An investigation of the mechanical properties and bonding mechanism of Ti/Al-laminated composites fabricated by ultrasonic consolidation

Abstract

Foils such as 1100 aluminum and TC4 titanium were used as matrix materials for ultrasonic consolidation test of dissimilar metal materials, and the samples of Ti/Al-laminated composites were prepared. The effect of amplitude and static pressure on the interfacial bonding strength of Ti/Al foil was studied by adhesion test. The mechanical properties of Ti/Al-laminated composites were tested by electronic universal testing machine. The microstructure of Ti/Al foil interface was observed by transmission electron microscope. The results show that ultrasonic consolidation can achieve a good bonding interface of Ti/Al foil, and the bonding strength of the interface increases first and then decreases with the increase of static pressure, and increases monotonously with the increase of amplitude. The optimum adhesion strength is 58.08 N cm⁻¹. The high temperature deformation constitutive model of Ti/Al-laminated composites is established and verified. The Ti/Al interface has metallurgical bonding, and the inner microstructure of Ti/Al matrix is obviously refined. The surface of titanium foil has formed nanocrystalline.

Ultrasonic consolidation technology is a novel method to achieve the low cost and green manufacture of advanced materials and structures ^1,2 . This technology utilizes the high-frequency vibration of ultrasonic wave by the means of layer-by-layer accumulation to achieve the solid metallurgical bonding between layers under the influence of static pressure land elastic vibration energy of the interfaces. The underlying mechanism of such a technology is the diffusion of metal atoms at the interfaces promoted by friction, temperature rise, and so on ³ . Therefore, ultrasonic consolidation owns the characteristics of both ultrasonic welding and laminated material augmentation manufacturing. To date, the fundamental theory of ultrasonic consolidation manufacturing technology is gradually established, and the process is relatively mature ⁴ . As a new methodology for the fabrication of high-performance composites, with great potential in the preparation of metal-laminated composite structure, ⁵ functional gradient material structure, fiber-reinforced metal matrix composite structure, ⁶ intelligent metal composite structure, ⁷ and so on, which enables the material structure prepared by ultrasonic consolidation to play a great role in the high efficient, high performance, and low cost manufacture of advanced materials. ⁸

The lamination and stacked manufacture of laminated metal materials is an important application of ultrasonic consolidation technology. Extensive investigations of theoretical calculation and numerical simulation have been performed regarding the multilayer metal foil physical model and ultrasonic consolidation energy transfer model. ⁹ The material candidate of metal layer for bonding can be stainless steel foil, brass foil, aluminum alloy foil, and nickel alloy foil. Under proper process conditions of ultrasonic consolidation, aluminum foils are able to be bonded with aluminum alloy or stainless steel foils with good interface bonding obtained. ^10,11 The results have shown that approaches of increasing the amplitude, static pressure, and welding temperature and reducing welding speed are beneficial to increase the welding density. ^12,13 However, the ultrasonic consolidation process for titanium foil and aluminum foil is limited studied.

In this article, the interfacial bonding force of ultrasonically consolidated Ti/Al layered composites was obtained by analyzing the mechanical properties and interfacial bonding mechanism of interface of 1100 aluminum foil and TC4 titanium foil fabricated by the ultrasonic consolidation. The constitutive model of Ti/Al-laminated composites was established, and the interfacial bonding mechanism was revealed.

Test materials and methods

The test materials were 1100 aluminum foil and TC4 titanium foil. The thickness values of titanium foil are 100,150, and 200 µm, while those for aluminum foil is 200 µm. Figure 1(a) shows the microstructure morphology of aluminum foil. Lamellar grains with a low dislocation density was observed, which is a typical rolled annealed state microstructure. The microstructure morphology of titanium foil is shown in Figure 1(b). The dislocation density is low, and the grains exhibit along strip-shaped structure. In addition, equiaxed crystals also exist.

Figure 1.

Microstructure observations of raw metal foils: (a) 1100 aluminum and (b) TC4 titanium alloy.

Ti/Al-laminated composites were prepared using the ultrasonic consolidation equipment. The width of ultrasonic processing head is 25 mm, and consolidation speed is 30 mm min⁻¹. The consolidation test temperature was 150°C, and static pressure was 1.0, 1.5, 2.0, and 3.5 kN. The selected amplitude of ultrasonic is 20, 25, 30, and 35 µm. With regard to the foil accumulation, all the foil surfaces were cleaned. An aluminum foil is first consolidated on the substrate followed by a titanium alloy foil attached. The above procedure is repeated until the required thickness of specimen was obtained. Figure 2 shows the microstructure morphology of specimen after ultrasonic consolidation treatment.

