Tokyo Institute of Technology School of Materials and Chemical Technology

Kumai-Muraishi laboratory

Numerical analysis of magnetic pulse forming and microstructure of high-speed deformed pure Cu

• High speed forming method by using electromagnetic force• Short Forming time (a few tens of micro-seconds)• High speed deformation with high strain rates

Magnetic pulse forming

<Problem>Direct observation of the metal behaviour during MPF is impossible. ⇒ Numerical analysis is required.

<Purpose of present study>

• High speed forming of pure Cu by MPF• Microstructure observation of the deformed Cu sheet• Reproduction of MPF process by using numerical analysis,

and investigation of deformation behaviour of the Cu sheet

Introduction Experimental & Analysis methods

<Experimental>• Machine：Bmax MP12.5/25• Metal sheet：C1020R

(190×70×0.75 mm)• Mold：SKD11

(Depth of groove：725 µm)• Capacitance：40 µF• Gap：1.15 mm• Charging enrgy：8 kJ

<Microstructure observation>• OM• TEM

Results and discussion

Conclusions

(1) Calculation of the impact velocity (Emag-Mechanical)

(2) Reproduction of the deformation of Cu sheet in the groove (AUTODYN)

<Calculation of impact velocity>

<Deformation of Cu sheet in the groove and local strain rate of each point>

<The effect of the local temperature change and plastic strain on microstructure of Cu sheet >

• The electromagnetic force and deformation behaviour of Cu sheet until collision with the mold were reproduced by using Emag-Mechanical, and impact velocity (v = 125 m/s) was obtained.

• The deformation of the Cu sheet in the groove at v = 125 m/s was reproduced by using AUTODYN. The numerical analysis result was in good agreement with the experimental result.

• Large increase in plastic strain and temperature of A and B was obtained. On the other hand, plastic strain and temperature increase of C was small during the deformation.

• Elongated grains were observed at A and fine grains were observed at B. Recrystallization might occur at B because the temperature increase and plastic strain predicted by numerical analysis were very large.

Discharge current through the coil, magnetic field and electromagnetic force ware calculated.

The deformation behaviour of the Cu sheet was reproduced. The velocity at the moment of the collision (impact velocity (V))

was obtained.

h：Smoothing length(Particle size)⇒ 2 µm

SPH method is applicable to analyze large deformation. Impact velocity obtained from Emag-Mechanical is given to Cu

sheet as the initial velocity. Local strain rate, plastic strain and temperature were calculated

by using the SPH method.

The deformation mechanism was examined by both microstructure observation and numerical analysis.

Very large temperature increase and plastic strain were obtained.

Recrystallization is considered to occur.Fine grains were observed.

The strain rate of C was low during the deformation.

high

low

Temperature increase and plastic strain of A were smaller than of B.

Heavy deformed grains remained.No evidence of recrystallization.

<Analysis methods>

Electromagnetic force and deformation manner of the Cu sheet were reproduced.

Deformation manner

Magnetic field

Mold

CuCoil0 µs 1 µs 5 µs 10 µs 13 µs

1 µs 5 µs 10 µs 13 µs

Impact velocity

Location A B C

Temperature increase (K) 60 850 5

Plastic strain 0.56 5.92 0.07A B

Initial gap = 1.15 mmCharging energy = 8 kJ

⇓Impact velocity = 125 m/s

The strain rate of B was large in early stage of deformation when B collided with mold.

The strain rate of A was low in the early stage of deformation, but when A collided with the mold, high strain rate deformation occurred.