Weight reduction and improved strength are two common engineering goals in the 19 joining sector to benefit various en gineering sectors ranging from transport, aerospace, nuclear to 20 many other s. Here, in this paper, we show that the suitable addition of carbon nanomaterials to a 21 tin-based solder material matrix ( C-Solder ® supplied by Cametics Ltd.) results in two fold strength 22 of solder ed composite joints . Single -lap shear joint experiments were conducted on soldered 23 aluminium alloy (6082 T6) substrates . The soldering material was reinforced in different mix ratios 24 (0.01 wt% and 0.05 wt% ) by carbon black, graphene and si ngle -walled carbon nanotubes . and 25 benchmarked against the pristine C -solder ®. The m aterial c haracterisation was performed using 26 Vickers micro -indentation, differential scanning calorimetry and nano- indentation whereas 27 functional testing involved mechanical shear tests using single -lap aluminium soldered joints and 28 creep tests . The 0.05 wt.% nanomaterials reinforced solders promoted progressive cohesion failure 29 in the joints as opposed to instantaneous fully disbond failure observed in pristine soldered joints , 30 which suggests potential application in high performance structures where no service load induced 31 adhesion failure is permissible (e.g., aerospace assemblies) . The novel innovation developed here 32 will pave the way to achieving low -cost high- performance solder joining without carrying out 33 extensive surface preparations. 34 Keywords: carbon black; graphene; single -walled carbon nanotubes ; tin solder; single -lap joint 35 36 1. Introduction 37 In the current era of lightweight hybrid structure s, design and manufacturing for aerospace and 38 automotive applications, fastener -less high quality and high strength joining methods capable of 39 joining dissimilar materials are being rigorously researched . The main methods used for structural 40 joining are mechanical fastening, adhesive bonding and high temperature joining such as welding, 41 brazing and soldering [1–4]. For joining of dissimilar materials, adhesive bonding is considered 42 superior to bolted joints as it produces a continuous connection and hence smooth strain transition 43 which greatly reduces stress concentration to benefit the fatigue performance of joints [5]. Although 44 adhesive bonding is similar to brazing and soldering in the sense that the surfaces being joined are 45 2 of 28 energised (e.g. via heating, chemical treatment or ultra -violet radiations ), the bonding formed during 46 adhesive joining (so-called secondary bonding) is not a metallurgical bond, rather, it is a chemical 47 bond (i.e., Van der Waals bond) [6]. However, the polymer bonding poses problems in curing time, 48 inspection , disassembly and environmental degradation [7–12], as well as the challenge to assure 49 bond quality [13,14] 50 Joining aluminium for structural application s has proven to be challenging using traditional 51 welding and brazing practices due to its high coefficient of thermal expansion (CTE) . The thermal ly 52 induced residual stresses at the interface leads to excessive strains causing CTE mismatch between 53 the substrate and the joining material [15]. While other fastener- less technologies such as adhesive 54 bonding, welding , brazing and solder based joining provide s an improved joint stiffness and 55 limitedly overcomes the problem of stress concentration [1 –4]. A solder joint is not usual for high 56 load applications due to it being brittle and its susceptibility to fail catastrophically. In adhesive 57 bonds, the damage tolerance is mainly facilitated by cohesion failure of the bulk adhesive at the 58 interface. 59 Soldering is a highly suitable method to join wide variety of materials, ranging from composites 60 to metals such as aluminium to ceramic materials [16]. The procedure of soldering involves applying 61 a melted soldering alloy on the heated surface of materials without requiring extensive surface 62 preparation in contrast to a time -consuming and sensitive surface treatment approach associated 63 with every adhesive bonding method . Contrarily, during adhesion [17,18] ,the same procedure would 64 require highly controlled surface treatment and possible process- induced tight disb ond. Similar to 65 adhesive bonding, the main parameters affecting the solder joint is the wettability which in turn 66 influences the interfacial strength of the soldered joint that is characterised using techniques such as 67 the single -lap joint shear test [12]. 68 Traditionally, tin (Sn) based solders are being used as an interconnection material for electronic 69 devices as they are thermally conductive , light weigh t, do not account for any significant volume 70 increase for the application s and their mechanical properties do not qualify for high loading 71 applications [19]. However, i n recent years , the development of graphene and carbon nanotubes 72 (CNTs) [10,11] has surfaced the idea of embedding carbon nanomaterials into the metal matrices 73 which has opened newer avenues for material joining. It allows to obtain enhanced specific stiffness, 74 strength, wear resistance, controlled CTE, fatigue resistance and electrical conductivity. The 75 nanomaterial reinforcements are also referred to as metal matrix nanocomposites [22]. Taking these 76 ideas forward, this work hypothesised that the mechanical properties of tin solder could also be 77 tailored and enhanced to withstand high structural loads and temperatures , specifically for joints 78 susceptible to interfacial adhesion failure . 