Effect Of Elements On The Properties Of A7N01 Aluminum Alloys Welded Joints Under High Humidity Welding Environment

A7N01 aluminum alloy is the main materials for key components manufacture of underframes for High Speed Trains. It is belonged to Al-Zn-Mg or Al-Zn-Mg-Cu series alloys. The content of elements had definitive effect on the aluminum alloy microstructure and then affected the mechanical properties.The aluminum alloy was in high sentivity to the hydrogen induced porosity when the alloys were under MIG welding, and most of the hydrogen are coming from the moisture and that is directed to the humidity.This paper is focused on the research of A7N01 aluminum alloy and will take the effect of different content of elements on the mechanical propeties into consideration. Four types of welded joints are fabricated under 10℃ temperature and 70% humidity in environment welding laboratory and studied the porosity formation and the mechanical properties of the welded joints.The equilibrium states microstructure of the A7N01 aluminum alloy are including MgZn2、Al-Cr-Mg-Mn、A16Mn and Mg2Si. There distribued a lot of porostiy inside the weld eapeciallly the fusion line area of the four type welded joints. The porosity were in large quantity with larger size and distributed along with the fusion line. So the humidity had great effect on the porostiy formation.The microhardness of the base metals and heat affected zone(HAZ) were in the same degree with the difference in the value 20HV and the softness in the HAZ zone was not obvious. The tensile sttrength was in great different,the highest value was 302MPa in the 7N01S1-T7 (H3#) sample but the lowest value was 262Mpa in the 7N01S1-T6 (H2#) sample.The 7N01S1-T7 (H3#)、7N01S1-T4 (H1#) sample had higher strength but the 7N01S1-T6 (H2#) sample had the highest toughness.The residual stress distribution of the four type welded joints were in great difference.For the longitudinal residual stress , the value of 7N01S1-T4 welded joint was 239Mpa, the vaule of 7N01S1-T7 welded joint was 302Mpa, the value of 7N01S1-T6 welded joint was 192.8Mpa, the value of 7N01S1-T8 welded joint was 225.0Mpa. For the transverse stress,the value of 7N01S1-T7 welded joint was 247.0Mpa, the vaule of 7N01S1-T8 welded joint was 134.0Mpa, the value of 7N01S1-T4 welded joint was 254.0Mpa, the value of 7N01S1-T6 welded joint was 129.5Mpa. The fracture of the base metals was M1# (38.17 )、M3# (34.34)、M2# (33.15)、M4# (23.93) in order but the order of the welded joints was H4#、H2#、H3#、H1#.The fatigue strength was decreased under the high humidity.