Assembling Aluminum Parts
Aluminum parts can be assembled by making use of a number of techniques, each specific to the performance requirements of the extruded profile.
The following joining methods are popularly used within the industry today:
Mechanical Joining Mechanisms
These mechanical joining mechanisms are designed into the extruded custom aluminum parts and are specifically catered for in the design stages. These mechanical joining mechanisms include screw bosses, snap joints, dovetail joints and bolt receptors.
Other types of joints also found here include:
Here joints can range for a simple t-joint using screws and expansion locking making use of a wedge shape, to a simple t-joint applying nut tracks, brackets and bolts, and a stable solution used for corner joints on square tubes.
Using press nuts or blind rivet nuts, the only element required is a hole, eliminating the need for extra thickness for longer threads. Common Examples: Press nuts are fitted from the back, using excentric presses, while blind rivet nuts are fitted using a rivet gun from the outside.
Here, various types of hinges are used, ranging from the simple hinge (with a ball diameter of no less than 5 mm) and hinges with screw ports, to self-locking hinges.
Tracks (for nuts and bolts)
The use of continuous tracks enables step-less fastening without having to machine the profile. By making use of special nuts and bolts, fastening is able to occur without the assembler having to slide the nut and bolt from the end of the track. Fasteners can also be fixed longitudinally into place by making use of stamping techniques.
In cases where rigidity and strength requirements are high in corner joints, the use of brackets are suitable. These brackets are usually cur from aluminum profiles and are designed to suit several fitting techniques. Some examples here include corner joints using pre-mounted bolts, a plate and joint combination, the U-Section and the Tie using sprung steel clips.
The most common method here is the use of screw ports to secure plastic or metal caps to a box aluminum parts. In round aluminum profiles, a removable end cap with sprung arms are common.
Here the screw ports can threaded in the normal way and, most commonly, screw ports are used directly for self-tapping screws.
Because of aluminum’s elasticity, custom aluminum parts are highly suited for these types of joints. Resulting in a quicker assembly (when compared to screws or welding), it is a widely used assembly method.
After steel, aluminum parts are the most frequently joined using adhesive bonding methods. Here, intermolecular forces will determine whether adhesive bonding is a possible solution. In cases where the aluminum surface is either contaminated or made up of low strength oxides that exceeds the critical thickness, no attraction between the two aluminum profiles will be present, making adhesive bonding inadequate.
The adhesive used in the process must be able to ‘wet’ the entire surface to be joined, whether aluminum is being joined to aluminum, or aluminum is being joined to another metal. In order to ‘wet’ the material to be joined, the adhesive must have a lower surface tension than the materials being joined.
Involving the formation of a rubber or plastic load-carrying element, the material in the cured adhesive bond is not as strong as the aluminium itself. This is counteracted in the design process by designing large contact surfaces.
Adhesives are known to cope better with shearing forces, whereas joints that are subjected to tensional forces are more often not suitable for adhesive bonding processes.
Choosing an Adhesive:
As a rule of thumb, stresses in adhesively bonded joints are greatest at the edges of the joint. Stiffer adhesives will result in a greater concentration of subsequent stress, in turn resulting in high levels of stress on both the adhesive and the surfaces it has bonded. In many cases, this stress is unnecessary.
The choice of the adhesive used will depend on:
- The way in which the specific adhesive works.
- What is required from the joint (sealing, filling, toughness, heat resistant etc.)
For an adhesive to be capable of ‘molding’ itself to aluminum parts, the adhesive used must:
- Have a good liquid property
- Must be able to harden into a bond that can handle the required stresses
It is also vital that the adhesive used in the bonding process has the required amount of time to properly bond to the aluminum surface’s micro-profile.
An adhesive bond’s change from liquid state to a solid bond is effected in the following ways.
|Solvent||The adhesive is in liquid form when it is heated.||
Contact between the adhesive and hardener.
Exposure to Moisture
In the absence of oxygen
Because water and solvents vaporize, adhesive that contain solvents will be unsuitable for use in cases where either both the materials being bonded are not able to let the solvent escape, and in cases where gap filling is needed.
Hot-melt adhesives are most widely used, but thermoplastic hot-melt adhesives are known to set too quickly on aluminum surfaces, resulting in a poor contact with the aluminum surface itself.
Hot-melt adhesives also have the following characteristics:
- A very low heat strength
- A very low creep strength
- Some of these adhesive are also known to become brittle in cold environments
Moisture Curing Hot-Melts:
Applied at lower temperatures and, when it is compared to thermoplastic hot-melts, have an excellent set of characteristics after curing processes have been completed. These types of adhesives are used in order to apply foil coatings to aluminum profiles, amongst other processes.
Here, a water based adhesive or an adhesive based solution is applied to the metal and then left to completely dry. The adhesive is then heated (so that it is ‘wet’ again) during the bonding process. These types of adhesives are also used to apply foil coatings to aluminium profiles.
