Materials & Advanced Manufacturing (M&AM) Microstructural - PowerPoint PPT Presentation

Materials & Advanced Manufacturing (M&AM) Microstructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8 Uchechi Okeke a, b , Carl Boehlert a a Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI b Tank Automotive Research, Development, and Engineering Center, Warren, MI DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Background – AA2139-T8 and FSW Manufacturing (M&AM) AA2139-T8 • Al-Cu-Mg-Ag alloy • T8 tempered – Solution treating (525 – 544°C) – Cold worked (rolled) – Artificially aged (163 – 256°C) • T8 yields the θ ’ and the Ω precipitates Friction Stir Welding (FSW) • FSW is a solid-state joining technique which involves a frictional heat flux and severe plastic deformation. • Develops unique microstructures due to dynamic recrystallization • This process refines the microstructure. • The mechanical strength of FSW materials is determined by the resulting microstructure. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Research Motivation and Objective Manufacturing (M&AM) • Research on FSW has been primarily performed on thin plates of Al alloys. • Limited transferable knowledge on FSW effects on the microstructure for thick Al alloys. • Limited transferable knowledge on FSW effects on the mechanical properties for thick Al alloys. • Limited information about the effects of the SZ microstructure on the tensile-creep behavior. • Due to growing interests and the application of lightweight alloys for vehicles, elevated temperature loading environments should be investigated. • This study aims to provide information on the tensile-creep deformation behavior of thick plate FSW Al-Cu-Mg-Ag alloys. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced FSW Parameters and Creep Test Conditions Manufacturing (M&AM) HAZ FSW processing parameters SZ samples Parameter Specification Top samples Shoulder Diameter 1.625” Mid Top Pin Length 0.972” 127 mm Mid Plunge Depth 0.02-0.005” Spindle Speed 150-250 RPM Mid Bottom Travel Speed 2 IPM Bottom Total Length 18” • Temperatures: 225 to 275°C Region Temp ( ° C) Stress (MPa) HAZ 250-275 50 • Applied Stress: 25 to 125 MPa Mid-Bottom-1 250-275 50 • Sample Prep.: HAZ 225 50-75-100 – Flat dog-bone shaped Mid-Bottom-2 225 50-75 – Gage dimensions: width=12.7mm, Bottom 250 25-35-50 thickness=1.78mm • SEM/TEM analysis Mid-Bottom-3 250 25-35-50 DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Stress increase creep behavior – 225°C, 50-125 MPa Manufacturing (M&AM) 1 225 o C HAZ CR=1.86x10 -9 s -1 0.8 SZ Mid Bottom-2 Creep Strain (%) CR=1.33x10 -8 s -1 0.6 CR=8.78x10 -10 s -1 0.4 CR=1.23x10 -9 s -1 CR=8.49x10 -10 s -1 σ = 125MPa 0.2 HAZ CR=5.53x10 -10 s -1 σ = 100MPa σ = 75MPa Mid-Bottom σ = 50MPa 0 0 100 200 300 400 500 600 700 800 0.75% strain to failure; V p =3.5% Time (hrs) 0.95% strain to failure; V p =4.3% The SZ Mid Bottom was less creep resistant than the HAZ. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Temperature increase creep behavior – Materials & Advanced Manufacturing (M&AM) 250-275°C, 50 MPa 2.5 σ =50 MPa SZ Mid Bottom 2 275 o C 250 o C Creep Strain (%) 1.5 CR=1.23x10 -7 s -1 1 CR=7.62x10 -9 s -1 HAZ 0.5 CR=3.07x10 -10 s -1 Mid-Bottom CR=1.5x10 -10 s -1 HAZ 0 0 50 100 150 200 250 300 350 2.4% strain to failure; V p =7.2% 0.25% strain to failure; V p =3.8% Time (hrs) The SZ Mid Bottom was less creep resistant than the HAZ. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

