Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Influence of various process conditions on surface finishes induced by the direct metal deposition laser technique on a Ti–6Al–4V alloy

The direct metal deposition (DMD) with laser is a free-form metal deposition process for manufacturing dense pieces, which allows generating a prototype or small series of near net-shape structures. One of the most critical issues is that produced pieces have a deleterious surface finish which systematically requires post machining steps. This problem has never been fully addressed before. The present work describes investigations on the DMD process, using an Yb-YAG disk laser, and a widely used titanium alloy (Ti–6Al–4V) to understand the influence of the main process parameters on the surface finish quality. The focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish. In order to understand the physical mechanisms responsible for deleterious surface finishes, we have carried out: (1) a precise characterization of the laser beam and the powder stream; (2) a large number of multi-layered walls using different process parameters (P(W), V(m/min), Dm (g/min), Gaussian or uniform beam distribution); (3) a real time fast camera analysis of melt pool dynamics and melt-pool – powder stream coupling; (4) a characterization of wall morphologies versus process parameters using 2D and 3D profilometry. The results confirm that surface degradation depends on two distinct aspects: the sticking of nonmelted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters. Last, a simple analytical model was proposed to correlate melt-pool geometries to resulting surface finishes

State of the Art of Laser Hardening and Cladding

In this paper an overview is given about laser surface modification processes, which are developed especially with the aim of hardness improvement for an enhanced fatigue and wear behaviour. The processes can be divided into such with and without filler material and in solid-state and melting processes. Actual work on shock hardening, transformation hardening, remelting, alloying and cladding is reviewed, where the main focus was on scientific work from the 21st century

High pressure minerals in the Château-Renard (L6) ordinary chondrite: implications for collisions on its parent body

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyropess, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general

Preparation and characterization of self assembled polymer nanocomposites

Polymerní nanokompozity na bázi polyhedrálních oligomerních silsesquioxanů (POSS) představují slibnou oblast výzkumu, která potenciálně může využít samouspořádávní při navrhování nových materiálů. Tato diplomová práce popisuje postup přípravy oktafenyl-POSS/PS, oktafenyl-POSS/PMMA a oktamethyl-POSS/PS systémů a charakterizaci jejich termomechanických vlastností v pevné fázi a reologických vlastností v roztoku. Získané výsledky jsou diskutovány s přihlédnutím k teoriím zabývajících se stavem disperze nanočástic.Polymer nanocomposites based on polyhedral oligomeric silsesquioxanes (POSS) are promising field which could potentially utilize self-assembly approach in designing new materials. In this thesis, a preparation protocol of octaphenyl-POSS/PS, octamethyl-POSS/PMMA and octamethyl-POSS/PS systems was described and thermomechanic properties in solid state and rheological properties in solution were investigated. The obtained results are discussed with focus on nanoparticles dispersion state theories.

Simulating the Effects of Substrate Pre-Heating on the Final Structure of Steel Parts Built by Laser Powder Deposition

Tool steel parts built by laser powder deposition often present a heterogeneous distribution of properties caused by the complex structural transformations that occur during the deposition process. A model describing these transformations has been developed. It couples finite element heat transfer calculations with transformation kinetic theory to predict the final microstructure and properties of the material and their variation across a laser powder deposited part. Pre-heating is often used to reduce the residual stresses and the risk of thermal distortion and cracking. However, this changes the heat transfer conditions and affects the final microstructure and properties. In this work the proposed model was used to evaluate the effects of substrate preheating on the final hardness distribution. The results show that the final hardness depends considerably on the initial temperature of the substrate.Mechanical Engineerin

Crystallization of Ge2Sb2Te5 films by amplified femtosecond optical pulses

Copyright © 2012 American Institute of PhysicsThe phase transition between the amorphous and crystalline states of Ge2Sb2Te5 has been studied by exposure of thin films to series of 60 femtosecond (fs) amplified laser pulses. The analysis of microscope images of marks of tens of microns in size provide an opportunity to examine the effect of a continuous range of optical fluence. For a fixed number of pulses, the dependence of the area of the crystalline mark upon the fluence is well described by simple algebraic results that provide strong evidence that thermal transport within the sample is one-dimensional (vertical). The crystalline mark area was thus defined by the incident fs laser beam profile rather than by lateral heat diffusion, with a sharp transition between the crystalline and amorphous materials as confirmed from line scans of the microscope images. A simplified, one-dimensional model that accounts for optical absorption, thermal transport and thermally activated crystallization provides values of the optical reflectivity and mark area that are in very good quantitative agreement with the experimental data, further justifying the one-dimensional heat flow assumption. Typically, for fluences below the damage threshold, the crystalline mark has annular shape, with the fluence at the centre of the irradiated mark being sufficient to induce melting. The fluence at the centre of the mark was correlated with the melt depth from the thermal model to correctly predict the observed melt fluence thresholds and to explain the closure and persistence of the annular crystalline marks as functions of laser fluence and pulse number. A solid elliptical mark may be obtained for smaller fluences. The analysis of marks made by amplified fs pulses present a new and effective means of observing the crystallization dynamics of phase-change material at elevated temperatures near the melting point, which provided estimates of the growth velocity in the range 7-9 m/s. Furthermore, finer control over the crystallization process in phase-change media can be obtained by controlling the number of pulses which, along with the laser fluence, can be tailored to any medium stack with relaxed restrictions on the thermal properties of the layers in the stack

