Citation: Faudree, M.C.; Uchida,
H.T.; Kimura, H.; Kaneko, S.; Salvia,
M.; Nishi, Y. Advances in
Titanium/Polymer Hybrid Joints by
Carbon Fiber Plug Insert: Current
Status and Review. Materials 2022,15,
Alessandro Pirondi and Chih Chen
Received: 5 March 2022
Accepted: 26 April 2022
Published: 29 April 2022
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Advances in Titanium/Polymer Hybrid Joints by Carbon Fiber
Plug Insert: Current Status and Review
Michael C. Faudree 1, * , Helmut Takahiro Uchida 2, Hideki Kimura 2, Satoru Kaneko 3, Michelle Salvia 4
and Yoshitake Nishi 2,3
1Faculty of Liberal Arts and Science, Tokyo City University, Yokohama-shi 224-8551, Japan
2Graduate School of Engineering, Tokai University, Hiratsuka-shi 259-1292, Japan;
firstname.lastname@example.org (H.T.U.); email@example.com (H.K.); firstname.lastname@example.org (Y.N.)
3Kanagawa Institute of Industrial Science and Technology (KISTEC), Ebina-shi 243-0435, Japan;
4Ecole Centrale de Lyon, CEDEX, 69134 Ecully, France; email@example.com
*Correspondence: firstname.lastname@example.org; Tel.: +81-(0)45-910-0104 (ext. 2906)
A literature review of up-to-date methods to strengthen Ti/carbon-ﬁber-reinforced polymer
(CFRP) hybrid joints is given. However, there are little or no studies on Ti/CFRP joints by carbon
ﬁber plug insert, which takes advantage of the extremely high surface adhesion area of ~6
Therefore, we cover the current status and review our previously published results developing hybrid
joints by a CF plug insert with spot-welded Ti half-lengths to enhance the safety levels of aircraft
fan blades. A thermoset Ti/CF/epoxy joint exhibited an ultimate tensile strength (UTS) of 283 MPa
when calculated according to the rule of mixtures (RM) for the CF cross-section portion. With concern
for the environment, thermoplastic polymers (TPs) allowed recyclability. However, a drawback is
easy CF pull-out from difﬁcult-to-adhere TPs due to insufﬁcient contact sites. Therefore, research on
a novel method of homogeneous low voltage electron beam irradiation (HLEBI) to activate a bare
CF half-length prior to dipping in a TP resin was reviewed and showed that the UTS by the RM of
CF/acrylonitrile butadiene styrene (ABS) and Ti/
CF/polycarbonate (PC) joints increased
154% (from 55 to 140 MPa) and 829% (from 30 to 195 MPa), respectively, over the untreated sample.
The optimum 0.30 MGy HLEBI prevented CF pull-out by apparently growing crystallites into the TP
around the CF circumference, raising the UTS amount closer to that of epoxy.
hybrid joint; thermoplastic; thermoset; titanium; spot-welding; carbon fiber; electron beam
1. Introduction and Background
Fiber-reinforced polymers (FRPs) have numerous applications, including airplanes,
space vehicles, automobiles, sports equipment, ships, bridge cables, and wind turbines, to
name a few [
], due to their formability, high strength, stiffness-to-weight ratios, and
advanced fatigue and corrosion resistance [
]. Recently, for aircraft such as the Airbus
A350 XWB and Boeing 787 Dreamliner, carbon-ﬁber-reinforced polymers (CFRPs) have
reached about 50 wt.% and have been applied to the wings, as well as the fan blades, of
turbo fan engines.
Titanium alloys, on the other hand, were ﬁrst developed for the aerospace industry [
and have since been utilized for jet engine components and airframe structure due to
their high strength, stiffness, toughness, fatigue, and excellent resistance to corrosion [
when coupled with CFRPs [
]. In addition, Ti alloys have the advantage of maintaining
mechanical properties at high operating temperatures and can be used for structures
requiring heavy loads such as wing–fuselage connections and landing gear [
commercial airlines have about 10 wt.% Ti; however, present modern aircraft such as the
Airbus A350 and Boeing 787 are built with slightly greater than 10 wt.% [
]. Ti alloys are
used for jet engine components with operation requirements of 673 to 773 K (400 to 500
Materials 2022,15, 3220. https://doi.org/10.3390/ma15093220 https://www.mdpi.com/journal/materials
Materials 2022,15, 3220 2 of 19
and can include low-pressure compressor parts, plug and nozzle assemblies in the exhaust
section, and fan blades [8,10,11].
To prevent impact damage and fracture, a Ti sheath is often mechanically ﬁtted,
assembled, and mounted to the leading edge of CFRP fan blades, having the advantage of
spontaneous adhesion with full contact at the Ti–epoxy interface.
Traditionally, fasteners have been used to join materials in aircraft. Although fasteners
have the advantages of simple processing, high joining strength, and small scatter of data,
disadvantages are increased in weight due to the fasteners themselves, and their sealing
performance is low. Also, bolt holes reduce the cross-sectional area of the part and, along
with threads, can act as stress concentrators [
]. Problems with drilling holes in FRP
laminate composites include ﬁber breakage, peeling of top plies at the hole entry, resin
degradation at the hole wall, and delamination of the laminate bottom plies [
], all of
which can result in crack generation or propagation during fatigue .
However, the beneﬁts of using adhesive joints are: (1) the sealing is complete; (2) a
weight reduction from the absence of fasteners; (3) the avoidance of stress concentration
from the fastener holes; (4) and an absence of drilling so that possible damage to the outer
plies is avoided. Hence, adhesive joints typically have higher fatigue strength than bolted
]. However, adhesive bonding has its disadvantages: selection is difﬁcult when
joining two different materials; additional steps are needed for degreasing and etching the
joining surfaces to attain high adhesion strength; and chemically treated adhesive joints
can degrade by oxidation in a few hours, decreasing the bonding strength [
]. Thus, it
can be challenging to achieve strong adhesive joints. Adhesion alone can make aircraft fan
blades more vulnerable to fracture at the Ti–CFRP interface by possible impact from bird
strike or other airborne debris during ﬂight, along with heterogeneous stresses generated
by the strong airﬂow of jet engine compressors.
