FEM simulation on forming vacuum hexagon socket screws using tube workpiece

This study proposes a new forming design concept to produce vacuum hexagon socket screws using tube workpiece, and uses the finite element simulation software DEFORM-3D to simulate the multi-pass forming process of vacuum hexagon socket screws. The simulated material is SUS-304 stainless steel, and the frictional conditions are assumed to be constant shear friction, the die is assumed to be a rigid body. In the past, the traditional vacuum hexagon socket screw manufacturing process used a multi-pass process to manufacture a standard screw using a solid workpiece (wire rod), and then drilling an air channel in the core of the screw. This process not only wastes time, but also needs to increase the processing cost. In this study, the time and cost of drilling the air channel can be reduced by forming the vacuum hexagon socket head screw through the hollow tube. The finite element simulation results confirm that this design is a feasible method. The pass schedule can be reduced from 6 stages to 3 stages, the maximum effective stress decreased by 28.13%, and the total forming force decreased by 57.66%. The research results can be provided to relevant industry as the reference.


Introduction
The Fasteners are the most important key mechanical components in the industry.The amount of fasteners used in a country is often regarded as the core indicator of industrialization.The definition of fasteners is to fasten two or more parts together.Common fastening methods: thread locking, riveting fastening, metal expansion locking, etc., are currently the most convenient and effective important components.The vacuum screws are mainly used in vacuum equipment or environments, and their principle is very simple, as shown in Figure 1.The main purpose is to pass the core of the screw through the hole.When the equipment or environment is vacuumed, the hole in the core of the screw can discharge the residual air from the teeth and blind holes.At present, vacuum screws are mostly used in semiconductor production line equipment, such as wafer factories, display factories, IC packaging factories and other vacuum process equipment or robotic arms.Automobiles, unmanned aerial vehicles, aerospace vehicles, national defense weapons etc. have been increased year by year, which has led to an increasing demand for semiconductors in the international market.Therefore, the application potential of vacuum series screws is limitless.Since vacuum screws are mostly used in clean rooms or vacuum equipment, the requirements for production lines are relatively strict, so corrosion-resistant and rust-resistant stainless steel is the first choice for material selection.Currently, the usage of SUS-304 stainless steel is the most bulk.Figure 2 is a photo of the SUS-304 stainless steel vacuum screw entity.Figure 2. Photo of the vacuum hexagon socket screw.Chen et al. [1] proposed a new single-step injection forging as an alternative way to the traditional multipass forging process for manufacturing automotive fasteners, and used finite element simulation software to simulate the forming process of vehicle parts to improve production efficiency, and raise die life as well as reduce wear resistance.Dubois et al. [2] proposed contact conditions for simulating the cold forming process, including two experiments of drawing and extrusion forming.The frictional conditions of each basic forging operation were analysed according to the lubrication parameters (lubricant: phosphate, drawing powder and cold heading oil, etc.) to predict the geometric changes of forged products related to the frictional conditions.Lee et al. [3] used DEFORM-2D finite element simulation software and experimental verification to realize the size difference between forging dies and forging parts.The major variation was influence of elastic deflection of dies and spring back of forged parts.The experimental results were in a good agreement.Doege et al. [4] explored different closed-die forging systems for the forming and manufacturing of spur gears and helical gears.The main purpose was to manufacture burr-free forgings.The research developed a new alternative closing device for forging dies to improve the shortcomings of the existing process.Dinger [5] adopted 3D finite element analysis of the thread forming process using the ABAQUS/Explicit software program.The thread forming screw assembly process was evaluated by numerical simulation and experimental methods.Pandya et al. [6] proposed that the use of finite element simulation could effectively design the forming conditions of the preformed pin during the hot forging process, which can minimize the forming load and equivalent stress to improve the life of the die, and the experimental verification was consistent with the finite element simulation results.Zhao et al. [7] studied various parameter, such as plate thickness, spacing, and connection length by means of finite element simulation applied to the shear resistance of the stainless steel screw connections, and the results were in good agreement experimental verification.Fernandes et al. [8] developed a new forming process for pipe fittings with gaskets, and the finite element method and boundary element (BE) method were combined to simulate to verify its feasibility.Perform discretization was avoided to improve final simulation performance.Gontarz et al. [9] utilized the finite volume method (FVM) and the finite element method (FEM) to simulate the head forging forming of the nail head forming process, and the results were verified experimentally.Through the comparisons of force and flow kinematics, the method could be proved to be feasible.Balendra et al. [10] proposed Injection forging was used to form complex component shapes.In this study, different forming experimental conditions were applied to the tube.And finite element simulation has been to verify its feasibility.Asnafi et al. [11] explores the fastener forming using the finite element simulation and experiment to verify the die stress analysis, and the research results were consistent with the finite element simulation.Qin et al. [12] utilized the finite element simulation to evaluate the situation of friction-related pressure loss during injection forging of solid and tubular parts, and several lubrication procedures were tested to find the optimal lubrication conditions.Lowering forming load and improving material flow were verified.Petrescu et al. [13] said that FEM's FORGE-2 software has been used to simulate the cold forging process of fastener steel to predict the failure conditions, and the simulation results have been verified by experiments as a feasible method.Rajiev et al. [14] used the DEFORM finite element simulation software to simulate the hot forging of pan head bolts.The simulation results could obtain the metal flow, die filling, contact pressure distribution and die temperature distribution.Moreover, the experimental research was carried out to compare the finite element simulation, the results were consistent with the experimental results.Chen et al. [15] proposed to use finite element simulation software to simulate and compare the multi-pass forming and injection forging of vehicle fasteners, and to verify with experiments to find out the best forming conditions for reducing the forming force and improving the dimensional accuracy of the parts.The research results of the foregoing references prove that the finite element method (FEM) is beneficial to the development and design of the forming die, and can also greatly reduce its time and cost.

