Failure Investigation on Failed Towing Arm of an Aircraft: A Case of Improper Material Processing and Failed Repair Welding of a Cast Al-Cu-Si Type Aluminium Alloy

Present work describes the metallurgical failure investigation of a failed towing arm/bar. Aircraft attached with this incident travelled a distance of 2 km with normal towing speed (8 km/hr). The component is said to have broken while turning to right from taxi track and then turning left about 15-20o. The mating fracture pieces of the failed towing arm fork along with bent towing pin were analyzed for failure investigation. Parent material of the towing arm has been found to be a cast Al-Cu-Si alloy closely matching with the alloy 295 used in T6 condition. Pin is made of mild steel. Failure of the towing arm fork has been initiated by weld solidification cracking of the weld region at close to the fusion line. Liquation cracks at heat affected zone (HAZ) have aggravated the situation further by weakening the microstructure and facilitating the crack propagation. Grain boundary precipitates of Al-Cu-Fe type in HAZ have facilitated the propagation of failure along with the liquation cracking. On the other hand, Al-Fe-Mn type of precipitates sitting along the grain boundaries in weld area seems to have not facilitated the failure initiation unlike the solidification cracking. Improper material processing has led to the weakening of the bulk microstructure and thus the component itself, by introduction of various defects (pores, grain fall out, thick Al-Cu-Fe type of continuous grain-boundary precipitates and cracking). Weld repairing has been found to be a failure in Al-Cu-Si system in the present component.Keywords: Towing arm, cast Al-Cu-Si, repair welding, hot cracking, solidification cracking, liquation cracking, intergranular fracture, Al-Cu-Fe grain boundary precipitates, Al-Fe-Mn grain boundary precipitates.