Joining dissimilar materials, whether metal-to-metal, polymer-to-polymer, or metal-to-polymer, has become increasingly important for a range of engineering applications, with each of these material combinations bringing their own benefits and challenges.
Dissimilar materials can improve design flexibility by using materials efficiently based upon their specific properties. Polymer-to-polymer joints can exploit strength properties close to those of the parent material, while metal-to-metal joins can be used for sacrificial corrosion or to gain specific properties from each metal.
Joining metals and polymers allows you to exploit the light weight and chemical resistance of the polymer with the strength and ductility of the metal. In this way, the metal provides strength and stiffness as the polymer imparts unique chemical properties to improve the functionality of the part. The aerospace and automotive industries, for example, have been creating hybrid components from lightweight magnesium or aluminium alloys alongside fibre-reinforced polymers. Polymers are also being used more widely in engineering structures due to their low weight, high strength and elastic modulus, reduced manufacturing cost, and greater design flexibility.
The key to successfully using dissimilar materials often lies in balancing the mechanical performance against the material weight and cost. However, there are also complications associated with joining dissimilar materials, especially through welding…
Although dissimilar materials can provide many desirable properties, they can be difficult to join and the behaviour of joints that have been adhesively bonded or welded through heating can be hard to ascertain.
Adhesives and mechanical bonding are both commonly-used methods for joining dissimilar materials, but they also bring limitations such as extra weight, stress concentration, surface preparation requirements, and the potential for harmful emissions into the environment.
Welding can be a solution to some of these limitations, but it also brings its own challenges that require an understanding of the behaviour of the different materials during welding as well as knowledge of the capabilities and limitations of each welding method.
As noted above, methods for joining dissimilar materials include adhesive bonding, mechanical fastening and welding. These processes can be used on their own or combined, but they all have their advantages and disadvantages, meaning that choosing the best method will typically depend upon the application and service requirements.
Adhesive Bonding
This joining method uses a polymeric adhesive that undergoes a chemical or physical reaction to form intermolecular bonds between itself and the workpieces. Industries such as aerospace and automotive have helped drive advances in adhesive bonding due to the weight reductions it offers compared to mechanical fastening (see below). Adhesion offers a homogenous stress distribution along the join under loading and modern, high-strength adhesives are able to withstand both alternating and static loads. The strength of the bond is typically informed by the surface of the materials being joined, which can be improved by cleaning or physically altering them. Pre-treatments like solvent cleaning can be used to help ensure a strong bond or the surface chemistry or topography can be changed, for example using abrasion to create scratches that aid adhesion. However, adhesive bonds require an overlapping joint configuration that can increase the thickness, weight and stress concentration of the finished part. The performance of an adhesive bond also depends on environmental factors, as humidity, moisture and temperature can all cause an adhesive to degrade. There are also concerns over the environmental impact of degrading adhesives, while the disassembly of adhesively bonded parts will invariably damage them, meaning material wastage. It is also problematic to non-destructively test adhesive bonds, making it difficult to determine the reliability and durability of the join. Adding to this problem is the tendency for bonded joints to fail instantly, rather than progressively.
Mechanical Fastening
Mechanical fastening allows for parts to be joined without fusing them together. It uses clamping methods like screws or rivets and can be used to join metals-to-metals, metals-to-polymers or polymers-to-polymers. This technique may include additional steps, such as drilling holes or creating screw threads, and rivets are sometimes heated prior to fastening so that they shrink and form a tighter bond as they cool. The simplicity of this process means that it is widely used to join components, however it can increase component weight as well as create stresses around the fastener holes in service, which can degrade the strength of the part and lead to corrosion-related issues. The joint configuration is dependent on the service conditions that need to be met, for example to account for thermal expansion or to create a leak-proof bond. When joining a polymer to a metal it is advised to place the metal underneath the polymer as the bottom layer undergoes higher levels of deformation. The nature of mechanical fastening means that the materials need to overlap in order to be joined, which can increase the weight, thickness and stress concentration in a structure.
Welding
The third solution for joining dissimilar materials is welding, which can be categorised under three headings; solid-state joining, partial solid-state joining, and arc joining methods. Each of these welding types are categorised according to whether the peak temperature exceeds the melting point of one or both of the materials to be joined, as follows:
- Solid-State Welding: The peak temperature stays below the melting point of the materials. Examples include diffusion bonding, friction stir welding, magnetic pulse welding and ultrasonic welding.
