Epoxy Bonding to Aluminum and Magnesium

I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered
homebuilts. The Conventional Wisdom bolts the thing to the firewall
using the clucth end bellhousing, often with an accessory case in
between.

The Ultracruiser Plus is different. Two aluminum angles are bolted to
the sides of the magnesium case, in the sump area, and, suitablely
reinforced, are used to bolt Berry mounts to "bed" type engine
bearers built up out of aluminum, that extend from the forward
fuselage.

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?

It's probably not carrying any loads. A basic tenet of structural
design is that the stiffest load path carries the load, and the bolts
through the angle and into the case (secured with nuts and washers
inside the sump, before the engine is assembled) seem a bunch stiffer
that the epoxy.

It's not a sure stop against leaks. Epoxy is a wonderful material, but
it doesn't bond particularly well or reliably to metals. Plus, it's
mechanical properties, from it's modulus of expansion, to it's
ductility, are far different that the aluminum, steel, and magnesium
sandwich is it the Mayonnaise of. My concern is the epoxy will
eventually crack.

I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase:

"The epoxy is to keep the angles from working and hogging out the
holes. The epoxy I use is a real good sealer, you've got to grind it
off."

My thoughts are this: LAP the angles to the side of the case (instead
of sanding with 80 grit), but use Permatex or Curil T to seal things.
Use close tolerance bolts in reamed holes in the side of the case and
the appropriate Loctite product to seal the bolts. And I'm inclined
to put the bolt heads INSIDE the engine.

Comments?

To see what this installation looks like:
http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal

To be honest this all sounds like a bit of a mess.

There are a number of issues here ... however it is difficult to comment in
any detail until I know what sort of epoxy is specified and without further
details of the surface preparation and details of the design.

1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx
90 deg C. If the epoxy is taken above that temperature then two things
happen ... first it softens and the stiffness and strength reduce
dramatically. Second irreveraible damage is done to the epoxy and it will
never be the same again - even when cooled. Unless you are using one of a
small number of epoxies that are designed for high temperature operation
(some of these have Tg of approx 400 F, 200 C I think without reaching for a
calculator) then it is certain I think that if used on an engine the 90 deg
C limit will be exceeded.

2. Next there is the issue of surface preparation. If the plans say prepare
by running with 80 grid paper then it is fairly clear that the guy who wrote
the plans knows little about what he is doing ! Epoxy metal bonding is
reliable if the surfaces are prepared properly but from what you have said I
doubt that is the case.

3. Bolted and bonded joints should be avoided because it is difficult to
predict the load transfer etc ... I won't go into detail on this - perhaps
later.

4. What are the differences in the thermal expansion coefficients of the
aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F
and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an
issue.

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

6. I think galling of the metal is a possibility but if appropriate
tolerances are used for the bolts and holes than this would be less of a
problem. Hard to say without seeing the drawings etc. Sounds to me like if
the holes are flogging out then the design has some fundamental problems and
that one should not be relying on epoxy that probably cannot withstand the
temperatures to fix it.

My gut feel is that you need something between the alumnium and the
magnesium for corrosion protection and possibly the help the galling issue.
I would assume that this is sufficiently ductile an rubbery that it will not
pick up any load and that the fasteners will transfer all the load. I would
use a rudder like sealant compound that can take the temperature ... not
epoxy. When you use sealant of this type in a joint with fasteners extra
largers of safety should be allowed because of the extra flexibility that
this produces in the joint (typically an extra factor of 1.5).



"Ryan Young" wrote in message
om...
I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered
homebuilts. The Conventional Wisdom bolts the thing to the firewall
using the clucth end bellhousing, often with an accessory case in
between.

The Ultracruiser Plus is different. Two aluminum angles are bolted to
the sides of the magnesium case, in the sump area, and, suitablely
reinforced, are used to bolt Berry mounts to "bed" type engine
bearers built up out of aluminum, that extend from the forward
fuselage.

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?

It's probably not carrying any loads. A basic tenet of structural
design is that the stiffest load path carries the load, and the bolts
through the angle and into the case (secured with nuts and washers
inside the sump, before the engine is assembled) seem a bunch stiffer
that the epoxy.

It's not a sure stop against leaks. Epoxy is a wonderful material, but
it doesn't bond particularly well or reliably to metals. Plus, it's
mechanical properties, from it's modulus of expansion, to it's
ductility, are far different that the aluminum, steel, and magnesium
sandwich is it the Mayonnaise of. My concern is the epoxy will
eventually crack.

I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase:

"The epoxy is to keep the angles from working and hogging out the
holes. The epoxy I use is a real good sealer, you've got to grind it
off."