There are several studies found in the literature to create stronger Ti-CFRP joints; the
most up-to-date are reviewed here to be useful to the reader. The methods include laser
treatment, improving bolted joints, carbon ﬁber nanotubes (CNTs), anodizing, brazing,
friction riveting, inductive heating, ultrasonic additive manufacturing, and novel bio-
Lately, the use of lasers has been a widely researched method ﬁnding success [
For Ti/short carbon ﬁber polyphenylene sulﬁde CFRP joints, laser welding was adopted,
achieving a maximum tensile shear load of 2052 N at an optimum 700 W laser power [
For Ti/thermoplastic PEEK CFRP joints, a laser to the Ti made successful joining possible
by creating a rough Ti surface for the molten PEEK resin to intricately ﬂow into, along
with the formation of a new phase of CTi
at the Ti–PEEK interface [
texturing found success: the texturing of a 0.2 mm wide grid on a Ti surface prior to hot
pressing was reported to raise the maximum shear force of Ti/CFRP joints three times
to 5286 N [
], while for a Ti/thermoplastic CFRP joint, laser texturing the Ti surface
signiﬁcantly increased wettability at the interface, raising the shear force 156% over the
untreated sample [
]. The effect of the scanning speed of laser joining was investigated
for a Ti/PEEK CFRP joint, revealing that higher scanning speeds created fewer defects
on the Ti surface and less bubbles in the CFRP, along with mechanical interlocking and
chemical bonding. For the Ti/PEEK CFRP joint, a scanning speed of 0.8 mm/min resulted
in a maximum shear force of 1024 N [
]. The method of laser-riveting Ti pins to Ti parts
followed by adhesive bonding and surface structuring of the Ti parts was demonstrated
to improve mechanical fatigue life over that of conventional Ti/CFRP joints, along with
higher stiffness with equal strength [
]. A metal surface laser plastic-covered technique
with high-speed laser rotational welding technology was reported to substantially improve
the shear strength and fatigue resistance of Ti/CFRTP joints by producing a hardened layer
on the Ti surface. A fracture was reported to occur within the CFRTP but not the interface,
demonstrating the strength of adhesion [
]. Another study reported that the pretreatment
method of laser cleaning thermoset CFRP with laser plastic-covered processing of the Ti
Materials 2022,15, 3220 3 of 19
surface to create a thermoplastic coating generated mechanical interlocking coupled with
chemical bonding for Ti/thermoset CFRP joints, enhancing strength .
There has been ample research to improve strength of bolted Ti/CFRP joints, and the
most current are covered here [
]. Notable is a recent study of an innovative PEEK
CFRP rivet cut from pultruded continuous ﬁber rods and heated directly into epoxy CFRP
plates. The resulting joint had nearly twice the shear and tensile strengths than that of Ti
]. Also, hybrid bonded/bolted (HBB) joints have been increasingly utilized in the
aerospace ﬁeld due to their higher tensile properties [
]. For example, the HBB joints of
three bolts aligned in the tensile direction with adhesive had higher tensile strength than
those with one or two bolts and were higher than pure bolted or pure bonded joints [
A study of bolted Ti/CFRP joints reported a “dynamic installation” method that reduced
typically undesirable damage to the top and bottom plies [
]. For single-lap Ti/polyimide
(PI) lap joints, two types of Ti alloy inserts, bushing and embedded conical nut, were
fabricated to repair the bearing damage zone [
]. The use of Ti rivets was coupled with a
laser riveting process, as mentioned above, to strengthen Ti/CFRP joints .
There have been recent studies on the testing and analysis of deformation and fracture
mechanisms of Ti/CFRP bolted joints [
]. For double-lap single-bolt Ti/CFRP joints,
a damage model was constructed to characterize ﬁtting tolerance effects on damage and
failure during quasi-static loads [
]. For single-lap pinned Ti/CFRP joints, a dynamic test
platform based on an electromagnetic loading technique was developed to analyze fracture
mechanisms, demonstrating, as expected, the most damage in the CFRP .
The use of carbon ﬁber nanotubes (CNTs) has been gaining signiﬁcant attention in
strengthening Ti/CFRP joints. The ﬂame method was utilized to deposit CNTs on Ti to
enhance the resistance-welding of hybrid Ti/TP composite joints. The CNTs acted as
“connectors”, increasing joint adhesion [
]. Reinforcing a PI matrix with multi-walled
CNTs (MWCNTs) was reported to strengthen the Ti–PI interface of a Ti/PI multilayered
alternating laminate joint [
], with MWCNT diameters from 2 to 20 nm, enhancing the
interface mechanical properties [
]: the average diameter of 8 nm increased the interface
mechanical performance almost 180% over that without MWCNTs [
]. A novel co-bonding
process was reported joining epoxy MWCNT-reinforced CFRP with Ti to make a ﬁber metal
laminate (FML) in which the joining of Ti to CFRP was performed simultaneously with the
CFRP manufacturing. Adding MWCNTs to epoxy resin increased fracture resistance over
140%, while, for safety, making structural health monitoring (SHM) possible to locate crack
propagation and stresses before joint fracture .
Research on anodizing the Ti surface prior to adhering to the CFRP includes the
application to ultrasonic welding for Ti/Nylon-6 CFRTP lap joints [
]. Other studies
include chromic acid anodization to adhere Ti to TP [
], as well as to the amorphous TPs of
polyphenylquinoxaline, glass-ﬁlled Ultem polyetherimide, unﬁlled Ultem polyetherimide,
and Victerex polyethersulfone of single lap joints [
]. The method of resin precoating
(RPC) after anodizing, grinding, or acid-pickling of the Ti surface was investigated [
The study reported NaOH anodizing with an RPC treatment resulted in 22.0 MPa bond
strength in single-lap shear tests, 105.3 and 70.1% higher than acid-pickled and ground,
In addition, a brazing method was utilized to fabricate Ti/short ﬁber PEEK C/C
]. A metallic foam interlayer was introduced, producing a homogeneous mi-
crostructure, changed stress distribution, and enhanced mechanical properties of the
In addition, for Ti/short ﬁber PEEK CFRP joints, employing a friction riveting process
with fast rotation speed, friction time, and forging pressure had success, with high pull-out
tensile strength ranging from 6.3 to 10.7 kN .
Another method, induction heating, was utilized for Ti/PPS (thermoplastic polyphene-
lene sulﬁde) tensile lap joints .
