Int J Adv Manuf Technol DOI 10.1007/s00170-015-7788-5
ORIGINAL ARTICLE
Correlation between surface characteristics and static strength of adhesive-bonded magnesium AZ31B Rui Zheng 1 & Jianping Lin 1 & Pei-Chung Wang 2 & Yongrong Wu 1
Received: 23 April 2015 / Accepted: 31 August 2015 # Springer-Verlag London 2015
Abstract The use of adhesive is posed to increase dramatically for application to the next generation of vehicle structures, as is the use of non-ferrous materials (e.g., magnesium and aluminum alloys). In this study, surface polishing treatment of 1.6-mm-thick Mg AZ31B with various sandpaper grits (i.e., 80, 240, 600, and 1200 grit) was utilized to understand the correlation between the surface characteristics and strength of the adhesive-bonded polished magnesium joints. Lap-shear joints were fabricated with an epoxy adhesive (Lord Versilok 253/254) and quasi-static tested. It was found that the strength of the adhesive-bonded Mg AZ31B was decreased by the surface polishing. To understand the effect of surface polishing on the surface characteristics of Mg AZ31B, surface morphology (i.e., surface roughness and surface area), surface chemistry component and surface free energy of the asreceived and polished Mg AZ31B substrates were measured and analyzed. Test results revealed that the removal of the surface oxide by polishing resulted in a decrease in actual surface area and content of oxygen element to decrease the surface free energy of the treated Mg AZ31B, and consequently, decreased the strength of the adhesive-bonded Mg AZ31B. Finally, examinations of the test results indicated that although the actual surface area of the bonding region correlated with the joint strength, the surface free energy provided a better index to indicate the strength of the adhesive-bonded Mg AZ31B.
* Jianping Lin
[email protected] 1
School of Mechanical Engineering, Tongji University, No. 4800, Cao’an Highway, Shanghai 201804, China
2
Global Research and Development Center, General Motors Corporation, Warren, MI 48090-9055, USA
Keywords Epoxides adhesive . Magnesium alloys . Surface characteristics . Static strength . Surface free energy
1 Introduction Epoxy-based, adhesive-bonded magnesium is known to have good stiffness and strength, providing a potentially wide range of applications, especially in lightweight vehicle structures [1, 2]. Despite these advantages, the use of adhesive for joining vehicle structures has been limited, partially because of concerns regarding the environmental durability [3]. In order to utilize the full potential of the adhesive-bonded magnesium, surface treatments have been usually employed to improve the corrosion resistance of magnesium [4]. As part of a study on the corrosion resistance of the adhesive-bonded magnesium alloys for automotive applications, a good understanding regarding to the correlation between the surface characteristics and static strengths of the adhesive-bonded, surface-treated magnesium joint is necessary. Many studies had been conducted to understand the effect of surface mechanical pre-treatment on the strength of the adhesive-bonded metal (e.g., aluminum [5, 6], steel [7], and titanium [8]) joints. The correlation between the surface characteristics (e.g., surface roughness [9], surface area, and surface texturing [10]) and joint strength was investigated. The investigation into the effect of surface mechanical pretreatment on the strength of the adhesive-bonded magnesium was still relatively sparse. Particularly, Song [4] has reported the effect of surface polishing on the corrosion resistance of Mg AZ31 and found that polishing treatment effectively removed the Fe-containing particles, and thus dramatically improved the corrosion resistance of Mg AZ31B sheets. Since surface polishing can be used to improve the corrosion resistance of magnesium, the understandings of correlation
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between the surface characteristics of surface mechanically treated magnesium and its adhesive joints’ strength have largely facilitated the application of adhesive bonding for magnesium in the vehicle architectures. The present study is to understand the correlation between the surface characteristics and strength of the adhesive-bonded magnesium. A two-component, rubber-toughened, acrylicmodified epoxy (L) adhesive was selected. Various grit size sandpapers (i.e., 80, 240, 600, 1200#) were utilized to polish magnesium substrates. The lap-shear joint configuration was selected in this study, which is the most widely used in vehicle structures. The specimens were fabricated and tested in an ambient laboratory environment. Quasi-static testing of the adhesive-bonded joints was performed and the results were analyzed. To understand the effect of surface treatment on the joint strength, the surface morphology, chemical elements, and surface free energy of polished Mg AZ31B were examined using Optical non-contact 3-D surface profile measurements, energy-dispersive spectrometer (EDS) analysis and contact angle measurement. Finally, the correlation between the surface characteristics of Mg AZ31B substrate and the strengths of the adhesive-bonded Mg AZ31B was discussed.
