The effect of pre-heating and pre-cold treatment on the formation of liquation and solidification cracks of nickel-based superalloy welded by laser beam

Abstract

The present study has evaluated the solidification and liquation cracks of GTD-111 superalloy by pulsed Nd:YAG laser techniques. For this purpose, samples were subjected to different temperatures under the conditions of heat treatment (1120, 1200 and, 1230°C), pre-cooling (−30 and 0°C), and pre-heating (100 and 200°C). The Rosenthal equation was used for calculation of mushy zone (MZ), partially-melted zone (PMZ), and heat-affected zone (HAZ). The values of the temperature gradient (G) and welding cooling rate (G×R) were calculated using the Rosenthal equation. The results showed that by reducing the temperature of the sample before welding (from 200 to −30°C), the G (from 3763.5 to 5821.5°C/mm) and the G×R (from 31,237 to 48,310.2°C/s) increased. Experimental test results were in agreement with Rosenthal's numerical results. Microstructural studies have shown that both solidification and liquation cracks are minimized by reducing the pre-cold temperature. This is attributed to the reduction in the segregation of alloying elements and the reduction in the volume of the weld pool. Further studies of the microstructure revealed that there was a correlation between the thickness of the grain boundary (GB) liquid and the formation of cracks. In this way, if the liquation material is from γ–γ′ eutectic and M3B2 boride, increasing the liquid film thickness (LFT) increases the sensitivity to cracking (Eq. (5) in the text), and if the liquation material is from γ′ phase and MC carbide, increasing the LFT reduces the sensitivity to cracking due to increased stress release.