My thoughts are this: LAP the angles to the side of the case (instead
of sanding with 80 grit), but use Permatex or Curil T to seal things.
Use close tolerance bolts in reamed holes in the side of the case and
the appropriate Loctite product to seal the bolts. And I'm inclined
to put the bolt heads INSIDE the engine.

Comments?

To see what this installation looks like:
http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal

"smjmitchell" wrote in message . au...

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!


Interesting. Where might I information like this for making decisions
on what metals can be safely bolted together w/r/t galvanic corrosion?
At issue right now is whether it'll be OK to thread my brass
fuel-system finger strainers and fuel drains into aluminum blocks, but
other such questions will certainly come up in future.

Thanks,
Greg

See below for data as requested:

Also see MIL-STD-899 for further info (you can download that from the web)
at http://stinet.dtic.mil/str/index.html


Metal Potential
Magnesium and its alloys -1.60
Zinc die-casting alloy -1.10
Zinc plating on steel -1.10
Zinc plating on steel, chromate passivated -1.05
Galvanised iron -1.05
Tin/Zinc (80/20) plating in steel -1.05
Cadmium-Zinc solder -1.05
Wrought Al-clad Al Alloys -0.90
Cadmium plating on steel -0.80
Aluminium alloy castings -0.75
Wrought aluminium -0.75
Non-stainless steel & grey cast iron -0.70
Duralumin type un-clad alloys -0.60
Lead -0.55
Lead-silver solder -0.50
Tin-lead solder -0.50
Tinned Steel -0.50
Cr plating (0.0005") on steel -0.50
Stainless 12% Cr -0.45
Tin plating on steel -0.45
Cr plating (0.00003") on plating steel -0.45
Chromium (99%) -0.45
Stainless High Cr (18/2) -0.35
Copper and its alloys (Brass, bronze, etc) -0.25
Nickel-Copper alloys -0.25
Stainless, Austenitic (18/8) -0.20
Silver solder -0.20
Monel metal -0.15
Nickel plating on steel Titanium -0.15
Titanium -0.15
Silver and silver plating on copper 0
Rhodium plating on silver plated Copper +0.05
Carbon +0.10
Platinum +0.15
Gold +0.15

Condition Max. Potential Difference
Marine and outdoor environment 0.3 Volts
Indoor environment 0.5 Volts
Interior assemblies, hermetically sealed No restriction




"Greg Reid" wrote in message
om...
"smjmitchell" wrote in message
. au...

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V
on
the galvanic table. That is a big different. You definitely need
something
to separate the two or the magnesium is going to get gobbled up !!


Interesting. Where might I information like this for making decisions
on what metals can be safely bolted together w/r/t galvanic corrosion?
At issue right now is whether it'll be OK to thread my brass
fuel-system finger strainers and fuel drains into aluminum blocks, but
other such questions will certainly come up in future.

Thanks,
Greg

Thanx for yourlearned and informed reply.

"smjmitchell" wrote in message . au...

To be honest this all sounds like a bit of a mess.

It does to me too, but on the other hand, there are a fair number of
flying installations like this, in spite of my "desk engineer"
concerns.

1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx
90 deg C. If the epoxy is taken above that temperature then two things
happen ... first it softens and the stiffness and strength reduce
dramatically. Second irreveraible damage is done to the epoxy and it will
never be the same again - even when cooled. Unless you are using one of a
small number of epoxies that are designed for high temperature operation
(some of these have Tg of approx 400 F, 200 C I think without reaching for a
calculator) then it is certain I think that if used on an engine the 90 deg
C limit will be exceeded.

That's about 190 deg F, which is a pretty comfortable oil temerature,
and this arrangement attaches to the side of the oil sump. But oil
temps up to 250 deg F need to be thought about. And that's higher
than most epoxy resins and adhesives, even post-cured.

I don't know what epoxy is used, I'll find out.

2. Next there is the issue of surface preparation. If the plans say prepare
by running with 80 grid paper then it is fairly clear that the guy who wrote
the plans knows little about what he is doing ! Epoxy metal bonding is
reliable if the surfaces are prepared properly but from what you have said I
doubt that is the case.

That's actually not outside the "standard of Care" for metal bonding
in low-stress operations. Gougeon brothers, makers of WEST and PRO-SET
epoxy, suggest sanding and etching for aluminum. The side of the
magnesium case is fairly rough, and not particularly flat. Sanding it
flattens it, removes the oxide coating, and still leaves some "tooth".

3. Bolted and bonded joints should be avoided because it is difficult to
predict the load transfer etc ... I won't go into detail on this - perhaps
later.

