Laser Melt Pool Research Articles

Evolution phenomena and surface shrink of the melt pool in an additive manufacturing process under magnetic field

The control of evolution of flow and heat transfer in melt pool is a key concern and hot topic to improve the produce quality, e.g. fissure and roughness, during the additive manufacture process. Researchers focus on controlling the evolution of melt pool by changing energy balance in melt pool. However, the momentum drive is seldom presented in literatures but fully necessary to further improve the control efficiency. Therefore, a promising magnetic force source is introduced to investigate the controlling effect on melt pool evolution. Based on a developed and validated laser melt pool model, the evolution results of three different magnetic field arrangements and intensities are obtained and analysed compared with that without magnetic force. In addition, the surface shrink characteristics during quenching process are studied to further evaluate the benefit of the added source. Results show that the melt pool evolution is significantly affected by the additional magnetic force. The increasing downward y-component force promotes the transport in melt pool which makes the surface deformation larger, as well as the melt pool dimension. Inversely, with the increase of x-component force, both the surface deformation and the melt pool dimension gradually decrease due to the induced transport limitation. During quenching process, the surface roughness is continuously reduced with a certain increase of the magnetic intensity caused by the increased melt pool duration and slightly vigorous surface flow. However, the improvement effectiveness becomes bad as the magnetic force exceeds a critical value owing to the increased shrink instability.

Investigation on the effect of the thermal dynamic, evaporation, and alternative material properties in a laser melt pool with a developed 2D model based on the VOSET method

ABSTRACTThe melt pool is the general process of the laser additive manufacturing process. An in-depth understanding of the evolution of the melt pool will facilitate obtaining an optimum mechanical performance. In this paper, a novel 2D transient model is presented, which considers most physical phenomena taking place in the laser melt process and incorporates a coupled multiphase capture method. This model is validated by comparing it to Han’s research. Then an investigation on the transient thermal dynamic in the melt pool including quenching time is carried out, and the effects of evaporation and alternative material properties on the evolution of the melt pool are discussed. The results show that the deformation of the free surface and the temperature in the workpiece both increase with energy input. After quenching, the temperature in the workpiece decreases sharply and the melt pool also shrinks. The evaporation effect is vigorous in the center of the melt pool and negligible in the periphery. The material properties have significant influence on the evolution process because the convection and diffusion in the melt pool are directly dominated by them.

Application of finite element, phase-field, and CALPHAD-based methods to additive manufacturing of Ni-based superalloys

Numerical simulations are used in this work to investigate aspects of microstructure and microsegregation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni–Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

Analysis of laser–melt pool–powder bed interaction during the selective laser melting of a stainless steel

The laser powder bed fusion (LPBF) or powder-bed additive layer manufacturing process is now recognized as a high-potential manufacturing process for complex metallic parts. However, many technical issues are still to overcome for making LPBF a fully viable manufacturing process. This is the case of surface finish and the systematic occurrence of porosities, which require postmachining steps. Up till now, the porosity origin remains unclear but is expected to be related to the stability of the process. As a LPBF part is made by the accumulation of hundreds of meters of small weld beads, it also appears to be important to understand all the phenomena that occur during the laser-powder-melt pool (MP) interaction for each single track. For this reason, in the first part of our study, using an instrumented LPBF setup and a fast camera analysis (>10 000 image/s), single tracks were fabricated and analyzed in real time and postmortem. Spatters ejections and powder denudation phenomena were observed together with variations of melt pool dimensions and melt-pool instabilities. In turn, the physical origin of this powder denudation and the dynamics of the MP were investigated and discussed.

