EP2535126B1

EP2535126B1 - Device and method for obtaining semi-solid sludges

Info

Publication number: EP2535126B1
Application number: EP11382201.9A
Authority: EP
Inventors: Iñigo LOIZAGA URBISTONDO
Original assignee: CIE Automotive SA
Current assignee: CIE Automotive SA
Priority date: 2011-06-16
Filing date: 2011-06-16
Publication date: 2017-04-05
Anticipated expiration: 2031-06-16
Also published as: ES2632181T3; EP2535126A1

Description

Technical Field of the Invention

The present invention relates to a device and a method for obtaining semi-solid sludges applicable in the metallurgical industry and more specifically in the field of manufacturing metal parts by means of any semi-solid state injection process in which closed vessels are used for preparing materials, preferably for preventing them from having a non-dendritic structure.

Background of the Invention

The shaping of materials by means of semi-solid state injection can be considered a process that is half-way between shaping by molding and solid state shaping. The semi-solid state shaping of aluminum alloys is a process comprising elements of several classic shaping techniques, such as high-pressure injection, squeeze casting and forging.
The advantages of semi-solid state shaping include the fact that it allows a great deal of freedom in the design of the mold and a high production rate, as well as high metallurgical quality and mechanical properties that are better than those from permanent mold casting or high-pressure injection casting.
The interest of these semi-solid state shaping techniques is based on the need to produce new parts and components with fewer defects and at a lower cost, and it is based on the thixotropy of certain materials whereby the rheological properties of the materials when a liquid phase and spheroidal solid phase coexist allow low viscosity, even with solid fractions, of 0.6 (expressed as per unit) depending on the material deformation rate, hence its name rheocasting.
Several processes for semi-solid state manufacturing are known today which, unlike the forging or casting process, allow obtaining a non-dendritic material structure which yields a material with thixotropic characteristics. Different technologies have been developed for this purpose, including the following with should be pointed out:

Passive agitation.
SRC (Shearing Cooling Rolling) process.
Ultrasonic treatment.
Electric discharge treatment.
Rapid solidification processes such as the Osprey process.
Air SlipTM.
SIMA (Strain Induced Melt Activated).
RAP (Recrystallization and Partial Melting).
Electromagnetic agitation (MHD).
New RheoCasting (NRC).
HITACHI Metals.
Semi-solid Rheocasting (SSR™).
Continuous Rheoconversion Process (CRP).
Direct Thermal Method (DTM).
Sub Liquidus Casting (SLC™).
Swirl Enthalpy Equilibration Device (SEED).
SLC developed by THT Presses.

In several of these technologies, agitation makes it necessary to pour the liquid into a crucible or cylindrical or frustoconical vessel, and after reaching the non-dendritic semi-solid structure, the slug, i.e., the semi-solid mass, also called semi-solid sludge, must be extracted. At this point of the process different problems arise which have a negative consequence of generating oxides on the surface, entrainments or material which remains adhered on the surface of the vessel. It must be taken into account that aluminum alloys oxidize almost immediately upon contact with the air.
One of the processes using vessels for handling material is the SEED (Swirl Enthalpy Equilibration Device) process mentioned above which is used below as the basis for the description of the problems of any process needing a closed vessel for preparing the material in a non-dendritic structure, especially in the case of cylindrical or frustoconical vessels.
The features of the semi-solid SEED injection process are as follows:

It is based on a liquid material (rheocasting).
It requires a lesser degree of control compared to other techniques.
It offers a relatively wide process window.
It reduces the tendency of the material to adhere to the mold.
The slug has a broad weight range (2-8 kg).
A wide range of alloys can be used:
- Casting alloys: 319, 356, 357, 514, 206.
- Forging alloys: 6061.

A schematic sequence of the current SEED process for manufacturing semi-solid sludges (slugs) used in aluminum injection cells for obtaining customized slugs has been depicted in Figure 1 and comprises the following sequential steps.

a) Melting aluminum by means of traditional methods: melting and pouring on a holding furnace, also referred to as a metering unit. Next, pouring the aluminum into monobloc cylindrical recipients referred to as crucibles. Each crucible is tipped to favor pouring and to reduce the generation of gases, as can be seen in the crucible of the prior art depicted in Figure 2.
b) Shaking the crucibles, usually on a vibration table, and cooling the aluminum until it reaches a suitable globular structure. The solid fraction will make up about 50% as a function of the percentage of Si, depending on the proximity to the eutectic point.
c) Optionally, after step b) the obtained semi-solid sludge can be drained prior to demolding.
d) Removing the crucible from the vibration table and transporting it with the slug therein to the container of high-pressure injection HPDC (High Pressure Die Casting) equipment, hereinafter the HPDC injector. Next, longitudinally extracting the slug, for example in automated equipment by means of an abrupt movement of the robot or by means of an auxiliary piston, and depositing it in the container of the HPDC injector. At this point it must be taken into account that the slug cannot be broken so as to prevent the generation of oxides.
e) Pushing the slug or semi-solid sludge with the piston of the HPDC injector in order to fill the steel mold.

