Phase Transformation and Residual Stress in a Laser Beam Spot-Welded TiAl-Based Alloy
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THE c-TiAl material is an attractive alternative candidate for aerospace and automotive applications because of its low density and good high-temperature properties.[1,2] Recently, TiAl has been implemented to low pressure turbine blades.[3] In the future, it is expected that TiAl will be widely accepted as a structural material in the aerospace industry, e.g., for turbine engines.[4,5] Thus, a good capability for joining TiAl alloy to itself or to other materials is urgently required. Intensive investigations have been carried out to perform a crack-free c-TiAl joint by supplying controlled heat treatments during laser beam welding, and crack-free joints with good mechanical properties have been achieved.[5–8] There are several intermetallic phases depending on the chemical composition in the equilibrium state, as indicated in the Ti-Al phase diagram in Figure 1. The region of interest, Ti-45Al, is indicated by the blue line. Throughout the temperature range, the predominant JIE LIU, formerly Postdoctoral Researcher, with HelmholtzZentrum Geesthacht, Institute of Materials Research, Max-PlanckStraße 1, 21502 Geesthacht, Germany, is now Researcher of Laser Welding Process with Robert Bosch GmbH, Am Bo¨rstig 2, 96052 Bamberg, Germany. Contact email: [email protected] NIKOLAI KASHAEV, Head of Department Joining and Assessment, STEFAN RIEKEHR, Scientist of Department Joining and Assessment, and MARTIN MU¨LLER, Director of Materials Physics, are with Helmholtz-Zentrum Geesthacht, Institute of Materials Research. Contact email: [email protected] PETER STARON, Head of Department X-ray Diffraction with Synchrotron Radiation, ANDREAS STARK, Scientist of Department Metal Physics, NORBERT SCHELL, Scientist of Department X-ray Diffraction with Synchrotron Radiation, and NORBERT HUBER, Head of the Division ‘‘Materials Mechanics,’’ Spokesman of the HelmholtzProgramme ‘‘Advanced Engineering Materials,’’ are with HelmholtzZentrum Geesthacht, Institute of Materials Research. ANDREAS SCHREYER, formerly Professor, Direct of Materials Physics with Helmholtz-Zentrum Geesthacht, Institute of Materials Research, is now Director for Science with the European Spallation Source ERIC, P.O. Box 176, 221 00 Lund, Sweden. Manuscript submitted January 27, 2015. Article published online October 24, 2016 5750—VOLUME 47A, DECEMBER 2016
phases in this region are the hexagonal a2 (Ti3Al) phase, the hexagonal a (Ti) phase, the tetragonal c (TiAl) phase, and the cubic high-temperature b phase. The occupation of Ti and Al atoms in the lattice is schematically shown. The terms a and c stand for the lattice parameters of each phase. Chen et al.[9] found that an addition of 8 at. pct Nb to TiAl alloys has pronounced effects on the phase transformation temperatures: (1) at high temperatures, the phase fields are shifted to a higher Al content; (2) the area of the b phase field is increased with a decreased b transus temperature (Tb) to 1728 K (1455 C), indicating that the addition of Nb stabilizes the b phase[4]; and (3) the a phase field
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