Aluminum Air Compressor Connecting Rod Repair
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  1. #1
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    Aluminum Air Compressor Connecting Rod Repair

    A customer brought in a damaged connecting rod from a Hitachi "wheelbarrow" style air compressor. It had a badly gauled up big end ID, and there was a crack all the way though the wall of the big end. Looked like it was run with the dipper starved of oil (I'm betting from either being run with a low oil level, or an excessive "tilt" angle.)

    A new connecting rod on one of these apparently costs a *lot* more than you'd probably guess, and since there was a "AL recycle" symbol cast into it, I decided might as well take a go at a repair.

    I vee'ed out the outside of the crack with a carbide burr, and found it welded decently. I then reamed out the galled material from the ID of the big end, and built the material back up using 4043 filler; TIG welded using an Everlast Super200P. I did the build up first around the edges with my usual "focused arc" settings. Then I filled in the middle areas, trying a "balanced arc" / 50% balance / DCEN setting (which was kind of fun, because of the mellower noise it made and also how it "wetted" the surfaces out nicely beyond the actual molten puddle.)




    After the welding operations were performed, I used a hand reamer to open up the big end to about .008" under final diameter, and honed it the rest of the way using a 240 & 400 grit flapper stick. This left a nice, smooth finish, cleaning up the visible honing chatter marks. Ended up with about .002" clearance.



    I did get a little bit of porosity in the welding operations, probably from contaminants inside the material (maybe from not reaming the galled material completely out), however I think those little voids might actually prove to be helpful little oil reservoirs.

  2. #2
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    Re: Aluminum Air Compressor Connecting Rod Repair

    awesome job there, hope it holds up well for your customer, looks good
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  3. #3
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    Re: Aluminum Air Compressor Connecting Rod Repair

    I am in awe.

  4. #4
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Nice job, but a rod? I never did an al rod, but have done similar to steel rods. Do everyone a favor and give a report whether it works well or fails. My concern is heat treatment is gone.
    Peter
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  5. #5
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    Re: Aluminum Air Compressor Connecting Rod Repair

    DCEN??????????

    You sure about that?
    If so explain how that did not melt into a puddle on the floor..
    I may NOT know inverter machines but aluminum done that way all "Shiney" and stuff is AC.
    DCEN with straight helium would just blow that away.

    And like Peter stated..(In not so many words)
    I believe it will fail also..

    ...zap!


    I am not completely insane..
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  6. #6
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    Re: Aluminum Air Compressor Connecting Rod Repair

    C'mon guys give it a chance, what is the worst that could happen?

    I know that is a repair that I could never do.

  7. #7
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by zapster View Post
    DCEN??????????

    You sure about that?
    If so explain how that did not melt into a puddle on the floor..
    I may NOT know inverter machines but aluminum done that way all "Shiney" and stuff is AC.
    DCEN with straight helium would just blow that away.

    And like Peter stated..(In not so many words)
    I believe it will fail also..

    ...zap!
    Damm Zap, I read quickly and didn't notice thd DCEN, but he has something about balance, so maybe he just goofed his writing? Moot point, it'll probably be used to make a window in the crank case.
    Peter
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  8. #8
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Thanks for the compliments, guys.

    castweld -
    I did express concerns to the customer prior to attempting this repair (although primarily, it was not being able to recreate the inside geometry accurately enough.) He originally brought it still attached to the crank, still installed in the crankcase, and asked if I could just weld it like that.

    I told the customer I could not make it as good as new, but he was still interested in a repair attempt, due to the high replacement part cost. He also intended to only use the compressor just as a second, backup. So I figured, why not let him pay me to learn?

    Anyhow, since the material is subject to highly cyclical, push-pull forces (pull sucking air in, push compressing it), fatigue related failure is likely the main concern at design stage. To combat this, it is likely the part is probably actually designed with relatively low material tensile loading. If I hear any feedback on how it works though, I'll post it here as a reply.

  9. #9
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by zapster View Post
    DCEN??????????

