Experimental characterisation and numerical simulation of fibre laser welding of AA 2024-T3 and Ti-6Al-4V
Creators: name: Ahn, Joseph
Publisher: name: Imperial College London
Year: 2016

description: The aircraft industry has long recognised the importance of climate protection and the benefits of reducing weight for the production of cost effective and fuel efficient aircraft structures. Fibre laser welding provides advantages over conventional riveting, mainly in terms of weight reduction and time saving. However, significant changes in microstructure, metallurgical state and associated mechanical properties occur in welded joints. Such changes can result in residual stresses, distortions and defects formation in the welded structure, thus significantly influencing the performance and service life. In order to maintain structural integrity of welded structures, the relationship between welding process and performance of the structure needs to be fully assessed. In this thesis, comprehensive relationships between materials, welding process, microstructure and mechanical properties of welded joints were established. Welding parameters including power density, laser power, welding speed, focal position, filler metal feed rate and shielding gas composition were optimised to produce high quality full penetration welds. Solidification cracking was found to be a critical issue in AA 2024-T3 when welding without filler metal. The addition of filler metal reduced its crack sensitivity but it was also necessary to provide the optimum feed rate to avoid welding defects and keyhole instability. Sufficiently high laser power and low welding speed were required for full penetration and also to minimise welding defects. Both argon and helium shielding gases were found to be effective since only weakly ionised laser induced vapour plume was formed rather than strongly ionised plasma. Softening in AA204-T3 deteriorated the plastic straining capacity of the weld due to confined plasticity development within the weld. A poor weld quality resulted in a mixed mode of brittle and ductile failure and contained micro porosities and hot cracks, whereas, a good weld quality led to a ductile mode with significantly less welding defects. In the case of Ti-6Al-4V, the strength was the greatest in the weld as a result of martensitic microstructure formed during fast cooling rates. Local plastic deformation was the lowest in the weld and therefore, failed in the parent material but at the cost of reduced ductility relative to the unwelded parent tensile specimens. The residual stresses and distortions due to time dependent and localised heating imposed during fibre laser welding were numerically simulated with thermal and mechanical boundary conditions integrated in the finite element models including post weld heat treatment, mechanical stress relieving treatment and various clamping arrangements. Mechanical boundary conditions had relatively small influence on residual stresses in thin sheets of butt welded specimens, whereas, greater restraints led to higher residual stresses and lower restraints led to lower residual stresses in T-joint specimens. Non-isothermal diffusional and diffusionless phase transformations in Ti-6Al-4V were modelled and their influence on residual stresses and distortions was examined. Phase transformations only had a small influence on the magnitude and distribution of residual stresses and distortions because the level of internal stresses due to phase transformation remained low unlike other materials which exhibit greater differences in the specific volumes between phases. Post weld heat treatment (PWHT) induced diffusional phase transformations via decomposition of martensite into α. It also decreased the magnitude of y stresses to the yield strength of Ti-6Al-4V at the treatment temperature by releasing the locked-in stresses. Mechanical stress relieving was also studied for reducing residual stresses and distortions, by means of plastic deformation applied during as well as after welding. When the load reached more than 50% of its yield strength, the stresses became compressive. Residual stresses were experimentally measured using X-ray and neutron diffraction techniques They were found to be dependent on the crystallographic hkl plane due to the presence of microscopic stresses. In the case of Ti-6Al-4V, the reflections were weak and only few times larger than the background due its highly incoherent cross-section. In addition, texture in Ti-6Al-4V weld also contributed to lower intensity counts observed during measurements. As a result, only certain peaks were detected in certain orientations. The Y residual stresses in the welding direction were very high but not as high as the yield strength of the material at room temperature for both AA 2024-T3 and Ti-6Al-4V. They were largely tensile in nature only within the weld and tended to be weakly compressive in the rest of the specimen. Comparative analyses between experimental and numerical results showed good agreements, proving the robustness of the finite element models.