[rb0087]

Hi everyone, I’m just getting started with welding, and I’m looking for some more experienced eyes to take a look at what I’m planning to do and give some advice and/or keep me from doing something stupid before I learn the hard way. I’m hoping to solve some pretty severe rust problems on my car (04 Cadillac CTS-V), starting with the rear upper spring mount, which is rusting away to nothing. I have access to a really nice TIG welder at work (Miller Dynasty 350), which I’ve been using to learn. After a few weeks of staying after work a couple hours each day to practice, I’m getting the hang of TIG welding, but haven’t done anything high stakes yet. I don’t have access to a MIG welder, which I know is typically the tool of choice for this sort of job.
Here are a few pictures of the problem area on my car. The upper spring mounts just acted as buckets to hold a bunch of salt, dirt, and moisture for the last 14 years, with predictable results. The frame is totally rusted through on the bottom/sides, as is the mount itself on the front half, with only the rubber spring pad covering the spring up.

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So I went to the junkyard and cut out some good replacements from a car that didn’t have as rough of a life (until it got T-boned and totalled). The spring mounts were spot welded in eight places, and I cut out the spot welds to separate the spring mount from the rail section. The steel is ~0.1” thick, in some places double that where it is lapped together. Based on spark testing it seems to be mild or possibly low alloy steel.

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So here is my plan:

• Keep practicing until I can weld 16ga steel overhead consistently
• Take the suspension off to get some working room
• Drill out the spot welds and take the old spring mount off after marking where it goes so I can align the new one.
• Use an air hammer / rust scaler to get the rust off the frame, and figure out how much needs to be cut out and replaced.
• Cut out the old frame section, and trim the replacement section to match. I think that just the bottom and front sides are rusted; the back seems to be good, so I would not be cutting completely through the frame.
• Clean everything to bare metal and paint the inside of the frame around where I’m going to weld with weld thru primer, and do the same on the new metal.
• Tack the new section in place with a small (~1/16”) gap around the edges
• Butt weld the new frame section all around, first stitch welding then filling in until it is welded all around. This is based on the GM frame sectioning recommendation, which is for a different part of the car, but it just says to stitch weld a butt joint. I figure that a full weld would be stronger, though, if I took my time and kept the temps under control.
• Cover the mating surface between the spring mount and frame with weld through primer
• Align the new spring mount, clamp in place
• Clean off the weld-through primer in the circle where I’ll be welding (I’ve heard that weld-through primer can cause problems with TIG… I haven’t tried yet, so I’m not sure if this is needed).
• Rosette weld at the factory spot weld locations

So what do you guys think? Does this sound reasonable, or am I crazy trying this as my first real project? Anything that I’m planning to do that you’d recommend against? What are your thoughts on butt weld vs lap weld on the new section? Thanks for any tips!

[Turbo]

Learning to tig weld is great, but you should probably make friends with someone with a mig. Seam sealer and undercoating burn and make a big sticky flaming mess.

[tweake]

why not see the local repair shop for advice?

i don't know where you are but over here a repair like that is not allowed for DIY'ers.
its restricted to certed welders only and they have paperwork to go with it.
one of issues afaik is type of steel used and heat treatment used.
certainly with late model vehicles manufactures have very specific repair processes. pays to find out what that is first.

[MarkL]

I've done a fair number of repairs on farm equipment that is similar to what you're trying to weld: a 12ga bush hog deck is always rusty and has paint and oil on it. Things I've found are that everything needs to be really clean and free of rust or I won't be successful with tig. I've tried two approaches, one is to make a patch that fits exactly for a tight butt weld. Tig doesn't like gaps, so the patch has to fit really well. The other approach which I don't think will work in your case is to have the patch overlap slightly and make it a lap weld. But don't try to weld a gap with tig unless you're really skilled. You'll also have to put in lots of tacks to keep things from warping and deforming. Your material is curved so it might not be as bad, but flat patches like to warp a lot so I put a lot of tacks and then gradually fill in the spaces between.

The other problem is getting rid of the rust and contamination on thin material. It's easy to grind on the edge of 0.1" material and end up with .050" material. The other problem is oil soaked into the metal. You can use a torch to heat the metal and boil some of that off, then keep wiping it down. I've used different chemicals to remove rust to avoid too much grinding. Naval jelly works if you can rinse it off well.

I've never used weld thru primer but I can't imagine it would work with tig. My experience is if a mosquito flies by and coughs on your weld, it will be a problem. Maybe there are products that work but I'd test them first.

You'll also probably need a torch control rather than a foot pedal, I've worked the pedal with the side of my head, my knee and my elbow, none of that gives a really good result.

You've picked a tough first project, I'd practice on a lot of similar material first. Get yourself some lightly rusted material and try to weld it so you see what happens. Put a little oil on and practice with that so you know what it looks like. If you can get another similar part from the junk yard, you could practice on that with pieces of scrap as patches before you try the real thing. You're doing 90% of the work getting everything prepped. You might go to a tig welder and have them give you a price to do just the welding for you once you do all the time consuming prep work. I don't think it would cost that much.

[rb0087]

Thanks for the replies!

I actually do have at least one friend with a MIG nearby, but I'm always hesitant to borrow and use another guy's tools when I don't know how to use them. I'm afraid that by the time I learn the process, I won't have a friend with a MIG anymore.

I haven't gone to a local shop for advice yet, but I have read up on GM's factory repair guide, because like you said, modern cars have very specific repair instructions. (For others who might be interested in looking that up for their car, it's available for free here . ) The good news is that the only exotic steels that they call out are in the side impact crash structure; the rest seems to be just conventional mild steel with some aluminum in a few places. The bad news is that GM doesn’t provide any repair procedures for this part of the vehicle, doesn’t sell this replacement part, officially advises against using any salvage structural parts, and says that if you can’t find a repair in the manual that means it can’t be done. So the only 100% safe, GM approved repair procedure is to junk the car and buy a new Cadillac. I can understand where they’re coming from to some extent, but that puts a body shop in a bad position if they were to agree to fix this for me. If I were them, I wouldn’t touch this with a 10ft pole; they would be doing something that GM specifically says not to do, and they’d be entirely liable if something bad happened. I’m an engineer, though, not a lawyer, and I’m not in any risk of suing myself, so when I look at this problem, I see a section of frame that is mostly gone but still hasn’t failed. Compared to this, a small knock down in the strength at the heat affected zone doesn’t seem like a big problem as long as the weld is good.

Thanks for all the tips, Mark, those are very helpful! I knew that prep was important for TIG, but I never thought about trying to weld some old contaminated metal for practice, that sounds like a great idea. I hadn't thought about the oil soaked into the metal, either. It definitely has some of that, especially after I sprayed some oil on it to try to keep the rust from growing further out of control. Doh. I will also definitely try the weld thru primer on something else first to see how/if it works with TIG. I also haven't tried a torch control yet, but there is one sitting on the shelf next to the welder, so I think I'll try that out next. It seems a bit trickier than using a pedal, but much easier than using my head/knee/elbow on the pedal !

Thanks for the advice on the joint, also. The other approach that I thought about was to add another piece of metal behind the butt weld, like this:

Do you think that approach might be better?

I definitely have considered getting help from a real welder, too, once I've done the prep work. There is a guy at work who is really good, and I've been getting feedback from him on my practice welds. I might see if he'd be willing to help.

[kiwi2wheels]

Get some draw ( wing nut) type clecos, they will make your life a lot easier and use their holes for your plug welds.

Use a staggered ,two row, layout if possible, so you have a good fit on the backside.