Aluminum Threads - Preventing Damage 2

[jbuckley2000]

Hello,

I am new to the forum and I did search a little bit, but I couldn't find any answers on my own.
Pretty simple question - I currently have an aluminum housing with a threaded hole in the base (2 1.2-12 SAE). I have to build an adapter plate to screw into the hole, but I am unsure about what materials to use as to not damage the aluminum threads.
I have always worked with steel housings and it was simple to have items made out of aluminum. Now I need something even softer than aluminum.
Thanks for any help you can give me - other engineers have made the plates out of plastic to prevent damage, but I want something more durable since it will be use with hydraulic oils.
Thanks,
JF

 

[TVP]

If I understand your question correctly you want something softer than aluminum (what alloy and manufacturing proces?) to screw into an internal thread. What type of forces act on the assembly? Why does the plate need to be softer than the aluminum? Aluminum bolts/screws can have similar hardness/strength as an aluminum housing and may be a satisfactory solution provided that a lubricant is used during assembly.

 

[EdDanzer]

SAE 660 or alloy 954 Bronze might be a good choice.

 

[kenvlach]

Hi jbuckley2000,
Welcome to Eng-Tips.
Agree with TVP's response. What aluminum alloy & temper?

I don't understand your thread size -- is it 1/2"-12? But good news that the bolt screw can be significantly stronger than the plate before causing mechanical damage (due to greater area at root of female threads). Did a calculation a few years back using equations from Machinery's Handbook. For 3/8"-16, I recall the male threads must be > 2x as strong before being able to strip female threads. Do the calculation for your thread size..

For aluminum threaded holes, commonly used bolts/screws are aluminum (risk of galling), galvanized or zinc-plated (risk of zinc corrosion causing seizure) steel & 18/8 stainless (risk of aluminum corrosion causing seizure). Bare steel or copper alloys can cause severe galvanic corrosion of the aluminum; see thread338-137607 for effect of copper. Also, a case where steel bolts snapped off in an aluminum housing; my diagnosis is aluminum corrosion seized the threads: thread281-147765.

I suggest any of the 3 choices: Al 6061-T6, yellow chromated zinc-plated* steel or 18/8 stainless, but essential to use an anti-seize compound on the threads. Avoid any containing copper, silver or graphite, these cause galvanic corrosion of aluminum.
*Either electroplated or mechanically plated, as these are available chromated.

You can also use DACROMET®-coated steel fasteners; some are compounded with aluminum to avoid the galvanic problem. Can be used with a PLUS® Sealer for even better protection of the aluminum.

http://www.dacromet.com/hcoat.htm

 

[EdDanzer]

kenvlach,
Interesting comments about galvanic corrosion.
I will agree threads in aluminum like to gall and corrode. We make some parts for an infrared camera where 32 tpi thread aluminum parts thread with stainless, aluminum, and 660 bronze. All must be assembled with nickel anti-seize or none will come apart.
I will ask if there is evidence of greater corrosion between the bronze parts and the stainless parts on older assemblies.

 

[jbuckley2000]

I am not 100% of the materials, but I believe the base was 6061 aluminum - probably whatever was commercially available.
The operating pressure should be around 150 to 200 psi.
Thanks to everyone for the responses!
It does sound like my boss wants to go with bronze or brass, but I wanted to get other opinions.

 

[kenvlach]

If the hydraulic fluid is completely water-free, then OK. But, not if it absorbs moisture like some types, e.g., most brake fluids.

Ed, same thing. A water-repellant gel is used on SS fasteners in aluminum for saltwater application. If the Ni anti-seize particles are embedded in a water-repellant matrix, OK. The Ni particles have an invisible oxide film from manufacturing which helps. Same priciple as Al-SS couples: both metals protected by an oxide film, so unless it's damaged, no galvanic reaction. But, I still want to warn against using any Cu alloy with Al. See the first link in my post above.
Ken

 

[EdDanzer]

So how do aluminum bronze bushings keep from self destructing? CA954 is 10% aluminum, 4% iron and balance copper.

 

[swall]

The aluminum is dissolved in the copper and forms a different phase than pure copper.It loses its identity as aluminum.It is not like having aluminum in contact with copper in a galvanic cell.That is the short answer. The long answer is beyond the scope of this thread.

 

[kenvlach]

Re EdDanzer'a question on Aluminum Bronze, UNS C95400. swall is correct. For a more quantitative understanding, consider the phase diagrams & microstructure. As-cast and annealed C95400 consists of about 50% each alpha and beta phases. -- ASM Handbook, Vol. 2, Non-Ferrous. Judging from the Cu-Al binary at 700 ^oC, alpha is Cu-9%Al while beta is Cu-11%Al. So, only a small galvanic difference between the phase. Surely affected by addition of 4% Fe & max 3.5% other alloying elements, but probably not much w.r.t. galvanic potential, as C95400 is fairly corrosion resistant and can be used for pump impellors.

However, for Al-rich alloys on the Al-Cu side, there is a large difference in composition between constituent compositions & hence emf's. At a 300 ^oC equilibrium, a 2xxx alloy consists primarily of Al-0.2%Cu and CuAl₂ (53.5 wt% Cu). The CuAl₂ is more electropositive than the Al-rich matrix, so causes galvanic corrosion in the presence of electrolyte. But, even worse corrosion results because after some alloy dissolves, dissolved Cu in the electrolyte deposits onto the alloy, and there is a huge difference in emf between Cu & Al.

 

[unclesyd]

Here is a thread lubricant (Tef-Gel) that is used with Al/Al screw threads or Al/any other metal. This grease prevents galling and help mitigate electrolysis. This is a must when we use Al/SS on boats. You can find this at nearly all marine supply stores.

http://www.tef-gel.com/index.htm

I see this one used quite a bit too.

http://www.super-lube.com/