Figure 2.

The microstructure morphology of specimen after ultrasonic consolidation.

The peel test was carried out using the Z100 electronic universal material testing machine (Zwick Roell), according to the GJB446-88 standard. The peel length was 30 mm, and the peel speed was 100 mm min⁻¹. Equation (1) was used to calculate the peel strength of the interface, and the influence of ultrasonic amplitude and static pressure on the interfacial bonding force of Ti/Al was studied

$\text{Peel strength}\left(\text{N}/\text{mm}\right)=\frac{\text{Stripping force}\left(\text{N}\right)}{\text{Width}\left(\text{mm}\right)}$

1

Ti/Al laminated was obtained under the preparation conditions of using material combination with thickness TC4/0.15 mm + 1100/0.2 mm at an amplitude 35 µm and a static pressure 2.0 kN. The tensile experiments were carried out using a LETRY-20T electronic universal material experiment machine (LETRY Ltd. China) to measure the mechanical properties of the Ti/Al layered composites. The test temperatures were room temperature, 400°C, 450°C, 500°C, and 550°C, and the strain rates used were 0.1, 0.03, 0.01, and 0.003 s⁻¹. Figure 3 shows the specimen dimension, the gauge length of specimen was 10 mm. The thickness and width of the specimen were 2 and 6 mm, respectively.

Figure 3.

Tensile pattern.

Results and discussion

Effect of ultrasonic consolidation parameters on the peel strength

Figure 4 shows the variation of peel strength of interface with different hydrostatic forces using titanium foils with different thickness. The ultrasonic amplitude was 35 µm. It can be seen that, with the increase of static pressure, the peel strength increases first and then decreases. Relatively larger peel strength was obtained at 1.5 and 2.0 kN. The reason is that the ultrasonic vibration at the machining head effectively functioned at the metal foil interface due to the static pressure. When the static pressure is too low, the ultrasonic vibration energy cannot be effectively transferred to the foil interface, and the ultrasonic energy loss slips on the surface between the machining head and the foil. The friction at the interface is weak resulting in failed bonding. However, when the static pressure is too high, the friction resistance between the foil interfaces becomes too large, which weakens the relative friction motion and decreases the effective amplitude. The best ultrasonic consolidation interface was obtained under the condition of using material thickness combination, TC4/0.15 mm + 1100/0.2 mm, at an ultrasonic amplitude of 35 µm and static force of 2.0 kN. The adhesion peel was 58.08 N cm⁻¹. Figure 5 shows the correlation between the peel strength of interface using titanium alloy foils with different thicknesses under a static pressure of 2.0 kN. It is found that the peel strength increases with the increase of ultrasonic amplitude. Under certain conditions of static pressure, with the increase of ultrasonic amplitude, the vibration amplitude between the foil interface increases, and the friction action intensifies, which is beneficial to the bonding of interface between foils and the improvement of the interface bonding quality.

Figure 4.

Correlation between peel strength and static force.

Figure 5.

Correlation between peel strength and ultrasonic amplitude.

Constitutive model of Ti/Al layered composite fabricated by ultrasonic consolidation

For the subsequent deformation of Ti/Al layered composites prepared by the ultrasonic consolidation, different deformation temperatures and strain rates have significant effects on the mechanical behavior. Simultaneously, the deformation behavior of titanium and aluminum themselves varies significantly with the change of temperature and strain rate. In this article, the deformation behaviors with a temperature range of room temperature, 400–550°C, and strain rate range of 0.003–0.1 s⁻¹ were studied.

For room temperature tensile tests, four different strain rates of 0.1, 0.03, 0.01, and 0.003 s⁻¹ were performed for Ti/Al layered composites. The obtained stress–strain curves are shown in Figure 6. As can be seen in this figure, with the increase of strain rate, the deformation resistance of the material is gradually increased, as well as the hardening. The determined yield strength and tensile strength are increased, and the elongation is gradually reduced.

Figure 6.

Stress–strain curves of uniaxial tensile test at room temperature using different strain rates.

By analyzing the forming conditions of Ti/Al layered composites, four testing temperatures of different strain rates (0.1, 0.03, 0.01, and 0.003 s⁻¹) were designed at 400°C, 450°C, 500°C, and 550°C. The stress–strain curves of different strain rates at a certain temperature were obtained and shown in Figure 7. As can be seen from the curves, for all test temperatures, with the increase of strain rate, the deformation resistance of the material increases gradually. In the meantime, the hardening capacity and elongation also increase gradually.

