Effect Of Elements Addition On Microstructure And Properties Of Al-Mg-Si Aluminum Alloy Used For Electrical Conductor

This paper has investigated the Al-Mg-Si aluminum alloy used as AAAC (allaluminum alloy conductor) for transferring electrical energy. The research work hasbeen carried out from aspects of casting, homogenization, rolling, heat treatments andproperties performance by means of microstructure investigation, tensile propertiestesting and electrical conductivity measuring. The principle objects of theexperiments were to improve the tensile strength, electrical conductivity andthermal-resistant properties of the alloys studied. In this paper, the effects of Zr, Laand Ce additions as well as thermomechanical treatment on the metallurgical structure,tensile strength, thermal-resistant properties and electrical conductivity of Al-Mg-Sialuminum alloys has been studied, and the existing form of the added elements hasalso been discussed which could provide theoretical bases of the reasonable amount ofelements addition. The key findings are as follows:(1) Zr addition could result in grain refinement in as-cast Al-Mg-Si aluminumalloy, besides the secondary dendrite arm spacing are refined from66μm to49.5μm.The recrystallization process is hindered in the Al-Mg-Si-Zr alloy during the solutionheat-treatment, and optical structure shows an uncompleted recrystallized structurewith deformed grains in the alloy. The yield strength increases from165MPa to230MPa and the tensile strength increases from220MPa to265MPa in the alloy withZr addition during aging treatment at190°C, besides the ulimate tensile elongationincreases from14.7%to20%. It is the fine grain strengthening, which results in theimprovement in both tensile strength and ductility of the Al-Mg-Si-Zr alloy. Based onthe Arrhenius Model it is possible to obtain an explicit expression for the criticalduration (t) of the material exposed to thermal-resistant treatment at temperature T.The duration of traditional Al-Mg-Si alloy for AAAC at130°C is only11yearsdescribed in the Arrhenius plot, while that of the Al-Mg-Si-Zr alloy could be40years.For the Al-Mg-Si-Zr alloy in this research the following equation represents thevariations of t and T:lg t=20.5+10486.2/T(2) La addition could result in a smaller grain size of the Al-Mg-Si-Zr aluminumalloy. The compounds containing La, Al and Si with the size of1~10μm were seen inthe alloy. The tensile strength decreases15MPa and the electrical conductivityincreases1.6%IACS when La amount reach0.22%. When La additions reach0.32%, the tensile strength decreases40MPa which falls by14%, and the electricalconductivity increases2%IACS. La elements could participate in forming somecompounds containing Si elements in the alloy which could reduce the adverse effectsof Si elements on the electrical conductivity, but it also leads to a less precipitationhardening effect of these alloys. Despite no improvement in the tensile strength ofalloy were seen, the beneficial effects on the thermal-resistant properties andelectrical conductivity of alloy with0.22%~0.32%La addition are validated, whichcould be a preferable way to manufacture thermal-resistant aluminum alloy conductor.(3) No improvement were seen in the as-cast microstructure of alloys with Ceaddition. Deformed grains are replaced by a new set of undeformed grains in the alloyduring solution heat-treatment, and the recrystallization process has not been hinderedwith Ce addition. Ce elements mainly exist in the form of compounds like AlCe orAlCeSi. The compounds show an appearance of unregular polyhedron. Thedeformation structure are retained in the alloy with a typical banded structure whensubjected to thermomechanical treatment. The thermomechanical treatment improvedalloy peak hardness (about30HB) and electrical conductivity (1.5~3%IACS).(4) Based on the comprehensive study of the fomer chapters, the resultingperformance of the alloys (70%deformation rate) are as follows:The tensile strength is225MPa, the electrical conductivity is56.7%IACS, theultimate elongation is9.9%and when subjected to thermal-resistant treatment at230°C for2h the tensile strength could maintain at a minimun90%of the initialvalue.