EFFECT OF RETROGRESSION AND REAGING (RRA) PROCESS PARAMETERS ON STRESS CORROSION CRACKING (SCC) RESISTANCE AND MECHANICAL PROPERTIES OF ALUMINUM ALLOYS 2024

• Main Author: Janitra Jodi, Eureka
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/75940
• Institution: Institut Teknologi Bandung

Air transportation is one of the most commonly used modes of transportation, it is even predicted that the number of airplanes will doubled on 2040 compared to 2020. The materials used for aircraft fuselage and wings must meet several criteria such as high strength, good fatigue resistance, high toughness, and good corrosion resistance. One of the alloys that is considered to meet these criteria and is commonly used is aluminum alloy 2024 (AA2024). Even so, AA2024 is susceptible to localized corrosion such as stress corrosion cracking (SCC). The operation of aircraft above sea level causes the deposition of chloride ions which facilitates the occurrence of SCC. SCC resistance can be increased in various ways, one of which is by heat treatment. However, in single stage heat treatment, strength is inversely proportional to SCC resistance so it is not possible to obtain a good combination of strength and SCC resistance. Retrogression and reaging (RRA) is a heat treatment method developed to increase SCC resistance without significantly decreasing the mechanical properties. The RRA heat treatment has been developed from 1989 for the 7000 series aluminum alloys, however the effect of the RRA heat treatment on other heat treatable aluminum alloys is still not well understood. In this research, the influence of the RRA heat treatment process parameters on AA2024 alloy was studied to gain an understanding of the effect of process parameters and obtain the optimum parameters to obtain the best combination of mechanical properties and SCC resistance. The parameters of the RRA process studied were preaging conditions, retrogression temperature, retrogression duration, and aging duration. The SCC resistance test was carried out using the constant load method with a loading of 75% of the yield strength value of AA2024-T6 or 250 MPa. The simulation for a corrosive environment containing chloride ions is by using NaCl solution with a concentration of 5% (percent by weight). Testing is carried out until the sample fails or is discontinued after 10 days. Susceptibility to SCC is determined based on the duration until fracture and the strain rate. In addition to constant load test in a corrosive environment, SCC resistance is also measured by measuring electrical conductivity. The studied mechanical properties are hardness, strength, and elongation at fracture which are carried out through hardness tests and tensile tests. Analysis using an optical microscope (OM) as well as a scanning electron microscope (SEM) was performed to see the effect of the RRA process parameters on the AA2024 microstructure. The SCC test sample that failed was seen using SEM to analyze the fracture surface. The preaging condition in the form of natural aging (NA) or temper designation T4 produces mechanical properties in the form of strength and hardness which are significantly higher than artificial aging (AA) or temper designation T6. Samples which were artificially aged shows that an increase in retrogression temperature, retrogression duration, and reaging duration causes a decrease in strength and hardness, on the other hand samples with preaging condition in the form of NA shows that an increase in retrogression temperature causes a decrease in strength, while an increase in the duration of retrogression and reaging causes an increase in strength and hardness. The formability of the samples preaged in the form of AA was higher than the samples preaged in the form of NA based on the strain hardening exponent value. SCC resistance in artificially aged samples will increase along with increasing retrogression temperature, retrogression duration, and reaging duration. Meanwhile, in samples preaged in the form of NA, an increase in retrogression temperature resulted in an increase in SCC resistance, while an increase in retrogression and reaging duration resulted in a tendency to decrease SCC resistance. Samples RT-1 and RD-3 have high strength and hardness, but have poor SCC resistance. Samples RT-2, RD-2, and SA-2 have strength and hardness that are close to samples RT-1 and RD-3 with significantly better SCC resistance, so they were considered to have the best combination of mechanical properties and SCC resistance.