Bonding Aluminum with Epoxy

Simon, interested in the techno so far i only went speedreading through the book and quoted what i read
i have a few more weeks before handing it back to librairy and will go through it more thouroughly and post
cheers

 

Ough, very interesting FLM’s fibre metal laminates, here an excerpt from book mentioned.
Glare is glass and Al sandwich (0.2 mm Al layers) its stronger and safer and has cost and weight savings over Al and Arall (aramid Al) sandwich witch was first and still used.
Al 2024 or 7475 is used and a chapter goes into a dozen different oxides and treatments.
Both sandwiches have excellent damage tolerance and are used in airplane flooring cargo doors outer skins, bulkheads etc.
mostly riveted but also bonded and are more durable and better impact and corrosion resistant than Al.
Its also used in containers for better blast resistance. My thoughts drifted to naval warfare.
The laminates sometimes replace double skins like in airplane flaps because of its low fatigue.
For the sandwich the topsheet is most important.
Milling and drilling may cause delamination as bending but pics show that may go a long way. Talking airplane production in the book of course.
On the net I read there are plans to use it in car production.
Buckling in high sheer stress (always in the middle) is possible but the material can be joggled.
Patches can be applied and ultrasonic inspection is done with electromagnetic induction.
Stretch forming and double curved sandwich panels are more difficult than monolithic materials but can be made in self forming moulds complete with stringers and internal doublers and cut outs with a vacuum mould autoclave.
A chapter on all sorts of splicing and these materials have better lightning protection as Al or carbon.
Fatigue on riveted joints chapter and Finite element modelling is elaborated on but I’ve not seen or heard of these materials in a program yet.
I think it’s a interesting change although not first choice for the backyard boat builder, book goes back to the library in exchange for some general engineering books.
This book is one out of a series of three.

 

If you firmly believe this, then you should NEVER fly on a commercial airliner made by the likes of Airbus, Boeing or McDonnel-Douglas since all the aircraft they have produced in the last 30 years rely extensively on bonded aluminum structures that carry primary loads.

Etch, Alodine and then prime the aluminum substrate with mil-p-23377 primer, then bond while the primer is in 'green' cure, preferably between 12 and 24 hours. To increase bond strength, heat the item to be bonded during cure (if possible/practical) to co-cure the primer and adhesive together.

Jimbo

 

Hello SeaSpark,
Generally speaking epoxy and methacrylates are perfect for joining aluminium to aluminium or aluminium to composite in a marine environment. The reason joints fail "usually" is poor joint design and thin bondlines. Speak to a good resin manufacturer or supplier near you and ask for the correct surface preparation of the resin they recommend. Its that simple. In regard to the joint design if you are not able to design the joint yourself speak to an engineer who can. Joints are best designed so the load is transferred in shear or compression, not tension or flexure and have adequate surface area.

Get a resin that has a shear strength of at least 25MPa and good elongation say 20% plus. Shear strength is what is required for the joint strength so higher is better eg toughened epoxy and PMMA's are about 30MPa strength. Elongation is required for toughness and good peel strength. If you use a polyurethane you can get very high elongations eg 100%.

Plus get the resin supplier to specifiy the correct bondline thickness if they can. Typical bondline thicknesses are 0.25 to 0.75mm. This can be controlled by the part design or by using glass tissue or linen (prior entry used glass beads) to control the bondline thickness.

Good luck, by the way all the comments about moisture ingress through the epoxy are incorrect. The usual problem is ingress along the bondline due to "neverbonded" bondlines not through the resin. If the surface can be wet sanded with epoxy this is a very good method. The epoxy needs to "wet" the surface of the metal, especially aluminium to create a good bond.

Peter S (Composite Engineer)

 

"Sanding in' the resin to form a good bond is a very old and effective trick to ensure that you are truly bonding to the aluminum and not the aluminum oxide layer which inevitably forms on exposed aluminum.

But it does not address the long-term stability of the aluminum surface being bonded and therefore cannot insure the durability of the bond itself.

Passivation followed by (chromium) conversion is the only known method that will accomplish both these goals. This is ultimately better than 'sanding in' the resin and likely much more practical, especially if the bonded area is large.

The method also allows for easy inspection of the bond surface for detecting the presence of contaminants or aluminum oxide. There's no way to do this while sanding in the resin.

 

Here is a complete specification for bonding aluminium from Hexcel Corp. I totally agree about passification and correct pretreatment.

www.hexcel.com/NR/rdonlyres/2BCC65F5-AB64-4D9B-810A-705E77ADD3D2/0/redux.pdf

Regards Peter

 

For Lotus Elise fans...

http://www.sandsmuseum.com/cars/elise/information/technical/asauto.html

 

How about bonding copper-nickel and wood/ply?

 

who has ANY proof (not "its in this book" as proof,,,proof as IVE done it) that ANY adhesive is ANYTHING other then a SECONDARY bond,,,almost every post ive read says we use it here,,or we use it all the time as a bonding material,,,,,when in fact,,welds, rivets,and so on are for the STRENGTH,, and the adhesive is to HELP the PRIMARY fastener with its job.

 

Secondary Bond

Firstly you need to define Secondary Bond. In the composite industry a Secondary Bond is a bond to a laminate that has already cured. Looking at your text though I think you mean that the structural element is the fastener and the bondline is the "backup". This is incorrect thinking. Fasteners are used near bonds for only two reasons:
1) To assist in aligning the parts during assembly until the adhesive is cured. Plus in some cases were the part is post cured to hold the part together in this process as the adhesive will soften while in the oven
2) To stop the joint peeling apart in service if peeling is a problem. If there is suitable geometry and space peeling can be designed for in the bondline so fasteners are not nessessary.

