Laser Remelting Research Articles

Multi-material additive manufacturing-functionally graded materials by means of laser remelting during laser powder bed fusion

Many processes may be used for manufacturing functionally graded materials. Among them, additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component. By adjusting the powder composition of the starting material layer by layer, a macroscopic and step-like gradient can be achieved. To further improve the step-like gradient, an enhancement of the in-situ mixing degree, which is limited according to the state of the art, is necessary. In this paper, a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting (LR) is described. The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process. Subsequent cross-sectional selective electron microscopic analyses were carried out. By applying LR, the mixing degree was enhanced, and the reaction zone thickness between the materials was increased. Moreover, an additional copper and iron-based phase was formed in the interface, resulting in a smoother gradient of the chemical composition than the case without LR. The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.

Investigation of the influence of process parameters on crack formation and mechanisms in Ti-48Al-2Cr-2Nb alloy via laser directed energy deposition

The Ti-48Al-2Cr-2Nb alloy was fabricated using laser directed energy deposition(LDED), and the impact of various process parameters on the macroscopic crack morphology was analyzed. The mechanism of crack formation was investigated through the analysis of crack microstructure, phase composition, crystal orientation, and elemental composition. The process parameters were optimized by response surface methodology(RSM) and the laser remelting method was used to suppress the crack formation. The results showed that the cracks were mainly caused by lack of fusion, residual stress during LDED and stress between different phases of TiAl alloy. The mismatch of process parameters results in insufficient energy for powder melting, ultimately leading to lack of fusion occurrence. To minimize crack formation, the response surface method was employed to optimize process parameters and reduce crack generation. The higher temperature gradient led to the existence of residual stress in the sample, and the higher stress between α 2 phase and B2 phase formed in the deposition process due to the difference of thermal expansion coefficients. The region where the two phases converge was the region with the highest crack sensitivity, and cracks occured in the region where α 2 phase and B2 phase converge in the form of excellent transgranular fracture. The samples prepared by using the optimized parameters can effectively restrain the cracks caused by lack of fusion, but can not restrain the cracks caused by the stress between phases. Laser remelting after LDED can not only reduce the temperature gradient and residual stress, but also remelt the unmelted powder on the surface of the as-deposited samples, effectively inhibiting the generation of cracks, and preparing crack-free samples.

Influence of buoyancy on phase separation during laser melting of Silicon and Iron ore

Phase separation is a well-known effect in liquid-liquid interactions, which can also occur during laser-related processes such as laser melting and laser alloying. However, the mechanism of phase separation between immiscible liquids on a millimeter scale during rapid laser processes has not been fully investigated, in which the extent of buoyancy’s contribution to it has remained unclear. Therefore, this investigation focused on the effect of buoyancy on liquid phase separation during the laser melting of silicon (Si) and iron ore. A simplified 2D numerical model was established to simulate the motion of a single Si liquid droplet in iron ore melt with and without the impact of gravity, respectively. The rise velocity of the droplet was calculated and analyzed under the effect of gravity. In addition, the phenomena of simultaneous laser remelting of Si and iron ore in layers were recorded with a high-speed camera, and the element and phase distributions of the target nugget were analyzed by Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). By combining the simulation, high-speed imaging, and SEM/EDS analysis, the effect of buoyancy on phase separation has been qualitatively analyzed. This investigation revealed that buoyancy is not the main driving force of liquid-liquid phase separation during rapid laser processing.

Laser-Powder Bed Fusion Additive Manufacturing of NiCrBSi Self-Fluxing Nickel Alloy, Material Health - Process Relationship

Laser Powder Bed Fusion (L-PBF) might be a promising solution for producing complex wear-resistant parts from NiCrBSi self-fluxing nickel alloys. This study investigates a large set of parameters with a baseplate heated up to 500 °C. Furthermore, laser remelting is also explored to further increase surface density and reduce cracking. Material health is deeply investigated by image analysis to quantify different defects (lack of fusion, porosity and cracks). Spatters likely induce lacks of fusion, while the low toughness and high hardness values cause cracks. The lack of fusion surface fraction and crack length decrease with the preheating temperature while the crack width increases. A surface density of 99 % is obtained with careful process optimisation leading to a laser power of 100 W, a laser speed of 750 mm.s-1 and a preheating temperature of 500 °C.

