Application of Novel Repair Methods to Advanced Materials

Cost reduction has become one of the major drivers in the aerospace industry today. In these efforts to reduce costs, virtually all aspects of manufacturing aircraft have been considered. One area that has received considerable attention is that of repair. Repair offers the opportunity to reduce first time manufacturing scrap, as well as the ability to extend actual service lives. Many of the recently developed repair approaches involve fusion joining type processes. Fusion joining (forming a melt zone, with joining occurring on solidification), however, can often be problematic for the metallurgy of many aerospace alloys. In particular, aerospace alloys can be subject to cracking during solidification, as well as the formation of deleterious phases on cooling. The latter, then, can lead to various forms of solid-state cracking either immediately after welding or in service.

In recent years, EWI has investigated a range of candidate repair processes for nickel-based superalloys. These have included micro gas tungsten arc (µGTA), micro plasma arc (µPAW), micro plasma transferred arc (µPTA), and laser beam welding (LBW), as well as electro-spark deposition (ESD). These processes all are capable of precision deposition with limited heat input. To best select and apply these processes, it is necessary to understand the trade-offs between productivity and the potential damage to component microstructures. To this end, a series of experimental studies were done creating build-ups on IN 718. Depositions were done on nominally 2.5-mm sheet stock in two configurations. These included both surface and edge locations. All processes were capable of build-ups in both locations. The experimental data was then used to extract a characteristic deposition rate for each process. These deposition rates ranged from roughly 5 mg/min to 5000 mg/min. Hardness data (indicative of the local microstructure) was also collected. Such deposit hardnesses ranged from 200- to 400-VHN, compared to a based metal hardness of 450-VHN.

Separately, the process data from those trials were used in conjunction with a series of closed form modeling solutions to estimate local cooling rates. These cooling rates ranged from less than 100ºC/sec to over 100,000ºC/sec. Comparison with the hardness data showed that the faster cooling rates led to a higher as-deposited hardness, reducing the need for postweld heat treatment. Cooling rates were also compared with deposition rates. This analysis showed that higher deposition rates generally also resulted in lower cooling rates, and increased the need for postweld aging.

Finally, it was noted that the use of powder increased both deposition and cooling rates. This was associated with the high surface to volume ratio of the particles, which allowed rapid melting (low heat input), thus higher depositions with reduced need for postweld aging. For more information, contact Jerry Gould at 614.688.5121 or jerry_gould@ewi.org.