**Yang Jia 1, Sicong Lin 1, Jizi Liu 1,2,\*, Yonggui Qin <sup>1</sup> and Kehong Wang 1,\***


Received: 27 September 2019; Accepted: 25 October 2019; Published: 28 October 2019

**Abstract:** An Al-Mg-Si alloy 6061 and an Al-Zn-Mg alloy 7A52 were joined by friction stir welding successfully. Pre- and post- heat treatment were employed to improve the strength of the weld. The results show a best weld joint with the lowest hardness of 100 HV in 6061 matrix, being achieved by post-solid-solution and subsequent two-stage artificial aging for the whole weld joint of the 7A52 and 6061 solid solution. Under this condition, the weld nugget zone (WNZ) is stronger than 6061 matrix but it has lower hardness than 7A52 matrix. The hardness of WNZ is contributed by the combination of η and L precipitates, dynamically changes along with the ratios between the number of η and L precipitates. The higher the number density of η precipitates, the hardness of WNZ is closer to that of the 7A52 matrix. Otherwise, the higher number density of L precipitates, the hardness of WNZ is closer to that of 6061 matrix. The coexistence of η and L precipitates is a direct result from the mixture of 7A52 and 6061 alloys achieved by stirring. Precipitates identification and composition analysis reveal a dynamic WNZ with constituent transition in hardness and composition.

**Keywords:** friction stir welding; aluminum alloy; heterogeneity; mechanical; microstructure
