Research On The Welding Thermal Simulation And Micro Area’s Corrosion Properties Of A7N01 Aluminum Alloy

A7N01 aluminum alloy is Al-Zn-Mg alloy of 7xxx series aluminum alloys.It has been widely used in high speed trains’car body manufacturing both in China and abroad.But its poor corrosion resistance has been the primary factor restricting the alloy which is widely applied.High-speed train body is full of welded structures,but its welding joints’mechanical properties are reduced so that they have been the weakest areas due to welding thermal cycle.For A7N01 aluminum alloy,not only the mechanical properties reduced,but also its corrosion resistance declined,especially for the welding heat affected zones’corrosion. It’s hard to study welding heat affected zones’microstructures and properties for that each of the zones is narrow.So it’s necessary to study the microstructures and mechanical properties of A7N01 alluminum alloy’s each welding heat affected micro zone by means of welding thermal simulation technique.This paper’s study objective is A7N01 aluminum alloy,including A7N01S-T5 and A7N01P-T4.The welding heat cycle curves and welding heat cycle parameters are got by laser-MIG hybrid welding method. In accordance with the test results, four different sets of welding thermal cycle parameters were designed,Then using Gleeble-3500 thermal simulation testing machine to simulate each micro welding heat affected zone.The A7N01 aluminum and its thermal simulated specimens were studied in their microstructures,second phases,tensile properties, flexural properties, impact toughness and surface hardness.Then their corrosion properties were studied by intergranular corrosion tests both in NaCl-H2O2 and NaCl-HCl solution, slow strain rate stress corrosion tests, electrochemical corrosion tests in 3.5% NaCl solution,exfoliation corrosion tests and electrochemical corrosion tests in EXCO solution.Microstructure analysis results show that,for A7N01S-T5 and A7N01P-T4,there are many strengthening phases of MgZn2 and a small amount of Al-Fe-Si and Mg2Si impure phases on the grain boundaries,and small MgZn2 in the interior of the grains.For high-temperature thermal simulation specimens,there are large number of strengthening phases of MgZn2 and impurity phases,such as Al-Fe-Si, Mg2Si and others.also,there are small amount of MgZn2 and Al-Fe-Si phases in the interior of the grains.For low-temperature thermal simulation specimens,there are large number of MgZn2 strengthening phases and small number of Al-Fe-Mn and Mg2Si impurity phases on the grain boundaries; within the grain boundary,there are small amount of fine MgZn2 phases.The results of the mechanical properties show that, for A7N01S-T5,the high-temperature thermal simulation specimens’ strength, elongation is higher than the base matal,than the low-temperature thermal simulation specimens;and the high-temperature thermal simulation specimens’ impact toughness is lower than the base material,than the low-temperature thermal simulation specimens. While for A7N01P-T4, two types of thermal simulation specimens’ strength and elongation are lower than its base metal,but they have little difference.The intergranular corrosion test results show that, Both in NaCl-H2O2 and NaCl-HCl solution,the thermal simulation specimens have poorer corrosion resistance than the base metal for A7N01S-T5,and its high-temperature simulation specimens’ corrosion resistance is the worst. Also for A7N01P-T4, its high-temperature thermal simulation specimens’ corrosion resistance is the worst, followed by the low-temperature thermal simulation specimens,and the corrosion resistance of the base material is optimal.Slow strain rate stress corrosion test results show that, for A7N01S-T5, the high-temperature thermal simulation specimen has the highest stress corrosion sensitivity,and its stress corrosion sensitivity index is 0.1262, showing a strong stress corrosion sensitivity. There are many secondary cracks in its fracture, and many sugar block grains, showing intergranular fracture mode; But the base metal and its low-thermal simulation specimens’ stress corrosion sensitivity are lower. Also for A7N01P-T4, its high-temperature thermal simulation specimen has the highest stress corrosion sensitivity,and its index of SSRT is 0.1101,showing a strong sensitivity to stress corrosion, and its brittle fracture is obvious;But the stress corrosion sensitivity of the base metal and its low-temperature thermal simulation specimen is lower.Exfoliation corrosion test results show that the corrosion’s degree of A7N01S-T5 and A7N01P-T4 alloys’ thermal simulation specimens are much heavier than that of their base metal in EXCO solution after 16 hours, and the erosion of high temperature thermal simulation specimens were significantly higher than the low temperature thermal simulation specimens.The electrochemical measurement results show that, both in 3.5% NaCl solution and EXCO solution,the A7N01S-T5 base metal’s corrosion resistance is optimal, followed by low-temperature thermal simulation specimens, while the high temperature thermal simulation specimens’ corrosion is the worst. Also for A7N01P-T4, the corrosion resistance of its high-temperature thermal simulation specimens is the worst, due to that its corrosion tendency is maximum and its corrosion rate is the fastest;followed by the low-temperature thermal simulation specimens,and the base metal has the best corrosion resistance,For that its corrosion tendency is the lowest,its corrosion rate is the minimum, and its polarization resistance is the maxminum.