Aluminum adherend

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

Fig. 9. Wedge test results of aluminum adherends with the following surface preparations FPL, PAA, and FPL followed by an NTMP treatment. Adapted from Ref. [42].

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

Fig. 10. Low-frequency electrochemical impedance of an epoxy-coated FPL aluminum adherend as a function of immersion time in 50°C water. Adapted from Ref. [46].

I08J (in a different form than for aluminum adherends) but Pasa Jell 107, Turco 5578 and alkaline peroxide [80] etches are also popular. 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

Abstract—The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive. 

Measured on aluminum adherends tested at 23°C after aging 200 h at 286°C. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

All specimens were constructed in a honded-beam configuration. Aluminum adherends were used in both the static and dynamic DCB configurations, and the composite adherends were used in the static and dynamic DCB and dynamic ELS and SLB configurations. Aluminum (6061) adherends were P2 etched to provide an adequate bonding surface and a bondline of either 0.8 mm or 0.5 mm was used. Because they were shipped in panel form, the composite adherends were bonded in 300 x 300 mm sheets. Each surface was abraded and then cleaned with acetone prior to bonding. The bondline was agmn set using either 0.8 mm or 0.5 mm wire at the center and 20 mm from both ends of the composite panels. Initially, this wire was only placed at the ends of the panels, but this resulted in inconsistent bondlines due to deformation at the center of the composite panels. Once the bondline thickness was set, a thermocouple was added to the center of the specimen to monitor the bondline temperature profile. 

The arrest values for the static DCB tests with aluminum adherends are shown in Figure 6. These values were calculated by inputting load, crack length, and openii displacement values collected following crack arrest into the corrected beam theoiy equations previously presented. 

Static DCB tests were also conducted using 12 and 36 ply adherends bonded with the same adhesive. Initially, precracks immediately grew into the composite adherends rather than within the adhesive layer. Results from these tests, which indicate the interlaminar energy release rate of the composite material, are shown in Fig. 9. The average energ> release rate of the composite material was approximately 550 J/m, 1250 J/m lower than the initiation energy release rate of the adhesive measured using the aluminum adherends. 

The bend strength of the composite bond containing the irradiated polyethylene is remarkable. The aluminum adherends must be bent through an angle of about 110° before failure of the bond occurs. On a number of occasions, bending the specimen has resulted in fracture of one of the aluminum adherends, rather than failure of the bond. 

The same effect as on aluminum adherend was also found with magnesium and PEI adherend. The optimum concentrations of each of the primers studied were applied to PEI (Ultem 1000) and magnesium alloy (AZ-91). Table 15.6 summarizes the single lap shear strengths of the resulting bonded joints. The results show that PAMAMs are effective in improving lap shear strength on... 

Fig. 15.1 Typical structures of PAMAM on aluminum adherend (X2000). The bar Is 5 pm.

Each material exhibits its own form of degradation and conditions under which the degradation occurs. For aluminum adherends, moisture causes hydration of the surface, i.e., the AI2O3 that is formed during the surface treatment is transformed into the... 

Bond failure can also occur if the surface is anodic relative to another joint component. An example would be clad aluminum adherends where a thin layer of pure aluminum overlays the base alloy. Such a surface layer is designed to be more corrosion resistant than the alloy, but to act as a sacrificial anode should corrosion occur. Although this approach works well for corrosion protection of the substrate material, it can be a disaster for bonded material if the adherend surface/interface corrodes. As a result, American companies tend to use unclad aluminum for bonding and provide other means of corrosion protection, such as painting [1,70]. On the other hand, European companies commonly use clad adherends, but with a thicker oxide (CAA) [6,18,71-73] that provides bondline corrosion protection. 

Figure 9 Wedge test results for FPL-etched aluminum adherends with FM-123 (moisture-wicking) adhesive, with FM-300 (moisture-resistant) adhesive, with an NTMP treatment and FM-123, and with BR-127 primer and FM-300. 

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