TIG Aluminum Strength Loss?

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I am planning on building an aluminum bicycle.  I have already bought all of the metal, the donor bike and was finalizing my plans when I ran across a post on another forum about welding aluminum bicycles:Hey All;I posted this on another thread but feel it appropriate to repost it here. Take heed.Just a heads up on my favorite materiel...Welding AL with TIG alone is not a safe thing to do -- Welding AL lowers the strength in the weld region from "T6" condition down to "T0" -- it cuts its strength in at least half. The problem is that AL gives little to no notice when it is going to fail. When a AL weld fails under pressure it is quick and catastrophic -- not the best thing for our applications. (wrote this before Brad's adventure)The answer is to get the completed frame heat treated -- this procedure raises the tensile strength back up to full T6 condition. This is expensive. The minumum cost is $240 where I get my frames heat treated.There is a danger of getting your frames ruined by heat warpage -- heat treating is a long procedure: 1 hour 15 minutes at 990 degrees and 8 hours at 350 degrees. After each heating, the bottom of the oven drops away and the frames fall into a bath of glycol which QUICKLY drops the temp. I have had a few frames warp -- none badly enough where they were unusable but they were noticable if you looked hard enough.I work in AL and love it. But, without access to heat treating, I would not feel it would be safe to indulge my AL passion. Just a safety note to all.
Reply:Yes, a welded connection will lose its heat treated condition. Aluminum hardens with age, and the heat treatment accelerates that process, so the hardened condition will come back in time. I don't recall the time frame, but its not quickly. People may have their opinions on your application (that should work, or that probably won't work), but they are not much use to you. You would have to do a structural analysis (by manual calculations if you are a genius, I use fea software...) to see what strength is required at each joint. Your application may work, it may be unacceptable, or you may be able to change the design to make it work.No simple answer...JasonPS sounds like a project, hope it works out for you.XMT 350 MPA Dynasty 280DXSuitcase 8RC / OptimaSpoolmatic 30AThermco Ar / C02 mixerAssorted O/A gear
Reply:I've always sent my work out to be heat treated. Sometimes with great results and other times not so great. Finding a good heat treat company will be a requirement to relieve stress and make the parts uniform strength after welding. I've had warping issues with some parts, they appeared as if they were stacked haphazardly in the oven. Being clear and specific with the company is important to get good results. Finding one that has experience doing frames would be ideal.Sabinna Den has a blog about frame building and this post is about heat treating:http://www.satincesena.net/?p=757
Reply:Thank you for the replies.  I will look into heat treatments.  I don't know how econical it is to pay $250 for a heat treatment for a bike that I will build for $300-350 total.I have done some more research.  I have some "quick" calculations now.Strength:  Aluminum to mild steel is 1.5 to 1 factor.  Welded Strength:  Aluminum loses 1/3 of its strength when welded.  Heat treating restores it back to full strength.Weight:  Aluminum of the same strength as steel will be about 1/3 lighter.So using those simple guildlines I think I will be OK.  The plans that I am using calls for 16 guage steel tubing.  To have the same strength in aluminum, I could use 3/32" aluminum tubing.  I am using 1/8" aluminum tubing.  So my tubing is about 1/3 stronger than called for in the original plans.  So if I lose 1/3 after being welded then I will be back to the original strength called for in the plans.  Using such thick aluminum means my weight savings will only be 15-20% which is still about 4-6 pounds.I will weld in some extra re-enforcement in a couple key areas of the frame.  A couple of extra oz. for piece of mind is worth it to me.Does this sound about right or are my rough calculations way off?
Reply:I am not an engineer, but...The science guys say that 6061 T-6 is not really "heat treated", it's actually artificially aged (as referenced by BCRD above). Considering most structures have a safety factor of 3 or 5 designed into them. My question is what is the bike designed for? Application is the real question BMX type use or street touring?6061 t-6 is one of the most widely used alums out there. With proper reinforcements, at critical attachment points, pretty tuff stuff.Good luck.Weldtek
Reply:Look into using 7005 instead of 6061.  It's often used in bike frames, where heat treatment cannot be done after welding.With 6061, you must heat treat after welding.
