Effect of Overheated Solution Treatment on Microstructure and Room Temperature Tensile Properties of 2218 Peak-aged Al-Cu Alloy

• Main Authors: Po-HanWang, 王柏翰
• Other Authors: Truan-Sheng Lui
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/77018042611737196582

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 100 === Al-Cu alloys are heat treatable Al alloys and are often applied as the structural component and in the aircraft industries. With regard to the structural component, the tensile ductility of Al-Cu alloy must be more than 10%. Overheated solution treatment were carried out in this study. At the first stage, 2218 alloy was subjected in the overheated solution treatment to modify the intermetallic compounds and to induce voids to the matrix such that the tensile ductility of 2218 alloy could be improved. The selected temperatures of overheated solution treatment were higher than the solution treatment temperature (T4). Following the first stage, peak aging was conducted to the 2218 alloy for increasing the tensile stress of 2218 alloy. After overheated solution treatment ranging from 490°C to 560°C and peak aging, specimens were tested in uniaxial tensile tests. The tensile properties of specimens after overheated solution treatment which is 535°C for 1 to 15 hours were also discussed in this study. During tensile deformation, the microstructure evolution was observed to realize the mechanism of crack growth in 2218 alloy. The specimen treated with 550°C overheated solution treatment had an obvious decrease in the tensile stress in comparison with specimens treated at other temperatures, which could be attributed to the coarsening of induced voids. When overheated solution treatment at less than 535°C, the clustering of Al7Cu4Ni particles made the connection between cracks more easy, which caused the decrease in tensile elongation of 2218 alloy. In 2218 alloy treated at 535°C, Al7Cu4Ni particles distributed on the cell boundaries and the amount of these particles decreased due to particles coarsening; therefore the average distance between particles increased. The distance of crack propagation was raised with increasing distance between particles. Besides, the stress concentration at the tip of the crack was released when the crack meet the void, which could inhibit the growth of the crack and raise the tensile ductility of 2218 alloy. With overheated solution treatment at more than 535°C, Al7Cu4Ni particles distributed along cell boundaries and the melting of second phases induced large voids in 2218 alloy, which caused intergranular fracturing, and therefore the tensile ductility of 2218 alloy declined. After peak aging, the tensile stress of 2218 alloy remained with increasing the duration time of overheated solution treatment at 535°C. Al7Cu4Ni particles coarsened with increasing duration time as a result of Ostwald ripening, and subsequently the amount of particles was also reduced. With increasing the distance between particles, it was inhibited that cracks propagated from one particle to another one, and this phenomenon could raise the tensile ductility of 2218 alloy. With overheated solution treatment at 535°C for 5 hours, the morphology Al7Cu4Ni particles and distribution of void in 2218 alloy was suitable for tensile ductility improvement.