    You sure about that?
    Perhaps "balanced arc" was not the right term. "Balanced AC" it should have been, perhaps? Should be equivalent to 50% electrode positive, 50% electrode negative.

    For whatever reason, I don't see the "edit" button near my original post, or I'd have edited it for clarity.

  10. #10
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Also to clarify what my usual "focused arc" settings for welding aluminum are:

    80% DCEN
    300 Hz pulsing
    sharp tungsten, with a flat on the tip
    tight arc

    oh and also can't forget (for Zap ):
    AC

  11. #11
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by castweld View Post
    Nice job, but a rod? I never did an al rod, but have done similar to steel rods. Do everyone a favor and give a report whether it works well or fails. My concern is heat treatment is gone.
    Peter
    Heat Treatment?

    How does the 'Heat Treatment' come into play when speaking about a 'CAST' Aluminum Part? Do you believe that it is a risk due to the part being fixed, thus weakening the 'original aluminum'? Wouldn't this repair be quite strong due to the 'new AL' covering the 'entire' uppermost part of the rod, including the crack that was filled in?

    Cheers,
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  12. #12
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    Re: Aluminum Air Compressor Connecting Rod Repair

    How did you get the rod off the crank? All the Hitachi compressors I found online had honda engines and I thought they had bolt on caps.

    I see a holy block in your future....

    This must have been the rod from the pump. Either way, it's a questionable repair at best. If repairing the rod cost less than replacing the rod, then you work too cheap. I would have replaced the rod even if the repair was free.
    Last edited by Boostinjdm; 10-14-2010 at 01:17 AM.
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  13. #13
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by jman View Post
    Heat Treatment?
    That is one ambitious repair. Good job.

    The big question is what cast alloy was the rod cast from? Some cast alloys like 356 are heat treatable, others like 208 are self aging. Most cast alloys simply do not have any real strength without heat treatment, they are soft and gummy. As a clue, many heat treated parts have been sand blasted as the heat treatment darkens the part. If I had to guess the rod was cast from 356 (or similar) and heat treated.

    I too am very interested to see how the repair holds up. I imagine the part will age harden as it's used... provided it makes it the first few minutes.

  14. #14
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Hey jakeru, got any more pics of those "fingers" that you used to prop up the rod?

  15. #15
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    Re: Aluminum Air Compressor Connecting Rod Repair

    boostinjdm - Yes, it was a rod from the air pump of the air compressor, not from the internal combustion engine. The crankshaft didn't have counterweights, so the rod could be slid (carefully maneuvered...) across the crank onto the machined rod journals. Anyhow, I just let the customer decide what to do and informed him of the risks, he had already researched replacement parts cost, so no reason to second guess or re-research his info there. Like I said before, I made the customer aware I had some concerns but they were willing to pay me to do it anyway. It wasn't real quick project, but didn't take me longer than a day either. It was kind of a fun project for me anyway. I like pushing the limits and doing new things. In this case, seeing what kind of tolerances I could muster with hand tools. But then the build up operation ended up being kind of fun too.

    forhire - thanks. I'm not sure what alloy it was, but can add that it looked die cast (maybe some kind of pressure cast?); like it was cast in a hard metal mold, not sand anyway, based on a very smooth finish and fine casting detail. I can also add that it had no problems with hot cracking / crack sensitivity when welding, which I've noticed some heat treatable Al-Mg-Si alloys (or at least, 6061, maybe 6063) can.

    Found this source on common die cast aluminum alloys, indeed there are a couple with Si+Mg > .1% = heat treatable alloys I think (A360, and A390.) And wow are those some high Silicon contents!
    http://www.kenwalt.com/DiecastingAlloys.pdf

    Shadil - here is a thread I made about the welder's fingers on welding tips and tricks forum (more pics inside):
    http://forum.weldingtipsandtricks.co....php?f=9&t=728

  16. #16
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    Re: Aluminum Air Compressor Connecting Rod Repair

    removed duplicate post
    Last edited by jakeru; 10-14-2010 at 03:49 AM.

  17. #17
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    Re: Aluminum Air Compressor Connecting Rod Repair

    ???? how did you make the big end was round and straight, and then measure it?

