TWI Industrial Member Report Summary 854/2006
By A Gregori
Nickel superalloys are highly susceptible to cracking and microfissuring during welding. Composition and ageing condition, together with thermal strain and stress developed during welding, are the main factors controlling cracking. High preheat temperatures have been used to control strain and avoid cracking during welding of these alloys. However, this technique presents several practical difficulties and a different approach using low heat input processes has been studied by TWI. Continuous-wave Nd:YAG laser welding and pulsed DCEN TIG, including the application of forced cooling and low arc energy (?0.3kJ/mm), were found to be effective in eliminating HAZ microfissures in aged, polycrystalline Waspaloy. [2,3]
Cracking was still found, however, when conventional TIG processes were applied to more highly alloyed single crystal and polycrystalline Ni-superalloys, such as CMSX-10 and Mar-M247, respectively. Pulsed TIG weld deposits on single crystal alloy resulted in cracking in the HAZ, and microfissures were associated with recrystallised regions. A manual micro-welding electrospark deposition (ESD) process providing very low heat input was also evaluated.[3,4] No recrystallisation or microfissures in the HAZ were found, but deposited metal was porous and the deposition rate was very slow compared with arc/power beam processes.
Previous TWI work[3,4] showed that reduction of the heat input to the superalloy component is necessary to minimise HAZ and weld metal cracking. However, conventional TIG is difficult to control at low arc energies, particularly during manual welding, and ESD productivity is too low. Therefore, a new high-frequency DCEN TIG welding process, operating with pulsing frequency of about 20kHz, was evaluated in this project for repair of CMSX-10 single crystal blades and Mar-M247 polycrystalline vanes. This high-frequency TIG process ensures very low heat input, while providing good arc stability during manual welding and a higher deposition rate than achieved with ESD. 
- To determine the feasibility of high-frequency pulsed DCEN TIG welding for repair of nickel superalloys.
- To make repairs to CMSX-10 single crystal blades and Mar-M247 vanes and inspect the welds for the presence, or otherwise, of flaws.
- To characterise the weldments metallographically.