Materials 2022,15, 3220 4 of 19
Ultrasonic additive manufacturing (UAM) was utilized to fabricate Ti/3-D CFRP struc-
tures, demonstrating that ultrasonic energy and surface roughness could yield increased
shear strengths .
Finally, a mechanically novel technique of fabricating bio-inspired adhesive single-lap
joints with a microstructural surface pattern resembling a gecko allowed fracture path
controllability for lap joints .
However, none of these technologies applied an insert intricately embedded into both
joint half-lengths for strong connection. The CF plug junction is employed, which is a
cross-weave that can be simply set into a Ti half-length slit prior to spot welding. Despite
the stress concentrators that can be generated at spaces within the weave, the weave pattern
itself is advantageous, reducing ﬂaw sensitivity and providing substantial mechanical
property improvement of the composites [
]. Wavelength dispersive X-ray spectroscopy
(WDS) analysis has shown that rapid spot-welding prior to rapid cooling solidiﬁes Ti metal
intricately between individual CFs in cross-weave CF plugs [
]. The advantages of
spot beams are that the beam is highly focused, the energy is controlled precisely to allow
rapid melting prior to rapid solidiﬁcation, and the vacuum atmosphere protects molten
metals from trace oxides and nitrides [10,11,46].
Therefore, to improve the adhesive force of the Ti–CFRP interface, we reviewed the
literature background [
] of joining Ti with CF cross-weave plug inserts to take
advantage of the extremely high surface areas of ~6 µm CFs for high adhesion.
It follows that the critical interface area (S
) to achieve the maximum tensile stress of a
single CF implanted in polymer can be calculated by the following equation :
For CF in epoxy, S
was experimentally determined to be 4.71
Lwere the CF radius (3
m) and length (5
m), respectively, for CF with an
extremely high tensile strength (
) of 6 GPa (6 GN/m
]. The reported results showed
in single-ﬁber tests implanted into epoxy glue that a 5.0 mm implant length Lapparently
gave the highest CF tensile strengths in the range of 2.7 to 4.8 GPa, as opposed to an Lbelow
2.5 mm giving 0.8 to 4.2 GPa. It was also reported in a single-ﬁber tensile test that, when
the CF implant depth in epoxy glue was more than 5 mm, tensile strength, i.e., adhesion
force at the median fracture probability P
= 0.50, was ~4.0 GPa, which was more than that
at a 1.5 mm depth (3.3 GPa) .
Thus, the critical resistant (shear) stress
) to pull out a single ﬁber of CF
from epoxy glue is a small friction resistance force of approximately 3.6 MPa calculated
σc= [πr2/Sc]σb= [d/2L]σb= 3.6 MPa (2)
Collectively, a high force of friction is produced between the high surface area of the
m diameter CFs and epoxy resin, creating strong adhesion with full contact [
based on this concept, new Ti/CF/epoxy joint to improve the strength of Ti/epoxy by
multiplying the contact area 450 times with a CF plug was innovated .
However, due to their crosslinked macromolecular structure, thermosets (TSs) are not
easily recyclable; degradation and disposal pose signiﬁcant problems for our environment.
On the other hand, thermoplastic polymers (TPs) are highly desired over TSs since they can
be melted and reformed for recyclability and sustainability, have shorter production times,
lower moisture absorption, increase crack resistance, and lower material costs [10,11].
An example of a TP is ABS, constructed with rubber structure phase ﬁne particles of
the elastomer polybutadiene (PB: –(CH
)–) distributed in an amorphous
phase matrix of acrylonitrile styrene (AS: –[(CH
–CH < –CN>)
–CH < –C
ABS has high crack resistance and recyclability, and it is only ~20% of the cost with ~10%
of the solidiﬁcation period of epoxies [
]. PC TP polymer is constructed of aromatic hard
segments and carbonate groups (PC: –(O– < –C
– < –C
recyclable, and has strong resistance to impact and high temperature .
Materials 2022,15, 3220 5 of 19
However, a disadvantage of TPs is low adhesive strength to CF due to easy ﬁber pull-
out from the typically difﬁcult-to-join TPs. While TS epoxy has strong interfacial adhesion
with CF around the entire ﬁber circumference, in the TP nylon-6 CFRTP, for example,
dendritic crystalline (hard segments) was found to grow incompletely and heterogeneously
around the CF circumference [
]. The initial crystallites heterogeneously nucleated at
sparse point contacts on the CF surface due to low wettability between the TP and CF.
Hence, the ultimate tensile strength (UTS) has been found to be inadequate in fabricated
Ti/CF/TP joints [10,11] in comparison to the full contact of TS epoxy .
The weak bonding between TPs and CF has also been attributed to CF lattice structure
having graphitic basal planes with nonpolar surfaces and chemical inertness due to the
manufacturing steps of high-temperature carbonization and graphitization [
the surface smoothness, negligible adsorption characteristics, and lipophobicity of CF lead
to insufﬁcient bonding with matrix materials [52,53].
However, activation by applying a light electron (e–) charge with homogeneous low
voltage electron beam irradiation (HLEBI) has been gaining attention, since it has been
found to improve many materials [
], including: enhancing wetting and mist
]; increasing the impact value of polycarbonate polymer [
the adhesion of glass ﬁbers to polymers [
]; adhering the ﬂat surfaces of metal/polymer
joints such as Al/PU [
] and Cu/PU [
]; and the joining of 18-8 stainless steel and
CFRP (18-8/CFRP) [
]. For an interlayered composite of three plies of carbon ﬁbers
between four layers of polypropylene sheets ([PP]
), a 0.22 MGy HLEBI dose directly
to carbon ﬁbers in a N
atmosphere to sized and 0.30 MGy to unsized carbon ﬁbers prior to
lamination assembly and hot pressing improved adhesion, raising the bending strength
of the composites [PP]
]. For [PP]
an HLEBI dose of 0.22 MGy was applied in an optimal 2000 ppm O
-rich atmosphere, the
bending strength was raised for all the fracture probabilities over the untreated sample [
However, it is not recommended to use unsized carbon ﬁbers in practical situations due to
an inferiority in processing and strength compared to sized CFs.
HLEBI is a comparatively simple technique that does not use atoms, catalysts, or
chemical treatments. Large platens can be treated.
