Aluminium Cracking

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According to the X-ray inspection and confirmed with the optical and SEM observations, sound tubular transitions of AA7075 to CFRP were produced using a stiff support mandrel to constrain the CFRP wall deformation. A minimum discharge energy of 2.5 kJ is required to ensure a tight interface however, the best results were found for 3 kJ of discharge energy. Increasing the coil number of turns, also increases the system inductance, slightly reducing current intensity peak of about 15 % when increasing the number of turns in the coil from 4 to 6, for these diameters, thus reducing the impact velocity.

Joining is produced by mechanical locking between the aluminium, which penetrates the epoxy resin matrix, and the fibre. For this materials combination the impact angle is not a key parameter since no weld is produced and thus the jetting, and its cleaning effect is not important. Also, the jetting does not need to be completely cleared from the interface.

It is also possible to produce joints on both ends of the CFRP tubes, without compromising the tube integrity, as shown in Figure 6.14.

Figure 6.14 – Example of a Al-CFRP sample produced by MPW.

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Figure 6.15 – Example of a cracked developed after welding

Three most probable causes for the cracks are galvanic corrosion resulting in stress corrosion cracking (SCC), stress concentration from the MPW process caused by the high strain rate deformation or lack of resistance section on the aluminium tube to withstand the contact pressure between both materials. The reference and the respective parameter sets from the samples that presented cracks are shown in Table 6.3.

Table 6.3 – Parameter sets of the cracked samples

Sample

Stand-off distance

[mm]

Coil position

Discharge Energy

[kJ]

Nr of coil turns

Impact Angle

[º]

Support

G7 1 Centre 2.5 6 17.4 HDPE

G10 1 Centre 2.5 8 13.1 HDPE

G14 1 Centre 3 6 17.4 Epoxy

G15 1 Centre 2.5 6 17.4 Epoxy

G16 1 Centre 2 8 13.1 Epoxy

G19 0.5 Centre 2.5 8 6.6 Epoxy

G27 0.5 Centre 2.5 8 6.6 Aluminium

To verify the failure cause, the initiation and propagation of the cracks need to be understood. From Figure 6.16 a) it is possible to observe that the cracks were initiated on the inner surface with no oxide formation or other compounds from corrosion. Also, the cracks propagated along the grains boundaries in an intergranular process typical of low

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acting forces (Figure 6.16 b)). The expansion force is created by the deformation imposed to the CFRP tubes which pushes the aluminium wall outwards.

Figure 6.16 – Micrograph details of sample G10. Welding parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil with HDPE mandrel: a) Location of cracks initiation points; b) Crack propagation detail

SEM observations were performed to identify the type of fracture and the possible reasons for it to occur. As shown in Figure 6.17, the fracture surface present signs of ductile failure. The dimples visible, especially in detail b), are evidence of the significant plastic deformation imposed to the material prior to failure. Such type of ductile fracture is commonly observed for aluminium alloys.

Figure 6.17 – Fracture surface of a crack on the aluminium G16 parameters: 2 kJ of discharge energy;

1 mm of stand-off distance; 8-turn coil with epoxy resin mandrel.

Comparing the images taken with secondary electrons (SE), and backscattered electrons (BSE) is possible to distinguish between compounds with different atomic weights (Figure 6.18). No corrosion products were observed on these inspections.

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Figure 6.18 – Interface Al fracture analysis G16 parameters: DE=2 kJ; SD=1 mm; 8-turn coil with epoxy resin mandrel: a) Secondary Electrons; b) Backscattered Electrons.

Since there was no evidence of galvanic corrosion, the failure could be due to stress concentration owing to the high strain rate deformation imposed by the MPW process. In MPW the deformation is highly localized and can sometimes lead to grain refinement [98]. Nevertheless, this was not observed on these specimens, since the differences in grain size are from the aluminium bars base material production method, as shown in Figure 6.19, with the grain size in the inner surface being nearly 10 times smaller than in the outer surface, reducing from about 30 µm to around 4 µm, on average.

Detail a) from Figure 6.19 is the base material as acquired with the pre-hole to machine it to tubes. The hardness profiles presented in Figure 6.19 are also marked on this figure.

Figure 6.19 – Base material: a) Macrograph from BS with the hardness profiles marked; b) inner surface;

c) outer surface.

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No visible difference was confirmed by the hardness profiles measured which were identical both for cracked and non-cracked tubes and for the tubes base material.