2 Experimental procedures 2.1 Materials Wrought 1.6-mm-thick Mg AZ31B sheet was used in this study. The chemical composition range (wt%) measured by us for magnesium substrate is Al (3.19), Zn (0.81), Mn (0.33), Si (0.02), Cu (0.05), Ca (0.04), and Mg (balance). A twocomponent, epoxy-modified, acrylic adhesive (Lord Versilok 253/254, hereafter referred to as L adhesive), containing 0.25mm diameter glass beads to control the bondline thickness, was used to join magnesium substrates. Per manufacturer’s data, the mechanical properties of Mg AZ31B and L adhesive are listed in Table 1. 2.2 Surface polishing To study the correlation between the surface characteristics and joint strength, various waterproof silicon carbide sandpapers (i.e., 80, 240, 600, 1200 grit corresponding to average grit Table 1
Mechanical properties of adhesive L and magnesium AZ31B
Materials
Adhesive L 1.6-mm-thickness magnesium AZ31B
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
3.7 232
8.9 310
28.6 6.9
sizes of approximate 192, 53.5, 16, 6.5 μm, respectively) were selected to manually polish the surface of magnesium substrates. Every batch (i.e., four pieces magnesium substrates) was aligned on a steel fixture. The surfaces of the substrates were polished with a rate of 2 times per second along the width direction of the specimen for ∼4 min. The middle two polished substrates were selected for sample fabrication. Ten replicates (to fabricate five samples of the adhesive-bonded joint) were performed for each sandpaper grid #. Table 2 lists Mg AZ31B specimens polished with various grit sizes. 2.3 Surface characteristics analysis 2.3.1 Surface morphology and chemistry Surface polishing may change the surface morphology and chemistry of Mg AZ31B [11]. To understand the variation, the surface roughness and area of the as-received and polished 1.6mm-thick Mg AZ31B substrates were measured with optical non-contact 3-D surface profile measurement (WYKO DT-X3, Veeco Company, USA). The 120-μm length of substrate of the polished region was selected to measure the surface roughness Ra, while the area of 120 μm×96 μm on the polished substrate surface was measured to acquire the surface area. The measurements at five different positions (shown in Fig. 1) were performed for each condition and the average value was reported. Scanning electron microscope (Hitachi S-3400 Japan) was utilized to examine the surface morphology. The surface chemistry of the as-received and polished Mg AZ31B substrates were measured by the energy-dispersive spectrometer (EDS). 2.3.2 Specific surface area To assess the effect of oxidation on the surface area, the Brunauer-Emmett-Teller (BET)-specific surface areas [12] of the as-received and oxidized Mg AZ31B specimens were measured. Mg AZ31B substrates were polished first to remove the surface oxide layer and then ground into powders (i.e., the average size of about 0.03 mm) as the requirement of BET-specific surface area measurement. The ground powders were divided evenly in two parts. One half of the powders was stored in a desiccator to minimize the oxidation at room temperature, and the other half was exposed at 480 °C for 20 min in air to oxidize magnesium powder [13]. Then, the BETspecific surface areas of the stored Mg AZ31B in desiccator and the oxidized powders (∼1.5 g) were examined from the amount of N2 adsorbed per gram of magnesium powders at Table 2
Magnesium AZ31B substrates polished with various sizes
Sandpaper grit #
As-received
80
240
600
1200
Sample No.
