Strength of Epoxy-Bonded Aluminium Alloy Joints After Sandblasting

The purpose of this article is to investigate the eff ect of sandblasting surface treatment, before the bonding process, on the strength of the adhesive joints of the three types of aluminium alloy sheets. Three values of pressure were used during the sandblasting process, i.e. 0.41 MPa, 0.51 MPa and 0.56 MPa. After mechanical treatment, the samples of aluminium alloys sheets were degreased three times with acetone by immersion. The elements were bonded with single-lap joints using two components epoxy adhesive. After the curing process, the shear strength tests of the aluminium alloy adhesive joints were conducted, according to the DIN EN 1465 standard. The surface topography of all samples was measured, and the most important surface roughness parameter and its waviness were determined. Based on the experimental results, it was noticed that, among others, while using the same pressure value during sandblasting of diff erent aluminium alloys, the roughness parameters Ra and Rz of the obtained surfaces diff ered depending on the type of alloy. It is also to notice that more than two times strength of the joints was obtained by those made of the aluminium alloy sheet, which was subjected to heat treatment.


INTRODUCTION
The strength of adhesive joints of various materials depends on many of factors [1][2][3][4][5][6][7]. These factors are, most of all, determined by particular stages of the bonding technology together with their characteristic features. All types of factors are important, i.e. structural, technological, material, exploitation and economic ones, as well as, those related to the protection of environment [3,8]. The bonded strength mostly depends on the fi rst technological operation, i.e. surface treatment method [4,5,[9][10][11][12][13][14][15][16]. This type of process increases the adhesive joints strength. Moreover de Barros et al. [5] underlined the importance of surface treatment of the substrate on the fatigue life. A proper preparation of the bonded surface includes the operations, thanks to which the surface layer will be characterised by very high activity [17][18][19][20][21][22]. This process enables to remove the dirt from the material, increase wettability, modify the surface's geometry, develop it properly, and increase the surface free energy [20][21][22][23][24][25][26][27][28][29][30]. Selection of the surface treatment method before the bonding process depends on, among others, the used material and its structure [2,5,12,22,27]. Other surface treatment methods are recommended for steel [26,[31][32][33], which is diff erent for metals and metal alloys e.g. aluminium alloys [12,18,[34][35][36][37][38] and titanium alloys [30,[39][40][41][42], and diff erent for composites [43][44][45] and various polymer materials [2,3,46]. Adhesive joints of aluminium and aluminium alloys are used in many industries, e.g. aviation, building construction and others. Therefore, it is very important to properly develop appropriate bonding technology, including the selection of the right method of surface treatment. Aluminium and aluminium alloy surfaces are most often subjected to chemical and electrochemical treatments [2,12,21,31,[47][48][49][50][51]. It should be mentioned that the first operation in the process of preparing the surface of aluminium and aluminium alloys is degreasing, which can be realized using various techniques and degreasing agents [2,15,34,47]. Aluminium and aluminium alloy surfaces can also be machined, as reported in many articles [18,[52][53][54][55][56]. Among the mechanical treatments, the most recommended ones are to use sandblasting [2,10,20,54,56], grit blasting [19,56,57] and ice blasting [35]. In case of sandblasting, it is recommended to use alumina, silicon carbide and quartz as an abrasive, which, due to their irregular shape and sharp edges, effectively provides roughness to the surface to be treated. When using mechanical treatments, attention should be paid to the geometry of the surface formed during this treatment, which sometimes may hinder its wetting by the adhesive, which in turn leads to poorer mechanical properties of the joints. Elbing et al. [35] emphasized that the adhesive strength of aluminium components could essentially be improved with the application of dry ice blasting. However, dry ice blasting causes only small change in the topographic surface structure of adherends, but the improvement of the adhesive joint strength is because of the removal of the distributing lubricant, aluminium-oxide, and magnesiumoxide films. Bockenheimer et al. [18] showed that the surface treatment of the aluminium alloy surface by grit blasting, with glass beads or alumina grit, changed both, the aluminium alloy surface topography and the chemical state of this adherends surface. The authors underlined that the chemical state of adherend surface were resulted from influence of the curing reaction of the epoxy adhesive. Harris and Beevers [19] studied the effect of grit blasting on the surface characteristic aluminium alloy and mild steel adherends. They found that slight differences in grit types led to noticeable changes on surface characteristic. Rudawska et al. [20] presented the issues of sandblasting processing on the surface properties of steel sheets. It has been found that sandblasting medium has a greater effect than treatment pressure. Mandolfino et al. [21] investigated the influence of sandblasting parameters, e.g. type of sand, impact angle and pressure, on the surface roughness and mechanical characteristic of bonded joints steel sheets. The authors stated that the surface properties of adherends after sandblasting have an influence on the bonded joints mechanical behavior. Based on the analysis of the results of sandblasting and grit blasting studies, it was decided that in this article, research related to the use of sandblasting as surface treatment would be conducted for materials with a different degree of surface preparation after the manufacturing process. Spaggiari and Dragoni [13] investigated the effect of the mechanical surface treatments on the adhesive bonded joints and they noticed that that a simple correlation with the surface roughness is not sufficient to predict the best joint performances. The obtained results demonstrated that mechanical treatment (e.g. sandblasting) the adherends gives a strong improvement in terms of performance. Zhan et al. [52] presented that with the increase of surface roughness of Al-Li alloy the tensile-shear strength of the adhesive joints increased and the groove structures made during mechanical abrading were considered as being responsible for this strengthening behavior.
The aim of the article is to investigate the effect of sandblasting surface treatment before the bonding process on the strength of the adhesive joints of the three types of aluminium alloy sheets. Comparative analysis of the surface roughness parameters after sandblasting and the adhesive joints strength of the aluminium alloy sheets are also presented. Based on the tests, it can be underlined that a main technological factor that has an impact on the shear strength value is a proper surface treatment of the adherend. The technological parameters of sandblasting also have an influence on the adhesive joints' strength of aluminium alloys sheets. Moreover, the type of material and its technological surface treatment (heat treatment) after the production process also play a significant role both in terms of surface roughness and the strength of the adhesive bonds of these materials.

