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Tade0

My friend is a car mechanic and his weekend car is a 1985 Nissan Z31, which he welded from two halves - front and back.

It's physically possible to do it and maintain reasonable safety but you need proper equipment, skills and time.

The frame is generally not designed to be welded like that, so without reinforcement it will be significantly weaker.

He did is as a hobby project but generally done properly such operations are not cost-effective.

Nevertheless it's still a service some shops provide at the cost of safety.

My aunt used to have such a car - she only noticed because she was blowing through sets of tires - the halves were misaligned(rotated) by less than one degree in each axis, but it was enough to make proper tire alignment impossible.

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> The frame is generally not designed to be welded like that, so without reinforcement it will be significantly weaker.

Welds can definitely be strong enough if done properly, as evidenced by the fact that welded parts involved in accidents can often be found to have broken somewhere other than a nearby weld.

However achieving the proper level of welding is easier said than done, and in particular being confident that a particular weld is done properly requires advanced equipment for non-destructive testing (x-ray, conductivity, echo ...)

You can hit it with a hammer and listen carefully! (No really, i've done some work on industrial robotics welding and it does this to produce a map of the weld it just performed for strength testing purposes - it can be done at the same time as peening)

Yes, that's a good idea to get an idea of the hardness of a weld. Better yet: anneal your welds.

Depending on where you live, buying a car with this modification may require disclosure. There are plenty of videos on youtube about people buying cars which end up being two parts welded together. I don't think it is safe at all.

Poland - we're in the "sweet spot" of having:

-Schengen(so a steady stream of cars imported from Germany with no additional fees).

-A Skilled workforce.

-Cost-conscious customers.

-Badly enforced regulations.

Which create an ideal environment for a cottage industry of shops which will take your storm-damaged aluminium roof sheets and turn them into an inspection, passing, moving car with a real VIN by next week.

I could create a Blade Runner'esque quote on stuff I've seen on the roads around here - especially regarding commercial vehicles.

It's less so like that than comparing to the 90s, but still.

Welding chassis of a car is a practice strongly recommended against, since it makes it significantly weaker at the welding point(s). It is very unsafe, although since it is an oldtimer restoration project, I salute your friend with respect!

Welding done correctly is as strong if not stronger than the base metal.

The weld will be stronger, but the heat-affected metal will become weaker than the rest of the metal, which is why if it breaks, it will be next to the weld.

Only when done improperly. Yes, this is a common failure mode, no, it does not have to be that way.

Typically the reason is not that the heat affected metal will be come weaker, typically the reason is that the weld is thicker than the material right next to it creating a stress point in the structure. The hardening can be taken care of by annealing.

The best way to fix cars is to increase the length of the weld if you can, the original weld points are there for a balance between economy and strength. If you start welding in 'original new and interesting places' you need to plan how you want to achieve the required strength since economy is likely the least of your worries.

Every stretch limo is essentially a demonstration piece to show that this does not need to be a problem, and that you actually can re-inforce a body quite nicely. Keep in mind that the wheelbase of a stretch limo pretty much guarantees much higher forces on all of the welding in the body including the existing seams.

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