Abstract
Microcracks developed in cleaning action region by GTA process have been investigated using an aluminum and aluminum alloys. Although no microcracks are observed in a pure aluminum and a commercially pure aluminum, microcracks are mainly developed in the region of Al-Mg alloys where the oxide film is broken due to the cleaning action. This tendency of cracks is quite the same as the weld solidification crack. The microcracks are easily developed with the increase in EN ratio of the welding current. Since dendrites are observed on the crack surface, the microcracks is intergranular due to the solidification crack. The cooling rate in this case was estimated to be about 104°C using the size of the dendrites. The microcracks are considered to be developed due to the phenomena that the region where the oxide film is broken is locally melted owing to the high current density and the cooling rate is very high. X-ray diffraction technique made clear that the continuous vague Debye-Scherrer rings are obtained from the region where the oxide film is broken even when using a single crystal. This shows that the single crystal changes to very small polycrystals in this region and the polycrystallized depth from the specimen surface was about 100μm. Fatigue strength at 1 × 107 cycles of specimens having the microcracks is lower than that of the base metal and decreases linearly with the increase in the crack depth.
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