Numerical simulation analysis of the temperature field in induction heating-assisted flow drilling of the 7075 aluminum alloy

Flow drill riveting (FDR) is a unique method designed for the joining of disparate materials such as aluminum alloys and carbon fiber-reinforced polymers (CFRPs). The success of FDR hinges on the frictional penetration process executed by the blind rivet. When deploying FDR for the joining of high-strength materials, a substantial penetration force often arises, leading to deformations in the workpieces and potentially causing failure in the joining process. To mitigate the excessive penetration force, the use of induction heating to pre-warm the workpiece before the FDR process has been suggested. This led to the development of the induction heating-assisted flow drill riveting (IHFDR) method. In this work, a FEM model was built for the IHFDR penetration process. This model was specifically designed for the AA7075-T6 sheet, utilizing the ABAQUS software. Induction preheating impact on the temperature of the workpiece during the rivet penetration process was scrutinized. The analysis revealed that the workpiece’s peak temperature during the rivet penetration process was elevated due to induction preheating. Furthermore, it was observed that as the preheating elevated the workpiece temperature, the rate of increase for the peak temperature lessened.


Introduction
To improve the weight of vehicle body reduction, lightweight materials, such as aluminum alloys and CFRPs, are used extensively on the vehicle body.Moreover, closed or semi-closed structural components have been increasingly applied to achieve weight reduction.Flow drill riveting (FDR) is a unique technology that is invented by Wang [1] in 2019 and can be used to realize the joining of lightweight materials (e.g., CFRPs [1], aluminum alloy [2]) and closed or semi-closed components.However, the rivet of FDR relies entirely on the extrusion mechanism to achieve rivet penetration, and the penetration force is always higher, especially for high-strength alloys.A higher penetration force is likely to cause excessive deformation of the connected parts and is a disadvantage to joining.
With the attempt to reduce the penetration force and improve the plasticity of the workpiece, a new joining method, induction heating assisted flow drill riveting (IHFDR) is proposed.As shown in Figure 1, the process of IHFDR includes two stages.In the first stage, when the workpieces are preheated to the target temperature by induction heating, the coil is removed and a blind rivet is simultaneously transferred to the top of the workpieces; in the second stage, a rivet with a high rotation speed penetrates into workpieces, after that, the mandrel of a rivet is pulled out to fasten the workpieces by folding the shank.Several preheating methodologies have been employed in the field of joining technologies.For instance, Raj and Biswas [3] used high-frequency induction heating-assisted friction stir welding (I-FSW) and conventional FSW to combine Inconel 718 plates.They observed that axial force and frictional heat were reduced by preheating, thereby enhancing the tool's lifespan.Campanelli et al. [4] developed and tested a laser-assisted FEW process on AA5754 alloy.Their findings highlighted that laser treatment promoted higher microhardness and decreased longitudinal residual stress within the weld zone.Additionally, Pitschman et al. [5] researched the electrically assisted FSW of AA6061 plates.They discovered that adding an electric current increased welding efficiency while consuming less energy.Moreover, 1.6 mm thick S12C low-carbon steel plates were subjected to high-frequency induction heating-assisted spot friction stir welding by Sun et al. [6].Their results showed a reduction in the frictional heat generated between the rotating tool and the workpiece.Chen et al. [7] integrated an electrical current into the conventional FSSW process for joining the AA6061 to TRIP 780 steel.Their results indicated that this electrical integration enhanced the flow of aluminum material.
The literature review illustrates that the studies examining the impact of preheating on mechanical joining processes have primarily been experimental in nature.In this study, however, the induction heating-assisted flow drilling riveting (IHFDR) process was applied to a 2 mm 7075-T6 aluminum alloy sheet.A FEM model was established to simulate the penetration process of the rivet.Subsequently, the impact of induction preheating on the workpiece's temperature during rivet penetration was studied.

Materials
As shown in Figure 2, a 7075-T6 aluminum alloy plate was used in this study and the thickness of the plate is 2 mm.To avoid the rotation workpiece with the rivet, the sheet was cut into rectangles and the size of the specimen was 38×60 mm 2 .The rivet is manufactured using mild steel, and Table 1 lists the mechanical characteristics of the workpiece and rivet.Thermal expansion coefficient (×10 −6 ) (1/°C) 25.2 4.9

Penetration experiments
As shown in Figure 3, an induction heating coil and an electric spindle were fixed on an industrial robot, and a fixture was fixed on the workbench.The fixture was used to clamp the workpiece and the coil was used to preheat the workpiece before the penetration of the rivet.The rivet was clamped by the fixture on an electric spindle to have a rotation speed and a feed rate during experiments.During induction heating, the width between the coil and the workpiece was 2 mm, and a thermocouple was used to realize the induction heating of the workpiece to the target temperature.The surface temperatures of the workpiece during the penetration of the rivet were measured by a thermal camera (Optris PI450).To ensure the accuracy of temperature measurement, the upper surface of the workpiece was evenly covered with graphite paint, and the emissivity of the workpiece surface was set to 0.9 by a calibrated thermocouple.In this work, the workpiece temperature when the rivet starts to penetrate the workpiece was set to 25, 100, and 200°C.Additionally, the feed rate of the rivet was given as 2 mm/s, and the rotation speed was set to 12000 rpm.