79 Moreover, the addition of transition metal to solder alloys has also been shown to enhance the 80 wetting of carbon materials at low temperatures thus enabling dissimilar mate rial joining at low 81 temperature. For instance, the addition of 2.5 wt .% to 5 .0 wt.% of Cr or Ni to Sn- 3.6Ag -0.7Cu alloys 82 (SAC alloys) showed enhanced wetting and joining of carbon nanotubes (CNTs) [23]. That is mainly 83 due to the uniform nonreactive dispersion of the transition metals in the SAC matrix , which diffuse s 84 into the carbon alloy border with no formation of brazing -like carbides. 85 3 of 28 C-Solder® is a commercial tin -based lead -free low -temperature soldering alloy manufacture by 86 Cametics Ltd. It enables joining of various carbon materials including carbon fibres or CNT fibres in 87 both carbon- carb on and carbon- metal arrangements [24] . During soldering, oxides form on the 88 surface of the molten solder preventing it from wetting the substrate. For that reason, forced wetting 89 or mechanical activation is applied by brushing/peening and ultrasonic agitation in the utilisation of 90 C-Solder® . 91 This research investigated the influen ce of reinforcing carbon nanomaterials (carbon black (CB), 92 graphene and single -walled CNT (SWCNT) ) to the C -Solder® [18]. The study includes a 93 comprehensive analysis of data from microstructur al characterisation, thermal analysis and 94 mechanical shear performance of soldered joints utilising the pristine solder, and carbon 95 nanomaterials reinforced solder with 0.01 and 0.05 wt.% loading contents . The joint strength was 96 evaluated using standard single -lap soldered joints made of aerospace grade aluminium substrates , 97 and under quasi -static loading conditions [12,25] . 98 2. Materials and Methods 99 2.1. Materials 100 The samples examined in this research are listed in Table 1 , categori sed into as-received C - 101 Solder® (CS) , solder mixed with carbon black (CB at 0.01 and 0.05 wt.% content ), graphene with 0.01 102 and 0.05 wt .% (G_0.01 and G_0.05, respectively) and SWCNTs with 0.01 and 0.05 wt.%. The samples 103 were characterised (i.e., melting temperature via differential scanning calorimetry, Vickers hardness 104 and creep behaviour via nano- indentation ands via optical and scanning electron microscopy, SEM) . 105 The nanomaterials were supplied as follows: 106 • SWCNT 755710- 1G supplied by Sigma Aldrich, 107 • Graphene CamGraph® G3 supplied by Cambridge Nanosystems 108 • Carbon black, MONARCH® 800 supplied by Cabot 109 The material preparation process entails mixing of C-Solder ingots and SAC305 powder with carbon 110 material which were initially loaded into vacuum induction melting furnace . They were mixed via 111 induction stirring in the temperature range of 1000 -1200°C , and cast ed in vacuum at 1200°C 112 Table 1. Solder samples studied and their methods of preparation 113 Solder sample s Induction mixed and cast in vacuum at 1200 oC CS C-Solder® CB_0.01 C-Solder® reinforced by 0.01 wt .% CB CB_0.05 C-Solder® reinforced by 0.05 wt .% CB G_0.01 C-Solder® reinforced by 0.01 wt .% Graphene G_0.05 C-Solder® reinforced by 0.05 wt .% Graphene SWCNT_0.01 C-Solder® reinforced by 0.01 wt .% SWCNT SWCNT_0.05 C-Solder® reinforced by 0.05 wt .% SWCNT 114 4 of 28 The as-received solder (no nanomaterial addition) was used as benchmark reference to join 115 aluminium substrates to test the integrity of C -solder® based single -lap soldered joints. High 116 performance, aerospace grade Al6082T6 aluminium alloys (tens ile strength of ~310MPa and 117 elongation at breakage of ~8%) [19] were used as substrate material s for the lap joining . 118 119 2.2. Characterisation of solder materials 120 To analyse the dispersion quality of the nano materials in the solder samples, the samples were 121 mounted in standard mounting epoxy resin, ground in three stages ( using 240, 1200, 2500 grit papers, 122 each at 200 rpm for 3 hrs. ) and polished in two stages , each at 250 rpm for 3 hrs. (using plano cloth 123 drenched with 3 -micron diamond suspension followed by OP-S Colloidal Silica ). SEM -EDS analysis 124 was conducted to identify the composition and their dispersion -distribution quality. The total weight 125 of the samples cut from solder rods was kept less than 10 mg. Three indentations were applied along 126 each solder sample with a load of 25 grams for 15 seconds. 127 128 Figure 1. Schematic illustration of c reep test using nano-indentation with a Berkovich indenter 129 The creep behaviour of the solders under nano- indentation loads was evaluated using a nano- 130 indenter with a Berkovich tip , as schematically shown in Figure 1 . Six indentations per sample were 131 made with spacing of 35 µm between indentations. Load controlled testing during the 132 nanoindentations with a minimum and maximum constantly held loads of 20 mN and 100 mN, 133 respectively , were used in the test scheme. The loading rate was 1 mN/s and the holding time was 134 120 s. When the load reached maximum value , the indentation depth versus time was recorded and 135 used for the creep performance evaluation , schematically shown in the inset in Figure 1. 