Comprehensive Analytical Modeling of Laser Powder-Bed/Fed Additive Manufacturing Processes and an Associated Magnetic Focusing Module

State-of-the-art metal additive manufacturing (AM), mainly laser powder-fed AM (LPF-AM) and laser powder-bed AM (LPB-AM), has been used to produce high-quality, complex-shaped, and end-user metallic parts. To achieve desirable dimensional, microstructural and mechanical features of as-built components through fast process optimization or feedback-control-based adaptive processing adjustment, high fidelity and calculation-efficient processing model is urgently needed. The thesis research has been motivated by the need for time-efficient process models of both LPF-AM and LPB-AM. To this end, comprehensive accelerated models for these processes have been built and experimentally verified. The comprehensive process model of LPF-AM was built by an innovative analytical approach. Firstly, a mathematical module that couples laser heat flux and powder mass flow was developed, while considering the attenuated laser intensity distribution and the heated powder spatial distribution. Correspondingly, a powder catchment module was built in terms of a three-dimensional (3D) melt pool shape and powder stream spatial distribution. Integrating these physical modules into the thermal modeling, a coupled heat and mass comprehensive model of the LPF-AM process was achieved. Experimental depositions of Inconel 625 proves the model’s high accuracy in predicting as-built deposits’ geometry (a maximum error of ~6.2% for clad width, ~7.8% for clad height) and powder catchment efficiency (maximum error of less than ~6.8%). It was found that the predicted real-time melt pool peak temperatures match well with the experimental results in Stainless Steel (SS) 316L deposition. The calculated micro-hardness has a maximum prediction error of ~16.2% compared with the measured results. The predicted microstructural evolutions show reasonable agreement with the experimental observations for both SS 316L and Inconel 625 depositions. Moreover, sensitivity analysis shows that the powder feed rate has the largest positive effect on the clad height. The time-efficient process model of LPB-AM was achieved by a novel analytical approach that couples the critical physics of the process, while considering the volume shrinkage and the melting regime. The proposed model can perform a time-efficient prediction of the localized-transient thermal field, melt pool temperature distribution, and multi-track overlapping dimension. The powder bed was treated as a homogeneous medium with effective thermophysical properties derived from the randomly packed rain model. In addition, different melting regimes of the LPB-AM process were considered in the built model. A 3D heat source model with variant penetration depths, together with the varying melting regimes, was utilized to solve the transient thermal field. Moreover, the density and top surface roughness of the final parts were empirically modeled using response surface regression under a Box-Behnken design. Subsequently, the mechanical properties of the part and the in-situ build rates were simultaneously optimized by combining the built analytical models and empirical models with employing a multi-objective genetic algorithm. Experimental results with SS 17-4PH show that the predicted melt pool dimensions have a high degree of accuracy under steady melting regimes, with a maximum of ~14% error for the width prediction and ~15% error for the depth calculation. Furthermore, an optimized parameter solution set was provided based on the built 3D Pareto fronts. The built models’ calculation time for the localized-transient characteristics for LPF-AM and LPB-AM are ~4 ms and ~1.2 ms, respectively. These findings confirm the great potential of the present research to be used for fast process optimization and in-situ process control. In addition, a new magnetic concentration approach designed with various configurations was explored. This approach is designed to focus the diverging metal particles in the gas-powder stream of LPF-AM, thereby improving powder catchment and deposition accuracy. It was shown that the proposed permanent-magnet-based configurations may not be suitable for concentrating submillimeter-sized particles. However, an additional development, a doublet-electromagnet-quadrupoles-based configuration with high frequency, may be capable of concentrating the non-ferrous metallic particles (e.g., aluminum particle) with a radius of r_p≥150 μm

