Title

Low-Pressure Sand Casting of Aluminum Alloy Cylinder Engine Parts Field of the invention

The present invention relates to the field of non-ferrous casting processes in foundries, and more particularly to an improved casting process in sand molds for manufacturing aluminum-alloy automotive parts, for example cylinder engine blocks and cylinder engine heads, and the like. The invention teaches a method and apparatus for filling precision sand molds with liquid aluminum alloys utilizing a low pressure furnace and an automated casting station with advantages over the prior art in investment and operational costs and increased productivity. Background of the invention

Automotive and aviation motor parts made of aluminum alloys (in this document for simplicity, reference to aluminum will be understood to include aluminum alloys) are produced in a number of casting processes and of types of molds ranging from the permanent metallic molds, semi-permanent molds and one-time usable sand molds. The quality of the cast products, meaning its mechanical properties and dimensional stability under all working conditions, depends on many factors which influence the final metallurgical microstructure of the casting, for example, the chemistry of the molten metal used, the type of molds and cores, the amount of inclusions and oxides occurring during the handling of the molten metal and molds pouring, the heat transfer rate and direction of cooling during solidification and subsequent cooling, as well as the heat treatment process undergone by the cast products.

It is known that the global automotive industry poses at all times challenging pressures upon the suppliers of the big automobile assembly plants because suppliers must provide high-quality products at low prices under a world- wide competition. There is a continuously felt need in the industry for improved and more versatile casting processes that enable the manufacturer to produce cast products, such as cylinder engine blocks and heads, of high quality and at the same time with low investment and operational costs.

Currently, the use of sand molds have proved to be the most advantageous process for massively casting large numbers of automotive parts, because the sand mold process requires low investment and operational costs. Aluminum casting in sand molds however requires special care of process details for producing good-quality products from the point of view of the mechanical properties, e.g. tensile strength, thermal conductivity, dimensional accuracy, [7219 ₅₅ .DOC]

casting soundness and machinability (meaning the metal hardness and ease of machining the cast product to give precision dimensional surfaces for assembling the motor components in the block).

There are a number of proposals in the prior art seeking to improve each of the process steps involved in the production of aluminum cylinder engine blocks and heads. Generally, the manufacturing process comprises the following steps: preparation of sand molds and cores, assembly of sand mold packages, filling of the sand mold package with liquid metal, solidification of the liquid metal, elimination of the risers, elimination of the sand cores from the internal spaces of the cast product, and heat treatment of the cast product. The present invention teaches a method and apparatus for casting aluminum cylinder engine parts which allows for combining the best features for each casting process step while more effectively automating the sand mold filling step to allow for an increased productivity of the overall casting process.

Applicants have found the following prior patent publications related to the background of the present invention.

U.S. Patent No. 4,733,714 to Smith discloses a method of making a casting wherein molten metal is caused to flow from a molten metal furnace acting as a primary source of molten metal. The molten metal is caused to flow through a heated launder system connecting through a rotary nozzle with a molten metal feeding refractory piece in communication with the mold by means of an electromagnetic pump or by pressurizing the holding furnace. This patent is mute regarding the method and means for automating the system by an automatic determination of the moment when the mold is filled up with molten metal.

U.S. Patent No. 5,163,500 to Seaton et al. teaches a low pressure casting process wherein sand molds are filled with molten metal from a low pressure holding furnace through a launder connected to an ingate in said mold located at a side of the mold. After the mold is filled up, it is rotated 180° and disconnected from the feeding nozzle. The time for rotating the mold is determined by means of a timer or of a pressure switch in the furnace. The molds are moved into the filling and rotating station by means of pushing bars which move the empty molds from a first conveyor and displace away the molds already filled up with molten metal to a second conveyor.

U.S. Patent No. 5,492,165 to Erana discloses a machine for filling sand molds with non-ferrous metals using a low pressure technique wherein a vacuum is applied to the mold [7219 ₅₅ .DOC]

for causing the liquid metal to fill the mold upwardly and when the mold is filled with molten metal, it is turned 180°. A level sensor located in the molten metal feeding furnace is used for determining that the mold has been filled up. This patent teaches feeding the mold through an opening in the bottom part thereof and relies on refrigerating the nozzle connecting to the mold ingate for creating a solid plug in the mold before disconnecting the mold from the nozzle.