电磁搅拌对激光熔池熔体流速及其凝固组织影响研究

电磁搅拌对激光熔池熔体流速及其凝固组织影响研究

Effect of Cr/C Ratio on Microstructure and Corrosion Performance of Cr3C2-NiCr Composite Fabricated by Laser Processing

The present study focuses on the effect of different Cr/C ratios on the microstructure, microhardness, and corrosion resistance of Ni-based laser clad hardfacings, reinforced by in situ synthesized chromium carbide particles. Cr3C2-NiCr composites have been laser processed with graphite/Cr/Ni powder blends with varying Cr/C ratios. Following phase analysis (x-ray diffraction) and microstructure investigation (scanning electron microscopy; energy dispersive x-ray analysis; transmission electron microscopy), the solidification of laser melt pool is discussed, and the corrosion resistances are examined. Several different zones (planar, dendritic, eutectic and re-melt zone) were formed in these samples, and the thicknesses and shapes of these zones vary with the change of Cr/C ratio. The sizes and types of carbides and the content of reserved graphite in the composites change as the Cr/C ratio varies. With the content of carbides (especially Cr3C2) grows, the microhardness is improved. The corrosive resistance of the composites to 0.2M H2SO4 aqueous solution decreases as the Cr/C ratio reduces owing to not only the decreasing Cr content in the NiCr matrix but also the galvanic corrosion formed within the carbide and graphite containing Ni matrix.

Effect of Fluid Convection on Dendrite Arm Spacing in Laser Deposition

Ni superalloys are widely used for hot section components in jet engines because they are very resistant to corrosion and maintain reasonably high strength at elevated temperature. However, the repair cost of the parts is high, partly due to the complexities of process variable optimization and control in laser cladding. In particular, optimizing the process parameters by experiments is time-consuming and costly. The microstructure and properties of the metal deposit are significantly influenced by values temperature gradient G and solidification rate R at the weld pool solidification boundary. Optimized values can help to reduce defects and improve properties of laser deposits. Optimization is hindered by the fact that the clad melt pool is hot and small, making in situ measurement of such solidification conditions difficult. Numerical simulation of the laser deposition process is a possible alternative to experimental measurement to obtain values of clad solidification parameters. In this investigation, G and R values at the weld pool solidification boundary were obtained from a three dimensional numerical simulation of laser deposition process and melt pool. The primary dendrite arm spacing and cooling rate of the deposited material were then correlated to these solidification conditions.

In vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting

ObjectiveSelective laser melting (SLM) is increasingly used for the fabrication of customized dental components made of metal alloys such as CoCrMo. The main aim of the present study is to elucidate the influence of the non-equilibrium microstructure obtained by SLM on corrosion susceptibility and extent of metal release (measure of biocompatibility). MethodsA multi-analytical approach has been employed by combining microscopic and bulk compositional tools with electrochemical techniques and chemical analyses of metals in biologically relevant fluids for three differently SLM fabricated CoCrMo alloys and one cast CoCrMo alloy used for comparison. ResultsRapid cooling and strong temperature gradients during laser melting resulted in the formation of a fine cellular structure with cell boundaries enriched in Mo (Co depleted), and suppression of carbide precipitation and formation of a martensitic ɛ (hcp) phase at the surface. These features were shown to decrease the corrosion and metal release susceptibility of the SLM alloys compared with the cast alloy. Unique textures formed in the pattern of the melting pools of the three different laser melted CoCrMo alloys predominantly explain observed small, though significant, differences. The susceptibility for corrosion and metal release increased with an increased number (area) of laser melt pool boundaries. SignificanceThis study shows that integrative and interdisciplinary studies of microstructural characteristics, corrosion, and metal release are essential to assess and consider during the design and fabrication of CoCrMo dental components of optimal biocompatibility. The reason is that the extent of metal release from CoCrMo is dependent on fabrication procedures.

A review of Computational Modelling of Additive Layer Manufacturing - multi-scale and multi-physics

This paper presents a review of the state-of-the-art in computational modelling of the Additive Layer Manufacturing (ALM) process, with a description and examples of models being developed at Swansea University, and a comparison to efforts from other commercial and academic organisations. As a process, ALM is strongly based on computational control and the production of net- shape parts, computational modelling is being used to understand and improve the process, as well as to create new and optimised functionality of components. Modelling efforts range from discrete element models of powder flow, to micro-and macro- scale models of the laser melt-pool, coupled thermal-structural models to predict residual stress, and automated optimisation methods for the development of new components. Modelling can also be used at a microstructural level for the prediction of the bulk properties. In all instances, the modelling is used with the purpose of increasing understanding to address known problems of the ALM process. Thus, as examples, residual stresses arise in metallic parts due to the requirement of building each layer from a base-plate with supports, and modelling can help reduce and optimise the support layout to produce as-built parts with minimal residual stress for maximum strength and minimum distortion. Understanding the heat transfer at the level of the melt-pool helps to reduce porosity and gas entrainment, as well as control the local heating rates which affect the formation of balling, which has been linked to surface roughness and also to porosity. If one looks at the evolution of computational models for casting as an indication of the way in which computation of modelling of ALM could be going, one of the long-term, but most highly coveted achievements would be a full process model, which allows the integration of micro- and macro-models of the process, with life-time component property prediction and the capability of simulating design and manufacturing iterations for a final component with optimised functionality.

