Apparatus and method to form metal containing nondendritic primary solids

An apparatus and method to form a metal containing degenerate dendrites are disclosed. Molten metal is cooled in a receiving means and agitated by rotating blades connected to a rotating shaft. Solid metal, such as dendrites, deposited on inner wall portions of the receiving means are removed by suitable means, such as a scraping blade, in operable combination with the rotating blades. The so produced metal can be cast by conventional metal casting processes.

BACKGROUND OF THE INVENTION 
This invention relates to the treatment of molten metal and more in 
particular to an apparatus and method adapted to treat molten metals to 
form a metal composition containing nondendritic primary solids. 
It has been determined that molten metal containing up to about 65 weight 
percent solids comprising degenerate dendrites exhibits thixotropic 
properties. The preparation of metal compositions containing such 
degenerate dendrites is described in, for example, U.S. Pat. Nos. 
3,902,544, issued Sept. 2, 1975, and 3,936,298, issued Feb. 3, 1976. As 
molten metal is cooled dendrites can be formed. The solidifying metal is 
vigorously agitated to prevent the formation of interconnected dendritic 
networks and to substantially eliminate or reduce dendritic branches 
already formed. The apparatus of U.S. Pat. Nos. 3,902,544 and 3,936,298 
have been found useful in preparing the desired metal. 
It is an object of the invention to provide an improved means to form metal 
containing degenerate dendrites. 
It is another object of the invention to provide an improved method to form 
metal containing degenerate dendrites. 
SUMMARY OF THE INVENTION 
The apparatus of the present invention is suited to form metal containing 
degenerate dendrites. This apparatus comprises in combination a means to 
contain a molten metal with an inlet suitable to feed metal and an outlet 
suitable to discharge the molten metal, a means to receive the molten 
metal from the containing means and to contain the molten metal during 
agitation and cooling of the molten metal. A means is in combination with 
the metal receiving means to control the temperature of the molten metal. 
A rotatable means is generally axially positioned within the receiving 
means. The rotatable means has a rotatable support with spaced apart 
molten metal agitating blades attached to the support and extending in a 
generally outwardly direction therefrom toward an inner wall portion of 
the receiving means. The agitating means are spaced apart from said inner 
wall portion during rotation of the rotatable means. 
A means to remove solid metal deposited on said inner wall portion from 
metal contained in the receiving means is in operable combination with the 
agitating means. 
In the method of the present invention the temperature of the molten metal 
in the receiving means is controlled to between the liquidus and the 
solidus temperatures of the metal, and the molten metal is agitated 
sufficiently to minimize formation of interconnected dendrites. During 
agitation, any metal deposited on the inner wall portions of the metal 
receiving means is continuously removed to minimize accumulation of solid 
metal deposited on such wall portions. The so-treated metal is solidified 
by well-known methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2 of the drawing, there is shown a molten 
metal treating apparatus 10. The treating apparatus 10 includes a means 12 
to contain the molten metal. The containing means 12 can be a suitable 
container resistant to the molten metal. Those skilled in the art are 
familiar with the specific materials of construction to use for such metal 
holding or containing means and for all other portions of the apparatus. 
For example, steel, ceramic and graphite have been employed for use with 
various molten metals. 
The containing means has an inlet to permit charging of solid and, 
preferably, molten metal. In FIG. 1, this inlet is sealed with a cover 
means 14 to minimize heat loss from the containing means 12 and to assist 
in reducing oxidation of molten metal 16 in the containing means 12. 