Figure 7.

Stress–strain curves of Ti/Al composites under different temperatures. (a) 400°C, (b) 450°C, (c) 500°C, and (d) 550°C.

When the Ti/Al layered composite prepared by the ultrasonic consolidation are deformed at high temperatures, the deformation behavior is affected by both temperature and strain rate. For the tensile test conditions, 400–550°C and 0.003–0.1 s⁻¹, there are both strain strengthening and strain rate strengthening. With the further proceeding of deformation, the softening effect also seriously affects the mechanical properties of the material. To more accurately characterize the flow behavior of Ti/Al layered composites at high temperatures, constitutive models are developed in this study. Considering the softening caused by temperature and strain, the softening factor $\text{exp}\left(bT+s\epsilon \right)$ is introduced into the Rosserd equation, so the expression of the constitutive model used is

$\sigma =K{\epsilon }^n{\dot{\epsilon }}^m\text{exp}\left(bT+s\epsilon \right)$

2

where n is the strain strengthening component, m is the strain rate strengthening component, and b and s are the temperature softening coefficient and strain softening coefficient, respectively.

Performing natural logarithms on both sides of equation (2)

$\text{ln}\sigma =\text{ln}K+n\text{ln}\epsilon +m\text{ln}\dot{\epsilon }+bT+s\epsilon$

3

When the temperature and strain rate are constant, $n=\text{dln}\sigma /\text{dln}\epsilon$ , that is, n is the slope of the line lnσ–dlnε. The range of n can be calculated as 0.1014–0.1623, and n = 0.131 by selecting the average value.

When the temperature and strain are fixed, $m=\text{dln}\sigma /\text{dln}\dot{\epsilon }$ , that is, m is the slope of the line $\text{lnσ}$ – $\text{ln}\dot{\epsilon }$ , as shown in Figure 8.

Figure 8.

Correlations between $\text{lnσ}$ and $\text{ln}\dot{\epsilon }$ at different temperatures. (a) 400°C, (b) 450°C, (c) 500°C, and (d) 550°C.

The calculated range of m is 0.0222∼0.0356. Selecting the average value, m = 0.131.

When the strain and strain rate are certain, b = dlnσ/dT, that is, b is the slope of the line lnσ–T, as shown in Figure 9.

Figure 9.

Correlations between lnσ and $\text{ln}\dot{\epsilon }$ at different strain rates. (a) 0.003 s⁻¹, (b) 0.01 s⁻¹, (c) 0.03 s⁻¹, and (d) 0.1 s⁻¹.

The calculated range of b calculated is between −0.00191 and 0.00163. After selecting the average value, b = −0.00177. When the temperature and strain rate are fixed, according to equation (3), $\text{ln}K$ + $m\text{ln}\dot{\epsilon }$ + $bT$ is a constant and substituted as K1, we can get

$\text{ln}\sigma =K1+n\text{ln}\epsilon +s\epsilon$

4

By introducing different strain values and establishing an equation set, the below equation can be obtained

$S=\frac{\text{ln}\frac{{\sigma }_1}{{\sigma }_2}-n\text{ln}\frac{{\epsilon }_1}{{\epsilon }_2}}{{\epsilon }_1-{\epsilon }_2}$

5

The calculated average value of S is −0.21175. The obtained n, m, b, s, and the stress and strain of the material under different conditions are substituted into equation (2), and the determined average value of K is 2351.5. Therefore, the constitutive equation obtained is given as follows

$\sigma =2351.5{\epsilon }^{0.131}{\dot{\epsilon }}^{0.0302}\text{exp}\left(-0.00177T-0.21175\epsilon \right)$

6

To quantitatively characterize the prediction accuracy of the established constitutive model, the correlation coefficient R between the predicted stress and the experimental stress, as well as the absolute value Average Absolute Relative Error (AARE) of the average error are calculated as follows

$R=\frac{{{\displaystyle \sum }}_{i=1}^N\left({\sigma }_{\text{E}}-{\overline{\sigma }}_{\text{E}}\right)\left({\sigma }_{\text{P}}-{\overline{\sigma }}_{\text{P}}\right)}{\sqrt{{{\displaystyle \sum }}_{i=1}^N{\left({\sigma }_{\text{E}}-{\overline{\sigma }}_{\text{E}}\right)}^2{{\displaystyle \sum }}_{i=1}^N{\left({\sigma }_{\text{P}}-{\overline{\sigma }}_{\text{P}}\right)}^2}}$

7

$\text{AARE}\left(\text{\%}\right)=\frac{1}{N}{\displaystyle \sum }_{i=1}^N|\frac{{\sigma }_{\text{E}}-{\sigma }_{\text{P}}}{{\sigma }_{\text{E}}}|$

8

where ${\sigma }_{\text{E}}$ and ${\sigma }_{\text{P}}$ denote as the experimental stress and the predicted stress, respectively; ${\overline{\sigma }}_{\text{E}}$ and ${\overline{\sigma }}_{\text{P}}$ denote as the average of the experimental stress and the average of the predicted stress, respectively; and N is the number of substitution data.

