Study On Heat-treatment Process Of Al-6.0Zn-2.2Mg-1.6Cu Aluminium Alloy

The purpose of this study is to provide the foundation for establishing the proper heat treatments in the process of the production of plates of the high-purity Al-6.0Zn-2.2Mg-1.6Cu aluminium alloy. For this alloy the homogenization of the ingot, the solution of the hot rolled plate and the ageing treatment of the pre-strained plate have been studied by means of tensile test, conductance measurement, exfoliation test, metallographic microscopy, scanning electron microscopy, transmission electron microscopy and the energy spectrum analysis. The results as follows:The low-melting product which began to melt at 478℃in the semi-continuous cast ingot of the alloy dissolve into its matrix at 465℃for 12 hours,except it transforms partly to S phase(Al2CuMg) with melting point about 490℃. The low-melting product could dissolve entirely into the matrix of the alloy at 465℃for 48 hours or by multi-step homogenization. Many retained phase(S-type phase) were found in the microstructure of the cast ingot after industrial homogenization , which indicated that industrial homogenization temperature was lower, below 470℃. In comparison to the former, it was not a large quantity of retained S-type phase in the cast ingot homogenized at 465℃for 48 hours in the laboratoryThe property of the alloy was not almost influenced by heating its cast ingot to 485℃in furnace following its first homogenization at 465℃for 24hours and helding for 10hours at 485℃. Following that two-step homogenization the cast ingot continued to be heatied to 500℃slowly and third-step homogenized at that temperature for 2 hours, so that the grain size of the alloy was not influenced very much,but its strength was increased a little and its conductivity decreased a little.A small amount of the low-melting product which began to melt at 478℃and much product with melting point about 490℃were found in the 15mm hot- rolled plate made from the cast ingot homogenized by industrial conventional homogenization treatment, so the temperature of the original solution treatment of the hot-rolled plate was required to be below 470℃. It was benefit to the increase of the strength of the alloy that the hot-rolled plate was treated again by the industrial conventional solution after first heated it to the industrial conventional solution temperature and cooled in air.The property of the pre-strained plate with thickness of 17mm made from the cast ingot homogenized by two-step treatment in industry was very sensitive to the temperature and time of second-step ageing, so they must be controlled strictly. In order to obtain the exfoliation degree of less than EB this plate should be aged at 163～160℃for 12～16h following the first step ageing at107℃for 7h. When the temperature of second step ageing was 160℃, the corresponding ageing time should be 16 hours.