Adherends
The single lap joints that were subjected to the tests were made of the following types of the aluminium alloy sheets: EN AW-2024 TO, EN  AW-2219 TO and EN AW-2014 T4. The EN AW-2024 TO, EN AW-2219 TO and EN AW-2014 T4 aluminium alloys are recognised as cast alloys of the second series, which means that the main alloy addition is copper and, in small quantities, magnesium and manganese. They are characterised by high strength and an average susceptibility to corrosion. The EN AW-2014 T4 aluminium alloy had been subjected to supersaturating and a natural ageing until it obtained a steady state. The EN AW-2024 TO and EN AW 2219-TO alloys had been hardened and were not subjected to heat treatment. Table 1 presents the chemical composition of the alloys listed above [58,59].

Adhesive
The epoxy adhesive with the designation Epidian 57/PAC/100:80 was used to make the joints. The epoxy adhesive contains epoxy resin based on Bisphenol A (Epidian 57 -commercial name, manufactured by CIECH Sarzyna, Nowa Sarzyna, Poland) and polyaminoamide curing agent (PAC -commercial name, manufactured by CIECH Sarzyna, Nowa Sarzyna, Poland). The adhesive preparation included mixing its ingredients in the proportion 100:80 (by volume). The Epidian 57 epoxy resin is an epoxy mixture created by the modification of the epoxy resin Epidian 5 with the saturated polyester resin Polimal 153. Table 2 presents the physicochemical properties of the Epidian 57 epoxy resin [60,61].
The polyaminoamide curing agent (PAC) is a modified polyamide curing agent produced by the polycondensation of polyamine with diameters of unsaturated fatty acid methyl esters. Its use increases elasticity and impacts the joints strength and that is why it is used to create deformationresistant adhesive joints. The curing time is from 4 to 7 days. However, it may be shortened to 8 hours by conducting the process at the temperature of 60°C. Table 3 presents the properties of the polyaminoamide curing agent [60,61]. The adhesive preparation was conducted mechanically at the adhesive preparation stand, with use of a horseshoe mixer, with the mixer's rotational speed of 460 rpm for 2 minutes, at the ambient temperature of 25±1°C and ait humidity 24±1%. The electronic balance TP-2/1 (FAWAG S.A. manufacturer, Lublin, Poland), having an accuracy of 0.01 g, was used in order to weigh the ingredients properly and precisely. While preparing  the epoxy adhesive, attention was paid to avoid creating the air bubbles as they could affect the joints strength.

