1
Joining of carbon fibre reinforced polymer (CFRP) composites
and aluminium alloys-A review
A. Pramanik
1*
, A.K. Basak
2
, Y. Dong
1
, P. K. Sarker
3
, M. S. Uddin
4
, G. Littlefair
5
, A.R.
Dixit
6
, S. Chattopadhyaya
6
1
Department of Mechanical Engineering, Curtin University, Bentley, WA, Australia
2
Adelaide Microscopy, the University of Adelaide, Adelaide, SA, Australia
3
Department of Civil Engineering, Curtin University, Bentley, WA, Australia
4
School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia
5
School of Engineering, Deakin University, Waurn Ponds, VIC, Australia
6
Department of Mechanical Engineering, Indian School of Mines, Dhanbad-826004,
Jharkhand, India.
Abstract
This paper investigates comprehensive knowledge regarding joining CFRP and aluminium
alloys in available literature in terms of available methods, bonding processing and mechanism
and properties. The methods employed comprise the use of adhesive, self-piercing rivet, bolt,
clinching and welding to join only CFRP and aluminium alloys. The non-thermal joining
methods received great attention though the welding process has high potential in joining these
materials. Except adhesive bonding and welding, other joining methods require the penetration
of metallic pins through joining parts and therefore, surface preparation is unimportant. No
model is found to predict the properties of jointed structures, which makes it difficult to select
one over another in applications. The choice of bonding methods depends primarily on the
specific applications. The load-bearing mechanism of bolted joints is predominantly the
friction that is the first stage resistance. Hybrid joints performance is enhanced by combining
rivets, clinch or bolts with adhesives.
Keywords: carbon fibre reinforced polymer (CFRP) composites, aluminium alloys, joining.
1. Introduction
Carbon fibre reinforced polymer (CFRP) is one of the most important materials for structural
applications, particularly in aviation industries owing to its high strength to weight ratio. CFRP
contains extremely thin carbon fibres (CFs) of about 0.005- 0.010 mm in diameter in polymeric
matrices leading to light weight composite structures. At a microscopic scaled level, carbon
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atoms of fibres are bonded together parallel to the fibre axis, and thus give rise to the
unidirectional alignment, which in turn contributes to superior tensile strength along with light-
weight structures and low thermal expansion. In most real-life applications, CFRP requires
joining with metal frames to form complete structures, which play an important role in hybrid
design. Hybrid design is an emerging process of joining composites and metals with desirable
and unique material characteristics such as higher strength and stiffness, resistance to physical
damage due to cracks, resistance to radiation damage, design versatility etc. [1]. The popularity
of such specific functional properties can meet enormous demands towards superior structures
to exploit the best performance of both metals and composites [2]. Thus it is very critical to
understand the issues associated with fabricating, machining and joining of composite
materials [3].
Stack-up formation is an effective means to build composite/metal structures with high bending
rigidity and insignificant increase in structural weight [4]. Furthermore, the sandwich stacking
formation is also well utilized particularly for manufacutre of composite panels in commercial
aircrafts like Airbus A380 or Boeing 787. CFRP/titanium, CFRP/aluminum and CFRP/CFRP
are some typical material formations that are commonly used in engine cowlings, fairings, and
fixed trailing edges, wing panels, helicopter blades, space optical benches , ship hulls, etc. [4].
It is also forseen thatthese types of composite structure formations will dominate the future
applications in Lockheed-Martin's X-33, Raytheon's Premier I, and tilt rotorcrafts from
Textron-Bell Helicopter or Boeing [5].
From a manufacturing point of view, joining of composite and metal stack-ups contributes to
substantial amounts of total manufacturing cost due to the number of steps from the beginning
to the final structural completion with high labor intensiveness. As reported in previous
literature [7] , such cost may be as high as the half of the total cost of the products. Conventional
mechanical fastening and adhesive bonding are generally used to fasten composites and metals
together in relatively simple structures. Mechanical fastenings using bolts or rivets usually
provide adequate joining strength, and thus can be widely used in engineering structures.