136 The creep response was evaluated using a power -law creep equation over a wide range of strain 137 rates known as the secondary or steady- state creep regime under constant loads [26,27] . Thus, 138 assuming a constant temperature and steady -state creep; the creep strain rate ( 𝜀𝜀̇) can be expressed as 139 [28]: 140 5 of 28 𝜀𝜀̇=𝐵𝐵�𝜎𝜎 𝐺𝐺�𝑛𝑛 , (1) where 𝐵𝐵 is a material constant, 𝑛𝑛 is the stress exponent, 𝐺𝐺 is the temperature dependent shear 141 modulus, and 𝜎𝜎 is the applied stress. 142 The temperature dependence of the shear modulus of pristine Sn has been adopted in this study, 143 in which G can be empirically calculated from [29]: 144 𝐺𝐺=16302−40.5(𝑇𝑇−273 ), (2) where T is temperature in Kelvin . 𝜀𝜀̇ and 𝜎𝜎 can therefore be expressed by [30,31] : 145 𝜀𝜀̇=1 ℎ𝑑𝑑ℎ 𝑑𝑑𝑑𝑑 and (3) 𝜎𝜎=𝑃𝑃 𝐴𝐴 , (4) where 146 𝐴𝐴=24.5 ℎ2, (5) and ℎ is the depth of indentation, 𝑑𝑑 is the creep time, and 𝑃𝑃 is the indentation load . Finally, using 147 Equations (1-5), the creep stress expo nent can be calculated: 148 𝑛𝑛=𝜕𝜕𝜕𝜕𝑛𝑛𝜀𝜀̇𝜕𝜕𝜕𝜕𝑛𝑛 (𝜎𝜎/𝐺𝐺) ⁄ (6) 2.3. Joint evaluation 149 2.3.1 The joining process 150 The Al6082T6 specimens were joined using C-solder® and its nanomaterials reinforced 151 variations (four specimens per variation) with the dimensions (according to Standard ASTM D1002 152 specimens) shown in Figure 2. 153 154 155 Aluminium substrate Side view: Top view: Shear performance (overlap) area 6 of 28 Figure 2. The dimensions of the Al6082T6 single -lap shear soldered joint specimens 156 The aluminium substrates (laps) were pre- heated on a ceramic hot plate (Stuart CN302) at 350°C. 157 The solder was deposited on the laps using a SUNBONDER USM -560 ultrasonic soldering iron at a 158 temperature of 400°C (the specified optimal wetting temperature for C -Solder®), an ultrasonic power 159 of 10 W and at an ultrasonic -frequency of 65 kHz. The lap joints were then pressed and left on the hot 160 plate to cool down (Figure 3) using steel press bars. The pressure fixture shown in F igure 3 was 161 formerly designed and assembled for research on single -lap bonded joints [17,18] in compliance with 162 the ASTM D1002 utilised herein . The thickness of the solder was controlled by using aluminium laps 163 of thickness 3.4 mm to even out the laps of the specimens (so that no misalignment occurs during 164 clamping in the gripping areas) , and allow a solder thickness of 0.2 mm. After the joining process, a 165 two-component Araldite 420 A/B epoxy adhesive was used to bond the aluminium tabs at the ends 166 of the spe cimens, followed by curing in an oven at 70°C for 5 hours. The end -tabs were added to 167 ensure longitudinal alignment of the samples’ ends when clamped in the test apparatus (Figure 4). 168 169 Figure 3. Aluminium laps joining process : 1) fixing aluminium lower laps, 2) apply the solder 170 material, 3) fix upper aluminium laps, 4) apply uniform pressure 171 2.3.2 Joint quality analysis 172 The joint quality was evaluated by cutting the cross- section of the overlap area of lap joints from 173 each category , mounting it in resin, grinding , polishing and examining it in the SEM and under an 174 optical microscope . 175 176 2.3.3 Single -lap joint tests 177 The single -lap shear tests were carried out using an Instron® universal testing machine shown 178 in Figure 4(a). The load cell fitted was a 30 kN load -cell. T he crosshead speed was set to be 1.3 179 mm/min to achieve quasi -static loading . To study the joint behaviour, a laser extensometer was used 180 to accurately measure longitudinal displacement data for the overlap area . The laser targets were 181 placed 10 mm away from t he joint ends as shown in Figure 4(b,c). 182 1 2 3 4 7 of 28 183 Figure 4. a) Schematic of fixed l ap shear test apparatus , b) Soldered aluminium sample under tensile 184 loading , and c) marks for laser extensometer (displacement ) measurements local to the soldered 185 overlap 186 The load and displacement data for each category of specimens were used to evaluate the 187 performance of the joints. The stress calculation in such single -lap joints would not provide 188 meaningful interpretation of the performance at localised area over the overlap region since stress 189 and strain distribution across the overlap region in a flat joint (soldered here) can be highly non- 190 uniform [17,18] with the maximum shear stress occur ring at the two ends of the joint [32] shown 191 schematically in Figure 5. Therefore, the joint shear and failure performance have been described 192 based on the load and displacement data for the localised areas, and the average stress -strain 193 calculations have been used to describe creep using the equations discussed above. 