MIG vs Arc Welding Advice Needed

Greetings,

The subframe tabs to which the lower control arms of some, older, Mercedes, sports cars are bolted had a tendency to crack. So Mercedes recalled some of these cars -- particularly those with iron blocks -- to allow dealers to weld 1/4" thick, steel gussets between the tabs and the subframe's steell cross-member, thereby reinforcing the tabs.

Unfortunately, Mercedes refused to recognize that the same cars with lighter, aluminum blocks also had a tendency to crack. So people with these cars have essentially been left to fend for themselves -- which is why I am posting this message.

More specifically, Mercedes recommends using a MIG welder to weld these gussets between the tabs and subframe's cross-member. However, a very experienced welder I know sees no reason why he can't do the job with the ARC welder he's been using on car frames and other things for the last 30 years.

My question is "Is there any reason why an arc welder can't or shouldn't be used to weld these gussets to my car's subframe?" The only factor I can think of is that the arc welder may have a greater tendency to burn the rubber bushings that surround the bolts passing through the tabs. OTOH, I have read that there are heat resistant materials that can be used to shield such things from the heat generated during welding?

I would therefore appreciate any advice anyone can give me on the best way to go here. Thanks very much in advance.

Tom

Reply to
Tom Johnson
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In as simple terms as I can think of that you can understand, it won't work. Arc welding is the LEAST efficient way to weld aluminum because of a lot of reasons.

Imagine this. Hammers can fix a lot of things. They cannot fix everything. Why can't I use this hammer to fix my watch when it fixed my bent steel tractor part?

Steve

Reply to
SteveB

Reply to
RoyJ

The MIG weld is usually preferred, easier to control in the odd positions, thin cross sections, and relatively clean metal involved. Plus most auto techs are not trained in stick welding, many more are trained in Mig. So the bulletin will specify Mig.

No particular reas> Greetings,

Reply to
RoyJ

What I got out of it was: the frame is being welded, it is steel.

The aluminum vs cast iron block only came into play to explain why the steel frame car, with the aluminum block, didn't get the gusset upgraded like the cast iron block cars did.

I don't see where welding aluminum was mentioned by the OP.

DJ

Reply to
IdaSpode

Top posted for your convenience:

I noticed the reference to block the first thing. But aren't we talking about two essentially exotic metals here, aluminum and cast iron?

Both are difficult to weld, even under the best of conditions. And both are hard to get to stay together even with the best of processes, heat treatment before and after welding, and all the other variables.

To have someone who is totally clueless and ignorant (no offense) ask why aluminum can't simply be welded by an arc process is unexplainable unless one wants to take the time to spend about a year with them.

True, an aluminum block may not stress a frame as much as a cast iron one, but they are lighter, hence the weight to horsepower ratio is increased and that alone skews everything about the stock body, frame, and engine. If an aluminum engine puts out the same horsepower as a cast iron equivalent, the difference in weight alone will make a lot of variations in torque and stress throughout the vehicle.

The question was not about aluminum or cast iron blocks, but gusseting frame cracks by the ineffective and failure prone method of arc welding aluminum. Notice the subject of fishplating with mechanical fasteners was not even broached?

Steve

Reply to
SteveB

Mebbe so. I believe few welders have the experience or confidence to weld this up. The weld may be good, but one has to ask how the driver managed to overstress the factory installed components in the first place. And will these stand up to a driver that obviously drives a car harder than intended, or drives roads that are causing failure:

I have taken a lot of pride in my career by overwelding things so I knew they would hold up UNLESS the operator overloaded the thing. In that case, I didn't lose any sleep at night if they caused it to fail on their own.

BTW, I hate the words "should" and "probably".

Steve

Reply to
SteveB

Yes, thanks for clarifying this. The steel gussets are supposed to be welded to the car's steel subframe, not to its aluminum block.

Actually, the bulletin specifies that the gussets "must be MIG welded," which is why I posted my question. I didn't know whether to interpret this as saying that a MIG welder is the only type of welder that can be used," or less adamantly.

use a 7xxx rod for strength. And certainly NOT use an oxy-acetylene torch.

Thanks again, He does intend to use a rod. The only question I still have is with respect to the bulletin's recommendation to loosen the bolts that attach the control arms to the tabs, and to move the rubber bushings surrounding the bolts away from the welding area. Does the heat generated by arc welding vs MIG welding carry a greater risk of melting these bushings, and if so, can a welding blanket protect them from the heat that arc vs MIG welding generates?

I also recently read about a gel that can be applied to things to render them more fire and/or heat resistant. Do you happen to know how effective that gel is?

Thanks for the additional clarification.

Tom

Reply to
Tom Johnson

First off, the factory installed frame components were undersized, had a problem with fatigue cracking that is aggravated by the heavy cast iron engine. The factory recall will only pay for the most pressing repairs, ie to the vehicles with the heavy engine block. Doesn't mean that at least some of the aluminum block models shouldn't get the same repair as a safety precaution.

I don't have access to the vehicle in question, nor to the tech bulletin that specifies how the repair should be made, nor to the metallurgical specs of the frame steel and the repair part. Without that data, I would HAVE TO use the word "probably" and "should". I work on a Porsche Cup car, we run into these issues all the time.

