Properties of welded joints of the Al – Zn – Mg – Ca alloy doped by microadditivies of zirconium and scandium

Abstract

This work is devoted to the study of the structure and properties of welded joints of hot-rolled sheets from the aluminum alloy Al – 4%Zn – 2.5%Mg – 2.5%Ca – 0.2%Zr – 0.1%Sc. The joints were obtained by manual argon-arc method (TIG-welding) using a wire of own production from the base metal composition (sample AlCa) and standard welding wire Of SvAMg5 alloy (sample AlMg). It is shown that in the cast state the alloy structure consists of an aluminum solid solution (Al) (which contains 2.9% Zn, 2.5% Mg, 0.28% Zr and 0.12% Sc) and eutectic crystals of the phase (Al, Zn)4Ca with a volume fraction of ~7.6%. Microstructural investigation of wrought products showed that the formation of a structure consisting of an aluminum matrix and uniformly distributed spherical particles with a diameter of less than 2 microns occurs as a result of hot rolling with a compression ratio of 95%. The quality of welded joints produced using different additives meets the requirements for the absence of internal defects. The structure in the middle of the joints corresponds to the cast structure of the alloy and has hardness (~80 HV) inferior to the hardness (~105 HV) of the main deformed alloy. Heat-affected zone retains a relatively high hardness (at 95 HV), which is associated with a stabilizing effect of dispersed particles of the eutectic phase (Al, Zn)4Ca, and also presence of nanoparticles of the phase Al3(Zr,Sc) in the original sheet. Additional annealing of welds at 350 oC resulted in degradation of the hardness of the AlMg sample to less than 70 HV and relative cross-sectional hardness equalization in the AlCa sample to more than 90 HV. This effect can be explained by the dispersion hardening in case of the AlCa sample due to the separation of coherent nanoparticles of the Al3(Zr, Sc) phase with the L12 structure during the decay of a supersaturated aluminum solid solution. The results of mechanical tests confirmed the advantages of the strength properties of the AlCa sample over the values of the AlMg sample. All samples in the initial state and annealed sample AlMg have strength coefficients relative to the base metal from 72.4 to 75.3. The greatest mechanical properties can be achieved in the welded joint AlCa in the annealed state: UTS = 274±2 MPa, YS = 181±20 MPa, El = 2.9±0.8 %, which corresponds to a coefficient of more than 80%. This paper was prepared in the framework of the Assignment No. 11.2072.2017/4.6 for implementing a project on the following subject: Developing a process for obtaining deformed semi-finished products made of aluminium-matrix eutectic composites hardened with L12 phase nanoparticles with no quenching applied.