0
1
2
3
4
Int J Adv Manuf Technol Fig. 1 Sketch of measuring positions for surface roughness and surface area
−195.7 °C with various relative pressures (i.e., from 0 to 1), which was performed on a Micrometrics ASAP2020 instrument. 2.3.3 Contact angle and surface free energy A high level of surface wetting is a prerequisite for good adhesion [14, 15]. Because the contact angles are closely related to wettability [16], they were measured and used to estimate the surface free energy of the polished magnesium adherends. Calculated surface free energy of magnesium adherends was then used to correlate with the strength and failure mode of the adhesive-bonded-treated magnesium joints. The surface free energy of the adherends was estimated by measuring the contact angle of three liquids on the adherends. Distilled water, diiodomethane, and ethylene glycol were selected as a probe to measure the contact angle of Mg AZ31B by the sessile drop method [17] with a Dataphysics OCA-20 contact angle analyzer under 23 °C. The test drop volume was 2 μL. The liquids were chosen to cover the broadest possible range from highly polar (water) to almost completely dispersive (diiodomethane). Table 3 lists the test results of the testing liquid surface tension and surface tension components at 23 °C [16]. The surface free energy of solids can be obtained by measuring the contact angles of liquids whose surface tensions are known. This measurement method is based on Young’s equation that describes the condition for equilibrium at a solid– liquid interface. γ L cosθ ¼ γ S −γ SL
In order to obtain γS, an estimate of γSL must be obtained. Owens and Wendt [18, 19] proposed a geometric mean approach to combine the dispersion and non-dispersion (polar) interactions, and the following expression for γSL was proposed: qffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ γ SL ¼ γ S þ γ L −2 γ S d γ L d −2 γ S p γ L p Combining Eq. (2) with Eq. (1) yields: qffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi γ L ð1 þ cosθÞ ¼ 2 γ S d γ L d þ 2 γ S p γ L p
ð3Þ
The contact angle of at least two liquids with known surface tension components (γL; γLd; γLp) on the solid must be determined to obtain γSd and γSp of a measured material. In this case, three liquids (distilled water, diiodomethane, and ethylene glycol) were used to determine γSd andγSp of Mg AZ31B substrates. 2.4 Sample Fabrication The lap-shear specimens were fabricated from 1.6 ×25× 100 mm Mg AZ31B coupons that were cleaned first by acetone and drying. Then, the specimens were prepared as follows: (1) apply the adhesive through a handheld injection gun on one of the two polished adherends; (2) position the adherends with and without dispensed adhesive using a fixture to keep the overlap area of the joints (i.e., 12.5 mm×25 mm); (3) bring
ð1Þ
Where γL is the experimentally determined surface tension of liquid, θ is the contact angle, γS is the surface free energy of solid, and γSL is the solid–liquid interfacial energy. Table 3
Test liquids and their surface tension components [15]
Liquids
Distilled water Diiodomethane Ethylene glycol
Temperature (°C)
23 23 23
Surface Tension (mN/m) γL
γdL
γpL
72.8 50.8 48
21.8 50.8 29
51.0 0 19.0
Fig. 2 Effect of surface polishing on the strength of adhesive-bonded 1.6-mm-thick Mg AZ31B
Int J Adv Manuf Technol Fig. 3 Effect of surface polishing on the fractography of the adhesive-bonded Mg AZ31B sheets. a As-received; b 80; c 240; d 600; e 1200 grit sandpapers
the adherends together by a fixture under ambient laboratory conditions; (4) apply pressure via the fixture to maintain a bondline thickness of 0.25 mm; (5) cure the specimens per supplier’s recommended curing procedure (i.e., 16 h in room temperature and then 20 min at 170 °C). All finished specimens were examined and the spew filets around the edge of the overlap were retained to simulate production conditions.
2.5 Quasi-static testing The cured specimens were kept 24 h at room temperature and then quasi-static tests were performed by loading each specimen to failure in a Zwick Z050 tensile tester according to the standard ASTM D1002-2001 for the determination of the joint strength. To minimize the bending stresses inherent in the testing of single-lap-shear specimens, filler plates (i.e., shims) were attached to both ends of the sample using masking tape to accommodate the sample offset. Load vs. displacement curves were obtained as the specimens were loaded at a stroke rate of 10 mm/min. The joint strength is evaluated by the peak load. Five replicates were performed, and the average peak loads were reported. 2.6 Analytical analysis
Fig. 4 Effect of surface polishing on the percentage of adhesive failure of adhesive-bonded 1.6-mm-thick Mg AZ31B joint
To understand the effect of surface polishing on the joint strength and failure mode, SEM was used to examine if the discrepancies are present in the joint. To determine the relative area of adhesive failure, a self-made software to distinguish the colors of the adhesive and magnesium substrate was employed to analyze the fractography of the adhesivebonded Mg AZ31B.