Reply:Sounds logical, but your calculations are off (but I can't say by how much). Wall thickness does not have a linear relationship with strength (you want to calculate "Z" - section modulus). Besides that, the strength of your connections is more complex than just wall thickness.Will it work? Maybe... your call.I would like to see pictures or sketches, just because I'm curious - not because it'll help...XMT 350 MPA Dynasty 280DXSuitcase 8RC / OptimaSpoolmatic 30AThermco Ar / C02 mixerAssorted O/A gear
Reply:I attached a picture of what the recumbent will look like when done.  It is not exactly what I am building but the basics are there.  It will be a road going bike with the 700c X 23mm skinny tires.The one in the attached picture is made with 16 guage steel tubing.  Mine will be all 6061 aluminum.  The frame of my donor bike is 6061 so the rest of the bike is going to be the same to match.  If my weld come out decent, then I am not going to paint it.  I'll just polish it up.I am a novice welder so I am not shooting for a stack of dimes welds.  If I can get solid caterpiller welds then I will be happy.  I did buy extra 1/8" aluminum plates to do a lot of practicing before I take my torch to the real thing. Attached Images
Reply:Originally Posted by NazracI am a novice welder so I am not shooting for a stack of dimes welds.  If I can get solid caterpiller welds then I will be happy.  I did buy extra 1/8" aluminum plates to do a lot of practicing before I take my torch to the real thing.
Reply:http://aluminium.matter.org.uk/aluselect/   Infomation regarding mechanical properties, temper designations and a whole lot more on aluminium alloys. Yeah it's in metric, deal with it The heat treatable alloys are 'precipitation hardening'. Artificial aging is one of the steps (there's also a solution heat treatment and quench). T6 temper is solution treated and artifically aged, welding such an alloy results in a HAZ containing annealed/partially annealed and over-aged areas. Different alloy groups behave differently in this regard (how much over-aging occurs and how quickly they naturally age), obviously dependant on initial temperThe aluminium matter site has detailed articles on the heat treatment of aluminium but a simpler overview can be gotten by googling 'precipitation hardening aluminium' and/or 'heat treatment alluminium'. Azom has a good overviewWhen something is engineered carefully rather than overbuilt PWHT is used to minimise weight i.e. no need to heat treat a frame but can make it lighter by doing so
Reply:FWIW, the filler strength is less than the base tube; the entire welded area is....less strong for a given. But that does not mean it is weaker. Aluminum needs to be able to flex a bit, and when done correctly, the welds will be ductile to allow some impact resistance rather than just all out 100% strength. For the bike you are building,..... I don't see a lot of jarring rides ahead. I don't see you jumping it like a BMX bike, I don't see downhill mountain bike riding in it's future. Truthfully, just weld it up, and go riding. Remember to keep it in context, over 98% of ALL aluminum welded products are not heat treated or hardened. Just welded up, and put into service. This includes damn near all aluminum bikes also.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:I am learning a lot as I research this more and more.  It turns out that it is true that Aluminum is considered temper 0 after welding.  That gives it a tensil strength of 16-22 ksi.BUT after 4 days at room temperature it is considered naturally aged and becomes T1 temper with a tensil strength of 26 ksi which is equal to T4.  Its yeild strength is still lower than T4 at 14 ksi vs 16 ksi.  T6 is 38 ksi tensil strength and 35 ksi yeild for comparison.I guess the bottom line is that if I am worried about the strength of the weld, it will become significantly stronger in less than a week.  I guess this kills my plans to do a Dukes of Hazard jump before the welds cooled.
Reply:Originally Posted by con_fuse9In practice, 'caterpiller' welds are tougher to do on aluminum.  On aluminum, the puddle doesn't move that well (you'll see what I mean).  Its actually easier essentially making a new puddle.  Just make this one closer to the edge of the old one.  You get the stack of dime look because the aluminum that was molten pretty much stayed put and 'froze', the next puddle you make will be in the new spot.  If you have a pedal, just lift about 1/2 way, move the torch to the exact new spot, and press again.  If you don't have a pedal, make your movements a little 'jerky'.  Keep the are in the puddle, just near the edge.Note: You cannot fusion weld aluminum, you must add filler (at least 6061).  If you don't it will crack as it cools.  So you will have to add filler.  Might as well learn to add it at the right time.  You will have to watch the angle of the torch and make sure your filler is big enough or it will melt off before you get into the puddle.  On aluminum, filler thickness vs. amps is important (at least way more important than with steel).See the videos on weldingtipsandtricks.com (sp?) and consider getting the HTP TIG welding video.  The HTP one is the best I've seen for actually viewing the puddle.Oh, and welding tubing is different than welding plate.  As much as possible, move the tubing.  Weld it in the flat.  Welding upside down, with
Reply:Check out the columbus 7005 set Nova has on October closeout. according to columbus it does not need to be heat treated. Set costs less than 160 I think.Also, some of the steel like s3 is real light and stiffthis is just a regular vs steel frame I did and it weighs in a little less than 19 lbs as seen here.Last edited by newlincoln; 05-20-2011 at 06:50 PM.