Finite element simulations 2.1. Traditional process of vacuum hexagon socket screw
The pass schedule for traditional process of vacuum hexagon screw were planned to 6 stages shown in Figure 3.The solid wire rod of SUS-304 is adopted as workpiece.According to the pass schedule, the multi-pass simulations can be performed using Deform-3D commercial software, the simulation results are able to summarize as shown in Table 1.

477ton
The pass 3 process is to form the hexagon hole in the screw head, especially depicts the effective stress and the effective strain in Figure 4.The maximum effective stress is 1600 MPa and the maximum effective strain is 7.82 mm/mm along the bottom of hexagon hole.It is noted that the total forming force is 2310 kN in Table 1.

New forming design of vacuum hexagon socket screw
With a view to reducing the pass numbers and the drilling cost, the tube workpiece of SUS-304 is used to do a new forming design.Figure 5  The vacuum socket screw is made of SUS-304.Stainless steel has high strength and high toughness mechanical properties.It is easy to generate high forming force and die stress during forming, which has a great impact on the wear life of die.Therefore, the new forming design develops three stages forming, as shown in Figure 5: The first stage (pass-1): The upper end of the tube workpiece is forged and preformed.
The second stage (pass-2): Preform a hole on the head to facilitate the next process.
The third stage (pass-3): Forge out the hexagon socket hole, and the hexagon socket screw can be manufactured.
Table 2 demonstrates the DEFORM-3D simulation conditions.The forging material is set to be stainless steel SUS-304, the number meshed is 100,000, the Young's modulus is 210GPa, and the Poisson's ratio is 0.3.The die is assumed to be a rigid body, the punching speed is 100mm/sec, the cold forging is adopted, the constant shear frictional factor is 0.12, the mesh is partially densified in the third pass by 0.3, and the damage condition of Normalized C&L is 0.8.

Simulation results
A series of simulation results are depicted from Figure 6 to Figure 8.The simulation results of threepass forming of vacuum socket screw are as follows: The first pass is mainly the head forming process, the maximum effective stress is 1130MPa, and the maximum effective strain is 1.93 mm/mm.The maximum value of the two mainly occurs at the free deformation of the head, as shown in Figure 6.In the second pass, the head with inner hole is preformed, and the maximum effective stress is 1140MPa.Due to the deformation limited by the die, it mainly occurs at the preformed hole, and the maximum effective strain is 7.50mm/mm also occurs at the preformed hole as shown in Figure 7.In the third pass, the inner hexagon of the head is forged.The maximum effective stress is 1150MPa.Due to the limitation of the die, the deformation mainly occurs at the pre-formed hole, and the maximum effective strain is 7.76mm/mm.At the inner hexagonal hole, the inner hexagonal screw is gradually formed in multiple passes as shown in Figure 8.   to the open die forging state, so the forming force is relatively low, and the maximum forming force is 220.8kN;The head with inner hole is preformed, which belongs to the closed-die forging state, so the forming force is higher than that of the first pass, and the maximum forming force is 454.0kN; the third pass forges the inner hexagon of the head, and the maximum forming force is 303.2kN.The total forming force is 978 kN.

Conclusions
The all forming characteristics are able to be summarized in Table 4. Comparing Table 4 with Table 1, the major results are as follows: 1. Traditional forming of hexagon socket vacuum screw using wire rod: 6 passes are used in this forming process, and total forming force is 2310kN (235.477ton).
2. New forming of hexagon socket vacuum screw using tube: 3 passes are used in this forming process total forming force is 978kN (99.69ton).The forming force comparing with traditional forming can be decreased 57.66%.3. Maximum effective stress at pass 3 of traditional forming is 1600MPa, whereas that at pass 3 of new forming is 150MPa, it can be reduced 28.13%.
In general, this study provides the concept of multi-pass forming process using tube workpiece to form vacuum hexagon socket screw instead of wire rod.The design is a feasible method according to the finite element simulation results.Moreover, the dimension comparisons of product between specification and FEM simulation in Table 5.The maximum error is around 5.175 %.The feasibility of new forming design can be verified, and the research results can be provided to relevant industries as the reference.