Int J Adv Manuf Technol Fig. 5 a Sketch of cross-section for the adhesive-bonded joint. b Cross-section of adhesive-bonded as-received 1.6-mm-thick magnesium. c Mg AZ31B polished with 80-grit sandpaper. d Close-up examination of the rectangular region in b. e Closeup examination of the rectangular region in c
3 Results and discussion 3.1 Effect of surface polishing on joint strength and failure mode Lap-shear joints made with and without the polished magnesium substrates were fabricated and mechanically tested, and results are shown in Fig. 2. Error bars in Fig. 2 indicate the standard deviations of results from the quasi-static tests. As shown, the strength of the adhesive-bonded polished Mg AZ31B was significantly weaker than that of the adhesive-bonded unpolished (i.e., as-received) Mg AZ31B. To understand the difference in Table 4 Effect of surface polishing on the surface chemical compositions of 1.6-mm-thick Mg AZ31B
joint strength, the fractography of the tested joints was examined and the results are shown in Fig. 3. Referring to Fig. 3, although the failure modes for the adhesive-bonded joints unpolished and polished Mg AZ31B had a mixed cohesive and adhesive failure mode, more area of adhesive failure mode was observed for the adhesive-bonded polished Mg AZ31B. On the other hand, an increase in sandpaper grit # (i.e., decrease in grit size) resulted in a decrease in strength for the joints made with the polished Mg AZ31B, referring to Fig. 2. Moreover, the relative area of adhesive failure mode was measured and the results are shown in Fig. 4. Referring to Fig. 4, the standard deviations for the relative area of adhesive
Grit Size
Mg
C
O
Al
Na
(#)
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
As-received 80 240 600 1200
66.38 84.69 74.41 81.18 80.81
56.02 77.26 64.17 72.98 72.52
5.34 6.33 8.84 6.97 6.85
9.12 11.69 15.43 12.69 12.45
25.38 6.50 13.59 8.27 8.84
32.55 9.0 17.81 11.30 12.06
2.12 2.48 2.11 2.60 2.39
1.61 2.04 1.64 2.11 1.93
0.78 – 1.05 0.98 1.10
0.7 – 0.95 0.93 1.05
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failure mode are indicated by the error bars. The relative area of adhesive failure increased with the increment of sandpaper grit # (i.e., 80, 240, 600, 1200 grit). To further understand the effect of surface polishing on the joint strength and failure mode, the cross-sections (shown in
Fig. 5a) of the adhesive-bonded joints made with the asreceived and Mg AZ31B polished by 80-grit sandpaper were examined by SEM and the results are shown in Fig. 5b–e. As shown, the bonds between the adhesives/magnesium substrates were compact, and few apparent discrepancies (i.e.,
Fig. 6 a Schematic of examining location on the polished substrate and effect of surface polishing on the surface topography of 1.6-mm-thick AZ31B magnesium alloys polished with various sandpapers with grit #: b As-received, c 80, and d 1200
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pores and cracks) were observed. Based on the results shown in Figs. 2, 3, 4, and 5, it can be deduced that the decrease in strength of the adhesive-bonded Mg AZ31B was not caused by the joint discrepancies. Instead, it was likely attributed to the surface characteristics of Mg AZ31B resulting from surface polishing. To understand the cause of the strength reduction mechanism, the effect of surface polishing on the surface chemical elements and surface morphology of Mg AZ31B was studied and will be discussed next. 3.2 Effect of surface polishing on surface characteristics (1) Surface chemistry Energy-dispersive spectrometer (EDS) was utilized to assess the surface chemistry of Mg AZ31. Table 4 lists the surface chemical compositions of the as-received and polished 1.6-mm-thick Mg AZ31B with various grit sandpapers. As shown, while the content of oxygen of the polished Mg AZ31B significantly decreased, the amount of magnesium element clearly increased and the elements of sodium, carbon, and aluminum changed little. These results suggest that oxide layer was removed from the surface of Mg AZ31B substrate by polishing. Because hydroxyl groups of epoxy resin interacted with magnesium oxide to form the hydrogen bonds that work as a main bonding force for the adhesive [20], the decrease in oxygen content may lead to the reduction in hydrogen bond between the oxide and adhesive L and resulted in a decrease in bond adhesion between the L adhesive and Mg AZ31B. (2) Surface morphology The surface morphology of 1.6-mm-thick Mg AZ31B polished with sandpapers of various grit sizes (i.e., 80, 240, 600, 1200 grit) was examined and the results of the as-received polished magnesium substrates by sandpapers of 80 and 1200 grits are presented in Fig. 6. As shown, surface polishing significantly