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Blowholes and their formation of Pulsed MAG welding was investigated. Among welding parameters such as arc length, average current, wire extension, welding speed and shielding gases, arc length was the most important factor. The average size of blowholes was as small as 0.62 mm. As results of analysing gas components of the blowholes by means of gas chromatograph, Ar, H2, and CH4 were detected and CO2 and CO and O2 were not in the blowholes. H2 and CH4 were also detected in blank test pieces. From SEM observation, the shape of the blowhole was like a sphere, the bottom of which was flat and the upper side was uneven like a corn having a slag spot at the top. The slag composition in the blowhole was Al, Si, Mn, Ca, La and Ce etc., and differed from the slag on the weld bead. From these results, the mechanism of blowhole formation in Pulsed MAG welding was considered as follows. Disturbance of the weld pool accompanied with spattering forms gas bubbles of the shielding gas in the pool. Small bubbles tend to be dragged into the bottom of the pool by whirling action, and be confirmed there to form blowholes close to the fusion line. CO2 in the blowholes are deoxidized to form the slag at the top of the blowholes.

Crack susceptibilites of weld crater and weld bead of synthesized-weld-metal Al-2%Zn-3%Mg alloy with and without zirconium up to 0.36% have been evaluated by means of GTA spot weld crater, Houldcroft type and Vatrah weld bead cracking tests.It was proved that zirconium was beneficial additional element to reduce crack susceptibility of both weld crater and bead with more addition than 0.24%. This beneficial effect of zirconium was closely related with grain refinement of weld structure. The grain refinement of weld bead resulted in the increase in the cracking threshold in BTR, although BTR was not changed.

It is very important to analyze accurately and practically the stress concentration factor due to structural discontinuity, Kts, in evaluation of fatigue strength at weld toes.In this paper, a practical method for analysis of Kts was proposed with the aid of finite element method using plate elements taking weld bead effect into account.As a result, concerning Kts of the plate with rib under uniform tension and bending, it was found that the calculated values using the proposed model of fine mesh agree with those using brick elements with an accuracy of ±10 percent. For five types of welded joints, the good agreement was obtained between calculated stress distribution and measured one near the weld toe.