Effects of minor Sc on microstructure and mechanical properties of Al-Zn-Mg-Zr alloy welded joint

Two kinds of Al-6.0Zn-2.0Mg-0.12Zr and Al-6.0Zn-2.0Mg-0.2Sc-0.12Zr alloy plates were prepared by ingot-metallurgy. The alloy plates with 3 mm thickness were welded by argon shield welding method,and the mechanical properties and microstructures of the two welded joints filled with Al-Mg-Sc welding wire were studied comparatively. The results show that firstly,minor Sc can raise the mechanical properties of the Al-Zn-Mg-Zr base alloy greatly. The reason for the increment is the fine grain strengthening,precipitation strengthening and the substructure strengthening caused by Al3(Sc,Zr). Secondly,η′ phase(MgZn2) and grain size in the heat-affected zone of the alloy without Sc become coarse obviously,the η′ phase(MgZn2) in the heat-affected zone of the alloy with Sc becomes coarse also,but the grain size has no visible change. Al3(Sc,Zr) particles are rather stable and can inhibit the movement of dislocation and sub-grain boundaries,overaging softening is not serious. Thirdly,adding minor Sc can raise the strength of welded joint remarkably,the tensile strength of alloy with Sc increases from 395 MPa to 447 MPa and the welding coefficient increases from 0.7 to 0.8 as well. The reason for the high strength of welded joint with Sc addition is the fine grain strengthening,precipitation strengthening and the increasing of resistance to thermal cycling softening caused by Al3(Sc,Zr).

Effects of electrochemical oxidation on surface and mechanical properties of dry-jet wet spun carbon fibers

The surface of dry-jet wet spun carbon fibers(CFs)is relatively smooth compared with those of wet spun CFs,which results in weak interfacial interactions between the fiber and the polymer resin.A surface functionalization treatment is particularly important to unleash the full potential of the superior mechanical properties of dry-jet wet spun CFs.In this study,the effects of electrochemical oxidation treatment time and current density on the surface structures and the mechanical properties of dry-jet wet spun CFs were investigated.The interlaminar shear strength of the CFs improves significantly from 69 to 84 MPa after 120 s of treatment.Further structural analysis reveals that the improvements are due to the addition of oxygen-containing functional groups and the optimization of the morphology on the surface of the CFs.