A perspective review on laser assisted microjoining

International audienceIn the era of miniature, laser microjoining has been evolving as a prospective smaller scale manufacturing process. The latest development in the field of laser technology brought new opportunities for laser to join wide variety of materials used in microsystems. In the growing technological field like micro-electro-mechanical systems (MEMS) and biomedical applications, the laser microjoining has the potential for application as encapsulation of miniature. However, the feasibility of microjoining depends on pulse modulation strategy and wavelength for selective range of laser power and laser irradiation intensity. Development and adaptation of new methodology also brings the flexibility in micro scale joining process with reduced defects and sound quality. A number of patents on the design of apparatus and methodology in this area show the signature of the development of the field. The current review article is focused on various aspects of micro scale joining process in the perspective of practical application. First, experimental investigation on the type of laser, process conditions, materials and feasibility of microjoining processes is reviewed. Secondly, on-line monitoring and control of the microjoining process is analyzed. An extensive part of the article is devoted to the review of numerical process model. It is anticipated that at ultrashot pulsed laser, the non-Fourier heat conduction analysis is more appropriate to signify rapid propagation of heat wave. Except from current prospective application, the future research direction and potential applications are also discussed. In a nutshell, the review article provides an overview of microjoining process which is extracted from literature and is required for promising development in microsystem technolog

Material Defects in Friction Stir Welding through Thermo–Mechanical Simulation: Dissimilar Materials with Tool Wear Consideration

Despite the remarkable capabilities of friction stir welding (FSW) in joining dissimilar materials, the numerical simulation of FSW is predominantly limited to the joining of similar materials. The material mixing and defects’ prediction in FSW of dissimilar materials through numerical simulation have not been thoroughly studied. The role of progressive tool wear is another aspect of practical importance that has not received due consideration in numerical simulation. As such, we contribute to the body of knowledge with a numerical study of FSW of dissimilar materials in the context of defect prediction and tool wear. We numerically simulated material mixing and defects (surface and subsurface tunnel, exit hole, and flash formation) using a coupled Eulerian–Lagrangian approach. The model predictions are validated with the experimental results on FSW of the candidate pair AA6061 and AZ31B. The influence of tool wear on tool dimensions is experimentally investigated for several sets of tool rotations and traverse speeds and incorporated in the numerical simulation to predict the weld defects. The developed model successfully predicted subsurface tunnel defects, surface tunnels, excessive flash formations, and exit holes with a maximum deviation of 1.2 mm. The simulation revealed the substantial impact of the plate position, on either the advancing or retreating side, on the defect formation; for instance, when AZ31B was placed on the AS, the surface tunnel reached about 50% of the workpiece thickness. The numerical model successfully captured defect formation due to the wear-induced changes in tool dimensions, e.g., the pin length decreased up to 30% after welding at higher tool rotations and traverse speeds, leading to surface tunnel defects

Modelling dynamic and static recrystallization using mean field and finite element methods

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A two-site mean field model of discontinuous dynamic recrystallization

International audienceThe paper describes a new model of discontinuous dynamic recrystallization (DDRX) which can operate in constant or variable thermomechanical conditions. The model considers the elementary physical phenomena at the grain scale such as strain hardening, recovery, grain boundary migration, and nucleation. The microstructure is represented through a set of representative grains defined by their size and dislocation density. It is linked to a constitutive law giving access to the polycrystal flow stress. Interaction between representative grains and the surrounding material is idealized using a two-site approach whereby two homogeneous equivalent media with different dislocation densities are considered. Topological information is incorporated into the model by prescribing the relative weight of these two equivalent media as a function of their volume fractions. This procedure allows accounting for the well-known necklace structures. The model is applied to the prediction of DDRX in 304 L stainless steel, with parameters identified using an inverse methodology based on a genetic algorithm. Results show good agreement with experimental data at different temperatures and strain rates, predicting recrystallization kinetics, recrystallized grain size and stress-strain curve. Parameters identified with one initial grain size lead to accurate results for another initial grain size without introducing any additional parameter

Tailoring coil geometry for secondary heating of substrate towards the development of induction heating-based wire additive manufacturing

