Hot Chamber Die Casting of Aluminum and Its Alloys

The present invention concerns a hot chamber die-casting apparatus which works under pressure and is used to cast articles of pure aluminum or its alloys as well as pure zinc, magnesium, copper and their alloys and all other alloys which strongly corrode ferrous materials. The apparatus permits the production of articles with an excellent surface finish without inclusions of oxides or anything else and without any contamination from the apparatus itself. In thin sections the articles produced are of uniform quality, improved ductility, and unweakened by iron dissolution.

In casting under pressure, two processes can be distinguished: cold chamber die casting and hot chamber die casting. The former is characterized by the fact that the molten metal is ladled into an unheated injection cylinder before each filling of the die. The second process is characterized by the fact that the injection cylinder is at the same temperature as the molten metal. The main disadvantages of the cold chamber process are:

when the molten metal is transported from the holding furnace to the chamber, a certain amount of oxide is simultaneously transferred

it is difficult to determine the exact quantity of molten metal ladled,

there is a variation of the oxide content of the molten metal which influences the quality of the parts,

further oxidation of the molten metal occurs during the filling of the injection cylinder,

metal contamination by the lubricant of the injection cylinder occurs,

there is no regular temperature evolution during casting.

The advantages of the hot chamber process are:

more regularity in the metal temperature at the die entrance,

the injection cylinder is filled without contact between the air and the metal being injected,

a uniform composition of liquid results.

Concerning Production

higher productivity results due to better utilization of the apparatus and a lower scrap rate regarding the casting and secondary operations.

the need for hand-ladling or auto-ladling is eliminated,

there is a greater potential for automation,

reproducible, predictable and controllable casting cycle results.

Concerning Die Peformance

there are multiple possibilities of positioning the die,

less internal pressure results,

there is less thermal variation as a result of faster and more uniform cycles,

there is less shrinkage of the casted part in the die,

the apparatus has longer life.

While there has been success in the hot chamber die casting of lead, tin and zinc alloys by way of a piston pump injection cylinder, the application of this technique has proved to be unsuccessful as far as aluminum and other corrosive metal alloys are concerned because ferrous material are quickly attacked by these metals when molten, thus limiting the life of the pump.

A gooseneck air machine has been used for aluminum alloys in order to obtain the advantages of the hot chamber process. However, the castings thus produced are not satisfactory in porosity and show increased iron content, the latter increasing the brittleness of the castings. At the present time, the casting of aluminum under pressure is essentially done with the cold chamber process, which is limited by the problems which arise from its unadaptability to automatization. The hot chamber process with the piston pump is a simple process which could make automatization possible (as in the case of zinc die casting) and thereby enable more economic production with better uniform quality.

Nevertheless, molten aluminum alloys are so corrosive that on the market at the moment, there is not a single properly adapted hot chamber die pump for casting these metals under pressure depite the encouraging results already obtained.

Indeed, Union Carbide Corporation has filed two patents (U.S. Pat. No. 3,319,702 and U.S. Pat. No. 3,586,095) concerning piston pumps, the elements of which (the piston and the cylinder), being very exposed to erosion and corrosion, are made of sintered TiB.sub.2 and ZrB.sub.2, and can withstand 50,000 cycles. Such pumps produce an increase of the iron content in the alloy which is less than 0.05% after 150,000 cycles (C. F. Fulgenzi, Trans Soc of Die Casting Eng 1968 paper 46). However, prolonged tests of these pumps on production machines (P. W. Marshall, Foundry Trade Journal, Nov. 26, 1970, 797) showed that their lifetime was irregular (from 600 to 57,000 cycles). The main reasons for the early degradation of the piston and cylinder are the poor resistance of TiB.sub.2 and TrB.sub.2 to thermal shock and the extremely large thermal expansion differences between cast iron and these materials which, in the long run, free-play and out of roundness of the parts. Again, the increase in the iron content of the alloy is very small (0.2%). This increase is nevertheless too large if alloys which contain very little iron are to be cast.

A Japanese patent (J P 74 074 523) describes an invention concerning a pump to be used only in aluminum casting which has been tested with molten aluminum. It can endure 120,000 cycles under a pressure of 150 kg/cm.sup.2, with a piston and cylinder made of hot-pressed TiC (75%)+Si.sub.3 N.sub.4 (25%) and 160,000 cycles under a pressure of 120 kg/cm.sup.2 with a piston and cylinder made of hot-pressed TiC (52%), Si.sub.3 N.sub.4 (25%), TiB.sub.2 (20%), and CrB.sub.2 (3%).