Improvement of corrosion resistance of AZ31 magnesium alloy by laser remelting/microarc oxidation/LDH composite layer

Laser remelting (LR) pretreatment, micro-arc oxidation (MAO) treatment and layer double hydroxides (LDH) sealing post-treatment are conducted on AZ31 alloy surface in sequence. The results show that the corrosion resistance of prepared LR/MAO/LDH composite layer is significantly improved. Compared with the solution-aged AZ31 alloy and LR/MAO layer, the corrosion current densities of AZ31-LR/MAO/LDH were reduced by 4 and 2 orders of magnitude, respectively, and the self-corrosion potentials were increased by 1.23 V and 0.25 V, respectively. Laser remelting pre-treatment increased the average thickness of the MAO layer by 5–10 μm. LR/MAO/LDH composite layers can effectively improve the corrosion resistance of AZ31 alloy, and have the potential for application.

Healing of keyhole porosity by means of defocused laser beam remelting: Operando observation by X-ray imaging and acoustic emission-based detection

One of the remaining challenges in Laser Powder Bed Fusion (LPBF) of metals is the control of the formation of keyhole pores, resulting from a local excessive energy input during processing. Such defects can lead to degraded mechanical properties and are typically detected and/or removed after the process through non-destructive quality-inspection procedures and porosity-removal treatments. Monitoring and controlling the formation of defects during the LPBF process can allow circumventing such time-consuming and costly post-process stages. This paper develops a new approach to perform in-situ healing of deep keyhole pores, using a positively defocused laser beam with finely tuned laser remelting process parameters. Synchrotron radiographic images of the process zone are acquired during laser remelting. The use of operando imaging enables the visualization of pore removal during processing, and unveils the effect of various remelting conditions on the healing efficiency. The acoustic signals generated during laser remelting are recorded using a high-sensitivity optical microphone, and analyzed in parallel with the X-ray images, allowing the acoustic signature of defect healing to be identified. The present paper demonstrates for the first time that an airborne acoustic sensor can be used to monitor the healing of keyhole pores during LPBF.

A novel approach to the deposition of Co-Mo-Cr-Si coating on Haynes 282 by laser fusion and laser remelting

This paper presents a novel hybrid method of the crack-free fusion of CoMoCrSi, which is a cobalt-based alloy with molybdenum additions with the composition analogous to Tribaloy® T400, on Haynes 282 for parts where extreme wear is problematic with high temperatures and corrosive media, such as industrial turbine applications, commercial jet engines, and rocket engines. To alleviate cracking of the coating during laser surface deposition/coating, previous attempts relied on preheating of the substrate to reduce cracks to alleviate a thermal mismatch between the substrate and molten powder. The primary aim of this work is accomplished by fusing crack free CoMoCrSi layers using a novel hybrid process of combining laser cladding and laser remelting processes. The optimal process conditions of coating crack-free CoMoCrSi on Haynes 282 were determined for both the laser powderbed fusion and remelting processes. Microhardness and chemical compositions of the coating layers produced using optimal process parameters with and without remelting the substrate were compared. The chemical compositions of different phases present in the coating layer were analyzed by Energy Dispersive X-ray Spectroscopy (EDS). The remelting process yielded no dilution at the surface, resulting in the preservation of the original CoMoCrSi coating properties.

Tensile Property of 7075 Aluminum Alloy with Strengthening Layer by Laser Remelting-Cladding Treatment.

