Fatigue strength and microstructure evaluation of Al 7050 alloy wires recycled by spray forming, extrusion and rotary swaging
(1. Programa de Pós-gradua？？o em Ciência e Engenharia de Materiais (PPG-CEM), Universidade Federal de S？o Carlos (UFSCar), Rod. Washington Luiz, Km 235, 13565-905 S？o Carlos - SP, Brazil;
2. Divis？o de Materiais, Instituto de Aeronáutica e Espa？o, CTA, Pra？a Marechal Eduardo Gomes, 50, 12228-904 S？o José dos Campos - SP, Brazil;
3. Departamento de Engenharia de Materiais (DEMa), Universidade Federal de S？o Carlos (USFCar), Rod. Washington Luiz, Km 235, 13565-905 S？o Carlos - SP, Brazil)
2. Divis？o de Materiais, Instituto de Aeronáutica e Espa？o, CTA, Pra？a Marechal Eduardo Gomes, 50, 12228-904 S？o José dos Campos - SP, Brazil;
3. Departamento de Engenharia de Materiais (DEMa), Universidade Federal de S？o Carlos (USFCar), Rod. Washington Luiz, Km 235, 13565-905 S？o Carlos - SP, Brazil)
Abstract: Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated (solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 °C, imparting higher tensile (586 MPa) and fatigue (198 MPa) strengths.
Key words: Al-Zn-Mg-Cu alloy; spray forming; thermomechanical processing; microstructure; fatigue