Joining in automobile construction part 1

Dry-film lubricants as an impact of automotive adhesive joints

The adhesive properties of our products are influenced by many different factors. In addition to influencing characteristics such as texture and surface chemistry, subsequent process steps are also decisive, especially those carried out at our customers. Knowledge and understanding of the relevance of these influencing factors enables the optimization of processes, selection of adhesives or the interpretation of commissioned adhesive tests.Dry lubricants are indispensable for a high-quality forming behaviour of aluminium sheets for automotive applications. However, those forming aids are often not removed before the joining step. Since structural adhesive bonding is widely applied in automotive industry and its importance is constantly growing [1], the influence of dry lubricants on the bond strength was investigated.The durability of adhesive bonds is of crucial importance and is strongly influenced by environmental factors. Various test methods are used in industry to simulate degradation at the interface between the adhesive and the metal surface under different climatic conditions [2]. Contamination or lubricant residues can also trigger chemical reactions under these climatic conditions, which damage the resistance of the bonded joints [3].The current study indicates the influence of dry lubricants on bond strength and corrosion behaviour of 6xxx aluminium alloy adhesive joints used for automotive industry. Based on the reproduceable results of an accelerated corrosion test, possible corrosion processes were examined and explained [4].

Abbildung_1_EN — **Figure 1:** Overview of the mechanical values during immersion testing. Corrosion resistance of references without (blue) and samples with dry-film lubricant (brown) are compared directly against each other

Results and discussion

In this investigation, the adhesive samples were aged directly in a NaCl solution for an accelerated corrosion test (immersion test). For the comparison, samples were prepared without and with dry lubricant (1 g/m²). Samples were taken and evaluated daily over a period of 7 days.The results in Figure 1 directly show the influence of the dry lubricant on the corrosion behaviour of the bonded aluminum samples. After a slight initial drop within the first 48 hours, samples without lubricant show a relatively constant residual strength over the entire test period. By contrast, samples prepared with dry lubricant show a significant loss in residual strength.The fracture pattern analysis of samples without DFL, shown in Figure 3 a - h, indicates no corrosive attack of the metal-adhesive-interface during immersion testing. The fracture patterns constantly show cohesive failure over the whole test period. By comparison, samples which were lubricated with DFL, show a corrosive attack already after the first 24 h of testing. With increasing corrosive infiltration of the bond line, as shown in Figure 3 i - p, the residual strength of the single lap shear specimen decreases. After completion the immersion testing, the fracture pattern of lubricated samples shows corrosive delamination over the whole bond line.

Abbildung_2_ENfuegen-im-automobilbau — **Figure 2:** Fracture pattern comparison between lubricated and non-lubricated samples before and after immersion

In addition, cross sections of the adhesive joints were characterized to gain more information about the lubricant induced corrosion mechanism. Figure 4 shows cross section SEM images of an adhesive joint of a lubricated sample after five days of immersion. The adhesive is delaminated over the whole bond line and already shows degradation in the center (Figure 4c). This behaviour is an effect of crevice corrosion based on its corresponding concentration gradient [5].

Displacement of DFL

Next to the effect on the corrosion behaviour, Figure 2 shows an influence of DFL on the mechanical properties of the adhesive joints already in the initial state. The lubricated samples show a reduced tensile shear strength up to 7 %. These findings indicate that the adhesive cannot absorb the DFL completely. Therefore, a single lap shear test with ascending DFL load quantities from zero to 5 g/m² was performed and is shown in Figure 5.The results confirm that the DFL reduces the tensile shear strength of adhered aluminium samples. The change gets smaller with increasing load quantity, which may be an indicator for lubricant displacement out of the adhesive joint. This assumption could be confirmed with microscopic images shown in Figure 6. The overhead view of an adhesive joint from a lubricated specimen indicates an incomplete absorption of the DFL from the adhesive. The lubricant gets displaced out of the joint and generates a wedge with a much larger engagement surface. This leads to a reinforced diffusion of corrosive ingredients, like Cl- or H₂O, of the immersion bath.

