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S. Ruan et al.
15.1 Introduction
The use of high strength steel sheets increases mainly for body-in-white parts for the
reduction of weight and improvement of strength. Hot stamping is common practice
in automotive industry for cover panels [1].
On the other hand, the rigidity takes advantages of hollow structures. Tube hydroforming is increasingly employed to manufacture hollow parts of high strength steels,
but it is not easy for severe requirement of equipment, small ductility of materials
and the inevitable spring-back [2].
Warm tube hydroforming is then employed to heighten the formability of high
strength steels. But using oil and water as pressure media limits the heating
temperature below 300
◦ C [3].
Hot metal gas forming (HMGF) is an innovative forming technology whose
concepts are based on tube hydroforming and hot to remove the limitation of the
heating temperature. HMGF process is coupled thermo-mechanical problem process.
The heating transferring could alter the flow-stress of the materials and a large cooling
rate is also necessary for martensite formation [4, 5].
Ferritic stainless steel showed at least 55% expansion at high temperatures during
bugling [6]. Then, the tube hot gas forming was introduced into the automotive industries by Fraunhofer Institute for Machine Tools and Forming Technology IWU [7–9].
Induction heating and additional heating elements was used for uniform temperature
distribution [10].
However, it is difficult to control the internal pressure in HMGF process since
the charging time is less than 1 s [11]. Thus, a simplified controlling strategy using
resistance heating and filling the air into seal tubes was introduced by Maeno et al.
[12], so that the tube could be bulged at low flow stress and the controlling of the
internal pressure is avoided.
The torsion beam is an important part in the rear suspension of the chassis. Its
torsion resistance and stiffness have always been crucial to the system. The tube
hydroformed torsion beams have been widely employed in the industry. A new gas
forming process of ultra-high strength steel hollow parts using air filled into sealed
tubes and resistance heating to omit the subsequent heat treatment was developed
[13].
Since direct HMGF process may induce a flange on the torsion beam, Zhang et al.
conducted a simulation study on an indirect HMGF process with furnace heating.
When the bugling finished, it showed the lowest temperature on the part was about
500
◦ C, which is below the martensite start temperature [14].
In this paper, an indirect hot gas metal forming process for a V-shape torsion beam
is introduced. An in-situ resistance heating method is employed to avoid the heat loss
during transition. The hot-rolled BR1500 press-harden steel is used in the process.
The consistent simulation model with all relevant sub-stages is then established in
LS-DYNA software and the reasonable pieces with high precision and low thinning
rate of 12% could be obtained.
S. Ruan et al.
15.1 Introduction
The use of high strength steel sheets increases mainly for body-in-white parts for the
reduction of weight and improvement of strength. Hot stamping is common practice
in automotive industry for cover panels [1].
On the other hand, the rigidity takes advantages of hollow structures. Tube hydroforming is increasingly employed to manufacture hollow parts of high strength steels,
but it is not easy for severe requirement of equipment, small ductility of materials
and the inevitable spring-back [2].
Warm tube hydroforming is then employed to heighten the formability of high
strength steels. But using oil and water as pressure media limits the heating
temperature below 300
◦ C [3].
Hot metal gas forming (HMGF) is an innovative forming technology whose
concepts are based on tube hydroforming and hot to remove the limitation of the
heating temperature. HMGF process is coupled thermo-mechanical problem process.
The heating transferring could alter the flow-stress of the materials and a large cooling
rate is also necessary for martensite formation [4, 5].
Ferritic stainless steel showed at least 55% expansion at high temperatures during
bugling [6]. Then, the tube hot gas forming was introduced into the automotive industries by Fraunhofer Institute for Machine Tools and Forming Technology IWU [7–9].
Induction heating and additional heating elements was used for uniform temperature
distribution [10].
However, it is difficult to control the internal pressure in HMGF process since
the charging time is less than 1 s [11]. Thus, a simplified controlling strategy using
resistance heating and filling the air into seal tubes was introduced by Maeno et al.
[12], so that the tube could be bulged at low flow stress and the controlling of the
internal pressure is avoided.
The torsion beam is an important part in the rear suspension of the chassis. Its
torsion resistance and stiffness have always been crucial to the system. The tube
hydroformed torsion beams have been widely employed in the industry. A new gas
forming process of ultra-high strength steel hollow parts using air filled into sealed
tubes and resistance heating to omit the subsequent heat treatment was developed
[13].
Since direct HMGF process may induce a flange on the torsion beam, Zhang et al.
conducted a simulation study on an indirect HMGF process with furnace heating.
When the bugling finished, it showed the lowest temperature on the part was about
500
◦ C, which is below the martensite start temperature [14].
In this paper, an indirect hot gas metal forming process for a V-shape torsion beam
is introduced. An in-situ resistance heating method is employed to avoid the heat loss
during transition. The hot-rolled BR1500 press-harden steel is used in the process.
The consistent simulation model with all relevant sub-stages is then established in
LS-DYNA software and the reasonable pieces with high precision and low thinning
rate of 12% could be obtained.