grade 9 titanium vs 7075 aluminum 3

[David108]

which metal has the better strength to weight ratio, i have heard many conflicting ideas. i am trying to build staffs for circus/juggling use. i want the lightest and strongest metal tube possible, so far i am looking at grade 9 titanium and 7075 aluminum, any thoughts?

 

[tbuelna]

The terms "light" and "strong" are rather meaningless. You need to provide some details of your staff design and the forces that the staff will be subjected to. What you'll likely find is that due to cost considerations some high-strength aluminum alloy will probably be the best choice. Titanium is quite expensive.

 

[David108]

thank you for the fast reply... the staff will be tossed, juggled, body rolled, and used as a pole vault. strength to weight ratio is the correct term? so lets say i have 2 poles. both 1600mm in length, both have a 23mm OD, and both have the same exact weight. which one will be the first to dent or bend? i want to make a staff that will not bend or dent because that makes the body rolling style not possible. ive been hearing that the 7075 has a better strength to weight ratio than the grade 9 titanium. im not sure i believe that. i hope this info is clear enough, and again thank you for taking the time to help.

 

[TVP]

Keep in mind that strength-to-weight ratio is only one consideration. When you talk about pole vaulting, stiffness (how much something deflects) is very important, and is a completely different criteria than strength. To answer your specific question, it depends on how the alloys are processed:

Typical properties for Grade 9 (Ti-3Al-2.5V)
Annealed: YS = 485 MPa, UTS = 621 MPa, Elongation = 15%
Cold Worked, Stress Relieved: YS = 724 MPa, UTS = 862 MPa, Elongation = 10%
Density: 4.51 g/cm³
So, YS/density = 107 for annealed or 160 for CWSR

Typical properties for 7075 (ASTM B221)
T6: YS = 503 MPa, UTS = 572 MPa, Elongation = 11%
T73: YS = 434 MPa, UTS = 503 MPa, Elongation = 13%
Density: 2.80 g/cm³
So, YS/density = 179 for T6 or 155 for T73

Since Al has a modulus (stiffness) of 71 GPa, and Ti has modulus of 100 GPa, the Al will deflect more than Ti for a given mass. A Google search of "bending stiffness stress tube" will give you some additional information on the subject.

 

[David108]

TVP, thank you for the informative reply... so in summary the 7075 is a better choice than titanium for my use?

 

[EdStainless]

This is a common design dilemma.
Since your OD and length are fixed it makes it a little easier.
The higher the Yield strength the less likely it will be to dent or bend, so advantage Ti.
The overall weight though is a separate issue. Since the Ti3-2.5 is so much stronger it will have a thinner wall than the 7075 for the same overall strength level. In fact for the same strength tube the Ti would have 2/3 of the all of the Al. This brings their overall weight very close to each other.
The Al tube (even with thicker wall) will flex more than the Ti, this may or may not be good for you.
You do need to decide how much overall strength (wall thickness) you want.

Not to make this more confusing but there are other choices also. You could use a high strength stainless steel (Custom 465) with a Yield strength of >1700MPa and a density of 7.83 g/cc.
This YS/d = 217, so per pound this material is stronger than either Ti9 or Al7075.
And Reynolds Technology in UK does make ultra high performance bicycle frame tubing from this alloy that they call 953.
But this tubing, even with a wall that is 1/3 of the Al will be much stiffer (less flex).

Reynolds also makes frame tubes in Ti6-4, a slightly higher strength alloy than the gr9. They might be able to make these for you in either Ti6-4 or 953. Give them a call.

If you made them:
953 23*0.5mm wall
Ti 6-4 23 x 1.0mm wall
Al7075 23 x 1.5mm wall
They would all weigh between 450 and 500g, and the strengths would be within a 5% range.


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Plymouth Tube

 

[weldstan]

Have you considered carbon fiber reinforced plastic?

 

[tbuelna]

Since this pole will be used as part of an acrobatic performance you might also consider safety issues. For example, what is the most likely way the pole will fail and what will happen to the performer as a result? Will the pole simply buckle gracefully leaving no sharp fractured edges? Titanium tends to be a bit notch sensitive, so surface damage like gouges or scratches may present a problem with this material unless the pole is carefully maintained.