For this application with reasonable material thicknesses, weld quality is much more a function of welder skill than the choice of Mig vs stick. How many times on this NG has someone mentioned "using a Mig to lay a nice looking bead that just sits on top and doesn't penetrate"?? In this case we are trying to second guess some German Engineer as to why he specified MIG welding. Perhaps he has a very telling and specific reason for Mig, perhaps he was too lazy to test out alternate approaches. This is a Mercedes, they are VERY firm that NOTHING be done on these vehicles outside an authorized service shop. Said shops will have MIG, will not have stick.

All that said, I' probably try to stay with with factory tech bullet>> The MIG weld is usually preferred, easier to control in the odd positions,

Reply to
RoyJ

The gusset is 1/4", but how thick is the frame. Is it a lap or butt joint, etc.? Material thickness, joint type and your ability to clean the weld area is probably the most important factors between MIG and STICK welding in this application. But if your welder is truly experienced, he should have no problem making the call. However, an inexperience operator could really screw this project up with MIG or STICK, so make sure you have chosen your man well.

The effects of heat on the rubber bushings will depend more on their distance from the weld area than the type of welding as well. If they are too close to keep them wet with either towels or a spray bottle, remove them until after the mod. Here again, you need an experienced professional to make the call.

This is not a project for someone's cousin's nephew's best friend's brother, ......... but it should be no problem for any truly GOOD welder.

Reply to
Maxwell

One reason might be because MIG is a low-hydrogen process, which might be called for if the frame is a higher carbon steel. With stick, I'd be tempted to use low hydrogen rod (7018)... --Glenn Lyford

Reply to
glyford

Apologies to the group. I misread and thought they were speaking of welding an aluminum frame. Something that is getting more common in automotive circles.

Steve

Reply to
SteveB

Hi Tom- MIG might be a better choice if the weld requires more than one pass, which seems likely. If your man ends up going with rod, make sure the weld is thoroughly cleaned up before and between passes. Any inclusions between passes will start cracks under load, and cracks are bad-bad.

weld blankets won't do much to stop heat transfer from metal to metal. You might be able to use a chill plate behind it, but if you have any doubts, it's best to move the melty bits from harms way.

If it doesn't work, you can always bust out the Rotary Weld Eraser and call do-overs.

Good luck.

Reply to
utahparx

In experienced hands, either stick or MIG are capable of doing the job. To MIG, the area will have to be very clean - free of grease, oil, paint, underseal etc. MIG will naturally produce a harder weld. Stick will make a softer, more ductile weld. Most of all, the base metals being welded must be taken into account. Newer vehicles will likely have lightweight, high strength steels that may predispose to a certain welding method.

Phil

PS. there is a heatsink material in the UK called 'Coldfront' that may be what you were thinking of in preserving your rubbers.

Reply to
Phil

"Phil" wrote

Most of all, the base metals being welded must be taken into account. Newer vehicles will likely have lightweight, high strength steels that may predispose to a certain welding method.

Phil

And heating them, welding them, or cutting with a torch will weaken the HAZ. An important thing no one has mentioned yet.

Steve

Reply to
SteveB

Am i nuts here or could that have been the original issue?

the part being Over hardened left it prone to cracking (a la DeHavilland Comet)

in which case allowing the HAZ to enter its annealed state may actually help the overall strength of the part because the steel being too hard left it brittle compared to what would normally happen on a vehicle suspension?

Just asking a silly question that no one can likely answer.

Brent Ottawa Canada

Reply to
Brent

Reply to
RoyJ

Not a silly question.

Many of the heavy trucks, Kenworth, etc. have frames made from heat treated steel. I have read Kenworth manuals that strongly warned against marking scratching, drilling, welding on, etc., frames. Even to the extent of cautioning "do not walk on frame with caulks in boots".

Bruce-in-Bangkok (Note:remove underscores from address for reply)

Reply to
Bruce in Bangkok

Thanks for the interesting disussion, guys. As it turns out, my subsequent research revealed it is illegal to use an arc welder on car frames in the state of Pennsylvania, and insurance companies will not pay for arc welding frames, supposedly because it can weaken or destroy the frame's metal. At least this is what two welders who specialize in repairing car frames told me, and neither had any reason to lie about it -- as far as I can tell -- since both had arc and MIG welders.So, the Mercedes technical service bulletin may have specified that the gussets 'must" be MIG welded for a good reason after all.

With respect to protecting the bushings, one of the MIG welders assured me that he could wrap the bushings in wet towels, and control the heat enough to prevent them from melting -- as did the welder who said he would arc weld it. OTOH, the other MIG welded told me that he would have to unfasten the control arm and move it enough to insure that the bushings wouldn't be damaged. So, even though that's still a grey area, I'm leaning towards using the guy with the MIG who assured me that the bushings wouldn't be a problem. (I guess if worst came to worst, and he ended up destroying the bushings, he would have to replace them for me or risk being sued).

In any case, the car is a 1981 Mercedes 380sl.

Regards,

Tom

Reply to
Tom Johnson

The heat affected zone of MIG is smaller than stick. And yes, you can wrap wet towels to protect SOME things.

Steve

Steve

Reply to
SteveB

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