TUESDAY PM Technical Program 58
4:10 PM Twinned Dendrite Formation in Al-Zn-Cr under Unusual Solidification Conditions: Güven Kurtuldu1; Philippe Jarry2; Michel Rappaz1; 1 Computational Materials Laboratory, Ecole Polytechnique Fédérale de Lausanne, Station 12, CH-1015 Lausanne, Switzerland; 2Constellium CRV, ZI Centr’alp, 0725 rue Aristide Berges, BP 27, Voreppe, FR-38341, France Twinned dendrites have been observed for the first time in an Al20wt.%Zn-0.1wt.%Cr alloy solidified under conditions of low thermal gradient, low cooling rate and with very limited convection in the melt. By comparison with the microstructure of Al-20wt.%Zn solidified under identical conditions, it is shown that 0.1wt.%Cr promotes the formation of growth twins, while all previous literature reported that twinned dendrites can only be obtained under strong thermal gradient and convection. A detailed analysis demonstrates that these twinned dendrites grow along <110> directions, with trunks split in their center by {111} twin planes. The possible mechanisms responsible of twinned dendrite formation are discussed and since this occurs under laboratory scale experiment conditions, this also opens new possibility to study their formation in situ, using ultrafast X-ray synchrotron tomography or radiography. 4:30 PM Analysis of Dendritic Primary Al Grain Ripening and Solid Fraction Measurement in A356 Alloy Semi-Solid Slurry Using Segregation Sensitive Reagent: Li Gao1; Yohei Harada1; Shinji Kumai1; 1Tokyo Institute of Technology Ripening of dendritic primary Al grain in semi-solid state has been paid much attention to since it is the most economical way to produce semi-solid slurry containing spheroidal Al grains for thixocasting. Also, solid fraction is the key factor in all the semi-solid processes. A segregation sensitive reagent (Weck’s reagent) can help to study both the two topics, revealing the inner-primary Al grain’s optical microstructure evolution during semisolid heat treatments (partial re-melting) of small A356 alloy samples cut from both as-DC cast and compressed ingots (Recrystallization and partial re-melting process). With the help of this etching technique, the influence of induced strain on the ripening mechanism of primary Al grains was investigated precisely. Furthermore, the peripheral part of the primary Al grain grown during water quenching was distinguished by the reagent. Therefore we could exclude this area when measuring the solid fraction and avoid overestimation by image analysis. 4:50 PM A Dual-Scale Segregation Model for the Direct Chill Casting Process: Ravindra Pardeshi1; Simon Barker2; Biswajit Basu1; Mark Gallerneault3; 1 Aditya Birla Science & Technology Co. Ltd; 2Novelis Global Technology Center, ; 3Novelis Global Technology Center, During Direct Chill (DC) casting, it is crucial to know the degree of segregation of solute at macro/ micro scale and phases formed during casting for devising downstream processing. For the accurate prediction of macrosegregation from a macroscopic model, proper representation of micro-segregation at a sub-grid level(secondary dendrite arm space) by accounting major micro-scale processes (back-diffusion, re-melting and eutectic solidification) is very important. In the present work, a general numerical approach of coupling the temperature and concentration fields using macro/micro dual scale solidification is applied for the DC casting process. The dual scale modeling framework implementation is done using a hybrid explicit-implicit solidification scheme. The advantage of using such approach is due to sub-grid micro-segregation model while doing macrosegregation calculations, where micro-scale information is available as input to downstream models. The application of modeling approach for simulating DC casting of aluminum alloy is done and merits/demerits of models are discussed.
Fatigue and Fracture Toughness Keynote
Program Organizers: William Cassada, Alcoa Technical Center; Hasso Weiland, Alcoa Technical Center; Anthony Rollett, Carnegie Mellon University Symposium Organizers: Krishnan Sankaran, Boeing; HJ Schmidt, AeroStruc Tuesday PM June 5, 2012
Room: Rangos I Location: University Student Center
Session Chair: Krishnan Sankaran, The Boeing Company 1:15 PM Keynote Fracture Mechanisms in Al-Cu-Li Alloys: Jean-Christophe Ehrstrom1; B. Bès1; J. Chevy1; F. Eberl2; 1Constellium CRV; 2Constellium Usine d’Issoire Compared to high toughness conventional 2000 alloys, 2198 for instance displays a 30% higher yield stress, with a still higher toughness. Whereas the 2000 alloys R-curve can be described by the combination of intra-granular damage evolution and constitutive behaviour, the shearing mechanism becomes important in Al-Cu-Li alloys especially when the thickness is reduced. In addition, intergranular failure contributes to the damage. These mechanisms are strongly related to the material texture and anisotropy, more so as the alloys are used in slightly underaged tempers. Failure mechanisms will be described and compared to those of conventional high toughness 2000 alloys and higher strength 7000 alloys like 7475.
Fatigue and Fracture Toughness 2 Program Organizers: William Cassada, Alcoa Technical Center; Hasso Weiland, Alcoa Technical Center; Anthony Rollett, Carnegie Mellon University Symposium Organizers: Krishnan Sankaran, Boeing; HJ Schmidt, AeroStruc Tuesday PM June 5, 2012
Room: Rangos III Location: University Student Center
Session Chair: Hans-Jürgen Schmidt, AeroStruc – Aeronautical Engineering 2:00 PM Invited Material and Structural Simulation Methodology Advancements for Fatigue and Damage Tolerant Critical Aircraft Structures – an Aluminum Supplier Perspective: Robert Bucci1; M. A. James1; M. Kulak1; M. B. Heinimann1; 1Alcoa Technical Center While aluminum alloys are available and capable of supporting static stress level increases needed to meet next generation weight reduction targets, it is the fatigue requirement that is expected to limit attainable weight and cost savings. This paper provides an overview on major areas of Alcoa research devoted to improving aluminum product fatigue and fatigue crack growth performance, including new structural concepts and related test/analysis methodologies designed to help both customers and the industry meet their design and life estimation goals. Alcoa aluminum products developed for the aerospace market provide the major application focus. Examples are provided to highlight significant achievements in areas of material fatigue behavior fundamentals, fatigue design/trade study and life prediction tools, and confirmation of new structural concept fatigue benefits all of which are currently being deployed to more rapidly introduce new and improved Alcoa aluminum products into the marketplace. 2:40 PM Fatigue Resistance of Al-Cu-Li and Comparison with 7xxx Aerospace Alloys: Armelle Daniélou1; Jean-Christophe Ehrström1; Jean-Patrick Ronxin1; 1Constellium-CRV Al-Cu-Li alloys are of great interest for aerospace applications due