13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- welding power. Our results are following an opposite trend. Our experimental criterion for the formation of a CLGB is nearly parallel to the incomplete penetration criterion. The third and final observation concerns the weld speed. In all welds done at a welding speed below 3.39 mm/s, a longitudinal grain formed. This weld microstructure was not reported by Dye et al. and is not acceptable for our application. A lower speed limit criterion was thus added to the map of Fig. 7a. a) Experimental b) Predicted by Dye et al. model Figure 7. Weldability map 5. Conclusion The TIG weldability of IN718 sheets was studied in order to reduce the occurrence of solidification cracking. The objective of the experimental work was to identify welding parameters that would prevent the formation of a centerline grain boundary within the weld microstructure. Three welding strategies were tested and the results are summarized below: Current pulsation Current pulsation did not improve our weld microstructure and modifying the current ratio had no significant effect on the weld microstructure. Constitutional supercooling Welding at high speed and power did not produce the expected fine and equiaxe microstructure. However, the formation of few equiaxe grains suggests that nucleation of grains from the weld pool could happen. Weld pool shape control By controlling the depth to width ratio, an improvement of the microstructure was observed. Grain growth inclination resulted in a microstructure free of centerline grain boundary, but partially penetrated welds were produced at heat input below 112 J/mm. Decreasing the welding speed had a detrimental effect since a longitudinal grain formed within the welds performed at welding speed below 3.39 mm/s. Results reported in a weldability diagram revealed that the weldable area of our IN718 TIG weld is very narrow, for the range of welding conditions tested. This is caused by the fact that the centerline grain boundary criterion is nearly parallel and very close to the incomplete penetration criterion. In practice, restricting the welding power in such a narrow band is limiting. Adjustments of the current intensity are often necessary to better control the dimensions of the assembly and the penetration of Centerline grain boundary region Incomplete penetration region Weldable area Centerline grain boundary region Incomplete penetration region Longitudinal Grain region
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