Joining Extrusions together
For many years the automotive industry has been using adhesive for joining extrusions together, indeed bonding Aluminium is commonplace in many industries.
It is sometimes difficult to convince the safety designer to use bonding and not conventional fixing methods such as Rivets, screws, or Welding. Though these all have their place in aluminium fabrications they will all have some adverse effect on the material adjacent to the point of joining.
Some of you will remember (if you are as old as me) the Comet airliner sometimes called the Whispering Giant and now the Nimrod. This plane was the first to have completely bonded wing spars and when the first one was decommissioned and broken down, the wing spars were found to be stronger than the day it was built.
The reason is clear, rivets cause fretting around the hole and can start a fracture so too will screw and although welding will give a lasting joint the adjacent aluminium will be stressed by the heat and can cause stress fractures.
Bonding, as long as the surfaces have been properly prepared will hold the two sections together without putting any stress on them, also due to there not being any movement between the bonded aluminium components there will be no fretting and subsequent breaking of the aluminium.
We have discussed the merits joining extrusions by bonding over mechanical fixings, Such as Rivets, Screws etc although they all have their place in the fabrication, however, if you are following the Bonding route you need to understand the bond line to ensure you don’t create a weak point in the structure.
The preparation of the Aluminium is key to getting a good surface for bonding, the aim is to remove if possible the Oxide layer which will form on the surface of the extrusion.
Abrasion, will take most of it off and produce a roughened surface which will help the adhesive key to the aluminium, suggest emery paper or Stainless steel Brush but NOT STEEL you must not contaminate the Aluminium with steel as you risk building a potential which can cause corrosion.
If at all possible you should have the material Anodised then the surfaces should be protected and the anodiser needs to know that you intend to bond, the reason is some anodisers use a Lanolin to enhance the finish after anodising, you will not bond to that surface.
Note Anodising is an insulating surface so if you require electrical continuity do not use this process see below.
Alochrome 1000 will give you a good surface for bonding and also give you an electrically conductive surface, This process is also recommended if you wish to paint the extrusion as the paint will key well to this surface.
Checking for grease A simple check before you bond is to put a few drops of water onto the surface of the aluminium, if it balls up it has grease on it, if it wets the surface easily it will accept the adhesive also.
The ideal Bond line will be as thin as possible keeping the surfaces as close as possible, more adhesive will not necessarily increase the strength of the bond as the further apart the two surfaces are the more you rely on the inherent strength of the adhesive and not the BOND.
Always follow the adhesive manufacturer’s preparation instructions.