User:Benatar.1/WE7407 Article 6

= Aluminum Joining =

Aluminum alloys are often chosen due to their low density, corrosion resistance, low cost, thermal and electrical conductivity. There are a variety of techniques to join aluminum together including mechanical fasteners, welding, adhesive bonding, brazing or soldering, etc. Various techniques are used based on the cost and strength required for the joint.

Mechanical fasteners
A simple and cheap method to join aluminum is using mechanical fasteners (i.e. bolts and nuts). Normally a hole is drilled into the base material and a fastener is placed inside. This type of joiner requires some type of overlapping material for a joint to be made. Aluminum bolts and nuts can be can be used; however, high-stress applications would require higher strength fastener material such as steel. This could lead to galvanic corrosion of different materials which have varying electrochemical potential. Significant corrosion would weaken the assembly over time and possibly lead to failure. In addition, different materials could result in thermal fatigue cracking from differing coefficients of thermal expansion. As the assembly is repeatedly heated stress can build up and enlarge the mounting hole. A commonplace mechanical fasteners are used is large aluminum panels with rivets on airplane exterior.

Adhesive bonding
Aluminum can be joined with a variety of adhesives. Aluminum may require some level of surface preparation and passivation to remove any unwanted chemical from the surface. Passivation could be as simple as rubbing alcohol or ultrasonic cleaning. Before bonding, a dry fit should be conducted to confirm proper fitting of the components. Adhesives may require heat, pressure, or both during curing; However, the adhesive application should be conducted as the adhesive manufacture instructions.

Surface Preparation
In order for a proper adhesive bond, some surface preparation is necessary. A surface cleaning to remove any impurities is made. The surface of the parts to be joined may be roughened with an abrasive such as sandpaper. This provides interlocking surface asperities and increases surface area for bonding. A chemical treatment may also be needed to increase the surface energy of the adherent and remove the oxide layer. Aluminum oxide is weakly bonded to the underlying aluminum metal and without removal, the adhesive joint is dramatically weakened. Oxides layers can separate from the metal substrate and is a key principle for adhesive failure theory, Bikerman weak boundary layer.

Type of adhesive
Adhesive selection can be dictated based on cost, strength needed ductility. Hobbyist commonly uses cyanoacrylate (super glue), epoxy, or JB Weld. Silicone may also be used in an application in which waterproofing is needed.

Welding
Most aluminum alloys can be joined by welding together; however, certain aircraft grade aluminum and other special alloys are unweldable using conventional methods. Aluminum is commonly welded with gas tungsten arc welding (GTAW). Due to aluminum's oxide layer, a positive polarity is needed break up the surface to ensure a proper weld. Alternating current (AC) is also used to allow the benefits of a negative polarity and enough positive polarity for a proper weld. Structural aluminum welding codes can be found in AWS D1.2. Aluminum welding typically creates a softened region in the weld metal and heat affected zone. Additional heat treatments may be needed to obtain an acceptable material for an application.

Brazing and Soldering
Aluminum can be brazed or solder to almost any material including concrete, ceramics, or wood. Brazing and soldering can be brazed manually or through an automated technique. Manual aluminum brazing can be difficult due to no observable color change before melting. Similar to other techniques, aluminum's strong oxide can prevent proper bonding. Strong acids and bases can be used to weaken the oxide or aggressive fluxes may be used. Brazing alloys for aluminum must have a relatively low melting temperature which is above aluminum's melting temperature (660°C). In addition, aluminum alloys with high magnesium can "poison" fluxes and depress the melting temperature which can result in a weak joint. In some cases, the aluminum parts can be cladded with a different material and brazed with a more common technique and filler material. Brazed joints require overlapping of parts. The amount of overlap can greatly affect the strength of the joint.