Precipitation Behavior Of Age-hardening Al-Mg-Zn Alloys And Improvement Of Their Intergranular Corrosion

Traditional 5xxx series Al-Mg alloys are used extensively in the marine and vehicle transportation due to their excellent corrosion resistance and weldability.These alloys are mainly strengthened by solid solution and strain hardening.The strength is lower than those ?-MgZn2 strengthened 7xxx series alloys with Zn/Mg ratios above 2.0.However,the corrosion resistance and weldability of 7xxx series alloys are poorer than 5xxx series alloys,and become an obstacle for the development and application of these high strength alloys.Thus,designing an alloy owning high strength,good corrosion resistance and good weldability is urgent for marine and vehicle industries.In this study,the author designed and suscessfully prepared the T-Mg32(AlZn)49 strengthened Al-Mg-Zn alloys with a Zn/Mg ratio below 1.0.The alloy compostion is different from traditional 5xxx series and 7xxx series alloys.Current studies in our group have demonstrated the suppression of solidification cracking tendency during fusion welding of the designed alloy.Thus both precipitation hardening and corrosion resistance of the designed alloys are focused in the study.With the help of hardness and tensile testing,differential scanning calorimetry,scanning and transmission electron microscope,and three dimensional atom probe,we have clarified the formation of clusters and their transformation to the subsequent T-Mg32(AlZn)49 strengthening phases,and characterized the competition of precipitates at an atomic level.The high temperature pre-treatment is developed to improve the intergranular corrosion resistance of the Al-Mg-Zn alloy.Al-5.1Mg-3.0Zn-0.15Cu alloy reaches the peak tensile strength of 489 MPa and yield strength of 417 MPa after solid solution treatment at 530?/10 min,then aged at 90? for 24 h and subsequently aged at 140? for 25 h.According to the diffraction information,and coherent relationships between particles and Al matrix,the precipitation sequence of T-Mg32(AlZn)49 phase is designated as SSSS? GPI zone?GPII zone(T")?T'?T-Mg32(AlZn)49.The pre-aging stimulates the formation of Mg-Zn clusters(precursor of T-Mg32(AlZn)49 phase)but inhibits the precipitation of Mg-Cu clusters(precursor of S-Al2CuMg phase),which accelates and enchances the age-hardening response of the Al-5.1Mg-3.0Zn-0.15Cu alloy at 180?.The poor peak hardness of alloys without pre-aging is from the synergetic effect of the hardening of S-Al2CuMg phase and the coarsening of T' or T precipitates.The high peak hardness of alloys with pre-aging results from fine,high number density and homogeneously distributed T" precipitates.Though the alloys after pre-aging at 90? for 24 h or natural aging for 168 h have the same hardness value,the formation of relatively stable GP zones during pre-aging rather than natural aging accelates and enchances the age-hardening response during subsequent artificial aging.Zn addition stimulates the precipitation of a T-Mg32(AlZn)49 phase but suppresses an S-A12CuMg and a ?-Al3Mg2 phase.With increasing Zn content,the age-hardening response is enhanced and accelerated significantly,resulting from the smaller size,increasing number density and volume fraction of the T"precipitates.The component similarity promotes the transformation from clusters into T" precipitates.Once clusters have formed in alloys with low Zn contents,most of them do not transform readily in the subsequent aging due to their high Mg/(Al+Zn+Cu).Thus,there are a higher number density clusters and few T precipitates in alloys,which results in a weaker age-hardening response.Although T-phases originate from diferent alloys,their Mg/(Al+Zn+Cu)ratios reach the constant value 3/7 for larger particle sizes.The intergranular corrosion resistance is enhanced with decreasing the pre-treatment temperature.The 410?/1 h+T6 temper can improve the intergranular corrosion resistance due to more discontinuous grain boundary precipitates and the lower concentration difference of Mg and Zn atoms between grain boundary precipitates and precipitate free zone.While it simultaneously keeps the similar strength to T6 temper because of the same microstructure in grains as T6 temper.And the essential reasons are the preferentially segregated Mg,Zn,and Cu atoms along grain boundaries and the maintained solid solution state in grains after 410?/1 h treatment,respectively.Among various factors,the discontinuous degree of grain boundary precipitates is the key reason for improving intergranular corrosion resistance.The above results can provide a guide for the composition design,sheet preparation,aging process design,microstructure design and performance regulation,and improvement of intergranular corrosion resistance of age-hardening Al-Mg-Zn alloys.