Study On Microstructure And Properties Of Sr Modified 5A12 Aluminum Alloy

The 5A12 aluminum alloy with high Mg content possesses excellent mechanical properties,but reveals poor corrosion resistance and plastic formability.There fore,it is of great significance to explore and improve the corrosion resistance and plastic formability of the alloy.In recent years,more and more attention has been paid to the improvement of the properties of 5xxxx aluminum alloys by microalloying.H owever,there are few studies on the improvement of corrosion resistance of Al-Mg alloy with high Mg content??7wt.%?and the addition of Sr element to microalloyed alloy.In this paper,the effects of Sr addition on the microstructure,mechanical propert ies and corrosion resistance of 5A12 alloy were studied.The effect of Sr element on the dynamic recrystallization behavior of 5A12 aluminum alloy at different deformation temperatures was investigated by hot compression simulation experiment and cuneiform plate rolling simulation.Finally,according to the effect of Sr element on the dynamic crystallization?Dynamic recrystallization,DRX?behavior of 5A12aluminum alloy,the alloy sheets with different degree of recrystallization were prepared by two rolling processes?conventional multi-pass and large strain rolling?,to analysis of its effect on the intergranular corrosion behavior of the alloy and reveal its mechanism.The main results of this paper are as follows:?1?The microstructural characteristics of 5A12-xSr alloy were analyzed,and the effect of Sr content on the microstructures of 5A12 aluminum alloy was studied.The results showed that when Sr element was introduced into 5A12 in the form of Mg-Sr master alloy,a new Sr-containing phase?not more common Al₄Sr?would be formed in the as-cast structure of the alloy.According to the atomic ratio of Al:Mg:Sr=81:7:12,it could be judged as?Al,Mg?₁₇Sr₂ phase.The addition of Sr had little effect on the grain size of as-cast alloy,but the?Al,Mg?₁₇Sr₂ phase distributing on the grain boundary inhibit the grain boundary migration during homogenization,so the grain size of as-cast alloy was refined.When Sr was not added,the grain size of the alloy increased by about 25?m from as-cast to homogenized state,while after Sr microalloying,the grain size increment was only about 7?m,and it could be concluded that the specific content of Sr had little effect on the grain size of the homogenized alloy.?2?The effects of Sr addition on the mechanical properties of 5A12 aluminum alloy at room temperature were studied in two commonly used states,O?complete annealing?and H321?stable annealing?.The results revealed that in the range of0-0.6wt.%for O-temper alloys,with the increase of Sr content,the tensile strength of the alloys first increased and then decreased,but the overall change was not significant,all of which were about 450 MPa,while the elonga tion showed a downward trend.This was due to on the one hand,the addition of Sr maked the grain refinement of the alloy producing fine grain strengthening effect.On the other hand,the size of Sr-containing phase raised with the increase of Sr content,and the large Sr-containing phase led to excessive stress concentration and separation from the matrix in the process of loading,which resulted in the decreased tensile strength and poor post-fracture elongation of the alloy at the same time.In addition,for H321 alloy,the fine-grained plate obtained by large strain rolling possessed a high proportion of large angle grain boundaries,which hindered greatly dislocation motion,so it revealed the better anti-aging softening ability.?3?The effect of Sr content on DRX behavior of 5A12 aluminum alloy was preliminarily explored by hot compression simulation and wedge rolling experiment.The results indicated that under the condition of strain rate of 9.1s^-1,the critical strain of DRX increased with the addition of Sr element at 360?,while decreased with the increase of deformation temperature?400 and 440??.when the deformation temperature was 400?and the Sr content was 0.2wt%,the critical strain of DRX was the smallestand its value was 0.054.?4?The effect of Sr content on the corrosion resistance of 5A12 aluminum alloy was studied,and the related corrosion resistance mechanism was explored.The results showed that the intergranular corrosion resistance of O-state alloys increased first and then decreased with the increase of Sr content in the range of 0-0.6wt%.The optimum Sr content was 0.2wt%.Under the ASTM G67 standard,the mass loss of the alloy was the smallest,and the weight loss of the alloy decreased 53%and 30.6mg/cm² relative to the matrix.In addition,for H321 alloy,the 5A12-0.2Sr alloy prepared by traditional multi-pass rolling process?without dynamic recrystallization?ha d the best intergranular corrosion resistance before and after sensitization treatment,and the weight loss of nitric acid corrosion after sensitization was 29.9 mg/cm².