WELDING AND DEPOSITION OF NICKEL SUPERALLOYS 718, WASPALOY AND SINGLE CRYSTAL ALLOY CMSX-10

摘要

Nickel superalloys are considered extremely difficult to weld and repair because of their susceptibility to heat-affected zone (HAZ) and weld metal cracking during fabrication, postweld heat treatment (PWHT) and subsequent operation. The weldability and repairability by TIG of annealed Alloy 718 and aged Waspaloy was investigated. Alloy 718 was investigated as this alloy provides a reference aerospace grade material. Forced cooling was applied during welding to control the temperature gradient, as this is known to be able to alleviate welding residual stress in aluminium alloys and stainless steels. The level of welding residual stress is one of the main factors controlling the weldability of nickel alloys, and minimising the amount of residual stress is beneficial to cracking resistance. An investigation into the shroud sealing of single crystal nickel superalloy CMSX-10 was also performed by TIG and electro-spark deposition (ESD) techniques with no preheat applied. Application of forced cooling aged Waspaloy resulted in the reduction or elimination of HAZ microfissures due to the reduction of thermal strain, distortion and stresses. Forced cooling during welding of Alloy 718 resulted in a substantial reduction of distortion and buckling. Weld metal and HAZ hardness were almost completely recovered after PWHT of Waspaloy deposits and SEM examination showed that γ'-phase re-formed in the HAZ during PWHT. CMSX-10 showed a high susceptibility to weld metal and HAZ cracking during standard TIG welding employing a Mar-M247 consumable. HAZ cracking was associated with recrystallisation of the parent material. No cracking or microfissures were found in the HAZ of ESD deposits on single crystal CMSX-10 alloy. No recrystallisation occurred in the parent material associated with Mar-M247 deposits produced by ESD.