Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Wed, 10 Sep 2003 02:14:50 -0400
Message-ID: <redirect-2574520@logan.com>
From: "Mike Hutchins" <mhutchins@attglobal.net>
Subject: Adhesive Bonding of Aluminum
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Hi Denny,

Adhesive bonding of aluminum components can be a tricky process, especially
if you want a permanent bond (20+ years). Surface preparation is extremely
critical, and amazingly time sensitive. The ultimate goal of surface
preparation is to eliminate any possibility that an Al-oxide has formed on
the surface of the bond region. The strength of the Al / Al-oxide interface
is significantly weaker than the base material, and therefore must be
eliminated. This is true whether you are creating an Al / Al bond or an Al /
composite bond.

Hysol has a Surface Preparation Guide that very briefly covers two surface
preparation methods suitable for Al. Throughout all of the processing steps,
it is absolutely critical that the bonding surfaces are not touched. Here
are the basic steps involved in preparing the bonding surfaces:

1. Degrease in a vapor bath of trichloroethane
2. Perform a Chromic Acid Etch (1 liter H20, 300g H2SO4, 60g Na2Cr2O7)
   a. Etch metal for 12-15 min @ 150-160 degrees F
   b. Immediately rinse in tap water for 5 min, then soak rinse with
distilled or deionized water
   c. Dry thoroughly at 120-140 degrees F
   d. Apply adhesive primer or bond within 16 hours, otherwise, start over
3. Perform Phosphoric Acid Anodizing (optional) - skip 2d and proceed with
process below
   a. Apply adhesive primer or bond within 16 hours, otherwise, start over

The Society of Automotive Engineers ("SAE") specifies a more rigorous
protocol
(http://www.sae.org/servlets/productDetail?PROD_TYP=STD&PROD_CD=ARP1524A),
which appears to have been developed by Boeing, as referenced in the Hysol
guide, above. The full process is described in SAE Aerospace Recommended
Practice 1524 (ARP1524) "Surface Preparation and Priming of Aluminum Alloy
Parts for High Durability Structural Adhesive Bonding". The steps detailed
in ARP1524 include:

1. Solvent Cleaning - Vapor degreasing with trichloroethane
The remaining "wet" steps must be performed in a continuous process, without
allowing the bonding surfaces to dry until the final drying after the
Phosphoric Acid Anodizing.
2. Alkaline Cleaning - nonsilicated alkaline cleaner
   a. 10-15 minutes in bath at 150 degrees F
   b. Rinse - Immersion/Spray with clean water for 2 minutes at 65-110
degrees F
   c. Rinse - Immersion/Spray with clean water for 5 minutes at 100-140
degrees F
3. Deoxidizing - this step replaces 2a-c, above, and is more environmentally
friendly (Chromic free)
   a. 10-15 minutes in bath of either
      1. Dilute Sulphuric Acid (17 parts of 94% H2SO4 plus clean water for
final volume of 100 parts) at 150-160 degrees F, or
      2. Dilute Phosphoric Acid (6.3 parts of 85% H3PO4 plus clean water for
final volume of 100 parts) at room temperature
   b. Rinse - Immersion/Spray with clean water for 2 minutes at 65-110
degrees F
   c. Rinse - Spray with deionized water at room temperature to remove tap
water
4. Phosphoric Acid Anodize
   a. Place in dilute H3PO4 as in 3.a.2 above at room temp (72 degrees F,
optimal)
   b. Apply DC voltage within 1 minute and raise to 10 volts over 2-5
minutes
   c. Maintain 10 volts +/- 1 volt for 20-25 minutes
   d. Rinse within 2 minutes of discontinuing power - Immersion with clean
water for 10-15 minutes at room temp
   e. Rinse - Spray with deionized water at room temperature to remove clean
water
5. Oven Dry
   a. 30 minutes at 175 degrees F
   b. Cool to room temp, maximum of 2 hours
6. Apply Primer (primer shall meet requirements of Aerospace Material
Specification 3107)
   a. Within 96 hours in low humidity environments and preferably within 2
hours, apply a corrosion-inhibiting, high-durability modified epoxy resin
primer (see spec -
http://www.sae.org/servlets/productDetail?PROD_TYP=STD&PROD_CD=AMS3107/4A)
to achieve a dry film thickness of 0.0001"-0.0002"
   b. Air dry for 30 minutes
   c. Oven dry for 1 hour at 255 degrees F
7. Perform Adhesive bond within 90 days, otherwise, strip part and start
over

Simple, huh?

After your bond has cured, it would probably be a good idea to secure the
ends of your stiffeners with a screw or a rivet to eliminate any chance for
the bond line to be subjected to "peeling" apart. It is surprising how
easily these bonds can fail in peel when subjected to a wedge test. For
example, I believe screws are now recommended at the ends of the hat
stiffeners used on all Lancair nose gear doors in lieu of rivets to prevent
this failure mode.

Whew!

Now, is all this really necessary for your application? Only time will tell,
but I suspect not. However, for any structural application, it would
probably be a good idea. One example that comes to mind is the bonding in to
the fuselage of shoulder belt anchor points.

Send us a picture or two once you get the stiffeners installed.

Best Regards,
Mike Hutchins