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Titanium or Aluminium: How to Decide Before You Send the Drawing

This decision gets made twice. Once properly, by an engineer weighing load against weight, and once by habit, when somebody copies the material callout from the last drawing. The second one is where money goes missing.

Here is the comparison that matters for a part you are about to have machined.

The numbers side by side

Property	Aluminium 6061-T6	Aluminium 7075-T6	Titanium Grade 5
Density	2.70 g/cm3	2.81 g/cm3	4.43 g/cm3
Minimum tensile strength	around 310 MPa	around 540 MPa	around 895 MPa
Young’s modulus	around 69 GPa	around 72 GPa	around 114 GPa
Useful temperature	to roughly 150 C	to roughly 150 C	to roughly 400 C
Thermal conductivity	around 167 W/m K	around 130 W/m K	around 7 W/m K
Corrosion in sea water	Needs protection	Poor, stress corrosion risk	Excellent
Typical carbide surface speed	300 m/min and up	300 m/min and up	around 50 m/min

That thermal conductivity row explains most of the machining difference. Aluminium carries heat away in the chip. Titanium keeps it in the cutting zone, roughly one twentieth the conductivity, which is why the machine has to slow down and the coolant has to reach the edge.

Where titanium is the only answer

• Sea water, brine or bleach. Aluminium corrodes, 7075 is particularly vulnerable to stress corrosion cracking. Titanium does not care.
• Above roughly 150 C. Aluminium alloys lose strength quickly past that. Titanium keeps working to around 400 C.
• Body contact. Biocompatibility is a titanium property, not an aluminium one.
• Strength in a small section. When there is no room to make the aluminium part thicker, titanium is the way out.
• Galvanic compatibility with carbon fibre. Aluminium in contact with CFRP corrodes. Titanium is close enough on the galvanic series to be safe.

Where aluminium is the right answer and titanium is habit

• The load case has margin and nobody checked.
• Weight matters but volume is free, so a slightly thicker aluminium section does the same job.
• The part is a prototype and the point is to learn something, not to fly.
• The part lives indoors, dry, at room temperature.
• Quantity is high and cycle time dominates the price.

The honest test is simple. If you cannot name the specific property that forced titanium, the part is probably an aluminium part.

Strength to weight, properly

The usual argument for titanium is strength to weight, and it is true but smaller than people assume.

Specific strength, tensile strength divided by density:

• 6061-T6: around 115 kN m/kg
• 7075-T6: around 192 kN m/kg
• Ti-6Al-4V: around 202 kN m/kg

Grade 5 beats 7075, but not by much. The bigger difference is that titanium holds that strength at temperature and in a corrosive environment, where 7075 does not. If your part is dry, cool and bolted, 7075 is a serious competitor and it machines four to six times faster.

Stiffness is the other side. Titanium’s modulus is around 114 GPa against 69 GPa for aluminium, but titanium is 64 percent heavier. For a stiffness driven part with room to grow, aluminium often wins on weight.

What each does to the quote

Line in the quote	Aluminium	Titanium
Stock price per kilo	Low	Several times higher
Buy to fly waste	Cheap to throw away	Expensive to throw away
Machine time	Fast, high metal removal rate	Slow, low surface speed
Tool life	Long	Short, abrasive and hot
Fixturing	Straightforward	Rigidity critical, often bespoke
Deburring	Burrs readily	Fewer burrs, harder edges
Finishing	Anodize in any colour	Anodize in interference colours only

A useful rule when a quote surprises you: in aluminium the price is usually machine time, in titanium it is usually the bar.

The decision table

Driving requirement	Material
Lightest possible, dry indoor service	7075-T6
General purpose, weldable, low cost	6061-T6
Sea water or chloride exposure	Titanium Grade 2
High load in a small envelope	Titanium Grade 5
Above 150 C	Titanium Grade 5
Implant or instrument	Titanium Grade 5 or Grade 23
In contact with carbon fibre structure	Titanium Grade 5
High volume, cost driven, mild duty	Aluminium

If you are genuinely undecided

Send the RFQ with both materials named and ask for two prices. Any shop that runs both will quote them together, and the gap is usually more informative than any general rule. Say on the form which properties are driving the choice, because that is what lets a shop tell you the aluminium version will not survive, or that it will and you are about to spend money for nothing.

Frequently asked questions

Keep reading

• Grade 2 vs Grade 5 Titanium: Picking the Right Grade for a Machined Part
• A Titanium Grades Chart for People Who Have to Buy Machined Parts