In addition to these sequential steps depicted in Figure 1, it is necessary to verify that no aluminum residues remain in the crucible and if necessary they must be cleaned as vigorously as possible to remove them. These aluminum residues can be oxides which deteriorate the following part to be obtained in the process. Standard crucible preparation steps must also be taken which comprise their cleaning, heating, etc. On the other hand, it is necessary to mount and secure the crucible on the vibration or shaking table and lubricant, usually boron nitride, must be sprayed on the crucible.
An auxiliary crucible preparation process should also be performed which comprises applying a lubricant on the crucibles with a brush or spray gun in a manner similar to how the paint is applied, and it is subsequently dried in a furnace. Depending on the lubricants, there will be between 3 and 5 application layers, and the application frequency logically varies depending on the applied layers.
As mentioned above, all these steps and the time and cost they entail are aimed at preventing the generation of oxides on the surface of the sludges and the crucibles or vessels, as well as entrainments or material that remains adhered on the surface of the vessels.

Description of the Invention

A first aspect of the present invention relates to an equipment for manufacturing metal injection parts as defined in claim 1.
A second aspect of the invention relates to a method for obtaining semi-solid sludges as defined in claim 4.
In processes for manufacturing semi-solid sludges of the prior art the molten aluminum is poured into a monobloc cylinder or crucible which is a recipient made of a refractory material used for melting a material at a very high temperature, while according to the invention the crucible is formed by two parts or shells which are preferably semi-cylindrical molten metal injection molds in which metal objects are melted, i.e., the invention proposes a crucible with two halves. The semi-solid or slug is therefore demolded transversely rather than longitudinally. The risks of the slugs breaking during demolding are thus eliminated and the slug is prevented from remaining completely or partially adhered to the crucible. Crucible cleaning processes are also significantly reduced.
The crucible according to the invention comprises two half-shells or parts, a half-shell fixed to the vibration table and a movable half-shell. Fixing means have been provided for fixing the moveable part to the fixed part and to the vibration table which allow configuring the crucible for pouring and solidification and logically allow separating the moveable part for demolding. According to a preferred embodiment, the fixing means are made up of a tiltable grip assembled in an articulated manner on the vibration table which can be coupled onto a perimetric flange of the movable half-shell. This is obviously a specific embodiment but this fixing could be done in many ways.
When demolding, the moveable shell is separated from the fixed shell and the slug is in the moveable shell that is displaced to the piston of the injector. This movement can be performed by means of a robot or any other type of automated installation.

Description of the Drawings

To complement the description that is being made and for the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description in which the following has been depicted with an illustrative and non-limiting character:

Figure 1 shows a schematic view of the sequence of operations comprising a semi-solid state injection process of the prior art.
Figure 2 shows an elevational view of a crucible of the prior art that can be used in the process depicted in Figure 1.
Figure 3 shows a schematic view of the sequence of operations comprising an embodiment of the method for obtaining semi-solid sludges according to the invention.
Figure 4 shows an elevational detail of a double-shell crucible comprising a fixed part and a moveable part, comprising an embodiment of the device for obtaining semi-solid sludges of the invention.

Preferred Embodiment of the Invention

In view of the mentioned drawings, it can be seen how in one of the possible embodiments of the invention one of the processes using vessels for handling the material is the process referred to as SEED (Swirl Enthalpy Equilibration Device) mentioned above which is the basis for the description of an embodiment of the invention; however, the invention is extensive and can be applied to any process needing closed vessels for preparing the material in a non-dendritic structure, especially cylindrical or frustoconical vessels.
The invention comprises a crucible recipient in two halves which can be used in manufacturing metal parts by means of any semi-solid state injection process (e.g. SEED) using closed vessels for preparing materials, preferably with a non-dendritic structure.
The differences between the two types of monobloc and double-shell crucibles are the following:

The double-shell recipient must have uniform thicknesses in order to assure a balanced heat discharge, guaranteeing the uniformity of the globular structure throughout the slug. There are clear geometric differences between both types of recipients because the double-shell recipient needs spaces for implementing securing systems between half-shells, securing with the shaking table and support-grip for handling the movable half-shell.

