The Influence of Alloy Chemistry and Powder Production Methods on Porosity in a P/M Nickel-Base Superalloy

INTRODUCTION Advanced nickel-base superalloys for use in gas turbine engines are produced using powder metallurgy (P/M) processing. The high alloy content of these alloys typically results in a high solvus temperature for the strengthening gamma-prime (y’) phase and so the heat treatment must be carried out at high temperature. This can allow entrapped gas from the atomization process to form Thermally Induced Porosity (TIP) with negative impact on low cycle fatigue. A designed experiment was undertaken to investigate chemistry and atomization parameters on porosity. The ahoy used was the advanced ahoy KM4 (U. S. Patent 5,143,563), which contains about 54 volume fraction of 7’ and has a y’ solvus of about 117O’C. The variables studied included carbon, boron, superheat, mesh size, and gasmetal (GM) ratio. In general, atomization parameters influenced loose powder porosity, with some secondary influence on TIP, while composition parameters intluenced TIP and had little intluence on loose powder porosity. Increased levels of boron strongly increased the amount of TIP but only slightly impacted loose powder porosity. Carbon had negligible impact on either type of porosity. Lower levels of loose powder porosity were associated with lower superheat, increased gas:metal ratio, finer powder size distributions, and reduced metal flow rate. Some of the responses were interactive. For example, higher metal flow rate promoted coarse mesh distributions. This experimental program studied factors controlling Thermally Induced Porosity (TIP) in powder metallurgy (P/M) alloys. It was actually the byproduct of a study intended to determine the influence of minor element chemistry on properties and processing of KM4, an advanced PIM disk ahoy ’ developed for the Dual Ahoy Disk (DAD) progran? Powder heats with various boron and carbon levels were produced at GE Corporate Research & Development Center (CRD) in Schenectady, New York, and were subjected to a variety of consolidation routes. An unusual amount of porosity was noted in many of the extrusions. The atomization parameters had been varied during the runs to support a concurrent fundamental atomization parameter vs. yield program, with the assumption that changes in those parameters would have little effect on the final powder. The initial data review indicated that this assumption had been wrong; in fact, the atomization parameters had a strong impact on porosity. Since the existing matrix of runs did not allow separation of chemistry impacts versus atomization impacts, an expanded matrix was developed, evolving into a core eight-run (LS) Designed Experiment with high and low levels of superheat, flow rate, and boron level. The LS experiment, combined with the initial runs and several additional supporting runs, provided a range of data allowing a more complete assessment of what factors impacted porosity. The different levels of influence on loose powder porosity versus TIP appeared to be due to different mechanisms. For boron, which impacts TIP but not loose powder porosity, the apparent mechanism was that boron impacted grain boundary strength or promoted local incipient melting. For the atomization parameters which influence loose powder porosity, the apparent mechanism was a change in how much entrapped gas was retained during formation of the powder particles. The results are discussed in terms of significance for ahoy development and process development. The results suggested that both ahoy chemistry and atomization had major influences on both loose powder porosity and on TIP. The results are discussed in terms of possible mechanisms and practical impact on ahoy design powder production, and component fatigue life.