In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (for example, 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67° rotation between layers. Understanding metal component microstructure during 3D printing remains a challenge. Here, the authors use local thermal parameters and the solidification microstructure to better understand how the printed microstructure varies with the laser scan strategy.

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The role of side-branching in microstructure development in laser powder-bed fusion

Numerical analysis and experimental investigation of welding residual stresses and distortions in a T-joint fillet weld

NeT bead on plate round robin: Comparison of residual stress predictions and measurements

Parts manufactured by laser powder bed fusion contain significant residual stress. This stress causes failures during the build process, distorts parts and limits in-service performance. A pragmatic finite element model of the build process is introduced here to predict residual stress in a computationally efficient manner. The part is divided into coarse sections which activate at the melting temperature in an order that imitates the build process. Temperature and stress in the part are calculated using a sequentially coupled thermomechanical analysis with temperature dependent material properties. The model is validated against two sets of experimental measurements: the first from a bridge component made from 316L stainless steel and the second from a cuboidal component made from Inconel 718. For the bridge component the simulated distortion is within 5% of the experimental measurement when modelled with a section height of 0.8 mm. This is 16 times larger than the 50 μm layer height in the experimental part. For the cuboid component the simulated distortion is within 10% of experimental measurement with a section height 10 times larger than the experiment layer height. These results show that simulation of every layer in the build process is not required to obtain accurate results, reducing computational effort and enabling the prediction of residual stress in larger components.

T. Wangsdan

Papers

Thermo-mechanical modelling of a single-bead-on-plate weld using the finite element method