YAG Laser Melting Research Articles

In-situ synthesis of TiC/Ti alloyed layer via pulsed Nd:YAG laser melting of Ti-6Al-4V titanium with preplaced carbon-based powder

The present research deals with fabricating an in-situ TiC/Ti alloyed layer on the Ti-6Al-4V titanium alloy through the preplacement of a mixture of graphite and nickel powders and subsequent pulsed Nd:YAG laser melting. The comprehensive effects of laser parameters, including scanning speed and energy density on the microstructural evolutions and hardness variations were investigated. It was found that the geometry of treated layers varied from 150 to 200 µm in thickness and 1200–1750 µm in width depending on the thickness of the preplaced layer and laser peak power. The promising results of X-ray diffraction patterns and scanning electron microscope observations manifested the formation of in-situ TiC and an increase in its content versus the energy density of the laser beam. The main mechanism of the in-situ formation of TiC in the matrix during laser irradiation was also discussed. The results indicated that the in-situ TiC phases had either dendritic or needle-like morphology. Laser parameters were crucial in establishing the TiC morphologies associated with temperature and chemical composition. The hardness values of the fabricated layers were directly proportional to the amount of TiC in the treated layers and varied from 1000 to 1400 HV0.3, which was nearly 3–4 times higher than that of the substrate.

Experimental Validation of a Laser Heat Source Model for Laser Melting and Laser Cladding Processes

Selective laser melting (SLM) and laser cladding are laser additive manufacturing methods that have been developed for application to the near-net-shape process and 3D printing. The temperature distributions and track profiles of SLM and clad layers require additional in-depth investigation to optimize manufacturing processes. This research involved developing a tailored laser heat source model that contains a comprehensive selection of laser beam characteristics and can be used in finite element analysis of the laser melting process. This paper presents a systematic experimental validation of the applicability of the proposed laser heat source model to single-track Nd:YAG and CO2 laser melting simulations. The evolution of the melt pool isotherms and the variation in track profiles caused by adjusting the laser power and scanning speed were numerically predicted and experimentally verified. Appropriate process parameters and the threshold power for continuous track layer formation were determined. The balling phenomenon on preplaced powder was observed at power levels below the threshold values. Nd:YAG laser melting resulted in a wide and shallow track profile, which was adequately predicted using the numerical simulation. CO2 laser melting resulted in a triangular track profile, which deviated slightly from the finite element prediction. The results indicated a high level of consistency between the experimental and the numerical results regarding track depth evolution, whereas the numerically predicted track width evolution deviated slightly from the experimentally determined track width evolution. This parametric laser melting study validated the applicability of the proposed laser heat source model in numerical analysis of laser melting processes such as SLM and laser cladding.

Hyperdoping of silicon with deep-level impurities by pulsed YAG laser melting

Hyperdoping with deep-level impurity is a promising method to prepare intermediate band semiconductors. We prepared silicon hyperdoped with deep-level impurities, sulfur and titanium, by ion implantation followed by pulsed YAG laser melting. The processes of sulfur and titanium hyperdoping are comparatively studied. The amorphous sulfur and titanium ion-implanted layers changed to monocrystal by following pulsed laser melting. The depth profile of sulfur impurity after pulsed laser melting is similar to that of ion-implanted sample, while large segregation is observed for titanium hyperdoping. The crystallinity and degree of segregation depend on the laser shot number and initially implanted titanium dose. There is a trade-off between crystallinity and depth profile of impurity for titanium hyperdoping. From a viewpoint material processing, formation of high-quality silicon monocrystal hyperdoped with sulfur is easier than that with titanium. Correlation between the mid-infrared optical absorption and photoconductivity is also discussed for sulfur-hyperdoped sample.

Surface modification of plasma sprayed Al2O3–40 wt% TiO2 coatings by pulsed Nd:YAG laser melting

Plasma sprayed Al2O3–40wt% TiO2 coatings were subjected to a pulsed Nd:YAG laser melting with power densities of 640 and 800kW/cm2 in order to minimize the surface porosity, micro-cracks, partially melted or unmelted regions, and achieving homogeneous microstructure. As-sprayed and the laser melted surface microstructure, chemical composition, microhardness and roughness were characterized by using scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, Vickers hardness and profilometer. Surface morphology of a laser melted coating showed that inhomogeneities are reduced, however, network of cracks were formed irrespective of power density. Columnar growth features were observed in laser melted regions when melted with 640kW/cm2 while these were eliminated when melted with in 800kW/cm2 power density. Predominant β-Al2TiO5 phase was observed upon both the laser melted coatings. With an increase of laser power density, laser melted coatings exhibited considerable increase in microhardness and decrease in surface roughness due to significant reduction in the surface defects.