A molten metal receiving means 18 with generally cylindrically shaped 
interior wall portions 19 is suitably attached to the containing means 12 
to permit molten metal to flow from within the containing means 12 through 
an outlet 20 and into an agitation chamber 22 within the metal receiving 
means 18. A rotatable means, such as an elongated support or shaft 24, is 
positioned to extend through the cover means 14, the containing means 12 
and the outlet 20 into a generally axial relationship with the interior 
wall portions 19 of the metal receiving means. A lower end portion of the 
shaft 24 sealably extends into an outlet 26 suitable to discharge metal 
from the receiving means 18. The shaft 24 has a plurality of agitating 
means, such as blades 28, connected thereto and extending in an outwardly, 
and preferably a generally radial direction, from the shaft 24 toward the 
wall portions 19. The blades 28 are spaced apart from the wall portions 19 
to avoid contact and possible jamming or wear during operation. Two sets 
of blades 28 on generally opposite wall portions of the shaft 24 are 
preferably substantially equal in weight to permit more uniform, i.e., 
minimum vibration, rotation of the shaft 24. The total weight of the 
blades 28 and a metal removal means, including shearing plates 30 and 
pivoting means 32 should also be considered when positioning the blades 
28, shearing plates 30 and pivoting means 32 in a weight balanced array 
around the shaft 24. The blades 28 in FIG. 1 are positioned at about a 30 
degree angle from horizontal to provide a shearing type stirring motion 
during rotation of the shaft 24. Other angles for the blade position and 
cross sectional configurations of the blade are satisfactory so long as a 
sufficient stirring motion is imparted to the molten metal to break apart 
the dendrites which form upon cooling of the molten metal. 
The shearing plates 30 are adapted to move outwardly from the shaft 24 and 
contact the wall portions 19 during rotation of the shaft in a clockwise 
direction. In the Drawing such outward movement is effected by centrifugal 
force pulling the shearing plates 30 against the wall portions 19 to 
scrape or shear off deposited solid metal, such as dendrites, which have 
deposited on such wall portions during cooling of the molten metal. 
Clearly, pivoting means 32 can be of other configurations, such as hinges. 
Likewise, means to retain the shearing plates 30 in a preferred 
substantially constant contact with the wall portion 19 in combination 
with, or in lieu of, centrifugal force are within the scope of this 
invention. 
A single, elongated shearing plate (not shown) can be employed instead of 
the shorter shearing plates 30. Also, the specific location, and number, 
of the blades 28 and the shearing plates 30 positioned around the shaft 24 
can vary; however, it is preferred that the combined shaft, blades, pivots 
and shearing plates be axially balanced to minimize vibration during 
operation. 
If desired, the shearing plates 30 or the blades 28 can be provided with a 
suitable means or extension (not shown) to minimize the possibility that 
such plates might rotate toward the shaft 24 before rotation thereof has 
started and not contact the wall portion 19 when the shaft rotates. 
Operation of the metal treating apparatus 10 is carried out in a similar 
manner to that described in U.S. Pat. No. 3,902,544, which, for brevity, 
is incorporated herein by reference. Metals suitable for treatment with 
the herein described apparatus are, for example, aluminum, copper, iron, 
nickel cobalt, lead, zinc and, preferably magnesium, and alloys thereof. 
Hereinafter, the description will refer to the preferred metal, magnesium 
and alloys thereof; however, it is to be understood that such description 
is equally applicable to other metals (including alloys). 
A magnesium alloy, such as a commercially available magnesium-aluminum-zinc 
alloy, is charged into the containing means 12 after removal of the cover 
14. If the magnesium alloy is molten when charged, it is only necessary to 
maintain the metal at a desired temperature which is the containing means. 
If desired, the metal can be charged in a solid form and melted in the 
containing means. Suitable commercially available heating means (not 
shown) can be used for the desired heating and/or melting. The shaft 24 
includes a metal baffel means 34. During operation the baffel means 34 
reduces the movement of metal between the containing means 12 and the 
agitation chamber 22 and aids in more accurately controlling the metal 
temperature. 
In a preferred embodiment, the shaft 24 is tubular and has a metal outlet 
plug 35 generally axially positioned therein. The outlet plug 35 extends 
through the center portion of the preferred tubular shaft 24 and is 
adapted to operate independently of the shaft 24. 
The plug 35 is moved downwardly to seal the outlet 26 and permit filling of 
the agitation chamber 22 with molten metal. The temperature of the metal 
in such chamber is controlled by suitable heating and/or cooling means 36. 
The metal is cooled to permit a portion of the metal to freeze. During 
such freezing the molten magnesium is agitated by rotating the shaft 24 at 
a sufficient rate to minimize, and preferably, prevent the formation of 
interconnected solid dendrites in the molten magnesium. The shearing 
plates 30 continuously remove metal deposited on the inner wall portion 19 
to minimize accumulation of solid magnesium deposited on such wall 
portions. The use of the shearing plates 30 in combination with the blades 
28 increases heat transfer between the heating and cooling means 36 and 
the molten magnesium in the agitation chamber 22, agitation or mixing of 
the molten metal, reduces the formation interconnected dendrites and aids 
in forming the desired degenerate, or broken dendrites substantially 
uniformly distributed throughout the metal in the agitation chamber 22. 
