Abstract
The purpose of this study is to apply a local heat treatment (LHT), in-situ, on the weld bead, using a defocused Yb: YAG laser beam on a continuous regime, in order to reduce residual stresses and decompose the brittle a’ martensite, into a lamellae and fine β phase. Laser scan experiments were firstly performed on a commercially pure titanium grade 2, with a wide range of parameters, in order to provide a “heat treatment window” without titanium melting. After optimization of the processing parameters, to obtain a sufficient width and depth for the scanning zone, experiments have been performed on a β-treated, fully martensitic Ti-6Al-4V sheet. For each processing experiments, the decomposition of a’, was studied based on metallographic cross sections. A local heating with a minimum energy density at 700 J.cm-2, has a sufficient effect to destabilize a’, while for an energy density at 1000 J.cm-2 , a diffusional transformation take place, with the formation of Widmanstätten microstructure. Finally, these optimized conditions were applied on a full penetration Ti-6Al-4V welds. The results of the LHT will be described in terms of the microstructural changes observed in the welded zone and hardness evolution.
Highlights
Laser welding is a rapid and accurate process to join thin components with complex geometry and minimal heat input
High cooling rate in the fusion zone (FZ), induces high level of residual stresses, and a significant local microstructural change, with a very complex microstructure, varying from a’ phase in the FZ, to a mixture of a and α’ phase, in the heat affected zone (HAZ) of titanium alloys [1]. This microstructural change reduces the elongation at 5% approximately, with a similar or slightly higher tensile strength compare to the base metal (BM)
The HAZ on plates are marked by the α→β allotropic transformation
Summary
Laser welding is a rapid and accurate process to join thin components with complex geometry and minimal heat input. High cooling rate in the fusion zone (FZ), induces high level of residual stresses, and a significant local microstructural change, with a very complex microstructure, varying from a’ phase in the FZ, to a mixture of a and α’ phase, in the heat affected zone (HAZ) of titanium alloys [1]. This microstructural change reduces the elongation at 5% approximately, with a similar or slightly higher tensile strength compare to the base metal (BM). The aim of this work is to propose a LHT on the welds, using a defocused laser beam, by passing standard heat treatments, implying use of larges furnaces under protective atmosphere
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