HLEBI activates the CF surface by decreasing the density of naturally occurring
dangling bonds in the hexagonal graphite structure detected by a reduction in electron spin
resonance (ESR) peak height [
]. Applying HLEBI to CF has been reported to strengthen
the CF itself, increasing fracture stress and elasticity, along with ductility [67,68].
Therefore, to increase the UTS of Ti/polymer joints, we present a research review of
introducing a CF plug with a high connecting surface area of ﬁne carbon ﬁbers [
and, for environmentally friendly Ti/TP joints, the activating of bare CF half-lengths by
homogeneous electron beam low voltage irradiation (HLEBI) prior to dipping in a TP
For simplicity, the joint designations will be (A) “Ti/polymer” for titanium/polymer
joint spontaneous adhesion with no CF or glue; (B) “Ti/CF/polymer” for those with CF
plug inserts; (C) “Ti/
CF/polymer” or “Ti/
CF/epoxy” for those with a CF plug that is
HLEBI (EB)-treated or a CF plug that is Ni-plated, respectively; and (D) “
CF/polymer”, or “
CF/epoxy” with a superscript ‘c’ for those whose UTS
was calculated for CF cross-section portions by the rule of mixtures.
A background of thermoset Ti/CF/epoxy joints with the same geometry as the TP
joints illustrated in Figure 1is included here to serve as a measure for more environmentally
friendly TP joints to attain [
]. The ultimate aim is the developing and strengthening of
Ti/CF/TP joints closer to or, if possible, beyond that of Ti/CF/TS epoxy joints with high
concern for the environment and safety.
Materials 2022,15, 3220 6 of 19
Figure 1. Schematic illustration of CF plug joint specimen [10,11,46].
2. Introducing CF Plug for Increased UTS of Titanium/Polymer Joints
Figure 1shows the schematic of a CF plug specimen with the following dimensions:
total length, width, thickness, CF plug length, and CF plug thickness of 60, 10, 3.0, 40, and
0.23 mm, respectively. The length of the CF plug into the Ti and polymer is 30 and 10 mm,
respectively. The Ti and polymer half-lengths are equal lengths at 30 mm each [10,11,46].
The Ti/CF/polymer joints were constructed by taking advantage of the extremely
high surface area of 6 µm CF cloth. To achieve this, a two-step process was employed:
The Ti/CF half-length was assembled by spot welding, as illustrated in
Schematic diagram of focused EB spot welding of CF with molten Ti under vacuum
The polymer/CF half-length was assembled by dipping the exposed CFs in molten
thermoplastics such as ABS , PC , or thermoset epoxy .
2.1. Ti/CF Half-Length: Assembly by Spot Welding and Examination
The first step of rapid spot welding (Figure 2) used to contact and wrap the CF with molten
Ti by capillary phenomenon is described here. The melting was performed by electron beam (EB)
at a 10 kV potential and a 25
5 mA current under a vacuum with 9.3
Pa residual gas
]. The EB welding process involved rapid heating above the Ti melting point (M.P.)
of 1943 K (1670
C) with a heating power term of 10 s followed by rapid solidification by heat
sink, where the solidification point of saturated C- and O-rich Ti occurred below the Ti M.P.
2.2. Polymer/CF Half-Length: Assembly
The remaining half-length of exposed CF cloth was dipped in the polymer at a tem-
perature above the melting point, dried, and solidiﬁed, resulting in the ﬁnished sam-
2.3. Tensile Testing and Analysis
The tensile tests were conducted with an Autograph tensile tester (Shimadzu Model AG-
10TE: Shimadzu Corporation, Tokyo, Japan) at 1.0 mm/min at room temperature [
The stress–strain curves were recorded according to crosshead displacement and confirmed
Materials 2022,15, 3220 7 of 19
via recording by video. Because the polymer half-length deformed more than the Ti/CF
during the tensile tests, true stress–strain curves could not be adaptable due to heterogeneous
deformation. Therefore, the UTS
(MPa) was obtained from the nominal stress–strain curves.
Since the UTS of the joints was smaller than that of Ti or CFRP, slip could not be observed.
After the tensile tests, sample cross-sections perpendicular to the tensile testing direc-
tion were cut 5 mm deep into the Ti/CF half-length for analysis. Element mapping of C,
Ti, titanium carbide (TiC), and titanium dioxide (TiO
) was carried out with an electron
probe micro-analyzer (EPMA-1610, 15 kV, 10 nA/Shimazu, Kyoto, Japan). X-ray diffraction
, MiniﬂexII, Rigaku, Tokyo) was performed using a 10
rate. Lattice structures of the compounds were determined by standard diffraction peaks
evaluated by the ICDD (International Centre for Diffraction Data). For more detail, please
refer to [10,11,46].
2.4. Results of Addition of CF Plug to Increase Tensile Stress of Ti/Polymer Joints
The initial attempts to employ CF plugs for Ti/polymer joints include Hasegawa et al.
(2016), who fabricated Ti/CF/ABS [
], which paved the way for further developments [
Figure 3summarizes our research results of CF plug addition to Ti/TPs (ABS and PC), along with
a Ti/TS (epoxy). Without a CF plug, Ti/ABS, Ti/PC, and Ti/epoxy joints adhere spontaneously
and have been experimentally found to have relatively low UTS amounts of 4.0, 1.0, and 3.5 MPa,
Improvements in ultimate tensile strength (UTS)
(MPa) of Ti/Polymer joints by addition
of CF plug. Data is from: Hasegawa, Faudree, Matsumuara, Jimbo, and Nishi (2016) [
Faudree, Enomoto, Takase, Kimura, Tonegawa, Jimbo, Salvia, and Nishi (2017) [
]; and Nishi,
Uchida, Faudree, Kaneko, and Kimura (2019) [
], for titanium joints with ABS, PC, and epoxy,
However, Figure 3shows that, by the addition of a CF plug taking advantage of
the broad interfacial surface area of d= 6
m CF cross-weave cloth, UTS could be raised
(+113%), 7 times (+600%), and 7 times (+630%) for the Ti/CF/ABS, Ti/CF/PC,
and Ti/CF/epoxy joints to 8.5, 7.0, and 25.5 MPa, respectively [10,11,46].