- Partial Solid-State Welding: The peak temperature exceeds the melting point of one of the materials to be joined. Examples include arc brazing and resistance spot welding.
- Arc Welding: The peak temperature exceeds the melting point of both materials being joined. This is rarely used, but can join dissimilar materials that are close in physical properties. Metal-to-metal joins can be created in dissimilar materials using conventional welding processes such as gas metal arc welding, gas tungsten arc welding, or shielded metal arc welding. However, the high energy inputs associated with these processes can create a metallurgical mismatch which hinders their use in metal-to-metal as well as metal-to-polymer or polymer-to-polymer joints. In addition, using high heat processes can prove difficult as polymers have a much lower melting temperature than metals, meaning they degrade before the metals reach their melting point.
Laser welding, friction welding and ultrasonic welding can be used for thermoplastic and thermoplastic elastomers, since they can be heated and softened before being reshaped, but chemically cross-linked elastomers and thermosets cannot be heated, remoulded or welded, as they are characterised by an irreversible crosslinking reaction that leads to degradation. We will look at these techniques in more detail below.
Laser welding, friction welding and ultrasonic welding have all been shown to be effective techniques for joining dissimilar materials, increasing productivity in metal-to-metal, polymer-to-metal, and polymer-to-polymer joins.
1. Ultrasonic Welding:
This is a solid-state joining technique that applies a localised, high frequency vibration energy to two workpieces that have been clamped together. Ultrasonic spot welding can join two thin sheets as well as a thin sheet to a thicker sheet using lap or butt joints. Ultrasonic welding can be used to weld metals and plastics, although the exact technique differs according to the material being joined. When welding metal, the direction of the ultrasonic oscillation is parallel to the weld area, whereas with plastic the direction of ultrasonic oscillation is perpendicular to the weld area. When welding metal, a solid state bond is created by the frictional action of the workpiece surfaces without melting them. However, ultrasonic welding causes melting and fusion in plastic materials. Experiments have shown that a 1mm sheet of aluminium alloy 5754 could be successfully joined to a2mm sheet of carbon-fibre reinforced polymer (CFRP). Amplitudes of around 40 µm allowed for improved contact between the materials as the CFRP matrix was displaced. As the welding process caused oxide layers on the metal sheet to peel away, intermolecular reactions were observed in the weld zone as the polymer matrix was displaced from the weld zone, allowing the ductile aluminium to adapt and mechanically interlock with the carbon fibres, increasing the joint strength. As plastic deformation took place in the aluminium sheet, the carbon fibres surrounded the aluminium alloy to create a successful weld.
2. Laser Welding:
Laser welding has been shown to be able to form joints that had been difficult or impossible with other welding methods, including dissimilar metals such as titanium/nickel alloy with stainless steel, and titanium alloy (Ti6Al4V) with lead. The strength of the material combinations in some of these joints have been shown to be greater than with the parent metal alone. Laser welding of polymers depends upon the laser wavelength being used, although high-power fibre lasers with wavelengths of around two metres allow polymers with different absorption rates to be joined without an additive. An additive can be used to help join polymers that would not otherwise absorb enough heat from the laser to make sure fusion takes place. Two dissimilar polymers that can both absorb the laser can be heated until they are close to their respective melting points before being pressed together to form a butt weld. Polymer-to-polymer welding can also be achieved using transmission welding if one of the polymers is transparent, allowing the beam to pass through it to the other polymer, which absorbs the beam, heats up and melts. The heat is conducted from the bottom polymer to the upper, transparent polymer, which melts and joins the two when pressed together. Laser welding can also be used for metal-to-polymer joining, also known as laser-assisted metal and plastic joining. To achieve these joins, the laser is focused at interface between the plastic and the metal so that a narrow band of the plastic is heated to melting temperature. This band of melted polymer forms bubbles along the interface, which increase the seam dimension by spreading and diffusing into the molten phase. The bond occurs in the molten-solid interface between the plastic and metal parts. This requires the metal and plastic parts to have an overlapping joint configuration, while the low thermal conductivity of the plastic ensures that the heat stays concentrated. The optical properties of the plastic will also impact how the heat behaves due to the molecular composition of the polymers and the wavelength of the laser beam, with more transparent plastics only forming a bond close to the interface with the metal where the laser beam is absorbed. Opaque plastics may require the beam to be focused on the metal, which conducts the heat back into, and melts, the plastic to form the join. Requiring a low heat input and highly adaptable, this process offers fast welding times and high strength bonds. Although it requires set parameters related to welding power and travel speeds to be followed to ensure the best-quality weld. It can be used with metals including aluminium, iron, steel and titanium.