Study on feasibility of producing an amorphous surface layer of Fe49Cr18Mo7B16C4Nb3 by pulsed Nd:YAG laser surface melting

This work aims to investigate whether an amorphous surface layer can be obtained when as-cast Fe49Cr18Mo7B16C4Nb3 alloy is submitted to pulsed Nd:YAG laser surface melting. The experiments were conducted in the various laser scanning speeds. The microstructures of laser treated zones were investigated by X-ray diffraction XRD and Field Emission Scanning Electron Microscope (FESEM) and their microhardness were measured, too. The chemical composition of different points of each sample was analyzed by energy-dispersive X-ray spectroscopy EDS. Although the estimated cooling rates in surface layers were higher than the required cooling rate to achieve full amorphization, but the present experiments were unable to retain complete glassy microstructure on surface and a mixture of amorphous (low volume fraction) and ultrafine grained phases were produced in surface of samples. Based on the findings, it was understood that the overlapping of successive pulses and element redistributions occurred in pulsed laser melting could severely restrict amorphization. The influence of laser scan speed and laser power on heat input, melting ratio, compositional changes and cracking in laser treated zone were discussed separately. It is suggested that the limited range of laser variables in pulsed Nd:YAG laser melting may help to produce a sound amorphous phase of as-cast Fe49Cr18Mo7B16C4Nb3 alloy.

Local Structures and Decomposed Phases of Laser-Melted ZrSiO<sub>4</sub>

Behaviors of CO2 and pulsed YAG laser melting of ZrSiO4 have been investigated using infrared (IR) spectroscopy. The laser-melted ZrSiO4 decomposed into binary oxides (ZrO2 and SiO2), while other phases or complex ZrSiO4 tetrahedron networks are also observed. The local structures and the phases of the quenched melts depend strongly on the quenching rate and melting conditions. Monoclinic ZrO2 are found to be the main ZrO2 phase in the samples treated by CO2 laser, although tetragonal ZrO2 was found near boundaries between the untreated and melted regions. High concentrations of tetragonal ZrO2 were detected in the samples treated by pulsed YAG lasers. Observations indicate that the formation of tetragonal ZrO2 is related to relatively high quench rates. Micro-IR data from areas near the boundaries between the quenched melts and untreated zircon show systematic variations of local structures and compositions. A small region with relatively low density between the untreated and melted boundaries was observed, which consists of tetragonal ZrO2 or glassy ZrO2, and SiO2. Broad vibrational bands occur in the wave number region where the characteristic frequencies of zircon are located. This observation could indicate the possible existence of small amounts of glassy ZrSiO4 in the melt state of zircon, although zircon tends to decompose above the melting point.

Effect of pulsed Nd:YAG laser melting treatment on microstructural and corrosion behaviour of AZ91C Mg alloy

In the present study, an attempt was made to improve the corrosion resistance and microhardness of a cast magnesium–aluminium–zinc alloy by laser surface melting. A 400 W pulsed Nd:YAG laser was used to melt and rapidly quench the surface of the cast AZ91C alloy to improve the corrosion resistance. This was evaluated using potentiodynamic polarisation and EIS studies. The microstructure of the melt pool was examined using optical microscopy and scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS), X-ray diffraction studies (XRD) and X-ray fluorescence (XRF). The work revealed that laser melting improves the corrosion resistance of as cast AZ91C alloy because of its fine microstructure and extensive solubility of aluminium in eutectic (β phase). The microhardness of the laser surface melted layer was also increased from 74 HV to 99–124 HV.

Some characteristics of YBa2Cu3Oy superconducting material obtained by arc-casting and laser melting

A superconducting Y-Ba-Cu-O with high Tc was prepared by arc casting to examine its effect on the crystalline phases in the annealed samples. X-ray diffraction (XRD) studies were carried out to investigate the orthorhombic superconducting phases. An a.c. susceptibility study was carried out and image analysis was conducted to obtain the density and grain size of the products. In addition, Nd:YAG laser melting of samples was carried out. It was found that the main crystalline phase was orthorhombic YBa2Cu3Oy and superconducting 1:2:3 crystalline phases were easily obtained by annealing in oxygen ambient and through laser melting.

Laser melting of Ti-15Al-20Nb alloy

Titanium aluminides have become promising materials for high-temperature applications, but the relatively poor oxidation resistance and elevated- temperature strength of these alloys limit their application to temperatures lower than 1000 °C. Niobium addition improves the properties of titanium aluminide. However, the mechanical, metallurgical, and corrosion properties of Ti- Al- Nb may be improved by treatment with a laser beam. Consequently, the present study examines the properties of Ti- 15Al- 20Nb alloy subjected to the Nd:YAG laser melting process. Hardness in the surface region increases to twice the base material hardness, and corrosion resistance improves considerably after laser treatment.