Xiaolan Han

Experimental Investigation on the Joining of Aluminum Alloy Sheets Using Improved Clinching Process

Aluminum alloy sheets have been widely used to build the thin-walled structures by mechanical clinching technology in recent years. However, there is an exterior protrusion located on the lower sheet and a pit on the upper sheet, which may restrict the application of the clinching technology in visible areas. In the present study, an improved clinched joint used to join aluminum alloy sheets was investigated by experimental method. The improved clinching process used for joining aluminum alloy evolves through four phases: (a) localized deformation; (b) drawing; (c) backward extrusion; and (d) mechanical interlock forming. A flat surface can be produced using the improved clinching process. Shearing strength, tensile strength, material flow, main geometrical parameters, and failure mode of the improved clinched joint were investigated. The sheet material was compressed to flow radially and upward using a punch, which generated a mechanical interlock by producing severe localized plastic deformation. The neck thickness and interlock of the improved clinched joint were increased by increasing the forming force, which also contributed to increase the strength of the clinched joint. The improved clinched joint can get high shearing strength and tensile strength. Three main failure modes were observed in the failure process, which were neck fracture mode, button separation mode, and mixed failure mode. The improved clinched joint has better joining quality to join aluminum alloy sheets on the thin-walled structures.

Comparative investigation of auxiliary processes for increasing the strength of clinched joints

Mechanical clinching has been widely used in the field of automotive industry in recent years. However, the clinched joint has a lower static strength than spot welding. In order to increase the static strength, a comparative investigation of two auxiliary processes for increasing the strength of clinched joints was carried out. One auxiliary process needs a rivet, while another auxiliary process needs a bumped die. Tension-shearing strength tests and cross-tensile strength tests were conducted to assess the static strengths of the joints after different auxiliary processes. Failure mode of all the joints was neck fracture in the tension-shearing strength tests and cross-tensile strength tests. Geometrical parameters of the joint profile were also investigated by comparing the two auxiliary processes. The values of energy absorption of different joints were obtained by measuring the areas between the force–displacement curve and x-coordinate. The two auxiliary processes were proved to be effective. The auxiliary process with a rivet has a better performance than the auxiliary process with a bumped die.

Influence of sheet thickness on mechanical clinch–compress joining technology:

A mechanical clinch–compress joining technology was investigated to join sheets with different thicknesses in the present study. A pair of flat dies was used to compress the clinched joint, and a rivet was used to increase the joining strength. Al5052 sheets with different thicknesses were used to conduct the joining experiments. Tensile strength test and shearing strength test were conducted to evaluate the quality of the compressed joint. Failure mode, feasibility of the joining method, neck thickness, tensile strength, shearing strength, and energy absorption were investigated to show the mechanical properties of the compressed joint. The main failure mode of the joints is neck fracture mode, which means that the neck thickness determines the strength in this study. The mechanical clinch–compress joining technology can increase the strength and energy absorption of the joint by increasing the neck thickness. The joint with a thick upper sheet also can get higher strength and energy absorption by producing a thicker neck thickness. The mechanical clinch–compress joining technology can be used in the visible areas where higher strength and lower protrusion are needed.

Optimization of geometrical design of clinching tools in flat-clinching

Owing to the importance of geometrical design of clinching tools, the flat-clinching were investigated numerically and experimentally to determine the optimal geometrical parameters in this study. The geometrical parameters in flat-clinching, including punch corner radius, draft angle of the punch, blank holder corner radius, blank holder radius, and punch velocity, were optimized using the orthogonal experimental design simulation method based on the evaluation of interlocking. The simulation results show that the punch corner radius and the blank holder radius have a significant impact on the clinched joint, whereas the draft angle of the punch and blank holder corner radius have little influence on the clinched joint. The simulation results were validated through an experimental setup testing on material aluminum alloy Al5052. The experimental results have a good agreement with the orthogonal experimental design simulation.

