Application of Wire-arc Directed Energy Deposition Process in the Al-Zn-Mg-Cu-Sc-Zr Alloys

Abstract

Wire-arc directed energy Deposition (WA-DED) process is a combination of arc and wire feeding additive manufacturing technology including either the gas tungsten arc (GTA) or the gas metal arc (GMA) process. And this technology has been applied in the aerospace manufacturing industry to reduce the time of product development and “buy-to-fly” ratios. In recent years, many researchers and scientists have begun to apply WA-DED technology to manufacture different aluminum alloy components.The high-strength Al-Zn-Mg-Cu (7xxxx series) aluminum alloy is called as a non-weldable aluminum alloy due to their high susceptibility of hot cracking during the welding process. Traditional processing technologies, such as casting, also face many challenges such as poor casting performance and extremely high buy-to-fly ratio during the manufacturing process of large-scale Al-Zn-Mg-Cu high-strength aerospace structural parts. Therefore, the feasibility of Al-Zn-Mg-Cu aluminum alloys in WA-DED applications is very necessary to be verified. However, the existing Al-Zn-Mg-Cu aluminum alloy filler wires are not suitable for WADED process due to their high susceptibility to hot cracks, which is the major challenge for 7xxxx series aluminum alloy applied in WA-DED.In this thesis, a novel Al-Zn-Mg-Cu-Sc-Zr aluminum alloy wire with ultra-high strength and excellent hot-crack resistance has been successfully prepared by optimizing the composition of Al-Zn-Mg-Cu alloying elements and adding appropriate amounts of inoculants Sc and Zr elements during the smelting process. And then, the thin-wall components without any hot cracks were successfully fabricated by using cold metal transfer (CMT) process. Several key steps including smelting process, wire process, deposition parameters, heat treatment processes were shown in this study. The microstructure and precipitated second phases under the as-deposited and different heat-treated conditions were discussed in detail. The average ultimate tensile strength (UTS), yield strength (YS) and elongation of the T6 heat-treated sample were measured to be 618 ± 4 MPa, 542 ± 6 MPa and 5.7%, respectively. The outstanding tensile strength successfully exceeded 600MPa.However, WA-DED technology is based on the fusion welding process, and porosity defects are inevitable during the manufacturing process of WA-DED aluminum alloys due to the significant difference in the solubility of hydrogen in the liquid and solid phases of aluminum alloys. Meanwhile, there are also lack of fusion (LOF) defects and severe grain boundary segregation during the solidification process. All these defects will seriously affect the mechanical properties of WA-DED aluminum alloys, especially the fatigue property which is always considered as the final criterion for the engineering application of a new material. In order to effectively address pore defects, LOF defects as well as solve the grain boundary segregation issue in WA-DED aluminum alloys, an innovative hybrid WA-DED + interlayer friction stir processing (FSP) method was applied to successfully fabricate thick-walled Al-Zn-Mg-Cu-Sc-Zr aluminum alloy component with enhanced strength-ductility and fatigue properties. The porosity defects caused by the WA-DED process were effectively addressed in the FSP effective zone, and the original continuous grain boundary eutectic structures were broken up and dispersed along the grain boundaries. The grains were also further refined with an average size of about 1.1 ± 0.2 μm in the stirring zone (SZ). After T6 heat treatment. The manufactured alloy achieves isotropy in mechanical properties, with a tensile strength and elongation of 645 ± 8 MPa and 12.6 ± 0.5%, respectively. The fatigue performance is significantly improved to 250 MPa. This method provides a feasible approach for eliminating defects and achieving isotropy in aluminum alloys during the additive manufacturing process.</p