affected the surface morphology. Many parallel valleys were observed on the surface of the treated Mg AZ31B and the magnitudes of these valleys decreased with an increase in sandpaper grit #. To quantify the effect of surface polishing on the surface morphology of the polished Mg AZ31B, the surface roughness Ra (i.e., the arithmetic average for the absolute values of the vertical deviations of the roughness profile from the mean line) and surface area index [21] (i.e., a ratio of actual total surface area to nominal flat surface area) of the overlap area (i.e., 12.5 mm×25 mm) of magnesium samples were examined and the calculated actual surface areas (i.e., the overlap area multiply by the surface area index) are shown in Fig. 7. Referring to Fig. 7, the standard deviations for the actual surface area and surface roughness are indicated by error bars. As shown,
Fig. 7 Effect of surface polishing on the surface roughness and actual surface area of Mg AZ31B substrate
there was an increase in surface roughness for sandpaper grit # between 0 and 80. Between grits 80# and 1200#, the surface roughness decreased. Examination of the surface roughness indicated that although significant experimental scatter was observed, there appeared a maximum surface roughness for Mg AZ31B substrate polished with grit-80# sandpaper. Different results were observed for the surface area where the actual surface area of the as-received magnesium was significantly greater than that of the polished Mg AZ31B magnesium and the grit size affected little the actual surface area of the polished Mg AZ31B. Careful analyses of the results shown in Table 4 and Fig. 7 revealed that the difference in actual surface area between the as-received and polished Mg AZ31B was likely attributed to the removal of the oxide layer resulting from polishing. To verify if the removal of oxide layer is related to the decrease in actual surface area of Mg AZ31B, the BET-specific surface area (i.e., total surface area of a material per unit of mass) of the as-
Fig. 8 Comparison of the BET-specific surface areas between the unoxidized and oxidized 1.6-mm-thick Mg AZ31B powders
Int J Adv Manuf Technol Table 5 Effect of surface polishing on the contact angle of testing liquids on Mg AZ31B substrate
Grit Size (#)
Distilled water Contact angle (Deg.)
Ethylene glycol Standard deviation
Contact angle (Deg.)
Diiodomethane Standard deviation
Contact angle (Deg.)
Standard deviation
As-received
88.1
1.28
55.8
1.20
43.4
1.82
80 240
79.5 90.2
1.84 0.90
65.0 77.2
3.32 1.12
53.4 45.3
2.49 2.22
600 1200
87.4 86.2
1.86 1.44
75.2 70.8
1.00 2.41
50.4 57.6
3.45 3.77
received (i.e., unoxidized) and oxidized Mg AZ31B were measured. The details of the sample preparation and measurements were described in Section 2. Figure 8 presents the comparison of the BET-specific surface areas of the unoxidized and oxidized 1.6-mmthick Mg AZ31B powders. As shown, the specific surface area of magnesium oxide was significantly greater than that of the unoxidized magnesium powders. These results confirm that the reduction of actual surface area was attributed to the removal of oxide layer resulting from surface polishing. Therefore, surface polishing reduced the actual surface area of Mg AZ31B, and consequently weakened the strength of the adhesive-bonded Mg AZ31B. (3) Surface free energy As the results shown earlier, while surface polishing changed the surface morphology and chemical elements of 1.6-mm-thick Mg AZ31B, it is likely that it would also influence the surface free energy of this material. To examine this, contact angles of three testing liquids on the polished Mg AZ31B were measured and the results are listed in Table 5. The surface free energies of the polished Mg AZ31B substrates were estimated based on these measurements using Eq. (3). Figure 9 presents the effect of surface polishing on the surface free energy of 1.6-mm-thick Mg AZ31B. As shown, there is a decrease in surface free energy for grits # between 0 and 80, while between grit-80# and grit-1200#, the surface free energy decreased slightly. The surface free energy decreased with increasing the grit # in which the dispersion component of the surface free energy decreased whereas the polar component slightly increased. The decrease in surface free energy was apparently related to the reduction of actual surface area (shown in Fig. 7) and variation of surface chemical elements (i.e., shown in Table 4). Careful examinations of the results revealed surface polishing with various grit sizes (i.e., 80, 240, 600, and 1200#) affected slightly the surface elements shown in Table 4 and the decrease in surface free energy resulting from surface polishing may cause by the reduction in actual surface area.