The Ni60-SiC-CeO2 strengthening layer with deep remelting pools was constructed on the surface of 7075 aluminum alloy using the laser remelting-cladding processing method, and a soft and hard interphase was prepared on the matrix by the interval of laser remelting, which was inspired by soft-hard interphase structure with excellent crack inhibition performance from the natural world. The microstructure and microhardness of the remelting region and the remelting-cladding region of the strengthening layer were studied. The tensile characteristics of two distinct strengthening layers were investigated in the laboratory. The results showed that the grain size of remelting pools is finer, and the microhardness is higher than that of the matrix, which makes crack propagation more difficult. In addition, the results show that the strengthening layer has compact and flawless microstructure and has been metallurgically integrated with the matrix, and the microhardness of the regions treated by laser cladding and laser remelting-cladding has been improved obviously. Toughness has improved, as has the problem of toughness reduction after cladding ceramic particles. The sample's strength is also significantly greater than that of the untreated sample.

Al-20Si/Cu-8P high‑silicon aluminum alloy coating enhanced by laser remelting and refinement: Microstructure, mechanical properties and wear resistance

An Al-20Si/Cu-8P high‑silicon aluminum alloy coating was prepared on A356 aluminum alloy by plasma spraying and was remelted by laser technology subsequently. Whereupon, the microstructure, mechanical properties and wear resistance of the coating before and after remelting were comparatively studied, aiming to investigated the combined enhancement effect of remelting and refining on high‑silicon aluminum alloy coatings. The results showed that the laser remelting basically eliminated the defects in the sprayed coating, and produced a metallurgical bonding interface between the coating and the substrate. The microstructure of the coatings before and after remelting was both composed of primary silicon and eutectic structure (α-Al + Si). Nevertheless, the Cu8P dissolved more fully and dispersed more evenly in the remelted coating, which was conducive to fully utilizing its refinement effect on primary silicon. Thus, the primary silicon in the remelted coating was obviously reduced in size compared with that in the sprayed coating, and its distribution was more even. Accordingly, the hardness, elastic modulus and fracture toughness of the remelted coating were greatly enhanced compared with the sprayed coating. Under dry friction condition, the wear resistance of the two coatings was significantly superior to that of the substrate. Compared with the sprayed coating, the friction coefficient of the remelted coating was slightly smaller, and the wear volume was reduced by 33.1 %. Therefore, the microstructure of Al-20Si/Cu-8P high‑silicon aluminum alloy coating was significantly improved and its properties was obviously enhanced by laser remelting and refinement.

Laser re-melting of modified multimodal Cr3C2–NiCr coatings by HVOF: Effect on the microstructure and anticorrosion properties

Abstract Modified multimodal (MMP) Cr3C2–NiCr coatings were fabricated by high-velocity oxy-fuel (HVOF) spraying deposited on a CuCrZr alloy substrate. However, due to the lack of its inevitable porosity, an additional laser re-melting (LRM) approach is highly required to improve the coating performance. Therefore, the LRM technique is employed in this study to improve the microstructure properties of an MMP Cr3C2–NiCr coating by HVOF. Solid-state phase transformation from Cr3C2 to Cr7C3 occurred during the LRM process. After the LRM process, the coating exhibits the presence of Cr3C2 nanoparticles that serve as reinforcement. These nanoparticles demonstrate minimal lattice misfit and exhibit high stability throughout the LRM process. The surface of the coating undergoes modification, resulting in the formation of homogeneous nano (20–130 nm), micron (150 nm to 0.3 μm), and submicron (2–3 μm) Cr3C2 structures, along with high-density microstructures, after the LRM process. Nano-Cr3C2 particle reinforced with high total work function and incredibly increased corrosion rate significantly improves coating corrosion resistance. Overall, porosity decreased from 3.9% of the HVOF coating to 0.3% of the LRM. As a result, the current density of anticorrosion decreased from 33.7 to 4.35 µA·cm−2, and the Vickers microhardness average values ranged from 1,050 to 1,300 HV0.3, indicating improved microstructure development and related properties.