A German patent (2 320 887) discloses the use of pump parts made of AlN alloyed with 0.1 to 10% in weight of Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sc.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Al.sub.2 O.sub.3 or metallic silicates. Pistons and cylinders made of these hot pressed substances have a lifetime of 100,000 to 130,000 pumping cycles of molten aluminum alloy.

A Japanses patent registered in Switzerland under the number CH 586 581 describes an injection pump characterized by a piston and cylinder or a coating of the latter which are manufactured from a sintered body consisting of a mixture of carbides and borides such as boron carbide 10 to 90% in weight, preferably 30-70%, titanium boride 5-60%, zirconium boride 5-60%, boron nitride 0.5-30%, and possibly 0 to 5% tantalum, molybdenum, tungsten borides, zirconium, silicon, tantalum, vanadium, chromium, tungsten, molybdenum carbides, aluminum, silicon, titanium, zirconium nitrides, aluminum and beryllium oxides.

The porosity of the sintered body is lower than 5%, its resistance to flexion above 400 kg/cm.sup.2 and its hardness above 1400 kg/mm.sup.2.

Tests with piston and cylinders made of such materials have been carried out. In certain cases, the main chamber body of cast iron, covered with graphite to protect it, becomes corroded by the fused metal (Al, 1.5-3.5 Cu, 10.5-12 Si, 0.3 Mg, 1 Zn, 0.9 Fe, . . . ) after 110,000 to 160,000 injection cycles under a pressure of 150-250 kg/cm.sup.2, although no sign of corrosion on the cylinder or the piston has been detected. However, it seems that this pump can only be used to cast zinc and magnesium alloys.

Thus it seems that there are certain materials adapted to the construction of the piston and cylinder even though they are not entirely satisfactory. However, the life of the pump and the quality of the cast pieces remain limited due to the corrosion and wearing out of the parts of the pump other than the piston and cylinder. The problems that must be solved in order to realize a pumping apparatus on a piston pump correspond to the four parts of this pump and may be stated as follows:

piston and cylinder corrosion, abrasion, and mechanical constraints such as loosening, poor fitting, alignment and others.

cylinder-end and gooseneck corrosion and erosion,

nozzle corrosion, erosion, oxidation in air, mechanical and thermal shocks,

crucible corrosion and oxidation in air.

The materials to be used to realize such a pump must be resistant to corrosion by molten aluminum, to abrasion and erosion, be quite hard and process expansion coefficients similar to those of ferrous materials. Also, for certain uses already stated, they must be able to resist oxidation and thermal and mechanical shocks. Finally, it is desirable to be able to use materials which are not wetted by molten metal and which are as dense as possible.

Table I shows several known properties of the materials which are the most resistant to molten aluminum corrosion, but other properties must also be taken into account as far as the realization of a pump is concerned.

The object of the present invention is to produce a pump allowing casting under pressure by the hot chamber process of articles made of aluminum alloys and other metals and alloys which are corrosive toward ferrous materials. It is a further object of the present invention to produce such a pump which has a long life--more than 150,000 cycles and which is reliable and does not result in any increase in the brittleness of the cast articles due to the iron content of the cast alloys.

The present invention is characterized by the nature of the materials used to protect the piston and cylinder from corrosion and wear i.e. Si.sub.3 N.sub.4 and Sialones, as well as by the protection of the whole or part of the other parts in contact with the molten metal and by the way the different parts are assembled.

FIG. 1 is a detailed cross-section illustrating the claimed invention;

FIG. 2 is a schematic cross-section illustrating another embodiment of the claimed invention.

The apparatus is composed of a crucible for the molten metal, an injection pump, a die and means for the heating and entrainment of the pump's piston. The pump, as shown in FIG. 1 in one form, is made of a crucible 1, a piston 2 and a cylinder 3, a cylinder-end 4, a gooseneck 5 and an outlet nozzle 7. The pump is made so that the various elements are protected from corrosion and/or oxidation and/or abrasion and/or erosion and/or thermal and mechanical shocks by materials resistant to those different stresses which are present as entire parts or as sleeves of parts.

Other than by its construction materials, the pump is characterized by the fact that the cylinder 3 is fixed to the bottom of the vessel by a cylinder support 8 so that only the piston 2 rises from the bath. This arrangement prevents the part of the piston which is above the molten metal from becoming soiled too quickly by the oxide formed at the surface of the bath, an oxide which would tend to wear out and block the piston-cylinder system.

The reciprocating motion of the piston is guided in two places, at the top with the aid of a piston holder 11 and at the bottom by the cylinder, from which it never exits entirely. The piston head is cut at its lower extremity, preferably by beveling or grooving, to have an oblique piston head so that at the peak of its course it can let the molten metal fill up the cylinder.

