MIG brazing for car body panels

[MCKDAVID]

It is not really anything to do with the strength of the joint it is how the applied heat effects the properties of the parent material now used for most car bodies

now that makes sense, thanks for that...

 

[PTvor]

"Not acceptable for anything structural, or within a given distance of anything structural, I think."

Maybe I'm getting senile, but I seem to remember that most of the older cars (70s / 80s) the roof / pillar sections were brazed to the body by the factories???? that would be structural...

Certainly the trick frames for the motorbikes of that era and before were all gas brazed, so the stength is clearly acceptable, perhaps its the individual method that was in question.... gas versus mig... both being done properly, I don't see the difference, the end strength result should be the same? I would guess that mig being the modern preference as faster in terms of production, with less distortion, or am I talking bolloks....

Here you go

http://www.motuk.co.uk/mot_appendix_c.htm

and here for the pdf.

Inspection Manual for Classes 3, 4, 5 and 7 Vehicles

http://www.motinfo.gov.uk/htdocs/

Appendix C Structural Integrity and Corrosion

10. Acceptable methods of repair
It is essential that repairs to corroded or damaged areas are properly
carried out. This requires that suitable materials of appropriate gauge or
thickness are used for repairs. Additionally:
• any plating or welding extends to a sound part of a load bearing
member
• the repair must appear to be virtually as strong as the original structure
with only continuous seam welding being acceptable for patch repairs
(even if the patch extends beyond the prescribed area).
• spot welded joints are acceptable where the original panel has been
replaced to an existing spot welded flange (provided the original
defective panel flange has been removed). Stitch or plug welding is
acceptable as an alternative to spot welding in these cases.
• MIG brazing, a combination of adhesive bonding and riveting, or
amalgamations of these with other joining methods may be the vehicle
manufacturers recommended method of repair. Such repairs must
therefore be accepted unless they are clearly inadequate.

11. Unacceptable methods of repair
Gas brazing, soldering, adhesive bonding, fibre reinforcement and body
filler are bonding processes and are not regarded as strong enough for
repairs to load bearing members although they are normally adequate for
other repair work.

They give diagrams showing what they consider to be load bearing and it looks as if roof pillars are.

Maybe there's a difference between what manufacturers are allowed to do, or were allowed to do, and what's considered an acceptable repair? They also talk about cars with a separate chassis, (Part 8 Vehicles with separate bodies), where the chassis is considered the main load bearing structure.

Back in the day when it was mainly mild steel, I can't see that the heat of welding would make a lot of difference. With modern cars with special steels being used it's all more critical and it looks as if the MOT test designers tell you to go with what the maker suggests, glue and rivets, MIG brazing or whatever, but definitely no gas brazing.

It looks as if there's a fair amount of discretion on the part of the tester to decide what's structural and what isn't.

Section 4 talks about "any load bearing or supporting structure or supporting panelling within 30cm of the mounting location."

Section 6 pretty well condemns any welded repair to steering and other highly stressed parts.

 

[AndyStobbs]

In years to come when these boron steels need repairing due to corrosion its all going to be pretty academic how they're joined in. Anything will be an improvement on rust.
I very much doubt people will be making patches from Boron steels either.

I realise that presently we are talking about crash damage repair not corrosion repair.

 

[arthur]

I did read that Honda now use friction bonding of steel to alloy, sounds great unless you negs to fix it. I'm not a new style fancy material man but a mate of mine is, he welds with mig brazing and uses some super duper bonding agents far stronger than welding, I was shocked how new techniques can bond steels and alloys so well!

 

[PTvor]

In years to come when these boron steels need repairing due to corrosion its all going to be pretty academic how they're joined in. Anything will be an improvement on rust.
I very much doubt people will be making patches from Boron steels either.

I realise that presently we are talking about crash damage repair not corrosion repair.

An interesting point.

When I asked at the garage I go to for MOTs about their MIG welder (a Migatronic 221) they said that 15 years ago when they bought it, they'd use it a couple of times a day. These days they use it once a week or once every couple of weeks. The cars they see in the main are up to 15 years old and rarely fail MOTs because of rust. Nothing like the 70s when your pride and joy would be riddled with rust in time for its first MOT.

I've seen them do MOTs on vintage cars, but these are lovingly looked after.

They don't do crash repairs.

I get the impression that cars are scrapped these days because they are old looking, or a big ticket item such as a cat converter or engine management system unit is fubar and the owner sees it as a better bet to spend the money on a known working motor.

I'd guess that crash repairs to cars with boron steel is something that insurance companies and makers have had to take account of. Corrosion repair 20 years down the line, they couldn't care less about and on the showing of 10-15 year old cars now, not many care.

 

[matt1978]

The high strength steels used nowadays also have a zinc coating on for corrosion resistance. Reduced heat methods of joining such as mig brazing are not only there to help the metal retain its properties but also to limit damage caused to the coating