Phase evolution in calcium phosphate coatings obtained by in situ laser cladding

Calcium phosphate coating was fabricated by in situ laser cladding using mixed powders of CaCO3 and CaHPO4, which presented a complex phase constitution since the reactions between CaCO3 and CaHPO4 would produce not only hydroxyapatite (HA) in the coating, but also other phases, such as Ca4(PO4)2O (TTCP) and α-Ca3(PO4)2 (α-TCP). In order to realize the control of the phase constitution, the effects of the Ca/P molar ratio of mixed powders, laser power, scanning velocity and heat treatment on the phase constitution of the coatings were investigated through X-ray diffraction analysis. It is found that the variation of the Ca/P molar ratio of the mixed powders, laser power and scanning velocity can adjust, to a certain extent, the proportion of HA, α-TCP, and TTCP in the coating. However, the α-TCP and TTCP cannot be eliminated from the coating due to the intrinsic high cooling rate of the laser melt pool during laser cladding. By suitable post heat treatment, the TTCP and α-TCP in the coating can be partially or completely transformed into HA. Therefore, HA coating or coatings with desirable proportion of HA, α-TCP and TTCP can be obtained by in situ laser cladding plus post heat treatment.

Three body abrasive wear of X12CrNiMo martensitic stainless steel laser alloyed with TiC

Martensitic stainless steels (MSSs) are often used for applications that require high wear resistance and hardness. A laser multi-track overlapping process was employed to fabricate TiC-reinforced metal matrix composite (MMC) coatings on X12CrNiMo steel substrate using a 4.4 kW continuous wave (CW) Nd: YAG laser. 50 and 75% overlap were produced. Comparative studies of the three-body abrasive wear resistance of the MMCs and the native material were carried out. The composite microstructures consist of homogeneous distribution of TiC grains and were free from cracks. Significant improvement in the hardness and wear resistance properties were obtained in the alloyed samples compared with the as-received MSS. The 75% overlap MMC has lower porosity, higher hardness and superior abrasive wear resistance than the 50% overlap. Key words: Laser alloying, microstructure, abrasive wear, porosity. INTRODUCTION Wear is a surface phenomenon and is primarily determined by the surface properties of a material rather than by the bulk properties. In this respect, surface modification techniques offer a solution to the problem of wear. Among the various techniques, laser alloying is especially attractive because of its unique features of process control (Candel et al., 2010). Laser alloying is enjoying increasing interest in many centres in the world. This type of surface treatment is used for improvement of hardness and abrasion wear resistance by influencing the structure change, mostly by introduction of powder particles into matrix on the material surface. Laser alloying is characterised by violent mixing of constituents in the melt pool, solidification of the remelted materials, convection motions in the laser melt pool which decides the final distribution of the alloying element in the remelted zone and a big temperature gradient. The powder is either introduced directly by the nozzle during alloying, or else

Laser repetitive pulse heating and melt pool formation at the surface

In this paper, laser repetitive pulse heating of steel surface is conducted to predict temperature and melt pool geometry in the irradiated region. The enthalpy porosity method is incorporated to account for the phase change in the irradiated surface. The flow field in the melt pool is simulated after considering the Marangoni effect. To examine the influence of the laser pulse intensity distribution on the formation and flow field in the melt pool, the laser pulse intensity parameter (β) is introduced in the analysis. An experiment is conducted to compare the melt pool geometry predicted from the simulation and that obtained from the experiment. The melt pool size is significantly affected by the intensity parameter (i.e., the melt pool is too shallow for high-intensity parameters). The predicted melt pool geometry is in agreement with the experimental results.

Growth mechanism of the tubular TiB crystals in situ formed in the coatings laser-borided on Ti–6Al–4V alloy

Coatings containing borides were fabricated on titanium alloy by laser boriding treatment. SEM images indicate that some rod-like TiB crystals with a hollow core (filled with α-Ti before etched) are formed in the coatings. The formation of rod-like hollow TiB is attributed to the following aspects: first, because the chemical composition in some areas in the laser melt pool is hypoeutectic and in some other areas is hypereutectic due to the boron atoms dissolved in the melt distributing unevenly during rapid solidification, β-Ti firstly crystallizes as primary phases in the hypoeutectic areas of the melt pool and acts as a heterogeneous nucleus for TiB to crystallize on it, and second, because the bond between B–B is stronger than that between Ti–B and between Ti–Ti. TiB grows along the B–B zig-zag chain as well as normal to the direction of the B–B zig-zag chain to form a tubular structure with a β-Ti core in it.

Seam gap bridging of laser based processes for the welding of aluminium sheets for industrial applications

Laser welding has a large potential for the production of tailor welded blanks in the automotive industry, due to the low heat input and deep penetration. However, due to the small laser spot and melt pool, laser-based welding processes in general have a low tolerance for seam gaps. In this paper, five laser-based welding techniques are compared for their gap bridging capabilities: single-spot laser welding, twin-spot laser welding, single-spot laser welding with cold wire feeding, twin-spot laser welding with cold wire feeding and laser/GMA hybrid welding. Welding experiments were performed on 1.1- and 2.1-mm-thick AA5182 aluminium sheets. The resulting welds were evaluated using visual inspection, cross sectional analysis with optical microscopy, tensile tests and Erichsen Cupping tests. The results show that the use of a filler wire is indispensable to increase the gap tolerance. A proper alignment of this wire with the laser spot(s) is crucial. With the single spot laser welding with cold wire feeding, a gap up to 0.6 mm could be bridged as opposed to a maximum allowable gap width of 0.2 mm for single-spot laser welding without filler wire. For 2.1-mm-thick sheets, the laser/GMA hybrid welding process can bridge even gaps up to 1.0 mm. Most welds had a high tensile strength. However, during Erichsen Cupping tests, the deformation of the welds is significantly lower as compared to the base material

Magnetic texture of Nd 2Fe 14B solidified in the surface layer of anisotropic sintered-magnets

The arrangements of the easy magnetization axis [001] of columnar Nd 2Fe 14B crystals in the laser scanned layer on anisotropic sintered Nd 15Fe 77B 8 magnets were investigated by XRD and the Bitter method. The results show that the common effects of both the heat flux and the substrate magnetization orientation constrain the columnar Nd 2Fe 14B solidified from the laser melting pool to form the c-axis texture orientated with the same direction as that of the substrate, when the geometric relationship between the heat flux in the laser scanning layer and c-axis texture orientation of the substrate is perpendicular to each other, and if the laser scanning velocity is no less than 25 mm·min −1. The c-axes of columnar Nd 2Fe 14B crystals are no longer randomly distributed in the plane normal to their preferential growing direction as they are randomly done in both ingots cooled by water-cooling copper mould and directionally solidified Nd-Fe-B rods.

Modelling the geometry of a moving laser melt pool and deposition track via energy and mass balances

The additive manufacturing technique of laser direct metal deposition allows multiple tracks of full density metallic material to be built to form complex parts for rapid tooling and manufacture. Practical results and theoretical models have shown that the geometries of the tracks are governed by multiple factors. Original work with single layer cladding identified three basic clad profiles but, so far, models of multiple layer, powder-feed deposition have been based on only two of them. At higher powder mass flow rates, experimental results have shown that a layer's width can become greater than the melt pool width at the substrate surface, but previous analytical models have not been able to accommodate this. In this paper, a model based on this third profile is established and experimentally verified. The model concentrates on mathematical analysis of the melt pool and establishes mass and energy balances based on one-dimensional heat conduction to the substrate. Deposition track limits are considered as arcs of circles rather than of ellipses, as used in most established models, reflecting the dominance of surface tension forces in the melt pool, and expressions for elongation of the melt pool with increasing traverse speed are incorporated. Trends in layer width and height with major process parameters are captured and predicted layer dimensions correspond well to the experimental values.