3. Activating CF Plug with HLEBI to Increase UTS for Ti/CF/Thermoplastic Joints
Since UTS amounts of the TP joints Ti/CF/ABS and Ti/CF/PC have been reported to
still be signiﬁcantly lower than that of TS Ti/CF/epoxy, HLEBI was used to activate the
Materials 2022,15, 3220 8 of 19
bare carbon ﬁber surfaces of the TP joints prior to dipping in a TP to make Ti/
and Ti/EBCF/PC joints.
The steps including HLEBI are as follows:
Step 1: Ti/CF half-length assembly by spot welding.
The new part of the process is, after Ti solidiﬁcation, the half-length of exposed CF
of the Ti/CF joint sample is surface-activated on both sides by HLEBI.
Step 3: Polymer/CF half-length assembly.
3.1. HLEBI Method
Prior to dipping in the polymer, the remaining CF half-length was treated on both
sides by an optimal 0.30 MGy homogeneous low voltage electron beam irradiation (HLEBI)
or (EB) to fabricate Ti/
CF/ABS and Ti/
CF/PC joints. Repeated irradiations to both
side surfaces of the samples were used to increase the total irradiation dose. The interval
between the end of one irradiation period and the start of the next operation was 30 s.
Details and parameters are given elsewhere in Hasegawa et al., (2016, 2017) [10,11].
3.2. Increase in UTS by HLEBI Activation
Figure 4shows that, by applying HLEBI activation at 0.30 MGy to the exposed car-
bon fiber half-length prior to dipping in a molten thermoplastic resin, the UTS amounts of
CF/ABS and Ti/
CF/PC were increased 114% and 200% to 18.2 and 21.0 MPa, respec-
tively, over those without HLEBI: Ti/CF/ABS and Ti/CF/PC of 8.5 and 7.0 MPa, respectively
(Figure 4) [
]. The increase was closer to the goal of Ti/CF/epoxy (25.5 MPa) [
addition, this was 4.6 times (360%) and 21 times (2000%) over that of the spontaneous adhesion
of Ti/ABS (no glue) (4.0) and Ti/CF/PC (no glue) (1.0 MPa) [10,11,46].
Improvements in ultimate tensile strength (UTS)
(MPa) of Ti/TP joints by HLEBI
activation of CF plug closer to that of TS epoxy. Data is from: Hasegawa, Faudree, Matsumuara,
Jimbo, and Nishi (2016) [
]; Hasegawa, Faudree, Enomoto, Takase, Kimura, Tonegawa, Jimbo, Salvia,
and Nishi (2017) [
]; and Nishi, Uchida, Faudree, Kaneko, and Kimura (2019) [
], for titanium
joints with ABS, PC, and epoxy, respectively.
Therefore, by setting the UTS of Ti/CF/epoxy equal to 1.00, the UTS (
) of Ti/
was increased from a 0.33 to a 0.73 fraction of Ti/CF/epoxy, while the UTS of Ti/
increased from a 0.27 to a 0.82 fraction of Ti/CF/epoxy.
Materials 2022,15, 3220 9 of 19
The increase in UTS of the TP joints is attributed to action of HLEBI taking advantage of
the high contact surface area of the CFs intricately connected to polymer matrix enhancing
adhesion over that of an untreated CF plug to make a stronger joint.
3.3. Results for Normalized (Corrected) cσb(cUTS) for CFRP Cross-Sectional Area Fraction by
Rule of Mixtures
Aircraft fan blades joining Ti with CFRP probably have an entirely CFRP cross-section.
Therefore, corrected tensile stress according to the CFRP cross-section (
) was calculated
for the samples in our studies by the rule of mixtures [10,11,46]:
cσb,JOINT =Σniσb,i =nCFRPcσb+nPσb,P (3)
are fractional cross-sectional surface areas perpendicular to the tensile testing
direction for components i(in this case, CFRP) and polymer/Ti P, respectively. Rearranging
cσb= [σb,JOINT −nPσb,P]/nCFRP (4)
were approximated as 1/13 and 12/13, respectively, according to
the specimen geometry in Figure 1.
Figure 5depicts a signiﬁcant increase in the
of TP joints by a 0.30 MGy dose
of HLEBI over that of the untreated sample when the cross-sections of the joints were
evaluated for the CFRP portion. For untreated
55 and 30 MPa, respectively. However, applying 0.30 MGy HLEBI to the thermoplastic joints
resulted in an increased
of 154% (from 55
140 MPa) for
CF/ABS and 829%
195 MPa) for
CF/PC. This was closer to the goal of
epoxy cTi/CF/epoxy of 283 MPa.
Improvements in corrected (normalized) UTS
for CF plug cross-sections of Ti/TP
joints by HLEBI for ABS and PC compared with that of untreated Ti/TS epoxy joint. Data is from:
Hasegawa, Faudree, Matsumuara, Jimbo, and Nishi (2016) [
]; Hasegawa, Faudree, Enomoto, Takase,
Kimura, Tonegawa, Jimbo, Salvia, and Nishi (2017) [
]; and Nishi, Uchida, Faudree, Kaneko, and
Kimura (2019) [
], for titanium joints with ABS, PC, and epoxy, respectively. As mentioned earlier,
superscript ‘c’ designates normalized (corrected) UTS according to CF portion of cross-section by
rule of mixture calculation.
Therefore, for the CFRP cross-section, by setting the
equal to 1.00, the
CF/ABS was increased from a 0.15 to a 0.49 fraction of
Materials 2022,15, 3220 10 of 19
cTi/CF/epoxy, while the cσbof cTi/EBCF/PC was increased from a 0.11 to a 0.69 fraction
3.4. Activation by HLEBI Increasing Adhesion of Carbon Fibers with Thermoplastic
Figure 6a shows that untreated CF exhibits a weak Van der Waals force with thermo-
plastic ABS with trace N
, and H
O gases in the chamber. On the other hand, as
shown in Figure 6b, activating the CF plug with 0.30 MGy HLEBI prior to dipping in the
TP resin increased covalent bonding with ABS, raising the UTS of Ti/
CF/TP joints over
untreated Ti/CF/TP [10,11]. The HLEBI acted to form active terminated carbon atoms on
the surface and activated vacant sites of dangling bonds. ESR studies have shown HLEBI
reduces dangling bond density on the CF surface; therefore, the excess charge probably
transferred through the highly conductive CFs to the thermoplastic, creating covalent
bonds, as depicted in Figure 6b. This reduces ﬁber pull-out.
