The influence of electron beam welding parameters on heat affected zone and weld metal cracking of nickel-based superalloys

Nickel-based Superalloys IN 738, Rene 80, and a proprietary single crystal material were studied in this project. These three alloys are commonly used in the hot section of gas turbine engines due to their excellent properties at elevated temperatures. In this study the influence of electron beam welding parameters on the cracking susceptibility of the alloys was investigated. The welding of the samples was performed at Bristol Aerospace located in Winnipeg, Manitoba. Focus coil current and welding speed were varied on samples of varying thicknesses of each alloy to simulate different types of blade repair. The total weld metal and heat affected zone (HAZ) crack lengths along with the weld and HAZ areas were measured using an optical microscope equipped with an image analysis system. The crack length per unit area, termed cracking index, was calculated and used to evaluate the cracking tendency of the alloy with respect to varying welding parameters. Orientation image microscopy was also used to determine if changes in crystal orientation were responsible for the cracking present in the single crystal and directionally solidified alloys. The experimental results showed that (1) The hardness of all three alloys can be reduced significantly by performing a special heat treatment called UMT. (2) IN 738 had a base microstructure of primary and secondary gamma' with intermetallics present. The microstructure of the post heat treated alloy consisted of coarsened primary and secondary gamma' plus the presence of M23C6 carbides. (3) The as-received microstructure of Rene 80 consisted of secondary gamma' along with a variety of carbides, borides and intermetallics. After heat treating the secondary gamma' was coarsened and M6C carbides were present. (4) The single crystal microstructure had very few phases present with only secondary gamma' and MC carbides visible. This is expected as fewer minor alloying elements are present as there is no need for grain boundary strengthening. Heat treating coarsened the gamma' phase. (5) Analysis of the cracking index of each alloy indicates that the welding parameters play a key role in determining the cracking susceptibility of the weld. The effects of focus coil current and welding speed on the cracking susceptibility are discussed. (6) The post weld microstructure of the directionally solidified and single crystal alloys have been analysed and the results tend to agree with the reports of other investigators.