The grain size & precipitate distribution varies Materials & Advanced Manufacturing (M&AM) through the depth of the weld. Top Mid Top Mid Mid Bot Bottom GS= avg. grain size V p = volume percent • The grain size decreases from the top of the weld towards the bottom. • The V p increases from the top of the weld towards the bottom. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Stress increase creep behavior through the depth Materials & Advanced Manufacturing (M&AM) of the SZ – 250°C, 25-35-50 MPa As-Welded As-Welded 2.5 SZ Bottom 250 o C 2 σ = 50 MPa Creep Strain (%) SZ Mid Bottom-3 σ = 35 MPa 1.5 σ = 25 MPa CR=1.06x10 -7 s -1 CR=2.06x10 -8 s -1 1 CR=2.72x10 -9 s -1 CR=1.34x10 -9 s -1 CR=1.07x10 -8 s -1 0.5 Bottom b) CR=7.03x10 -10 s -1 Mid-Bottom 0 0 100 200 300 400 500 600 1.6% strain to failure; V p =11.3% 2.4% strain to failure; V p =12.5% Time (hrs) • The SZ Mid-Bottom failed before the SZ Bottom. • SZ Bottom had a higher creep strain to failure. • Creep failure varies through the depth of the SZ. • Precipitates have coarsened  Unstable microstructure! DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced TEM of the base metal Manufacturing (M&AM) <100> <110> • Fine precipitate phases 34 nm are observed ( θ ’ and Ω ). • These precipitates strengthen the alloy. 110 nm 123 nm 24 nm DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

TEM of creep tested SZ through the depth of the 2.5 Materials & Advanced SZ Bottom 250 o C Manufacturing (M&AM) weld at 250°C, 25-35-50 MPa 2 σ = 50 MPa Creep Strain (%) SZ Mid Bottom-3 σ = 35 MPa 1.5 <100> - Bottom <110> - Mid Bottom-3 σ = 25 MPa CR=1.06x10 -7 s -1 CR=2.06x10 -8 s -1 1 CR=2.72x10 -9 s -1 CR=1.34x10 -9 s -1 CR=1.07x10 -8 s -1 0.5 177 nm CR=7.03x10 -10 s -1 θ ’ 0 0 100 200 300 400 500 600 273 nm θ ’ Time (hrs) Ω phase is not present • The microstructure was unstable during testing. • Precipitation and coarsening of the precipitates was observed in both areas. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Summary Manufacturing (M&AM) • The SZ was less creep resistant than the HAZ. • The refined microstructure of the SZ led to reduced creep resistance. • The SZ microstructure was dynamic/unstable during testing. – Precipitate coarsening and dissolution may have contributed to this. • The microstructure in the SZ is a function of the thickness through the weld. • The resulting creep properties were a function of the microstructure. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Acknowledgments Manufacturing (M&AM) This work was supported by the National Science Foundation Division of Material Research • (Grant No. DMR1107117). Center for Nanoscale Materials at Argonne National Laboratory via the U.S. Department of • Energy (Contract No. DE-AC02-06CH11357). – Special thanks to Dr. Yuzi Liu who was a great assistance in acquiring the as-welded and deformed TEM images. Institute of Materials Research, Tohoku University through the National Science Foundation • East Asia & Pacific Summer Institute Fellowship (Award No. 1515111). – The authors are grateful to Dr. Makoto Nagasako and Mr. Shun Ito for acquiring the TEM images. DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Tensile Behavior at RT Manufacturing (M&AM) 500 BM 400 Engineering Stress (MPa) SZ 300 200 BM FSW UTS (MPa) 433 364 100 0.2% YS 300 265 (MPa) 0 0 1 2 3 4 5 6 7 8 Strain (%) DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Secondary Creep Mechanism Analysis Manufacturing (M&AM) 2.5 1.4 10 -7 FSW, 50 MPa FSW, 50 MPa 1.2 10 -7 2 -7 s -1 CR=1.24x10 T=275C 1 10 -7 Creep Strain (%) -1 ) 1.5 Creep Rate (s 8 10 -8 6 10 -8 1 -8 s -1 CR = 1.45x10 T=250C -8 s -1 4 10 -8 CR = 1.45x10 T=250C 0.5 2 10 -8 0 0 0 50 100 150 200 0 50 100 150 200 Time (hrs) Time (hrs) DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Fractographs of 225°C, 50-125 MPa Manufacturing (M&AM) 225°C, 50-75 MPa SZ Mid Bottom 225°C, 50-125 MPa HAZ DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Fractographs of 250-275°C, 50 MPa Manufacturing (M&AM) 250-275°C, 50 MPa SZ Mid Bottom DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Materials & Advanced Fractographs of the SZ at 250°C, 25-35-50 MPa Manufacturing (M&AM) 250°C, 25-35-50 MPa SZ Mid Bottom 250°C, 25-35-50 MPa SZ Bottom DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited