Line Energy Density Research Articles

Effect of laser process parameters on the pores, surface roughness, and hardness of laser selective melting of dental cobalt-chrome alloys.

To address the quality problems caused by high porosity in the preparation of dental cobalt-chrome alloy prosthetics based on selective laser melting (SLM) technology, we investigated the influence mechanism of different forming process parameters on the microstructure and properties of the materials. Moreover, the range of forming process parameters that can effectively reduce defects was precisely defined. The effects of laser power, scanning speed, and scanning distance on the pore properties, surface roughness, and hardness of dental cobalt-chrome alloy were investigated by adjusting the printing parameters in the process of SLM. Through metallographic analysis, image analysis, and molten pool simulation, the pore formation mechanism was revealed, and the relationship between the porosity and energy density of SLM dental cobalt-chrome alloy was elucidated. When the linear energy density was higher than 0.18 J/mm, the porosity defect easily appeared at the bottom of the molten pool. When the laser energy density was lower than 0.13 J/mm, defects occurred in the gap of the molten pool due to insufficient melting of powder. In particular, when the linear energy density exceeded the threshold of 0.30 J/mm or was below 0.12 J/mm, the porosity increased significantly to more than 1%. In addition, we observed a negative correlation between free surface roughness and energy density and an inverse relationship between macroscopic hardness and porosity. On the basis of the conditions of raw materials and molding equipment used in this study, the key process parameters of SLM of molding parts with porosity lower than 1% were successfully determined. Specifically, these key parameters included the line energy density, which ranged from 0.13 J/mm to 0.30 J/mm, and the scan spacing should be strictly controlled below 90 μm.

Numerical simulation of single-pass selective laser melting of Mg-Y-Sm-Zn-Zr alloy

A three-dimensional finite volume method (FVM) thermal-fluid coupling model was developed to simulate the temperature field distribution and dimensions of the melt pool in selective laser melting (SLM) of Mg-Y-Sm-Zn-Zr alloy powder. This model aims to accurately determine the temperature field and surface morphology of the melt pool by considering the laser's impact on powder particles, free surface dynamics, convection, and multiphase heat transfer. The study explores how laser power and scanning speed influence temperature distribution, melt pool size, and flow dynamics. The findings show that increasing laser power or reducing scanning speed consistently affects the melt pool size, peak temperature, and cooling rate. During the evaporation of the alloy in the melt pool, fluid flow is primarily governed by recoil pressure and Marangoni convection. Notably, increasing the laser power from 40 W to 100 W and the scanning speed at 0.3 m/s enhances the depth of the melt pool indentation from 13.86 µm to 23.35 µm. Furthermore, the indentation depth correlates positively with the laser line energy density.

Effects of Laser Scanning Strategy on Bending Behavior and Microstructure of DP980 Steel

Laser bending is a kind of cumulative forming technology and bending efficiency is one of its most important indexes. This study investigates the bending behavior and the microstructure of DP980 steel plates under different laser scanning strategies, using an IPG laser system. Two sets of experiments varied the accumulated line energy density (AED) by altering the laser scanning velocity and number of scans. The results show that, for the single laser scanning process, the bending angle of the plate increases with AED, due to a larger temperature gradient through the thickness direction; however, this relationship is nonlinear. A higher AED led to a sharper initial increase in bending angle, which then plateaued. Under the same AED conditions, the bending angle of the plate undergoing multiple laser scans increases by at least 26% compared to the single one, due to the microstructure changes. It is revealed that the bending efficiency is affected by both the AED and the resultant microstructure evolution in the DP980 steel. Higher AED values and appropriate peak temperatures facilitate better bending behavior due to the formation of uniform martensite and grain refinement. Conversely, excessive peak temperatures can hinder bending due to grain growth.

Microstructure and mechanical properties of Ti6Al4V/W bimetallic structure via selective laser melting

The titanium alloy Ti–6Al–4V (TC4) and tungsten (W) possess excellent complementary properties that would be useful for a wide range of industrial applications if they could be suitably combined. Selective laser melting was applied to prepare TC4/W specimens with a bimetallic structure. A strong bond without interfacial macrocracks was obtained by adjusting the laser line energy density to <0.75 J/mm. The bond interface of the bimetallic structure reached a microhardness of 503 ± 20 HV, which was harder than that of the two base materials, and the transverse and longitudinal ultimate compressive strengths were 1510 ± 40 MPa at strains of 20%–22.5%. Three-point bending experiments showed that when TC4 was used as the base material, the flexural strength of the specimen was 2210.5 ± 44.5 MPa. The high strength of the bond interface was attributed to sufficient agitation of the molten pool and reinforcement by fine crystalline grains. The agitation was provided by the melting and elemental diffusion of TC4 owing to the high energy input during the formation of W, and the fine crystalline grains were obtained by the dispersed W particles providing multiple nucleation sites for liquid TC4. The bimetallic structure retained the strength of W and toughness of TC4, which bodes well for its industrial application.

Fine-Tuning Melt Pools and Microstructures: Taming Cracks in Powder Bed Fusion—Laser Beam of a non-weldable Ni-base Superalloy

Powder Bed Fusion – Laser Beam (PBF-LB) of high γ’ strengthened Ni-base superalloys, such as CM247LC, is of great interest for high temperature applications in gas turbines. However, PBF-LB of CM247LC is challenging due to the high cracking susceptibility during PBF-LB processing (solidification cracking) and heat treatment (strain age cracking, mostly caused by residual stresses). This study focuses on understanding the impact of process parameters on microstructure, residual stresses and solidification cracking. Laser power (P), speed (v) and hatch spacing (h) were varied while the layer thickness (t) was fixed. The melt pool size and shape were found to be key factors in minimizing solidification cracking. Narrower and shallower melt pools, achieved using a low line energy density (LED = P/v ≤ 0.1 J/mm), gave low crack densities (0.7 mm/mm2). A tight hatch spacing (h = 0.03 mm) resulted in reduced lack of fusion porosity. Electron backscatter diffraction investigations revealed that parameters giving finer microstructure with 〈100〉 crystallographic texture had low crack densities provided they were processed with a low LED. Atom probe tomography elucidated early stages of spinodal decomposition in the as-built condition, where Cr and Al cluster separately. The extent of spinodal decomposition was found to be affected by the LED and the hatch spacing. Samples with low LED and small hatch spacing showed higher degrees of spinodal decomposition. X-ray diffraction residual stress investigations revealed that the residual stress is proportional to the volumetric energy density (VED = P/(v. h. t)). Although low residual stresses can be achieved by using low VED, there is a high risk of lack of fusion. Hence, other parameters such as modified scan strategy, build plate pre-heating and pulsed laser mode, must be further explored to minimize the residual stresses to reduce the strain age cracking susceptibility.

Microstructure and mechanical properties of Cu-Cr-Zr alloy prepared by electron beam additive manufacturing and laser-MIG hybrid welding

Cu-Cr-Zr alloys are widely used as medium-strength and high-conductivity copper alloys in aerospace, transportation and electronic information fields. The application of additive manufacturing technology is expected to realize the preparation of Cu-Cr-Zr alloy parts with complex structure and superior property. However, the commonly used laser additive manufacturing technology is not applicable to the preparation of copper alloys due to its intrinsic high reflection to laser. In this study, we adopted two technologies, i.e., electron beam additive manufacturing (EBAM) and laser-metal inert gas hybrid welding (laser-MIG), for the preparation of Cu-Cr-Zr alloys. Finite element simulation was employed to simulate the temperature field and residual stress field of the material during solidification. The microstructure, relative density, microhardness, tensile properties and electrical conductivity of the as-fabricated samples were characterized. The results showed that samples with better moldability were obtained by both methods at a line energy density of 600 J/mm. In the top area, both methods exhibit medium-sized equiaxed grains with more precipitated phases, showing the best comprehensive properties. The EBAM method demonstrates superior formability, better relative density and electrical conductivity due to more stable temperature field changes. On the other hand, the laser-MIG method obtains finer grains and higher mechanical properties because of a faster cooling rate, but leads to a decrease in relative density and electrical conductivity. This study presents new experimental data for Cu-Cr-Zr alloys prepared by EBAM and laser-MIG methods, contributing to extend the application of additive manufacturing in the preparation of Cu alloys.

Single-track investigation of additively manufactured mold steel with larger layer thickness processing: Track morphology, melt pool characteristics and defects

Large layer thickness is essential to increase building rate, but excessive layer thickness easily lead to defects which negatively affect component quality. The evolution of track morphology, melt pool characteristics and defects of additively manufactured mold steel by using various larger layer thickness (60 and 90 μm) while also varying the laser energy density was investigated systematically. Results indicated that the morphologies of single tracks shifted sequentially from discontinuousness to continuous irregularity and continuous regularity with the increasing line energy density at 60 μm layer thickness. The width and depth of melt pool increased substantially with the line energy density increased from 0.4 to 1.3 J/mm, while the height of melt pool was nearly constant. Correspondently, transition of the melt pool based on the depth-to-width ratio from balling to conduction and keyhole modes was observed. When the layer thickness increased from 60 to 90 μm, the average build percentage of the single tracks decreased by 43 % with increasing line energy density from 0.26 to 0.51 J/mm, and more spatters around the surface of tracks were found at 0.8 J/mm. Interestingly, the average width of the melt pool decreased by 37 μm, while the height of melt pool increased by 45 μm. More small spherical gas pores (5–15 μm) occurred in bottom of keyhole due to trapped vapor at high energy density. The decrease in melt width with increased layer thickness is most likely due to the weakened Marangoni convection. Finally, formation mechanism of lack-of-fusion defects and gas pores with larger layer thickness processing were analyzed based on melt pool characteristics, such as spheroidization, keyhole and spatters.

Microstructure and corrosion property of TC4 coating with Al0.5CoCrFeNi high-entropy alloy interlayer by laser cladding

In this work, TC4 coating was prepared on 45 steel surface successfully by laser cladding and Al0.5CoCrFeNi high-entropy alloys (HEA) coating was designed as an intermediate layer. Microstructure, microhardness and electrochemical corrosion characteristics of TC4/HEA composite coating were evaluated in detail. The interfaces without cracks of HEA layer/substrate and TC4 layer/HEA layer are clear and straight, and a metallurgical bonding is obtained when the line energy density is 315 J/mm2 and 180 J/mm2. A reactive layer consisted of TiAl and TiFe intermetallic compounds (IMC) is formed at TC4/HEA interface. The HEA transition layers are composed of uniformly distributed equiaxed matrix (FCC) and intercrystalline phase (BCC), while a transition from Widmanstätten structure to coarse acicular crystal was found in TC4 layer. The microhardness of HEA coating is slightly higher than the substrate and lower than the TC4 coating, and the microhardness value of TC4 layer is up to 745.8 HV0.2. Potential dynamic polarization curves test results show that a comparatively small corrosion zone is concluded for TC4/HEA composite coating sample and the corrosion resistance of the composite coating is significantly improved with an increased value of 55.9 % in Ecorr (−0.325 V) and a decreased value of 99.3 % in Icorr (0.1 μA/cm2) compared to the substrate.

Investigating double-scan strategies for reducing heat-affected zone in laser surface melting

Laser surface melting (LSM) is a technique for modifying surface topography without adding or removing material. LSM may result in large heat-affected zones (HAZs) that may be undesirable in several applications. The current work aims to investigate double-scan strategies to reduce HAZ. This strategy used two laser scans, each at half the line energy density used for a single scan. A numerical model is developed for LSM to predict the MZ and HAZ and is validated against experimental results using Ti6Al4V. In addition, the effect of the double-scan strategy on the surface roughness, hardness and residual stresses is evaluated. The experimental results show the HAZ depth decreased by 26%, the surface hardness was improved, and the resultant residual stresses were preserved for the double scan strategy compared to the single scan. However, it resulted in a relatively rougher surface finish.

Numerical simulation of melt pool size and flow evolution for laser powder bed fusion of powder grade Ti6Al4V

The study of molten pool characteristics is a powerful means of determining the quality of laser additive manufacturing forming. In this paper, a three-dimensional transient thermal flow field numerical model of Ti6Al4V powder processed by LPBF is developed based on FLOW-3D software, and the dynamic evolution of the molten pool with fixed process parameters is quantitatively described using dimensionless numbers in computational fluid dynamics. It is shown that the main heat transfer mode of the molten pool is thermal convection; the evaporative back sitting pressure, surface tension and Marangoni shear force are the main driving forces for the evolution of the molten pool. Furthermore, the influence of key process parameters on the heat flow field of the molten pool was analyzed. When the laser power was 300 W, the depression depth and molten pool depth caused by the recoil pressure were significantly larger, and the isothermal density of the solidification region was more intensive; when the scanning speed was increased from 0.4 m/s to 0.8 m/s, the molten pool length was reduced from 524 μm to 410 μm, and the line energy density was reduced making the amount of melted powder decrease. The wettability and fluidity of the molten state metal becomes worse; when the scanning interval increases to 120 μm, a large number of incompletely melted powder tracks cannot lap correctly at the midline of the double track. The results of the above simulations achieve a high degree of agreement with the surface quality of the specimens during the experiments, which provides guidance for quantitative analysis of molten pool evolution and prediction of Ti6Al4V forming quality.

Microstructure evolution of the W-C hard coatings using directed energy deposition on tungsten alloy surface

Herein, we propose a method for producing WC composite ceramic coatings on tungsten alloy surfaces using directed energy deposition (DED). Different microstructures of DED single tracks were obtained with different line energy densities. Two parts of DED tracks were discovered, which contained the WC fusion zone and the WC-Ni-Co melting injection zone. The WC fusion zone mainly comprised four typical phases, WC, W2C, W, and C(diamond, graphite (D, G)), which were present in the microstructures of the tracks in different combinations. The key factor for determining the microstructure evolution was C content. With higher line energy density, more tungsten alloy melted, which decreased C content in the melt pool. As C content decreased, the composition of the microstructure transformed to C(D, G) + WC + W2C → WC + W2C → W2C + W + a small amount of γ-(Ni, Co). C content gradually decreased from the top to the bottom of the melt pool, resulting in a gradient microstructure transition. The microstructure and property could be tuned by controlling the C content. The hardness was the highest, 2300 HV, at 40.7 at.% C with WC-W2C eutectoid. This study provides practical insights for producing coatings on tungsten alloy surfaces with optimized process parameters and for tailoring mechanical properties.

Research on selective laser sintering process of ternary composite materials (PP/MCMB/CNT)

The composite material composed of polypropylene and conductive filler has good corrosion resistance, light weight and good conductivity, and is one of the preferred materials for fuel cell bipolar plates. The formation of ternary composite materials composed of polypropylene, mesocarbon microbeads and carbon nanotubes is realized by selective laser sintering. The influences of laser power, scanning speed, scanning spacing and defocus amount on forming quality are studied. The formability of mixed powders with different composition proportions as well as the conductivity, bending strength and hydrophobicity of the formed samples are investigated. The results show that when the input energy density is insufficient, it is easy to form defects such as spheroidized particles and worm-like structures. The parameters with good forming quality are in the following range: defocus amount is + 3 mm; line energy density is 2.94–3.85 J/mm2; peak power density is 1.13 × 104-1.48 × 104 W/mm2 and scanning spacing is 0.1–0.14 mm. The conductive filler in composite material is beneficial to improving the conductivity, but it will reduce the formability of the mixed powders and reduce the bending strength and hydrophobicity of the sample. The effect of mesocarbon microbeads on improving conductivity or on reducing hydrophobicity is smaller than that of carbon nanotubes.

An integrated methodology for estimating the interface temperature and effects of powder diameters in laser transmission welding process

The powder diameter affects the heat generation and further influences the quality of weldments when the MZA powder is used as laser-absorbents to carry out laser transmission welding (LTW). It is difficult to measure the temperature due to its extremely short duration. A mathematical model considering the multiple reflections of metal powder is established to predict heat generation. The mathematical model is verified and then used to describe the influence of line energy density and powder diameter on the temperature profile of the LTW process. The results demonstrate that the temperature from experiments and simulation are consistent. Therefore, the established model is suitable to predict the heat generation of metal powder laser-absorbents in the LTW process.

A new control parameter to predict micro-warping-induced job failure in LPBF of TI6AL4V titanium alloy

Laser powder bed fusion (LPBF) includes a few printing techniques widely used, in recent years, concerning the additive manufacturing of Ti6Al4V alloys. These produced parts, typically utilized in sectors such as aerospace and biomedical, are characterized by very high added value. It is therefore fundamental to identify the influence of process parameters typical of LPBF technology on the occurrence of warping leading to process failure. This study deals with the characterization of single-track and “micro-scale” level warping phenomena which may lead to protrusion of material over the powder bed and process failure before normal termination. This phenomenon was investigated as a function of process parameters, referring also to the strength and ductility characteristics of the manufactured samples. With this purpose, several samples were printed using variable process parameters both in terms of line energy density (LED) values and in terms of laser power and speed combinations such as to guarantee constant LED values. For the samples that did not show significant micro-warping phenomena, in addition to the transversal and longitudinal geometric characterization of the single track, tensile tests were performed to determine both the resistance of the material and the ductility characteristics. The single tracks, for given process parameters, were printed on a homogeneous material substrate. For every single track, a microstructural and morphological transverse and longitudinal characterization has been carried out and the measured geometrical features were correlated to the process parameters. The obtained results allowed the identification of a new threshold parameter, indicating the limit operating conditions beyond which significant warping phenomena and process failure occur.

Integrated modelling and simulation of NiTi alloy by powder bed fusion: Single track study

In-depth understanding of the layer-by-layer process is critical for the quality control of additive manufactured (AM) components. Besides the development of experimental techniques, computational modelling is another important way to study the AM mechanisms in detail. However, currently there is still lack of a modelling platform that integrates all the necessary physics involved in the AM process. In the present study, we develop a modelling framework that integrates a phase field method for microstructure evolution, a lattice Boltzmann method for melt pool dynamics, a modified ray-tracing method for laser-material interaction, and a minimum gravity energy algorithm for powder bed generation to simulate the single-track, powder bed fusion (PBF) process of NiTi shape memory alloy. Our simulation shows that different from keyhole depth, melt pool size does not obey the scaling law with line energy density, the keyhole depth and the overlapping ratio of laser spot vs. keyhole opening together determine the laser absorptivity. Based on the simulation finding, we also develop a self-consistent analytical model to predict the keyhole depth and the absorptivity for given scanning parameters. The present work lays a solid foundation towards quantitative understanding of multi-layer AM process.

Study on Surface Roughness Improvement of Selective Laser Melted Ti6Al4V Alloy

To improve the surface quality of Ti6Al4V parts formed by selective laser melting (SLM), this paper systematically studies the effects of laser power, scanning speed and inclination angle on the different surface morphology and roughness of parts. On this basis, the effect of surface remelting and multi-layer profile scanning process strategies on improving the surface quality of parts is explored. The upper surface roughness varies parabolically with increasing line energy density, the line energy density value that minimizes the upper surface roughness is around 0.22 J/mm, and the minimum Ra value is 4.41 μm. The roughness of upper and lower sides increases significantly with the increase in scanning speed. As the inclination angle increases, the roughness of the upper and lower sides gradually decreases, which is caused by the combined influence of powder adhesion and step effect. The surface remelting process strategy can reduce the upper surface roughness by 35.68% and reduce its Ra value to 2.65 μm. The multi-layer profile scanning process strategy can reduce the upper side and vertical side roughness by more than 50%, down to Ra 5.10 μm and Ra 4.61 μm, respectively.