Schematics of (
) untreated CF and (
) HLEBI-treated CF with ABS thermoplastic. HLEBI
activation area is in yellow. Not drawn to scale.
Based on the mean density
) of carbon ﬁbers (1760 kg m
) and the irradiation
potential at the specimen surface (V: keV), the penetration depth Dth (m) of HLEBI was
calculated by the following equation [11,69–71]:
Dth = 66.7V5/3/ρ(5)
to be 123
m. Since HLEBI was applied to both sides of a CF plug with a thickness 0.23 mm
m), the CF plug was activated throughout its thickness. With a diameter of 6
extremely high surface area was activated for increased adhesion with the thermoplastic.
4. Ti/CF Half-Length: Metallographic Changes Due to Spot Welding
As for the Ti/CF half-length, CF pull-out was not found due to strong adhesion at
the Ti–CF interface by the rapid spot welding; hence, fracture occurred in the Ti/Polymer
half-length mostly in the form of CF breakage and pull-out [10,11,46].
The rapid spot welding appeared to prevent the excessive formation of embrittling
TiC in the Ti/CF half-length with no or minimal damage to the CFs. The XRD results
detected trace amounts of crystalline TiC, evidenced by slight peaks at the 2
of 36, 40, 62,
and 76 deg, although sharp TiC peaks were not detected [10,11,45].
was detected (in pure form, anatase, rutile, and brookite) and reported as
small peaks at the 2
of 30, 35, 41, 57, 60, 69, 75, and 76 deg [
was reported to
enhance the interfacial adhesion of Ti/PC and Ti/CF .
Materials 2022,15, 3220 11 of 19
WDS mapping showed CFs retained their sizes and shapes [
], indicating the rapid
spot-welding method acted to minimize high-temperature contact time, preserving the CFs.
Furthermore, the Ti was observed to solidify intricately between individual CFs [
4.1. Ti/CF Half-Length: Metallographic Process
To describe the metallographic process during rapid heating above the Ti M.P. of
C), a phase of C- and O-rich Ti molten liquid with Ti crystallites was formed
from the CF and ~300 ppm trace O
in the EB chamber. Subsequently, during rapid
solidiﬁcation below 1943 K, Ti crystallites containing C and O in Ti molten liquid were
formed, although most solids were amorphous or composed of very small crystal grains
due to the supercooling. The amorphous structure had advantages over crystal because
stress-concentrating grain boundaries were avoided, while reduced grain size increased
strength over the larger size. The C- and O-rich Ti molten liquid was in equilibrium with
the TiC and TiO2solids formation until total solidiﬁcation.
Figure 7illustrates the Ti-C phase diagram [
], which represents the metallographic
process during spot welding. A three-phase Ti-C-O would be beneﬁcial; however, as
far as the authors know, it was not found in the literature. From XRD analysis, TiC and
, along with Ti and C, were detected at the CF–Ti interface layer for the Ti half-length
Figure 7. Ti-C phase diagram adapted from Bandyopadhyay, Sharma, and Chakraborti (2000) .
The process starts during rapid heating when C contamination up to 1.2 wt.% in the
Ti liquid alloy decreases the melting point from 1943 to 1919 K (1670 to 1646
C) at the
eutectic point. In the thin layer around the carbon ﬁbers, higher C contamination from 1.2
to 43 wt.% tremendously enhances the liquidus from 1721 K (1448
C) to a maximum of
3338.85 K (3065.7 ◦C) . Hence, Ti solid solutions with possible trace crystalline TiC are
formed at the CF–molten Ti interface [45,67].
Moreover, oxygen addition, as evidenced by TiO
peaks by XRD, occurring probably
from trace O
in the vacuum chamber elevates the liquidus line to the maximum in the Ti-O
phase diagram (not shown) of 2158 K (1885
]. The specimens are then put through
rapid quench by cold water and solidify in supercooled amorphous form or with small
grains. Caution is advised however because, while the joint as a whole is strengthened,
excessive contact with the hot molten Ti lowers the strength of CFs .
4.2. Ti/CF Half-Length: Diffusion
WDS mapping observation of the Ti/CF half-length showed that C atoms were found
to diffuse into the Ti matrix, but Ti diffusing into the closely packed hexagonal graphite C
structure was not observed [
]. Additional observation showed C atom diffusion
Materials 2022,15, 3220 12 of 19
into Ti was particularly around the carbon ﬁber circumferences in a thin ﬁlm about ~1 mm
thick , which probably included the trace TiC detected by XRD.
To discuss the diffusion rates of C, O, and Ti in Ti, the reported diffusion coefﬁcients,
/s) are listed in Table 1. It shows that, for
-Ti from 1113 to 873 K (840 to 600
the Dof C and O cover the same range: for C, 9
/s; and for O, 0.2
to 2 ×10−9to 6 ×10−13 cm2/s (although the range for O was wider) .
Diffusion coefﬁcients for carbon, oxygen, and titanium atoms in titanium. Data is from
Nakajima and Koiwa (1991) .
Diffusion Coefﬁcients, D(cm2/s) in Ti
Dof C in a-Ti Dof C in b-Ti
T(K) D(cm2/s) T(K) D(cm2/s)
1113 9×10−11 1693 8×10−5
873 2×10−12 1353 2×10−6
D of O in a-Ti Dof O in b-Ti
1113 0.2 to 2 ×10−91693 1×10−6
873 6×10−13 1173 0.6 to 1 ×10−7
Dof Ti in Ti (self diffusion)
The left column of Table 1shows that, at higher temperatures above the phase transi-
-Ti, there appears to be a hierarchy of increasing diffusion rate in
→C, with the C atoms having the highest value .
If a thin TiC layer was generated at the Ti–CF interface, the diffusion coefﬁcient Dof
the C in TiC was ~5
/s at ~1723 K (~1450
]. This was seven orders
of magnitude lower than the Dof C in
-Ti of 8
/s at a similar temperature
of 1693 K (1420
C) (Table 1), indicating that, despite embrittling, the TiC layer can be
advantageous for preventing C diffusion into Ti.
