Drilling Non-Ferrous Aerospace Alloys: 6061, 7075, and Titanium

6061-T6 Aluminum: Fast, But Watch the BUE

6061-T6 is one of the most drillable aerospace alloys — soft, thermally conductive, and free-cutting by most metal standards. It supports high surface speeds and aggressive feeds. Standard starting parameters for HSS in 6061: 200 to 300 SFM, 0.005 to 0.010 IPR feed for 3/8" diameter, scaling proportionally. CNC production drilling in 6061 commonly runs at 400 SFM or higher with carbide tooling.

The dominant failure mode in 6061 is built-up edge (BUE) — aluminum's tendency to cold-weld to the drill edge under cutting pressure, forming a deposit of aluminum on the rake face and margins. The BUE changes the effective cutting geometry, increases cutting forces, and eventually tears away in chunks that damage the real cutting edge. The result is a poor bore finish and accelerated drill wear even though the material is soft.

Solutions for BUE in 6061: high-helix drills (35-40 degree helix) with polished flutes — the high helix evacuates chips faster so they don't compress and weld. Cutting fluid with good lubricity (cutting oil or high-concentration soluble oil). TiAlN or bright-finish coatings on the drill — uncoated tools show more BUE than polished or coated equivalents. And running at the upper end of the recommended speed range rather than the lower end — at higher speeds, the chip formation is more adiabatic (heat stays in the chip) and less adhesive.

Hole quality in 6061 with proper setup is excellent. Ra 63 or better in the bore is routine. Diameter accuracy within 0.001" of nominal. Deburring requirements are minimal with sharp tooling and correct feed — the chips peel cleanly rather than forming the rolled burr that appears with wrong geometry or dull tools.

7075-T6 Aluminum: Higher Strength, Different Rules

7075-T6 is significantly stronger than 6061 — about 83,000 PSI tensile strength versus 45,000 for 6061 — due to higher zinc, copper, and magnesium content. It's the primary structural aluminum for aerospace applications where weight is critical: wing spars, bulkheads, high-stress fittings. It's also harder to drill well.

The strength difference means higher cutting forces for the same cross-section, more heat generated, and more abrasive wear on the drill because the harder second-phase particles in 7075 act as abrasives during cutting. Reduce speed to 150 to 250 SFM for HSS (or 300 to 400 SFM carbide), keep feed at 0.004 to 0.008 IPR for 3/8" diameter.

BUE is less of a problem in 7075 than 6061 — the higher alloying elements reduce the tendency to cold-weld to the tool. However, abrasive wear on the margins is more significant. Drills that last 500 holes in 6061 might run only 300 in 7075 under identical conditions. High-helix geometry still preferred, but the priority shifts from BUE prevention to maintaining sharp margins.

Coolant is more important in 7075 than 6061. The harder material generates more heat, and thermal softening of the HAZ in 7075 workpieces affects fatigue life — a concern in aerospace structural applications. Machine the part with adequate flood coolant to control heat input and minimize any thermal effects on the material immediately surrounding the holes.

Titanium Grade 5 (Ti-6Al-4V): The Hard Case

Titanium Grade 5 is the most common aerospace titanium alloy — strong, corrosion-resistant, and biocompatible. It's also one of the most challenging drilling materials in common shop use. Three properties make it difficult: very low thermal conductivity (about 1/6 of steel), strong tendency to work harden, and high chemical reactivity with tool materials at elevated temperatures.

Thermal conductivity: most heat generated in cutting steel goes into the chip and workpiece and conducts away from the cutting edge relatively quickly. In titanium, the heat stays concentrated at the tool edge because the material can't conduct it away. Temperatures at the cutting edge in titanium can be 300 to 400 degrees higher than in steel at the same SFM. HSS softens and fails quickly at these temperatures; even carbide suffers accelerated diffusion wear.

Work hardening: similar to stainless steel, titanium work hardens significantly in the layer just ahead of the cutting edge. Light feeds allow the tool to ride on this hardened layer rather than cutting below it. This is why the "go slow and easy" instinct is wrong for titanium — you need feeds that penetrate below the work-hardened zone.

Starting parameters for Ti-6Al-4V with M42 cobalt HSS: 20 to 30 SFM (barely turning), 0.003 to 0.005 IPR feed for 3/8" diameter, full flood coolant at maximum available pressure, peck drilling mandatory above 2xD depth with 0.5x diameter peck depth. With solid carbide and through-spindle coolant: 50 to 80 SFM, slightly higher feeds. The slower speeds feel inefficient, but running titanium at steel speeds destroys drills in 5 to 10 holes versus the 50 to 100 holes achievable with disciplined parameters. Patience in setup and operation pays directly in tooling cost and production reliability.