Effect Of Microelement Sr On Textures And Forming Properties Of Al-Mg-Si-Cu Based Alloy Sheets

The world auto industries are facing more and more serious problems: energy, environment and safety. Therefore, to reduce the weight of cars becomes an important measure to reduce energy consumption, pollution and to improve efficiency, through which all car manufacturers can sharpen their competitiveness. Today, it is an effective way for reducing car weight by manufacturing cars with aluminum alloy. Al-Mg-Si (-Cu) based alloy is most usually used as alloy auto sheets. But this kind of alloy has a disadvantage of forming properties. Although Al-Mg-Si(-Cu) based alloys have been a hot issue about the study of auto body sheets outside, they have hardly been investigated inside. 6000 system alloys have comprehensive properties, but their forming properties cannot be satisfactory. Forming properties depend on many factors, last but not least, they depend on textures, hence textures are the criteria of the performance of forming properties. In this dissertation the author tries to provide basis for improving forming properties by studying effect of Sr on textures and forming properties of Al-Mg-Si-Cu based alloy sheets.The aim of this thesis is to get a kind of Al-Mg-Si-Cu based alloy automobile body sheet by optimizing the alloy composition and processing technology. The best composition of this alloy is: AH. 2Si-l. 35Mg-0. 8Cu-0. 35Mn-0. 2T1-0. 033Sr (wt%). Its comprehensive properties are excellent: in T4 condition, its mechanical properties: yield strength, tensile strength and elongation ratio is up to 200Mpa, 355Mpa, 23%(relatively), respectively. Its forming properties: n value,r value and cupping value is up to 0.296, 0.726 and 7.92 respectively.In this thesis, the effect of microelement Sr on microstructure and precipitation kinetics is investigated by using metallograph microscope, microhardness instrument, DSC, TEM, XRD, etc.. In the meantime, the effect of microelement Sr and heat treatmentsystem on the textures of alloy sheets is analyzed by using the ODF figures-the orientation distribution function figures.The analysis of microstructure, mechanical properties and precipitation kinetics results: (1) When the content of Sr is 0.033 wt % in Al-Mg-Si-Cu based alloy, it can hinder the expansion of the crystalline grains boundary and make the crystalline grains get fine. It also makes the second phase particles distribute evenly on the boundary and in the crystals. These particles can hinder or suppress the movement of dislocation, which has improved mechanical properties. (2) When the content of Sr is 0.093 wt % in Al-Mg-Si-Cu based alloy, the crystalline grains are larger, and the second phase particles distribute mainly in the crystals. So the mechanical properties is relatively bad. (3) The addition of Sr into Al-Mg-Si-Cu based alloy cannot change the precipitating temperature of the ageing phases, but can change the active energy of the precipitations of P " andQ. When the content of Sr is 0.033 wt % in Al-Mg-Si-Cu based alloy, the active energy of the two phases reduces, so the two phases precipitate easily. When the content of Sr is 0.093 wt % in Al-Mg-Si-Cu based alloy, the active energy of the two phases rises again, so the two phases precipitate difficultly.The analysis of textures results as follows: (1) Under the condition of no mid-anneal, the addition of Sr into Al-Mg-Si-Cu based alloy affects the matrix lattice, reduces the energy of the fault layer, hinders the cross slide and changes the types of textures. When the content of Sr changes from 0 to 0.019wt% and 0.093wt%, the distortion of lattice becomes heavier while the content and the types of textures change largely. (2) Under the condition of no mid-anneal, the addition of Sr into Al-Mg-Si-Cu based alloy does not benefit to produce rotation cube texture and face texture. When the content of Sr changes from 0.019wt% to 0.093wt%, the content of the two above-mentioned textures gets reducing more sharply. (3) Al-Mg-Si-Cu based alloy with mid-anneal at 350C/lh comparing with Al-Mg-Si-Cu based alloy without mid-anneal, the content...