With this new recipient (crucible) architecture, one of the half-shells is permanently secured to the vibration (shaking) table and the other one will be moveable. The moveable one will have three positions:

1^st position: Secured to the table and to the other half-shell at the time of pouring.
2^nd position: It is then released, transporting and depositing the slug on the container of the HPDC injector.
3^rd position: Cleaning and preparing for the next cycle.

The movable half-shell has a type of grip plate which allows grabbing it and handling it by means of robot, manipulator or manually. Taking into account that the thicknesses must be uniform in the recipient, an insulator is introduced between the movable half-shell and the grip plate. This grip plate must ideally be located in the upper part of the recipient, at a height which the liquid aluminum/slug never reaches.
The bottom of the half-shells coming into contact with the shaking table can be open or closed, whichever is appropriate, because the drain hole is not required.
The cleaning process is much simpler in the double-shell system because the movable half-shell is brought closer to the cleaning-lubricating head which is always accessible and in the same position.
The double-shell system does not require much lubrication because the transverse demolding of the semi-crucibles is very easy due to the fact that it already has natural demolding. As less lubrication is needed, the thickness of the oxide layer enveloping the slug is less and the risk of displacing said oxides to the part when filling the mold is lower.
The resulting process is as follows:

A) Melting aluminum by means of traditional methods: melting and pouring on holding furnace (also referred to as metering unit).
- Pouring the aluminum on double-shell cylindrical recipients (crucibles). The crucible is tipped to favor pouring and to reduce the generation of gases.
B) Shaking the double-shell crucibles (on a vibration table) and cooling the aluminum until it reaches a suitable globular structure. The solid fraction will make up about 50% as a function of the percentage of Si (proximity to the eutectic point).
C) Turning the crucible until it is horizontal and separating the movable half-shell such that the slug remains therein to be transported to the container of the HPDC injector.
D) Depositing the slug on the container is reduced to turning the half-shell and letting the slug fall by rolling-gravity. Pushing the semi-solid slug with the piston of the HPDC injector to fill the steel mold.

The process also comprises the following auxiliary steps:

Standard crucible preparation process: cleaning, heating and lubricating.
Operation for mounting and securing the crucible on the vibration (shaking) table.

The crucible recipient in two halves (half-shells) allows the transverse demolding of the slug and accordingly:

Simplifies the lubrication process and reduces the thickness of the oxide layer generated on the skin of the slug.
Eliminates the risks of the slugs breaking.
Eliminates the risks of the skin of the slug being adhered to the recipient. It increases the quality of the parts and reduces deterioration.

By eliminating the longitudinal extraction of the slug, the risk of the slug adhering to the crucible (completely or partially) is reduced, reducing interruptions caused by insufficient lubrication. In addition to the interruptions, the recipient cleaning processes are significantly reduced, especially when the skin remains adhered to the crucible.
The transport and pouring of the slug into the container of the HPDC injector is simplified and the risks of the slug breaking are reduced. This advantage will be more important as the size of the slugs increases.
An important part of the crucible lubrication and preparation processes is eliminated. The process becomes more linear and more efficient, and it provides a prior verification that no residues are left in the crucible with the subsequent need to eliminate them.

Claims

Equipment for manufacturing metal injection parts, comprising a vibration table, a high-pressure injection equipment (5) and a device for obtaining semi-solid sludges, characterized in that the device for obtaining semi-solid sludges comprises a crucible (1, 2) comprising at least one fixed part (1) which is fixed to the vibration table, and at least one moveable part (2) which can be attached to the fixed part (1) through fixing means (3).
Equipment according to claim 1, wherein the fixing means (3) consist of an articulated clamp which allows fixing the moveable part (2) to the fixed part (1).
Equipment according to any of the preceding claims, comprising a fixed part (1) and a moveable part, both being semi-cylindrical.
Method for obtaining semi-solid sludges, characterized in that it comprises using the equipment according to any of claims 1 to 3.
Method according to claim 4, which comprises:
A) melting aluminum and pouring said molten aluminum into the crucible formed by at least one moveable part (2) and at least one fixed part (1), tipping the crucible to favor pouring and to reduce the generation of gases,

B) shaking the crucible and cooling the aluminum,

C) turning the crucible until it is horizontal and separating the moveable part (2) such that an obtained solid mass (4) is housed in said moveable part (2) for transport to high-pressure injection equipment (5), and

D) depositing the semi-solid mass (4) on the container of the high-pressure injection equipment (5) by means of turning the moveable part (2), letting said semi-solid mass (4) fall due to the effect of gravity, and pushing the semi-solid mass (4) with the piston of the high-pressure injection equipment (5).
Method for shaping materials which comprises obtaining semi-solid sludge by means of the method according to any of claims 4 and 5.