Here we will take a look at the main methods of materials joining; adhesives, mechanical fastening, and welding.
1. Adhesive Bonding
Adhesive bonding uses bonding chemicals to stick two or more parts together. It can be used to join polymers, metals and ceramics, either to themselves or each other. Adhesively bonded joints can withstand compressive, shear, and tensile stresses, but can struggle to resist peeling. Adhesives can be used to provide permanent, semi-permanent or temporary fixing depending on the application and industry. Adhesives can be categorised as follows:
- Anaerobic Adhesives: These acrylic-based adhesives are designed to cure in the absence of air
- Cyanoacrylates Adhesives: Also called instant glues, these are designed to cure in the presence of moisture and UV
- Epoxy Adhesives: Capable of withstanding extreme temperatures, these adhesives cab also be exposed to to high shear and peel
- Hot Glue: Hot glues solidify as they cool
- White Glue: These adhesives require contact and pressure as they solidify. Primarily used for wood, cloth, paper, and other porous substrates
Adhesives are often used in conjunction with mechanical joining methods in order to increase the joint integrity.
2. Mechanical Joining
This process joins parts through the use of clamps, screws, bolts or rivets. This provides a versatile and easy-to-use process that not only has the ability to join dissimilar materials, but also allows you to dismantle an item for maintenance, inspection or repair. Mechanical joins can offer a high strength connection with a variety of types and component sizes available. Despite these advantages, the holes used to join materials can be prone to fractures or corrosion and screws or bolts can loosen over time.
3. Welding
Widely used for joining metals, his process includes fusion welding, brazing, soldering, pressure welding, and solid-state welding among the methods used. Fusion welding processes involve the melting and resolidification of the material in the zone where the join is required. Brazing and soldering uses an added filler material to create the join between two or more workpieces and solid state welding requires the deformation and diffusion of materials and so requires no filler materials. Pressure welding uses a mixture of electrical or laser energy alongside the application of pressure to form the join.
Choosing the best method for joining two or more materials will impact the longevity and security of a joint, so it is important to use the most suitable method for your application and requirements. Some of the factors that are worth considering when choosing a joining method include:
- Materials: The first thing to consider is the materials to be joined. Are they the same, similar, or completely different from one another? Some materials can withstand welding and other cannot. Equally, some adhesives work better with some materials than others. The different properties of materials can be influential in your choice of joining method.
- Required Durability of the Joint: Will your join need to be disassembled or is it to be a more permanent fixing?You will also need to assess any stresses the joint will be subjected to, such as tension or compression, and whether the join needs to be as strong as the materials themselves.
- Environment: The environment that the joint needs to exist in should be assessed too – is it a dusty or greasy environment? Is there the potential for corrosion? Is there high, low or fluctuating temperatures around the join?
- Surface Preparation: Do the materials to be joined require any surface preparation for your chosen joining method? Preparations can include drilling, grinding or sanding in order to make the material ready to accept the joining technique.
- Aesthetics: The final appearance of the join or part can be important and can make a difference to the exact joining technique used.
- Maintenance Requirements: Will your part require maintenance and repair and how often will this take place? Semi-permanent or temporary joints may be better for items that require frequent maintenance.
- Budgets, Time and Resources: These factors include the cost of making the joint as well as potentially repairing or replacing it. This also includes time-scales for making the joint and how long it is expected to be in service.
Of course, these are just some of the factors that could determine your type of joining method, which can be further honed to find the best exact technique, such as spot welding or friction welding.
Different joining methods and techniques offer different expectations with regards to joint integrity, and different industrial applications will also have different acceptable failure tolerances. While any failure can be a problem, those that impact critical structures will tend to have larger implications, which is why it is also important to understand the factors that affect the integrity of different joining methods.
Of course, even when the most suitable joining method has been chosen, there are other factors that can affect the integrity of a joint. These may include process failures, human error, secondary failures as a result of a failure elsewhere, and more. This is why it is important to have a robust monitoring strategy assets and products.
However, there are some potential issues that different joining methods are prone to, as follows:
- Adhesive Joints: Common problems can include the material surfaces not being prepared for the adhesive (too wet, dirty, etc) or not enough pressure being applied during the process. Adhesives may also fail as a result of operational issues such as harsh environmental conditions or excessive levels of fatigue that the adhesive is not designed to withstand.
- Mechanical Joints: Aside from errors in design, clamping force or fretting, common issues with mechanical joints include fatigue issues around the joint (rivets, screws, etc), cracks, loosening, vibrations, and stress concentrations.
- Welded Joints: The welded area is typically the weakest point of a joined item and where the effects of stress will be most keenly felt. In addition, the welding process can impart defects into the weld itself.
Material joints need to be reliable and suitable for their purpose, whether a temporary, semi-permanent or permanent bond is required. With the variety of different materials that are available, whether metals, wood, ceramic, or polymers, as well as the large number of applications across different industries, it is important to choose the correct method for your requirements.
Common material joining methods can be broadly split into three types; adhesive joints, mechanical joints and welded joints. Each of these has their own uses, advantages, and drawbacks. Welding can create permanent, strong bonds in materials through the application of heat to the materials and/or a filler material. The level of heat and therefore the associated factors differs depending on the type of welding process used. Related processes, such as brazing and soldering, are also included in this type of joining. Adhesives can create an invisible join that doesn’t damage or alter the materials that are being joined, however, adhesives can be adversely impacted by factors such as fatigue and environmental conditions. Mechanical joints, meanwhile, offer an alternative that makes it easy to disassemble and reassemble parts. Despite this, mechanical fastening methods can be prone to damage where any holes are drilled to allow for the fastener and the joints can loosen as a result of vibrations or movement over time.
Whichever, joining method is used, it is important that the joint is created in accordance with any standards, especially for critical joints, and they should be inspected and maintained to ensure their ongoing integrity.