2099 is a third-generation aluminum-lithium alloy developed specifically for extruded aerospace structural shapes, such as fuselage stringers and frames, rather than sheet or plate. It carries 2.40-3.00% copper and 1.60-2.00% lithium, a relatively high lithium loading that pushes the alloy's characteristic weight and stiffness advantage further than lower-lithium grades in this family: each percent of lithium added by weight lowers density by roughly 3% and raises elastic modulus by roughly 6% compared with conventional aluminum. What sets 2099 apart compositionally from the rest of the FM Carbide Al-Li lineup is its zinc content of 0.40-1.00%, notably higher than the other grades, which is characteristic of alloys optimized for extrusion processing and structural section properties.

Like the other third-generation members of this family, 2099 was developed to deliver better fracture toughness and corrosion resistance than first-generation Al-Li alloys such as 2090, while still capturing most of the density and stiffness benefit that makes lithium-bearing aluminum attractive for aircraft structure. It is normally supplied as certified extrusion stock for qualified aerospace applications rather than general-purpose bar or plate.

2099's relatively high lithium content makes it more reactive at the cutting edge than conventional aluminum alloys, raising the risk of built-up edge with a dull tool or overly conservative speeds. A sharp, polished, positive-rake insert combined with a coating suited to non-ferrous machining helps keep the cut clean, and steady coolant or lubrication reduces both that reactivity and the heat generated during the cut.

Because 2099 is produced as extruded structural shapes like stringers and frames, it's often machined with more complex, thin-walled cross-sections than flat sheet or plate stock, which makes chip evacuation and cutting-force control especially important. Long, continuous aluminum chips need to be broken deliberately with the right chipbreaker geometry, and keeping cutting forces and heat low helps preserve dimensional accuracy on these thin, close-tolerance extruded sections.

Despite the extra care lithium requires, 2099 still cuts at speeds well above steel or titanium thanks to aluminum's low shear strength and high thermal conductivity. Running the recommended speed range with a sharp, well-maintained edge produces better results than slowing down, which tends to promote edge buildup rather than avoid it.