Thermal Cutting

Oxyacetylene cutting

This type of cutting is a very convenient and low-cost process for fabrication. Oxyacetylene cutting of titanium can use the same techniques as those for steel, even at greater speeds.

Straight cutting, cutting of shapes and thick sections are relatively easy. Just as for steel, the edge must be pre-heated before starting to cut. lf the material is very oxidated, it may be necessary to remove scales from the surface by grinding.

This method obviously produces a contamination by gas used for cutting at high temperatures and the original chemical quality of this area must be restored by means of grinding in order to obtain a clean surface.

This is especially important if the cut edges of plates must be welded, because the contaminated elements could spread along the welding bead and cause dangerous brittleness.

ln addition, the area subjected to cutting is altered by heat and this causes microstructural transformations in the metal, which do not normally exceed 2 or 3 millimeters in depth.

 

Plasma cutting

This type of cutting, reaching much higher temperatures, enables cutting speed to be increased, and it is advantageous for thicknesses under 50 mm.

As a result of a higher thermal input in the cutting area, the edges are sometimes more contaminated than in oxyacetylene cutting and the thermal altered zone is deeper.

However, some plasma cutting machines inject water around plasma and this greatly reduces previous phenomena.

lt has been found that area, thermally altered by plasma cutting, can be limited to between 1 to 6-7 millimeters for plates with thickness between 5 to 50 mm.

The cutting speed can reach some tens of meters per hour, according to the types of gas and machine used, besides the thickness of plate to be cut.

Laser cutting

High-power lasers are being increasingly used to cut plates. The high concentration of energy generally makes laser cutting cleaner, faster and more accurate than other methods.

With titanium, the problem nevertheless remains that the molten metal has great affinity with the gases in the air or with the gas jet (normally oxygen or nitrogen) used to increase the effect of laser cut.
Good cutting results can be obtained with titanium if gases like argon or helium are used for shielding.
With a judicious combination of parameters, such as gas pressure, focalization of the beam, and adjustment of power, well-finished edges can be obtained with small (or Without) contamination.

Cutting speeds depend to a large extent on the laser type and power used, and they may vary from a few meters per minute (up to 5-6) for thin work pieces (less than 1 mm) to lower values (about l mm per minute) for thickness of 10÷15 mm.

 

Water cutting

The principles of this technique have been well known for several years in the mining field and they have been developed specifically in recent times by Aeronautical industry for different types of material (composite, honeycomb, stratified); now this method is beginning to be used widely in several lndustrial fields.

The cut is obtained by the strong abrasion exerted by a very high-pressure jet of water (3.000-4.000 bar) to which abrasive powders (garnet or aluminium oxide) are added in order to cut hard materials and metals.

The negligible heat generated by the attrition caused by cutting means that the quality of the cut edge (completely uncontaminated) is very close to that of a mechanical cut. The only difference is that numeric controls can be used to achieve any sort of cutting shape, without having any tool wear problems.

The sides are in fact very clean and cutting accuracy is quite sufficient for framework whilst roughness varies from 1 .5 to 6 Ra, depending on plate thickness.

Cutting speed is certainly not very high (see diagram p.29), but water cutting, in comparison with other methods, has sometimes an advantage, because the edges, after cutting, do not need to be prepared for welding.
We think that, in the future, this method will be very important in titanium industry.