When the desired concentration of solids in the molten magnesium has been 
reached, the shaft is raised to open the outlet 26 to remove the metal 
from the agitation chamber 22. Continued rotation of the shaft 24 assists 
in removing metal from the agitation chamber 22. The liquid-solid mixture 
produced can be cast by well-known means into desired shapes. 
If desired, solid metallic and/or nonmetallic substances can be mixed into 
the molten metal in the agitation chamber 22. 
The following example further illustrates the invention: 
EXAMPLE 
A metal treating apparatus substantially is the same as that shown in the 
Drawing was employed for this Example. A standard magnesium base alloy 
(AZ91B) with a nominal composition of 9 weight percent aluminum, 0.7 
weight percent zinc, 0.2 weight percent manganese and the balance 
essentially magnesium was melted in a melting container separate from the 
metal treating apparatus. About 20 pounds of the molten magnesium alloy 
was transferred to the containing means and agitation chamber of the metal 
treating apparatus. An argon atmosphere was maintained within the 
apparatus to prevent the magnesium alloy from burning. The molten metal 
temperature of the metal charged into such apparatus was approximately 
615.degree. C. 
Initially, the metal contained within the agitation chamber was heated by 
heating coils positioned around the exterior periphery of the agitation 
chamber. The agitating means, including the shaft, agitating or mixing 
blades and shearing plates, was a total of approximately 4 inches in 
diameter. When this agitation means was rotated at about 300 revolutions 
per minute, the heat applied by the coils was turned off and the metal 
within the agitation chamber cooled by passing air through three tubular 
coils surrounding the periphery of the agitating chamber. This resulted in 
the molten magnesium alloy being concurrently cooled and sheared. 
The metal within the agitation chamber was maintained at 580.degree. C. 
during agitation. At this temperature, which is below the liquidus 
temperature of AZ91B alloy, the solid portion of the metal being rapidly 
mixed constitutes about 27 weight percent of the metal contained within 
the agitation chamber. The agitation provided by the apparatus employed in 
this example was sufficient to break apart interconnecting dendrites and 
minimize the formation of an interconnected dendrite structure. The 
shearing blades continuously scraping on the inner wall portion of the 
agitation chamber minimized the accumulation of solid metal deposits on 
the interior wall portions of the agitation chamber, even though heat was 
being removed through such wall portions by means of the cooling coils. 
A substantially uniformly mixed or homogenous mixture of the solid and 
liquid metal produced at the 580.degree. C. temperature was removed from 
the agitation chamber through an opening in the lower portion of such 
chamber by raising an axially positioned, metal outlet plug and continuing 
to rotate the shaft and plug to assist in removing metal from such 
opening. When approximately 0.66 pounds was withdrawn from the chamber, 
the metal outlet plug was repositioned to seal such outlet and prevent 
leakage of molten metal thereform. Approximately 0.66 pound of molten 
metal was automatically transferred from the containing means into the 
agitation chamber. The metal temperature within the agitating chamber 
increased 1.degree. to 2.degree. C. when the molten metal was charged 
thereto since the metal within the upwardly positioned containing means 
was maintained at 610.degree. to 620.degree. C. It took about an average 
of 0.6 minute for the temperature within the agitation chamber to be 
cooled to the original 580.degree. C. by the cooling coils. 
The above procedure was repeated until the desired amount of product was 
produced. To maintain the molten metal temperature within the upper 
containing means substantially uniform, molten metal was poured through 
the hole in the cover after about 4 pounds of product had been obtained. 
The product removed from the agitation chamber was satisfactorily 
solidified by pressure die casting into parts of a desired configuration. 
The solid-liquid metal produced by this process was found to have 
thixotropic properties. 
As is apparent from the foregoing specification, the device of the present 
invention is susceptible of being embodied with various alterations and 
modifications, which may differ from those described in the preceding 
description. For this reason it is to be fully understood that all of the 
foregoing is intended to be illustrative and not to be construed or 
interpreted as being restrictive or otherwise limiting the present 
invention.