Laser Molten Pool Research Articles

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF LASER SOLID FORMED NICKLE BASE SUPERALLOY INCONEL 718 PREPARED IN DIFFERENT ATMOSPHERES

Due to the rapid melting and solidification in laser molten pool,the laser solid forming (LSF) samples have generally much finer microstructure than that of the conventional cast,and are also free of macrosegregation.To date,more and more researchers have reported the application of LSF technology in the forming of superalloy,which contains some easily segregated elements such as Nb,Ti and Al.To prevent the molten pool from oxidation,the LSF process usually should be performed within a protective atmosphere.But in practice,the forming of oxidation-resistant metals are always performed in the air atmosphere and only the molten pool is protected by noble gas. Therefore,it is important to investigate the microstructures and properties of LSF materials prepared in the air and noble gas.In the present study,the microstructures,tensile properties and high cycle fatigue properties of LSF Inconel 718 alloy samples prepared in the air and argon atmospheres were investigated.It is indicated that the microstructures in as-deposited and heat-treated LSF Inconel 718 alloy samples prepared in both atmospheres are similar.The as-deposited microstructure consists of columnar dendrites grew epitaxially along the deposition direction from the substrate,and changes to the equiaxed grains after solution heat treatment due to the recrystallization.The tensile properties of the two kinds of the samples after heat treatment reached the standard for wrought Inconel 718 alloy. The tensile strength of the sample prepared in air is slightly higher than that of the sample prepared in the argon atmosphere,and the high cycle fatigue lifetime of the latter is 30%higher than that of the former,which is related to the fewer defects such as micro pores and small size oxide inclusions existed in the former.The lower high cycle fatigue property of LSF Inconel 718 alloy than that of wrought Inconel 718 alloy is attributed to the metallurgical defects and the coarsening and uneven distribution of the recrystallized grains.

Design of Forming Shape Measurement System for Laser Molten Pool in Laser Fabricating

In order to study forming shape distribution of laser molten pool in laser fabricating, a CCD-based image detection system was developed. The system mainly consists of a CCD-based detector, optical elements, an image acquisition card and its special software. Experiments on shape measurement of the laser molten pool were carried out. It was shown that 2D morphology and the dimension of laser molten pool could be measured in real time. The (X, Y) dimension of the laser molten pool in laser cladding was (2.6 mm, 2.5 mm) with an approximately ellipsoidal shape. Laser cladding parameters were optimized according to the measurement results. It was a non-destructive measurement method, which would be a new method for online measurement and feedback control in laser fabricating.

基于数字信号处理器的图像处理系统在激光熔池温度场检测中的应用

基于数字信号处理器的图像处理系统在激光熔池温度场检测中的应用

Measurement of Temperature Field in Laser Molten Pool by CCD Based on DSP

In this paper, a measurement system of temperature field in laser molten pool by CCD based on DSP is presented. The paper also presents the system of hardware, software process, and the method of image processing by DSP. To solve the real time problem in the processing of measuring the temperature field, the system uses DSP as the main CPU, processing the image without a computer. By this method, the system can become simple and efficient and measure the laser molten pool temperature field quickly and exactly. Gradient image of temperature field is displayed on LCD after real time processing by this system. The final picture of temperature field can be directly analyzed.

Study on Measurement of Melting Process of Molten Pool Formed by Laser Scanning Mirror

The profile and temperature distribution of laser molten pool are mainly factors which have directly effect on quality of laser processing (laser melting and laser cladding). It is very necessary to study measurement method of melting process in laser molten poo1 on-line, for improving processing design and inspecting laser processing quality. A system of dynamic process measurement for laser molten pool was developed. The melting process of laser scanning molten pool formed by high power CO2 Laser was measured. Its temperature distribution was analyzed by special analysis software. It was shown that a section of integrated molten pool would come into being in the middle of the laser scanning line spot after a period of scanning time, and then the molten pool got increased in length with the time, and a little increased in width at the same time, the result was consistent with that of computer numerical simulation. Compared with laser focusing spot, laser scanning spot was more uniform in temperature distribution, that could be propitious to improve the quality of laser processing.

Effect of Ni contents on the microstructure and mechanical properties of TiC–Ni cermets obtained by direct laser fabrication

Abstract TiC–Ni cermets have been produced using direct laser fabrication on a low carbon steel substrate, with 20, 30 and 40 vol.% Ni, respectively. The effects of Ni content on the microstructure and mechanical properties of TiC–Ni cermets were investigated. The TiC particles were produced by an in-situ reaction of titanium and graphite during the direct laser fabrication, instead of being directly injected into the laser molten pool. The results showed that in the cermets only TiC and Ni were present as phases, indicating that TiC was synthesised by the in-situ reaction of Ti and C in the powder mixture and the amount of Ni content did not influence the presence of phases in the final products. Moreover, the results revealed that the increase of Ni content will result in the decrease of bending strength and micro-hardness.

双方程边界偶合法激光熔覆传质过程数值模拟

The coupled numerical simulation on fluid flow, heat transfer, and mass transfer in the process of laser cladding is undertaken on the basis of the continuum model. In the simulation of mass transfer in the laser molten pool, the concentration distribution in the regions on different sides of the interface between cladding layer and substrate is calculated separately and coupled at the co-boundary. The non-equilibrium solute partition coefficient is obtained from equilibrium solute partition coefficient according to the Sobolev model. By using the developed software which is based on the commercial software PHOENICS 1.4, the distribution of Fe in laser molten pool in an experiment of cladding Stellite 6 on 12CrMoV is calculated. The obtained results well coincide with the experimental ones.