Abbildung_3_DEfuegen-im-automobilbau — **Figure 3:** Cross section of an lubricated adhesive joint after 5 days of immersion showing bond line corrosion

Abbildung_4_EN — **Figure 4:** Single lap shear test results of specimen with ascending lubricant load quantities

Swelling of the adhesive

Epoxy adhesives, such as the one used in this study, tend to absorb water, which leads to an increase in volume. Table 1 shows the swelling, i.e. the volume change, of an adhesive sample according to DIN EN ISO 62:2008 when immersed in deionized water (DI) and 5 wt.% NaCl solution at 70 °C. The precise size of the 40 x 40 mm² samples were measured using a calliper gauge with an accuracy of 0.01 mm. The corresponding volumetric change was calculated using Eq. 1, where V1 and V2 are the volumes before and after immersion.

The immersion in DI-water leads to a higher volumetric change than the immersion in 5 wt.% NaCl-solution based off reverse osmosis behaviour [6]. Furthermore, the literature reports that swelling of the adhesive forms cavities in the vicinity of inorganic fillers inside the polymer network due to different expansion rates of both, polymer, and inorganic ingredients. This creates a lot of free volume into which water or immersion solution can penetrate, leading to further diffusion [7]. Figure 6 confirms the presence of cavities in the vicinity of inorganic fillers inside the polymer network of the adhesive after the immersion in 5 wt.% NaCl-solution for 7 days at 70 °C. These cavities promote the diffusion of the immersion solution, which consequently leads to corrosion at the interface between the aluminium and the adhesive.

Abbildung_5_ENf_uegen-im-automobilbau — **Figure 5:** Overhead microscopic view of an adhesive joint with protruding dry lubricant

Abbildung_6_fuegen-im-automobilbau — **Figure 6:** SEM image of the adhesive after 7 days of immersion. The polymer network shows cavities in the vicinity of anorganic fillers

Change of the polymer matrix

At the aluminium-adhesive-interface the DFL is directly in contact and can interact chemically with the adhesive. Therefore, three different polymer blends with DFL concentrations from zero to 1 wt.% were prepared to simulate the interaction effect of a lubricant on the mechanical properties of the adhesive. The result of the tensile test with all three blends is shown in Figure 8 and indicates a change of the mechanical properties of the adhesive due to the DFL.With increasing lubricant concentration, the tensile modulus as well as the breaking stress reduces. The addition of 1 wt.% DFL into the adhesive results in a 7 % reduction of both, the tensile modulus and the braking stress. The increase of the breaking elongation with increasing DFL concentrations is a result of a lubricant induced plasticisation of the adhesive. This plasticisation was also observed in the dynamic-mechanical analysis (DMA analysis).

Immersion time [Days]	Volume change in DI-water [vol.%]	Volume change in NaCl-solution [vol.%]
7	17	12
14	18	14
21	19	15

Table 1: Overview of the volumetric change after immersion

Abbildung_7_EN — **Figure 7:**Tensile test results of adhesive blends with varying DFL concentrations

The loss factor tan δ is a measure of the samples dissipated energy and the peak maximum of its graph can be used for determination of the glass transition temperature [8]. As shown in Figure 9, the DFL reduces the glass transition temperature of the epoxy polymer by 2.5 degrees. This finding constitutes the double confirmation of a DFL induced change of the mechanical properties of the adhesive. Besides the actual modification of the adhesive’s mechanical properties due to the lubricant, DSC analysis of the adhesive blends show a DFL induced change in the thermal properties of the epoxy polymer. The lubricant reduces the on-set temperature, which leads to an earlier curing reaction. Also, the peak maximum of the heat flow graph is reduced by 5 degrees due to the DFL and the polymer network formation during curing has in total a higher exothermicity. Those findings are shown in Figure 10.

Abbildung_8_EN — **Figure 8:** DMA analysis plot showing a DFL induced reduction of the adhesive´s Tg

Abbildung_9_EN — **Figure 9:** DSC analysis reveals change in the thermal properties after interaction of the adhesive with DFL

Corrosion mechanism

corrosion mechanism seems to be a combination of three factors with different effects. Adhesive degradation during accelerated corrosion testing is an expected process and occurs also on non-lubricated samples. But in this case the DFL promotes the degradation due to the incomplete absorption of the lubricant from the adhesive. The logical consequence is a displacement out of the joint which generates a wedge with a much larger engagement surface (see Figure 11 number 3). Additionally, the swelling of the adhesive, shown in Figure 11 number 2, caused by reinforced diffusion of the immersion solution due to the larger contact area of the DFL wedge, favours further diffusion. Swelling of the adhesive creates cavities in the vicinity of inorganic fillers inside the polymer network. The increased free volume allows the adhesive to absorb more immersion solution which promotes bond line corrosion at the aluminium-adhesive-interface. Residual lubricant stands directly in contact with the adhesive at this interface (see Figure 11 number 1). Interactions at the interface lead to changes of mechanical and thermal properties of the epoxy polymer matrix.

