FR2607736A1 - Apparatus for thermoforming sheets or strips of Al - Google Patents

Abstract

The invention relates to a device for thermoforming sheets or strips of Al or alloys thereof. The device comprises a lower chamber 3 and an upper chamber 4 which are movable with respect to each other, as well as clamping jaws 12 for clamping the sheet 1 on the rods 2 for supplying current from the secondary 10 of a transformer 9, the current supplies 10 consisting of several conductors 10a, 10b, 10c, 10d which are mounted in parallel and can be individually insulated. The device permits the forming of sheets or strips of Al or alloys thereof ranging up to 2.5 mm in thickness.

Description

APPARATUS FOR THEIMOFOIMAGE OF EXCAVATIONS
OR A1 BANDS
The invention relates to a device for thermoforming sheets or strips of Al or its alloys.

 ether forming of Al strips or sheets is known; it consists in heating a sheet or strip of Al initially planar by any means, holding the sheet in a suitable blank holder and then applying the sheet on a matrix, either using a punch or using pneumatic action (pressure or vacuum).

This process is described in particular in US patents A-3,342,049 or FR.A-2,004,410.

One of the difficulties encountered during the implementation of this process, in particular, during the heating of the sheet by Joule effect under the influence of a high intensity and low voltage electric current, is the control of the temperature of the sheet. to train. The temperature profile obtained before forming must be adapted to the desired deformations; one may wish, for example, that the temperature be as uniform as possible to obtain a homogeneous deformation or on the contrary impose a heterogeneous temperature profile if the deformations must be unevenly distributed.

The device according to the invention solves this problem.

The device according to the invention therefore comprises, as is known, a metal sheet or strip of Al or one of its alloys supported on the upper opening of a lower sealed chamber. A sealed upper chamber, generally movable along a vertical axis, can come to bear by its lower face on the sheet or strip, opposite the edges of the lower chamber, by pinching the Al sheet, on any its periphery ensuring sealing between the strip and the chambers.

In the vicinity and inside the edges of the lower chamber are two metal bars, arranged transversely to the length of the strip, and on which it rests.

Upper jaws actuated by jacks can also rest on the lower bars by pinching the strip.

The lower bars as well as the sheet (or strip) are part of the secondary circuit of a heating transformer whose power is adjustable for example using thyristors varying the intensity at the primary.

The chambers are provided with the organs necessary to place therein either a die or a punch-and adjust their relative positions and means for pressurization or evacuation.

The relative movements of the various members are generally ensured by hydraulic cylinders.

The characteristic of the invention is that the current leads to the bars are made up of several conductors, mounted in parallel and which can be disconnected or not, as desired, from the transformer secondary by suitable switches; it is thus possible to preferentially pass the current through one zone or the other of the sheet as a function of the desired thermal profile.

Furthermore, in order to better maintain the sheet at its periphery during forming, the bars are offset vertically by a few mm relative to the upper edge of the lower chamber.

The invention will be better understood with the aid of the following examples, illustrated by FIGS. 1 to 4.

Figure 1 schematically shows a longitudinal sectional view of the device according to the invention.

Figure 2 shows a plan view of the current leads to the bars.

Figure 3 shows the equivalent electrical diagram of the transformer secondary.

FIG. 4 represents a transverse temperature profile before (I) and after (II) disconnection of a lateral current supply.

FIG. 1 represents a sheet of Al strip resting at the upper part of a lower sealed chamber 3 containing a lower tool holder 7 as well as a tube 8 for pressurization or evacuation.

The superior room 4 also contains. an upper tool holder 5, to which is fixed a matrix 6 of refractory cement, pierced with pressurization or evacuation conduits, connected to an external tube 8 '. These chambers are metallic but carry, at their opening, an electrical and thermal insulating rim.

The upper chamber as well as the tool holders are vertically movable by means of hydraulic cylinders 11. In the vicinity and outside of the chambers 3,4 are two metallic copper bars 2, which act as a current supply to band 1.

They are located substantially at the opening of the lower chamber 3.

In front of these bars, there are two clamping jaws 12, made of copper, moved by the jacks 11 'which will ensure good electrical contact of the strip 1 on the bars 2.

The bars 2 and the strip 1 are part of the secondary 10 of a transformer 9 low voltage and high current.

Bars 2 are connected to the secondary in a transformer
10 by several conductors, here 4, (10a, 10b, 10c, 10d) mounted in parallel
secondary and isolable from secondary 10. (Figure 2).

The corresponding electrical diagram is shown in Figure 3. it is therefore possible to isolate at will one or more conductors 10a to 10d both to the right and to the left of the forming device.

 To ensure better retention of the sheet 1 during forming, it is preferable that the bars 2 are located slightly above or below the opening of the lower chamber, at a distance of up to a few millimeters.

EXENPLE.l
A 5754 strip 0.8 mm thick and 350 mm wide, in the H111 state, was brought to the temperature of 45000 and was formed under the following conditions: the strip is introduced into the device, and the jaws 12 are applied to the bars 2 by the jacks 11.

Immediately, the transformer is connected. At a voltage of 4 volts, an alternating current with a frequency of 50 They and an intensity of 14000 amps flows through the strip. After 21 seconds of current flow, the temperature reached by the sheet is 47,000 in the center and 25,000 on one of the edges. The temperature profile of the sheet transversely to the flow of current is given in FIG. 4, case I. This is due to the fact that, for practical reasons, the transformer is not located on the axis of symmetry of the device.

By repeating this heating cycle after disconnecting the conductor 10a from the bar, a symmetrical temperature profile is obtained, represented in FIG. 4, case II. This symmetry avoids the preferential forming of one of the sides of the strip, which is very harmful in the case of complex parts.

 At the end of these 21 seconds, the intensity is reduced by 60% of its initial value to maintain the metal at a substantially constant temperature, and this for 16 seconds. Simultaneously, 15 seconds after starting the heating, a pressure of 0.5 NPa (5 bars) is applied to the lower part of the sheet by the tubing 8, while the tubing 8 'is connected to a primary vacuum pump. After 34 seconds, the chambers are vented. The final part has a height of 90 mm and an area equal to 1.5 times the initial useful area (300x300 mm2). The average elongation is therefore about 22.5% and the reduction in thickness locally reaches nearly 50%.

This device allows the forming of sheets or strips of Al or its alloys which can go up to a thickness of 2.5 mm.

Claims (2)

IIvmlcATIOIIS 1. Device for thermoforming sheets or strips of Al or its alloys, comprising a lower chamber (3) and an upper chamber (4) movable relative to each other, as well as jaws (12) ensuring the tightening of the sheet (1) on the secondary current supply bars (2) of a transformer (9) characterized in that the current supplies (10) consist of several conductors (10a, 10b, 10c, 10d) mounted in parallel and individually isolable. 2. Device according to claim 1 characterized in that the upper level of the bars (2) is offset, up to a few mm, relative to the level of the opening of the lower chamber (3).