ITBO20130146A1 - MACHINE AND METHOD OF PRESSOCOLATE IN SEMISOLIDO - Google Patents

Description

DESCRIPTION

â € œMACHINE AND SEMI-SOLID PRESS-COOKING METHODâ €

TECHNIQUE SECTOR

The present invention relates to a machine and a semi-solid diecasting method.

ANTERIOR ART

In the automotive industry, the introduction of increasingly restrictive regulations by legislators with regard to pollutants continues to constitute an ineluctable push towards the search for solutions aimed at reducing consumption both through the efficiency of combustion of the engine and, above all, with the introduction of light alloys or technopolymers to contain the overall mass of the vehicles.

In this context, aluminum alloys can represent an optimal solution in consideration of low density values, adequate technological characteristics and complete recyclability. However, even if the aluminum transformation processes, in particular the smelting ones, make use of more and more refined simulation and process control methodologies, in fact in the last decades there have not been any upheavals that have changed the scenario. production: die casting, gravity casting and low pressure casting make up almost all of the forming processes.

Die-cast components, in particular, thanks to the possibility of obtaining thin sections and integrated solutions, allow significant weight and cost reductions but do not allow the achievement of structural characteristics which generally remain pertinent to gravity castings.

The efforts of the foundries to improve the quality of the diecastings with vacuum injection techniques or in the laminar mold filling regime are therefore understandable to avoid the incorporation of air and therefore allow the execution of the heat treatment of reclamation. However, the elimination of solidification shrinkage is much more difficult since, in diecasting, it is very difficult to feed the casting during the solidification phase. For this reason, when the first patents relating to the methods of obtaining particular microstructures were filed a few decades ago, a prelude to the possibility of performing semi-solid phase forming processes, at the same time expectations for the development of new casting technologies grew. that could combine low transformation costs with excellent metallurgical and mechanical characteristics.

At the beginning of the 70s, at the M.I.T. of Boston, it was demonstrated that it is possible to obtain a globular (non-dendritic) microstructure of the solid phase as long as the metal bath is kept under vigorous agitation during solidification and therefore it is practically impossible to grow the dendrites. An alloy with a globular microstructure possesses, at temperatures included in the solidification range, pseudoplastic and thixotropic properties, ie its viscosity decreases with increasing agitation and with time. These characteristics allow forming operations by means of conventional deformation or casting processes as the semi-solid, subjected to flow, practically retains the characteristic properties of liquids.

As regards diecasting, in particular, the main advantages deriving from the use of an alloy in the semi-solid state are now known and concern both the product and the process.

The essential condition for obtaining a microstructure with thixotropic properties consists in promoting the formation of the largest possible number of nuclei followed by solidification conditions that achieve the â € œmultiplication of the dendritesâ €: this is made possible by a rapid cooling of the metal alloy with simultaneous agitation during the first stages of solidification.

Over the last few decades, the most varied techniques capable of obtaining metal alloys with a globular microstructure have been developed and partly industrialized.

In addition to the known ones based on mechanical and electromagnetic mixes, the ones inherent to the treatment of a metal bath with the immersion of an ultrasonic probe should also be remembered: in this case the mechanism for the formation of the nondendritic fine structure is based on high power ultrasonic vibrations which induce cavitation and acoustic sliding phenomena in the metal alloy: the entity of the phenomena involved in the growth and collapse of unstable bubbles generates hydraulic waves that produce artificial sources of solidification nuclei.

As regards the operating methods for the production of castings, a first technological classification is based on the possibility of using the metallic material with a globular microstructure for the production of castings directly starting from the liquid phase (Rheocasting) or on the use of material previously prepared and subsequently preheated starting from the solid phase (thixocasting).

In the thixocasting process, the thixotropic billet is cut into blocks (of weight corresponding to the casting to be produced) which are positioned on an induction heating table, brought to a temperature included in the solidification range and picked up by a robot which they are loaded into the injection container of a diecasting machine for subsequent injection into the mold. After extraction from the mold, the casting is sheared to separate the piece from the feed channels and overflow sprues which are sent for recycling generally â € œout-sourceâ €.

For many years the various component productions have been made (and in part still are) according to the thixocasting process on the basis of evident industrial advantages: on the one hand the producers of raw materials able to produce globular microstructure billets starting from continuous casting processes, on the other hand transformers, or foundries with exclusive skills for the production of castings.

Despite the initial technological enthusiasm and a progressive general interest in semi-solid forming processes on the part of vehicle manufacturers, the difficulties linked to true mass production were already perceived starting from the early 1990s in consideration of a series of factors such as the costs for the acquisition of the raw material, additional costs related to the recycling of waste and, above all, a know-how that is still too specialized for the realization of the castings.

In any case, the problem of industrial costs, relating to the need to invest in specific equipment for the preheating of the thixotropic material, placed the semi-solid transformation processes in an industrial environment that was not well defined: the semi-solid technology certainly presented itself competitive compared to forming processes such as forging or die casting (albeit with the limit of some performance characteristics), but with significant additional costs compared to conventional die casting (albeit with recognized metallurgical and mechanical improvements of the parts produced). Consequently, the actual industrial applications were reduced in number compared to a much more significant potential. For example, it was difficult to economically compete with die-cast parts too often defined as â € œgood-enoughâ € and at times it could even seem unrealistic to compare oneself from a metallurgical sanity point of view with pieces produced in gravity. It was only at the end of the 90s that a new renaissance of semi-solid molding could be witnessed: starting from this period new techniques of the Rheocasting process were developed, that is the technological variant based on the ™ immediate use of the thixotropic material (â € œslurry on demandâ €).