194 195 Figure 5. Schematic illustration of shear stress distribution in an adhesively bonded joint [32] 196 Failure of the joint was predicted to initiate at the edges first, when the effective stress exceeds 197 the yield strength of the joint . Adams et al. (1997) [28] showed that the strength of the adhesive joint 198 increases linearly with longer overlap length, until a certain threshold beyond which no stronger 199 bond s are formed [33]. It occurs after the threshold plastic deformation of the aluminium substrate 200 becomes dominant which induces bond failure. This has been further dis cussed in the next sections . 201 2.3.4 Failure Mode investigation 202 The mode of failure was mic roscopically investigated by observing the shear area after lap shear 203 test using an Oxford Instrument SEM and a Nikon Eclipse Me600 optical microscope . 204 3. Results 205 Shear stress Overlap length Laser extensometer marks Single -lap soldered joint Clamps 8 of 28 3.1. Solder Characterisation 206 3.1.1 Microstructural Analysis 207 The microscopic images for the induction mixed solders with and without the carbon 208 nanomaterials are presented in Figure 6. The se images show non- uniform distribution of the 209 nanomaterials; the best dispersed quality amongst all was observed for the case of CB_0.01 and 210 SWCNT_0.01 samples. Agglomeration of nanomaterials were observed in all samples. However, t he 211 largest nanocluster agglomerations were observed for the samples containing 0.05 wt.% of 212 nanomaterials (CB, Graphene, SWCNT) . It may be noteworthy that the two other mixing methods 213 were also trialled (as described below) but the one shown here provided the best practice for 214 achieving better dispersion. Typical SEM -EDS analysis of the reference solder’s microstructure (no 215 nanomaterial addition) resulted in the composition shown in Figure 7 , showing Sn, Cr, Ag 3Sn and 216 Sn5Cu 6. 217 218 a) C-Solder® b) CB_0.01 c) CB_0.0 5 d) G_0.01 9 of 28 Figure 6. SEM micrograph s of C-Solder® samples induction mixed with carbon 219 nanomaterial via identical mixing parameters ( captured at 5kV ); a) pristine solder, b) 0.01 220 wt.% CB reinforced solder, c) 0.05% wt.% CB reinforced solder, d) 0.01 wt.% graphene 221 reinforced solder, e) 0.05 wt.% graphene reinforced solder, f) 0.01 wt.% SWCNT 222 reinforced solder, g) 0.05 wt.% graphene reinforced solder 223 224 Figure 7. Typical composition of the reference solder material with no nanomaterials 225 addition obtained from the EDS analysis, dominantly present by Tin ( Sn), Cr, Ag 3Sn and 226 Sn5Cu 6 227 3.1.2 Thermal Analysis 228 e) G_0.05 f) SWCNT _0.01 g) SWCNT_0.05 200 𝜇𝜇m 10 of 28 Figure 8 shows the DSC profile of C -Solder® and its nanomaterials reinforced variations . A 229 distinctive single endothermic peak was observed with t he onset temperature in all materials, defined 230 as the starting temperature for endothermic reaction , between 218.09°C and 219.749°C, and the 231 melting temperature in the range of 219.99°C to 222.68°C. 232 233 234 Figure 8. DSC profile s for: a) C-Solder® (no nanomaterial addition) , b) 0.01wt.% carbon black 235 reinforced solder, c) 0.05wt.% carbon black reinforced solder, d) 0.01wt.% graphene nanoparticle 236 reinforced solder, e ) 0.05wt.% graphene nanoparticle reinforced solder, f) 0.01wt.% SWCNT 237 reinforced solder, g ) 0.05wt.% SWCNT reinforced solder 238 (a) C-Solder ® (b) 0.01 wt.% CB (c) 0.05 wt.% CB (d) 0.01 wt.% graphene (e) 0.05 wt.% graphene (f) 0.01 wt.% SWCNT (g) 0.05 wt.% SWCNT 11 of 28 239 240 Figure 9. Melting temperatures of solder with and without nanomaterials 241 As can be seen from figure 9, the change in melting temperature between the samples was not 242 significant but a slight reduction in the melting temperature was observed for the G_0.01 (219.99°C) 243 whereas an increase for the SWCNT_0.05 (222.68°C) [25]. 244 245 In comparison with conventional lead -free solders such as the Sn-3.5Ag, the melting 246 temperatures were slightly different [34]. This behaviour is consistent with the studies conducted on 247 the addition of Ni -CNTs to Sn -Ag-Cu solder alloy [22, 23] and the observations on the add ition of 248 carbon black to Sn-3.5Ag [34] . Since the temperatures required to melt these reinforced solders are 249 very low relative to melting temperatures of joining substrates (aluminium), it can be emphasised 250 that the thermal stresses induced i n the substrates, during the joining process, were minimal. 251 3.1.3 Vickers Hardness 252 The hardness improvement after addition of nanomaterials in to metal matrix has been reported 253 to arise from either of the four main mechanisms [37] : 254 • load transfer strengthening 255 • Dislocation bowing or dislocation interference ( a.k.a. Orowan strengthening) 256 • strengthening due to thermal, elastic moduli, or geometric mismatch between nanoﬁller and 257 metal matrix 258 • grain improvement strengthening 259 Slight increase was observed in the transition from the as -received C-Solder® to 0.01 wt% CB, 260 and a significant increase in 0.05wt% CB enhanced C-Solder® (Figure 10) by nearly 0.85 HV . The 261 increase was also seen in the case of SWCNT addition at 0.01wt% and 0.05wt% however lower at 262 0.05% wt.