Optimization of geometrical design of clinching tools in clinching process with extensible dies

Due to the importance of geometrical design of clinching tools, the clinching process with extensible dies was investigated numerically and experimentally to seek for optimal parameters of clinching tools in this study. The joining parameters, including punch corner radius, sliding distance, die depth and bottom thickness, were optimized using the orthogonal experimental design simulation method based on the evaluation of tensile strength. The simulation results were validated through an experimental setup testing on material aluminum alloy Al5052. The orthogonal experimental design simulation results showed reasonably good agreement with the experimental results. To further investigate the validation of the simulation model, the different bottom thicknesses within a reasonable range of value were studied. The results also indicated that the simulation model could be employed to predict the joint forming by the clinching process with extensible dies.

Experimental research on the compressed joints with different geometrical parameters

In recent years, mechanical clinching has been widely used to join aluminium alloy sheets to build the automotive body. The high protrusion on the joint may limit the use of the mechanical clinching in the visible places. In order to get a lower protrusion, a compressing method was investigated in this work. Clinched joints with different geometrical parameters were used to conduct the experiments. AL5052 was taken as the material of the sheets. The protrusion of the clinched joint was compressed by two flat dies. A rivet placed in the pit of the clinched joint was used to control the metal flow in the compressing process. The top die moved downward to compress the protrusion, and the bottom die was fixed. The study shows that the tension-shearing strength and cross-tensile strength can be increased by the compressing method. The compressing method can increase the tension-shearing strength and cross-tensile strength by increasing the neck thickness. The compressed joint has larger neck thickness and lower protrusion than the clinched joint. Neck fracture mode is the main failure mode of the joints. The energy absorption can also be increased after the compressing process.

Investigation of Flat Clinching Process Combined with Material Forming Technology for Aluminum Alloy

In recent years, the use of aluminum alloy has tended to increase for building lightweight automobiles to reduce their automotive weight, which is helpful to save energy and protect the environment. In order to join aluminum alloy, a flat-clinching process combined with material forming technology was investigated to join aluminum alloy sheets using an experimental and a numerical method. Al1060 was chosen as the material of the sheet, and DEFORM-2D software was used to build the numerical model. After the numerical model was validated by the experimental results, the influences of punch diameter and holder force on the materials deforming behavior of the clinched joint were analyzed using the numerical model. Then, the material flow, joining ability, and joining quality were investigated to assess the clinched joint. The results showed that an increase in punch diameter could give rise to an increase in neck thickness and interlocking length, while an increase in blank holder force induced a decrease in interlocking length and an increase in neck thickness. The joining quality could be increased by increasing the forming force. It can be concluded that a clinched joint has better joining quality for joining light-weight sheets onto automotive structures.

Microstructural evolution of AlCu2.4 alloy prepared by electromagnetic stirring

The effects of stirring current and stirring frequency on the microstructure of semisolid AlCu2.4 alloy prepared by electromagnetic stirring (EMS) were investigated. The microstructural evolution of the partial remelting semi-solid AlCu2.4 alloy under different stirring currents, stirring frequencies, and isothermal holding time was also investigated. Results showed that with increased stirring current and frequency, the microstructures of semisolid AlCu2.4 alloy evolved from coarse dendrites to fine, small, and quasi-globular solid particles uniformly distributed among the eutectic. Moreover, after partial remelting, these microstructures became smaller and more spherical and were distributed more evenly with increased stirring current and stirring frequency. When the stirring parameters were 30 A and 30 Hz, the average grain size was approximately 80 μm and the shape factor was about 0.76. During partial remelting, both average particle size and the degree of spheroidization constantly increased with prolonged holding time because of Ostwald ripening. The current coarsening rate constant of the semisolid AlCu2.4 alloy prepared by EMS was 567 μm3s−1 at 590◦C. Comparison between the present work and other literature results indicated that the coarsening rate was associated with the microstructure prior to partial remelting, as well as with the alloy composition.