Investigations of Join-Ability and Energy Absorption of Clinched Joints in Titanium and Aluminum-Lithium Sheet Materials
2016 年 57 巻 10 号 p. 1849-1852
詳細
2016 年 57 巻 10 号 p. 1849-1852
The join-ability and energy absorption of extensible die clinched joints in titanium and aluminum-lithium sheet materials were investigated in this paper. Tensile-shear tests were carried out to characterize the mechanical properties of different clinched joints made of the similar and dissimilar alloy sheets combinations. The quality assessment, load-bearing capacity and energy absorption of different clinched joints were studied. Results showed that the load-bearing capacity and energy absorption of clinched joints with titanium as upper sheets are higher than that of the clinched joints with aluminum-lithium as upper sheets.
The demand for lightweight structures has resulted in a significant increase in the use of lightweight alloys. Normally these lightweight alloys are difficult to weld with conventional spot welding. Considerable effort has gone into developing new joining processes suitable for use with lightweight alloys1–3). Mechanical clinching has also been used in many industrial sectors including aerospace and automotive4,5). Clinching is a joining method in which sheet alloy parts are deformed locally without the use of any additional elements. The mechanical property of clinched joints has been the subject of a great amount of numerical and experimental studies.
The join-ability of different rigid thermoplastic polymers with aluminum alloy sheets by mechanical clinching has been investigated by Lambiase et al.6,7) In a recent study, a numerical model describing the evolution of ductile damage was developed to predict the onset of fracture during the clinch joining of thin aluminum sheets8). The effect of corrosion phenomena in critical environmental conditions on the mechanical performance of steel/aluminum hybrid clinched joints has been studied by ageing in salt spray environment9). Mucha and Witkowski10) presented the strength analysis of clinched joints with various values of bottom thickness. The maximum joint strength was obtained by performing lap joints, H-shaped samples and T-shaped samples strength test. The influence of the clinching process parameters on the join-ability of high-strength steel was studied using finite element method (FEM)11). Mechanical performances for clinching similar and dissimilar titanium, aluminum and copper alloys have been investigated in a recent study12).
Aluminum-lithium alloy sheets are being increasingly utilized in aerospace engineering. The use of aluminum-lithium alloy sheets brings notable benefits resulting from their high strength and corrosion resistance, but also creates joining problems. Mechanical clinching method can be used to join a wide range of materials, but no investigation on such clinched aluminum-lithium alloy joints has been reported so far. Present paper reports the join-ability and energy absorption of extensible die clinched joints in titanium and aluminum-lithium sheet materials. Force-displacement curves were obtained to characterize the tensile-shear properties of the joints. The load-bearing capacity, energy absorption and failure modes of different clinched joints were studied.
Configuration and boundary condition of a single lap clinched joint are shown in Fig. 1. The titanium TA1 and aluminum-lithium 1420 sheet materials tested were of dimensions 110 mm length × 20 mm width and were joined together in the central part of lap section. Table 1 is used for describing the single lap clinched joints with different sheet material combinations.
Configuration and boundary condition of a single lap clinched joint (dimensions in mm).
| Clinched joints | TA1-TA1 joint | TA1-1420 joint | 1420-TA1 joint | 1420-1420 joint |
|---|---|---|---|---|
| Upper sheet | TA1 | TA1 | 1420 | 1420 |
| Lower sheet | TA1 | 1420 | TA1 | 1420 |
The strengths of the clinched joints are dependent on the process of clinching in which both upper and lower sheets undergo plastic deformation and resilience to form a mechanical connection. During the clinching process, the upper sheet undergoes a significant thinning near the punch corner radius. The typical failure modes of clinched joints are button separation mode and neck fracture mode. The button separation mode normally results from small undercut, and the neck fracture mode normally results from small neck thickness. Thus the strength of an extensible die clinched joint depends on the joint profile, particularly on the neck thickness (tN) and the magnitude of the produced undercut (tU) as shown in Fig. 2(a). The bottom thickness (X) will also affects the strength of clinched joints but is not as important as the neck thickness and the undercut.
Quality assessment of different clinched joints (dimensions in mm).