% SWCNT , attributed to the extensive agglomeration shown in Figure 6 (g), compared to 263 12 of 28 that of the 0.01wt% sample observed to attain a more uniform SWCNT dispersion in Figure 6(f) . The 264 hardness of G_0.01 was found lower than that of the C-solder® by maximum 0.2 H V, and G_0.05 265 approximately the same as the pristine solder. It is not clear whether the slight downfall in the G_0.01 266 material is due to the morphological changes in graphene nano- platelet (as opposed to spherical 267 cluster formed in the CB and SWCNT cases) or is attributed to a weak bonding between the 268 nanomaterials and the solder matrix . Figures 6( b,d) show similar dispersion quality for G_0.01 and 269 CB_0.01, while CB_0.01 presents a slightly more uniform distribution. On the other hand, Figure 6(f) 270 presents SWCNT_0.01 in better dispersion and distribution tha n the other two. This suggests that the 271 distribution quality may have played a role (though not quantified) . A definite statement based upon 272 morphological changes or distribution effects would be forfuitious, and would require further 273 investigations. Therefore, further investigations were carried out in section 3.1.4 where the 274 nanoindentation data and its scatter are provided and discussed. 275 276 Figure 10. Vickers hardness of solders with different compositions 277 Nevertheless, as a comparative remark, i n comparison to other solders like the Sn-3.5Ag and SAC305 278 (HV≈ 14-15), C -Solder® in Figure 10 showed a higher hardness value [21 ,25], mainly due to the 279 presence of chro mium in the solder matrix which restricts dislocation motion s [39]. This restriction 280 can be overcome by addition of carbon nanomaterials according to our phenomenological study 281 above , i.e. the promotion of dislocations restriction but with carbon nanomaterials mimicking 282 chromium role. 283 3.1.4 Creep behaviour during nano- indentation 284 Typical indentation load vs. depth curve for the C-Solder® samples conducted at room 285 temperature under maximum load of 100 mN is presented in F igure 11. Following figure 11, five 286 loading/unloading nano- indentation cycles were performed for all samples: 287 288 13 of 28 289 290 Figure 11. Typical load vs. indentation depth curve for the C -Solder® 291 292 ABCD points on the curve represent the first loading/unloading cycle. AB represents the first 293 loading process, BC the constant load holding process and CD o n the curve represents the unloading 294 process. FG show the change in the indentation depth, held for 120s when the maximum load of 295 100mN is reached and held constant . Creep is operative during this holding time , and was extracted 296 from further analysis (results shown in Figures 12 and 13) . The solder samples exhibited significant 297 permanent (inelastic) deformation and slight elastic recovery, as apparent from the unloading curve 298 (GH) . No discontinuities were obser ved on the curves which indicate d that no fracture occurred 299 during the indentation tests [40]. Figure 12 compares the average indentation depth for all samples 300 at the maximum load (100mN) . As seen, all reinforced solders exhibit increased indentation depth 301 under creep conditions, with 0.05 wt.% nanomaterials reinforced solders ( CB_0.05, G_0.05 and 302 SWCNT_0.05) exhibiting the highest creep indentation compared to their 0.01 wt.% companions , with 303 G_0.01 showing the largest scatter . Therefore, creep deformation under constant load was high er in 304 solders with higher contents, indicative of promoting deformation mechanisms enabled by higher 305 nanomaterials contents which have increased dissipation of strain energy via provision of relatively 306 higher contact areas between the nanomaterials and the solder matrix . Such performance was seen 307 to improve pristine solder’s brittleness towards ductility under constant load creep conditions . 308 Similarly in Figure 10 , the hardness was higher or equal in high content nanomaterial s reinforced 309 solders than low content ones (with the exception of SWCNT_0.05 due to relatively higher 310 agglomeration and poor distribution) . This , together with the creep indentation data, suggests a high 311 strengt h material with high creep deformation at high carbon nanomaterial contents . As seen, G_0.01 312 (the case with the lowest hardness) shows the highest scatter and only a slight creep indentation in 313 020406080100120 0 500 1000 1500 2000 2500 3000 3500 4000 4500Load (mN) Indentation depth (nm) C A B D G E F H 14 of 28 Figure 12. Further analysis requires full investigation throughout the creep regime alongside 314 microscopic images, conducted and described in Figure 13: 315 316 Figure 12. Average creep nano -indentation depth of solder s with and without nanomaterials 317 Figure 13 presents the creep indentation depth vs. holding time, averaged between six 318 indentation s per sample category (indexed by _1 to _6 in the figure ) under constantly held loads . The 319 shaded area between the two dashed lines in all sub -figures shows the range of data for the non- 320 reinforced solder (i.e., C- Solder®). Such area is repeated in all sub -figures in the interest of a 321 comparative analysis. Such data are accompanied by the microscopic images for each material, taken 322 from Figure 6. The high disparity in creep performance is due to the non -uniform distribution and 323 dispersion of nanomaterials in the reinforced solders obse rved in the microscopic images. It is noted 324 that the data on the left column sub -figures are not directly comparable to the images since the 325 indentation was performed on different location, however for a clearer size comparison of the 326 positions at which the six -indentation may have occurred compared to the size of the microscopic 327 images presented here, three grid area have been identified on locations with localised high, medium 328 and low nanomaterials dispersion, identified by H, M and L respectively . Each grid represents the 329 positions in six subsequent elements over which the indentation may have occurred (each element 330 size was spaced by 35 micron) . The difference in the creep data in relation to their high 331 scatter/disparity can be investigated from the microscopic images in a using the grid areas and 332 arbitrary selection of one row or column of six -subsequent elements. To simply put: if the six - 333 indentation paths were conducted over the L region s, the scatter would have been lower than the 334 case tha t the path is conducted over the H and M region. The reason is that the distribution of 335 nanomaterials in the L regions over six subsequent elements (representative of the six indentation 336 spacings) is relatively more uniform than that in the H and M region. This assumes that the material 337 immediately underneath the visible surface follows the same pattern on the surface distribution. The 338 15 of 28 analysis is then suggested to follow the concepts of dispersion- distribution schematic ally illustrat ed 339 in Figure 14. According to the figure, 0.01wt.% CB has poor distribution and dispersion of the CBs 340 while 0.05 wt.% CB showed good distribution while having poor dispersion. Both graphene 341 reinforced solders (0.01 wt.% and 0.05 wt.%) are poor in distribution and dispersion, also the case 342 with the 0.05 wt.% SWCNT. The only reinforced solder material with almost both good distribution 343 and dispersion qualit y is 0.01 wt.% SWCNT which is the only material with the lowest scatter 344 observed in Figure 13( e). This suggests the direct link of the scattered data to the distribution - 345 dispersion quality where the nanoindentation data with lowest disparity appears from a 346 nanocomposite with reasonably good dispersion and distribution. 347 348 349 200 𝜇𝜇𝜇𝜇 (a) 0.01 wt.% CB H L M 200 𝜇𝜇𝜇𝜇 (b) 0.05 wt.% CB 16 of 28 350 351 352 200 𝜇𝜇𝜇𝜇 (c) 0.01 wt.% graphene 200 𝜇𝜇𝜇𝜇 (d) 0.05 wt.% graphene 200 𝜇𝜇𝜇𝜇 (e) 0.01 wt.% SWCNT 17 of 28 353 Figure 13. Indentation depth vs . time at constant loaded creep for a) 0.01 wt.% CB, b) 354 355 (f) 0.05 wt.% SWCNT 200 𝜇𝜇𝜇𝜇 18 of 28 356 Figure 14: Schematic illustration of dispersion and distribution quality 357 358 Moreover, t he observation s for SWCNT_0.05 along with those in Figure 10 (hardness ) and Figure 359 12 (creep) , and compared to those for SWCNT_0.01, indicate that an addition of 0.05wt% of SWCNT 360 has a deteriorating effect on the strength but an enhancement in creep performance owing to 361 promoting deformation mechanisms (alternatively increased creep rate and strain) . Assuming 362 constant solder matrix and process parameters, one may stipulate that the major reason for such 363 performance at high nanomaterials content is due to the morphological changes which upon 364 numerous agglomeration points can induce locali sed stress co ncentration [41]. 365 From the slope of the indentation vs. time curves in Figure 13, t he creep stress expo nents for the 366 solder materials were calculated using Eq. 6. The results are presented in Figure 15. 367 Poor dispersion Poor distribution Poor dispersion Good distribution Good dispersion Good distribution Good dispersion Poor distribution Typical carbon nanomaterial reinforced solder in high black & white contrast 100 𝜇𝜇𝜇𝜇 19 of 28 368 Figure 15. Creep s tress expo nents of solder samples 369 It is common to use the value of the powe r-law creep stress exponent as an indicator of the rate and 370 mechanism driving the creep deformation in metals . As seen in Figure 15, the exponent for all solder 371 materials is near ly three, indicati ng that the main governing deformation mechanism is creep 372 dislocation glide [42] which is common for low- temperature creep deformation mechanism . That 373 partially ratify o ur former discussion that the addition of carbon nanomaterials mimics the role of 374 chromium in solder material in terms of dislocations enhancement. For G_0.01 and SWCNT_0.05, a 375 high disparity in the exponent data is seen, obtained from fitting the power -law equation (Eq. 1) to 376 the actual creep data which exhibited the highest scatter, i.e., Figure 13 (c) and 13(f)). 377 3.2. Single -lap soldered joint evaluation data 378 3.2.1 Joint Quality Analysis 379 All joints soldered by different material categ ories were cut -sectioned and investigated via optical 380 microscop y, and none show ed any void formation owing to the controlled pressurisation and 381 assembly procedure described in section 2.3.1. The voids were inherently present in the materials 382 prior to pressurisation (Figure 6) . Typical optimal images of an aluminium -aluminium soldered joint 383 assembled using G_ 0.05, CB_0.05 and pristine C-Solder® are shown in Figure 16. The images for 0.01 384 wt.