THE EFFECT OF DOUBLE STAGE SOLUTION HEAT TREATMENT AND VARIATION OF AGING TEMPERATURE AND TIME ON THE MICROSTRUCTURE AND HARDNESS OF AL 7075 ALLOY

• Main Author: Nathaniel Lauwis, Andrian
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/67657
• Institution: Institut Teknologi Bandung

Al 7075 alloy is one of the metal alloys widely used in aircraft structural applications owing to its valuable properties, such as light, strong, and good fatigue resistance. The properties of the Al 7075 alloy are mostly acquired because of the treatment process traversed, one of which is heat treatment. Al 7075 alloys in an untreated state, have properties that are undesirable, such as segregation of the second phase particle in the microstructure. To overcome this, a heat treatment process was developed that was able to eliminate the segregation while providing optimal properties for the 7075 Al alloy. In this study, 7075 Al alloy was obtained from PT. Dirgantara Indonesia was studied by determining the influence of twostage solution heat treatment and variations in the aging process on the microstructure and hardness of the alloy. A series of solution heat treatment and aging processes and testing was carried out to determine their effects on the microstructure and hardness of 7075 Al alloys. The solution heat treatment experiment was divided into two, one stage and two stages. For the one-stage process, it is carried out at 470ºC for 2 hours. For the twostage process, they were carried out at 460ºC for 1.5 hours and continued at 485,495, and 505ºC for 45, 30, and 15 minutes. Aging experiments were carried out at temperatures of 120, 140, and160oC for 24, 18, and 16 hours. All experiments were carried out in an inert condition. After the experiment, characterization testing using optical microscope (OM), scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) and hardness test were carried out. Experimental results data were then analyzed to determine the optimal solution heat treatment process and study its effects and the aging process. The effect of two-stage solution heat treatment resulted in a higher dissolution than the one-stage solution heat treatment. This was characterized by a lower second phase particle area fraction because the two-stage solution heat treatment involves a higher temperature. The two-stage solution heat treatment was able to minimize the increase in grain size compared to the one-stage solution heat treatment. The hardness value of the two-stage solution heat treatment obtained was higher as well and based on the hardness value, the 7075 Al alloy relies more on the age hardening strengthening mechanism. The optimal process was obtained in the two-stage solution heat treatment sample with the second stage temperature at 495ºC for 30 minutes. The effect of aging time produces different results depending on the temperature used. While the effect of aging temperature shows that high aging temperature will result in faster peak hardness being achieved and high precipitates area fraction due to high diffusion, but the nucleation was more difficult and not all transition phases were formed, resulting in lower peak hardness than the lower aging temperatures.