FE model
ABAQUS was selected as the simulation solver in this research.Figure 4(a) demonstrates that the coil structure was simplified under the premise of ensuring the size of the coil end.As depicted in Figure 5, there were two regions on the i.e. the region covered by a coil with a mesh size about 0.05*0.05*0.2mm 3 and the outer region with a mesh size about 0.5*0.5*0.2 mm 3 .When analyzing electromagnetic fields and temperature fields, the grid type was EMC3D8 and DC3D8, respectively.In addition, the properties of the model in induction heating simulation are given in Table 2. Table 2. Material parameters used in the FEM simulation of the induction heating process [11].

Coil
Workpiece Air Electrical conductivity (×10 -8 Ωꞏm) 1.67 5.20 0 relative permeability 1 1 1 As depicted in Figure 6, a 2 mm thick aluminum alloy sheet and a rivet are the only two components of the FE model of the rivet penetration process.To obtain the temperature field generated from the induction heating process, the grid structure and the element type of the workpiece were the same as that of the FE model of the induction heating process.Furthermore, the interface coefficient of friction between the workpiece and rivet was set to 0.3 [12].The model was set up based on taking into account flow behavior using JC constitutive model and the constants are summarized in Table 3.  3. Johnson-Cook material constants for the aluminum alloy AA7075-T6 [13] .

Results and discussion
To demonstrate that the finite element modeling is reliable, the results of the FEM analysis were juxtaposed with actual experimental findings.These comparisons were made under conditions where the workpiece was preheated to 200°C.The simulation results revealed that the workpiece temperature adjacent to the rivet approximated 400°C (Figure 7 Figure 9 illustrates the impact of preheating on the workpiece temperature at the interface between the workpiece and the rivet during rivet penetration.As depicted in Figure 9(a), it is noticeable that when the workpiece's initial temperature is 25°C, the workpiece temperature increases instantly due to frictional heat upon contact with the rivet.However, when the workpiece's initial temperature is elevated to 100°C and 200°C through preheating, the workpiece temperature first rises and then drops as the penetration time extends.This is because there isn't enough frictional heat generated to make up for the heat lost through conduction from the induction heating process to the environment due to the small contact area between the rivet and the workpiece material.
In Figure 9(b), it is shown that the peak temperature of the workpiece during rivet penetration increases proportionally from 340°C to 348°C and 390°C as the workpiece temperature is raised from 25°C to 100°C and 200°C.These results suggest that induction heating can effectively augment the peak temperature of the workpiece during rivet penetration.Furthermore, it can be seen that the increase in the workpiece's peak temperature during rivet penetration is 310°C, 248°C, and 190°C, respectively, when the initial temperature is set at 25°C, 100°C, and 200°C.This is because the decreased strength of the workpiece material with increasing temperature reduces frictional heat generation.

Conclusions
This study introduced a novel mechanical joining technology, the IHFDR, which combines the FDR process and induction heating methodology.A FEM model was created to explore the impact of induction preheating on the evolution of workpiece temperature during the frictional penetration process of the rivet.Simulation outcomes revealed that induction preheating effectively elevated the peak temperature of the workpiece during rivet penetration.However, the softening effect of induction preheating on the workpiece material reduced frictional heat generated during rivet penetration.Consequently, the rise in the peak temperature of the workpiece during rivet penetration decreased as the workpiece's initial temperature, increased by preheating, ascended.The FEM model, thus, proves to be valuable in understanding the progression of workpiece temperature during the IHFDR process.

Figure 1 .
Figure 1.An illustration of the induction heating-assisted flow drill riveting process.

Figure 2 .
Figure 2. Illustration of dimensions of (a) workpiece and (b) rivet (unit in mm).

Figure 4 .
Figure 4. Illustration of FE model of induction heating process: (a) simplified structure of coil, (b) load and boundary conditions of the model.

Figure 5 .
Figure 5. Illustration of the mesh for FE model of induction heating process.

Figure 6 .
Figure 6.Illustration of the mesh for the FE model of the rivet penetration process.

Figure 9 .
Figure 9.Effect of induction heating on workpiece temperature during riveting penetration at the rivet hole: (a) temperature history, (b) the peak temperature of the workpiece.