Surface treatment
The surface preparation was conducted in two stages: • The first stage -sandblasting of the aluminium alloys surface, which was performed in order to increase the surface roughness. The variable parameter of sandblasting was pressure of 0.41 MPa, 0.51 MPa and 0.56 MPa.
Other parameters kept constant. • The second stage -degreasing by immersion in acetone in order to remove the impurities created especially during the mechanical processing performed beforehand. After that the samples were left to dry completely.

Sandblasting
Sandblasting was performed with use of the abrasive blasting machine. The KC 1600 cabinet, thanks to a stable construction made of steel and a closed working space (1320×1320×950 mm), provides safety during processing. It needs to be pointed out that the blast cabinets ensure the best results possible in comparison to other construction tools aimed at sandblasting. It stems from the fact that the operation is conducted in a tight chamber that prevents the impurities from the outside to get in touch with the processed elements. The EB F54 aloxite, made of calcined bauxite, was used as a blasting medium [62]. The aloxite grit is very hard (9 in the Mohs hardness scale) and has very sharp edges that self-renew while working. The use of aloxite enables to clean the surface to a high extent. The F54 aloxite grit's size is between 355 to 300 μm. Due to the fact that aloxite is a reusable abrasive, the size of grit during processing of the aluminium alloys might have been slightly smaller.
The samples made of the aluminium alloy were placed inside the cabinet. During sandblasting the aloxite was getting out from the sandblasting machine's nozzle with use of the compressed air and was hitting against the processed materials' surface. After that, it was falling into the charging hopper; from there it was being poured to the abrasive pressure container again. Each aluminium alloy was subjected to sandblasting with use of three different pressure values, i.e. 0.41 MPa, 0.51 MPa and 0.56 MPa. This operation enabled us to obtain a clean and rough surface, even in hard-to-reach places, especially edges and corners. Table 4 presents the pressure variants used during sandblasting.

Degreasing method
A degreaser used in this stage was acetone, which dissolves fats, oils, lubricants, paints, as well as soft polymers. It is not toxic, however, when inhaled; it may cause headaches and irritation of the mucous membranes of the nose, mouth and eyes. Acetone is a colourless transparent liquid of a characteristic smell and high volatility.
After the sandblasting operation, the surface of the processed elements was covered with dust, which was removed during degreasing by immersion. The degreasing time was 4 seconds, and after that, the samples were left until acetone evaporated completely. This operation was conducted three times in order to remove all the impurities. Omission of that stage would result in weaker adhesive joints with low strength.

Shape, dimensions and conditions of adhesive joints preparation
The EN AW-2024 TO, EN AW-2219 TO and EN AW-2014 T4 aluminium alloys were used to make 54 joints (6 single-lap joints made of all 3 aluminium alloys subjected to sandblasting with use of 3 pressure values) with use of the Epidian 57/ PAC/100:80 two-component epoxy adhesive. Figure 1 shows the adhesive lap joint scheme. Table 5, in turn, presents the geometric dimensions of the real joints after curing. After the adhesive was prepared, it was instantly applied on the surface of one of the bonded elements. Then, the bonded elements were steadied in a locking special device. Instant application of the adhesive and bonding the elements right after the adhesive mass was prepared to preserve the adhesive's properties. The adhesive life, especially in case of the epoxy ones, changes with time. The curing process was performed at ambient temperature and under a pressure of 0.018 MPa. The curing time was 7 days. The pressure enabled the adhesive mass to fl ow in all pores and cracks and to distribute the adhesive on the whole surface. The processes of the adhesive compound preparation, application and conditioning were conducted in the conditions described in Table 6.