However, mechanical fastenings suffer from weight increase and low sealing capacity. In
addition, the cross-sectional area of structures decreases due to the presence of bolt holes with
the stress increase. In addition, drilling process towards the formation of bolt holes causes
cracks in composite structures. In view of that, adhesive joints are more favourable in that the
process offers sealing effect with less significant stress concentration as well as flaw-free effect
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in composite structures. The selection of proper adhesives is critical for joining dissimilar
materials because the adhesive degradation with time can significantly reduce the bonding
strength. To tackle this issue, co-curing during the joining is an effective means by using
excessive resins as adhesives, which ensures that curing and joining take place at the same
time. Since this process is free from additional curing process, labour consumption is reduced
accordingly. However, a significant increase in joining strength with respect to conventional
adhesive bonded joints has not been expected [8]. . Consequently, welding and hybrid bonding
are recommended in order to address the weakness of above- mentioned methods. In hybrid
bonding, mechanical fastening on the top of adhesive bonding is added to improve the overall
joining strength. In general, manufacturing time, performance and cost are vital factors in the
selection process of a specific mechanical joint. With challenging technology ahead, blends of
mechanical joints with adhesive bonding are anticipated [6-8].
Much research work in the field of CFRP and metal joining is available in literatures with
numerous results. . Nonetheless, it is difficult in having a good understanding of this field due
to disorganised and less linked scientific results obtained. Our current investigation studied
different types of joining methods for CFRPs and aluminium alloys available in literatures. The
main objective of this paper is to find the knowledge available in the joining CFRPs and
aluminium alloys and link such knowledge for comprehensive understanding of joining effect.
In this study, all the possible joining methods of these two materials were critically analysed,
and the information such as, bonding process and mechanism as well as mechanical properties
are presented holistically. In this way, industrial partners and researchers can benefit from this
comprehensive review and overcome associated limitations and drawbacks in order to meet the
future challenging in joining such materials.
2. Adhesive bonding
Adhesive bonding is the process of binding two components using a suitable binder (i.e. an
adhesive). Applications of adhesives for joining elements made of dissimilar materials are
commonly employed in aviation, automotive and building industries [9-12]. Joining of CFRPs
with aluminium alloys via adhesive bonding is by far the most conventional method with both
advantages and limitations. Since adhesive bonding is an irreversible process, attempts to
dissemble the joints can be expensive, which results in the complete material damage involved
in the joints. Adhesive bonding not only seals the joints but also prevents crevice and galvanic
corrosion between two dissimilar materials. Almost any pair of dissimilar materials such as
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metals, polymers or ceramics can be joined with this method. Adhesive bonding is the only
viable method to achieve structures involving the joining of thin-walled elements, among
which an element has substantial dissimilar thickness. Adhesive bonding offers light-weighted
structures with respect to other assembly technologies and developments, particularly in
aviation industries. In addition, stress concentration becomes less significant without the
requirement of bolt holes, thus avoiding structure weakening [13].
The adhesives as the main elements in adhesive bonding should have good wettability with
respect to joining components, such as CFRPs and aluminium alloys, which are generally in a
semi-solid state to facilitate associated applications. However, to fabricate load bearing joints,
liquid adhesives have to be used, which have inbuilt ability to harden without curing in elevated
temperatures. Exceptionally, pressure sensitive adhesives are permanently sticky and basically
perform their function in a sticky state [14]. The selected adhesive must have lower surface
tension than that of CFRP/Al 6061 alloy to ensure uniform wetting of entire surfaces so that
the occurrence of droplets is avoidable. Uniformly spread adhesives improve molecular
contacts between adherents, thereby increasing the joint strength. To achieve the ultimate
strength from the joint, adhesives should be allowed to have enough time to set and to follow
surface profile (i.e. roughness profile) of adherents. However, this is not possible by using fast
setting and highly viscous adhesives. As such hot-melt thermoplastic adhesives are not suitable
for such applications [15]. In this case, two components of elastomers can be used to form
rubber-like joints retaining their elasticity at low temperatures and elastic epoxy adhesives
depending on different applications. Epoxy adhesives result in product high strength and
durability after being cured at high temperatures [16].
