Effects of trace addition of vanadium and depression amount on recrystallization temperature and mechanical performance of 5182 belts
GAO Jia-cheng(高家诚)1, CHEN Zhi-qiang(陈志强)1, MING Wen-liang(明文良)2,
WANG Yong(王 勇)1, CUI Xian-you(崔先友)1, YUAN Li-jun(袁礼军)2
1. College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China;
2. Southwest Aluminium Fabrication Plant, Chongqing 401326, China
Received 28 July 2006; accepted 15 September 2006
Abstract：Because the mechanical performances of 5182 belts used for carbonated drinks cover decrease after baking, the effects of trace addition of V and depression amount in last step on microstructure and properties of 5182 belts were investigated. The microstructure, mechanical performances and recrystallization temperature of 5182 belts and 5182V belts in different steps were analyzed comparatively with metallographic microscope, micro-hardness tester, electron universal materials test machine and differential thermal analyzer. The results show that the mechanical performances of the belts are remarkably improved by the trace addition of V and the reduction of depression amount in last step. In addition, the recrystallization temperature of the belts is also increased but not obviously. As the precipitation of V is not full, there are not enough disseminatedly distributed particles, and the recrystallization temperature increases little. However the solution strengthening and the fine grain strengthening are enough to improve the mechanical performances to satisfy customer requirements. The effects of reduction of depression amount in last step on mechanical performance were explained in view of energy. Moreover, the strengthening mechanism of V-compound interlocking grain boundary was also discussed.
Keywords: 5182 belts; vanadium; recrystallization temperature; mechanical performance
1 Introduction
The 5××× series aluminum alloys which cannot be strengthened by heat treatment belong to the Al-Mg alloys of wrought aluminum alloys. And the 5182 aluminum alloy is an alloy with excellent welding performance, corrosion resistance, mechanical working property and low-temperature performance. It has been widely used in manufacturing aerospace outer covering, ship, pressure container as well as auto part[1-2]. The belts used in can cover and can suspension link need coating with lacquer and baking before punching, which often cause its mechanical performance to decrease, even to the level unable to satisfy the service performance requirement of manufacturer. Recently, there have been many reports related to recrystallization or strengthening methods of aluminum alloy. However, most of them discuss mainly the separate influence of trace addition of element or elements, and the reports about suppressing recrystallization and reducing the strength loss after baking are rare, especially those on the belts [3-9]. In this paper, the effects of trace addition of V and depression amount in last step on microstructure and properties of 5182 belts were investigated. Also, their mechanism was discussed.
2 Experimental
In casting, appropriate readjustment of the 5182 chemical composition was performed. Table 1 shows the chemical compositions of 5182 alloy before and after adjustment.
The belts process used for can cover and can suspension link was: the blank→cold rolling→annealing in processing→end product rolling→withdrawal straightening→coating and baking→splitting-packing. The sheets after hot rolling were 2.3 mm thick and the amount of deformity reached 92%, and then through the processes which are similar to the factory: the blank→cold rolling (1.0 mm for process 1 or 1.4 mm for process 2) →annealing in processing→end product rolling (0.24 mm), the sheets were rolled to the finished size. Baking test was performed after then.
In each working step, the tensile samples along rolling direction were used, and the tensile tests were performed with the CMT5105 electron universal materials test machine. The metallography samples were obtained along rolling direction and rolling normal direction separately. After electro-polishing with perchloric acid ethyl alcohol solution and anode filming, their microstructures were observed with metallography microscope Axiovert200 MAT. Surface micro-hardness values were obtained on the HV-1000. In addition, thermal analysis of the end belts before baking was performed with the NETZSCH STA 449 differential thermal analyzer.
3 Results
3.1 Mechanical properties
The mechanical performances of sheets and belts in each working step are listed in Table 2. The yield strength and tensile strength have diverse increments after rolling; however the elongation ratio has diverse reduction. After annealing, the yield strength and tensile strength have obvious drop, and the material strength and plasticity return to the values before rolling. However, after the second rolling, the strength and plasticity of material restore the level of work hardening condition. After baking below the recrystallization temperature of belts, the material strength slightly drops, however the plasticity slightly increases. In comparison with the primitive 5182 belts (Table 3), the improvement in mechanical performances is remarkable, which can fully satisfy the application requirements.
Fig.1 shows the hardness experimental results. The change in hardness correlates with that of materials strength.
3.2 Thermal analysis
The thermal analysis results are shown in Fig.2. After the addition of 0.02%V, the recrystallization temperature enhancement of the belt is not obvious. From the thermal analysis result of the belts without V and the belts made by original process with V, the recrystallization temperature rises from onset value of 297.2 ℃ to peak value of 299.3 ℃. From the thermal analysis result of the belts without V and the belts made by original process 1 with V, the recrystallization temperature rises from onset value of 297.2 ℃ to peak value of 300.1 ℃. The recrystallization temperature enhancement of belts is conservatively estimated to be only 2-3 ℃. However in the deformation reduction in last step, the recrystallization temperature also has small scope enhancement. The recrystallization temperature of the belts with V rises from peak value of 300.1 ℃ to onset value of 307.5 ℃, only enhanced by 7-8 ℃. However, for the whole enhancement effect, the recrystallization temperature rises from onset value of 297.2 ℃ to onset value of 307.5 ℃, approximately 10 ℃.
