Design Rules for Solid-State Join-ability of High Temperature Structural Alloys

Inertia Friction Welding (IFW) is a solid-state joining process capable of consistently producing high quality welds. When implemented in the quality-sensitive aerospace industry, IFW of Ni-base superalloys is important due to high temperature structural strength requirements of these alloys. A trial-and-error approach often successfully determines proper welding parameters, but is expensive and time consuming. Such issue is even more pronounced when developing a robust processing window for dissimilar Ni-base superalloys with different mechanical properties at elevated temperatures.

The overall objective of this project is to develop and validate a physics-based process model which can predict the joint microstructure and strength by inputting the initial chemistry and microstructure of Ni-base superalloys and IFW parameters. The key steps of developing this predictive model include the following: [1] Existing property prediction tools such as JMatPro will be used to predict thermo-physical properties such as flow stress based on initial microstructure and chemistry. The predicted flow stress at key temperatures and strain rates will be validated using experimental data obtained though a Gleeble based hot torsion test. [2] The physical material property data will be input into a fully-coupled thermo-mechanical model to predict the temperature, stress, and strain evolution for a given set of processing parameters. [3] The calculated thermal and strain history will be input to a microstructure model to predict changes such as grain size, strengthening precipitate size distribution and volume fraction, and intermetallic formation. [4] Microstructure and deformation produced by the model will be compared to experimental welding trials. If successfully validated, the predictive model will be readily extended to study IFW of dissimilar Ni-base superalloys.

A 2-D axisymmetric thermo-mechanical model has been developed using DEFORM software provided by Scientific Forming Technologies Corporation, which accounts for both friction and deformation heat generation. The results show friction coefficient and subsequent heat generation has a major impact on deformation. The predicted effective strain at the interface ranges from zero (m=0.63) to excessive (m=0.77).

The predicted heating is consistent with experimental data up to approximately 800°C; however, there is a discrepancy between the predicted and experimental data above this temperature. Research is ongoing to develop an improved description of friction as a function of temperature and sliding velocity. Future work will involve extensive microstructure characterization of base material and IFW joints.

Graduate Student: Daniel Tung

Faculty Advisor: Wei Zhang (OSU)

Industry Contact: David Mahaffey