Note that the wt.% of O atoms in the Ti crystal structure was reported to slow the
diffusion of C in TiC .
5. Developments in Thermoset Ti/CF/Epoxy Joints
For comparison to Ti/TP joints, the background of developing thermoset Ti/TS/epoxy
joints by novel CF plug insert is given here [
]. The successful innovation of Ti/CF/epoxy
joints could be achieved, but with an epoxy matrix [
]. The spot welding of the Ti/CF
half-length and the joint dimensions were identical to those of TPs mentioned above [10,11].
Figure 8shows a summary of our research up to now on increasing the UTS of
Ti/epoxy joints [
]. The UTS of a TS Ti/epoxy spontaneous joint with no glue was
= 3.5 MPa, while that of Ti/glue/epoxy joint was 5.9 MPa [
]. As men-
tioned earlier, the CF plug addition boosted the UTS to 25.5 MPa [
]. The Ti/CF/epoxy
joint was further strengthened to 35 MPa by a novel process of Ni-coating of the CFs of
CF/epoxy joints prior to welding with Ti [
]. This was compared to the UTS of epoxy
resin of 69 MPa [
]. However, the CFRP portion of the cross-section for the
joint was calculated by the rule of mixtures to be above that of epoxy resin at
= 283 MPa.
Ni-plating of the CFs prior to welding with Ti for the
CF/epoxy joint increased the
cσbfurther to 45% over the cTi/CF/epoxy joint with 413 MPa .
Materials 2022,15, 3220 13 of 19
Advances in UTS of Ti/epoxy joints with CF plug. Data is from Hasegawa, Inui, Shi-
raishi, Ishii, Kasai, Matsumura, and Nishi (2016) [
]; and Nishi, Uchida, Faudree, Kaneko, Kimura
(2019) . Reported data of UTS of epoxy resin from Shackelford (2000) is also shown .
The rule of mixtures calculation for the CFRP portion of the cross-section indicated
that a CF plug can make it possible to increase UTS orders of magnitudes higher than
that of Ti/epoxy (no glue) with spontaneous adhesion. The
CF/epoxy joints were 80 times (~8000%) and 113 times (~11,700%) larger, respec-
tively, than the σbof Ti/epoxy at 3.5 MPa .
Metallographic Process of Increasing Adhesion by Ni Coating on Ti/CF Half-Length for
This paper focuses on increasing the UTS of Ti/TP and Ti/CF/TP joints by HLEBI.
However, the metallographic mechanisms for Ti/TS and Ti/
CF/epoxy joints are brieﬂy
Although the CF plug greatly enhances UTS over spontaneous adhesion, untreated CF
does not bond well due to its inert surface and low wettability, as well as having chemical
instability with metals including iron, all of which limit its mechanical properties [
Therefore, to raise the UTS of Ti/CF/epoxy joints further, Ni has been used to coat CFs. A
Ni plating prevents the encroachment of molten metal at high welding temperatures and
excess brittle carbide formation at the CF–metal interface from reactions between carbon
and the metal [
]. A Ni coating acts as a buffer with mutual diffusion between Ni and Ti
as a gradient absorbing energy during tensile testing to increase strength. As shown in
Figure 8, Ni-plated CFs increased the UTS of a Ti/
CF/epoxy joint to 413 MPa over that
of an uncoated Ti/CF/epoxy joint at 283 MPa.
An observation of the Ni-coated area by XRD indicated that trace NiTi and Ni
tallites were formed at the Ni–Ti zone [
] as a diffusion layer in the Ti/
The diffusion coefficients for Ti-Ni
Ti were from 7.5
/s at 1173 K
], which were higher than those of the O in
-Ti at 0.6 to 1
/s and the
Ti (self-diffusion) in Ti at 6
/s, both at the same temperature of 1173 K [
mentioned earlier, metallic elements were not detected in the carbon fibers.
6. Summary of Our Research Increasing UTS of Ti/TP and Ti/Epoxy Joints
For easy reference, Tables 2and 3give a summary of our research up to now advancing
the UTS of hybrid Ti/polymer joints showing: an increase in the UTS (
) (MPa) of treated
(HLEBI or Ni) CF plugs over an untreated CF plug condition, as well as UTS improvement
over a Ti/polymer (no glue) condition.
Materials 2022,15, 3220 14 of 19
Summary of improvements in UTS (
) (MPa) of: treated (HLEBI, or Ni plating) over un-
treated CF plug joints (left two columns); and total improvement of treated CF plug over Ti/polymer
(no plug, no glue) joints (right two columns). Data is from: Hasegawa, Faudree, Matsumuara, Jimbo,
and Nishi (2016) ; Hasegawa, Faudree, Enomoto, Takase, Kimura, Tonegawa, Jimbo, Salvia, and
Nishi (2017) [
]; and Nishi, Uchida, Faudree, Kaneko, and Kimura (2019) [
], for titanium joints
with ABS, PC, and epoxy, respectively.
Treated CF-Plug over Treated CF-Plug over Ti/Polymer (No Glue)
Joints sb(MPa) Joint sb(MPa)
Ti/CF/ABS 8.5 Ti/ABS (No Glue) 4
Ti/EBCF/ABS 18.2 Ti/EBCF/ABS 18.2
imp. 114% imp. 355%
Ti/CF/PC 7 Ti/PC (No Glue) 1
Ti/EBCF/PC 21 Ti/EBCF/PC 21
imp. 200% imp. 2000%
Ti/CF/Epoxy 25.5 Ti/Epoxy (No Glue) 3.5
Ti/NiCF/Epoxy 35 Ti/NiCF/Epoxy 35
imp. 37% imp. 900%
Summary of improvements in normalized (corrected) values of UTS (
) (MPa) for CF cross-
section portions of joints calculated according to rule of mixtures in Table 2for: treated (HLEBI or Ni
plating) joints over untreated CF plug joints (left two columns); and total improvement of treated CF
plug joints over Ti/polymer (no plug, no glue) joints (right two columns). Data is from: Hasegawa,
Faudree, Matsumuara, Jimbo, and Nishi (2016) [
]; Hasegawa, Faudree, Enomoto, Takase, Kimura,
Tonegawa, Jimbo, Salvia, and Nishi (2017) [
]; and Nishi, Uchida, Faudree, Kaneko, and Kimura
(2019) , for titanium joints with ABS, PC, and epoxy, respectively.
