EFFECT OF DOUBLE RETROGRESSION AND REAGING HEAT TREATMENT (DRRA) ON STRESS CORROSION CRACKING AND MECHANICAL PROPERTIES OF 2024 ALUMINIUM ALLOY

• Main Author: Janitra Jodi, Eureka
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/66149
• Institution: Institut Teknologi Bandung

Aluminum alloy is an alloy that is very commonly used in everyday life ranging from construction, food and catering, electronics, to transportation such as aircraft. One of the most common alloys used in aircraft is aluminum 2024 (AA 2024). AA 2024 has high strength, good fatigue resistance, and high toughness, however AA 2024 is prone to failure in the form of stress corrosion cracking (SCC), especially when used as a construction material for aircraft due to chloride ion deposits from seawater. Increasing the resistance of SCC can be done in various ways, one of the most common is by heat treatment. Nevertheless in one stage heat treatment, strength and SCC resistance is inversely proportional so that it is impossible to gain both good strength and SCC resistance. In this study, various heat treatments were studied, especially double retrogression and reaging (DRRA) on SCC and the mechanical properties of AA 2024. The heat treatment variations used were T62, T72, retrogression and reaging (RRA), and DRRA. The SCC experiments were carried out using the constant load method with a loading of 75% of the yield strength of AA 2024-T6 or 250 MPa. This value is taken because it is considered sufficient to pass the SCC threshold stress value. The solution used to simulate a corrosive environment is NaCl solution with a concentration of 5% (percent by weight). The test is carried out until the sample fails or is stopped after 7 days. Susceptibility to SCC is determined based on the duration to fracture and the rate of crack growth. The mechanical properties studied were hardness, strength, and elongation to fracture which were carried out through hardness tests and tensile tests. The SCC test sample that failed was seen using scanning electron microscope (SEM) to analyze the fracture surface. The highest resistance to SCC was found in T72 sample which did not fail until the 7th day with a lower elongation than the DRRA sample which also did not experience fracture. The lowest SCC resistance was found in sample T62 which failed on day 2. The highest strength and hardness was obtained in sample T62 with values of 366.1 MPa and 120.3 HV respectively. The strength of the RRA and DRRA samples was similar, but the SCC resistance and work hardening potential of the DRRA samples were significantly higher. Therefore, DRRA heat treatment is considered to produce the most optimal combination of SCC strength and resistance.