behavior are calculated and the vaporization of the molten slag is considered. The properties of the arc plasma which is a heat source of the process is given by a simplified heat source model in the simulation model. Radiative energy into the flux and pressure to keep an arc cavity are considered as heat source properties for SAW process simulation. The simulation model can reproduce the shape of the penetration and solidified slag covered the bead of the experimental result by adjusting the heat source parameters. Moreover, influence of the flux on the penetration formation is discussed by the model. When the flux exists, the bead width is wider and the bead height is lower than those without flux. This is because, the temperature of the side region of the bead becomes higher before the heat source passes, because the energy given into the flux is transported to the base metal when the flux exists.
  • A New Approach to Controlling Metal Transfer by Dynamic Modification in Gas Composition of Arc Atmosphere

    pp. 363-378

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    DOI:10.2207/qjjws.38.363

    It has been well known that, in GMA welding, various welding phenomena are largely affected by the type of shielding gas used. However, up to the present time, almost no attempt has been made to utilize ‘gas composition’ for controlling welding phenomena. This study focused on this point and, thus, an investigation was made to understand how dynamic gas compositional modification affected various welding phenomena. In this study, pulsed Ar gas was injected periodically into CO2 shielding gas so that the gas composition of arc atmosphere largely modified in a very short period of time. It was found that this quick gas compositional change from CO2 to Ar led to a large decrease in arc voltage and an arc shape change from a constricted shape to a flare shape and that a molten droplet formed at the electrode tip during the CO2 period was released due to a sudden change in force balance between the molten droplet and the arc. The increase in current stimulated by the voltage drop may have assisted the metal drop detachment due to an increase in electromagnetic pinch force. If the cycle of this periodical Ar gas injection was selected to be shorter than that of repelled metal transfer in CO2 atmosphere, the repelled transfer was avoided and stable drop transfer occurred instead with far less spatter. Appropriate selections of the duration time of pulsed Ar gas injections avoided spray transfer which might cause undesirable finger-like penetration profiles. This new conceptual welding method, namely ‘Pulsed Gas MAG Welding’, required only a small amount of Ar gas to obtain the effects mentioned above.
  • Experimental study on the metal transfer control by using pulsed Ar addition in CO2 arc welding process

    pp. 379-391

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    DOI:10.2207/qjjws.38.379

    CO2 gas shielded arc welding, which is widely used in the industrial world, has been recognized as an efficient and economical GMA welding process. However, it’s characteristic repelled transfer produces a large amount of spatter which results in costly cleanup and, thus, practical methods to overcome this weakness have been highly demanded. In the series of this study, ‘Pulsed Gas MAG’ welding was proposed as a new approach to reducing spatter, in which Ar gas pulses were periodically injected into CO2 shielding gas so that gas composition of arc atmosphere was largely modified in a very short period of time and a metal droplet formed at the electrode tip during CO2 atmosphere was released before repelled transfer had occurred. In this report, the influence of parameters of Ar gas injections, such as frequencies, average gas flow rates and gas injection periods, on the metal transfer was investigated. It was found that synchronous metal transfer stimulated by pulsed Ar gas injections was achieved at any frequencies from 35 to 65Hz under the conditions of gas injection periods longer than 4ms and of Ar gas volumes larger than approximately 1mL/pulse. This is because Ar gas injections below these critical levels resulted in lack of time for arc shape changes and metal drop detachment. From these experimental results, a model to explain the metal transfer phenomena was developed. The amount of spatter was reduced to the levels as low as that of the conventional MAG welding with 80%Ar+20%CO2 shielding gas mix and penetration profiles were round shape like those obtained in CO2 arc welding.
  • Numerical study of the effects of short-circuiting current on a controlled short-circuit transfer process

    pp. 392-402

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    DOI:10.2207/qjjws.38.392

    Gas metal arc welding has wide industrial applications so that there is a high demand for increased efficiency and quality for it. A controlled short-circuit transfer process, which means a process realized by appropriate current and wire feed control in this paper, is a low heat input gas metal arc welding process. In this process, short-circuit transfer is stably and periodically repeated to enable the low heat input and the high deposition rate. Recent years, this type of process has been investigated to be applied to dissimilar materials welding, wire arc additive manufacturing and lap joint welding of thin plates of high tensile strength steel, these are current important topics. However, control factors of the process are not clear enough because of the complexity of its welding phenomena and the lack of investigations for them. In this study, a controlled short-circuit transfer process is modeled in two-dimensions to clarify the metal transfer and weld pool phenomena during the process. In this report, the effects of short-circuiting current on the process are investigated. We revealed that the electromagnetic force induced by short-circuiting current has a large effect on the weld formation process. Short-circuiting current induced the high pressure in the liquid column that is formed between electrode and base metal while short-circuiting, and the strong convection flow induced by the large pressure gradient between the liquid column and the weld pool promoted the weld formation. We also revealed that the electromagnetic force affects on the droplet transfer process. Short-circuit transfer cycles became more instable when the short-circuiting current is higher.

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