Boride Zone Formation in Transient Liquid Phase Bonding of Pairings of Parent Superalloy Materials with Different Compos
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NTRODUCTION
JOINING of nickel-base superalloys exhibits potential to simplify manufacturing of blades and vanes in modern gas turbines. The size of these blades is ever increasing and so is the probability of casting defects. The problems during manufacturing could be avoided by fabricating the blades out of several smaller castings, which are joined together afterward. In addition, such a modular approach presents us with the possibility of using different materials in different parts of the same blade depending on the level of stress applied or corrosion resistance needed. Consequently the interest in possible joining techniques has increased over the last years. An established repair technique for high-temperature components is transient liquid phase (TLP) bonding. TLP bonding was first reported by Duvall et al.[1] in the 1970s for nickel-base superalloys. This method is based on the diffusion of the melting point depressant (MPD) element out of the molten filler into the parent material and subsequent isothermal solidification of the braze alloy due to increasing melting point. Several studies[2,3] show its application for nickel-base superalloys. Boron is a common MPD because of its capability of reducing the melting point by a suitable amount and because it diffuses rapidly in nickel-base superalloys. There is one problem reported in the literature[4–7] regarding boron as MPD: As the solubility of boron in nickel-base alloys is very limited, brittle borides precipitate in the parent alloy in the vicinity of the joint during the brazing process. These precipitates will deteriorate the mechanical properties of the joint,[8] in particular, S. STEUER, Research Associate at the Chair, and R.F. SINGER, Professor and Head of the Chair, are with the Institute of Science and Technology of Metals, Department of Materials Science and Engineering, University of Erlangen, 91058, Erlangen, Germany. Contact e-mail: [email protected] Manuscript submitted September 13, 2012. Article published online January 17, 2013 2226—VOLUME 44A, MAY 2013
LCF properties. So the aim is to minimize or completely avoid boride precipitation during the bonding process. If borides precipitate, the literature[4,9] predicts a peak in the boron profile next to the joint in the parent material. However, so far this peak has not been experimentally verified. The intention of the present study is to join pairings of parent superalloy materials with different compositions and grain structures, namely, one single crystalline and one columnar grained, by TLP bonding. The resulting boride structure is analyzed and the boron concentration profile measured with GDOES. The diffusion profiles are analyzed with regard to brazing temperature and brazing time and compared to the results predicted in the literature. In another part in this series of articles, we will discuss how boride precipitation can be completely avoided and the influence of the parent material.[10]
II.
EXPERIMENTAL
A. Material Two different directionally solidified nickel-base superalloys
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