Titanium Machining Glossary

Alpha case

A brittle, oxygen-enriched surface layer that forms when titanium is heated above roughly 315 C in open air, common after welding or heat treatment without shielding. It has to be removed, usually by machining or chemical milling, before the part is put into service.

Alpha-beta alloy

A titanium alloy, such as Grade 5 Ti-6Al-4V, containing a mix of the two crystal structures titanium can take. Alpha stabilisers (aluminium) and beta stabilisers (vanadium) are added to get a balance of strength and ductility.

AMS

Aerospace Material Specification, published by SAE International. AMS 4928 is the common aerospace call for Grade 5 bar, tighter than the general industrial ASTM B348.

Anodizing (Type II / Type III)

An electrochemical process that grows a controlled oxide layer on titanium. Type II is functional, for wear and anti-galling. Type III is decorative, where the colour comes from light interference set by voltage, not dye. See the anodizing guide.

ASTM B348

The general industrial specification for titanium bar and billet. The default a shop buys to unless a drawing calls out something tighter, such as AMS 4928.

Beta alloy

A titanium alloy stabilised mostly in the beta phase, giving high strength after heat treatment. Less common in general machining than alpha-beta alloys like Grade 5.

Buy to fly ratio

The weight of stock bought divided by the weight of the finished part. High in titanium because the bar is expensive and machining from solid can leave most of it as chips. See the buy to fly calculator.

Certificate of conformity

A document from the machining shop stating the parts were made to the drawing and material requirements, usually referencing the mill certificate and heat number behind the material.

Chip load

The thickness of material each cutting edge removes per revolution. Too low in titanium causes rubbing and work hardening instead of cutting.

Chloride stress corrosion

A cracking mechanism that affects some metals in chloride environments. Titanium is highly resistant to it, one of the main reasons it is chosen over stainless steel in marine and chemical service.

Commercially pure titanium (CP)

Unalloyed titanium, Grades 1 through 4, distinguished mainly by oxygen content. Corrosion resistant rather than high strength. Grade 2 is the most common.

Crevice corrosion

Localised corrosion in a tight gap, such as under a gasket or in a threaded joint, where oxygen cannot reach to maintain titanium's protective oxide. Grade 7 and Grade 12 resist it better than Grade 2.

Cutting fluid pressure

The pressure at which coolant is delivered to the cutting edge. In titanium, high pressure through spindle coolant, 70 bar and above, reaches deep features and clears chips that flood coolant cannot.

EN 10204

The European standard naming mill certificate types. 3.1 is the normal call for machined titanium parts, issued by the mill's own inspection department. 3.2 adds independent third party witness and is slower and more expensive.

First article inspection (FAI)

A full dimensional and material check of the first part made from a new process, reported against every drawing characteristic. AS9102 is the common aerospace format.

Galling

Cold welding between two sliding or threaded surfaces of the same or similar metal. Titanium galls against itself readily, which is why titanium threads into titanium often need an anti-seize compound or an anodized surface.

Grade 2

Commercially pure titanium, the most common CP grade, around 345 MPa minimum tensile strength. Chosen for corrosion resistance rather than load. See the grade reference.

Grade 5 (Ti-6Al-4V)

The default structural titanium alloy, roughly 6 percent aluminium and 4 percent vanadium, around 895 MPa minimum tensile strength. The grade a shop assumes if a drawing just says "titanium".

Grade 7

Commercially pure titanium with a small palladium addition for improved crevice corrosion resistance in hot reducing acids. More expensive than Grade 2 and less commonly stocked.

Grade 9 (Ti-3Al-2.5V)

A near alpha alloy with strength between Grade 2 and Grade 5, usually supplied as tube. Common in bicycle frames and aircraft hydraulic lines.

Grade 23 (Ti-6Al-4V ELI)

The extra low interstitial version of Grade 5, with tighter limits on oxygen, nitrogen, carbon and iron for better fracture toughness. The standard grade for load bearing implants.

Heat lot traceability

A process that keeps a bar's heat number attached to every blank cut from it, through machining and any outside process, so the finished part can be traced back to its original material test.

Heat number

The unique identifier for one melt of material at the mill. Every mill certificate is tied to a heat number, and traceability means keeping that number attached to the material as it is cut up.

Interstitial elements

Small atoms, mainly oxygen, nitrogen, carbon and hydrogen, that sit in the gaps of titanium's crystal lattice. More of them raises strength and lowers ductility and toughness. ELI grades cap them tightly.

ISO 5832-3

The international standard for Ti-6Al-4V bar for surgical implants, the equivalent of ASTM F136.

Mill certificate

The document a mill issues for a heat of material, giving its chemical composition and mechanical test results. See EN 10204 for the certificate types.

Near net shape

A stock form, usually a forging, that is already close to the finished part geometry, reducing the material removed by machining. Worth the tooling cost mainly on repeating parts with a poor buy to fly ratio.

Passivation

A chemical treatment that removes embedded iron and other contamination from a titanium surface, mainly picked up from steel tooling and fixtures during machining. Different from anodizing, which changes the oxide deliberately for colour or wear.

Segregation (material)

Physically separating different titanium grades in a shop's stores, most importantly Grade 5 from the visually identical Grade 23, to prevent a mix-up that would compromise an implant order.

Solution treated and aged (STA)

A heat treatment for Grade 5 and other alpha-beta alloys that increases strength above the as-annealed condition. Specified on the drawing when the part needs the higher strength.

Surface speed

The speed at which the cutting edge moves across the workpiece, in metres per minute. Titanium runs at a fraction of the surface speed of aluminium because it keeps heat at the cutting edge instead of carrying it away in the chip.

Thermal conductivity

How readily a material conducts heat away from the cutting zone. Titanium's is roughly a twentieth of aluminium's, which is the root cause of most of its machining difficulty.

Titanium sponge

The porous metallic form of titanium produced from ore, before it is melted into ingot. The starting point of the mill supply chain.

Work hardening

A local increase in hardness caused by deforming rather than cutting the material, typically from a dull tool or too light a chip load. Once work hardened, the surface is harder to cut on the next pass, which can compound tool wear.

No term matches that. Ask us on an RFQ and the answer goes here.