The materials processing research base of the Materials Processing Center

The goals and activities of the center are discussed. The center activities encompass all engineering materials including metals, ceramics, polymers, electronic materials, composites, superconductors, and thin films. Processes include crystallization, solidification, nucleation, and polymer synthesis

Investigation of UV and IR Laser Processing of Single- Crystalline 4H:SiC and Characterisation of Laser Grown Graphene Derivatives

The formation of graphene (G) on the surface of silicon carbide (SiC) has gathered interest over recent years as a potential component in high power nano and microdevices. However, it is still in the early stages of research, therefore there are many challenges to overcome. Among the existing problems, the formation of good quality graphene/SiC is one of the most critical factors that determine the behaviour of this heterostructure. Here we report a full study of the formation of graphene and its derivative structures on SiC using different laser systems with different controlled irradiation conditions.Laser ablation experiments on polished 4H-SiC wafers using a 193 nm ArF laser over a fluence range of 0.3Jcm−2–5Jcm−2 are reported. An onset of material modification was measured at a laser fluence of 925 ± 80 mJcm−2, and a concomitant etch rate of ∼200 pm per pulse. Laser ablation sites have been analysed using optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman microscopy and white light interferometry (WLI). Different surface modifications were observed. The influences of the laser fluence, number of pulses, and scanning velocity on the position of the microchannel are discussed. At a laser fluence in the region of 1.0 Jcm−2, the irradiated site removed material forming a uniform crater. At a higher laser fluence, in the region of 2.7 Jcm−2, nodule-like structures form on the base of the ablation crater. An increased fluence led to a smoother surface with higher etched depth and ripple formation. The dissociation of laser irradiated 4H-SiC was discussed. Graphene oxide (GO) and reduced graphene oxide (rGO) formed on the SiC surface by 193 nm laser- induced high-temperature thermal decomposition of the SiC substrate. The decomposition resulted in the presence of silicon (Si), especially on the edge of the irradiated site.Graphene formation on the 4H:SiC surface by high power CO2 laser. Two distinct ablation threshold energies of 4.3 mJ and 73 mJ were found. The etch rate was dependent on the applied pulse duration, laser power, the scanning velocity and the number of pulses. High temperature thermal decomposition of the SiC substrate was achieved with a CO2 laser over a power range of 1-30 W. The structure was different from the structure obtained from the UV laser irradiated samples. More rough surfaces were prepared with small islands of graphene, GO and rGO on SiC in addition to the ripples. Monolayer and Multilayer graphene was also achieved. The laser-induced surface decomposition of the SiC was controlled spatially. The processing was held at room temperature, and the operation carried out in either a vacuum chamber or at atmospheric pressure. A fast graphene growth rate was achieved. This method is achievable, scalable and compatible with semiconductors technology due to the onsite direct writing of graphitic structure formed by the laser. This method is cost-effective as it does not necessitate SiC pre- treatment, there is no need for a processing vacuum chamber, and it can be achieved on the nano/microsecond time scale.Analytical and Finite element simulations using COMSOLTM MetaphySiCs, 5.3 have been used to calculate laser-induced temperature rise of 4H-SiC as a function of laser fluence. The simulated temperature was always less the temperature anticipated analytically. The 193 nm laser fluence required to reach the melting points of silicon, silicon carbide, and carbon, have been calculated and correspond to ∼0.97, 1.95 and 2.6 Jcm−2, respectively. Extreme heating and cooling rates controlled the growing process of graphene and its derivatives. The CO2 laser-induced temperature rise was also estimated. The CO2 laser acted as a heat source for the SiC. High power was used to reach the high temperature needed to decompose the SiC. Pulse duration also played a significant role in controlling the temperature and the depth distribution inside the SiC.This work reports the graphene formation on the surface of SiC by laser-induced thermal decomposition for electrical characterisation. Current-voltage (I-V) measurements show a decrease of the electrical resistance per unit length by nine orders of magnitude. The lowest resistance per unit length was obtained using a laser fluence of ~1.5 Jcm-2, a pulse repetition frequency of 10 Hz and using a sample translation speed of 0.01 mms-1. Temperature simulations have been performed using the finite element method (FEM) to assist in understanding the dissociation mechanisms of SiC and hence optimise the experimental variables. 2D axis-symmetric FEM calculations predict a surface temperature of ~2500K at a laser fluence of 1.5 Jcm-2. Laser-irradiated 4H:SiC is an efficient and controllable method of producing highly reproducible electrically conducting tracks. An increase in the conductivity was observed when the graphitic structure was produced with the CO2 laser. However, the conductivity was less than the graphitic structure produced by the 193 nm laser. It is expected that the different graphene interfaces, including Ohmic contact and Schottky contact, was created