Absolute stability of the solidification interface in a laser resolidified Zn–2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn–2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210 mm/s. An implicit expression of the critical solidification velocity for the cellular–planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M–S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652 mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the η phase. This indicates that HGAS was successfully achieved in the present experiments.

Three-dimensional analytical model on laser-powder interaction during laser cladding

To compute the attenuation of laser power by powder stream and the temperature distribution of the powder particles arriving at different sites on the surface of the workpiece, a three-dimensional analytical model was presented based on Lambert-Beer theorem, Mie’s theory, and heat equilibrium principle. The obtained results revealed that for a given stream spread and speed of powder particles, the laser intensity distribution of laser spot and the temperature distribution of the powder particles reaching the surface of workpiece are both greatly affected by the powder feeding rate and the feeding angle. It was suggested that the presented results should be considered in the laser cladding process in order to control the dynamic processes in laser molten pool.

Effects of powder feeding rate on interaction between laser beam and powder streamin laser cladding process

A theoretical model was presented to calculate the laser intensity distribution and the particle temperatures at different sites of the workpiece in the laser cladding process. By using this model, the effects of the powder feeding rate on the laser intensity distribution and the particle temperatures were investigated, the calculated results under the condition of different injection angles were also plotted. It is shown that with increasing the injection angle, the laser intensity distributions are similar but the peak value of the laser intensity decreases. Simultaneously, the peak value of the particle temperature increases and the distribution of the particle temperatures gets central symmetrical gradually. These tests results should be considered in model of laser cladding due to their subtle effects on the dynamic processes in laser molten pool.

MODELING OF TURBULENT TRANSPORT IN LASER SURFACE ALLOYING

A three-dimensional, transient model is developed in order to address the turbulent transport in a typical laser surface alloying process. Turbulent melt-pool convection is taken into account by using a suitably modified high-Reynolds-number k–ϵ model in the presence of a continuously evolving phase-change interface. The phase-change aspects of the problem are addressed using a modified enthalpy-porosity technique. This newly developed mathematical model is subsequently utilized to simulate a typical high-power laser surface alloying process, where effects of turbulent transport can actually be realized. In order to investigate the effects of turbulence on laser molten pool convection, simulations with laminar and turbulent transport are carried out for some problem parameters. Significant differences in the molten pool morphology are observed on comparing the laminar and turbulent simulation results. It is also revealed that turbulent simulations yield a much better match with the experimentally obtained species concentration distribution within the alloyed layer, compared to that in corresponding laminar simulations.

Interaction between laser beam and powder stream in the process of laser cladding with powder feeding

Based on Lambert–Beer theorem and Mie's theory, the attenuation of laser power by a powder stream was calculated and the laser intensity distribution at the laser spot on the surface of a workpiece was obtained. Simultaneously, the temperature distribution of the powder particles arriving at different sites on the surface of the workpiece was computed following the heat equilibrium principle. The computed results revealed that for a given stream spread and speed of powder particles, the intensity distribution of the laser spot and the temperature distribution of the powder particles reaching the surface of the workpiece are both greatly affected by the powder feeding rate and the feeding angle. It is suggested that the presented results should be considered in the laser cladding process in order to control the dynamic processes in a laser molten pool.

Development of microstructures in laser surface remelting of DD2 single crystal

Detailed experiments were carried out to investigate the effects of crystallography on the microstructure evolution during the solidification of the laser molten pool with a DD2 single crystal specimen using a 5 kW CO 2 laser. The experimental results showed that crystallographic effects can influence dendrite growth by favoring growth along a preferred crystallographic direction, and crystallographic effects are greater than that of heat flux during dendrite growth. On the same crystal face, the microstructures in the molten pool are very different when laser beam scanning along different crystal orientation. The same conclusions can be obtained by laser beam scanning along a fixed direction on different crystallographic surface. A growth velocity analysis was carried out during laser surface remelting processing based on a minimum velocity or minimum undercooling criterion and the relationships between dendrite growth velocities in the three <100> directions, moving velocities of the solid/liquid interface and laser scanning velocities for the different crystallographic orientations were also set up. According to this analysis, the reconstructed and predicted molten pools concurred with the experimental results very well.

On cellular spacing selection of Cu-Mn alloy under ultra-high temperature gradient and rapid solidification condition

In order to study the cellular/dendritic spacing selection under ultra-high temperature gradient and rapid solidification condition, laser surface remelting technology have been adopted for its special advantages. During laser surface remelting, there is an extremely high temperature gradient (>10{sup 6}k/m) in the laser molten pool, which leads to directional growth of various microstructures. Two main advantages of the experiment are (1) no nucleation is involved, i.e., epitaxial growth; (2) the local growth rate and the corresponding microstructure can be determined directly. In the present work, detailed experiments have been carried out to investigate the cellular/dendrite spacing selection of Cu-Mn alloy under ultra-high temperature gradient and rapid unidirectional solidification condition, and the experiment results are compared with KGT model.

Microstructure evolution of Cu–Mn alloy under laser rapid solidification conditions

Laser rapid solidification experiments were performed on a Cu–Mn alloy to investigate its microstructure evolution as a function of growth rate. By taking transverse and longitudinal sections of the laser traces, the resulting morphology and the corresponding growth rate were quantitatively measured by scanning electron microscopy (SEM) and image analysis. As the growth rate increased, the morphological transition from dendrite→fine cellular crystal→banded structure→fully cell-free microstructure, involving planar front growth, was clearly observed in the laser molten pool. By determining these transition conditions and comparing them with the predictions of theoretical models, we found that the origin of the banding can be well elucidated by the current Carrard–Gremaud–Zimmermann–Kurz (CGZK) phenomenological model, while the critical condition of planar front growth was in reasonable agreement with Mullins–Sekerka (M–S) stability theory.