Aluminum Corporation of China obtained 6048 aluminum alloy TTP curves by quenching quenching method, calculated quenching factors in the quenching temperature range, and combined quenching factor analysis methods to predict the hardness of the alloy at different quenching rates. The results show that the "nose tip" temperature of 6082 aluminum alloy TTP curve is about 335 °C, the quenching sensitive temperature range is 225 ~ 460 °C; when the quenching cooling rate of the alloy at quenching sensitive temperature range 225 ~ 460 °C is greater than 16 °C/s, The hardness of the alloy can reach 90% of the larger hardness value.
6082 alloy belongs to Al-Mg-Si series heat treatable aluminum alloy, with moderate strength and good welding performance and corrosion resistance, is mainly used for transportation and structural engineering, such as bridges, cranes, roof structures, transportation vehicles and Transport ships, etc. [1]. Aluminum production process must be accurately controlled production process to obtain excellent overall performance, especially the quenching process, the quenching rate is too slow will affect the aging effect, quenching rate too fast will lead to increased residual stress [2]. Therefore, it is of great significance to study and improve the quenching system by studying the quench sensitivity of the alloy [3-5]. Many foreign scholars studied the quenching susceptibility by measuring the alloy TTP curve, and combined quenching factor analysis method to predict the hardness, strength and corrosion resistance of the alloy, and obtained good results [3-7]. In this paper, the time-temperature-hardness curve of 6082 aluminum alloy was determined by the method of graded quenching. The hardness of the alloy at different quenching rates was predicted by combining with the quenching measured cooling curve, which provided experimental basis for the development of the online quenching process of aluminum alloy profiles.
1 Experimental materials and methods 1.1 Experimental materials Experimental materials A 6082 aluminum alloy produced by an aluminum company was selected and the state was in an extruded state. The alloy composition is shown in Table 1. The aluminum alloy was cut into 20 mm x 20 mm x 4 mm square small specimens in the extrusion direction. Aluminum alloy cylindrical quenched sample design shown in Figure 1. The thermocouples were respectively mounted on the centers of the test bars (marked as A, B, and C, respectively) of 5 mm, 10 mm, and 60 mm from the water spray end face of the sample, as shown in FIG. 1 with three deep holes.
1.2 Experimental method Small samples of aluminum profiles were treated at 530°C for 2 hours and then treated with isothermal treatment in salt baths at different temperatures for different periods of time. They were then immediately quenched into room temperature water and artificially aged at 175°C for 6 hours. Hardness test. The temperature range in the salt bath furnace is 200~500°C, taking 20 temperature points in total. To ensure the accuracy of the experiment, the temperature in the salt bath should be adjusted to fluctuate by ±3°C. Insulation time varies from 5s to 500s. The greater hardness of the alloy was treated with solution at 530°C for 2 h, and water quenched at room temperature and then artificially aged at 175°C for 6 h. The cylindrical quenched specimens of aluminum rods were quenched at 530°C for 3 h and quenched at the end [9,10]. During the quenching process, data was collected using a self-designed temperature acquisition system, and then subjected to artificial aging at 175°C for 6 hours. Hardness Testing.
Since the solubility of alloying elements Mg and Si in Al decreases with decreasing temperature, the supersaturated solid solution undergoes a desolvation transition during the isothermal heat treatment process. The rate of the desolvation transition depends on the nucleation rate of the desolvation phase and Grow rate [11]. When the isothermal temperature is low (≤225°C), though the supersaturation is high and the nucleation rate is high, due to the lower temperature, the solute atom migration rate is small and the growth rate is slow, so the conversion rate is small and the hardness of the alloy is low. Slower decline with time; at high isothermal temperatures (≥460°C), although the diffusion rate of solute atoms is large, due to low supersaturation, low desolvation driving force, and low nucleation rate, the phase transition rate Small, the hardness of the alloy decreases more slowly with time; and when the isothermal temperature is in the middle range of 225°C to 460°C, the degree of supersaturation is large enough, the driving force for desolvation is also large enough, and at the same time, the solute atom migration rate is guaranteed. Large enough to nucleate and grow faster. As the precipitation of dissolved phase grows up, the surrounding solute atoms are consumed, the supersaturation of the solid solution is reduced, and the subsequent aging strengthening effect is suppressed. Therefore, the hardness of the alloy decreases rapidly with time in the intermediate temperature range. It can be seen that the quenching sensitivity of the 6082 aluminum alloy in the high temperature zone is very low, but the quenching sensitivity in the intermediate temperature zone is extremely high, and the sensitivity in the low temperature range quenching is somewhere in between. This is the reason that leads to a "C" shape in the TTP curve. Therefore, in the online quenching process of aluminum profiles, in order to improve the good performance of the profiles and reduce the residual stress after quenching, the quenching rate in the middle and low temperature zone should be increased as much as possible, and the quenching rate in the high temperature zone should be appropriately reduced.
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