2.1 Joining Methods by adhesive bonding
Prior to a bonding process, adherents (i.e. CFRPs and aluminium alloys in this case) are
required to be thoroughly cleaned, which means that any contamination removal should be
made by degreasing either via mechanical polishing or by using wipe cloths in order for the
surfaces to be bonded. Hence the preparation depends primarily on adherents and adhesives to
be used in the joining process. Emery papers of different grades and solvents like acetone can
be used for this purpose. Usually etching or light abrasion is followed by the solvent wipe to
get rid of grease and other loose dirt. Etching can be carried out in a chemical manner by using
hydrochloric acid and water (e.g. 20 to 80%). Despite such a quick process, it discolours metal
surfaces owing to the oxidation effect. A universal etchant, used for aluminium alloys, involves
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chemical etching before microstructural contrasting in polarised light. Detergents must be
avoided for both components in that they can further aggravate contamination [19]. More
details of surface preparation coupled with their effectiveness are discussed in subsequent
sections.
In case of aluminium alloys, surface films in the formation of aluminium oxide (Al
2
O
3
) are
unavoidable upon exposure to air or water, resulting from very low wetting capability. such
tenacious films are hard to remove with the requirement of extensive chemical treatment [17].
Therefore, the surface should be chemically modified in order to prevent such film formation
in first place. This can be done either by adding coupling reagents or by anodizing [18].
Coupling reagents form such strong and irreversible covalent bonds between surface oxides
and hydroxides, which are in turn linked with adhesive during the curing process. On the other
hand, anodizing results in the formation of rough and water-resistant oxide films at micro scale
level by using sulphuric, phosphoric or chromic acid. Sulphuric acid treatment is used in lightly
stressed joints to obtain the best results for the application of elastic adhesives. Anodizing with
chromic and phosphoric acid is performed for highly stressed joints, which are meant to be
used in the corrosive environment. This process actually forms regular micro pores in oxide
layers towards underneath metal surfaces. During the curing process, adhesives fill up those
micro pores and eventually reach the metal surfaces. The treatment with phosphoric acid gives
best results when used with low viscosity primer [19]. If proper steps are followed to clean the
surfaces using such strong oxidising agents, the results of this can lead to excellent surface
finish without deteriorating their properties. Afterwards the application of primer prepares the
surface for adhesion with stronger and more uniform bonding [20].
In addition to chemical treatment, acetylene and nitrogen plasma can also be used to modify
aluminium panel towards adhesive bonding [21]. Figures 1 and 2 show the volume effect of
different gases during plasma treatment as well as treatment time on contact angle of
aluminium with water. This plasma treatment modifies the surface characteristics of structures,
as evidenced by the change in contact angle between aluminium and water from 82° to 135°.
The contact angle was minimum in a gas mixture of acetylene/ nitrogen with a volume ratio of
3:7 for the exposure time of 90 s as opposed to 5:5 for 30 s.
![Fig. 35: Delamination of CFRP laminates in hole-clinched joints [117].](/figures/fig-35-delamination-of-cfrp-laminates-in-hole-clinched-10kwiz43.webp)
![Fig. 1: Effect of volume ratio of acetylene /nitrogen on contact angle of aluminium with water [21].](/figures/fig-1-effect-of-volume-ratio-of-acetylene-nitrogen-on-2s32fufh.webp)
![Fig. 2: Effect of exposure time on contact angle between aluminium and water for acetylene /nitrogen at the volume ratio of 5:5 [21].](/figures/fig-2-effect-of-exposure-time-on-contact-angle-between-16bp8h8y.webp)
![Fig. 21: A comparison among (a) maximum load, (b) energy absorption and (c) stiffness for different joints with CFRP cross-ply and angle-ply laminates [77].](/figures/fig-21-a-comparison-among-a-maximum-load-b-energy-absorption-3jqbvm2d.webp)
![Fig. 22: Failure types of SPR joints with (a) CFRP cross-ply and (b) CFRP angle-ply [77].](/figures/fig-22-failure-types-of-spr-joints-with-a-cfrp-cross-ply-and-2ts6zkl7.webp)
![Fig. 34: Cross-section of hole-clinched joints between Al6061-T4 alloys and CFRPs [116].](/figures/fig-34-cross-section-of-hole-clinched-joints-between-al6061-kbchwhhp.webp)