Heat transfer and melt flow of keyhole, transition and conduction modes in laser beam oscillating welding

Melting modes have significant effects on the heat transfer and fluid flow in laser beam oscillating welding (LBOW), which in turn affects the welding quality and solidification microstructure. In this paper, a computational fluid dynamics (CFD) model for LBOW of 2219 aluminum alloy is developed to study the melting/solidification, fluid flow pattern, keyhole dynamics and energy absorption under different welding modes: keyhole, transition and conduction mode. A hybrid heat source model is developed to describe the influence of the vapor heat and multiple reflections of the laser rays on the keyhole surface. The predicted molten pool dimensions and weld profiles are in good agreement with the experimental results. By increasing the oscillating frequency, the line energy density significantly decreases due to a higher scanning speed that results in a shallower keyhole with wider opening. The laser rays are more likely to leak out after 2-3 reflections which reduces the energy absorption. Thus, the melting mode will transfer from keyhole to conduction mode. With further increase of the oscillating frequency, the heat transfer will be dominated by heat conduction when the energy density is below the threshold for keyhole formation. Our work lay the foundations for optimizing the LBOW process to obtain sound joints.

Microstructural evolution and mechanical behavior of additively manufactured tantalum produced by electron beam powder bed fusion

Tantalum components were manufactured by electron beam powder bed fusion (EB-PBF) at different line energy densities in this study. It was found that, the relative density, grain size and mechanical properties are significantly influenced by line energy density. Tantalum component with relative density above 99% can be achieved when the linear energy density exceeding 960 J/m. The tantalum manufactured at 1200 J/m has a superior mechanical property with tensile yield strength of 331 MPa and elongation of 31.1%, which is better than that prepared by other conventional methods. The high strength is mainly attributed to the fine grains and dislocation network substructure, while the defect-free microstructure and low content of interstitial oxygen atoms are responsible for the good ductility.

High wear resistance WC-Co reinforced GCr15 bearing steel composite prepared via selective laser melting (SLM)

Among numerous reinforced particulates, WC-Co particles are regarded as a promising reinforcement for Fe matrix composites to enhance the tribological properties. Because of the particles’ high chemical stability, and high hardness. In this study, WC-Co particles were chosen as reinforcements to produce GCr15 bearing steel matrix composites by SLM. The microstructures and friction performance of the composites were observed and analyzed. The effect of the laser line energy density (LED) on phases, microstructure and friction performance features is investigated systematically. The phase of samples added with WC-Co particles changed obviously, from mainly α-Fe to γ-Fe. At the same time, Fe7C3, WC, CrSi2, M3W3C phases were found in the composites. The composite prepared by SLM has a good inter-reaction interface between the GCr15 bearing steel matrix and WC-Co particles. Two special microstructures, nail-shaped and cellular microstructure were found in the composites, and with the increase of LED, the size of the cellular microstructure increased, maximum average size is 1.45 μm. After adding WC-Co particles, the properties of GCr15 bearing steel were significantly enhanced, and the microhardness increased from 651 ± 61 HV0.2 to 1093 ± 139 HV0.2, and the COF and wear rate decreased from 0.5091 and 1.82 ± 0.17 × 10−6 mm3N−1 m−1 to 0.4116 and 0.58 ± 0.11 × 10−6 mm3N−1 m−1, respectively

Study of mechanical and tribological properties of Ti–6Al–4V alloy fabricated by powder bed fusion laser beam

ABSTRACT Powder bed fusion laser beam, as one of the most promising forming technologies, offers unmatched benefits over traditional processing, particularly in the production of Ti–6Al–4V. The influence of laser line energy density (LLED) on the forming surface, phase composition, micro-hardness, tensile characteristics and wear resistance of Ti–6Al–4V alloy were explored to disclose the evolution of mechanical and tribological properties. According to the findings, the LLED causes ‘depressions’ and ‘highlands’ between nearby scanning tracks. The phase composition did not alter appreciably as LLED increased. Micro-hardness and tensile characteristics increased initially, then declined, and the value of maximum micro-hardness and ultimate tensile strength were 388.17 HV0.2 and 1197.5 MPa, respectively. Furthermore, when the LLED is 0.24 J mm–1, the wear resistance is optimal under the aviation lubricant medium, with an average friction coefficient of 0.1505 and volume wear rate of 6.95∗10−8 mm2∗N−1, and a wear mechanism of mild furrow wear and adhesion wear.