13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- between dendrites, and the deformations and stresses due to shrinkage of the material grow gradually as solidification is ending, tearing the joint apart. The amount of minor addition elements such as B, Zr and Nb, have a significant influence on the weldability of nickel alloy IN718. During solidification, segregation of Nb leads to the formation NbC and Lave phases which are widening the solidification range increasing the size of the crack susceptible mushy zone. Moreover, NbC and Lave phases are brittle and deleterious to the weld ductility even after solidification is completed [5]. Since B, Zr and Nb are added intentionally in superalloy IN718 for strengthening purposes, it is important to promote the formation of a homogeneous solidification structure. If the last liquid to be solidified (rich in alloying elements) is concentrated in one region, the weld ductility can be reduced significantly [6, 7]. This is the case when a centerline grain boundary (CLGB) forms. Weld microstructures with a CLGB are susceptible to crack propagation along the length of the weld just behind the melt. Welding can be successful (no cracking) for welds having a CLGB. However, the unevenly distributed fragile phases will create a weak plane that can compromise the weld mechanical properties [8]. For these reasons, a straight centerline grain boundary is an unwanted feature often related to solidification cracking [4]. 2. Methodology 2.1. Nominal welding conditions and microstructure The TIG welds for which cracking occurred are but-welded IN718 sheet rings having an average thickness of 1.33 mm. The nominal chemical composition of the sheet material and the nominal welding parameters are reported in Table 1 and 2 respectively. Table 1. Chemical composition of nickel superalloy IN718 (weight %)1,2 Ni Cr Nb Mo Ti Al Co Mn Si Cu Ta C B Min. Max. 50.00 55.00 17.00 21.00 4.75 5.50 2.80 3.30 0.65 1.15 0.20 0.80 1.00 0.35 0.35 0.30 0.05 0.08 0.006 1Remaining is iron 2 Phosphorus and sulfur content must be below 0.015 Table 2. Welding parameters DC Current I (A) Voltage U (V) Welding Speed WS (mm/s) Filler wire speed (mm/s) Diameter of filler wire (mm) Electrode angle (o) Linear Heat input HI (J/mm)1 86 8 5.27 5.72 0.889 45 140 1Calculated by ࡴࡵൌࢁ ࢃ ∗ ࡿ ࡵ Visual and optical microscopy observations were performed on cracked specimens to identify the failure mechanism. The longitudinal aspect of the crack, its position at the center of the fusion zone, and its interdendritic path led to the identification of a solidification cracking problem. The same features were observed by Kerrouault [9] and Shinozaki et al. [10] while monitoring solidification cracking in austenitic stainless steels. Moreover, the microstructure of all specimens observed contained a centerline grain boundary (CLGB) as shown in Fig. 1.
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