Tests and statistical analysis
The tests conducted for the purpose of the present article included determining both the geometrical structure of the adherends after the surface treatment, as well as, conducting the strength tests of diff erent variants of the adhesive joints made of the aluminium alloy, taking into consideration diff erent sandblasting variants.
The surface topography of all samples was measured with a tracer method with use of the Hommel-Etamic T8000 RC120-400 device (JEN-OPTIC Industrial Metrology Germany GmbH, Schweninngen, Germany). The most important surface roughness parameters and its waviness were determined. Also, a 3D topography of the surface prepared with use of sandblasting, during which diff erent values of pressure were used, was prepared. All the tests were conducted as recommended with the standards: PN-EN ISO 11562, PN-EN ISO 4287 and PN-EN ISO 25178. The topographic structure measurements of the analysed surfaces consisted of mapping the material profi le by passage of the measuring tip on the surface at the speed of 1.20 mm/s. The sheet surface scanning range was 4.80 mm × 4.80 mm and 1920 points of measurement were set. The following  roughness parameters were analysed in both 2D and 3D systems: • Ra -mean roughness profi le deviation, • Rz -ten-point mean roughness, • Sa -arithmetical mean height, • Sz -maximum 3D profi le height, • Sku -kurtosis.
The surface roughness measurement was conducted on three samples of each aluminium alloy (10 mm from every sample's edge, whereas the third measurement was made in the middle, i.e. 50 mm from the sample's edge), whose surface had been prepared according to three diff erent variants. Each sample was measured at 1920 points, and this is why the tables present the mean measurement values. The shear strength tests of the aluminium alloys adhesive joints were performed on the Zwick/Roell Z150 testing device (ZwickRoell GmbH&Co. KG, Ulm, Germany), according to the DIN EN 1465 standard, at a constant speed of 5 mm/min. The adhesive joints were fi xed at the testing machine with the screw-wedge clamps. Test-Expert software was used to visualise the test results. After carrying out the strength tests, a visual analysis of the failure of adhesive joints was carried out based on the PN-EN ISO 10365 standard.
The shear strength (R t ) was determined on the basis of the obtained value of the failure force and the geometric bonding surface, using the basic formula for strength (1): where: P -failure force, N; A -geometric bonding surface, mm 2 .
The strength test results were then subjected to a statistical analysis based on ANOVA due to the fact that the number of the analysed groups was higher than two. The analysis was conducted with use of the Statistica software. The statistical analysis of the obtained results enabled us to compare the mean strength value taking into consideration the analysed pressure variants applied during the mechanical processing. The Shapiro-Wilk test (normal distribution), Levene test (variance homogeneity) and the Tukey's HSD test at the assumed probability level α = 0.05 was used. The probability level 0.05 is recognised as a boundary value of the acceptable error rate. The Tukey's HSD test is a post-hoc test (or a multiple comparison test) and may be used to fi nd means that are signifi cantly diff erent from each other in a distribution of the analysis of variance. Tables 7 to 9 present the results obtained during the topographic structure measurement of the materials, whose surface was subjected to sandblasting. However, for comparative purposes, Table 10 presents the results of surface roughness of aluminum alloy sheets before sandblasting. When analysing the data in Figure  2, it was observed that the higher the pressure value during sandblasting of the given material, the higher the arithmetical mean value of the profi le    Based on the results presented in Figure 3, it was observed that this parameter increases together with the increase of the pressure value  aluminium alloy (V1 variant), and the difference was of 7.13 μm (29.67%). Table 7 presents the obtained results for the aluminium alloys, whose surface was subjected to sandblasting according to V1 variant.

Figures 2 and 3, and
The graphics presenting an exemplary topography of the alloys' surface showed the irregularities caused by sandblasting. The highest value of the arithmetical mean height, i.e. the Sa  Table 8 presents the results of the surface roughness measurements of the aluminium alloys, whose surface was prepared using V2 variant. The exemplary topographic maps show the micro irregularities caused by sandblasting.  Table 9 presents the results of the surface roughness measurements of the aluminium alloys, whose surface was prepared using V3 variant. The surface topography showed the irregularities Comparing the results presented in Figure  2, Figure 3 and Table 10, it was noticed that after the mechanical treatment of the surface of the aluminium alloy sheets, i.e. sandblasting, a significant increase of the considered surface roughness parameters Ra and Rz was observed, more than several times. This means that sandblasting influences the geometric development of the surface, which in the gluing processes is important; first of all, it increases the real surface of the bonding, and thus increases the share of the mechanical adhesion.