3.3 Correlation between surface characteristics and joint strength To understand further the correlation between the surface characteristics and the strength of the adhesive-bonded Mg AZ31B, the correlation between the surface morphology and surface free energy and joint strength, respectively will be discussed next. 3.3.1 Correlation between surface morphology and joint strength To understand further the relationship between the surface characteristics and joint strength, the effect of surface morphology (i.e., surface roughness and actual surface area) on the strength of the adhesive-bonded Mg AZ31B is discussed next. Figure 10a, b present the effects of surface roughness and actual surface area on the strength of the adhesive-bonded Mg AZ31B, respectively. As shown in Fig. 10a, the joint strength increased with an increase in surface roughness (Ra) and reached a maximum at a surface roughness of 1.094 μm. Between 1.094 to 1.456 μm, the joint strength decreased.
Fig. 9 Effect of surface polishing on the surface free energy of 1.6-mmthick Mg AZ31B
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Fig. 11 Correlation between the surface free energy of Mg AZ31B substrate, strength, and relative area of cohesive failure for the adhesive-bonded Mg AZ31B
Fig. 10 Correlation between the a surface roughness Ra and b surface area of Mg AZ31B substrate and lap-shear strength of adhesive-bonded 1.6-mm-thick magnesium AZ31
These results indicate that surface roughness alone correlated weakly with the strength of the adhesive-bonded Mg AZ31B. Different results between the actual surface area and joint strength were observed in Fig. 10b. As shown by the fitting curve using OriginPro 8.0 [22], a logarithmic function with a variance (R2) value (i.e., red line in Fig. 10b) of 0.879 was observed. These results suggest that the joint strength correlated with the actual contact area better than the surface roughness (Ra) at the overlap of the joint. Similar findings were reported by Zielecki [23]. 3.3.2 Correlation between surface free energy and joint strength As the results shown earlier, although the actual surface area correlated with the joint strength, it hardly captured the role of oxygen content resulting from surface polishing on the joint strength. To address this, the joint strength was correlated with the surface free energy that reflects the surface chemistry and
surface morphology of Mg AZ31B. Figure 11 presents the correlation between the surface free energy and the strength and failure modes of the adhesive-bonded Mg AZ31B. As shown, the joint strength increased with an increase in surface free energy. The increase in joint strength was likely attributed to an increase in the adhesive/adherend bond adhesion resulting from the increase of surface free energy. The relationship between the surface free energy and joint strength shown in Fig. 11 was linearly fitted with a variance R2 of 0.934, which was significantly better than that of the surface area and joint strength (i.e., 0.879). Furthermore, it can be seen from Fig. 11 that the relative area of cohesive failure also increased with an increase in surface free energy of the treated Mg AZ31B. These results suggest that the surface free energy of Mg AZ31B provides a better qualitative index than actual surface area to indicate the strength of the adhesive-bonded Mg AZ31B.
4 Conclusions Experiments were conducted to understand the effect of surface polishing on the static strength of adhesive-bonded 1.6mm-thick Mg AZ31B substrates. 1. Surface polishing significantly decreased the strength of the adhesive-bonded Mg AZ31B. The decrease in joint strength was attributed to the changes in surface morphology and surface chemistry of Mg AZ31B; 2. The changes in surface morphology and surface chemistry of Mg AZ31B by polishing treatment resulted from the removal of surface oxide to decrease the surface area and oxygen content of Mg AZ31B surface, which would likely decrease the surface free energy of Mg AZ31B to weaken the bond adhesion between the adhesive and Mg AZ31B;
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3. The surface free energy provided a better qualitative index than actual surface area to indicate the strength of the adhesive-bonded Mg AZ31B.
10.
11. Acknowledgments This work was funded by General Motors Global Research and Development Center (grant no.: PS21025708). The authors gratefully acknowledge Robert Szymanski for material preparation.
12. 13.
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