Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Laser Beam 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 high temperature properties, such as low density, high specific yield strength, high creep, and oxidation resistance.[1–6] This material is considered for applications in turbine construction. Great efforts have been made to produce sound joints of the material. However, the material is intrinsically brittle. Due to the large residual stress generated by the welding, it is easy to observe defects such as cracks and pores in the welds.[3,7] The residual stresses were found to be over 1000 MPa in an as-welded specimen (13 9 50 9 2 mm3, Ti-45Al-5Nb-0.2C-0.2B, at. pct, the chemical composition is in atomic percent in this article) and they were reduced to 460 MPa after in situ post-weld heat treatment by a defocused laser. The stresses were totally relieved after the specimen was heat treated in a furnace for 2 hours at 1533 K (1260 C).[5] On the other hand, the microstructure transformation by heat treatment also plays an important role in controlling the intrinsic JIE LIU, Ph.D. Student, VOLKER VENTZKE, PETER STARON, and NORBERT SCHELL, Scientists, NIKOLAI KASHAEV, Head of Department, and NORBERT HUBER, Head of the Division Materials Mechanics, Spokesman of the Helmholtz-Programme Advanced Engineering Materials, are with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany. Contact e-mail: [email protected] Manuscript submitted March 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
brittleness of the material.[8] The effect of post-weld heat treatment on phase transformation was also investigated by Arenas and Acoff[9] on c-TiAl-based material (Ti-46.8Al-1.8Cr-1.8Nb-0.02Fe-0.004Si). It was pointed out that the amount of brittle a2 phase increased significantly in the as-welded region. The dendritic a2 grains transformed gradually into ductile equiaxed c at 1273 K and 1473 K (1000 C and 1200 C) as heating time was increased. The investigations of tensile properties and fracture mode throw light on the understanding of the relationship between microstructural and mechanical properties. Lots of effort has been placed on the near c-TiAl alloys. It is reported by Nieh and Wadsworth[10] that for the high strained c-TiAl-based alloy deformed at a low strain rate at temperature of around 1073 K (800 C), the most common considered mechanisms for superplastic flow involve grain boundary sliding (GBS), accommodated by grain boundary migration, recrystallization, diffusional flow, or some dislocation slip process. Dislocation slip is the main mechanism for the accommodation processes. The soft phase in the alloy could be helpful to absorb the dislocations emitted by GBS in order to reduce the stress concentration and delay cavity formation.[11] It was also found that during creep, these cavities nucleate at grain boundaries, triple points, and ledges where there are stress concentrations and interface decohesions.[12] These cavities grow by stress-dr
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