| Aluminum and Al alloys exhibit low density, high strength, good electrical and thermal conductivity etc. and they are applied in the electrical industry such as wire and cable, electrical switches more and more widely. But, for aluminum alloy conductors, good electrical conductivity and excellent mechanical properties are contradictory and cannot be improved simultaneously, which limits their application obviously. In this paper, low alloying and hypoeutectic Al-Si alloys were selected as the main main research subjects. Using complex boron treatment process combined with extrusion deformation, heat treatment etc., to reduce the content of main transition metal elements and Ca, improve the morphology and distribution of second phase to make them own good electrical conductivity and mechanical properties at the same time and realize the integration of structure and function.The main results are as follows:1) Melt treatment process of complex boron treatment and grain refinement for low alloying Al alloysFor low alloying aluminum materials, with the melt treatment process of complex boron treatment and grain refinement, good electrical conductivity and excellent mechanical performance of Al alloys can be achieved at the same time. Different processing orders will influence the treatment efficiency. It is good for the improvement of electrical conductivity and mechanical properties by adding boron and then adding grain refiners.During boron treatment process, the forms of Al-B master alloy, addition, holding time and trace elements such as Ti are closely related to the efficiency of boron treatment. Results show that Ti can promote the precipitation of TM elements with forms of polynary borides, achieving high boron treatment efficiency. Based on this, the corresponding theory model of boron treatment reaction mechanism was established. For AA1070A1, with 2%A1-3B addition, boron treatment efficiency was 77.7% and the electrical conductivity can be improved to 64.6%IACS from 60.8%IACS, improved by 6.3%. However, with Ti assistant, more TM elements were promoted to precipitate in forms of doped borides. In this condition, boron treatment efficiency can be improved to 93.3% and the electrical conductivity can reach 65.3%IACS. Similarly, for other low alloying Al alloys, such as Al-0.5Fe-0.2Si, with 4% Al-3B addition holding for 15min, the electrical conductivity can be improved to 58.3%IACS from 53.0%IACS, increased by 10.0%.Boron treatment combined with grain refinement was done to improve the mechanical properties and electrical conductivity of the low alloying Al alloys, simultaneously. According to the above study, by the combination of 1200ppmB and 0.5% Al-5Ti-0.8B-0.2C treatment and then ?14±0.5mm AA1070A1 rods were fabricated, the electrical conductivity of the alloy can be improved to 65.0%IACS, the UTSRT and elongation can be improved to 85MPa,58% from 66MPa,42%. The mechanical properties and electrical conductivity were significantly superior to GB/T3954-2014 electrical round aluminum rod in the corresponding national standard. By making use of the complex boron treatment and grain refinement, the electrical conductivity, UTSRT and elongation of Al-0.5Mg-0.35Si, Al-0.5Fe-0.2Si and Al-0.8Fe-0.2Cu alloys can also be improved simultaneously.2) Study about the forming mechanism of Cr-and Ca-containing metal borides and the influence of borides on electrical conductivity of Al alloysIn this paper, Al-Cr(Ca)-B alloys were fabricated by in situ synthesis method in Al melts and the forming mechanisms of CrB2 and CaB6 and the effect of the borides on electrical conductivity Al alloys were studied.For Al-Cr-B alloy, preparation temperatures can influence the phase composition, reaction path, growth mechanism and crystal morphology of CrB2. When Cr agent was added into Al melts at 650? and then with the temperature increasing to 850?, intermediate phase AlCr2B2 was formed and then CrB2 with hexagonal prism in shape was formed; when Cr agent was added into Al melts at 850 ?, dissolved B reacted with Cr forming CrB2 with hexagonal plate morphology; however, when the preparation temperature was higher than 1000?, apart from CrB2 particles, lots of AlB12 particles with great size were formed. Besides, the influence of Mn on CrB2 crystal growth and morphology evolution was also studied. Results show that Mn was easily to mix in chromium boron compounds causing the structural and compositional evolution, forming doped (Cr1-xMnx)B2 and (Cr1-xMnx)3B4 respectively. For Al-CaB6 alloy, during the the process of synthesis, the morphology features of CaB6 at different growth stages can be reserved and most of CaB6 exhibit cube morphology, following the optimal cycle growth mechanism.CrB2 and CaB6 particles can improve the mechanical properties of Al alloys, but show different effects on electrical conductivity of Al alloys. For pure aluminum conductors, adding CrB2 particles reduced its electrical conductivity. However, for Al-Si alloy, with certain addition of CrB2, the electrical conductivity of this alloy can be improved. CaB6 can improve the electrical conductivity of AA1070A1 conductors to some extent. Thus, CrB2 is a kind of applicable reinforced particles to Al-Si alloys and CaB6 is a kind of suitable reinforced particles for low alloying conductive Al alloys.3) Study about Si morphology evolution and its relationship with electrical conductivity and mechanical properties of hypoeutectic Al-Si alloysThe electrical conductivity of hypoeutectic Al-Si alloy is closely related with the morphology evolution of Si. The electrical conductivity of Al-4Si as cast is 41.8% IACS. Through melt boron treatment and Sr modification complex treatment, homogenization heat treatment and hot extrusion deformation, the content of TM elements in solid solution was decreased and the morphology evolution and distribution changes of eutectic Si were from needle flake (distribute along the grain boundary) to fibrous/globular form morphology (distribute along the grain boundary) to graininess (distribute along the grain boundary) to graininess (dispersion distribution), the corresponding electrical conductivities were improved to 46.6%IACS,53.9%IACS and 60.0%IACS from 41.8%IACS. Results show that during the evolution process of eutectic Si, the scattering of eutectic Si on electron flow decreased and the electronic transmission channels increased and when the eutectic Si particles are small, dispersion distribution and with high density twins, the corresponding electrical conductivity is excellent.In order to further improve the mechanical properties of Al-Si alloy, suitable amounts of Mg, Fe, Cu elements were added to Al-Si alloy and combined with the melt treatment mentioned above, Al-4Si-0.8Mg-0.2Fe-0.1Cu alloy was fabricated and then by extrusion processing ?10.5mm rod was made. In this condition, the eutectic Si, Mg2Si and AlFeSi etc. were small and distributed homogeneously in Al matrix and eutectic Si particles own high density twins. The electrical conductivity of the alloy is 59.1%IACS, thermal conductivity at room temperature is 198.949 W·m-1·K-1, thermal conductivity at 150? is 196.017 W·m-1·K-1, the average liner coefficient of thermal expansion at 26?100? is 16.8×10-6 K-1, the average liner coefficient of thermal expansion at 100?300? is 23.4×10-6 K-1, brinell hardness is 121 HBW, UTSRT is 345MPa, elongation is 13%. The electrical conductivity and thermal conductivity of the alloy were similar with pure aluminum and mechanical performance is superior to pure aluminum and thus a new material with excellent electrical conductivity and mechanical properties was fabricated. |
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