Rate of Melt Removal from the Cut Kerf

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.

3.2 Rate of Melt Removal from the Cut Kerf

The melt flow velocity and the melt film thickness are modelled in this study. The molten layer is continuously sheared and accelerated down the cut kerf by the pressurized assist gas jet thus maintaining a minimum melt film thickness at the cutting front. As the laser cutting process progresses, the entire melt surface is in contact with the gas jet as shown in the schematic representation of the laser cutting front in Figure 17. The x - axis is directed in the cutting direction, y is the coordinate perpendicular to the cutting front and z coordinate is along the cut depth.

3.2.1 Melt flow velocity

By applying the principles of conservation of mass and momentum to the boundary layer flow, the expression for the melt flow velocity can be obtained. The melt ejection from the laser cut kerf is mainly driven by the shear force at the gas/melt interface and the pressure gradient. The melt flow velocity profile across the melt layer from one kerf wall to the other kerf wall is presented in Figure 18. The cut kerf width is very small (< 1.0 mm) and the melt layer has a high viscosity so that the boundary layer can be assumed to cover the entire kerf. Therefore, the maximum melt flow velocity is at the centre of the cut kerf and the boundary layer thickness can be taken to be equal to half the kerf width. The melt velocity, u_Z(y), in the boundary layer increases from zero at the kerf walls where y0 to maximum velocityUat the centre of the kerf whereyw/2.

Figure 18. Melt flow velocity profile in the boundary layer (w is kerf width)

The melt flow velocity profile is developed in Publication 2 as:

)

And the maximum melt flow velocity U at the gas/melt interface is:

)

where is the gas viscosity,  is the melt kinematic viscosity, _g is the gas density, U_g is the characteristic gas velocity inside the cut kerf, d is the workpiece thickness, and w is the kerf width.

3.2.2 Melt film thickness

For a steady laser cutting process, the mass balance between the rate of melting and the rate of melt removal from the cut kerf is given by:

)

Considering a melt film of unit thickness, the total flow Q is given by:

)

And the mean melt velocity, u_m averaged across the cut kerf is:

)

3.2.3 Separation and transition of melt flow

The separation and transition of flow depends on the pressure gradient in the cut kerf (Figure 19). A laminar boundary layer flow - in which the melt particles move in smooth streamlines - exists under conditions of zero external pressure gradient (i.e.P/z0) such that the velocity gradient,u_Z/y, takes the preferential general form in which

y u_Z 

 / is greatest at the kerf wall and falls steadily to zero at the outer edge of the boundary layer (i.e. at the melt/assist gas interface). Under conditions of extreme adverse pressure gradient ⁷⁷ in the cut kerf (i.e.P/y0), the velocity profile u_Z(y) becomes increasingly distorted until the velocity gradient at the kerf wall (u /y) becomes zero

and the melt flow separates from the kerf wall. There will be a back-flow adjacent to the kerf wall downstream from the separation point and the laminar boundary layer flow transitions into a turbulent boundary layer flow in which the melt particles move in random paths. The transition to turbulent flow in the boundary layer can also occur when the disturbances in the laminar boundary layer become amplified until turbulence is developed.

These disturbances in laser cutting may be caused by fluctuations in processing parameters

73.

During laser cutting of a metal workpiece using an inert assist gas jet, the retardation of melt streamlines in the boundary layer due to the viscous shear in the boundary layer can result in flow separation as the melt layer thickens rapidly in order to satisfy continuity within the layer. And the point along the cut edge where this occurs is referred to as the boundary layer separation point (BLS).

Figure 19. Effect of pressure gradient on the flow velocity profile and separation.

4 EXPERIMENTAL INVESTIGATIONS

In order to achieve the objectives of this study, experimental investigations were performed using the high power ytterbium fibre laser. The cutting tests were specifically designed to investigate four research problems concerning the laser cutting of thick-section metal. These research problems are outlined below:

(i) The laser power requirement for cutting of a steel workpiece and the effects of processing parameters on the energy balance at the cutting zone and the resulting cut edge quality in laser oxygen cutting of mild steel.

(ii) The processing parameters that influence the rate of melt removal during laser cutting of thick-section stainless steel using an inert assist gas jet.

(iii) The different categories of cut edge quality for different combinations of cutting speed and laser power.

(iv) The requirements for optimization of cut edge quality during laser cutting of thick-section stainless steel using an inert assist gas jet.

The results of these experimental investigations are outlined in the review of publications presented in chapter 5 and are reported extensively in the research papers that make up the second part of this thesis.