Aluminum fatigued bike frames and parts

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Hello all and Happy New Year,I routinely repair bicycle frames mostly aluminum.  I have a question that came up because I will be taking a part to be heat treated and was curious about how metal fatigue affects these parts.  I read an AWS article on aluminum fatigue and one of the points it made is that metal fatigue is cumulative.  A part that is inspected, magnafluxed, x-rayed, etc. and found to be crack free could have built up fatigue wear so that such an inspection does not give a clue as to its wear life.So my question is:  Can the heat treating process on aluminum (annealing, heating and quenching) restore the metal to its original condition or in other words roll back the clock on its life cycle as far as metal fatigue?  If so, it could help those with obsolete bikes that would like to repair their cracked or broken frames.  This came up because someone I know had their bike part repaired and plated (repair of a crack) only to have it crack in another location.  This made me think that the part has reached the end of its life cycle and that no amount of repairs and plating will prevent future cracks from developing.Anyone have any experience or thoughts on the subject?Thanks,Tony
Reply:When you say the bike was "plated," what exactly are you referring to?   "Electroplating" the surface, or "fish-plating" on top of a discovered crack?Lincoln Power Mig 216Lincoln AC/DC-225/125Miller  625 X-Treme PlasmaMiller 211 Forney 95FI-A 301HF 91110Victor Journeyman O/PMilwaukee DaytonMakita  Baileigh NRA Life Member
Reply:Hi,no, I mean that the cracks once welded had plates put over the repair.  Like fish plates.
Reply:I can't tell what we are looking atSent from my iPad using TapatalkMillermatic 211Everlast 200DX Thermal Arc 181iKlutch ST80i lunchbox stick welderLincoln Weld-Pak 100Century Stick welderPowermax 30 plasma cutterHenrob 2000 torchLogan 200 lathe (60 years old)Clausing 6339 Lathe (for sale)
Reply:Sorry.  Its a chain stay on a bike frame that had a crack in it.  Here is another one.  The crack is welded and then a plate put over the area, sometimes on both sides.  If its a serious off road rider I will strongly suggest they have it heat treated to restore the T6 hardness they leave the factory with.  If they are mild trail riders or street riders then the repairs hold up fine.  The temper is lost initially (T-0) but then air hardens back up to a  T4 hardness within a few days.  When I took one to be heat treated I had them test it before the process.  The weld zone areas were T4 and the rest of the part was T6.So I was wondering if in the case of a part that is cracking due to fatigue, if the tempering process will restore it back to original fatigue life or will the part have to be scrapped despite the re-hardening process? Attached Images
Reply:Can't magnaflux aluminum  1st on WeldingWeb to have a scrolling sig! HTP Invertig 400HTP Invertig 221HTP ProPulse 300HTP ProPulse 200 x2HTP ProPulse 220MTSHTP Inverarc 200TLP HTP Microcut 875SC
Reply:Reinforcing things sometimes gets complicated.The original temper gets damaged,and parts that are designed to have a a small amount of flexure have another problem.If you remove the ability of a part to slightly flex along its whole length, all of the flexing forces will be concentrated along the edges of the reinforcing.
Reply:@ therrera - image of the classic Lockheed Stress Analysis Memo 192a.   Most metallurgists in the aerospace/precision fields agree that welded 6061-T6 (solution heat treated and naturally aged alloy), without applying any post-weld temper "restoration", will trend towards the T-4 condition in time.  Subsequent heat treatments will not return the material to pre-welded conditions.  Note the Efficiency column in the chart.Fabricators tend to focus on the strength of the actual weld of an aluminum structure, when in fact the design limiter in the structure is the resulting strength within the HAZ after welding.  HAZ strength is ~ 20% the weld line strength.  That's why ample use of gussets on aluminum structures is key. Attached Images"Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:If he doesn't get to this thread send a PM to Zank. He is the bicycle expert around here. Sent from mobile.  Not responsible for TyposTiger Sales:  AHP Distributor    www.tigersalesco.comAHP200x; AHP 160ST; MM350P,  Spoolmatic 30A; Everlast PowerTig 185; Thermal Dynamics 60i plasma.  For Sale:  Cobra Mig 250 w/ Push-pull gun.  Lincoln Wirematic 250
Reply:Hi Gentlemen,thanks for the feedback.  I will pm Zank and run this by him.  I get the initial reply as far as how effective heat treating is when re-tempering.  So what I'd like to get a sense of if if heat treating will affect the grain structure of the aluminum such that the fatigue wear is reversed in any way.  I also get the idea of how strengthening one part of a bicycle structure can result in the stresses moving down the line and affect another area as what may have been the case in the part I first spoke about where it was repaired and gussetted and then a new crack appearedWhat I have to find out is if he crack appeared near the repair or in a totally different part.  My assumption was that the part was fatigued and just gave way, but it could have been the result of strengthening one area and having the stress move to another.Thanks,Tony
Reply:Tony,A full annealling heat treatment after welding, followed by a quench and tempering heat treatment, would repair SOME fatigue 'damage'.  It will not heal cracks or other macroscopic damage.  But it will allow some of the damage accumulated at the microscopic level to 'heal'.  To put this in context, let's say that you repaired a crack in a well used aluminum bicycle frame.  Then you took the frame to a heat treater and had them fully anneal the frame and temper it to restore the hardness.  All the aluminum in the frame would then possess the desired strength and toughness.  Areas of the frame that had the earliest stages of fatigue would be "repaired". However, cracks in areas that you did not repair would still exist and would continue to grow and expand once the frame was back in service.  The area you repaired would be as strong as possible and any weakness in the weld heat affected zones would be healed by the heat treatment process.Lastly, the annealing heat treatment would cause any and all residual stresses in the frame to dissipate.  This would likely cause the frame to distort; perhaps severely enough that it wouldn't be usable after the heat treatment was completed.  The only way to insure this doesn't happen is to mount the frame in a steel jig(something unaffected by the aluminum annealing temperatures) that would pin any critical areas and prevent them from moving during the aluminum annealing heat treatment.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Thanks for the information and opinions.  Generally speaking all my frame repairs have held up over the past 11 years I have been doing them.  It is not my main work but I get several a month and want to get into this line of work more and leave behind the heavier welding work for the younger guys.  As such I want to know as much as I can about the mechanics and metallurgy behind bicycles frames and repairs.  I use plating extensively basically using the same principles I have learned about plating and reinforcement on heavy equipment.  For the past year I have been encouraging hard off road riders to have the parts hardened after repair and have started a relationship with a local heat treating company to that end.  I ordered some plans for building a frame jig so I can straighten and align bent frames but have not gotten into that part of yet.  Its on my "things to do" list.Thanks again,Tony
Reply:Really interesting question. I would defer to Mr. Benson. Wish I knew more on the subject!Mike ZanconatoZanconato Custom Cycles @mzank on Instagram