US Patent No. 5,725,043 to Schaefer et al. teaches a particular valve design made of graphite for controlling the flow of molten metal between a molten metal holding furnace and a low pressure furnace in a casting process, allowing for quick replacement of the valve. Schaefer also discloses a method of operating the molten metal holding and pressurizing vessels relying on predetermined time periods for the casting molds to be filled up for repeating the mold pouring cycle.

U.S. Patent No. 6,540,007 to Meyer discloses a process for molding a casting made of a light alloy wherein a sand mold is rotated 180°. Before disconnecting the mold from the metal feeding nozzle, the mold ingate is mechanically sealed by means of a suitable metal plate or with a sand plug. This mechanical closure of the mold adds complexity and increases costs and time to the casting process.

U.S. Patent Application No. 2004/0050525 Al discloses a process and apparatus for discharging a dose of molten metal from a pressure pouring furnace into casting molds. The control of the molten metal feed to the pressure furnace is made by means of a level sensor in the pressure furnace which emits a signal for actuating to seat or unseat a plunger and sealing plate combination which is located in the wall separating a heating chamber from the pressure chamber of the furnace. Kennedy teaches using the differential pressure measured before and after dosing the molten metal for an accurate control of the quantity to the mold. An algorithm for the relationship between this differential pressure for each type of mold has to be developed and applied.

PCT International Application Publication No. WO 2007/079482 teaches a metal casting system using an engineered sand mold without conventional gating design and which utilizes a vacuum pressure for causing the molten metal to fill up the mold. The sand mold is encapsulated for allowing the vacuum negative pressure to raise the molten metal into the mold cavity. This publication does not include any reference to method and means for determining the moment when the mold is full and does not mention rotation of the mold. [7219 ₅₅ .DOC]

DE OS 196 49 014 Al teaches a mold design which is fed with molten metal against gravity from a low pressure furnace through an inclined launder through an ingate located laterally in said mold. The ingate channel is built inside the mold also inclined with respect to the horizontal to prevent the molten metal to flow back to the feeding furnace when the mold is rotated 180°. The inclined design of the feeding channel inside the mold adds unnecessary complexity to the manufacture of the molds. This patent publication does not teach any method or means for automating the casting operation by a suitable determination of the moment when the mold is filled up with molten metal. Additionally, this patent teaches that the filled molds each remain in their respective casting station for cooling; thus lowering the productivity of the casting stations (which are mounted in a rotary platform) with the consequent disadvantages of (1) a low productivity of the whole set of stations when one of the stations requires maintenance and (2) the operational cost of continuously moving the rotary platform. Objects of the invention It is therefore an object of the present invention to provide an automated low-pressure sand casting process for manufacturing high-quality aluminum-alloys cylinder engine parts at low cost and increased productivity.

It is another object of the invention to provide a process and apparatus for sand casting of cylinder engine parts of high productivity utilizing a metallic chill as part of the mold package, feeding the mold with molten metal against gravity and inverting the mold with respect to the direction of gravity for ensuring that the solidification is carried out while molten metal in the risers of the mold feeds the mold cavity during said solidification stage.

It is still another object of the invention to provide an automated process and apparatus for sand casting of cylinder engine parts achieved by the present invention wherein liquid molten metal level sensor mounted on a casting station is used for determining the time when a mold is filled up, and automatically actuate on a mold rollover system for immediately inverting the mold with respect to the direction of gravity and on a programmable robot for removing the sand mold from the mold filling station, thus increasing the overall productivity of said sand casting process. Summary of the invention

The objects of the invention are generally achieved by providing an apparatus for casting cylinder engine parts of aluminum alloys comprising a molten metal holding furnace; a low-pressure mold feeding furnace; a conduit communicating said metal holding furnace [72195 ₅ .DOC]

and said low-pressure furnace; valve means for controlling the gravity-driven flow of molten metal from said holding furnace to the low-pressure furnace; a mold-filling station where a sand mold is automatically positioned in a filling position and a post-filling position; a heated launder connecting said low-pressure furnace and said mold filling inlet; a sensing device located in said filling station for determining the moment when a sand mold is filled with molten metal; and a programmable controller which acts on said mold-filling station in response to a signal from said sensing device for positioning the mold filled with molten metal in said post-filling position. Brief description of the drawings Figure 1 is a diagrammatic representation of a casting installation according to the present invention using one mold-handling robot for production of high quality cylinder engine parts with increased productivity.

