Why weld joints are stronger than the base material itself?

I wonder it for a long time and still couldn't find a reliable source. I've seen some posts on the internet saying that welds are mostly stronger than the base material but at first glance it contradicts with my intuition. I'm working on a bicycle frame for myself and the welds are important for me any mistake can cause injury so I have to take my time.

What is the physics behind it? (I've taken Mechanics and material courses so feel free to use related technical terms)
 

jrmichler

Science Advisor
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A typical fillet weld on thin wall tubing will have cross sectional area much larger than the tube wall thickness. The weld material can have lower yield strength than the tubing, but be stronger because of the larger area.

I once welded up a recumbent bicycle frame from 4130 tube. I used a torch and coat hangers for filler. After 3000 miles of use and abuse, there were no cracks. I even stress tested it by riding down the steps of the local public library.

4130 is an alloy specifically designed for easy welding and for high fatigue strength. It was originally designed for steel tube aircraft fuselage use.
 
4130 is an alloy specifically designed for easy welding and for high fatigue strength. It was originally designed for steel tube aircraft fuselage use.
Well, actually I was thinking of made the whole frame with 6061 aluminum. Weight is important for me. What do you suggest? Can you also give another suggestions you think I must consider please.
 

Baluncore

Science Advisor
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Energy propagating as noise waves along the tubes is reflected from the lug or weld discontinuities. That increases the local stress at a stepped junction as multiple waves are present in the tube. That is why gentle tapers are important in the welds. Notice the shape of the old lug joints that were silver soldered to the tubes, there were external tangs that coupled energy into the tube over some distance. That prevented step reflections and concentrations of forces.

Surrounding every weld in the base metal is a “Heat Affected Zone”. Cracks may form in that zone which weaken the parent material. To prevent that cracking, the joint should first be preheated, then after the weld is placed, allowed to cool slowly in the oven, or under a blanket.

Some removal of material from the tube surface may take place adjacent to a weld. Unfortunately, that reduces tube thickness at the point where it needs to be thickened. The same thinning of the tube can occur if the area of the weld is mechanically ground to taper the profile. Select a welding method that does not reduce tube thickness, and one that does not require mechanical finishing that may damage the tube surface. There was a time when joining tubes with lugs and silver solder achieved the best results. Working in a dark environment made it possible to monitor the solder penetration into the joints between the lug and tube. The style of the lug and the meniscus of the solder formed the taper.

Some aluminium alloys will work harden. Be sure to select an aircraft grade tube that is specified for TIG welding, if that is the procedure you will use. The choice of filler rod is important. You might cut your own rods from the same alloy.

Practice your welding on cheap tube. Then destructively test the weldment to identify weaknesses. Repeat the trying, testing, learning process cycle until you can get consistently good results. Only then should you weld your frame.
 

jrmichler

Science Advisor
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Given the practical realities of steel and aluminum, you may find that a steel frame will end up weighing the same as an aluminum frame.

Try search term welding 6061 aluminum. One of the hits, and not the only good one, is: https://www.lincolnelectric.com/en-us/support/welding-how-to/Pages/aluminum-design-mistakes-detail.aspx.

If you choose 4130 steel tubing, look to the homebuilt aircraft people for sources. Bicycles and homebuilt airplanes have many of the same requirements: Light weight, reliable, safe, and that can be built in a garage without megabuck tooling. Here is a good discussion of welding methods: http://www.vansairforce.com/Community/archive/index.php?t-11267.html. Here is what the FAA has to say about repairing steel tube structures: https://www.faa.gov/documentlibrary/media/advisory_circular/ac_43.13-1b_w-chg1.pdf. A very good place to start looking: https://eaa.org/eaa/aircraft-building/building-your-aircraft.
 
In a previous life I was a bicycle frame builder. At the time most handbuilt bikes used steel tubing. One of the distinguishing features of bicycle tubing sets which set them apart from aircraft tubing was that the tubes were "butted". For the uninitiated this essentially meant that the wall thickness varied to add strength to the area where the tubes would be joined at their ends and to reduce weight in the middle section.
Most steel and aluminum alloys are heat treated / artificially aged / work hardened to some extent to improve desired properties of strength, hardness, ductility, toughness or whatever is appropriate for the purpose. The addition of heat in the joining process usually detracts from the effect of the previous treatments. (But not always. )
Aluminum alloy 6061 is solution heat treated to a T6 temper and artificially aged to create a high strength alloy suitable for MIG or TIG welding with 4043 filler.
If you use plain gauge tubing (not butted) to build your frame the nett result will be that if the wall thickness at the joint is sufficient to compensate for the welding, it will be thicker and heavier than needed in the mid section of the tube. Factory built frames overcome this problem by heat treating their frames back to the T6 state after fabrication.
If you buy a 6061 bicycle frame tubeset from a company such as Reynolds it will be designed on the assumption that you are going to heat treat the completed frame.
As an alternative you might like to consider buying a Reynolds 853 set. This is a steel tubeset which air-hardens to a higher strength after brazing.
Depending on your location you may be able to find a local supplier who can sell you a tubeset from another specialist company.
The tubeset makers can supply you with all the technical information and advice you could possibly need. They are paranoid about inexperienced frame builders using their material to make frames that break.
 

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