weld thread repair strength? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: August 09, 2020, 02:30:05 am
content: I saw a video on youtube some time ago where the guy repairs a somewhat damaged weld, like a external piece of metal cracked off and it left the thread half intact, and he fixed it by screwing a bolt in there, then just dripping aluminum on top of it and then removing the bolt when he was done and doing a little filing so it looks right. This was for something with a engine.How does the strength compare to totally refilling it with metal, drilling it out, then tapping it? The problem of course is that you would need to locate the hole precisely. I know cast metal threads are not the best, but how about aluminum.. cast on top of steel?

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2020, 03:08:39 am
content: So he needed the bolt to align the threads before welding.Without cutting away along the two cracks so that he can get full weld penetration, it will be weaker whether he preserves the threads or not. If he does cut away the cracks before welding, then I think it would be practically as strong.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 09, 2020, 03:15:08 am
content: Sometimes you just don't need that much strength, and this was obviously good enough for the U-Tuber, but based on your description I can't imagine it having anything like the original strength. It seems very unlikely that there is an actual bond between the molten aluminum dripped on and the base material, even if that is also aluminum. But if the joint break was rough enough there might be a pretty good mechanical bond.

author: coppercone2
author_rank: Super Contributor
timestamp: August 09, 2020, 03:19:14 am
content: Well lets say you have a crack that partially goes down the bolt hole from the exterior. Then, you cut off all the cracked area (imagine taking a slice out of a cored apple). But, there is enough left so you can put a bolt into place. Say 50% of the apple remains. Like you cut it diagonally so the bolt can do a few full threads, and 100% of the area behind the bolt remains. So, you are not welding the cracked part of an apple, you are cutting out the crack, then overfilling it.So then you put the bolt in, and fill it, with a big mound on top, then you use a file to make it look right. There should be 100% penetration right? I know you can add aluminum to fill a hole, I have done it with a gas torch on a aluminum plate (1/8 th), with nothing behind it.. you just get a stalagmite of aluminum you need to grind off the back. Normally people put a copper block behind it to fill a hole with less material, but it is possible. When you grind it away, it looks the same. I assume a high power tig torch can add aluminum on top of something. It should work like a 3d printer right? I mean, you can tell the top melted. Usually when I try to fill a hole with gas, I end up making the hole bigger first, before I figure out the exact distance/torch motions/fire size/dabbling motion to add metal.. Then you make a shitty layer that you melt more stuff on top of, the bad stuff goes down like zone purification of silicon rod. Normally I fill it, then put a bunch more flux on top, then over fill it, then cut off the stalactite. If I invest in some copper 'anvils' I can probably do better. Takes a while to do. Very wasteful but it seems to work for a panel.I assume with tig, the bolt acts like the common torch technique of putting copper backer behind a hole to refill it. But is a a thread. Does the thermodynamics allow for a good thread to form from casting? I know with steel you get it weak. Even a cast hole is weak in steel. But I am unsure about aluminum. And I am unsure if this is true for welding work, I just know its weak to cast an object vs machine it out of a properly formed billet (i.e. bridge Ibeams.. thats why modern structures are stronger, we can actually afford to drill out holes instead of precast them for bridges, structures, etc.. this has to do with how the metal does not cool properly if there is a hole in it). But, given the fact that molds are made from sand, and this is a steel 'core', it should have better thermal properties.. but again the rate of heat flow is different so I expect some kind of thermal problems in the crystallization of the stuff.. if aluminum behaves the same... I don't know if it does.It was like a threaded hole to put the cover on the engine or attach something to a engine (exhaust) or something like that.

author: eKretz
author_rank: Frequent Contributor
timestamp: August 09, 2020, 11:26:45 am
content: Depends on the process. Doing that with TIG is pretty likely to contaminate the puddle, since the sharp points of the threads will probably melt if they're hit by the arc. Regardless, no that won't give you a full strength thread. Whether it's good enough would depend on the application.By the way, "correct" repair is not just welding up the existing hole. To perform a proper repair, the hole should be machined oversize so that there's no original hole surface remaining. Then the hole should be filled with weld. Finally the top surface should be milled or ground flush and the hole drilled and tapped.Attempting to just fill the hole with weld is likely to introduce contamination to the puddle since there's always grime and grit down in threaded holes. It's also preferable to have the fusion/transition line between weld and parent material nowhere near threads, since if there is any porosity or slag inclusions in the weld, the weld will often peel away under the high forces produced by torquing a bolt.