Heat treatment of welded joints made on 7020 alloy with the AlMg5 alloy

Abstract

The basic problem of joints welded from the high strength aluminium alloys hardened by heat treatment is a significant drop of the welded material properties in HAZ (Heat Affected Zone). To improve the mechanical properties of welded joints and to prevent crack formation in HAZ, a multi-stage heat treatment can be used. The study examines changes in the properties and structure of HAZ in a welded joint deposited with the Al5Mg alloy on a 7020 alloy (AlZn4, 5Mg1) in the T6 condition. The impact of a two-stage heat treatment (solutioning at 470 degrees C and aging at 90 degrees C/8h + 145 degrees C/16h) on structure and properties of HAZ was examined. Considerable degree of elimination of the HAZ effect on structure and properties of a welded joint was observed. For the 7020/Al5Mg welded joint, the mechanical properties (Rm = 348MPa, Rp(0.2)=301MPa and A50=11,4%) comparable with the properties of 7020 alloy in T6 condition were obtained. Structure examinations were carried out by light microscopy and transmission electron microscopy, while mechanical properties were determined by hardness measurements and a static tensile test. An attempt was made to measure the residual stresses in an around-weld area.