Microcracks in aluminium alloys developed in the cleaning action region

Microcracks developed in the cleaning action region during the GTA (TIG) process have been investigated using aluminium and aluminium alloys. Although no microcracks are found in pure aluminium and commercially pure aluminium, microcracks are mainly developed in Al-Mg alloys where the oxide film is ruptured by the cleaning action. The microcracking trend found is analogous to the weld solidification crack susceptibility of Al-Mg alloys, and the microcracks developed in the cleaning action region are predicted to be related to solidification cracks. The microcracks readily develop with an increasing EN ratio of the welding current. A pattern of solidified dendrites is clearly visible, suggesting that the microcracking has developed by the cathode spot formed leading to local melting. The microcracking is also intergranular. The cooling rate during solidification can be estimated from the dendrite size, being put at around 10 4 °C/sec. Microcrack development is considered to be due to the fact that the region where the oxide film is ruptured is locally melted by the high current density and the very high cooling rate. The X-ray diffraction results suggest that continuous faint Debye-Scherrer rings are obtained from the region where the oxide film is ruptured even when a single crystal is used. The single crystal here changes to very small polycrystals in this region, the polycrystallised depth from the specimen surface being around 100 μm. The fatigue strength of microcracked specimens at 1 × 10 7 cycles is lower than that of the base metal, decreasing linearly with an increasing crack' depth.