Figure 2 is a diagrammatic representation of a casting installation according to the present invention similar to that shown in Figure 1, using two mold-handling robots. Figure 3 is a diagrammatic representation of a casting station according to the present invention. Detailed description of some preferred embodiments

For a better understanding of the spirit and scope of the invention, it will be here described with reference to the appended figures 1 , 2 and 3. It is known that the quality and cost of the aluminum alloys cast products depend on a number of factors involved in the different process steps and tools utilized that eventually determine the final metallurgical microstructure, which in turn produces the desired mechanical characteristics of the cast part and its dimensional stability and performance under the working conditions of modern vehicles. Some of the most relevant factors and process steps affecting quality are: 1) Type of molds and cores; 2) Mold filling process; 3) Solidification process; 4) Cooling process and rate of the cast products; and 5) Heat treatment of the cast products.

According to a preferred embodiment of the invention, the preferred type of molds and cores are those made from silica sand which provide good precision dimensions and low cost for massive production of cylinder engine blocks and heads. The sand cores preferably use a water-soluble binder which may be organic (e.g. based on starch) or inorganic (e.g. based on sodium silicate and sodium phosphate). The invention however is fully applicable to sand molds with any type of binder. [72195 ₅ .DOC]

A metallic chill forming part of the mold is preferably used as a heat sink for setting a direction of the solidification process. Usually the chill is located opposite to the riser section of the mold, so that liquid molten metal feeds the mold cavity during the solidification stage that starts in the sections of the mold where molten metal has contact with said chill. The invention is also fully applicable to sand molds without metallic chills.

The preferred pouring process is a low-pressure feed of molten metal against gravity which provides a quiescent (non-turbulent) flow preventing the formation of bubbles and inclusions. The molten metal and pressure furnace vessels are covered and a non-oxidizing gas is used as a source of pressure in order to prevent oxidation of the aluminum alloys. The level of the mold feeding furnace is automatically replenished at each pouring cycle by the action of a ceramic plunger valve which allows molten metal to flow into the pressure pouring furnace from the holding furnace just by the level equalization when the plunger (or graphite plug) valve communicates between both furnaces.

A heated launder system connected between the pressure pouring furnace and the mold is designed having an upward inclination with respect to the horizontal direction of feeding flow (preferably about 10°) for causing molten metal in the launder to return to the pouring furnace by gravity when the pressure in said pouring furnace is relieved after the mold is filled up. This feature of the launder avoids the need of separate shutting off devices or complex anti-spill nozzles and special gating and riser designs. The feeding nozzle has a non-spill sealing spherical design which permits the rotation of the filled-up mold about a horizontal axis passing through the mold's molten metal feeding inlet, while the higher pressure of the pouring furnace is maintained on the mold.

After the mold is filled up with molten metal, a sensor mounted on the casting station sends a signal to a PLC controller which then acts on the mold rollover motor whereupon the sand mold at the station is inverted about 180°, so that the solidification starts in the lower part of the mold where the chill is located and the metal shrinkage draws more liquid metal from the upper riser section of the mold, favorably flowing downwardly in the direction of gravity. The pressure exerted in the pouring furnace by a non-oxidizing gas (usually nitrogen) is maintained during rotation of the mold to assure that the liquid aluminum fills all the spaces in the mold complex and thin-walled geometry.

In Figure 1 cooling of each mold is done at a cooling position (not shown) separate from the filling position; so that the productivity of the casting station is maximized (since the casting station is fixed in place and is used only for the filling and subsequent rotation of [72195 ₅ .DOC]

the mold). In Figure 2, the use of a plurality of casting stations and of robots permits the retention of the mold in its casting station during initial cooling. After solidification of the cast product, the sand cores and chill are removed from the mold at a recovery position, and the cast product may be cooled down by a water mist and/or further subjected to the predetermined heat treatment to obtain the desired mechanical properties and overall quality.

Referring to Figures 1, 2, and 3, like numerals designate like elements. Starting with reference to Figure 1 illustrating a casting system, sand molds 10 having an integrated chill 12 are transported by a conveyor belt 14 up to a point where a robot 13 having arm 16 and mold handling means 18 places a mold 10 in a casting station 20 where the mold is fixed to the casting station frame 23 by means of suitable holders 21.

The sand molds 10 are filled with molten aluminum alloy from a low pressure furnace 22 through a heated launder 24 that has an upward slant with respect to the horizontal feed direction (typically 10°). This inclination of launder 24 assures that liquid aluminum in the launder flows back down and returns to the low pressure furnace 22 when the mold 10 has been filled and rolled over, and the pressure in furnace 22 has been released or sufficiently decreased to avoid spilling liquid aluminum upon disengagement of the launder from the mold.

