Laser Power Requirement

The melt film is generated by the melting action of the absorbed laser power from the incident focused high intensity laser beam and the oxidation reaction power (in case of oxygen assist gas). The generated melt film is then sheared and blown away from the cutting zone by the action of the pressurized assist gas jet - acting coaxially with the laser beam - to create the cut kerf. The cut kerf generated during laser cutting is shown schematically in Figure 16.

Figure 16. A schematic of the laser cut kerf; w is the cut kerf width, d is the workpiece thickness, and v is the cutting speed.

3.1.1 Laser cutting of metal using an active assist gas jet

The power balance at the cutting front for steady-state laser oxygen cutting of a steel workpiece using an oxidizing assist gas jet is given in equation 11. The proportion of the kerf volume that is vaporized was considered to be negligible because thick-section metal

cutting is considered in this analysis. Vaporization of the kerf volume is minimal during thick-section metal cutting due to the high conduction losses which scale with workpiece thickness. incident laser power, P_R the reaction power provided by the exothermic oxidation reaction,

 is the workpiece material density, w is the cut kerf width, d is the workpiece thickness, V is the cutting speed, C_P is the specific heat capacity of the workpiece material, T is the temperature change of the melted kerf volume, L_m is the latent heat of melting, and P_Loss is the conductive power loss to the substrate metal.

It is assumed in this analysis that all the generated melt is oxidized into FeO and is removed in the molten state through the bottom of the cut kerf. Powell et al. argued that the FeO generated in laser-oxygen cutting of mild steel does not boil because it does not have a gas phase but would dissociate when heated to high temperatures of which the dissociation process consumes much energy and could lead to a collapse of the cutting process ⁷⁵.

The conduction power losses to the substrate metal are considered as the only significant means of power loss from the cutting zone while the convection and radiation power losses are considered to be negligible. The heat conduction from the cutting zone to the substrate metal in the cutting direction is regarded as power utilized for cutting and the melt/solid interface is at the melting temperature throughout the cut thickness. Therefore, the conduction power loss from the cutting zone to the substrate metal is considered to be only due to the temperature gradient on the kerf walls.

When a cut slot is made in a workpiece of thickness d at a cutting speed V and the cut slots are made at a distance L from each other, the conductive power loss,P_Loss, to the substrate metal is given as:

Schulz et al. ⁷⁶ developed an analytical approximation of the heat conduction losses during laser cutting of metals and provided an expression for the temperature change,T_Loss, which is used here to estimate the temperature change in the substrate metal as:

)

The boundary separating the molten kerf volume and the solid kerf walls is at the melting isotherm so that the kerf walls are at the melting temperature T_m and the edges of the workpiece are at the ambient temperature T_amb (room temperature 298 K).

The Peclet number, Pe, is given by:

)

The reaction powerP_R is estimated from the power

O2

P made available by the oxygen flow into the interaction zone or from the powerP_Fe made available by the molten iron flow into the interaction zone ⁶. The minimum value of these two powers is the maximum reaction power added to the cutting process by the exothermic oxidation reaction because the reaction is limited by the flow rate of the rarer type of reactant (either oxygen or iron).

The reaction powers

O2

P and P_Fe are estimated using equations 15a and 15b.

)

During the laser cutting process, only a small proportion of the oxygen jet is consumed in the reaction, part of it is lost across the top surface of the workpiece or down the backside of the kerf and the rest is used as thrust to blow the melt out of the cut kerf. And approximately 50% of the molten iron reacts with the oxygen in the cut kerf to form FeO.

3.1.2 Laser cutting of metal using an inert assist gas jet

The power balance contributions during inert gas assisted laser cutting of metal include the absorbed laser power as the only incoming power contribution to the cutting zone. For a pure fusion cutting process where the kerf volume must only be melted but not vaporized, the power balance at the cutting front is given in equation 16. Thick-section metal cutting is considered in this analysis whereby vaporization of the kerf volume is minimal due to the high conduction losses.

) 16 ....(

...

...

...

...

)

( _{P m Loss}

L wdV C T L P

AP    

where A is the absorptivity of the workpiece to the incident laser radiation, P_L is the incident laser power,  is the workpiece material density, w is the cut kerf width, d is the workpiece thickness, V is the cutting speed, C_P is the specific heat capacity of the workpiece material, T is the temperature change of the melted kerf volume, L_m is the latent heat of melting, and P_Loss is the conductive power loss to the substrate metal.

The conductive power loss from the cutting zone to the substrate metal is estimated using equations 12-14 in section 3.1.1.