Thermo- Mechanical Modeling of the Volvo Varestraint Test with the Aid of Finite Element Method

Abstract

In aerospace industry there are several complex geometries to be fabricated with only a few companies that posses the skills to cast or forge the complex Nickel-based superalloys. The aim is to find an alternative manufacturing approach to avoid the monopoly on the market, and still have the same or better standard of the product. Volvo Aero Corporation (VAC), has turned to the idea of welding cast, wrought and forged segments together with complex geometry that are to be utilized as structural components in the hot rear end of a jet engine. By that it goes without saying that knowledge and control of the welding process is of great importance. VAC have designed and developed a simulative weldability test, a so called Varestraint machine, to run real-life testing on plates by applying augmented strain and welding simultaneously. The aim is to fundamentally study the cracks by forcing them to occur. The purpose of this master thesis was to design and validate a thermo-mechanical model (TMM), by using finite element analysis and evaluate the physics why the cracks propagate. The material of interest is a solution heat treated Ni-based superalloy named Haynes® 282®. The thesis is divided into two parts, Part I that describes the creation of the TMM. Part II contains a method for finding a crack criterion. The work has included convergence tests, validation with real tests, developing material data, analyzes of the test plate geometries and configurations for the Varestraint test. In addition, real tests with the VAC Varestraint test has been performed and simulated to gain information about what strains and temperatures that the specimen endures during the welding procedure. This analysis has been correlated with the cracking results of the real tests. The real tests were carried out with the aim to find the threshold strain for cracking which is the maximum strain for where cracking do not occur. The main result of this thesis is an accurate and validated thermo-mechanical model of the Varestraint test. The augmented threshold strain for Haynes® 282® in Varestraint testing was found. It was concluded that the local strain during a Varestraint test was significantly higher than the predefined augmented strain that has been widely used in the past to represent a materials resistance to weld cracking. Further conclusions is that the simulated local strains at the trailing edge of the weld pool are not applicable to use for a crack initiation criterion due to the simplifications, as simulating the weld pool as a solid for example.