Treated CF-Plug over Treated CF-Plug over [Ti/Polymer] (No Glue)
csb(MPa) Joint csbor sb(MPa)
cTi/CF/ABS 55 Ti/ABS (No Glue) 4
cTi/EBCF/ABS 140 cTi/EBCF/ABS 140
imp. 154% imp. 3400%
cTi/CF/PC 21 Ti/PC (No Glue) 1
cTi/EBCF/PC 195 cTi/EBCF/PC 195
imp. 829% imp. 19,400%
cTi/CF/Epoxy 283 Ti/Epoxy (No Glue) 3.5
cTi/NiCF/Epoxy 413 cTi/NiCF/Epoxy 413
imp. 45% imp. 11,700%
Table 2shows that, for the TPs, Ti/
CF/ABS and Ti/
CF/PC were increased by
114% and 200% over untreated Ti/CF/ABS and Ti/CF/PC and by 355% and 2000% over
Ti/ABS and Ti/PC, respectively. For the TSs, Ti/
CF/epoxy was increased 37% over
untreated Ti/CF/epoxy and 900% over Ti/epoxy.
Likewise, Table 3shows an increase in the normalized (corrected) UTS (
according to the CF portions of the cross-sections. For the TPs,
CF/PC were increased 154% and 829% over untreated
Materials 2022,15, 3220 15 of 19
and 3400% and 19,400% over Ti/ABS and Ti/PC, respectively. For the TSs,
was increased 45% over untreated cTi/CF/E=epoxy and 11,700% over Ti/epoxy.
Figure 9graphically shows the advances in UTS with reported values for Ti, ABS,
PC, and epoxy. In summary, with the experimental data reported, the CF plug appears to
tremendously increase the UTS of Ti/polymer joints.
Graphical summary of current status and review showing our previously published results
developing Ti/polymer hybrid joints by CF plug insert in terms of fracture probability P
(MPa). Normalized (corrected)
for CF plug cross-sections are shown with data
from: Hasegawa, Faudree, Matsumuara, Jimbo, and Nishi (2016) [
]; Hasegawa, Faudree, Enomoto,
Takase, Kimura, Tonegawa, Jimbo, Salvia, and Nishi (2017) [
]; and Nishi, Uchida, Faudree, Kaneko,
and Kimura (2019) [
], for titanium joints with ABS, PC, and epoxy, respectively, along with
bulk Ti from Barksdale (1968)  and data of ABS, PC and epoxy from Shackelford (2000) .
A review was conducted of the latest studies found in the literature to create strong
Ti/CFRP joints. Up to now, methods have included laser treatment, improving bolted joints,
carbon ﬁber nanotubes (CNTs), anodizing, brazing, friction riveting, inductive heating,
ultrasonic additive manufacturing, and novel bio-inspired adhesives. However, none of
these technologies has applied a carbon ﬁber (CF) insert intricately embedded into both
joint half-lengths for strong adhesive force, taking advantage of the broad interfacial surface
area of d= 6
m CF cross-weave cloth. Therefore, we then reviewed our literature on the
strengthening Ti/polymer joints by CF plugs by ﬁrst spot-welding the Ti to CF, followed
by dipping the remaining half-length in polymer resin.
Employing a CF plug for Ti/polymer joints resulted in a substantial increase in the
ultimate tensile strength (UTS) over spontaneous adhesion without glue: 2.1 times (+113%),
7 times (+600%), and 7 times (+630%) for Ti/CF/ABS, Ti/CF/PC, and Ti/CF/epoxy joints
to 8.5, 7.0, and 25.5 MPa, respectively.
However, since thermoplastic polymers (TPs) have poorer adhesion to CF than thermoset
(TS) epoxies, CFs were treated with homogeneous electron beam irradiation (HLEBI) prior to
dipping in the TP resin. The resulting UTS amounts for acrylonitrile butadiene styrene (ABS)
CF/ABS and polycarbonate (PC) Ti/
CF/PC joints were increased 114% and 200%
Materials 2022,15, 3220 16 of 19
to 18.2 and 21.0 MPa, respectively, over those of Ti/CF/ABS and Ti/CF/PC. This was closer
to that of the epoxy joint Ti/CF/epoxy at 25.5 MPa. When calculated according to the rule
of mixtures (RM) for CF cross-section portions, the UTS of Ti/
CF/ABS and Ti/
were increased 154% (from 55 to 140 MPa) and 829% (from 30 to 195 MPa), respectively, over
untreated samples, closer to that of Ti/CF/epoxy at 283 MPa.
The strengthening mechanism of the action of HLEBI prevented CF pull-out by appar-
ently creating covalent bonding at the CF–TP interface, as well as growing crystallites into
the TP around CF circumference.
Our research employing CF plugs to join Ti and TPs is progressing to reach the ultimate
goal of raising the UTS of thermoplastic Ti/CF/TP joints to that of thermoset Ti/CF/epoxy
for safety and environmental sustainability.
Author Contributions: Conceptualization, Y.N., M.C.F. and M.S.; methodology, Y.N., M.C.F., H.T.U.
and M.S.; software, M.C.F.; validation, M.C.F. and Y.N.; formal analysis, Y.N. and H.T.U.; inves-
tigation, M.C.F., Y.N. and H.T.U.; resources, M.S., H.K. and S.K.; data curation, M.C.F. and Y.N.;
writing—original draft preparation, M.C.F. and Y.N.; writing—review and editing, M.C.F. and Y.N.;
visualization, Y.N. and M.S.; supervision, Y.N., H.K. and S.K.; project administration, Y.N. and M.S.;
funding acquisition, M.C.F., Y.N. and M.S. All authors have read and agreed to the published version
of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author. At the time the project was carried out, there was no obligation to make the
data publicly available.
The authors sincerely thank A. Mizutani, H. Hasegawa, S. Ishii, Y. Miyamoto, A.
Tonegawa, N. Inoue, K. Oguri, and Y. Matsumura of Tokai University. Sincere gratitude goes to the
Japan Society for the Promotion of Science (JSPS) Core-to-Core Program for their great support.
Conﬂicts of Interest: The authors do not declare any conﬂict of interest.
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