Induction heating (IH), a clean energy source, is potentially used to develop wire additive manufacturing (AM) system. The optimised parameters that simultaneously melt the mild steel wire and raise the substrate temperature is established. A fully coupled thermal-electromagnetic model of AM system is developed to perform numerical experiments on temperature development. The proposed hybrid helical-pancake coil with circular cross-section melt the metallic wire and raise the substrate temperature to 1490 K. The hybrid coil provides rapid heating to the wire (2819 K) and substrate together by enhancing magnetic field strength. The experiments using high-frequency IH system (550 A and 353 kHz) with a 3-turn helical coil is validated with model results for 2 mm mild steel wire.</p

Mean Field and Finite Element Modeling of Static and Dynamic Recrystallization

International audienceQuantitative prediction of grain size and recrystallized volume fraction is still a real challenge for many alloys, and even for simple materials when subjected to complex thermal/mechanical histories, as in multi-pass (industrial) processing. A first step is therefore taken in the direction of multiscale modelling of recrystallization, by considering digital polycrystalline microstructures. These synthetic mesoscopic microstructures are meshed adaptively and anisotropically, with refinement close to the grain boundaries. Crystal plasticity finite element (CPFEM) simulations are combined with a level set framework to model primary recristallization, following plastic deformation. In the level set method, the kinetic equation describing interface motion uses the calculated stored energy field provided by CPFEM calculations, and works on the same mesh. Discontinuous dynamic recrystallization can be modelled within the same approach, effectively coupling plastic deformation with nucleation and growth processes. Parallel to the finite element approach, a mean field model is developed in the general context of multi-pass processing. The model considers categories of grains based on two state variables : grain size and total dislocation density. As opposed to the finite element approach, there is no crystallographic or topological information. It is computationally much cheaper and therefore suitable for direct coupling at the scale of forming processes, for industrial applications. The parameters of the model can be identified from inverse analysis, using experimental stress-strain curves, recrystallized volume fractions, and grain sizes. Mean field and finite element models are compared, and it is shown that the detailed information provided by finite element simulations can be used to calibrate or optimize the mean field method

Modelling deformation and recrystallization in metals using digital microstructures and mean field methods

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Macrocyclic lanthanide(III) complexes of iminophenol Schiff bases and carboxylate anions: syntheses, structures and luminescence properties

The mixed-ligand lanthanide(III) complexes derived from the tetraiminodiphenolate (L¹H₂) or octaiminotetraphenolate (L²H₄) macrocyclic ligands and acetate (OAc⁻) or benzoate (OBz⁻) anions have been synthesized and structurally characterized. With the ligand combination L¹H₂ and OBz⁻, either the dimeric complexes [{Ln(L¹H₂)}₂(μ₂-η¹:η¹-OBz)₄](ClO₄)₂ (Ln = La (1), Nd (2) and Eu (3)) or the monomeric complexes [Ln(L¹H₂)(η²-OBz)₂](ClO₄)·n solvent (Ln = Tb (4), Lu (5) and Y (6)) have been obtained. The X-ray crystal structures of 1, 4 and 6 have been determined, of which 4 and 6 have the same structure. L¹H₂ and OAc⁻ combination affords the dimeric complexes [{Ln(L¹H₂)(η²-OAc)(μ-OAc)}₂](ClO₄)₂ (Ln = La (7), Nd (8) and Y (9)) and the structure of 9 has been determined. In complexes 1–9, L¹H₂ uses two imino nitrogens and the two phenolate oxygens for metal coordination, while the other two uncoordinated imine nitrogens are protonated and intramolecularly hydrogen-bonded to the phenolate oxygens. The metal centre in all the cases is 8-coordinate and has distorted square antiprism geometry. With the octaiminotetraphenolate macrocyclic ligand (L²H₄) dinuclear complexes of composition [Ln₂(L²H₄)(OAc)₂(NO₃)₄]·nH₂O (Ln = La (10), Pr (11), Nd (12) and Y (13)) have obtained. The structures determined for 11 and 12 have shown that they are isostructural and there are two independent molecules (A and B) in the unit cell. The molecule A has the bond types [Ln₂(L²H₄)(η²-OAc)₂(η²-NO₃)2(η¹-NO₃)₂] and its 9-coordinate metal centre obtains a tricapped trigonal prismatic geometry, while the molecule B is [Ln₂(L²H₄)(η²-OAc)₂(η₂-NO₃)₄] whose 10-coordinate metal centre has a sphenocorona geometry. In these compounds, four uncoordinated imine nitrogens are protonated and hydrogen-bonded to the phenolate oxygens. The luminescence spectra of [{Eu(L¹H₂)}₂(μ-OBz)₄]²⁺ (3²⁺) in acetonitrile (at 298 K) and in acetonitrile-toluene glassy matrix (at 77 K) have been studied

