Concentration mode of the powder stream in coaxial laser cladding
Introduction
The blown powder laser cladding is one of the rare processes which is capable of giving a fused bond with low dilution over large or small areas. It has thus found applications in industry and is the subject of much research around the world. Most work to date has used an off axis powder feed system to avoid the powder melting and sticking to the feed nozzle or oxidising prior to cladding. However, a coaxial feed would make this promising process omnidirectional and therefore easier to use. This would open up new possibilities for repair and for the newly developed laser direct casting process (LDC) [1], [2]. In this new technique, 3D metal components can be directly built from a CAD drawing, suggesting a new route for low volume manufacturing.
The stream structure of the coaxial jets has been the subject of many experimental studies and reported in extensive literature reviews of the available coaxial jet mixing data for gaseous systems [3], [4]. However, it should be pointed out that until very recently it was impossible to find in the literature a well-documented experimental study of powder streams for coaxial laser cladding. The level of corresponding information relating to two-phase turbulent coaxial jet, especially the effects of the laser heating, nozzle geometry, particle loading ratio, shield gas velocity and particle size on the structure of two-phase coaxial jets (namely mean and fluctuating velocity distributions of both phases, particle concentration and particle size distributions), are virtually non-existent for laser cladding.
In this article, image analysis and optical sensor methods, which are based on the light scattering and attenuation on the particles, were adopted here to detect the structures of cold powder stream from a coaxial laser cladding nozzle. The aim of the cold flow experiments was to measure the concentration mode in the coaxial flow with various settings of nozzle exit and shield gas velocity. The results could provide a good understanding of the parameter selection in coaxial laser cladding.
Section snippets
Analysis of the concentration mode of powder stream
For a continuous source as illustrated in Fig. 1, a steady concentration distribution of particles from a line source along the y direction with a travel speed, V, in the z direction in a medium could be established. However, the exact calculation of this problem is difficult. The subject of analysing the relative magnitudes of convective and diffusive transfer of line source particles in a medium has been discussed in Ref. [5], from which the following approach is taken.
The particle
Measurement of the powder concentration
The experimental arrangement is shown in Fig. 2. A CCD camera with image analysis system was used in this work. The shutter speed for the flow image was set at 50 frames/s. A cylindrical lens (fused silica, 60×30 mm2, focal length of 100 mm) was used to deliver a plane light sheet from a tungsten lamp across the powder stream to illuminate the powder concentration without turning on the CO2 laser. The powder material was 304L stainless steel of a diameter range from 45 to 105 μm at an average
Light scattering through the powder stream
When a beam of light illuminates an assembly of transparent particles, some of it is transmitted, some absorbed and some scattered. The scattered radiation includes the diffracted, reflected and refracted parts of the original beam and the absorbed radiation in re-transmission at a longer wavelength which is usually not picked up by the detecting device.
In the experiments the CCD camera receives light scattered by a number of particles, simultaneously illuminated in an optical sensing volume,
Light attenuation in the powder stream
If a light beam falls on an assembly of macroscopic particles the attenuation is given byBy integrating Eq. (6) with constant powder properties, it becomes, are known as the Bouguer–Beer law, where I is the intensity of the light beam on the sensor, I0 is the intensity of the light beam first entering the medium, n is the number of the particles per unit volume, A is the mean particle cross-sectional area, E is the extinction coefficient and l the powder medium
Image analysis of the powder streams
The powder concentration along the transverse direction was visualised by a CCD camera from a 45° projecting angle to a light sheet crossing the powder stream as illustrated in Fig. 2. The light sheet has an average thickness of 2 mm and was arranged to cross the powder steam at various stand-off distances. Typical visualisation results are shown in Fig. 5. Detailed concentration profiles of the powder stream were evaluated by image analysis.
Based on the scattering effects of the illuminated
Results of luminance analysis
The concentration of cold powder stream in the transverse direction was analysed on the image taken by the CCD camera, which focused on the stream centre with a stand-off distance of 20 mm. The variation of the luminance intensity at the centre line of the image was analysed. Due to the light scattering phenomenon, the light intensity is a function of the wave length, the view angle, the particle size, the shape and its number as expressed in , . Integrating the scattered light intensity in the
Scanning powder sensor
In order to directly measure the powder concentration of the jet stream, a scanning sensor was used as shown in Fig. 7, which is based on a photo detector (response wavelength 400–1300 nm) with a 3 mW He/Ne laser (λ=0.63 μm) and a convex lens (focal length=12 mm). The sensor was designed to measure the spatial distribution of the powder concentration. Detailed specifications of the photo diode are listed in Table 1. The basic instrument design uses a silicon photo diode to receive the radiation
Scanning sensor calibration
The sensor was calibrated by a device, where a copper tube with an inner diameter of 3 mm was directly connected to the powder feeder. The sensor was arranged to measure the powder concentration at the centre line of the copper tube. Assuming a uniform distribution of powder in the tube, the mean concentration could be calculated by dividing the mass flow rate of the powder by the flow rate of the carrier gas. The signals of the receiving beam intensity are plotted in Fig. 8 for various stream
Results and discussion
The scanning results are plotted in Fig. 9 of a cold powder stream (powder mass flow rate of 0.03 g/s, carrier gas velocity of 2 m/s) with various shielding gas velocities. The sensor was located at various stand-off distances and scanned in the transverse direction. The signal drop due to the received beam radiation is shown in Fig. 9. A volumetric distribution of the powder concentration at different gas settings is clearly shown. The concentration profiles along the beam centre are plotted
Conclusions
The mode of cold powder streams was experimentally characterised by a CCD camera with scattering luminance analysis. A scanning powder sensor was designed and accurately calibrated to detect the powder concentration based on the light extinction effects. Both techniques show similar Gaussian mode in the transverse direction of the powder streams. These techniques might be applied to the process automation in the near future for on-line detecting the stream structure of the coaxial laser
Acknowledgements
Many thanks are due to Professor W.M. Steen and the colleagues of the Laser Group at the University of Liverpool for their invaluable suggestions to this study, which was funded by NSC, Taiwan and performed under the LEMA (Laser Engineering for Manufacturing Applications) project sponsored by EPSRC, UK.
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