When the mold 10 is filled up, the casting station 20 rotates 180° about an axis 30 passing through the ingate 25 of said mold while the pressure of molten metal is maintained. The casting station 20 may rotate in one direction only or can be designed to have a reciprocal motion.

When the rollover of the mold is complete, the fixture clamps 21 and fill probe 26 are retracted so that the mold 10 can be extracted from the casting station automatically.

After rollover of mold 10, the chill 12 is in the lower position of the mold (as illustrated in dash-dot outline in Figure 3). Robot 13 may move the inverted mold 10 from the casting station 20 to a cooling position (not shown in Figures 1 or 3, for simplicity of illustration), where the molten metal is cooled down and solidified. If the process involves the use of an integral bulkhead chill 12, sufficient time will be allowed for the mold 10 to cool to permit safe removal of the chill 12 from the casting 60. Thereafter, molds 10 are placed in a recovery station 58; where the chills, sand molds and cores are removed, and cast products 60 are then transported away by means of conveyor 62 for further processing.

Molten metal, for example aluminum alloys, is prepared in melting furnace 54 having an opening 55 for charging aluminum scrap and ingots as well as the necessary alloying [7219 ₅₅ .DOC]

elements. It is to be understood in this specification that a reference to aluminum for simplicity also includes aluminum alloys (unless otherwise indicated from the context). Molten aluminum is transferred from melting furnace 54 to holding furnace 52 via conduit 59. Holding furnace 52 provides a source of molten metal for replenishing the level of molten metal in low pressure furnace 22. A suitable valve 51 is used for allowing molten metal to refill pressure furnace 22 via conduit 57 after each mold filling operation. Valve 51 also seals the inner space of low pressure furnace 22; so that the pressurizing gas holds the desired pressure level for causing the molten metal to flow into the mold.

After the mold filling and inversion, the casting is consequently oriented so that the risers are upright thus providing a gravity feed of molten metal to the casting as it solidifies.

The casting station 20 may be loaded and unloaded with molds 10 by any suitable mechanical means including but not limited to a gantry, robot or conveyor.

Figure 2 shows the casting installation of Figure 1 modified to use two mold handling robots: 13 and 15. Robot 13 handles the empty molds 10 from conveyor 14 and positions said molds into one of the casting stations 41, 43, 45 or 46 mounted on a rotary turntable 48. Robot 15 takes filled molds 10 from such stations and places them in a recovery station 58 where the metallic chill, the sand molds and cores are removed and cast products 60 are then transported away by means of conveyor 62 for further processing. Rotary turntable 48 may include a plurality of casting stations (other than four) as may best fit for the particular design of a casting installation, presenting the advantage of providing a time for solidification of the liquid aluminum after molds roll-over while other molds are being filled.

Referring now to Figure 3, the precision sand mold 10 is placed (automatically) by a robot 13 into the casting station 20. Clamping means 21 are activated, and launder 24 is advanced into the precision sand core package 10. The precision sand core package 10 will be positioned so that the runner and risers are filled before the rest of the sand mold package. In a preferred embodiment of the invention, a fill probe 26 mounted on the casting station 20 detects and emits a signal 27 to programmable controller 32 when the mold is full, which in turn sends signal 29 to motor means 28 triggering rotation of the casting station 20 about a horizontal axis 30 preferably passing through the ingate 25 of mold 10. Rotation of casting station 20 (substantially 180°) about horizontal axis 30 occurs automatically when the mold is full. Launder 24 remains connected to mold 10, and therefore mold pressure is maintained during the rotation process. The launder comprises a ceramic tube as well as a ceramic nozzle and has a rotary tip connecting with the ingate 25 of [721955.DOC]

mold 10 and suitable sealing means allowing molten aluminum to fill the mold cavity and preventing liquid aluminum from spilling (while maintaining the pressure level inside the mold). Launder 24 is also capable of fine adjustment when it is located against the ingate port of the mold. The pouring launder 24 has a unique ceramic elbow design which has an access port on top for ease of cleaning and also acts as a trap for oxides, preventing them from entering the mold during pouring.

