Effect of Pre-Post TIG Welding Heat Treatment on Cast NI Superalloy

The effect of a pre (before) and post (after) heating welding treatment on the microstructure and mechanical properties of the scrap blades made of cast INC738LC superalloy is the main goal of the present investigation. The filler used in TIG welding was a INC 625 solution hardened superalloy as the proposed solution for hot cracking of the INC738LC cast superalloy in literature. The TIG welding was processed with respect to the constantly optimized parameters (current, voltage, speed, gas flux rate and number of passes) to make a mechanical properties comparison between the as received superalloy and the welded superalloy with heat treated specimens. The characterization techniques employed in this study are hardness measurements, tensile tests, optical microscopy and scanning electron microscopy. We found that the proposed preheating improves the TIG welding of the INC 738 LC superalloy specimens and the post welding heat treatment enhances its mechanical properties.

Effect of Double-Step Solution Treatment on Rejuvenation Heat Treated Microstructure of IN-738 Superalloy

This work investigates the effect of rejuvenation heat treatment, with double-step solution treatment at the temperature from 1150 °C to 1200 °C, on the recovered microstructure of IN-738 cast superalloy. The superalloy has been long-term exposed as a turbine blade in a gas turbine prior to this study. After double solution treatment and aging at 845 °C for 12 h and 24 h, the recovered microstructures were examined by using a scanning electron microscope. Coarse γ΄ particles, that have presented in damaged microstructures, could not be observed in the samples after the rejuvenation heat treatment. In addition, the image analysis illustrates that the reprecipitated γ΄ particles in the samples with double-step solution treatments increase significantly in sizes during aging than that in the samples with the single-step solution treatment. Furthermore, the measurement of the samples hardness presents that the as-receive sample hardness is improved after rejuvenation heat treatment studied in this work.

MICROSTRUCTURAL AND MECHANICAL ASPECTS OF FRICTION WELDED DISSIMILAR JOINTS FOR AERO ENGINE APPLICATION

Gas turbine engines demand material with unique properties like high-temperature oxidation and corrosion resistance, high specific strength, etc. All over the world material development to meet these requirements has led to the development of novel alloys. While Titanium base alloys are used in the low-temperature regime of the gas turbine engine, Nickel-basedsuperalloys are used for hot end components of the engine. With the increase in the temperature requirement for the turbine parts, the form of the Ni-based superalloys changed from wrought to cast superalloys. As an inherent process of investment cast superalloy blades and vanes which has serpentine passages for air cooling, these passages are required to be closed after casting. The numerous adapters also need to be joined on the cast superalloy casings for various instrumentation, lube oil ports. These cast superalloys are nonweldable and joining these pose a challenge. In this present investigation, the joining of the Nibasedsuperalloy BZL12Y and martensitic stainless steel AE961W using rotary friction welding process. The mechanical properties of the dissimilar joints were evaluated as per the ASTM standards. Microstructural features of various regions of welded joints using optical microscopy (OM) and scanning electron microscopy (SEM). The material is welded in different condition to obtain the maximum tensile strength of the weld joint. From this investigation, it was found that the combination of aging and h & t condition weld joint gives good strength and a stable hardness value. Correlation between tensile properties and microstructural features were analyzed and reported in this paper.

Evaluating the Microstructures and Mechanical Properties of Dissimilar Metal Joints Between a New Cast Superalloy K4750 and Hastelloy X Alloy by Using Different Filler Materials

Two kinds of filler materials were used to join dissimilar alloys between a new cast superalloy K4750 and Hastelloy X by tungsten gas arc welding (GTAW). The segregation behavior, interfacial microstructure and mechanical properties of the dissimilar joints were evaluated. The results show that both filler materials can be used to obtain sound dissimilar joints successfully. Microstructural observation show that no obvious cracking is observed in the joints achieved by both filler materials. The segregation extent of various elements in Hastelloy X weld metal is more severe than that in the K4750 weld metal. No unmixed zones were observed at the interfaces. Transition areas with the chemical compositions various between the K4750 alloy and the Hastelloy X alloy were found at the joint interfaces. The maximum width of the transition area between the K4750 weld metal and Hastelloy X base metal is smaller than that between the Hastelloy X weld metal and K4750 base metal. The ultimate tensile strength and yield strength of the joints with Hastelloy X filler material are slightly higher than those with K4750 filler material, however, the K4750 filler material results in a higher total elongation and fusion zone microhardness than those with Hastelloy X filler material. Both dissimilar joints fractured with a ductile feature which exhibits tearing edges and dimples. Hastelloy X filler material is suggested to be more suitable for joining of K4750 superalloy and Hastelloy X dissimilar metals in terms of obtaining superior comprehensive mechanical properties.