Depending on the manufacturing method chosen, the crucible 1 can be linked to a melting pot 12 by a channel 13 as in FIG. 2 or be within a melting pot as in FIG. 1. The preferred type of channel is the submerged sort which prevents air or oxide passing from the melting pot to the pump unit.

In order to prevent oxide from forming on the surface of the bath, consists of protecting the bath may be protected from any contact with air by a neutral atmosphere 16 such as Ar or N.sub.2, or any other non-oxidizing gas or mixture of non-oxidizing gases (as in FIG. 2). In FIG. 2, the melting pot is separated from the pump unit but non-oxidizing atmospheres can be used also where the pump unit is enclosed within the melting pot.

The gooseneck 5 can be formed of only rectilinear parts so that tubes may be inserted.

The loosening (free play) due to the thermal expansion differences which appear between the inserted pieces and the surrounding structure may be compensated for by a system of bevel-edged rings.

The vessel 1 is made of cast iron, preferably very resistant to aluminum corrosion, e.g. aluminum cast iron (I. B. Serelryakova et al, Protection of Metals 3 (1) 1967 95) or chrome cast iron (H Fr Honsel et al, Aluminum 39 (1963) 675).

The piston 2 and cylinder 3 are made of Si.sub.3 N.sub.4 or from a composition of Si-Al-O-N called Sialones, hot-pressed, very hard materials perfectly resistant to molten aluminum and oxidation, unwetted by molten aluminum and resistant to abrasion and erosion.

The cylinder-end 4 and the gooseneck 5 can be protected by an ALN, chromized or sulphurized coating or by the tube inserts made of alumina, beryllia or other pure or mixed oxides such as aluminum titanate, stumatite (natural alumino-silicate), of Sialones, of AlN, of sintered Si.sub.3 N.sub.4, of chromium, in metallic form or bound in a cermet (TiB.sub.2 -Cr), borides of titanium, zirconium, chromium, tantalum, molybdenum and tungsten; carbides of silicon, titanium, zirconium, tantalum, vanadium, chromium, tungsten, molybdenum; and of other fairly hard materials which resist molten Al corrosion and erosion and which are hardly or not at all wetted by the latter.

The seals 6 applied by an appropriate technique can be made of BN or graphite or Cr. Elsewhere, the gooseneck can be protected by an AlN coating, by chrome or sulphur. The cylinder support 8 can also be made with one of the substances stated above which resist corrosion and possess sufficient mechanical properties.

The outlet nozzle 7 is made of hot-pressed Si.sub.3 N.sub.4, hot pressed Sialones or lined with pieces of these materials. Due to the non-wetting properties of these materials with molten aluminum, the danger of the formation of a solid lump of alloy after each injection is greatly reduced.

The crucible 1 is protected from corrosion and oxidation by a coating 9 of Cr, Al.sub.2 O.sub.3 or other oxides such as Al.sub.2 O.sub.3 -TiO.sub.2, of TiB.sub.2, ZrB.sub.2, CrB.sub.2 or other pure or mixed borides, AlN, Si.sub.3 N.sub.4, BN, Sialones or other nitrides, applied in an appropriate way, such as slurry, flame spraying or covering by plates even if they are not placed in an impermeable way (the sealing off is caused by the formation of compounds like Fe-Al or Fe-Al-Si CH 588 320).

The apertures necessary to enter the gooseneck and to machine other parts of the pump may be closed by conical plugs 10 held in place by screws. The plugs are made of materials which resist corrosion and erosion such as Si.sub.3 N.sub.4, AlN, SI-Al-O-N, BN, CR metal, TiB.sub.2, ZrB.sub.2, Al.sub.2 O.sub.3, BeO, aluminum titanate, stumatite, graphite or others.

Other parts of the machine, i.e. the dies, particularly those made of cast iron and die steel are subject to corrosion by the molten metal, although to a lesser degree because of the lower temperature than that of the pump. The dies are above all subject to oxidation, to a certain erosion by the molten metal, to thermal shock during casting, to a degradation as some pieces weld to the die and to the thermal cycles during cooling of the castings and to mechanical shocks during withdrawal from the die.

The present invention also concerns the covering of dies with materials which are neither corroded nor wetted by the metals to be cast, which improve the surface of the cast pieces, which facilitate their withdrawal from the die and which increase the die's lifetime. To this effect, dies of cast-iron or steel, or other materials, are lined or coated with a dense, adherent layer, of good surface condition and thick enough to guarantee long lifetime, composed of Si.sub.3 N.sub.4, AlN, Si-Al-O-N, BN, graphite or pyrolitic carbon, pure or alloyed.