FAR-UK is an engineering company with an automotive mindset that designs and manufactures lightweight vehicles and safety structures. It has a strong R&D focus, one area of which is in joining dissimilar materials. It believes that there is no one best material for optimum design, rather it aims to answer the question of how to put the right material in the right place. Creating multi-material structures from the huge range of structural materials available means that the joining conversation is important, because the way in which dissimilar materials are fixed to each other can drastically affect the mechanical properties of the structure.
Bonding dissimilar materials together often involves adhesives, but this is often not trivial. There are several different engineering aspects that need to be considered to extract maximum performance. For example, the traditional high-performance adhesives that are used in the aerospace and the automotive industry have generally been based upon epoxies, which have either needed longer cure times or elevated temperatures to be cured successfully.
A benefit of adhesives is that they provide a variety of bond gaps that would not be possible with a joining method like welding. But wide bond gaps are not possible to achieve with all formulations. For example, fast-reacting adhesives such as MMAs, which typically cure within 15 or 20 minutes, produce a lot of heat. To increase a bond gap, a user must put in more adhesive material, which means that the curing reaction is going to get hotter. Used incorrectly in this way, it isn’t unusual for fast-reacting adhesives to essentially damage themselves by overheating and producing local embrittlement. (Adhesive manufacturers are now continually developing these products to allow for increased bond gaps without damaging performance.)
When joining materials, peel loads are often the most challenging for adhesives, and can be detrimental to the assembly’s performance. There are three options to avoid the limitations of weak peel strength in a glued joint. First is to get rid of the joint. That typically comes down to a trade-off between more complex tooling, in which a greater number of parts in a final assembly are made as one part, avoiding the need to join them, versus the increased cost of more complex tooling. Second is to move the joining surface by, for example, adding a bonding flange to move the load somewhere else. The third option is to add a peel stopper, such as a self-piercing rivet, which can introduce its own challenges.
Self-piercing rivets have a long history of successful use with metal-to-metal joints. Their performance is not the same with composite materials, which lack metals’ strength and ductility. Composites do not deform plastically before failure, as metals do. When experiencing an increasing load, composites will withstand the load up to a maximum, and then delaminate, often explosively, and without a huge amount of warning. There is a lot of work being done to try to make riveting more composite-friendly.
To describe how well something sticks to something else, we often boil things down to discussions of surface ‘adhesion’. This term encompasses a huge number of complex interactions taking place at the interface between the adhesive and the substrate. When joining dissimilar materials, the two most commonly-discussed are chemical bonding and mechanical interlocking processes. Surface preparation is key to maximising both.
Adhesives latch on chemically to the outermost surface of a substrate. If the surface is contaminated, the adhesive will latch on to the contaminant instead, which may only be loosely attached to the substrate. This means that the joint will quickly fail when force is applied. Even a little contamination can lead to stress points. Users must always remove any form of contamination before bonding. Another type of surface preparation is activation, treating the surface so that it’s more chemically attractive, through processes such as plasma treatment. A third process is surface stabilisation, chemically treating surfaces so that they don’t deteriorate over time. A good example is anodizing metals, which helps prevent oxides from forming. Industry practice, and what Far-UK does, is to mix and match all three.
One interesting aspect of design that is very topical at the moment is sustainability and end-of-life, both of which are affected by the joining techniques used. There is increasing interest in recycling products. But those imperatives run counter to traditional design processes, whose whole purpose is to resist structures falling apart in use. In the future, we will want adhesives that not only offer the same performance as before, but also extra flexibility for component disassembly. There is much research being done in this area. One example looks at putting additives into adhesives that are thermally-activated; when they heat up, the additives expand and can effectively pop the joint open. There is a significant amount of development now of joining technologies which can maintain the strength and processability of the part while being more sustainable for the future.
In summary, joining dissimilar materials is tricky, and getting harder simply because, through materials technology innovations, there are more of them on offer.
BOX: HIGHER PERFORMANCE
An interesting result of Far-UK’s research on crash structures is that the use of bonded dissimilar materials can fundamentally change how structures will perform. Far-UK has done a lot of research looking at the effect of bonding together carbon fibre composite to a metal crash can (either steel or aluminium). It found that if carbon-fibre composite is added, it will effectively increase the amount of energy that the metal part is able to absorb before failure, without actually affecting the energy absorption of the composite itself. A component made out of dissimilar materials (pictured above) is hence capable of absorbing more energy than the fully metal or fully composite versions of that component. Such results are helpful for optimising automotive crash structure performance, because then designers can then start reducing its weight, saving fuel and CO2.
However, modelling these aspects in FEA [finite element analysis] is quite challenging. Accurate modelling depends on accurately portraying the characteristics of each material. Not only must one have accurate data on both of these materials and their behaviour when loaded, but also the adhesive’s performance must be factored in too, to understand the behaviour of the whole system. Far-UK has built up significant experience in joining dissimilar materials and continues to invest in R&D. -Ross Minty
This article is an edited version of ‘Designing with dissimilar materials and implications for joining them together’, a presentation given at the 2021 Engineering Design Show.