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The present invention relates to a magnesium sheet for a diaphragm of a speaker. More
particularly, the present invention relates to a magnesium sheet for a speaker obtained by rolling
a directionally solidified ingot, which is produced by a heating mold type continuous casting
method. There are various types of speakers, such as audio, television, telephone handset,
earphones, etc., but they all emit sound due to vibration of diaphragms such as metal, glass and
paper. . In order to reproduce subtle sounds, it has been considered important that the
diaphragm be thin and light. In particular, the diaphragms of small earphones and telephone
handsets must be made of a high rigidity, light and extremely thin material. Magnesium is the
lightest metal among practical metals with a specific gravity of 1.7. Conventionally, the specific
gravity of Ti that has often been used for the diaphragm of the speaker is 4.5, and magnesium is
a metal suitable for the diaphragm of the speaker, which is much lighter than this. However,
since magnesium has a crystal structure consisting of a close-packed hexagonal lattice, it is
difficult to form a plate with poor processability, so a diaphragm of pure magnesium speaker
could not be produced. Conventionally, casting of a metal ingot, which is a material to be
processed, is performed using a cooling mold. Therefore, the ingot consists of polycrystals
including equiaxed products, and there is a columnar zone with poor workability at the outer
periphery of the ingot, and inside! In many cases, casting defects such as J-holes, air bubbles, and
segregation of impurities occur. Roll such ingots and make five thin foils! When manufacturing,
generation of pinholes due to these casting defects and generation of ear cracks during
processing are difficult problems. It was impossible, however, to swing the super-single speaker
diaphragm of pure magnesium having a dense hexagonal lattice type island structure. In the past,
zinc and manganese were added to form magnesium alloy ingots by adding zinc and manganese
to give magnesium formability, and then hot-worked at 250 ° C or higher to form a sheet (used
as a speaker diaphragm. did. However, even if such an element is added, in order to form a thin
ridge having a thickness of 0.01 mm, primary grains remaining in the foil at the time of rolling,
such as edge cracking or a history of cast m weave inside the foil In other words, cracks in the
solidified grain boundaries are likely to occur, and the yield rate of the healthy foil is extremely
low. Therefore, super thin foils of magnesium alloy were extremely expensive and were not used
except for special speakers. Also, the addition of an element having a large specific gravity such
as zinc or manganese to magnesium results in an increase in the weight of the diaphragm, so the
appearance of a method for producing a thin and light pure magnesium diaphragm without
adding such an element Has been strongly desired.
The present inventors have conducted many experiments on a method of using a pure
magnesium and obtaining an extremely thin diaphragm for a speaker using pure magnesium
without adding heavy elements such as zinc and manganese to magnesium. , The temperature of
the inner wall of the mold is kept above the solidification temperature of the cast metal, and the
cooling of the ingot is performed only outside the mold, using the heated mold continuous
casting method (Japanese Patent No. 1049146). By casting a single-crystal plate-like ingot and
rolling it, it is possible to form a very thin magnesium diaphragm, such as sound O, 01 mm
without pinholes or cracks at grain boundaries, without occurrence of edge cracking I found that
it was quite easy. That is, the horizontal graphite mold having a hollow cross section having a
width of 1.00 mm and a height of 1 to 10 mm was heated so that the temperature of the inner
wall was 251 ° C. or higher. The molten magnesium was supplied to the mold, and cooling of
the ingot was carried out using water only outside the mold, thereby performing continuous
casting of magnesium strip ingots of 1 to 10 mm in size. Near the solidification edge of those
ingots, they consist of polycrystals, but as the casting progresses, the number of crystals
decreases as a result of competitive growth of crystals, and finally a single crystal is obtained. It
was found that rolling was performed at various temperatures of C or higher and processing
could be performed to a thickness of 0.01 mm without occurrence of cracking. Moreover, by
changing the thickness of the ingot, which is the material to be processed, to change the degree
of processing of the plate, and by appropriately selecting the processing temperature, a
temporary of any hardness can be easily obtained, such as hard or soft. We have found that
diaphragms for various sound quality can be made of light pure magnesium. The present
invention provides a pure magnesium diaphragm for a speaker, which could not be obtained by
the method of processing an ingot obtained by a conventional cooling mold as a raw material,
and greatly contributes to the development of acoustic equipment. It seems to be.
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