Lightweight materials such as magnesium alloys, aluminum alloys, titanium alloys, etc. are widely adopted in manufacturing industries. These lightweight materials have almost replaced the use of steel in car bodies [1–4]. The connection methods of these materials have been studied by many researchers. Clinching technologies is a joining method which can join the sheet materials without any auxiliary part. It was introduced by the German patent issued in 1897. At that time, the development of the clinching technology was limited by the widespread use of steel sheets. Clinching technologies was firstly widely used in the manufacturing industries in 1980s because the lightweight alloys were widely adopted in manufacturing. Clinching technologies was used at Audi Company in 1985 [5]. Subsequently, more and more manufacturer used clinching technology to join body sheet materials. Recently, clinching technology was widely employed to join parts of the car body [6–8].
Clinching technologies join metal sheets by creating an interlock, which is formed by local cold deformation between the sheets. The tools for the clinching process are simple and do not need auxiliary parts, such as bolts and rivets [9]. However, the strength of traditional clinched joints is not high, which is also the main drawback hindering the application of clinching technology. Many researchers have researched the clinching process, such as deformation mechanism, tool parameters, failure mode, punch force, and so on, to refine the strengths of clinched joints. Lee et al. [10] designed tools of the clinching process for joining the Al6063 alloy sheet materials to refine the quality of clinched joints. He et al. [11, 12] investigated the clinched joint strength and energy absorption with extensible dies by numerical and experimental method. The authors found that the strength and elongation rate of the joints can be increased by the adhesive layer. Lambiase et al. [13, 14] studied the effects of parameters of clinching process on the joint strength, and optimize the clinching tools by numerical and artificial intelligence methods. The study proposed the undercut can be increased by reducing the diameter of the bottom die, and the thickness of joint neck can be improved by the larger punch and die, smoother corner radius and shallow dies.
Furthermore, due to the special requirements of industry for the surface of clinch joints, many researchers have innovated the clinching process and clinching tools, resulting in higher strength and lower protuberance. Wen et al. [9] reduced the protuberance of the joints and increased the strength of the joints by a reshaping process, which involves compressing the clinched joints with a pair of reshaping tools. The results of their studies found that the protuberance of the joint can reduce dramatically, and the static strength of reshaped joints has better performance than clinched joints. The protuberance height can be decreased by a new reshaping process using a cambered die and flat anvil (or two flat anvil), which was proposed by Chen et al. [15, 16]. The neck thickness and the interlock of joints can be refined by this process since the materials of the protuberance flows to the around the neck. Moreover, Chen et al. [17–21] also proposed methods of reshaping joints with and without a rivet. The heave of joints was reduced, and the strength of joints was all increased by these reshaping methods.
However, these reshaping methods above can only reduce the protuberance of joints to a certain extent, but cannot make it completely flat. In this paper, a new reshaping method, named two-strokes flattening clinching (TFC) process, was proposed. The TFC method not only increases joint strength but also results in a significant reduction in protuberance of joint. Al1060 sheet materials were used for experimental test. The influence of forming forces on TFC process was investigated. Furthermore, the mechanical properties, including tensile-shear strength, failure modes, material flow and energy absorption, were compared between conventional and TFC clinched joints. From results of the test, the protuberance of joints was flattened, and the strengths and the neck thicknesses of the joints were improved. The tension-shear strength of TFC joints is increased by 30.3% compared with conventional clinched joints at the forming force of 30 kN. The failure modes of joints were all neck fracture. The results also showed that the energy absorption of the TFC joints is better than that of conventional clinched joints.