2091 is a copper-lithium aluminum alloy from the 2xxx family, distinguished within the FM Carbide Al-Li lineup by its unusually high magnesium content of 1.10-1.90%, well above the other grades in this group. That magnesium addition works alongside 1.80-2.50% copper and 1.70-2.30% lithium to build strength while also supporting the alloy's damage tolerance. As with every alloy in the aluminum-lithium family, the lithium content is the headline feature: each percent of lithium added by weight lowers density by roughly 3% and raises elastic modulus by roughly 6%, so 2091 offers a meaningful weight and stiffness advantage over conventional 2xxx and 7xxx aluminum grades.

2091 belongs to the broader generation of Al-Li alloys developed to serve aerospace structural applications where reducing airframe weight has a direct payoff in fuel efficiency and payload. Like other alloys in this family, it is engineered and supplied for aircraft skin, structural, and fastener applications rather than general-purpose fabrication, and it is typically machined in relatively thin sections where dimensional control and surface finish are critical.

2091's high lithium content, over 1.7% at the low end of its range, makes it noticeably more reactive at the cutting edge than conventional aluminum alloys. Lithium reacts more readily with tool materials and tends to promote built-up edge if the tool isn't sharp or the speed is too low, so a polished, positive-rake edge and a coating suited to non-ferrous machining are worth the extra attention. Steady coolant flow or lubrication also helps manage both the chemical reactivity and the heat generated at the cutting zone.

Chip control needs deliberate attention on 2091, as it does on every alloy in this family: aluminum naturally forms long, continuous chips, and without the right chipbreaker geometry those chips can tangle with the tool or the workpiece. Because 2091 stock is almost always machined into thin aerospace skin or structural sections, controlling cutting forces and heat is also important for holding tight dimensional tolerances and avoiding part distortion.

With a properly prepared edge, 2091 still cuts at speeds well above what's typical for steel, since it shares aluminum's low shear strength and high thermal conductivity. Running with confidence at the recommended speed range, rather than backing off, generally produces a better finish and less edge buildup than a hesitant approach.