<title>Investigations of the gas flow of conic-cylindrical and supersonic nozzles in a laser cut kerf</title>

Abstract

Ill the laser cuffing process the gas flow is of main importance for exhausting molten and evaporised material from the cut kerf. The present paper investigates the gas flow of conventionally used conic-cylindrical nozzles and newly designed supersonic nozzles in a cut kerf A quantitative description of pressure gradients and velocity profiles of the flow field is given as a result of numerical simulation. The calculations are compared with experimental Schlieren photographs. For both nozzle types the dependence ofthe flow field on the variation ofthe nozzle adjustment and the nozzle stand-off distance is investigated. In case of the conic-cylindrical nozzle the non-linear pressure and velocity distribution inside the kerf indicates both the shock formation caused by expansion to ambient pressure and the detaching of the flow field. The important result for the supersonic nozzle is the independence of the flow field from the nozzle stand-off distance which leads to an unchanged cut quality. Therefore the supersonic nozzle is of special interest for 3D laser cutting and cutting ofthick materials.

Effect of nozzle orientation on the gas dynamics of inert-gas laser cutting of mild steel

“…Flow separation has also been seen in previous model kerf studies [8,9]. Zefferer et al [8] were able to push the separated region out of the bottom of the kerf with po values greater than 1.5 MPa (220 psig).…”

“…Since the gas-dynamics interactions inside the actual laser kerf are extremely difficult to observe, a geometrically similar model kerf similar to those used by previous investigators [8,9] was used. The model kerf was based on the experimental case of a 3 mm thick mild steel plate.…”

“…Other investigations [3][4][5] have used various supersonic nozzles in an effort to improve the assistgas effectiveness. Tandem nozzles in an off-axis configuration have also been tested [6], and the gas-dynamic interactions inside the kerf have been studied both by scale-modeling [7,8] and by computer modeling [9,10].…”

Laser Cutting: A Review on the Influence of Assist Gas

“…Stand-off distance is monitored and controlled during industrial practice by using distance sensors, such as capacitive sensors (used with conducting workpieces), or “tactile” (contact) height sensors (for insulating workpieces). Leidinger and Schuöcker found the extreme dependence of cut quality with the stand-off distance for highly underexpanded jets by means of numerical simulations and Schlieren photographs [43]. They also computed the pressure distribution of underexpanded jets emerging from converging nozzles, finding a maximum for a stand-off distance of 1.5 mm, i.e., when the workpiece is just before the position of the first Mach shock disk.…”

Three-dimensional modeling of the gas field distribution of a high-pressure gas jet used in laser fusion cutting

“…1,2 The commonly used cutting nozzles, produced by a simple machining process, belong to the subsonic type such as the parallel, conical, and convergent types. [11][12][13] Laser fusion cutting experiments were also carried out by various investigators 10,14 -16 using different cutting nozzles. There exist some strong oblique and normal shocks in the jet which cause poor edge quality.…”