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. 3

  • Characteristics of Material Flow in Friction Stir Spot Welding Tools in Consideration of Cutting Tools

    pp. 125-133

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    DOI:10.2207/qjjws.38.125

    This paper investigates the material flows in A5052/A6061 lap joints when friction stir spot welding tools with cutting edges and rake angles similar to a cutting tool are used. In this study, several kinds of modelling experiment were used to visualize the material flows during the friction stir spot welding. The majority of the material flows in A5052/A6061 lap joints was diagonally upward from the lower sheet, whether the rake angle of the probe with the cutting edge similar to a cutting tool was negative or positive. When the tool shoulder failed to contact the upper sheet surface, the stirring ability of the probe alone was to an extent high with a negative rake angle and very low with a rake angle of 0° and with positive rake angles. However, when the tool shoulder came into contact with the upper sheet surface and the chips generated by the probe were clogged, the stirring abilities of the tool with a rake angle of 0° and with positive rake angles were very high. In addition, in the tool with a rake angle of 0° and with positive rake angles where the discharge of chips was smooth, the lower sheet material moved upward as the chips were stirred and produced new material flows. From these results, when these tools are used, it is suggested that the generation, the discharge and the stirring of chips affect the material flow upwards, which is crucial, and also downwards, in A5052/A6061 friction stir spot lap joints.
  • Numerical simulation and experiment on brittle fracture surface morphologies in steel

    pp. 134-146

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    DOI:10.2207/qjjws.38.134

    A numerical simulation model of brittle crack propagation was developed incorporating fracture surface irregularity, and small-scale crack arrest experiment of a steel plate was conducted to validate the model. Mechanisms of the formation of brittle fracture surface irregularities including chevron markings in the steel were discussed based on the model calculations and the experiment. Formation of the chevron markings was found to depend on applied stress intensity factor. The model calculations and the experiment showed that the chevron markings are nucleation and continuation of ridges, which are formed between two cleavage crack terraces with different height levels. The dependence of the extent of the chevron markings on applied stress intensity factor is understood as that a deep ridge reduces local stress intensity factor by shear stress acting on the ridge, and the deep ridge can develop only at high stress intensity factor level but only shallow ridges are possible if stress intensity factor level is low. This tendency agreed between the experiment and the calculation. Relationship between crack arrest toughness and fracture surface irregularities is discussed.
  • Study of burn-through prediction in MAG arc welding using molten pool monitoring technique

    pp. 147-156

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    DOI:10.2207/qjjws.38.147

    In this paper, in order to predict the occurrence of excessive penetration and burn-through that occur in groove MAG welding, we observed the molten pool with cameras, and considered parameters correlating with the occurrence of the excessive penetration and burn-through. In the case of groove MAG welding using a robot, excessive penetration and burn-through that occur due to the effects of disturbances such as gap between base materials caused by heat distortion and member accuracy are obstacles to improvement of welding quality and production efficiency. So, we constructed a simultaneous observation system for the top and bottom side of the molten pool using two cameras. Molten pool width (W) and reduction of molten surface height (R) are extracted as feature quantities, and compared with the state of the bottom side of the molten pool. Although the decrease of W and R was confirmed, they alone were not enough to predict the occurrence of burn-through. Therefore, W+R was set as a new parameter (P) in consideration of the molten pool cross-sectional shape change. As a result, the decrease of P was confirmed before the occurrence of burn-through, and it was shown that P is an effective parameter for predicting the generation of burn-through.
  • Observation of Micro Fatigue Cracks at GMA Welded Joint of Thin Steel Sheet

    pp. 157-163

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    DOI:10.2207/qjjws.38.157

    Fatigue tests were performed on GMA welded joints using a prototype steel sheets with varying amounts of C. In addition, the origin of the fatigue cracks in the thin steel GMA welded joints were investigated through observation the micro cracks in the specimens that were judged to be not fractured by fatigue limit stress in the fatigue test. As a result of plane bending fatigue test of bead-on-plate welding sheet, fatigue limit of the welded specimen did not differ much between the steel sheets. Observing the specimens which were not fractured at the fatigue limit, micro cracks often occurred at the bottom of the ripples on the weld metal surface and in the ferrite structure. It was founded that fatigue cracks in thin steel sheet GMA welding using mixed gas occurred parallel along the bottom of the ripples and parallel cracks merged with the crack growth, resulting in a zigzag propagation path on the weld joint surface. The effect of ripple on the fatigue limit was predicted by comparing estimating the fatigue limit without considering the ripple with the fatigue limit obtained from experiment. Finally, the effect of ripple on the fatigue limit was more than 10% and less than 20%. And it was implied that higher the strength of the material, the greater the effect of ripple on the fatigue limit.
  • Observation of welding phenomena with blowholes for detection of welding defects

    pp. 173-182

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    DOI:10.2207/qjjws.38.173

    Arc welding is important technology for manufacturing that is directly linked to quality and productivity. Especially GMAW with Ar or CO2 shielding gas is widely used in various industries because of its high productivity. To achieve high quality and productivity, in-process welding quality control technology will be required. In this study, the authors observe welding phenomena with Blowholes(BH) by using high speed cameras to develop in-process welding defects detection technology with image sensing methods. By observing welding phenomena with BH bubbling phenomena are observed just under the arc and on the surface of moltenpool. It was found that the bubble generation on the surface of the molten pool is correlated with the pinhole generation, and the bubble generation just under the arc is correlated with the blowhole generation.
  • Influence of Thickness Reduction in Base Plate on Elastic Stress State of Patch Plate Joint by Combination of Welding and Bonding

    pp. 183-192

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    DOI:10.2207/qjjws.38.183

    For investigating influence of thickness reduction in base plate on elastic stress state of patch plate joint by combination of welding and bonding, a series of experiment and analysis were conducted. A static tensile loading experiment was carried out. The thickness reductions of 2.5mm were applied to the both top and bottom surfaces of base plate with a thickness of 12mm. The thickness-reduced parts of base plate were covered by the patch plates with a thickness of 6mm. The joining methods of patch plates were only fillet welding, only bonding and the combination of welding and bonding. The load-carrying effect at the center of joint was improved by the combination of welding and bonding compared to the case with only welding. However, the stress reduction effect around the welded part could not be clearly confirmed from the experimental result. 2D elastic analysis was performed for simulating the experiment and examining the stress reduction effect around the welded part in detail. Furthermore, the influence of degree of thickness reduction in base plate on the stress state around the welded part was investigated. The results indicated that the stress reduction effect by the combination of welding and bonding was larger around the weld root rather than the weld toe.

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