We were recently asked to develop 3D laser processing parameters for cutting thick section (8-11 mm) high strength low alloy (HSLA) steel. The main objectives of this work were to develop parameters for producing cuts with negligible dross and minimum taper within a specified cycle time. The customer also requested that consumable costs be minimized.
Of the parameters tested, the type of assist gas was shown to be an important factor in both piercing time and quality and overall cut quality.
Laser cutting is a multi-parameter process. It is sometimes difficult to characterize the interrelationship between the many parameters which include:
- Laser beam properties: average and peak power, focusing lens focal length, focused spot size at the workpiece.
- Transport properties: cutting speed, focus position relative to the workpiece.
- Assist gas properties: gas type, gas pressure, nozzle design and standoff.
Independent of material type and thickness, laser cutting involves two processes: (1) piercing and (2) cutting. Results for the three gas types evaluated for these two processes are summarized in the following table.
|Assist gas||Piercing results||Cutting results|
|Nitrogen||Spatter free; pierce time similar to that using air||Dross that is difficult to remove|
|Compressed air||Spatter free; pierce time similar to that using nitrogen||Dross that is difficult to remove|
|Oxygen||Excessive spatter; large pierce diameter; results are sensitive to pressure||Negligible dross|
As indicated in the above table, nitrogen and air produced spatter free pierces but there was dross attached to the bottom edge of the cut. Oxygen assist gas resulted in a pierced hole much larger in diameter than the kerf width and excessive spatter attached to top surface. However, the cut quality was much better in terms of the amount of dross and the cutting speed was higher.
While the process window for the pressure of both nitrogen and compressed air is quite wide, the window for oxygen gas pressure is relatively narrow, especially for thicker sections. With too little oxygen pressure, the molten material adheres to the parent material, forming dross or even sealing the hole. Too high pressure results in burning and significant degradation of cut quality. For HSLA thickness greater than 5 mm, the tolerance band (‘too high’ vs. ‘too low’) for oxygen decreases with increasing thickness. For example, for 9 mm thick material, the maximum tolerance band for oxygen pressure is 10 psi (0.7 bar).
Various assist gas combinations were evaluated to produce parts with clean pierce and dross free cuts. The cutting tests show that for 8-11 mm thick materials the best results were achieved using two gases – compressed air for piercing (Figure 1) followed by oxygen for cutting (Figure 2). Hardware for quick changeover between the gases is key to achieving an acceptable cycle time.
|Figure 1: 11mm thick HSLA steel pierced using SmartPierce with compressed air at 6 bar in 0.3. second||Figure 2: 11mm thick HSLA steel cut with oxygen assist gas at 0.2 bar and at a speed of 900-1200m/min.|
Cutting tests in thick section HSLA steel have highlighted:
- Compressed air is the best choice for producing a clean and fast pierce while minimizing cost.
- Low gas pressure oxygen is the best choice for cutting given the ability to produce dross free cuts at relatively high cutting speed.
- A combination of LASERDYNE SmartPierce™ and special hardware for quick changeover between air and oxygen is required to achieve pierce times much less than 1 second.