Zn含量对Al-Mg-Si-Cu合金拉伸性能及晶间腐蚀敏感性的影响

Tensile Properties and Intergranular Corrosion Sensitivity of Al-Mg-Si-Cu Alloy with Different Zn Coatents

采用拉伸测试、浸泡腐蚀试验、差示扫描量热(DSC)、扫描电镜(SEM)及透射电镜(TEM)等手段,研究了Zn含量(0~1.2%,质量分数)对自然时效(T4)和峰值时效(T6)状态Al-Mg-Si-Cu合金拉伸性能和晶间腐蚀敏感性的影响。结果表明:较低的Zn含量(0~0.8%)对T4和T6合金强度无显著影响;T4合金无晶间腐蚀敏感性,而T6合金发生严重晶间腐蚀且最大腐蚀深度随Zn含量增加而下降,并于0.8%Zn含量时降至320μm,较无Zn合金减小220μm。当Zn含量增大至1.2%,晶内预脱溶原子偏聚区(GP区)和β″相析出过程得到促进,使得合金强度显著提高,T4合金屈服强度和抗拉强度分别为196 MPa和363 MPa,较无Zn合金分别提高30 MPa和44 MPa, T6合金屈服强度和抗拉强度分别为368 MPa和443 MPa,较无Zn合金分别提高44 MPa和57 MPa,同时,两种时效状态合金均能保持较高的延伸率;T4和T6合金显著增强的晶间腐蚀敏感性应与高Zn含量加大了晶界析出相与无析出区(PFZ)之间成分差异有关。此外,实验合金拉伸断口形貌特征基本不受Zn含量影响,而与时效状态密切相关:T4合金为韧窝型穿晶断裂,T6合金为沿晶断裂与韧窝型穿晶断裂共存模式。

Due to the combination of low density,medium strength,high corrosion resistance,good weldability and plastic deform⁃ability,the Al-Mg-Si-Cu alloys had found wide applications in the field of automotive,rail transportation and aviation.However,the addition of Cu element could increase the strength of the alloys in peak-aging state but at the cost of corrosion resistance.In order to im⁃prove the strength and corrosion resistance simultaneously,the effects of Zn content on the tensile properties and intergranular corro⁃sion(IGC)susceptibility of Al-1.0Mg-0.95Si-0.9Cu-x Zn-0.6Mn-0.12Zr alloys(x=0,0.4,0.8,1.2)in the natural aging(T4)state for 14 d and peak aging(T6)state at 180℃for 8 h,were investigated by Vickers hardness test,tensile test,immersion corrosion test and differential scanning calorimetry(DSC),scanning electron microscopy(SEM),transmission electron microscopy(TEM).The re⁃sults showed that the addition of low Zn contents(0~0.8%)had no significant effect on the characteristic of hardness variation and ten⁃sile properties(strength and elongation)of T4 and T6 alloys.The yield strength,ultimate tensile strength and elongation of T4 alloys were in the range of 156~163 MPa,304~308 MPa and 27.5%~28.1%,respectively,and T6 alloys had the yield strength,ultimate tensile strength and elongation in the range of 336~340 MPa,395~404 MPa and 14.4%~14.8%,respectively.Furthermore,T4 alloys almost had no IGC susceptibility,whereas serious IGC occurred in T6 alloys,which decreased with the increase of Zn content.When Zn content of the alloy was 0.8%,the maximum corrosion depth was 320μm,which was 220μm less than that of Zn-free alloy.When Zn content of the alloy increased to 1.2%,the precipitation of Guinier Prestion zone(GP zone)andβ"phase was greatly promoted within the grains and caused significant improvement of age-hardening rate and the alloy strength.The yield strength and ultimate ten⁃sile strength of T4 alloy were 196 MPa and 363 MPa,respectively,which were increased by 30 MPa and 44 MPa compared with Zn-free alloy.And the T6 alloy had the yield strength and ultimate tensile strength of 368 MPa and 443 MPa,respectively,which were 44 MPa and 57 MPa higher than those of Zn-free alloy,respectively.At the same time,T4 and T6 alloys could still achieve high elonga⁃tions of 25.3%and 12.6%.The increased IGC susceptibility of T4 and T6 alloys should be related to the compositional change of the grain boundary precipitates and precipitate free zones caused by the addition of high Zn content,which resulted in a larger gap be⁃tween their corrosion potential.In addition,the tensile fractographies of the four alloys were not affected by the content of Zn,but de⁃pendent mainly on the aging state.The dimple induced transgranular fracture was observed for T4 alloys,whereas T6 alloys were char⁃acterized by the combination of intergranular fracture and dimple induced transgranular fracture.Furthermore,the microcracks were initiated only around those second phase particles such as coarse AlFeMnSi constituents and sub-micron dispersoids,which led to the dimple induced transgranular fracture for T4 alloys.By comparison,the low strength of the precipitate free zones at grain boundaries in T6 alloys was also responsible for the initiation of microcracks and led to the intergranular fracture.In general,the appropriate amount of Zn alloying in Al-Mg-Si-Cu alloy could accelerate the aging hardening rate of the alloy and significantly improve the strength of the alloys in T4 and T6 state by promoting the precipitation of aging strengthening phase,while maintaining high level of elongation and corrosion resistance,but only when Zn content was relatively high,such as 1.2%.Or IGC resistance of the alloy could be greatly im⁃proved by bridging part of the high corrosion potential caused by Cu element without sacrificing the aging hardening rate and tensile properties,such as 0.8%Zn content.