Preventive measures for over-burning defects of aluminum alloys using medium frequency induction heating machines

7A04 aluminum alloy is an important aluminum alloy material in aircraft manufacturing. Most of the aircraft structural parts such as center beams, lower beams, wing frames, beams, stringers, and front cover shells are made of 7A04 aluminum alloy. 7A04 aluminum alloy has high toughness and high specific strength, as well as high fatigue resistance and corrosion resistance. Heat treatment usually adopts solution treatment, 465-475℃, water cooling, artificial aging, 115-125℃x3h with furnace heating to 155-165℃x3h air cooling.

As we all know, attention must be paid to preventing over-burning defects in workpieces during heat treatment production of aluminum alloys. This is because the heating range of aluminum alloys is narrow, and if the heating temperature is slightly higher than the standard or the heating time is too long, overheating and overburning defects of parts often occur. Overheated defects in workpieces are difficult to repair, resulting in product scrap. In this regard, a productive test analysis was conducted and beneficial test results were obtained.

The test material is 7A04 aluminum alloy, the test piece is a high-strength extruded rod with a diameter of 85mm, and an inner hole with a diameter of 30mm is drilled. The test temperature is 470-500℃, with one gear every 5℃, and each gear adopts different heat preservation time. After heating, inspect the appearance quality and metallographic structure of the specimen. The specimens were heated at 480°C and held for different times before quenching. Although the solid solution heating temperature of 480°C is not a dangerous temperature, overheating may occur if the heat preservation time is too long. The large size of this specimen is prone to segregation. When the holding time is too long, low melting point eutectic aggregates are easily formed and overburning occurs.

According to the above test results, it is inappropriate to use 480°C as the upper limit temperature for solution treatment of 7A04 aluminum alloy, and 475°C is more suitable. Tests have shown that the structure of 7A04 aluminum alloy parts is still normal after being kept at 475°C for 165 minutes, which is consistent with the recommended temperature above. The test results at different heating temperatures show that when the heating temperature is higher than 495°C, the specimen will be severely overburned, and 495°C is the dangerous temperature where overburning defects will occur during solid solution heating.

Based on the above tests and analysis, in order to prevent overburning during solution aging heating of 7A04 aluminum alloy parts, Zhengzhou Gou's recommends using a medium frequency induction heating machine to carry out corresponding improvement measures for aluminum alloy overburning defects. Among them, the recommended process temperature of 465-475°C should be used, and the heating time is calculated based on the effective heating thickness or diameter of the workpiece.

In order to strictly prevent the heating temperature of the workpiece from being too high, the heating temperature should not exceed 495°C. When the workpiece is heated at 480°C, the heat preservation time of the 7A04 specimen must be strictly controlled. Tests have shown that 7A04 aluminum alloy will appear overburned when heated and held at 480°C for 150 minutes, which is a warning line; when the holding time is 90 minutes, the metallographic structure of the workpiece is normal.