Typical cross-sections of different clinched joints are shown in Fig. 2. It can be seen that for the similar metal sheets clinched joints, both TA1-TA1 joint and 1420-1420 joint are good clinched joints. The neck thickness tN of 1420-1420 joint is bigger than that of TA1-TA1 joint whereas the undercut tU of TA1-TA1 joint is bigger than that of 1420-1420 joint. For dissimilar metal sheets combination of titanium TA1 and aluminum-lithium 1420 alloy, it is clear that TA1-1420 joint can be a good clinched joint which has satisfied undercut but the connection cannot be formed well in 1420-TA1 joint as the undercut is normally small. That means when clinching the dissimilar metal sheets combination of titanium TA1 and aluminum-lithium 1420 alloy, titanium TA1 sheets should be used as the upper sheets.
A servo-hydraulic testing machine was used to carry out tensile-shear tests of different clinched joints. As shown in Fig. 1, two spacers with the sheet thickness were used on both ends of the joints to reduce the influence of additional bending on the joints during the tests. A quasi-static downward displacement load was applied to the lower end and the upper end of each of the joints was fixed. Ten samples were mechanically tested for each type of clinched joints. Continuous records of the applied force-displacement curves were obtained during each test.
The failure modes of different failed clinched joints are shown in Fig. 3. It is obvious that for TA1-TA1 joints, 1420-TA1 joints and 1420-1420 joints, all specimens were failed with the neck fracture mode. In TA1-1420 joints, however, four joints were failed with neck fracture mode and six joints were failed with button separation mode. In the tensile-shear load condition, the neck of upper sheet bear a main shear load by geometrical interlocking. When the shear stress reaches the yield criterion of upper sheet material, a crack is initiated from the interfacial surface of the upper sheet and grows into the upper sheet thickness. Finally the inner button is sheared off at the neck. The button separation mode in TA1-1420 joints could be attributed to relative small yield strength of lower sheet material.
Failure modes of different clinched joints.
Figure 4 shows the force-displacement curves of different clinched joints with fracture processes. For easy comparison, all curves were drawn using the same coordinate scales. It is noteworthy from Fig. 4 that the load-bearing capacity of clinch joints with titanium TA1 as upper sheets is much higher than that of the clinched joints with aluminum-lithium 1420 as upper sheets. It can be seen from Fig. 4 that in the cases of the clinch joints failed with neck fracture mode (Fig. 4(a), (c) and (d)), the force suddenly drops after the peak. In the case of the clinched joints failed with button separation mode, however, after the peak the force decreases gradually, as shown in Fig. 4(b). It is also interesting to note that the load-bearing capacities of 1420-1420 joints are much lower than those of TA1-TA1 joints and TA1-1420 joints though the neck thickness tN of 1420-1420 joint is bigger than those of TA1-TA1 joint and TA1-1420 joint as shown in Fig. 2. This could be attributed to relative small yield strength of aluminum-lithium 1420 sheet material. It can be concluded that the load-bearing capacity of clinch joint mainly depends on the mechanical property of upper sheet material.
Force-displacement curves of different clinched joints with fracture processes.
It is clear from Fig. 4 that in the cases of the clinch joints with TA1 titanium as upper sheets, especially TA1-1420 joints, the maximum loads are shown at higher deformation levels, and the failure processes involve much higher energy absorption. Figure 5 shows average energy absorption of different clinched joints. It is clear that in any case, the clinch joints with TA1 titanium as upper sheets give better performance.
Average energy absorption of different clinched joints.
This paper deals with the mechanical properties of extensible die clinched joints in titanium and aluminum-lithium alloy sheet materials. Load-displacement curves were obtained for characterizing the mechanical properties of different clinched joints. The join-ability, load-bearing capacity, failure modes and energy absorption of joints were studied. The major findings of this research can be summarized as follows:
(1) The similar sheet materials and the dissimilar sheet material combinations of titanium TA1 and aluminum-lithium 1420 all can form mechanical joints by clinching.
(2) The load-bearing capacity of clinch joints with titanium TA1 as upper sheets is higher than that of the clinched joints with aluminum-lithium 1420 as upper sheets.
(3) In the cases of the clinch joints with titanium TA1 as upper sheets, the failure processes involve much higher energy absorption.
This study is supported by the National Natural Science Foundation of China (Grant No. 51565023) and Major Program Foundation of the Education Department of Yunnan Province, China (Grant No. ZD201504).
