Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones

Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

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Optical trapping and manipulation of nanostructures

https://research.birmingham.ac.uk/portal/files/20155386/Accepted_manuscript.pdf

On the role of melt flow into the surface structure and porosity development during selective laser melting

Non-Gaussian beam profiles such as Bessel or an- nular beams enable novel approaches to modifying materials through laser-based processing. In this review paper, proper- ties, generation methods and emerging applications for non- conventional beam shapes are discussed, including Bessel, an- nular, and vortex beams. These intensity profiles have important implications in a number of technologically relevant areas includ- ing deep-hole drilling, photopolymerization and nanopatterning, and introduce a new dimension for materials optimization and fundamental studies of laser-matter interactions.

Bessel and annular beams for materials processing

Selective laser melting (SLM) is an attractive technology, enabling the manufacture of customised, complex metallic designs, with minimal wastage. However, uptake by industry is currently impeded by several technical barriers, such as the control of residual stress, which have a detrimental effect on the manufacturability and integrity of a component. Indirectly, these impose severe design restrictions and reduce the reliability of components, driving up costs. This paper uses a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in SLM. A complex interaction between transient thermal history and the build-up of residual stress has been observed in the two laser scan strategies investigated. The temperature gradient mechanism was discovered for the creation of residual stress. The greatest stress component was found to develop parallel to the scan vectors, creating an anisotropic stress distribution in the part. The stress distribution varied between laser scan strategies and the cause has been determined by observing the thermal history during scanning. Using this, proposals are suggested for designing laser scan strategies used in SLM.

https://nottingham-repository.worktribe.com/preview/974815/ADDMA-D-16-00022.pdf

Understanding the effect of laser scan strategy on residual stress in selective laser melting through thermo-mechanical simulation

https://publik.tuwien.ac.at/files/PubDat_198014.pdf

Metal Ablation with Short and Ultrashort Laser Pulses

Simulation of Laser Beam Melting of Steel Powders using the Three-Dimensional Volume of Fluid Method

In order to get a deeper understanding of laser beam welding, a process model was developed at the Chair of Manufacturing Technology. It is based on the continuity equation, the equation of heat conduction and the Navier–Stokes equation. The model includes effects of Fresnel absorption, vapor pressure, surface tension, melting and evaporation enthalpy and energy loss due to evaporating material. This paper presents the results of a three-dimensional, transient finite volume simulation of a laser beam deep penetration welding process based on this model. The simulations show periodic keyhole oscillations and the complex fluid dynamics of the melt pool. A comparison of the evaporation rates calculated from the simulations and the experimentally observed process emissions shows good correlation. Furthermore, the simulations show pore formation at higher feed rates, the influence of a gap on the welding process and give an explanation for the welding behavior of zinc coated steel sheets.

A 3D transient model of keyhole and melt pool dynamics in laser beam welding applied to the joining of zinc coated sheets

https://publik.tuwien.ac.at/files/PubDat_202687.pdf

Numerical Simulations - A Versatile Approach for Better Understanding Dynamics in Laser Material Processing

We introduce "microdeflectometry," a novel technique for measuring the microtopography of specular surfaces. The primary data are the local slope of the surface under test. Measuring the slope instead of the height implies high information efficiency and extreme sensitivity to local shape irregularities. The lateral resolution can be better than 1 μm, whereas the resulting height resolution is in the range of 1nm. Microdeflectometry can be supplemented by methods to expand the depth of field, with the potential to provide quantitative 3D imaging with scanning-electron-microscope-like features.

Karl-Heinz Leitz

Papers

Metal Ablation with Short and Ultrashort Laser Pulses

Simulation of Laser Beam Melting of Steel Powders using the Three-Dimensional Volume of Fluid Method

A 3D transient model of keyhole and melt pool dynamics in laser beam welding applied to the joining of zinc coated sheets

Numerical Simulations - A Versatile Approach for Better Understanding Dynamics in Laser Material Processing

Microdeflectometry--a novel tool to acquire three-dimensional microtopography with nanometer height resolution.