Laser processing of titanium alloys to increase the strength of adhesive joint with cfrp

Titanium alloys are widely used as fasteners, hinges, power elements in a spacecraft (SC). In particular, titanium alloys are used to create one-piece adhesive joints of carbon fiber structures. As traditional connection methods (welding, soldering) used for metal parts cannot be used for parts made of composite materials (CM) or complicate the design (threaded, riveted joints, etc.) [1–3], CM is glued to metal elements through which the connection is provided. The fitting of the SC lock, which is an adhesive pair “titanium - carbon fiber” is shown in Fig. 1.

Despite the visible advantages of adhesive bonding, the strength of such knots is rather low. This is due to the fact that titanium alloys are hard-to-glue materials, since there is always a thin oxide film on the alloy surface, which prevents the formation of interatomic and intermolecular bonds between the adhesive and the substrate. The adhesive strength of titanium alloys can be increased by pre-treatment before bonding. Titanium processing methods to increase the adhesive strength during bonding are considered in [4; 5]. Laser processing is a promising method for increasing the adhesive strength of a titanium alloy [6]. This method has high productivity and stability of the properties of the treated surface, and, in contrast to chemical methods of surface treatment, the environmental friendliness of the process. In [7–11], the mechanisms of laser processing on metals (Al, Ti) by lasers of various types are considered.

Рис. 1. Механическое устройство батареи солнечной:

1 – силовой трубчатый элемент (углепластик); 2 – фитинг (титановый сплав)

Fig. 1. Mechanical device of solar battery:

1 – power tubular element (carbon fiber); 2 – fitting (titanium alloy)

Previous studies have been carried out [12; 13], where the influence of the titanium alloy surface treatment method on the strength of the adhesive joint of the titanium-titanium pair was determined. It was found that laser ablation of the surface has a positive effect on the adhesive strength of the titanium alloy surface.

Experiment

In this study, samples of titanium alloy OT-4 with dimensions of 70×20×2 mm and samples of bidirectional carbon fiber reinforced plastic KMU-4 with dimensions of 70×20×2 mm were used. The samples were glued together with a three-component adhesive VK-9 (OST 1-90281-86) with an overlap. Gluing was carried out according to OST 92-0949-74. Adhesive joint area S=300 mm2. A batch of 5 samples was tested to ensure statistical reliability. The geometric dimensions of the test specimen are shown in Fig. 2.

Рис. 2. Геометрические размеры образца клеевого соединения титана с углепластиком

Fig. 2. Geometrical dimensions of the sample of adhesive joint of titanium with carbon fiber

To determine the strength of the adhesive bond of the pair “titanium - carbon fiber”, shear tests were carried out. Mechanical testing was carried out 24 hours after gluing on a universal tensile testing machine (Eurotest T-50, S.A.E. IBERTEST, Spain) at a speed of 5 mm/min.

Laser treatment of the surface was carried out on an ytterbium pulsed fiber laser (IPG, YLPM-1 -4×200-20-20). In a previous study [13], characteristic surface textures were identified, which were taken as a basis in this work. Modes of laser processing are presented in Table 1. Laser wavelength is 1.064 µm, pulse repetition rate is 40 kHz, and pulse energy is 0.5 mJ. The laser beam movement pattern is bidirectional shading along one axis. The microstructure of the titanium alloy surface, depending on the processing mode, is shown in Fig. 3.

Titanium Alloy Laser Surface Treatment Parameters

Table 1

Mode No.	1	3	12	13
Laser power, W	10	20	20	10
Processing speed, mm/s	200	200	200	400
Number of stripes per 1 mm	15	15	25	25

Рис. 3. Микроструктура поверхности титана при лазерной обработке: a – режим № 1; b – режим № 3; c – режим № 12; d – режим № 13

Fig. 3. The microstructure of the titanium surface during laser processing: a – mode № 1; b – mode № 3; c – mode № 12; d – mode № 13

For comparison, specimens were made and shear tested with mechanical methods of surface treatment of titanium alloy: manual surface grinding in two directions and with sandblasting. The roughness of the treated surface was measured with a profilometer (TR110, TIME Group Inc., China). The results of surface roughness are presented in Table 2. The average value of CFRP roughness is 0.85 µm.

Рис. 4. Результаты испытаний прочности образцов на сдвиг

Fig. 4. Results of testing the strength of specimens for shear

Table 2

Surface Roughness Value of Titanium Alloy with different processing methods before gluing

Processing method	Ti without processing	Manual grinding	Sandblasting	Laser processing mode No. 1	Laser processing mode No.3	Laser processing mode No.12	Laser processing mode No.13
Ra, мкм	0.78	1.23	1.15	6.5	9.65	5.37	1.76

Results and discussions

In Fig. 2 it can be seen that laser treatment of the titanium alloy surface favorably affects the strength of the adhesive bond of the titanium-carbon fiber pair. Comparing the results, the laser treatment increased the strength by more than 160% compared to the untreated surface. Since the metal is machined in production to increase the bonding area before bonding the metal to the composite, it is more accurate to compare the strength of laser-cut specimens with that of hand-ground specimens. Consequently, the strength of laser-treated samples increased by more than 80% relative to the average value of mechanical grinding

The highest value of shear strength was shown by samples with laser processing No. 1 and 3. This is due to the largest increase in the surface bonding area, as well as mechanical locking of the adhesive in the microrelief of the structure. Mode No. 13, having a slight increase in surface area, as can be seen from the surface roughness, showed a significant increase in the strength of the adhesive joint. This means that the improvement in shear strength caused by laser surface treatment is a mixed effect of increasing surface area, mechanically locking the adhesive, and changing the surface chemistry. The chemical composition of the surface structure under the influence of laser scanning is gradually transformed from Ti and Ti 2 O 3 to crystalline TiO 2 . Laser processing promotes the transformation of metal elements and oxides with low valence into an oxide with high valence [14; 15], which contributes to the adhesive bonding with the surface.

The nature of the destruction of the adhesive bond in the samples with laser processing is predominantly cohesive, but samples with the destruction of the carbon fiber material were also observed, i.e., the shear stresses in the composite material exceeded the adhesive strength. The nature of the destruction of samples without processing, with manual grinding and sandblasting showed adhesive and mixed destruction. It is worth noting that tests of samples with manual grinding showed a large spread of values, which indicates a low reliability of this gluing method.

If the results of the previous study [5] are compared with those presented, a decrease in the strength of the adhesive joint in the same laser processing modes by 25%, which is associated with the influence of the preparation of carbon fiber before gluing, can be observed.

Conclusion

The study showed that laser treatment of the titanium alloy surface before bonding to CFRP increases the adhesive strength by more than 80%, compared with modern bonding technology (machining). The highest strength index was shown by samples with processing modes that provide the largest bonding area (mode No. 1 and 3).