In all the joints I've analysed or designed the bonded joint has been stronger then a joint using rivets or fasteners in the same space. There is an adhesive for every application now just ask 3M, Lord, Epoxy suppliers or Sika for the solution.

 

well in my boat industry they told me a secondary bond is the second bond method in 1 application,,bonding something to an already cured "whateva" is a mechanical bond ,,the bonding of the 2 pieces and then another layer on top,,(within so many hours) is a chemical bond,,,or have i missed out on the newsletter? hehe

 

A mechanical bond is a when a keyed surface is used to "join" to and there is no chemical bond present. Polyester/Vinylester and epoxy all chemically bond to each other but some combinations are better then others. Mechanical bonding is not a good approach to use in boatbuilding. eg if you grind a laminate and bond to it with the aim of a mechanical bond it would be unsuccessful. It would be like grinding a PVC pipe and trying to bond a polyester laminate to it, it may look like its stuck but it hasn't.

eg 1 if you have an old polyester laminate and try to bond to it with polyester resin it will probably fail. This is due to the polyester having completly cured and no new crosslinking sites are available for further bonding. This is a secondary bond (its called a secondary not to say it is inferior, but to say it is the second application of laminate)
eg 2 if you use epoxy or acrylic adhesive on an old polyester laminate it will work as the epoxy and acrylic use different bondsites to the polyester resin. This is again a secondary bond. The primary laminate is the first laminate built.
eg 3 If you build a polyester laminate you have a few days before all the bondsites are taken up. I don't know the offical time it would depend on the resin and temperature. But if you then laminated to it using polyester it would be a chemical bond as there are available sites for the crosslinking to occur.

Hope this helps
Peter

 

I am surprised that methacrylate glues are not mentioned more. (Peter did...)

Many aluminium parts are succesfully bonded with methacrylates in marine, airospace and automotive environment.

I am familiar with the ITW Plexus range. They carry a primer (PC-120) which etches the oxide layer. Then a durable glue bond can be made using MA-300, MA310 or MA420.
However, methacrylates demand more accuracy then epoxy glues. Temperature is a factor, mixing is more critical, and bondline thickness needs to be observed as well (1mm for Plexus). Most manufacturers have all the procedures in the specsheets.

I have had some aluminium bonded parts in my osmosis tester (submerged, 60 degrees C) which never came apart. The must have been there at least for 6 months.

When using epoxy to glue aluminium, keep track of the elongation at break. This should indeed be high (thank you Peter). Normal epoxies do not have an elongation which is that high, so use a "rubber toughened" epoxy, and even then check the spec sheets.

 

Polyester over epoxy


By Jeff Wright

Most production fiberglass boats are made with polyester resin. WEST SYSTEM epoxy is a wonderful material for repairing polyester fiberglass boats. One reason for this is the ability of epoxy to form a stronger mechanical bond to a damaged laminate than polyester resin. Epoxy also provides a better moisture barrier than polyester resin.
Understanding the materials
Polyester resin laminates achieve their highest strength when the bonds between layers of fiberglass are chemical or primary bonds as opposed to mechanical or secondary bonds. The manufacturing process for polyester boats has been developed so that a chemical bond exists between the gelcoat and the laminate. When a polyester boat is built, polyester gelcoat is first sprayed onto the mold surface. The first layer of the laminate is then applied to gelcoat, which has not completely cured. The two layers eventually cure together with a chemical bond between them.

Applying gelcoat to a cured laminate relies on a mechanical bond. Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat. We developed some tests, to determine whether or not the mechanical bonds achieved between gelcoat and properly prepared, cured epoxy were strong enough to achieve a durable repair.
so west system says we're all right,,,secondary and mechanical are the same thing,,so that kinda makes us a little wrong too,,hehe
some more alum stuff
(1) Department of Materials Science & Engineering, University of Cincinnati, 45221-0012 Cincinnati, Ohio
(2) Present address: Department of Polymer Science & Engineering, University of Akron, 44325 Akron, Ohio

Received: 21 December 1993 Revised: 1 July 1994
Abstract The adhesion-in-peel test was used to determine peel strength and adhesion characteristics of a cured-in-place silicone elastomeric joint sealant on aluminum substrates. The sealant used was a Dow Corning Type 3145 RTV Adhesive Sealant. The results showed that the silicone sealant had poor adhesive bonding to the untreated aluminum. Plasma polymerization of hexamethyldisiloxane (HMDS) onto the aluminum was seen to move the locus of adhesive failure to being between the plasma film and the silicone. Plasma polymerization of HMDS with oxygen carrier gas produced excellent adhesion and cohesive failure in the silicone was observed.
http://www.springerlink.com/content/h7w872lj3x37tv48/?p=dfc74956036549e0b4adff8c4879b5d6&pi=4
wowz,,,dem sum big werds,,hehe

 

This is an interesting thread,in the early 90s my son and i had a snowboard manufacturing company,prior to that i had been a boatbuilder for 20 years an thought i knew how to glue things,boy was i wrong.When we started there wasnt a manufacturer in the world who could build a board which would stay together for more than a season.There were a lot of different materials with different properties all pressed together into a very flexable product that then got abused unmercifully in sometimes sub zero conditions,by the time most of the manufacturing ceased in the us and moved to Asia we were building reliable product. One of the most important keys to this was acid etched rubber foil,this stuff was only about .010" thick and provided a slightly flexible interface between,say,the hardened steel edge or aluminum tip and tail protectors and the glass epoxy laminate.We of course also used toughened press mold epoxies.
Steve.