Electrolytic Titanium Al-Mg-Si Alloy Sheet And Performance

Microstructure and properties of an electrolytic titanium aluminum alloy (Al-Mg-Si) were investigated by Transmission Electron Microscope(TEM), Scanning Electron Microscope(SEM) incorporating energy dispersive X-ray analysis(EDX), High Resolution Transmission Electron Microscope(HRTEM), Differential Thermal Analyzer (DSC) and Electronic Tensile Tester. The microstructure of the half-continuously cast electrolytic titanium aluminum alloy (Al-Mg-Si) and the phase structure of the cast and uniform heat-treated electrolytic titanium aluminum alloy (Al-Mg-Si) were investigated emphatically. The influence of heat treatment parameters on microstructure and properties of the Al-Mg-Si alloys was studied and the HRTEM analysis of aging-treated Al alloy was carried out. The activation energies ofβ" andβ' phase in a T4 heat-treated state were calculated by measuring the exothermic peak of the corresponding DSC curves. The effect of excess Si, Mg₂Si and Ti/B contents on the microstructure and properties of the Al-Mg-Si alloy was analyzed. An in-situ SEM observation of the fracture process during tensile test was carried out to understand the initiating and propagating mechanism of cracks. The results obtained present theoretical and practical significance to the research and development of the electrolytic titanium aluminum alloy (Al-Mg-Si).The influence of the Mg content, uniform heat treatment and T4 status on the microstructure and phase structure of the electrolytic titanium aluminum alloy (Al-Mg-Si) was investigated by SEM, EDX and XRD. The area and line scanning analysis by SEM revealed that the Mg and Si clustering at the grain boundaries generated non-equilibrium microstructure. The crystalline phases of cast electrolytic titanium aluminum alloy (Al-Mg-Si) is consisted of Al_1.9CuMg_4.1Si_3.3,Al_0.75MnSi_1.25,Mg₂Si, and element Mn and Fe are of mutual replacement. Al_1.9CuMg_4.1Si_3.3 is sphere-like and dissolvable into the matrix. A1MnSi type phases include Al_0.75MnSi_1.25 and Al₅Mn₁₂Si with irregular morphology, and the content of these phases reduces with an increase in Mg content. The cast alloy is consisted of Mg₂Si phase with black block appearance, the corresponding content increase with an increase in Mg content and uniform heat-treatment has no obvious influence on Mg₂Si phase.The effect of pre-aging treatment, pre-aging treatment plus simulatively baking and pre-aging treatment + pre-tensile + simulatively baking on the microstructure and properties of the Al-Mg-Si alloy were investigated for the first time. A short time pre-aging treatment of the alloy after water quenching resulted in inhomogeneous distribution of elements in the Al-matrix solid solution, restrained the hardening effect during natural aging process, and relieve the effect of residence time on the properties of the alloy. The formation of a large amount ofβ" nucleus induced by pre-aging treatment promoted the precipitation ofβ" phase and increased the hardening effect significantly during the sequent artificial aging process. A good combination of both high strength (337MPa) and good plasticity (19.9%) (marked alloy 4) was achieved by the optimal treating parameters, i.e., solid solution treatment at 550℃for 30min+ pre-aging treatment at 170℃for 5min+natural aging treatment for 5 days+silulatively-baking treatment at 180℃for 30min, which nearly approachs to the strength of wrought aluminum alloy 6009 heat-treated at T6 status (345MPa). The optimal heat treatment parameters for pre-tensile treatment of alloy 8 is solid solution treatment at 550℃for 30min+pre-aging treatment at 170℃for 5min+natural aging treatment for 5 days+silulatively-baking treatment at 180℃for 30min, by which the strength and extensibility are 373 MPa and 22.2% respectively, which is a little higher than the strength (345MPa) and the extensibility (12%) of wrought aluminum alloy 6009 heat-treated at T6 status.HRTEM analysis revealed that the interplanar distance of the spinodal-decomposed phase is 35% larger than that of the matrix since the clustering of Mg and Si on the plane of (200) ofα(Al) (the atomic radius of Ai and Mg is 0.1432nm and 0.1602 nm respectively.) resulted in distortion of crystal lattice.DSC analysis revealed that the primary aging stage of the electrolytic titanium aluminum alloy (Al-Mg-Si) (at around 150℃) was characterized by GP zone with needle- or sphere-like appearance. The needle-likeβ" phase having a total coherence relationship with the matrix appeared at about 250℃, a partial coherenceβ' phase formed at 320℃, and a stableβ(Mg₂Si) phase formed finally, which means that the aging sequence is GP→β"→β'→β(Mg₂Si). The thermodynamic equation of activation energy of precipitating phase by aging treatment is, by which the activation energy ofβ" andβ' in T4 status is 63KJ/mol, 121 KJ/mol respectively, it indicates that theβ" is easier to form thanβ' phase as well as the strengthening effect ofβ" is superior toβ'.The strength of the alloy is significantly improved and extensibility of the alloy is increased to a certain extent by an increase in excess Si content. The enhance extent of the strength of the electrolytic titanium aluminum alloy (Al-Mg-Si) before simulatively baking treatment was reduced but the corresponding extendibility of sheet material kept constant with an increase in excess Si content. The mechanism of improving strength of the alloy can be understood to be of the clustering precipitation of excess Si and the precipitation of Mg₂Si. The addition of Ti and B into the alloy effectively refined the grain size; the grain size of the as-cast alloy was refined with a decrease in the ratio of Ti to B.The in-situ tensile test was firstly carried out under SEM observation for the electrolytic titanium aluminum alloy (Al-Mg-Si). The fracture process of the alloy is similar to normal plastic materials. Cracks initiated at locations of stress concentration and the interface between the coarse precipitating phase and the matrix. Cracks propagated parallel to slipping stripes and along the grain boundaries. Cracks meeting coarse Mg₂Si phase propagated along the interface between Mg₂Si and the matrix. The conect of cracks resulted in final fracture.