Browsing by Author "Kujanpaa, Veli"

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    Characterization of the melt removal rate in laser cutting of thick-section stainless steel
    (Journal of Laser Applications, 2010) Wandera, Catherine; Kujanpaa, Veli
    The efficiency of the laser cutting process depends on both the rate of melting and rate of melt removal from the cut kerf. The depth of flow separation and the dross attachment on the lower cut edge relate to the efficiency of the melt removal process and can be used to characterize the rate of melt removal from the cut kerf. The melt flow velocity and melt film thickness are formulated in this study by consideration of the fundamentals of viscous incompressible fluid flow. The calculated melt flow velocity and melt film thickness are correlated with the depth of flow separation on the 10 mm stainless steel AISI 304 EN 1.4301 laser cut edge. The effects of process parameters—including assist gas pressure, nozzle diameter, nozzle standoff, focal point position, and cutting speed—on the depth of flow separation and the dross attachment on the lower cut edge are investigated. The assist gas pressure, nozzle diameter, and focal point position are found to significantly affect the efficiency of melt removal from the cut kerf.
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    Inert gas cutting of thick-section stainless steel and medium-section aluminum using a high power fiber laser
    (Journal of laser applications, 2009) Wandera, Catherine; Salminen, Antti; Kujanpaa, Veli
    Inert gas assisted laser cutting of 10 mm stainless steel plate and 4 mm aluminum sheet was performed with a 5 kWfiber laser. The effects of laser power, cutting speed, focal point position, and assist gas pressure on the cutting performance and cut quality were investigated. Clean cut surfaces without or with minimal dross were achieved with some combinations of process parameters and attempts were made to define parameter windows in terms of cutting speed and laser power for good quality cutting. The maximum cutting speeds for acceptable cut quality were determined at different power levels. The range at which complete through cutting could be achieved so-called parameter window was limited upwards by insufficient power intensity to obtain through cutting at high cutting speeds and downwards by heat conduction at slow cutting speeds. The effects of focal point position and assist gas pressure on the striation pattern cut surface roughness were also examined. Low surface roughness was achieved with the focal point position inside the workpiece showing the need for a wider kerf for better melt ejection in thick-section metal cutting. There was also a reduction in surface roughness with increase in assist gas pressure, but there was no significant reduction in surface roughness above the gas pressure of 16 bar, which could be due to the gas flow dynamics inside the narrow cut kerf at high assist-gas pressures.