My point exactly.

4. What are the differences in the thermal expansion coefficients of the
aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F
and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an
issue.

There is also the steel bolts to consider, but their lower coefficient
of expansion actually tightens up the joint. Perhaps to the point of
fracturing the epoxy! I can't find good numbers on cured resin alone,
but cured composites have pretty low coefficients, like 2.0e-6.


5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at
the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of
cars, with WATER running through their engines, have cast iron blocks
and aluminum heads. In this situation, there is no ready source of
electrolyte. I don't see this as a huge concern. I'm more bothered
by oil leaks.

6. I think galling of the metal is a possibility but if appropriate
tolerances are used for the bolts and holes than this would be less of a
problem. Hard to say without seeing the drawings etc. Sounds to me like if
the holes are flogging out then the design has some fundamental problems and
that one should not be relying on epoxy that probably cannot withstand the
temperatures to fix it.

Amen! Albeit, there are those pesky flying examples to be explained
away....

My gut feel is that you need something between the alumnium and the
magnesium for corrosion protection and possibly the help the galling issue.
I would assume that this is sufficiently ductile an rubbery that it will not
pick up any load and that the fasteners will transfer all the load. I would
use a rudder like sealant compound that can take the temperature ... not
epoxy. When you use sealant of this type in a joint with fasteners extra
largers of safety should be allowed because of the extra flexibility that
this produces in the joint (typically an extra factor of 1.5).

This aligns with my thinking. High Temperature Room Temperature
Vulcanizing Silicone rubber looks good for this. The security of the
joint would be in the close fit of the mating parts, not in the epoxy.
The bolts would be a tight fit in reamed holes, backed up with
Loctite Red.

"Ryan Young" wrote in message
om...
I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered
homebuilts. The Conventional Wisdom bolts the thing to the firewall
using the clucth end bellhousing, often with an accessory case in
between.

The Ultracruiser Plus is different. Two aluminum angles are bolted to
the sides of the magnesium case, in the sump area, and, suitablely
reinforced, are used to bolt Berry mounts to "bed" type engine
bearers built up out of aluminum, that extend from the forward
fuselage.

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?

It's probably not carrying any loads. A basic tenet of structural
design is that the stiffest load path carries the load, and the bolts
through the angle and into the case (secured with nuts and washers
inside the sump, before the engine is assembled) seem a bunch stiffer
that the epoxy.

It's not a sure stop against leaks. Epoxy is a wonderful material, but
it doesn't bond particularly well or reliably to metals. Plus, it's
mechanical properties, from it's modulus of expansion, to it's
ductility, are far different that the aluminum, steel, and magnesium
sandwich is it the Mayonnaise of. My concern is the epoxy will
eventually crack.

I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase:

"The epoxy is to keep the angles from working and hogging out the
holes. The epoxy I use is a real good sealer, you've got to grind it
off."

My thoughts are this: LAP the angles to the side of the case (instead
of sanding with 80 grit), but use Permatex or Curil T to seal things.
Use close tolerance bolts in reamed holes in the side of the case and
the appropriate Loctite product to seal the bolts. And I'm inclined
to put the bolt heads INSIDE the engine.

Comments?

To see what this installation looks like:
http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at
the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of
cars, with WATER running through their engines, have cast iron blocks
and aluminum heads. In this situation, there is no ready source of
electrolyte. I don't see this as a huge concern. I'm more bothered
by oil leaks.

Auto engines have head gaskets between the electro different metals,
plus if you don't run an inhibitor is a mutli-metal water cooled engine
you will corrode the aluminum away, period.

Yours
Vern

Vernon Klukas wrote in message .. .
5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at
the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of
cars, with WATER running through their engines, have cast iron blocks
and aluminum heads. In this situation, there is no ready source of
electrolyte. I don't see this as a huge concern. I'm more bothered
by oil leaks.

Auto engines have head gaskets between the electro different metals,
plus if you don't run an inhibitor is a mutli-metal water cooled engine
you will corrode the aluminum away, period.

Yours
Vern

Vern is 100% correct. I have repaired several automotive engines where
this has been a problem. I used Belzona Super Metal (1111) to repair
the problem. Also have used it to fix a multitude of diesel engines.
These were do to electroylisis and cavitation. This is caused because
the owners did not change or maintain the proper chemical balance of
the antifreeze.
Toyota engines require the Toyota antifreeze. Dexcool is not approved
and according to a Toyota engineer, it will eventually damage the
engine. He also told me not to mix Dexcool with glycol antifreeze.
To make a long story short, follow the engine manufactures
recommendations.