Transformation of Ceramic Phase in Laser Melting Pool of Zn-Al Based Ceramic Composited Film

Transformation of Ceramic Phase in Laser Melting Pool of Zn-Al Based Ceramic Composited Film

Microstructure and properties of laser clad TiC+NiCrBSi+rare earth composite coatings

TiC particle reinforced Ni-based alloy composite coatings were clad to AISI 4140 chromium molybdenum alloy steel substrate using a laser. The microstructure and tribological behavior of coatings were studied experimentally by means of scanning electron microscopy, transmission electron microscopy and wear tests. Partial solution of TiC particles occurs in the laser melting pool during the heating stage. Twins and dislocations were observed at the interface of TiC particles and the matrix. The wear resistance was found to correlate with the load and amount of the TiC particles present in the clad layer. By adding La 2O 3, the hardness of clad coatings was significantly increased, and the wear resistance of clad coatings was enhanced.

SiC p/Ti6Al4V functionally graded materials produced by laser melt injection

With a well-controlled laser melt injection (LMI) process, for the first time the feasibility is demonstrated to produce SiC particles (SiC p) reinforced Ti6Al4V functionally graded materials (FGMs). SiC p are injected just behind the laser beam into the extended part of the laser melt pool that is formed at relatively high beam scanning velocities. The process allows for the minimization of the decomposition reaction between SiC p and Ti6Al4V melt, and also leads to FGMs of SiC p/Ti6Al4V instead of a homogeneous composite layer on Ti6Al4V substrates. An injection model is designed based on the temperature/viscosity field of the laser pool for a deeper understanding of the mechanism of formation of the FGMs with LMI. The model is based on finite element calculations of the temperature field in the melt pool, physical considerations of the LMI process and it is supported by experimental observations. Three types of reaction layers are observed around SiC p, namely a thin monocrystalline TiC layer, a cellular polycrystalline TiC layer and a thick mixed layer of TiC with Ti 5Si 3. Among them, only the monocrystalline TiC layer exhibits particular orientation relationships (ORs) to the SiC p lattice, i.e. (111) TiC∥(0001) SiC and 〈110〉 TiC ∥〈1 1 ̄ 00〉 SiC or (111 ) TiC ∥(10 1 ̄ 2 ) SiC and 〈1 1 ̄ 0〉 TiC ∥〈1 2 ̄ 10〉 SiC . These two kinds of TiC reaction layers act as a barrier against the interfacial reaction and its swift formation during rapid cooling hinders the dissolution of SiC p in the Ti-melt.

Functionally Graded Materials produced with High Power Lasers

AbstractWith a well-controlled laser melt injection (LMI) process, for the first time the feasibility is demonstrated to produce SiC particles (SiCp) reinforced Ti6Al4V functionally graded materials (FGMs). SiCp are injected just behind the laser beam into the extended part of the laser melt pool that is formed at relatively high beam scanning velocities. The process allows for the minimization of the decomposition reaction between SiCp and Ti6Al4V melt, and also leads to FGMs of SiCp/Ti6Al4V instead of a homogeneous composite layer on Ti6Al4V substrates. An injection model is designed based on the temperature/viscosity field of the laser pool for a deeper understanding the mechanism of formation of the FGMs with LMI. The model is based on finite element calculations of the temperature field in the melt pool, physical considerations of the LMI process and it is supported by experimental observations. Three types of reaction layers are observed around SiCp, namely a thin monocrystalline TiC layer, a cellular polycrystalline TiC layer and a thick mixed layer of TiC with Ti5Si3. Among them, only the monocrystalline TiC layer exhibits particular orientation relationships to the SiCp lattice, i.e. (111)TiC//(0001)SiC and <110>TiC//<1100>SiC or (111)TiC // (1012)SiC and <110>TiC//<1210>SiC. These two kinds of TiC reaction layers act as a barrier against the interfacial reaction and its swift formation during rapid cooling hinders the dissolution of SiCp in the Ti-melt.