Fixation of carbon dioxide by macrocyclic lanthanide(III) complexes under neutral conditions producing self-assembled trimeric carbonato-bridged compounds with µ<SUP>3</SUP>-η<SUP>2</SUP>:η<SUP>2</SUP>:η<SUP>2</SUP> bonding

A series of mononuclear lanthanide(III) complexes [Ln(LH<SUB>2</SUB>)(H<SUB>2</SUB>O)<SUB>3</SUB>Cl](ClO<SUB>4</SUB>)<SUB>2</SUB> (Ln = La, Nd, Sm, Eu, Gd, Tb, Lu) of the tetraiminodiphenolate macrocyclic ligand (LH<SUB>2</SUB>) in 95:5 (v/v) methanol-water solution fix atmospheric carbon dioxide to produce the carbonato-bridged trinuclear complexes [{Ln(LH<SUB>2</SUB>)(H<SUB>2</SUB>O)Cl}<SUB>3</SUB>(µ<SUB>3</SUB>-CO<SUB>3</SUB>)](ClO<SUB>4</SUB>)<SUB>4</SUB>·nH<SUB>2</SUB>O. Under similar conditions, the mononuclear Y<SUP>III</SUP> complex forms the dimeric compound [{Y(LH<SUB>2</SUB>)(H<SUB>2</SUB>O)Cl}(µ<SUB>2</SUB>-CO<SUB>3</SUB>){Y(LH<SUB>2</SUB>)(H<SUB>2</SUB>O)<SUB>2</SUB>}](ClO<SUB>4</SUB>)<SUB>3</SUB>·4H<SUB>2</SUB>O. These complexes have been characterized by their IR and NMR (<SUP>1</SUP>H, <SUP>13</SUP>C) spectra. The X-ray crystal structures have been determined for the trinuclear carbonato-bridged compounds of Nd<SUP>III</SUP>, Gd<SUP>III</SUP> and Tb<SUP>III</SUP> and the dinuclear compound of Y<SUP>III</SUP>. In all cases, each of the metal centers are 8-coordinate involving two imine nitrogens and two phenolate oxygens of the macrocyclic ligand (LH<SUB>2</SUB>) whose two other imines are protonated and intramolecularly hydrogen-bonded with the phenolate oxygens. The oxygen atoms of the carbonate anion in the trinuclear complexes are bonded to the metal ions in tris-bidentate µ<SUB>3</SUB>-η<SUP>2</SUP>:η<SUP>2</SUP>:η<SUP>2</SUP> fashion, while they are in bis-bidentate µ<SUB>2</SUB>-η<SUP>2</SUP>:η<SUP>2</SUP> mode in the Y<SUP>III</SUP> complex. The magnetic properties of the Gd<SUP>III</SUP> complex have been studied over the temperature range 2 to 300 K and the magnetic susceptibility data indicate a very weak antiferromagnetic exchange interaction (J = -0.042 cm<SUP>-1</SUP>) between the Gd<SUP>III</SUP> centers (S = 7/2) in the metal triangle through the carbonate bridge. The luminescence spectral behaviors of the complexes of Sm<SUP>III</SUP>, Eu<SUP>III</SUP>, and Tb<SUP>III</SUP> have been studied. The ligand LH<SUB>2</SUB> acts as a sensitizer for the metal ions in an acetonitrile-toluene glassy matrix (at 77 K) and luminescence intensities of the complexes decrease in the order Eu<SUP>3+</SUP> &gt; Sm<SUP>3+</SUP> &gt; Tb<SUP>3+</SUP>