The spray forming of Ni based superalloys
The main aim has been to investigate the effect of process parameters on the spray forming of UDIMET 720 and in particular to understand the mechanism of grain size evolution in the deposit using a combination of experimental and computer modelling techniques. Samples of two spray formed Ni superalloys, MAR-M-002 and UDIMET 720, have been re-heated into the solid/liquid region and the fully solid region just below the solidus temperature for a series of times to try and reproduce the situation of grain growth in a spray formed deposit. Grain growth in the solid/liquid region follows the equation: d3=d03 +Kt, where d is the grain size, d0 is the initial grain size, K is the coarsening rate constant and t is the time. Coarsening rate constants have been determined for temperatures in the solid/liquid region and they increase with increasing temperature/decreasing solid fraction. Existing spray forming equipment for Al alloys has been modified to manufacture UDIMET 720 deposits. Process conditions were monitored continually during spray forming, in particular the temperature of the deposit by embedded thermocouples and infra red thermal imaging of the deposit top surface. Above a deposit temperature of ≈1250°C the microstructure consists of equiaxed fine grains (20μm-35μm) and the porosity is low (<1%). Below this deposit temperature the microstructure consists of droplet "splats" and the porosity is higher (2-4%). The measure grain size increases with increasing deposit temperature and solidification time and agrees reasonably well with the predicted grain size using the above equation. A commercial finite difference based fluid dynamics software program, FLUENT, has been used to model the 2-dimensional dynamic and thermal behaviour of UDIMET 720 droplets during gas atomisation and spray forming. The effect of atomising gas pressure, spray distance and melt mass flow rate on the equilibrated droplet spray temperature has been examined and shows similar variations with process parameters as the measured maximum deposit temperature. The predicted spray temperature at the substrate is always higher than the measured maximum deposit temperatures under all conditions, and increases with (i) decreasing gas pressure, (ii) decreasing spray distance and (iii) increasing MFR. Mean droplet temperatures and velocities are strongly dependent on droplet size, the mean droplet temperature decreases and mean droplet axial velocities increases with decreasing droplet size.