The results obtained from equations (7) and (8) are shown in Figure 10, where R = 0.9734 and AARE = 4.9%. The correlation coefficient R indicates the linear relationship between the experimental stress and the predicted stress, and the closer the value R to 1, the more accurate the predicted stress. The absolute value AARE of the average error is a statistical parameter, the smaller the AARE, the smaller the error of the predicted stress. Therefore, the calculated model can more accurately predict the deformation behavior of Ti/Al-laminated composites prepared by ultrasonic consolidation at high temperatures.

Figure 10.

The comparisons between predicted stress and experimentally determined stress.

Analysis of the bonding mechanism of ultrasonic consolidation interface

After performing ultrasonic consolidation, the microstructure of aluminum foil is shown in Figure 11. As shown in this figure, the material occurred obviously refinement after ultrasonic consolidation, which is due to the severe microplastic deformation of the material processed by the ultrasonic machining head. The fracture within the grains occurs exhibiting as the strip-like grain is broken directly into several segments to form several grains. In the meantime, recrystallisation can be observed in Figure 12, which is due to the large number of dislocations accumulated in the material due to the severe plastic deformation and the formation of subgrains. With the proceeding of deformation, the recrystallized grains are finally formed. Meanwhile, the dislocation density inside the material decreases. Therefore, the microstructure of the material after ultrasonic consolidation enables to refine the initial grain. There are two main mechanisms of grain refinement. One is that the lath grain is directly broken resulting from ultrasonic vibration energy to form a plurality of long strip size smaller grains; the other is that under the action of ultrasonic vibration friction, a large number of dislocations are produced inside the grain, and the dislocation piling-up is formed inside the grain to form subgrain boundaries, resulting in the occurrence of recrystallization.

Figure 11.

The microstructure morphology of aluminum foil using ultrasonic consolidation.

Figure 12.

The microstructure of recrystallisation.

After ultrasonic consolidation treatment, the obtained microstructure of Ti foil is shown in Figure 13. A large number of dislocations are generated inside the material, which is due to the severe microplastic deformation within the material due to induced dislocations inside the grain arising from the micro-vibrations.

Figure 13.

The dislocation morphology of titanium foil.

Meanwhile, after ultrasonic consolidation treatment, nanograins appear inside the material. As shown in Figure 14, under the condition of severe plastic deformation, the metal grains are broken to form tiny grains with the scale reaching the nanometer level. The diffraction pattern shows a polycrystalline diffraction ring. At this time, there is a high distortion energy inside the grains, and the grain orientation is highly disordered. Therefore, obvious grain boundaries are unable to be identified in the microstructure morphology of the metal.

Figure 14.

The microstructure morphology after ultrasonic consolidation. (a) microstructure of nanograins (b) Diffraction pattern

Conclusion

1. The interface of the Ti/Al-laminated composite is flat and uniform, and the quality of the interface bonding is good. The interfacial bonding strength increases first and then decreases with the increase of static force. The bonding strength increases monotonically with the increase of amplitude. Under the conditions using the amplitude of 35 µm and the static force of 2.0 kN, the optimal ultrasonic consolidation interface is obtained with a peel strength of 58.08 N cm⁻¹.

2. With the increase of strain rate, the deformation resistance of Ti/Al composite increases gradually, and the hardening increases gradually also. The yield strength and ultimate tensile strength increase, while the elongation decreases gradually, a high temperature constitutive model of Ti/Al-laminated composites is established: $\text{σ}=2351.5{\epsilon }^{0.131}{\dot{\epsilon }}^{0.0302}\text{exp}\left(-0.00177T-0.21175\epsilon \right)$ .

3. Under the influence of ultrasonic vibration, the ribbon-shaped grains in aluminum foil are directly disconnected to form a plurality of strip-sized smaller grains, while a large number of dislocations are produced inside some grains. The formed dislocation piling-up inside the grains results in subgrains and recrystallisation subsequently. Nanograins are formed in the microstructure of titanium foil.