Reactive fusion cutting

In reactive fusion cutting, an active assist gas jet capable of reacting exothermically with the molten material is used and acts as another heat source to the cutting process ^{2, 62}. During the reactive fusion cutting process, the active assist gas jet (usually oxygen or compressed air) passing through the cut kerf plays two important roles, namely exerting the necessary thrust to blow the molten material out the cut kerf, and also reacting exothermically with the melt thus providing additional heat to the cutting process. The incident laser beam melts the workpiece and ignites and carries on the exothermic reaction between the molten material and the active gas jet. The additional heat added by the exothermic reaction enables higher cutting speeds to be achieved than in the fusion cutting process which uses an inert assist gas jet. The amount of energy supplied by the exothermic reaction varies with the material; with mild steel and stainless steel it is about 60% of the energy used for cutting, and with a reactive metal like titanium it is around 90% ².

The most productive method in industry for laser cutting of low alloy steel is to use oxygen or compressed air as the assist gas jet. However, the downside of this process is the presence of the oxide layer on the cut edge which may influence the final quality during further processing operations e.g. welding or painting. The oxide layer may require to be removed in a cleaning operation prior to further processing of the part.

2.3 Laser Cutting Parameters

The quality of the laser cutting process – and consequently the resulting cut edge quality – is governed by a number of parameters related to the laser system, material, and the process ² (see Figure 8). The laser system parameters include the wavelength of the laser radiation, maximum output laser power, and laser beam quality; the material parameters include the material type and thickness; and the processing parameters include the used laser power, cutting speed, focal length of the focusing lens, focal point position relative to workpiece top surface, type and pressure of assist gas, nozzle diameter, and nozzle stand-off distance. For the cutting of a specific material and workpiece thickness using a particular laser system, the processing parameters can be altered by the operator so as to optimize the cutting process and obtain high cut quality at a high cutting speed for high productivity. The laser system parameters - which are characteristics of the used laser system - can not be modified by the operator.

Figure 8. Laser cutting parameters

The used laser power determines the maximum cutting speed that can be applied for cutting of a given workpiece thickness. The power balance at the cutting front is such that part of the absorbed laser power is utilized in melting the kerf volume and part of it is lost from the cutting zone through heat conduction to the substrate metal. As cutting speed is increased, penetration of the workpiece may not be achieved at the cutting speeds beyond the maximum achievable cutting speed for the used laser power.

The focal length of the focusing lens determines the minimum focused spot size essential for high power intensity required for laser cutting of a metal workpiece. The relationship between the lens focal length and the focused spot diameter is defined as ⁷:

raw beam diameter on the focusing lens, and BPP is the Beam Parameter Product of the incident laser radiation.

The focal length also determines the depth of focus which is the effective distance over which the minimum focused beam diameter is maintained. The depth of focus is the distance over which satisfactory cutting can be achieved. The relationship between the lens focal length and the depth of focus is given as ⁷: consequently a longer focal length may be necessary in some applications where a longer depth of focus is essential for good cut edge quality like in thick-section metal cutting.

The focal point position is the location of the minimum focused spot size relative to the workpiece top surface and affects the laser power intensity on the workpiece. There is an optimum focal point position - for a particular workpiece thickness - which produces minimum cut edge surface roughness and minimum dross attachment on the lower cut edge. There are also focal point positions where cutting through the material can not be achieved because of the reduced power intensity on the workpiece.

The principle role of the inert assist gas jet - e.g. nitrogen or argon - during laser cutting of a metal workpiece is to accelerate and expel the molten layer from the cut kerf. The use of an active assist gas jet (usually oxygen) for laser cutting of a metal workpiece influences the energy balance at the cutting zone. The oxygen gas jet plays two major roles during the laser cutting process, firstly the oxygen gas reacts exothermically with the molten metal resulting into energy addition to the cutting zone, and secondly the oxygen gas jet exerts the necessary thrust required to eject the oxidized melt from the cut kerf.

The nozzle delivers the assist gas jet to the cutting front. The design of the nozzle orifice determines the shape of the cutting gas jet at the cutting front and influences the efficiency of melt ejection. The nozzle stand-off distance is the distance between the nozzle and the workpiece top surface. The nozzle stand-off distance influences the gas flow dynamics at the entrance of the cut kerf and consequently influences the gas flow patterns at the cutting front. The gas flow patterns at the cutting front have a strong bearing on the resulting cut edge quality especially during high pressure inert gas assisted laser cutting ^70-72.

The maximum achievable cutting speed at a given laser power level and the resulting cut edge quality are governed by the selection of appropriate processing parameters. The choice of practical processing parameters for a particular material and workpiece thickness depends on the cut edge quality desired, and the cutting speed required. Optimization of the processing parameters enables high cutting speeds to be achieved and also enhances the melt removal from the cut kerf so as to prevent the undesired dross adherence on the lower cut edge and reduce the cut surface roughness. The current trend is towards improving cut edge quality while maintaining a high cutting speed through adaptive control of processing parameters.

2.4 Laser Cut Quality Characteristics

The characteristics of the laser cut edge that can be used to define the laser cut quality include: the cut kerf width, dross attachment on the lower cut edge, cut edge squareness deviation (perpendicularity), cut edge surface roughness, and boundary layer separation point. These characteristics are explained in detail in the following sections.