Larry,
That could be, and I certainly don't mean to question your sources
and experience. I think our different findings can be explained by the
following: I had been doing alot of reading on composites lately, and
seeing how many metal MGB panels I could replace with a FRP panel. What
I found was that the newer High-Strength auto steel used in
post-gas-crunch cars (~75 on in a few, and many more starting in the
mid-1980's) is very thin and manufactured with a complex heat-treatment
to give it extra strength. It doesn't respond to heat well at all. The
metal becomes very brittle in the HAZ, and thus far weaker than an
equivalent thickness of mild steel. There were all kinds of problems in
the early years after the introduction of the high-strength steel with
body-shop repairs, and people using traditional methods for repairs.
Careful MIG and TIG welding can make a passable repair, but gas, stick
and spot (in general) really screw up the metal, leading to cracks,
corrosion, etc. etc. etc., so the manufacturers looked for heat-less or
low-heat methods of repair. The epoxy method rose to the top. It is
used for panel repairs, and at least for all non-load-bearing skins at
the factory, and now windshield attachment (now a load-sharing
component). It could be used for more, and possible some structural
assembly, but I don't honestly know. I just don't recall reading about
any. Suspension fittings and pillars all seem to be spot-welded on all
the late models I've had a chance to look at. At the factory, they can
program the robot welders to the bare minimum heat for factory
spot-welds and do so w/o harm.
I think we are both right, but with a twist:
Epoxy repairs and bonds are stronger than conventional welding to
High-Strength steel, because the heat would make the surrounding metal
weaker than the epoxy bond, not that epoxy is stronger than metal.
On mild steel, which we have on our MG's, welding is stronger, because
welding doesn't damage mild-steel.
This is all based on piecing several documents and sources together, not
on any one authoritative thesis. Do we have any metalurgists in the
group that can set me straight? In the space buisiness, we use all
kinds of metal/epoxy bonds, but spacecraft are still hand-built, and
have all kinds of funky fasteners to provide a large surface area for
bonding. Here, it seems that metal is used for bearing surfaces
(pivots, etc.) mostly. We spend alot of time trying to design a part
so that the metal and composite have equal, but opposite coefficients of
thermal expansion, so that overall length doesn't change. (diameter is
another matter)
James J.
p.s. If the surface area of the mating surfaces is large enough, and it
is covered entirely w/ epoxy, it can be made stronger than a stitch weld
or a string of spot welds, for the same reason a 3/4" aluminum rod is
stronger than a thread of steel (in tension), even though the maximum
STRESS allowable by steel is far higher than aluminum.
Peace.
Larry Hoy wrote:
>>Epoxy bonding (between metal and metal) is applicable where
>>brazing or soldering is acceptable, in my humble opinion based upon my
>>
>>
>
>
>
>>observations (not having seen any OEM documentation describing their
>>decision process).
>>
>>
>
>James J, It's my understanding that testing has found the bonded panels
>to be stronger than the traditional methods of assembly. (I'm assuming
>spot welding).
>
>The only product I'm familiar with is FUSOR. It is a two part epoxy
>that is mixed at the time of use by the tool that is used to apply it.
>It bonds the steel in thirty minutes and is fully set in 24 hours.
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