% materials are not shown as they looked identical to their 0.05 wt.% pairs as observed across a 385 large range of the interface. A fully adhered interface by soldering has been observed for the 386 specimens examined, at 0.01 and 0.05 wt.% except that a slight irregularity is seen in the soldered 387 interface in the CB_0.05 specimen . Though such observation can be made for all specimens 388 depending on the magnified area s (i.e., further magnification than that presented in the figure can 389 lead to the observation of mo re irregularities in the specimens), the authors could identify relatively 390 more numbe rs in a 1 mm-area in the CB_0.05 interface, hence providing a comparative analysis 391 together with the mechanical shear performance. 392 20 of 28 393 394 395 Figure 16. Typical o ptical microscop ic image s for aluminium joint interface with a) C-Solder® , b) 0.05 396 wt.% CB reinforced solder, and c) 0.05 wt.% graphene reinforced solder 397 3.2.2 Single -lap joint testing 398 The shear load vs. displacement data for all soldered joints are shown in Figure 17 . Two single - 399 lap joint specimens per nanomaterial type have been presented in the interests of clarity, representing 400 the weakest and highest ductile performance (lowest and highest failur e strain exhibited respectively ) 401 100 𝜇𝜇m Aluminium C-Solder® Soldered interface Presence of voids Direction of pressur isation Direction of shear loads post pressurisation 100 𝜇𝜇m Aluminium CB_0.05 Soldered interface 100 𝜇𝜇m Aluminium G_0.05 Soldered interface Presence of voids 21 of 28 of the joints in each category . The results of the joints made of the pristine solder are shown in solid 402 lines. The data with the 0.01 wt.% nanomaterials content (Figure 17a) show a load -displacement 403 performance falling between the weakest and strongest pristine soldered joints. However, t he scatter 404 in load -displacement data for the reinforced soldered joints is lower than that of the pristine joint. 405 The SWCNT_0.01 and CB_0.01 show ed weaker performanc e than the G_0.01 in terms of the highest 406 load reached and the ultimate displacement at failure . Recalling from Figures 10 and 12, G_0.01 407 possessed the lowest hardness and creep indentation depth, while it performs better than the other 408 0.01 wt.% carbon types in t he joints herein . This is due to either introduction of a better bonding 409 quali ty with aluminium or pressurisation during development of the soldered joints, or both. No 410 research has particularly studied the bonding quality of graphene nanomaterial w ith aluminium or 411 aluminium oxide, however a number of articles have investigated the effect of graphene a ddition to 412 aluminium in metal matrix composites (MMC) [43,44] . 413 On strength, stiffness and hardness in numerous graphene types, processes and contents , and 414 reported upto nearly 200%, 80% and 30% in tensile strength, stiffne ss and toughness, comparable to 415 those of the CNT addition in aluminium based MMC . Therefore, a direct conclusion cannot be 416 reliably drawn . Further investigation is required with focus on the bonding quality of such joints 417 examined here. 418 The aluminium joints soldered by the 0.05 wt.% carbon nanomaterials reinforced solder 419 materials (Figure 17b) also exhibit a superior performance in case of graphene in the ultimate failure 420 displacement and the peak load reached. The other 0.05 wt.% reinfor ced types also perform ed better 421 from their 0.01 wt.% cases. 422 423 (a) 22 of 28 424 Figure 17: Shear load versus displacement data in single -lap joints soldered by pristine and 425 nanomaterials reinforced solders; a) comparison of pristine solder joint performance with 0.01 wt.% 426 carbon nanomaterials content, b) comparison of pristine solder joint performance with 0.05 wt.% 427 carbon nanomaterials content 428 429 The failure load in almost all joints soldered by 0.01 wt.% reinforced solders was lower than the 430 pristine solder . This can be attributed to the fact that in some areas in the s older matrix, the carbon 431 nanomaterials com ing into contact with each other , form clusters due to strong Van der Waals forces 432 between them [27 , 25]. This may hinder the bonding between the nanomater ials and the solder matrix 433 which in turn results in cluster induced porosity that act as potential stress concentration site and 434 promote joint failure. However, when the weight percentage of nanomaterials increases from 0.01 435 wt.% to 0.05 wt.% , the failure load generally increases . Despite the ongoing agglomeration there 436 remains sufficient nanomaterials to develop bonding with the matrix . This is speculative based on 437 the phenomenological data presented in Figure 17(b) but i s in contrast to the existing literature which 438 reported that microscopic level porosity starts to affect the strength of the solder with the addition of 439 more than 0.05wt .