Table 1 Chemical compositions of wrought aluminum alloy 5182 (mass fraction, %)
Table 2 Mechanical performances of sheets and belts in each working step
Table 3 Mechanical performances of primitive 5182 belts
3.3 Metallograph
The microstructures of samples during each working procedure are shown in Fig.3. The grains in working step 3 are obviously smaller than the grains in primitive conditions. The equiaxed grains at the process 1 are obviously smaller than those at the process 2. The fiber structure exhibited in the working step 5 changes little contrary to that in the working step 4, which means that the belts do not recover and recrystallize.
Fig.1 Hardness experimental results
4 Analysis and discussion
4.1 Effects of trace addition of V on 5182 belts
Vanadium belongs to transition metals. A trace addition of V in the aluminum alloy leads to generate the infusible compound Al11V. It not only has the effect of refining casting alloy grains, suppressing alloy to recrystallize, refining grains and increasing recrystallization temperature, but also has the effect of enhancing the alloy strength, plasticity and high-temperature performance [5]. Thus it usually serves as the alterant to refine grains, and to form intermetallic compound to enhance the aluminum alloy strength at high temperatures with dispersion strengthening[3-5]. Although the major transition elements can increase the aluminum alloy recrystallization temperature, V has little effect, which can cause the recrystallization temperature to increase 15 ℃ at most [6]. However, a trace addition of V can obviously enhance the tensile strength and the elongation ratio. It was reported that V can remarkably enhance the tensile strength of Al-Cu alloys. When the addition of V is about 13%, the strengthening effect is most notable [7].
Fig.2 Thermal analysis results: (a)Without V; (b) With V at original process; (c)With V at process 1; (d) With V at process 2
Fig.3 Microstructures of samples: (a) Primitive 5182; (b) Step 2 at process 1; (c) Step 2 at process 2; (d) Step 3 at process 1; (e) Step 3 at process 2; (f) Step 4 at process 1; (g) Step 4 at process 2; (h) Step 5 at process 1; (i) Step 5 at process 2
The addition of 0.02% V leads to suppress the recrystallization process. Because the solid solubility of V is very low in the aluminum alloy, and the diffusion velocity is low, the diffusion of V is incomplete when alloy solidifies, therefore the oversaturation solid solution forms. The tiny, disseminatedly distributed Al11V particles precipitate from the oversaturation solid solution during subsequent heat treatments. These particles are coherent with the matrix, interlock intensely the movement of sub-boundary and the dislocation, and suppress recrystallization. Since the solid solubility of V is up to 0.41% and the addition of V is low, the precipitation of V is not full. The most of V continue to retain in the solid solution. Therefore, the enhancement of recrystallization temperature is very low [4, 7-8].
In the 5182 belts aluminum alloys, the addition of 0.02% V enhances the materials strength and the surface hardness obviously. This is probably the integrated effect of solution strengthening, the fine grain strengthening and the dispersion strengthening. The addition of 0.02% V distorts the matrix lattice. The stress field of distortion integrates with elastic stress field around, which hinders the dislocation movements and causes the solution strengthening. The addition of V in casting causes the fine grain as alterant, which generates many dislocations to hinder dislocation movements and causes strengthening. Moreover, the coherent Al11V particles dissemination-distributed with the matrix precipitate from the oversaturation solid solution, which also hinders the dislocation movements and causes strengthening. However, the addition of V is low, therefore the dispersion strengthening is not obvious. This is also the right reason why strength and surface hardness are enhanced remarkably after the addition of V, but the recrystallization temperature enhances not so obviously[5, 9].
4.2 Effects of deformation degree on 5182 alloy
Generally speaking, when the deformation degree increases, the recrystallization temperature reduces. After the deformation degree reaches a certain value, the recrystallization temperature tends to a certain value. In this experiment, the final amount of deformation is from about 83% (1.4 to 0.24 mm) to about 76% (1.0 mm to 0.24 mm). From the thermal analysis results, this process alteration enhances the recrystallization temperature by approximate 10 ℃. But the reduction of deformation degree has harmful work hardening and causes the loss of strength and the surface hardness loss [5, 10].
In view of energy, the reduction of deformation degree, which is the reduction of metal cold-work storage energy after cold-work, is helpful to the reduction of energy release speed in the recovery or the recrystallization and causes it to release cold-work storage at a higher temperature, which enhances the material recrystallization temperature slightly. However, the reduction of cold-work storage can actually reduce the effect of work hardening[5, 11].
5 Conclusions
1) In aluminum alloy 5182 belts, the addition of 0.02% V increases the recrystallization temperature by approximate 3 ℃, but the material strength and hardness are enhanced obviously.
2) The depression amount in last step makes the recrystallization temperature increased by approximate 10 ℃. Although it can satisfy the application require- ments, the strength of the belts with V decreases actually contrary to the original process.
3) The enhancement effect of the materials strength and the surface hardness with the addition of 0.02% V is probably the integrated results of solution strengthening, the fine grain strengthening and the dispersion strengthening. And the fact that the coherent Al11V particles dissemination-distributed with the matrix precipitate not enough is the reason why the recrystallization temperature does not enhance so obviously.
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(Edited by YUAN Sia-qian)
Corresponding author: GAO Jia-cheng; Tel: +86-23-65102821; E-mail: gaojch@cqu.edu.cn