Adhesive joints strength
Shear strength results of the adhesive joints made of three types of the aluminium alloy sheets: EN AW-2024 TO, EN AW-2219 TO and EN AW-2014 T4, whose surface was subjected to sandblasting with use of three pressure values before the bonding process, are presented in Figure 4. Based on the presented shear strength results of the aluminium alloys (Fig. 4), whose surface was prepared using the three diff erent variants of sandblasting, it may be stated that the adhesive joints of the EN AW-2024 TO aluminium alloy are characterised by the lowest shear strength. However, the highest shear strength of that alloy was obtained after sandblasting according to the V3 variant, whereas the lowest one was obtained using the V2 variant (0.51 MPa). The diff erence between the highest and the lowest shear strength values increased to 3.52 MPa (66.67%). In case of the joints made of the EN AW-2219 TO aluminium alloy, the highest strength value was obtained for the V3 variant, and the lowest one for the V2 When analysing the strength of joints made of diff erent aluminium alloys that were prepared using diff erent pressure values, it was observed that there was a signifi cant diff erence between the joints characterised by the highest and the lowest shear strength. The joints of the EN AW-2014 T4 aluminium alloy had more than three times higher

Failure analysis of adhesive joints
The visual analysis of the failure of adhesive joints was carried out based on the DIN EN ISO 10365 standard. Based on failure analysis of adhesive joints after the mechanical tests, it can be observed that all the tested types of adhesive joints reveal the presence of failure inside the adhesive layer. None of the tested types of aluminium alloy sheets showed failure inside adherend -aluminium alloy sheets. The results of the different types of adhesive layer failure are shown in Table 11 and Figure 5.
When visually assessing the samples of adhesive joints in the place of joining after the mechanical strength tests, it can be seen that in any variant of surface preparation by sandblasting, the fourth type of failure of the adhesive joint did not occur -ACF (p) adhesive and cohesive failure with peel. It was also noted that in most cases there were two types of failure: adhesion failure (AF) and also special cohesion failure (SCF).
The photos in Fig. 5 show examples of EN AW-2014 T4 aluminium alloy adhesive joints, because for this material it will obtain the highest shear strength of adhesive joints. It has been found that for the adhesive joints of this alloy, increasing the pressure value during sandblasting caused a greater amount of failure to the adhesive joint. Perhaps the higher pressure causes the unfavourable shape of the surface roughness and considering the mechanical adhesion. After the V3 variant, no greater real bonding surface was obtained, and thus the adhesive was less anchored in the surface micro-unevenness and the failure image was rather classified as adhesive failure (AF). On the other hand, in the V1 variant surface treatment for this type of aluminium alloy sheet, a greater number of samples were noticed showing the properties of the cohesive failure (SCF) of the adhesive joint. On this basis, it can be assumed that the sandblasting variant with lower nozzle pressure may contribute to obtaining a more favourable stereometrics structure of the surface of the analysed aluminium alloy sheets. As a result, a larger real bonding surface (wetted by the adhesive) is obtained. This can be confirmed by the strength results for those joints, where the adhesive joints of aluminium alloy sheets made in the first surface treatment condition (V1) are characterized by greater strength. It should be noted that the authors are aware that the visual assessment of the failure character of adhesive joints based on the DIN EN ISO 10365 standard may be a subjective assessment, but efforts were made to make the presented results as objective as possible.

Statistical analysis of adhesive joints strength results
The first stage of the analysis includes preparing the descriptive statistics that characterise the analysed variables. The results are presented in Tables 12 to 14.
The next stage of the statistical analysis includes verification of adjustment of the empirical research of the analysed variables' result to the normal distribution with use of Shapiro-Wilk (S-W) normality test. The statistical analysis (at the