Curing consist of a large group of structural adhesives used for joining both standard and custom aluminum parts, and is cured in the following ways:
One-component acrylate adhesives are capable of curing in one tenths of a second when exposed to blue light (with a wavelength of >400 nm) or UV light (with an approximate wavelength of 350 nm). Acrylate adhesive are suitable for joints aluminum profiles and glass, and are often known to be transparent.
Epoxy adhesive have also been developed that harden when exposed to UV light.
Contact between the Adhesive and Hardener:
Here, the adhesive is on one surface while the hardener is on the other surface (to be joined together). Referred to as SGA adhesives, they are not suitable where gap filling is required, but does have an excellent impact and peel strength. SGA adhesives have been largely replaced by modified acrylic adhesives, which are mixed directly from their packaging and has the added benefit of forming thick joints.
In this process, one-component epoxies are commonly used. Requiring heating for curing (of a minimum of 100°C), curing times are as short as 60 seconds are achievable because of aluminum’s heat induction properties. Heat-hardening films are extensively used in the aero industries and takes a minimum of 30 minutes to harden at a minimum temperature of 125°C.
Exposure to Moisture:
Here, cyanoacrylate adhesives are used and are capable of hardening very quickly when in contact with moisture. Note that a joint between two aluminum parts take longer to bond, as a joint between aluminum and plastic or rubber. Best suited for small joints and thin bonds, these adhesives normally has a low impact and a low peel strength.
One-component polyurethane elastomers are also cured by humidity of air, and is used in the bonding of car windows and in manufacturing of vehicle bodies and containers. A slow bonding process, the time needed for the completion of the joining process depends on the relative air humidity and the joint geometry. Also present here are MS Polymers as well as polyurethane elastomers (which can be cured by both moisture and heat).
Done by mixing a component, this group commonly has the two-component epoxy and two-component polyurethane adhesives. Stiff and elastic, epoxy adhesive that has an elongation at fracture of up to 120% are available, as are elastic epoxy adhesives, commonly used on bonds that are sensitive to heat. Epoxy adhesives deliver higher strength bonds and an improved durability when cured at higher temperatures. Two component polyurethane adhesives delivers a rubber like bond that will remain elastic at minus °C temperatures. Also used here are two-component silicon adhesives that are capable of curing quickly at room temperatures.
In the absence of Oxygen:
Referred to as locking fluids or anaerobic adhesives, these types of adhesives cure when it they come into contact with the metal ions. Not generally recommend for joining aluminum as an activator is required, resulting in a bond that has a lower strength. Note however that there are adhesives in this group that are capable of hardening on aluminum surfaces without the use of an activator.
It is important to note here that the performance of the joint will depend on the design of the two aluminum profiles joined.
More commonly found in the industry today is the use of Friction Stir Welding (FSW) to join large, flat sections. This process involves the two aluminum extrusions to be held adjacent, while a rotating tool presses down on and runs along the joint.
This tool used generates temperatures in the abutting material that is adequate to bond the two desired material together in what is called a plastic state. This is achieved without melting the aluminum.
Welding methods used to join custom aluminum parts:
Used on aluminum with a thickness of 1 millimeters and up, filler metal is added in the form of wire, which is fed through the welding torch. Capable of being performed in any position and for all joints, its higher current density is able to deliver a high welding speed that seen in TIG Welding. This is beneficial as a higher welding speeds has a positive effect on shrinkage and distortion.
A suitable welding process for aluminum with a thickness of under 1 millimeter, it has a thickness limit of ± 10 millimeters. Capable of being performed in any position, an experience welder is capable of delivering fault-free welds.
This process involves the introduction of filler metal from the side and is recommended where the width of gaps vary.
Learn more about TIG Welding.
The robotized MIG Welding process has its advantages in cases of long production runs. Delivering an increased productivity, this process allows for the easy control of the work piece, but note however that problems can occur in cases where the work piece is of a very thin nature or has uneven gaps.
Friction Stir Welding (FSW)
Designed to exploit aluminum’s capabilities of withstanding extreme plastic deformation at high temperatures (note not above melting point), the clean surfaces of the aluminum parts are heated through friction, created by a rotating tool, and then pressed together.
This technique results in the formation of a new homogeneous structure, without making use of filler metal or shielding glass.
Compared to other welding techniques, FSW delivers:
- Joints that are principally flush wit the surface.
- An increased joint strength.
- An increased process repeatability.
- Joints that are free of void and impermeable (leak proof joint).
- A reduction in thermal deformation.
Standard and custom aluminum parts can also be fabricated to achieve desired results. If you would like to learn more about fabrication, please visit our Aluminum Fabricators page.
Back To Main Page: Aluminum Extrusions
Further Suggested Reading:
- Aluminum Grades Used in Extrusion including Chinese Equivalents
- Tolerances in Aluminum Extrusion
- Finishes that can be used on Aluminum Extrusions
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