The hardness profiles were measured as shown in Figure 6.20 for the tubes that presented cracks to evaluate the difference on the hardness on both sides of the crack, and in opposing sides for non-cracked tubes, shown in Figure 6.21. A similar approach was used for the aluminium base material with two lines starting on opposing sides towards the centre. The hardness plots are presented according to the tube wall thickness, t in [mm], starting from the outer diameter inwards, where X=0 is the inner wall.

Figure 6.20 – Sample G7 cross section view and hardness profile.

Welding parameters: DE=2.5 kJ; SD=1 mm and 6-turn coil with HDPE mandrel.

The lines in the macrograph correspond the hardness profiles.

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Figure 6.21 – Sample G11 cross section view and hardness profile.

Welding parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil with HDPE mandrel.

The lines in the macrograph correspond the hardness profiles.

Comparing the hardness profiles, from samples that cracked with samples that did not crack, no visible difference was found, as seen comparing Figure 6.20 with Figure 6.21. If there was stress concentration due to the joining process the hardness profile should change after cracking, since residual stresses would be relieved when the sample ruptures. The profiles are also coherent with the base material hardness (Figure 6.22).

Concluding, the difference in the grain size was not due to the MPW process but rather due to the tube extension with grain refinement.

Figure 6.22 – Aluminium base material hardness profile

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Since the other two causes are not possible, the probable cause for the aluminium failure is the excessive tangential stress, imposed by the elastic deformation stored on the CFRP tube which acts as a spring expanding the aluminium tube outwards, for the resistant section used. To confirm this, the residual deformation of the CFRP tubes after the MPW process was measured on the X-ray images. With these measurements it was found that the CFRP tubes were compressed between 0.25 and 0.5 mm along the radius.

Therefore, the potential mechanical energy was stored on the CFRP tubes which act as an isobaric expansion pressure (σr) on the inner surface of the aluminium tube.

Assuming that the aluminium tube behaves as a pressure vessel disregarding the axial pressure, for these calculations, Hooke’s law can be applied considering the CFRP Elastic Modulus and that all the deformation is within the elastic regime. The axial forces can be disregarded since the aluminium tube is not constrained along the axis. For homogenous pressure distribution, plane strain state can be considered, thus, the nominal extension computed was in the radial and angular directions ranging between 25.0×10^-3 and 50.0×10^-3. Thus, the calculated applied pressure ranges between 41.3 and 82.5 MPa approximately, using equation 6.1 and considering an aluminium ultimate tensile strength of 575 MPa.

With this data it is possible to dimension the aluminium tube wall thickness to avoid plasticization, using the aluminium yield strength in equation 6.1, disregarding the energy used to initiate and propagate the crack and the strain hardening due to the MPW process.

t pr

yy =

 ^(6.1)

In equation 6.1 p [Pa] is the applied pressure between the aluminium and the CFRP tubes, r [m] the inner radius of the aluminium tube and t [m] the wall thickness.

Solving this equation for the maximum and minimum pressures calculated, it was found that the minimum aluminium thickness should range between 0.72 and 1.43 mm to resist to the internal pressure applied by the CFRP tubes outwards. This accounts for the samples that did not crack, since the deformation of the CFRP tube was not always sufficient to store enough energy to crack the aluminium tube. Depending on the used

151 energy, the aluminium wall thickness need to be increased to sustain the contact force between both materials.

Lap shear tests were performed to establish a minimum resistance of the joint specimens. The specimens tested were produced using a 10-turn coil at tube centre, 3 kJ of discharge energy and 1 mm of stand-off distance. The specimens only had one aluminium fitting to test accurately the joint strength. Non-sliding criteria was imposed, that is, the resistant force considered is the maximum before the sample parts start to slide.

The joint behaviour is presented in Figure 6.23 with the failing point marked with an arrow.

The mandrel was machined tight to the CFRP tube inner diameter to provide support both to produce the joint and to fix the carbon fibre part to the tensile test machine.

Regarding the aluminium part a piece from a solid round bar was used to have a solid basis to fix to the testing machine while the other side was machined to a tube with 25 mm of outer diameter and 1.5 mm of wall thickness to produce the joints

Figure 6.23 – Force-Displacement curves for two different samples.

As observed, the two specimens tested presented similar behaviour in the elastic regime achieving a peak load of 2.9 kN, before starting sliding. The minimum force to slide is similar in both specimens which shows good repeatability for the contact established with the tested conditions. However, after sliding started the behaviour can be quite different. These differences on the resistance profile are due to different precision on the mandrel diameter which influences the contact along the tube axis after starting to

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slid, which is in good agreement with the need to use stiff mandrels to ensure homogenous contact on the joint. Specimen 1 had a tighter mandrel resulting in the ripple typical of sliding and the resistance increment is also due to the jetting which is deposited on the adjacent region of the joint, acting as a barrier to sliding owed to the tight fit between both parts.