DISCUSSION
A main technological factor that has an impact on the strength of adhesive joints is a proper surface treatment of the adherends. The method used during the tests described herein was sandblasting, which caused some irregularities, cracks and pores on the adherends surface. Thanks to that, the adhesive-adherend contact surface was increased and the anchor points for the adhesive were created. Comparative analysis of the surface roughness' parameters and the adhesive joints strength of the aluminium alloy sheets were based on the correlation coefficient of two values: shear strength and surface roughness parameters (Ra and Rz). The correlation coefficient (ρ) serves to determine the relations between two values and it may be defined with a following dependence as reported in Ref. [20]. Table 22 presents the selected Ra and Rz surface roughness parameters of the surface of adherends, whose surfaces were subjected to sandblasting according to three variants (Table 4) and the shear strength of the adhesive joints of the analysed sheets.
When considering the dependence between the values of the Ra and Rz surface roughness parameters of the aluminium alloy sheets and the strength of the adhesive joints of the analysed sheets, in case of the variant 1 a positive correlation between these values may be observed. However, it is not as strong as for the variant 2. The correlation coefficient for the distribution for the compared parameters Ra and Rt is 0.66, and for the compared parameters Rz and Rt it is 0.67.
In case of the second variant (V2), when comparing the analysed surface roughness parameters, it was observed that together with the increase of these parameters' value, the strength of the adhesive joints of the aluminium alloy sheets increases as well. The correlation coefficient for the distribution for the compared parameters Ra and Rt is 0.98, and for the compared parameters Rz and Rt it is 0.99.
For the third variant of sandblasting V3), an inverse correlation may be observed: together  Tables 15-17. The next stage of the statistical analysis included verification of the assumption of the variance homogeneity, with use of the Levene's test. The Levene's test results are presented in Table 18.
As the assumptions of the normality of distribution and the homogeneity of variance were proved to be true, the parametric tests (Tukey's HSD) were used in the next stage of the statistical analysis. Its results were presented in Tables 19-21.
For the EN AW-2024 TO alloy, considering three values of pressure used during sandblasting, the multiple comparison analysis did not show any statistically significant differences between the mean values of the obtained results (Table 19). The results presented in Table 20 show that there were some differences between V1 and V2 variants and V2 and V3 variants of sandblasting. There were no statistically significant differences between the V1 and V1 variants. In case of the results for the EN AW-2014 with the decrease of the roughness parameters value, the strength of the joints increases. The correlation coefficient is negative and for the cpmapred parameters Ra and Rt it is -0.96, and for the compared parameters Rz and Rt it is -0.74.
de Barros et al. [56] in their research showed that the influence of individual average surface roughness parameters (e.g. after mechanical treatment in the form of grit-blasting and sandblasting) is not statistically significant when it is correlated with the bond strength. In turn, Bockenheimer et al. [18] emphasized that the effect of mechanical surface treatment for aluminium alloy bonding was very complex, compromised the topography structure of adherends surface and changed the chemical state of the adherends surface. On the basis of the present tests, it can be added that technological factors of machining can shape a specific surface structure, which ultimately affects the strength of adhesive joints, considering the aspect of mechanical adhesion. This statement was also supported by the results of research presented in the study prepared by Rudawska et al. [20]. Moreover Boutar et al. [53] and Harris and Beevers [19] concluded that the changes in the adhesive joints properties associated with the roughened surfaces cannot be explained simply by the increased roughness characteristic, such as mechanical anchoring and increased effective real bond area. The properties and changes of both physical and chemical surfaces of joined materials should be considered simultaneously. Spaggiari and Dragoni [13] underlined that that a simple correlation with the surface roughness is not sufficient to forecast the best adhesive joint execution.
With regard to the results obtained, it is also important to notice that more than two times the strength of the joints was obtained by those made of the EN AW-2014 T4 aluminium alloy, which had been subjected to heat treatment, i.e. solution heat treatment and natural ageing until obtaining a stable state. This procedure resulted in the material's hardening, and this was the only material that was not deformed after sandblasting. The adhesive joints of the EN AW-2014 T4 aluminium alloy obtained more than three times higher strength than those made of the EN AW-2024 TO aluminium alloy. In case of the two other aluminium alloys sheets, i.e. EN AW-2024 TO and EN AW-2219 TO (not subjected to heat treatment), many samples were deformed during sandblasting. This proves the fact that sandblasting of these materials, is not recommended due to lower hardness and high susceptibility to deformation. For these materials it is recommended to use a different surface preparation method.

CONCLUSIONS
Based on the conducted tests, it can be underlined that a main technological factor that has an impact on the shear strength of adhesive joints is a proper surface treatment of the adherend. The technological parameters of sandblasting have an influence on the adhesive joints strength of the aluminium alloys sheets. The type of material and its technological surface treatment (heat treatment) after the production process also play a significant role both in terms of surface roughness and the strength of the adhesive bonds of these materials. When using the same pressure value during sandblasting of different aluminium alloys surface, the roughness parameters Ra and Rz of the obtained surfaces differed depending on the aluminium alloy type. The adhesive joints of the aluminium alloy sheets, the surface of which has been heat treated after the production process, showed a much higher strength than the other joints for which sheets were not subjected to heat treatment.
The conclusion is that an important element in the bonding technology is not only the surface treatment prior the bonding process, but also the surface treatment of adherends after the production process (temper state). However, the surface treatment method should be selected for each type of the material individually, depending on its properties.