Effect Of Mn On Microstructure Properties And Dispersoids Of 6xxx Series Aluminum Alloy

Practice has shown that improving the formability of aluminum alloy sheets is the key of its wide application in the outer body of cars.The smaller size and higher density of dispersoids are important factors of the formability of aluminum alloy sheets.At present,Al-1.3Si-0.42Mg type Al-Mg-Si alloy,which is most promising in the field of automobile body sheets,usually adds trace elements such as Mn to refine grains and inhibit recrystallization,and Mn precipitates dispersoids at homogenization.The size,density and distribution of dispersoids have an important influence on the strength,plasticity and formability in the alloy sheet.In order to reveal the influence of Mn content(including the interaction of Fe impurities)on the formation of dispersoids and effectively control dispersoids and further improve formability,this article will systematically study the effect of Mn on microstructure properties and dispersoids of Al-1.30Si-0.42Mg with 0.2%Cu alloy.The main results are shown as follows:1)Before the Mn content increased to 0.5%in the alloy,the amount of dispersoids increased,the size decreased and the distribution became more uniform.After more than 0.5%of the Mn content,although the alloy formed a high-density dispersoids,a wide dispersoids non-precipitated region occured on both sides of the grain boundary.The dispersoids were a-Al(MnFe)Si phase with sc structure in the Mn containing alloy.When the Mn content increased from 0.2%to 0.5%,the Mn/Fe of the dispersoids increased from 7.64 to 11.77.The number of dispersoids increased and the distribution was more uniform and the size change was not obvious in the trace amount of Fe(less than 0.2%)alloy.With the increase of Fe content,the dispersoids of the alloy were still the ?-Al(MnFe)Si phase with the sc structure but the Fe/Mn value increaseed and the change of the number,size and distribution of the dispersoids were not obvious while the numbers of coarse second phase increased.The change of the amount and distribution of the dispersoids in the alloy with 0.1%Cr were not obvious.2)With the increase of Mn content(before 0.5%),the yield strength of the alloy T4 sheet decreased gradually in three directions,and the tensile strength changed little.When the Mn content exceeded 0.5%,the yield and tensile strength increased.When Mn content was close to 0.5%,the transverse strength of the sheet was significantly higher than longitudinal strength.The 0.5%Mn alloy sheet had the lowest yield ratio.Elongation decreased with increasing Mn content.The cup value,the n15 value in the three directions and the average value of n15 reached the maximum at 0.5%Mn alloy.When the Mn content continued to increase,the value decreased.The r15 value did not change significantly with Mn content in the longitudinal and transverse directions,but it increased first and then decreased with the increase of Mn content in the oblique 45° direction,which eventually caused the average value of r15 increased first and then decreased with the increase of Mn content.When Mn content was 0.5%,r15 reached the maximum value.3)As the Fe content increasing(to 0.5%),the strength of the alloy T4 sheet increased slightly,the elongation decreases and the r15 value increased slightly.The change of the cup value,the n15 value of three directions and the average value n15 was not obvious and the yield ratio was the lowest when Fe was 0.2%.The addition of 0.1%Cr had the effect of increasing the strength of the T4 sheet of Al-1.30Si-0.42Mg alloy containing 0.2%Fe,and the r15 value did not change significantly with the Fe content increasing.4)Pretreatment at 250? or 350? before normal homogenization treatment could increase the density of dispersoids and made the distribution became more uniform in the semi-continuous ingot of Al-1.3Si-0.42Mg alloy.However,when the pretreatment at 250? or 350? exceeded 72 h or 24 h respectively,the change of the density and distribution of the dispersoids was not obvious.The density and distribution of dispersoids in the thin plate after solid solution treatment was similar to the ingot and the recrystallization was less sufficient,and the density and distribution of the dispersoids of the final thin plate were similar after pretreatment at 250?×72 h and 350?×24 h;The coarse second phase of the sheets without homogenization treatment was slightly smaller in size and larger in number while the dispersed phase was finer and less in number.5)Compared with the normal homogenization treatment,the pretreatment at 250? or 350? had little effect on the strength and cupping value of the Mn-containing Al-1.30Si-0.42Mg alloy sheet after solution treatment but the pretreatment could improve its extension.Pretreatment at 350? could improve the n15 value and the r15 value of the alloy sheet with higher Mn content,but the effect of pretreatment at 250? was less.The change of strength of the alloy sheets without homogenization treatment was not obvious,but the elongation and the cupping value decreased.When Mn content was less than 0.2%,the n15 decreased slightly.When the Mn content was less than 0.5%,the r15 value increased.