Figure 1 .
Figure 1.Diagram of vacuum screws.Figure2.Photo of the vacuum hexagon socket screw.Chen et al.[1] proposed a new single-step injection forging as an alternative way to the traditional multipass forging process for manufacturing automotive fasteners, and used finite element simulation software to simulate the forming process of vehicle parts to improve production efficiency, and raise die life as well as reduce wear resistance.Dubois et al.[2] proposed contact conditions for simulating the cold forming process, including two experiments of drawing and extrusion forming.The frictional conditions of each basic forging operation were analysed according to the lubrication parameters (lubricant: phosphate, drawing powder and cold heading oil, etc.) to predict the geometric changes of forged products related to the frictional conditions.Lee et al.[3] used DEFORM-2D finite element simulation software and experimental verification to realize the size difference between forging dies and forging parts.The major variation was influence of elastic deflection of dies and spring back of forged parts.The experimental results were in a good agreement.Doege et al.[4] explored different closed-die forging systems for the forming and manufacturing of spur gears and helical gears.The main purpose was to manufacture burr-free forgings.The research developed a new alternative closing device for forging dies to improve the shortcomings of the existing process.Dinger[5] adopted 3D finite element analysis of the thread forming process using the ABAQUS/Explicit software program.The thread forming screw assembly process was evaluated by numerical simulation and experimental methods.Pandya et al.[6] proposed that the use of finite element simulation could effectively design the forming conditions of the preformed pin during the hot forging process, which can minimize the forming load and equivalent stress to improve the life of the die, and the experimental verification was consistent with the finite element simulation results.Zhao et al.[7] studied various parameter, such as plate thickness, spacing, and connection length by means of finite element simulation applied to the shear resistance of the stainless steel screw connections, and the results were in good agreement experimental verification.Fernandes et al.[8] developed a new forming process for pipe fittings with gaskets, and the finite element method and boundary element (BE) method were combined to simulate to verify its feasibility.Perform discretization was avoided to improve final simulation performance.Gontarz et al.[9] utilized the finite volume method (FVM) and the finite element method (FEM) to simulate the head forging forming of the nail head forming process, and the results were verified experimentally.Through the comparisons of force and flow kinematics, the method could be proved to be feasible.Balendra et al.[10] proposed Injection forging was used to form complex component shapes.In this study, different forming experimental conditions were applied to the tube.And finite element simulation has been to verify its feasibility.Asnafi et al.[11] explores the fastener forming using the finite element simulation and experiment to verify the die stress analysis, and the research results were consistent with the finite element simulation.Qin et al.[12] utilized the finite element simulation to evaluate the situation of friction-related pressure loss during injection forging of solid and tubular parts, and several lubrication procedures were tested to find the optimal lubrication conditions.Lowering forming load and improving material flow were verified.Petrescu et al.[13] said that FEM's FORGE-2 software has been used to simulate the cold forging process of fastener steel to predict the failure conditions, and the simulation results have been verified by experiments as a feasible method.Rajiev et

Figure 3 .
Figure 3. Pass schedule for traditional process of vacuum hexagon screw.

Figure 4 .
Figure 4. Effective stress and effective strain in Pass 3 forming process of screw.

Figure 5 .
Figure 5. New forming pass schedule of vacuum hexagon socket screw.

Figure 6 .
Figure 6.Effective stress and effective strain in the first pass.

Figure 7 .Figure 8 .
Figure 7. Effective stress and effective strain in the second pass.

Figure 9
Figure9depicts the forming force diagram for each pass.The first pass is head forming, which belongs to the open die forging state, so the forming force is relatively low, and the maximum forming force is 220.8kN;The head with inner hole is preformed, which belongs to the closed-die forging state, so the forming force is higher than that of the first pass, and the maximum forming force is 454.0kN; the third pass forges the inner hexagon of the head, and the maximum forming force is 303.2kN.The total forming force is 978 kN.

Figure 9 .
Figure 9.The forming force for each pass.

Table 1 .
Summary of FEM simulation for traditional process of vacuum hexagon socket screw.

Table 3 .
Comparisons of initial sizes between tube and wire rod workpieces.

Table 4 .
The forming force, effective stress, effective strain for each pass.

Total Forming Force: 978kN = 99.69tonTable 5 .
Dimension comparisons of product between specification and FEM simulation.