Abbildung_10_EN — **Figure 10:** Overview of responsible corrosion mechanisms. Number 1 represents the change of the polymer matrix, number 2 displays the swelling of the adhesive and number 3 illustrates the displacement of the DFL with its forming

Conclusion

Dry lubricants are indispensable for high-quality forming behaviour of aluminum sheets in the automotive sector. However, they are often not removed in the interest of efficient process control during downstream assembly and joining processes. Remaining quantities of lubricants or impurities on the metal surface are a significant factor influencing the durability of the bonded joints. In comparison with a non-lubricated reference the DFL promotes increased damage to the bonded joint during ageing. The fracture pattern analysis indicates bond line corrosion already after 24 hours of testing. A cross section of a tested joint showed corrosive delamination of the adhesive which lead to the loss of the mechanical properties of the joint. Furthermore, the load quantity of the dry lubricant influences the tensile shear strengths already in the initial state. Microscopic characterisation showed a displacement out of the joint which indicates an incomplete absorption of the DFL from the adhesive. Repression favours the diffusion of corrosive media. This diffusion leads to a swelling of the adhesive and forms cavities in the vicinity of inorganic fillers inside the polymer network. Those voids additionally favour the diffusion of immersion solution which promote bond line corrosion at the aluminium-adhesive-interface. Furthermore, the dry lubricant is directly in contact with the adhesive at this interface. The interaction between both, the DFL and the adhesive, lead to a change in the mechanical and thermal properties. Tensile tests and a DMA analysis revealed a lubricant induced plasticisation with a decrease of the glass transition temperature of the adhesive. The dry lubricant also affected the curing behaviour which was observed by DSC analysis. According to the manufacturer, epoxy adhesives can generally absorb up to 5 g/m² of surface oils. However, the reproducible tests carried out show a significant negative effect of the lubricant on the adhesive properties, at least for this specific selection of adhesive and ageing method. An even distribution of the lubricant and sufficient surface cleaning prior to bonding counteracts this effect and will improve the adhesive strength of the bonded part.

Customer benefits

The continuous development of our processes enables us to offer premium products, including the highest quality and customised optimization solutions. By expanding our adhesives expertise, we can provide our customers with comprehensive advice.

References

[1] European Aluminium Association, EAA Aluminium Automotive Manual - Joining, The Aluminium Automotive Manual, European Aluminium Association: Ljubljana, Slovenia, 2015, 1-5[2] D. Mercier, J.-C. Rouchaud, M.-G. Barthés-Labrousse, “Interaction of amines with native aluminium oxide layers in non-aqueous environment: Application to the understanding of the formation of epoxy-amine/metal Interphases”, Appl. Surf. Sci. 254 (2008) 6495-6503[3] J. Hirsch, “Recent development in aluminium for automotive applications”, Trans. Nonferr. Met. Soc. China 2014[4] R. Gruber, T. D. Singewald, T. M. Bruckner, L. Hader-Kregl, M. Hafner, D. Stifter, „Influence of Dry-Film Lubricants on Bond Strength and Corrosion Behaviour of 6xxxAluminium Alloy Adhesive Joints for the Automotive Industry”, Lubricants 11 (2023) 43[5] N. LeBozec, D. Thierry, “Influence of test parameters in an automotive cyclic test on the corrosion and mechanical performance of joined materials”, Materials and Corrosion 66 (2015) 1051-1059[6] R.C.L. Tai, Z. Szklarska-Smialowska, “Effect of fillers on the degradation of automotive epoxy adhesives in aqueous solutions: Part I Absorption of water by different fillers-incorporated automotive epoxy adhesives”, J. Mater. Sci. 28 (1993) 6199-6204,[7] T. M. Bruckner, T. D. Singewald, R. Gruber, L. Hader-Kregl, M. Klotz, M. Müller, G. Luckeneder, M. Rosner, C. Kern, M. Hafner, C. Paulik, “Water absorption and leaching of a 1K structural model epoxy adhesive for the automotive industry”, Polymer Testing 117 (2023) 107870[8] A. Chateauminois, B. Chabert, J.P. Soulier, L. Vincent, “Dynamic mechanical analysis of epoxy composites plasticized by water: Artifact and reality”, Polym. Compos. 16 (1995) 288-296

Back