Titanium - Aluminum Bolted Joint 1

[spedtoe169]

Hi all, I'm thinking about using titanium bolts for an application I'm designing. The bolts we're looking at are 6A1-4V unplated. They'll be going directly into tapped 6061-T6. I'm not very concerned with the strength of the bolts, they should be more than adaquate as there are a lot of them but I am curious if there are any other issues I should be addressing. I.e. corrosion, the low MOE of titanium etc.

Thanks!!

 

[CoryPad]

two concerns: galvanic corrosion (Al preferentially corrodes when connected to Ti) and thermal expansion mismatch (possible high fastener stress due to Al expansion).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Screwman]

What property of the titainium parts are you selecting them for? We need to know more about the aplication before we can make an intelligent comment. Loading, environment,criticallity, etc. all come into play.

 

[spedtoe169]

I was afraid of the galvanic corrosion issue. It is a problem in all environments, or just the more severe?

The Ti is being selected for its strength to weight ratio. Basically I have a bolted joint that needs to be as light as possible. In the past, stainless steel fasteners have been used but my calculations show that we can nearly halve the weight with Ti at no cost to strength.

Loading is highly variable but not severe due to the number of fastners. The joint is quite critical but the redundancy is extreme. Normally I would reduce the number of fastners and use the more well known and proven stainless fastners but this is not possible without a redesign. This is not currenly an option.

Environment also varies. The Ti will likely only be used for our own personal applications so I will have a high degree of control over the environment. Temperatures should never exceed 300degF or so. They will get wet from time to time but will never be exposed to moisture (other than humidity) for extended periods of time. They will never be exposed to any corrosives and should be free of any electrical concerns. Anything I missed in there?

Thanks for the quick replys!!! Keep them coming!

John

 

[CoryPad]

If the environment will not have high conductivity liquids (and low chloride ion concentration in particular), then galvanic corrosion should be minimal. Your temperature range is large enough to warrant a calculation of thermal stresses. I recommend using the guidelines in VDI 2230, available at:

http://www.beuth.de

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Screwman]

Threading directly into the aluminum, you will likely have some significant galling issues. If you can get some silver plated screws that should help the galling, but depending on your environment, it might create additional galvanic issues. Given your description, I think you will be OK with respect to galvanic.
Watch out for your length of thread engagement into the aluminum, you are going to need a significant amount of engagement in the aluminum to be able to develop full strength in the fasteners without having nut strippage as a failure mode. Off hand, not knowing the exact alloys involved, my gut feel is that you will need about 2.5 screw diameters of full thread engagement in the nut to get screw breaage as your failure mode.

 

[spedtoe169]

I checked out the engagement using formulas from the Machinery's Handbook and everything checked out ok.

Frequent assembly/disassembly is not an issue. Its unlikely that this stuff will ever come apart.

Would a small dab of anti-seize help prevent the galling? Silver plated screws are a good idea, but not really practical for us.

Thanks guys!

 

[yates]

Screwman is right on the galling. I would not worry about any galvanic corrosion, because it is highly unlikely you will be able to screw in bare Ti6Al4V bolts into tapped 6061-T6 anyway !
I would thoroughly recommend coating the bolts with an aluminum-rich epoxy (eg. NAS4006) to minimize galvanic corrosion, and also lubricating the mating surfaces with MoS2 or graphite upon assembly in order to obtain enough preload without stripping the aluminum thread.