Detailed explanation of heat treatment process of aluminum alloy telescopic ladder bushing

The shaft sleeve is one of the main parts of the gear pump, which is installed at both ends of the high-speed gear to support the bearing. It must have sufficient strength and good abrasion resistance. In order to ensure the performance requirements of the parts, our factory uses non-standard aluminum-tin alloys, and the role of Cu is to strengthen the matrix. Sn can form a softer, low-melting Al-Sn eutectic, which increases wear resistance. The original heat treatment process is water cooling at 515°C for 6 hours, and air cooling at 180°C for 8 hours. There are two problems with this process:

(1) The Al-Sn eutectic was overfired and tin was added to form an Al-Sn eutectic containing 99.5% tin, and the melting point was 229°C. When the workpiece is heated to 515 °C, the Al-Sn eutectic is overfired, and remelted pellets are formed during quenching and cooling. In the process of forming remelting pellets, on the one hand, the grain boundary is oxidized, which reduces the strength of the grains, and on the other hand, many microscopic voids are generated, which increases the grain interface energy and reduces the strength of the metal, which is easy to pass through during use. early failure.

(2) The production time of the process is long, up to 16h, the production efficiency is low, and the energy consumption is large.

It can be seen from the metallographic observation and mechanical property test data that the 250℃×7h treatment process is ideal. The Al-Sn eutectic is distributed along the grain boundary in a broken network. On the one hand, the distribution of the Al-Sn eutectic is relatively uniform, which ensures good wear resistance. On the other hand, the Al-Sn eutectic does not divide the matrix, so that the alloy has better plasticity and toughness. The higher hardness can be obtained by the treatment process of 250℃×7h, because the alloy contains less Cu and the cooling speed of the metal mold is fast. During the cooling process of casting, it has been ensured that the Cu melts into the solid solution and plays a role in quenching. This is also demonstrated by the increased hardness through natural aging. The tensile strength of the 250℃×7h air-cooling process is 47% higher than that of the 515℃×6h water-cooling+180℃×8h air-cooling process. The temperature of the 250℃×7h air cooling process is slightly higher than the melting point of the Al-Sn eutectic, in order to obtain the broken network distribution of the Al-Sn eutectic through a short holding time without overburning the eutectic. On the basis of the process test, a small batch (400 pieces) of trial production was carried out. After the hardness, metallographic inspection and product delivery test on the test bench, all of them were qualified. The process was officially put into production in 1996, and hundreds of thousands of bushings have been produced, all of which are qualified. The work efficiency is increased by 1.3 times, and the capital can be saved by about 70,000 yuan a year, and the electricity saving exceeds 80,000 kW.h.