Fe-Mn-Al-Ni Shape Memory Alloy Additively Manufactured via Laser Powder Bed Fusion

Fe-Mn-Al-Ni is an Fe-based shape memory alloy (SMA) featuring higher stability and low temperature dependency of superelasticity stress over a wide range of temperatures. Additive manufacturing (AM) is a promising technique for fabricating Fe-SMA with enhanced properties, which can eliminate the limitations associated with conventional fabrication and allow for the manufacture of complicated shapes with only a single-step fabrication. The current work investigates the densification behavior and fabrication window of an Fe-Mn-Al-Ni SMA using laser powder bed fusion (LPBF). Experimental optimization was performed to identify the optimum processing window parameters in terms of laser power and scanning speed to fabricate Fe-Mn-Al-Ni SMA samples. Laser remelting was also employed to improve the characteristics of Fe-Mn-Al-Ni-fabricated samples. Characterization and testing techniques were carried out to assess the densification behavior of Fe-Mn-Al-Ni to study surface roughness, density, porosity, and hardness. The findings indicated that using a laser power range of 175–200 W combined with a scanning speed of 800 mm/s within the defined processing window parameters can minimize the defects with the material and lead to decreased surface roughness, lower porosity, and higher densification.

The forming quality of Tungsten-Carbide coatings synthesized in situ by laser remelting

The work aimed to improve the forming quality of multi-pass coatings synthesized by laser remelting in situ. The influence-relation model of the overlap rate on surface flatness and dilution rate of coatings was constructed by the single-factor method. The functions of laser power, scanning speed, defocusing distance, and scanning mode on the surface flatness and pore area of remelted coatings were studied by mixed horizontal orthogonal test, and the influence law of process parameters on surface flatness and internal pore area was revealed. Flatness increased first and then decreased with the increased overlap rate. Energy acting on the substrate decreased, and the dilution rate decreased. When the dilution rate was satisfied, the overlap rate corresponding to the maximum value of the flatness fitting curve was 26.7%. The scanning speed and laser power of remelting significantly affected coating flatness and the signal-to-noise ratio of the pore area. The signal-to-noise ratio of flatness to the pore area was positively correlated with laser power and negatively correlated with the scanning speed. When the scanning speed of remelting decreased, the duration of the molten pool increased, and its solidification rate decreased. Therefore, bubbles in the molten pool had sufficient time to overflow, which increased coating flatness and decreased the internal pore area.The optimum process parameters under multi-index optimization of integrated flatness and pore area were as follows using grey relational analysis: the laser power of 1,600 W, scanning speed of 3 mm/s, a defocusing distance of 10 mm, and parallel cladding direction. Sticky powders in the coating disappeared under the optimal remelting process parameters, and flatness and the internal pore area were improved. Maximum hardness was increased from 1,281.7HV to 1,454.2 HV, with an increase of 13.46%. The grain size was small and hardness was improved due to the increased tissue density in the remelting area of the coating.

Effects of laser remelting on microstructure and tribological properties of plasma–sprayed Fe45Mn35Co10Cr10 high–entropy alloy coating

To further improve the Fe45Mn35Co10Cr10 high–entropy alloy coating prepared by plasma spraying, laser remelting was used to improve its structures and tribological properties. The effects of laser remelting on the microstructure, element composition, phase and hardness of obtained coating were investigated using a super depth of field microscope, energy dispersive spectrometer, X–ray diffraction and hardness tester, respectively. The tribological properties were measured via a ball–on–disk tester at room temperature and 500 °C, and the wear mechanisms were also discussed. The results show that the laser–remelted HEA coating is still composed of face–centered cubic structure, and its hardness is increased by ~301.4% compared with the plasma–sprayed coating, which is attributed to the grain refinement and solid solution strengthening effects. The average coefficient of friction and wear rate of laser–remelted HEA coating is superior to the plasma–sprayed HEA coating, showing that the laser remelting process plays the roles of friction reduction and wear resistance. The wear mechanism of plasma–sprayed HEA coating at room temperature is fatigue wear, abrasive wear and oxidative wear; while that of laser–remelted coating is abrasive wear and oxidative wear. Moreover, the wear mechanism of plasma–sprayed coating at 500 °C is adhesive wear, abrasive wear and oxidative wear, while that of laser–remelted HEA coating is adhesive wear and oxidative wear, which is attributed to the reduction of plastic deformation by the softening effect at high temperature.

