Analysis on the Causes of Point Shaped Defects in 7050 Aluminum Alloy Forged Plate

The point-shaped defects were found in the 7050 aluminum alloy forged plate through the ultrasonic testing along the thickness direction. The metallographic examinations, scanning electron microscopy analyses, and energy dispersive spectroscopy analyses were performed to investigate the cause of the defects. The results indicate that point shaped defects are internal small cracks. The fracture surface of the crack is a brittle intergranular fracture, with a large amount of Al₂CuMg and Ti-Zr-Al ternary alloy phases on the grain boundary surface. Analyses suggests that due to the existence of Al₃Zr phases with large size in the Al-Zr intermediate alloy and the inappropriate addition time of the Al-Ti intermediate alloy, the large Al₃Zr phases in the ingots for forged plates were not completely dissolved during the melting process, and the coarse Ti-Zr-Al ternary alloy phases with high melting point were formed with the later added Ti element and segregated locally. This locally enriched defect was retained in the ingot structure, significantly reducing the alloy’s plasticity and making the local area brittle. During the forging process, the brittle ternary alloy phase cracked under forging load. The dense distribution of small Al₂CuMg phases at grain boundaries and the weak grain boundary bonding force of 7050 aluminum alloy at high temperatures were also important reasons for local intergranular cracking.