An Investigation of the Fatigue Properties of the Aluminium-copper Alloy 2014

Abstract

Investigating fatigue properties is very important to researchers since the knowledge of the fatigue life is required in both design and predicting the life of aircraft and other structural materials. Through experimental study of fatigue properties, prevention of tragic events like the infamous Aloha airline disasters can be achieved. Owing to its relatively high strength and low density, the aluminium alloy AA 2014 has been the primary structural material for aircrafts. In military and commercial airplanes, rivets are been replaced by welds so as to improve on cost and structural integrity. Porosity and hot cracking are some of the major welding defects encountered during welding of high-strength AA 2014 aluminium alloy. Upon repetitive loading on these welded structural components cracks develop and grow leading to catastrophic failures. This study therefore investigates the fatigue properties of AA 2014 aluminium alloy. Fatigue crack growth (FCG), low cycle fatigue (LCF) and high cycle fatigue (HCF) properties of AA 2014 aluminium alloy as-received were investigated. Welding also affects the microstructure, mechanical properties and fatigue properties of the heat affected zone (HAZ). Therefore, the effect of the welding process on fatigue properties of AA 2014 aluminium alloy was also analyzed. Since investigating HAZ of real welded joints is difficult because of the narrowness of the HAZ, a suitable process of thermal simulation was employed to prepare specimens used to study the various sub zones of the heat affected zone. This process involved thermal simulating the two regions of HAZ to an already predetermined welding temperature. Thermal simulation done was one that resulted in as close as possible to the thermal cycle histories studied earlier by actual welding obtained using alternating current gas tungsten arc welding (GTAW). The simulation process was done using a muffle furnace that had a peak temperature of 1200 °C. Specimens were put in the muffle furnace to simulate the two regions of the HAZ with peak temperatures of 590 °C (region C) and 650 °C (region D) representing regions 5mm and 4mm respectively from the weld center line. The HAZ region located at 5 mm from the weld centerline was found to offer least resistance to fatigue crack growth, had shorter fatigue life and displayed lowest value of hardness and fatigue strength compared to both the base metal (BM) and the HAZ region located at 4mm from the weld center line. The optical micrographs of the base metal and the two regions of the heat affected zone were carried out using a universal optical microscope named OPTIKA B-353 v MET. The optical micrographs were taken at a magnification of X200 with the BM and the two regions of HAZ oriented in the longitudinal transverse direction (L-T). Microstructure characterization was also carried out. The HAZ region C recorded the highest grain size value compared to the base metal and heat affected zoned region D. In this study, HAZ region C was confirmed to be the weakest link in the HAZ which is significantly affected by the thermal cycle profiles developed during the thermal cycle simulation process. The potential precipitates in Al-Cu (2xxx series) alloys particularly AA 2014 are CuAl2, and Al5Cu2Mg8Si5. The possible cause of weakness in HAZ region C was associated with dissolution of strengthening phases. Precipitate dissolution in AA 2014 occurs as the particles of this alloy are exposed to thermal cycle temperatures higher than 400 °C. Therefore, degradation of the strengthening phases of AA 2014 aluminium alloys occurs severely at 590 °C due to dissolution of CuAl2 precipitates in aluminium matrix.