This result is promising since the joint resistance is based on friction which directly correlates with the tube dimensions as described below. The low friction coefficient exhibited by the CFRP tubes can be increased witha coating by increasing the roughness of the surface resulting in a higher friction coefficient. If a metal coating is used, welding between it and the flyer can be achieved which improves the joint strength.

For the calculations made, the contact was considered as presented in Figure 6.24 and following the available literature [99]. These calculations were made assuming a homogenous pressure distribution on the contact area and no aluminium wall thickness reduction.

Figure 6.24 – Scheme of the joint contact: Dc, is the contact diameter; Ac is the contact area; Lc is the effective contact length; Aal is the Aluminium resistant section; tal is the aluminium wall thickness

The joint axial resistance is given by equation (6.2) which is only dependent on the friction coefficient, Cf [-] and the contact force, Fc [N] between the sliding surfaces.

c f

f

C F

F = 

Cf simply depends on the materials in contact and the surface roughness which do no change with the different joint dimensions. Thus, to increase the joint axial resistance the contact force needs to be higher for the resistance to improve without the use of an interlayer.

153 Since the aluminium tubes ruptured with the internal pressure it is possible to compute this using the aluminium ultimate strength (σr). For this, it was considered that the pressure distribution was homogenous and that the aluminium tube behaves like a pressure vessel disregarding the axial pressure and that all the deformation measured for the CFRP tubes is stored as elastic energy (equation 6.3).

c al r

c D

P = 2t (6.3)

Where t, in [mm], is the aluminium tube wall thickness and Dc, in [mm], is the contact diameter, which in this case is the CFRP tube outer diameter. From Equation 6.3, for a 515 MPa of yield tensile strength, to dimension the joint within the elastic regime, [100], the maximum internal pressure which can be withstand by the aluminium tube is of 51.5 MPa.

With this, and computing the contact area (Ac) between the aluminium tube and the CFRP tube, the contact force can be computed by equation 6.4.

c c c

c c

c

P A P D L

F =  =    

A Cf value of 0.18 was calculated with equation 6.2 for the combination studied, which is smaller than the values found in literature for similar combination which was of 0.23 [101]. In this study, the aluminium alloy and the carbon fibre used are different, as well as, the epoxy resin used which along with the smooth finishing on the contact surfaces can account for the differences found.

Combining the three equations, the joint resistance (Ff) depends on the aluminium wall thickness, t, and the effective contact length, Lc, which is the sum of all contacting asperities between the aluminium and the CFRP tube, as shown below (equation 6.5).

c al

r f c c c

al r f

f D L C t L

D C t

F =  2   =  2 

(6.5)

Therefore, by increasing the aluminium tube wall thickness and the contact length it is possible to increase the joint resistance. The contact length is directly correlated with

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the number of turns in the coil thus, it can be assumed that the target resistance can be achieved.

As expected, and as shown in Figure 6.25, the failure occurred at the joint interface since it is a friction based joint. The aluminium adhesive tape visible on one side of the CFRP tube was used to avoid damage to the fibres when clamping the specimen on the testing machine.

Figure 6.25 – Tensile test specimen after testing

Jetting was confirmed with EDS inspections, shown in Figure 6.26, where aluminium traces were found attached to the CFRP tube outer wall on the section adjacent to the impact zone (marked in the previous figure). On detail d), two large aluminium pieces are visible but also scattered particles on the CFRP surface projected from the jetting.

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Figure 6.26 – EDS analysis of the CFRP outer surface near the impact zone. a) section analysed Elements dispersion b) C; c) O; d) Al; e) Si; f) Zn.

With the aluminium fitting sliding against CFRP fibres could have been torn, however, the visible marks are from the jetting which is only composed of aluminium, and shallow surface scratch marks from the relative sliding between both parts.

There is the possibility to improve the joint resistance with simple geometry changes, such as a new aluminium fitting design or changing the surface roughness to increase the friction force. The aluminium thickness and the contact length between both materials, along the joint axis, are the geometrical features to increase the joint resistance, as shown by the equations, while the friction coefficient can be increased by different finishing on the contact surfaces to increase roughness.

A coating can also be used to either avoid damage due to impact during the MPW process by using a ductile coating to absorb the impact, or to increase the joint axial resistance since the friction coefficient would change and, for metallic coatings, welding may be possible.

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