Tailoring the microstructure and mechanical properties for Hastelloy X alloy by laser powder bed fusion via scanning strategy

Irregular large pores induced by laser powder bed fusion (LPBF) are inherent defects, which greatly domain the mechanical properties of as-built components. Laser remelting is an effective method to diminish the porosity and increase the energy input, while excessive heat accumulation is generally at the expense of powder spatter. In the study, LPBF-fabricated Hastelloy X alloy was built by combining laser remelting and five scanning strategies (three meander and two chessboard strategies) to investigate the scanning strategy effects on the related microstructure and mechanical properties via experiment and simulation. In comparison, results showed that the average temperature of the powder model rises from 622.02 K to 880.88 K, and the cooling rate approximately increases from 106 K/s up to 107 K/s for the chessboard strategy. The chessboard strategy with a large scanning island yields the highest density (99.34%). More homogeneous grain also nucleates due to the evident 〈111〉 micro-texture distribution. The refined microstructure achieved by the chessboard strategy with laser remelting leads to a superior combination of tensile strength (∼820 MPa) and elongation (∼30%). Grain boundary strengthening and dislocations hardening are the dominant mechanisms of as-built Hastelloy X alloy.

Plasma sprayed Al2O3–40%TiO2 coating by laser remelting: Structural evolution, tribological properties and DFT calculation

A plasma sprayed Al2O3–40%TiO2 coating was processed by laser remelting to improve its tribological properties. The effect of laser remelting on the tribological properties of obtained coatings was investigated using a ball–on–disc wear tester, and the wear mechanisms were discussed by the density functional theory calculation. The results show that the phase of plasma sprayed Al2O3–40%TiO2 coating is changed from TiAl2O5 to Al2O3/Ti3O5 by laser remelting, and the grains of laser remelted Al2O3–40%TiO2 coating become refined to improve the coating hardness. The average coefficient of friction and wear rate of plasma sprayed Al2O3–40%TiO2 coating is decreased by laser remelting, which is attributed to the higher binding energy of Al–O in laser remelted Al2O3–40%TiO2 coating.

Correction to: Effect of layer‑by‑layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Correction to: Effect of layer‑by‑layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Intelligent Additive Manufacturing Architecture for Enhancing Uniformity of Surface Roughness and Mechanical Properties of Laser Powder Bed Fusion Components