3. Friction Spot Joining:
This process, a variant of linear friction stir welding (FSW), was originally invented to replace resistance spot welding for aluminium sheets. However, it has since been proven to also join dissimilar metals like aluminium and magnesium alloys. Similar to FSW, friction stir joining (FSJ) also uses a high-speed rotating tool with a probe pin that is plunged into the material. A shoulder on the tool determines the depth of the rotating tool, creating friction against the workpieces and causing plastic deformation in the material around, creating the join. Once achieved, the tool is retracted from the workpiece to complete the process. The difference between FSJ and FSW is that the tool moves linearly along the workpiece in FSW while it is inserted and held in a single position with FSJ. It is difficult to achieve a sound join between metals and polymers using the same technique as with dissimilar metal-to-metal joints due to the poor thermal conductivity and structure of thermoplastics. Instead, the metal and polymer workpieces can be overlapped and clamped together, with the metal on the top of the polymer, before the tool’s sleeve and pin are rotated in the same direction and the sleeve is pressed to the metal to create frictional heat. The pin is inserted into the metal, which plasticises the material and leaves a reservoir, into which plasticised metal is extruded as the pin is withdrawn. This tool plunging does not touch the polymer layer, so does not damage the fibre reinforcement in the polymer workpiece below. The join is created through the conduction of frictional heat through the metal layer to the polymer, which softens and melts to the metal above. The join is consolidated by the application of pressure once the tool is retracted from the metal.
4. Friction Stir Welding:
This technique has been investigated for metal-to-polymer joining, but the thermal differences between metals and plastics makes this difficult. However, it has been used for metal-to-metal welds. Although dissimilar polymers can be joined using this method, they require a different tool design to compensate for their low thermal conductivity and molecular structure.
The most challenging dissimilar joins are those made between polymers and metals due to their different properties. However, allying the light properties of polymers with the structural rigidity of metals has led to a focus on joining these materials. Here we present an overview and comparisons of the different metal-to-polymer joining methods, including welding processes:
1. Mechanical Joining:
This can deliver reliable joints between metals and polymers with high levels of resistance, such as with riveting. However, the nature of mechanical fastening means that joint designs are constrained to allow for the fastenings. Production rates are also relatively slow as the joint shape and positions tend to be fixed mechanically.
2. Adhesive Joining:
Widely used to join plastics to metals, this technique is simple and offers a high level of design flexibility. However, adhesives can show low mechanical and chemical resistance, limited effective temperature ranges, require surface preparation, and are difficult to assess for durability and life-cycle purposes.
3. Ultrasonic Welding:
This has proven effective for metal-to-fibre-reinforced-polymer joining. With low energy use and fast welding times, ultrasonic welding doesn’t cause microstructural changes in the metal part. Uniform mixing and intermolecular contact between the dissimilar materials promotes mechanical interlocking for high joint strengths.
4. Laser Welding:
Laser welding can create stable, high strength bonds between metals and polymers, even though it doesn’t melt the metal. Instead, the polymer is heated to create bubbles that physically and chemically bond with the metal. With fast welding times and a small heat input, laser welding has joined polymers to aluminium, iron, steel and titanium. However, parameters such as welding power and travel speed can have an influence on the quality of the final weld, while more transparent polymers are less effective at laser beam absorption. There are also design limitations, with lap joints being preferred as they allow the laser beam absorption into the polymer.
5. Friction Spot Joining:
This solid state method provides a uniform mix of the metal and polymer at the joint interface, quickly and easily creating joints with good mechanical performance. While welding equipment for this process is widely available, it is not suitable thick metals and requires metals with low melting points, such as aluminium and magnesium.
Whether made up of metals, polymers, or a combination of the two, dissimilar materials have shown their use across industry for the range of different properties they can deliver. While the improved functionalities and performance of these combined materials are highly desirable, they can provide challenges when it comes to joining them, particularly when welding. The challenges are heightened further when making metal-to-polymer welds due to their differing thermal and physical properties.
Dissimilar materials can be adhesively bonded, mechanically joined, or welded together, and sometimes a combination of these techniques is used. However, each technique has its own strengths and drawbacks, which need to be understood for them to be used effectively and safely.