  18. #18
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    Re: Aluminum Air Compressor Connecting Rod Repair

    I used some adjustable hand reamers. I bought a set from harbor freight (came in a wood box, made in India) just for this job. I ended up using the two largest sizes (the largest is spec'ed for up to 1.5" diameter):



    Rotating the nuts at either ends causes the multiple cutting blades to slide up / down ramped grooves machined into the tool, which causes them to expand or contract uniformly. The chattering I got would have probably been reduced if I had a proper handle for the tool, which I did not. (I did most of the reaming by holding the workpiece by hand, spinning it around the reamer clamped in a vice. Supporting something lightweight by hand is not a very rigid way of doing it.) The tool was OK, left round holes, but could also probably use a little TLC / "blueprinting" I didn't feel like doing to possibly reduce the chattering also. Not sure if I'm going to do invest that effort now and keep them or not, it felt I was fighting it a bit.

    I measured diameters using 6" digital calipers, which I find are useful for all kinds of things. Quick, precise to .0005" / .01mm, and accurate / repeatable. Button toggles between mm/in:

  19. #19
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by jman View Post
    Heat Treatment?

    How does the 'Heat Treatment' come into play when speaking about a 'CAST' Aluminum Part? Do you believe that it is a risk due to the part being fixed, thus weakening the 'original aluminum'? Wouldn't this repair be quite strong due to the 'new AL' covering the 'entire' uppermost part of the rod, including the crack that was filled in?

    Cheers,
    Most al castings are hardened with one of the many hardning process, many loosly call this heat treatment. A hardened part will be dead soft after welding, in haz. Some parts will age harden to near orig state if the correct filler is used. (reason I use A356 on head castings) 6061 was mentioned, but I doubt this was made from it. 6061-0 is about 18,000 tensile while 6061-t6 is about 42,000 tensile. Without knowing alloy and hardning process we don't know how much strength was lost, but could possibly be >50%.
    jakeru did a great job repairing this especially with his limited tooling, but I will be surprised if holds up in the long run.
    Peter
    Equipment:
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    1 bad back

  20. #20
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Well put peter, if the rod was solution/aged hardened then that will be lost near the weld.

    I also agree with your first post, the issue has been decided, and the part repaired well (at least I think so considering what jakeru started with) so now the results are important.

    If it fails do you forever fear, or do you learn more about the animal your playing with. I have to admit I'd be pretty timid about trying this... Leaving it to a younger person seens like a good idea!

    Matt

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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by Matt_Maguire View Post
    Well put peter, if the rod was solution/aged hardened then that will be lost near the weld.

    I also agree with your first post, the issue has been decided, and the part repaired well (at least I think so considering what jakeru started with) so now the results are important.

    If it fails do you forever fear, or do you learn more about the animal your playing with. I have to admit I'd be pretty timid about trying this... Leaving it to a younger person seens like a good idea!

    Matt
    Yea, a few years ago I would have said thats a repair that can not be done. Changes in technology make repairs similar to this possible. Cold spray would build the inside of rod without destroying hardening. http://www.asminternational.org/pdf/...cold_spray.pdf
    Of course, is it cost effective?
    Peter
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  22. #22
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    Re: Aluminum Air Compressor Connecting Rod Repair

    If you don't mind divulging... What did you charge for this repair?

  23. #23
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Thanks for your interest, airwolf, but not interested in publicly sharing that info.

  24. #24
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    Re: Aluminum Air Compressor Connecting Rod Repair

    please update on if it holds together or not!
    I am curious what RPM the pump spins at.. I am sure it is WAY less than an IC engine rod, so the stresses are probably more compression than tensile, and it might be just fine long term!

  25. #25
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    Re: Aluminum Air Compressor Connecting Rod Repair

    Quote Originally Posted by jdchmiel View Post
    please update on if it holds together or not!
    I am curious what RPM the pump spins at.. I am sure it is WAY less than an IC engine rod, so the stresses are probably more compression than tensile, and it might be just fine long term!
    x2
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