Why is everything in aerospace Ti-6Al-4V? Is titanium really that special?
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TL;DR: At room temperature annealed Ti-6Al-4V is not dramatically stronger per kilogram than 7075-T6. It wins on temperature, corrosion and strength in small sections, and it loses on heat conduction, stiffness and cost. The situation: titanium Ti-6Al-4V shows up in airframes, engines, fasteners, spacecraft hardware and implants, and the usual explanation is "it is strong and light." True, but incomplete, so I ran the numbers from our archive. The numbers (Boze datasheet) Density 4.43 g/cm3. Yield strength 830 to 950 MPa annealed, 1000 to 1100 MPa solution treated and aged. Elastic modulus 113.8 GPa. Melting range 1604 to 1660 C. Thermal conductivity only 6.7 W/(m.K) at 20 C. Strength per density Divide yield by density: annealed Ti-6Al-4V is about 187 MPa per g/cm3, and the aged condition 226 or more. 7075-T6 aluminium (Kaiser data, 503 MPa over 2.8) comes out at about 180. A quenched and tempered 4140 steel (740 MPa over 7.85) is about 94, and 6061-T6 (276 MPa over 2.7) about 102. So titanium clearly beats steel and 6061, but at room temperature it is not far ahead of 7075-T6. So why titanium? - Temperature. Aluminium alloys lose strength quickly as they warm up, while titanium keeps working where aluminium is out of the game. That is general engineering knowledge, and it is why engine and hot-structure parts go titanium. - Corrosion. It tolerates seawater and many chemicals, which is also why it is the implant material of choice. - Strength in a small section. Aged titanium reaches 1000 MPa and more, which is hard to match with aluminium when space is tight. The downsides people skip - Heat. 6.7 W/(m.K) is far below aluminium (167 W/(m.K) for 6061), so heat stays where it is made. That hurts electronics enclosures and makes machining hard, because heat piles up at the cutting edge. - Stiffness. 113.8 GPa is a bit more than half of steel's roughly 200 GPa and about 1.6 times aluminium's, so a titanium part can flex more than a steel part of the same size. - Cost. Expensive to buy and to machine. My rule: if the part runs hot, sits somewhere corrosive, or must carry high loads in a small section, titanium makes sense. If it is a room-temperature bracket where 7075 or even 6061 would do, titanium is often a status choice rather than an engineering one. Where have you seen titanium chosen when aluminium would have done the job, or the other way round?