Studies On The Theories And Technology Of High Strength 6XXX Series Aluminum Alloys By Multi-Components Alloying

The processing theories and technology of 6069 aluminium alloy were studied systematically in this thesis. The strength of the alloy was increased by microalloying, which broke the patent limit of AA6069 alloy, and as a result the high strength 6XXX aluminum alloys with Chinese characteristics were developed.In this thesis, the effects of main alloying elements (Mg, Si and Cu) on microstructures and mechanical properties of AA6069 alloy were studied firstly. Then effects of processing parameters on the type and shape of V-containing phases in Al-V master alloys and roles of V in the AA6069 alloy were investigated. Furthermore, the effects of various amounts of Mn addition to AA6069 alloy, taking the place of V by Zr in AA6069 alloy and increasing Fe content in the alloy with V replaced by Zr on microstructures and mechanical properties of the alloys were studied. As a result, the high strength 6XXX aluminum alloy containing Zr with its own characteristics and higher strength than that of AA6069 alloy and the alloys with the strength over 500MPa were developed. The theoretic and technical foundation was provided for the applications of the high strength aluminum alloys in industry.At First, based on the standard composition of AA6069, effects of Mg/Si ratio, the contents of Cu and Mg on the microstructures of AA6069 alloy and mechanical properties of the T6 extruded bars were studied. The optimal main alloying elements contents of the alloy were consequently Al-1.6Mg-1.15Si-1.0Cu (wt.%), which resulted in the alloy having a good combination of strength and plasticity.Then effects of V content, melting temperature, holding time and cooling rate on the configuration, shape and size of V-containing phases in Al-V master alloys were studied. The optimum processing technology for the production of Al-V master alloy was determined as follows:the V addition amount was 4 wt.%; the melting temperature was 1100?; the holding time was 10min and finally the melt was cooled by chilling. By using this method, an Al-4 wt. % V master alloy with accurate V content, as well as large amounts of fine and petal-shaped AI3V particles with dispersive distribution will be obtained. Additionally, this process has low energy consumption.Microstructure observation showed that V in AA6069 alloy mainly precipitated in the form of fine elliptical shaped Al(VCrTi) phases which could refine the microstructure during the solidification and inhibit recrystallization significantly during heat treatment and extrusion. As a result, the ultimate tensile strength (UTS) and yield strength of T6 extruded bar of the AA6069 alloy reached 419Mpa and 361 MPa respectively, and its elongation was 18.8%.When Mn was added to AA6069 alloy, the strength of the T6 extruded bar of the alloy increased with the increase of Mn content at first and then decreased. The optimum Mn content was 0.2 wt.%. The fine (<100nm) and metastable Mn-containing dispersoids with spherical or elliptical shape were present in the alloy with minor Mn addition. These Mn-containing phases would inhibit recrystallization and impede dislocation movement. Consequently, the UTS and yield strength of T6 extruded bar of the alloy were improved to 431Mpa and 372MPa respectively, and its elongation was 15.6% with a little loss.The fine and metastable Ll2-Al3Zr particles and thicker DO22-Al3Zr particles were formed when Zr replaced V in the AA6069 alloy in the same amount (0.15 wt.%). The minor Zr addition resulted in the remarkable refinement of as-cast microstructure, effective inhibition of recrystallization during extrusion and simultaneously the promotion of precipitation of Q' phases, which increased the strength of the alloy further. As a result, the UTS and yield strength of T6 extruded bar of the alloy were enhanced to 440Mpa and 361 MPa respectively. Additionally, the optimum homogenization treatment of the alloy was to homogenize the ingot at 540 ? for 24h and then air cooling. The T6 heat treatment of the extrusion of the alloy was to carry out the solid solution at 550 ? for 2h and water quenching and then aging at 170? for 12h. This alloy broke the patent limit of AA6069 alloy and a new alloy with distinguishing features was developed.The Al(CrFe)Si particles with the spherical or rod shape were formed when a certain content of Fe was added into the Al-1.6Mg-1.15Si-1.0Cu-0.15Cr-0.15Zr-0.03Ti (wt.%) alloy. The increase of Fe content promoted the precipitation of fine and elliptical DO22-Al3Zr phases with the dispersive distribution. The combination of Al(CrFe)Si and DO22-Al3Zr particles inhibited recrystallization significantly and promoted precipitation of Q' phases in large amount. As a result, when Fe content of the alloy was 0.356 wt.%, its UTS and yield strength of T6 extruded bar were improved to 503Mpa and 478 MPa respectively and its elongation was 12.25%. While when Fe content of the alloy was 0.601 wt.%, its UTS and yield strength of T6 extruded bar were further improved to 520MPa and 503 MPa respectively and its elongation was 10.38%. These alloys broke the patent limit of AA6069 again, and became the 6XXX alloy with highest strength reported at present.