In the Laser Powder Bed Fusion (L-PBF) process, 3D components with complex geometries are fabricated in a layer-by-layer fashion by using a controlled laser beam to selectively melt particular regions of the metal powder bed. However, due to the stochastic nature of the L-PBF process, the top surface roughness of each solidified layer tends to be different even when the optimal processing conditions for the different positions on the build plate are employed. As a result, the mechanical properties of the built components frequently vary from one component to the next. Accordingly, this study proposes an Intelligent Additive Manufacturing Architecture (IAMA) for controlling the surface roughness of each build layer through an appropriate adjustment of the laser re-melting parameters. The IAMA architecture comprises five modules, namely In-Situ Metrology (ISM), Ex-Situ Metrology (ESM), Automatic Virtual Metrology (AVM), Additive Manufacturing Simulation (AMS) and Intelligent Compensator (IC). The feasibility of the proposed architecture is demonstrated by comparing the top surface roughness of cubic and mechanical strengths of tensile test samples built using the proposed method with those built using a traditional L-PBF approach without surface roughness control. It is found that the samples fabricated using the IAMA approach have an average top surface roughness of 1.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m and a standard deviation is 0.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m. By contrast, the samples produced using the traditional L-PBF approach have an average surface roughness of 13.45 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m and a standard deviation of 2.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m. In addition, the specimens produced with the assistance of IAMA architecture have an average tensile strength of 1013 MPa with a standard deviation of 69.5 MPa, while those printed without surface roughness control have an average tensile strength of 903 MPa with a standard deviation of 101.4 MPa <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —As L-PBF produce part in a layer-by-layer manner, therefore, the roughness on the top surface of previous layer have a strong influence on the printing quality of current layer. The variations of surface roughness will lead to the fluctuation of mechanical properties of the fabricated components. Additionally, to the best of author’s knowledge, current commercial L-PBF machines can not actively control the surface roughness of samples during the process. The proposed IAMA in this work can predict and control the roughness on the top surface of each layer during L-PBF process. Therefore, the constructed architecture has strong potential for integrating into the commercial L-PBF machine for controlling the surface roughness on the top of the parts during the manufacturing process. As a result, the quality of the fabricated components is expected to be in consistent. Accordingly, L-PBF machine equipped with IAMA will have a strong potential in applying for mass production of the components in aerospace and automobile industries.

Material Health of NiCrBSi Alloy Parts Produced via the Laser Powder Bed Fusion Process

Laser powder bed fusion (L-PBF) is a novel process representing a possible solution for producing resistant parts using NiCrBSi hard-facing nickel alloys with complex geometry. Process parameters for more common alloys are explored with a standard Renishaw AM400 device (Renishaw, Wotton-under-Edge, UK) and an SLM Solution 250 device (SLM Solutions Group AG, Lübeck, Germany) modified with a baseplate preheated at high temperatures (300 °C and 500 °C). Laser remelting is also investigated in hopes of further improving material health. The origin of the main defects is studied. A lack of fusion is likely to be generated by spatters ejected from the melting pool while cracks are induced by the alloy’s lack of toughness. Using image analyses, those defects are quantified and correlated with processing parameters. Lack of fusion and total crack length decrease with an increase in baseplate’s preheating temperature. However, crack width increases with preheating temperature. Therefore, via a careful optimization of process parameters, samples with a surface density of 99% and narrow cracks are obtained.

Effect of microstructure and chemical composition on cell orientation and proliferation viability of titanium alloy processed by pulse lasers

Samples with different surface microstructures and elemental composition were fabricated by pulsed lasers with different pulse duration, including the millisecond laser remelting (RE), the bioceramic coating (BC), and nanosecond laser processing (NL) samples. The morphologies, chemical elements, phases, and valences of these samples were analyzed and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The contact angles of the samples were measured in air and under water. Their cell activity and behavior were analyzed by cell proliferation and cytoskeleton staining experiments. The results show that the hydrophilicity and aerophobicity increase after laser treatment, and the wetting behavior of underwater bubbles is strongly related to the wettability of the sample in the air. None of the samples examined in the experiment are cytotoxic. During the initial culture, the greater unevenness of the BC and NL sample surfaces hinders the initial expansion of the cells. In contrast, the cell spreading area of the ground (G) and the RE samples is larger. The inclined sidewall in the V-shaped structure weakens the energy barrier in the groove-ridge combination (NL sample), allowing cells that rarely adhere to the sidewall to adhere successfully. In addition, the orderly distribution of adhesion sites created by laser processing further promotes cell adhesion on the inclined sidewalls. When the cells have been cultured to Day 7, the NL sample forms a clear directional distribution of cells with its unique V-shaped structure mimicking the placoid scale on the shark skin surface.

Microstructure, Mechanical Properties and Oxidation Behavior of Refractory Multi-principal Element Alloys by Laser Remelting and Conventional Manufacturing

Microstructure, Mechanical Properties and Oxidation Behavior of Refractory Multi-principal Element Alloys by Laser Remelting and Conventional Manufacturing