% of ca rbon nanomaterials [35]. Considering the discontinuity in the failure load 440 pattern pinpointed in SWCNT_0.01 and the fact that all SWCNT_0.01 joints utilised solder from the 441 same ingot, solder quality issues might be addressed as a result of the contamination of the solder , 442 i.e., excessive so lder oxidation. 443 Moreover, t he failure loads and shear strengths of soldered joints are comparable to adhesively 444 bonded aluminium joints. According to a study [45], the maximum lap shear load withstood by an 445 ARALDITE Rapid epoxy -based adhesive mixed with aluminium powder/aluminium joint was 446 approximately 3.7 kN and the corresponding shear strength was nearly 9 MPa whic h is significantly 447 lower than the failure loads and shear strengths of all the soldered joints tested in this work . Such 448 considerable improvement by the carbon nanomaterials suggests an alternative joining process over 449 bonding process which exploits enviro nmentally sensitive polymer adhesive. Another research 450 conducted by Ufferman et al. (2018) [46] assessed single -lap adhesively bonded coated aluminium 451 (b) 23 of 28 5052 laps shear tests. The maximum force attained was 10. 06 kN. However, the shear area was large 452 (≈ 40x40mm) and the calculated shear strength was approx. 6.3 MPa which is significantly lower than 453 the shear strength obtained in the soldered aluminium joints examined here. 454 3.2.3 Failure Mode Investigation 455 456 Figure 18. Failure of Al6082T6 joint during lap shear testing 457 458 During the lap shear tests, the failure initiated at the end of the joint and propagated until full 459 failure occurred (i.e. detachment ) as shown in F igure 18. This confers that the joint end, where shear 460 stress is expected to be at a maximum level, was the failure initiation site. All soldered joints failed at 461 the bulk of the solder progressively, displaying a damage toleran t performance in which no 462 instantaneous damage propagation was observed ( referred to as adhesion failure, an undesirable 463 damage mechanism as opposed to cohesion failure in adhesively bonded joints) . The rough surface 464 of a typical bulk of the solder material existing on both sides of the aluminium laps are shown in 465 Figure 19 post ultimate failure, a clear indi cation of no -interface failure case. 466 467 468 Figure 19. Optical microscopy of the shear area of Al6082T6 joints 469 4. Conclusions 470 This article investigated the merits of using carbon nanomaterials reinforcing agent to mix with 471 commercial tin-based solder material, C-S older® . A comprehensive microscopic examination 472 followed by the mechanical testing procedure involving microstructure, hardness, melting 473 temperature, creep nanoindentation behaviour and single -lap shear joining testing with 0.01 wt .% 474 24 of 28 and 0.05 wt .% carbon black, graphene and single walled carbon nanotubes reinforced solder 475 materials were investigated. 476 The melting temperature of all the carbonaceous tin- based solder achieving unprecedented joint 477 strength was of the order of approximately 200°C suggesting that no additional equipment is 478 required to use these solder materials for joining process es. This also means that the thermal stresses 479 on the aluminium substrate remained minimal compared to other joining techniques like welding 480 and brazing. 481 The results of the creep nano- indentation testing suggest ed an optimal creep behaviour occurs 482 for the nanomaterial addition of both 0.01 wt.% and 0.05 wt% with the best performance for the 0.05 483 wt.% addition . As for the hardness, it was observed that the increase in w eight percentage of the 484 nanomaterials in the solder matrix increases the hardness. 485 The single -lap join ing of Al6082T6 substrates using the pristine and carbon nanomaterials 486 reinforced solders showed comparable performance in terms of the average shear strength (i.e., 487 failure load divided by the overlap area) and failure displacement with in-use structural adhesive 488 bonding with a significant improvement in the exhibition of progressive failure in the bulk of the 489 solder rather than the interface, an undes ired adhesion failure in the bonded joints . 490 In conclusion, th e newly developed solder joint material and process provides a n opportunity to 491 develop low cost high strength structural -grade joining of aluminium substrates with minimal 492 surface preparation , a potential alternative to the existing adhesive based structural joining. 493 494 Acknow ledgments : 495 The authors would like to acknowledge the support s from Cametics Ltd. , and the UK EPSRC 496 funded research (Ref. EP/R016828/1 ), the EPSRC DTP grant for 'Microstructural Study of 497 Multifunctional Nanocomposites', Ref. EP/R513027/1 ) and H2020 European Regional Development 498 Fund - IMAGE (Ref. 31R17P01422 – CU027) . The authors would also like to greatly acknowledge the 499 UKRI support in establishing the CDT in Ultra Precision Engineering ( EP/L016567/1) and the EPSRC 500 DSM NetworkPlus ( EP/S036180/1) beside the VC Fellowship from Cr anfield University to sponsor 501 the PhD research of Sara Hawi . 502 Data Statement: 503 The underpinning data can be accessed at Cranfield University’s repository via 504 https://doi.org/10.17862/cranfield.rd.14815404. 505