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.
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.
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.