Pressure furnace 22 has a gas inlet pipe 40 provided with valve 36 allowing communication with a gas, for example nitrogen (though other inert gases also may be used for example: argon, helium or other inert gases), for pressurizing furnace 22 and causing molten aluminum to flow upwardly through launder 24 as a quiescent flow without turbulence to fill the mold 10.

When the 180° rotation is complete, controller 32 sends a signal 31 to controller 34 to release the pressure on the low pressure furnace 22 by opening valve 42 on the gas outlet 44. Controller 32 then signals robot 13 which removes sand mold 10 from casting station 20 (in the embodiment shown in Figure 1), or from the corresponding casting station 43 of turntable 48 (shown in Figure 2), and places it on a recovery station 58 where the metallic chill, the sand molds and cores are removed from the casting.

Upon removal of sand mold 10 from casting station 20, controller 34 sends signal 33 to open valve 51 to allow molten aluminum to flow from holding furnace 52 to low pressure furnace 22 to recover the level of liquid aluminum in said low pressure furnace 22. Molten aluminum furnace 52 is larger than said low pressure furnace 22 so that the level in furnace 22 is recovered by just opening valve 51, thus dispensing with the use of a pumping means for transferring the liquid aluminum. Aluminum make-up to furnace 52 may be similarly carried out by transfer means known in the art. This arrangement of furnaces provide a casting system of improved productivity while maintaining a high quality control of the molten aluminum by placing filters 56 in conduits 57 and 59.

The mold is filled with the runner and risers in the downward position. When the mold is filled and rotated it is allowed to cool with the runner and risers in the upward position to maintain head pressure during the solidification process. Mold 10 may or may not include an integral metallic chill or smaller localized chills to cool the casting bulkheads rapidly during the casting process.

Probe 26 in one preferred embodiment has an electricity conducting bar76 which may be inserted and retracted through a suitable opening in the sand mold, for example by means [721955 DOC]

of a pneumatic piston, for determining when the mold is full. The molten metal upon reaching the desired level in the mold contacts the conducting bar 76 thereby closing a circuit and sending a signal to controller 32. Probe 26(by way of illustration and not of limitation) can also be any one of a number of sensing devices, such as a capacitive sensor, an eddy current probe, or a laser level sensor, depending on the core package design. The sand mold package may be designed; so that it will have a clearance or an opening for the metal sensing device to detect the level of molten metal inside said mold.

A predetermined mold filling profile is developed for each specific product being produced. The actual filling flow of the metal is controlled by a feedback signal 37 from a transducer 39 that detects the level of pressure in the low pressure furnace 22. This signal 37 is compared with the predetermined filling profile by controller 34 to make fine adjustments to valves 36 and 42 thus compensating for any shortfall or excess of pressure in the furnace during the pouring process.

The system also allows the operator to set one or more pressure levels over discrete time periods during a mold fill profile according to mold configurations and the remaining amount of melt in the pressure chamber.

The furnace 22 is pressurized using nitrogen, so that molten aluminum flows through launder 24 and into the precision sand core package in a quiescent manner. A signal 37 from pressure transducer 39 that is mounted on the low pressure furnace is taken by controller 34 for actuating on valve 36 through signal 72 and/or valve 42 through signal 74 so that any loss or surplus of pressure in the furnace 22 is carefully controlled during the pouring process. Transducer 39, along with controller 34 allow the system to automatically compensate for any pressure leaks that may exist in the system and permit a very fine control of the internal pressure in the furnace 22. The system of the invention has both the ability to inject and release gas during the mold's filling process. For example, if the mold fill profile requires at a certain moment a particular set-point of 3.0 psi and the valve 36 opens to inject nitrogen into the furnace 22, and the pressure inside the furnace is actually 3.2psi, the system acts on valve 42 and releases pressure in order to bring back the furnace to the 3.0 psi level. When the fill probe 26 detects that the mold is full, it triggers the PLC controller to rollover the casting station 20 for 180° while the furnace holds a constant mold pressure to maintain head height preventing any fluctuation in mold pressure. [7219 ₅₅ .DOC]

In one aspect of the invention, a casting system and method are provided having an increased productivity through an automatic operation. In another aspect of the invention, a method and apparatus are provided for controlling the filling up of molds with liquid aluminum alloys utilizing a set of three pressure transducers in a low-pressure casting system. It is of course to be understood that only some preferred embodiments of the invention have been described as a way of illustration so those skilled in the art may understand the spirit and scope of the invention, and that numerous changes may be made to the embodiments herein shown and described without departing from the scope of the invention, which is defined by the appended claims.