Vernon Klukas wrote:
5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at
the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of
cars, with WATER running through their engines, have cast iron blocks
and aluminum heads. In this situation, there is no ready source of
electrolyte. I don't see this as a huge concern. I'm more bothered
by oil leaks.


Auto engines have head gaskets between the electro different metals,
plus if you don't run an inhibitor is a mutli-metal water cooled engine
you will corrode the aluminum away, period.

Yours
Vern

Rotary engines don't. Some people are running straight H20. Never
heard of that particular problem...period.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

I was thinking ProSeal or similar for the interface...

"smjmitchell" wrote in message u...
To be honest this all sounds like a bit of a mess.

There are a number of issues here ... however it is difficult to comment in
any detail until I know what sort of epoxy is specified and without further
details of the surface preparation and details of the design.

1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx
90 deg C. If the epoxy is taken above that temperature then two things
happen ... first it softens and the stiffness and strength reduce
dramatically. Second irreveraible damage is done to the epoxy and it will
never be the same again - even when cooled. Unless you are using one of a
small number of epoxies that are designed for high temperature operation
(some of these have Tg of approx 400 F, 200 C I think without reaching for a
calculator) then it is certain I think that if used on an engine the 90 deg
C limit will be exceeded.

2. Next there is the issue of surface preparation. If the plans say prepare
by running with 80 grid paper then it is fairly clear that the guy who wrote
the plans knows little about what he is doing ! Epoxy metal bonding is
reliable if the surfaces are prepared properly but from what you have said I
doubt that is the case.

3. Bolted and bonded joints should be avoided because it is difficult to
predict the load transfer etc ... I won't go into detail on this - perhaps
later.

4. What are the differences in the thermal expansion coefficients of the
aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F
and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an
issue.

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

6. I think galling of the metal is a possibility but if appropriate
tolerances are used for the bolts and holes than this would be less of a
problem. Hard to say without seeing the drawings etc. Sounds to me like if
the holes are flogging out then the design has some fundamental problems and
that one should not be relying on epoxy that probably cannot withstand the
temperatures to fix it.

My gut feel is that you need something between the alumnium and the
magnesium for corrosion protection and possibly the help the galling issue.
I would assume that this is sufficiently ductile an rubbery that it will not
pick up any load and that the fasteners will transfer all the load. I would
use a rudder like sealant compound that can take the temperature ... not
epoxy. When you use sealant of this type in a joint with fasteners extra
largers of safety should be allowed because of the extra flexibility that
this produces in the joint (typically an extra factor of 1.5).



"Ryan Young" wrote in message
om...
I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered
homebuilts. The Conventional Wisdom bolts the thing to the firewall
using the clucth end bellhousing, often with an accessory case in
between.

The Ultracruiser Plus is different. Two aluminum angles are bolted to
the sides of the magnesium case, in the sump area, and, suitablely
reinforced, are used to bolt Berry mounts to "bed" type engine
bearers built up out of aluminum, that extend from the forward
fuselage.

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?

It's probably not carrying any loads. A basic tenet of structural
design is that the stiffest load path carries the load, and the bolts
through the angle and into the case (secured with nuts and washers
inside the sump, before the engine is assembled) seem a bunch stiffer
that the epoxy.

It's not a sure stop against leaks. Epoxy is a wonderful material, but
it doesn't bond particularly well or reliably to metals. Plus, it's
mechanical properties, from it's modulus of expansion, to it's
ductility, are far different that the aluminum, steel, and magnesium
sandwich is it the Mayonnaise of. My concern is the epoxy will
eventually crack.

I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase:

"The epoxy is to keep the angles from working and hogging out the
holes. The epoxy I use is a real good sealer, you've got to grind it
off."

My thoughts are this: LAP the angles to the side of the case (instead
of sanding with 80 grit), but use Permatex or Curil T to seal things.
Use close tolerance bolts in reamed holes in the side of the case and
the appropriate Loctite product to seal the bolts. And I'm inclined
to put the bolt heads INSIDE the engine.

Comments?

To see what this installation looks like:
http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal

"Blueskies" wrote in message m...
I was thinking ProSeal or similar for the interface...

I was too, until I looked at the temperature resistance of
polysulfides (ProSeal being that). It's pretty low, it releases
pretty completely at about 250 deg F, whereas the cheapest RTV starts
from there, and they have readily available compounds that go up to
700 deg F or higher.

I've always looked down my nose at RTV, and I've seen many jobs
bodgered up using it, but in this case, I'm beginning to believe it's
the right goop for the deal.