All these new techniques were inspired by the simple consideration that it is sufficient to operate a rapid cooling of the metal bath (to favor the increase of the solidification cores) and at the same time an adequate mixing (to be applied in any case only in the first stages of solidification) to promote dendritic multiplication (in the phases in which the dendrites are extremely thin and unstable in consideration of a high ratio between surface and volume) and, above all, the thermal homogenization of the semi-solid charge being formed.

The advantages deriving from the immediate use of the semi-solid material are undeniable: the use of preheating systems of the thixotropic material is no longer necessary (generally carried out with induction or forced convection techniques) and it is also possible to recycle directly â € œin-houseâ € sprues, sprues and any waste.

These techniques are generally able to generate metallic material with a globular microstructure and to allow its immediate use in the known semi-solid forming processes.

Common aspects, albeit with different operational variants, concern the methods of withdrawing the molten material from a holding furnace to an intermediate vessel for the preparation of the semi-solid (1st transfer) and the subsequent phases of pouring the metal charge from said container in the injection container of the various types of presses (2nd transfer).

And even if the operations aimed at creating the desired microstructure are adequate, nevertheless it may be the above-mentioned steps of transferring the metallic material that significantly penalize the metallurgical characteristics and the defectiveness of the castings produced. In fact, each pouring of the metal corresponds to an incorporation of oxides (â € œbi-filmsâ €) which have deleterious effects on the health and mechanical characteristics of the pieces; moreover, if the metal, after mixing, must necessarily be poured into the injection container of the diecasting machine, it is obvious that the fraction of solid (depending on the extent of the thermal transfer performed) must necessarily be limited: otherwise the 2nd payment would not be practicable with the consequence of negatively affecting the amount of solidification shrinkage inside the castings.

A process capable of reducing the aforementioned criticalities is the â € œSub Liquidus Castingâ € (SLC). In it, the preparation of the metal is carried out directly inside the injection container of the press: in this way the second pouring is avoided and, consequently, no restrictions are placed on the size of the fraction of the injectable solid. In another technological variant, called EMS, unlike the SLC process which does not require any type of mixing, an electromagnetic mixer is used to accelerate the preparation of the semisolid. The aforementioned technologies, however, require vertical injection presses which, in addition to being not very widespread, are already dedicated to semi-solid technology.

It is necessary to underline that the commercial success of the individual processes is linked not only to the pursuit of the desired metallurgical and mechanical characteristics of the castings produced but, above all, to the investments and management costs of the various equipment used and to the reliability of the processes. productive: otherwise the new technologies are applied to market niches with the results of poor penetration for mass productions.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a machine and a method of semi-solid diecasting which are free from the drawbacks described above and are at the same time easy and economical to produce.

According to the present invention, a machine and a method of semi-solid diecasting are provided, as claimed in the attached claims. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the attached drawings, which illustrate some non-limiting examples of embodiment, in which:

â € ¢ Figure 1 is a schematic view of a semi-solid diecasting machine made in accordance with the present invention;

â € ¢ Figures 2-5 are schematic views of the semi-solid diecasting machine of Figure 1 during subsequent stages of the diecasting process; And

â € ¢ Figure 6 is a schematic view of a variant of the semi-solid diecasting machine of Figure 1.

PREFERRED FORMS OF IMPLEMENTATION OF THE INVENTION

In figure 1, the number 1 indicates as a whole a semi-solid diecasting machine.

The semi-solid diecasting machine 1 comprises a diecasting mold 2, inside which a molding chamber 3 is obtained which reproduces in negative the shape of the piece to be made and is intended to receive the metal casting. melted for the subsequent solidification of the casting itself. The diecasting mold 2 is composed of a mobile die 4 and a fixed die 5 which, during the feeding of the molten metal jet, are mechanically coupled together to form the molding chamber 3; once the cast of molten metal is sufficiently solidified, the mobile die 4 is moved away from the fixed die 5 to allow the extraction of the piece from the mold 2.

The diecasting mold 2 is supported by an injection press 6, which pressures the jet of molten metal into the molding chamber 3. The injection press 6 comprises a vertical support plane 7, to which the fixed matrix 5 of the mold 2 is mechanically fixed. The injection press 6 comprises a cylindrical tubular injection container 8, which extends horizontally and is arranged through a corresponding cavity obtained in the support plane 7 and through a corresponding cavity obtained through the fixed matrix 5 of the diecasting mold 2.

Inside the injection container 8 there is a cylindrical preparation chamber 9 into which the molten metal (liquid) is poured before injecting the molten metal (semisolid) itself inside the molding chamber 3. At an opposite end to the diecasting mold 2, the preparation chamber 9 is delimited by an injection piston 10 which is mounted axially sliding along the preparation chamber 9 itself; in correspondence with the diecasting mold 2 (i.e. on the opposite side of the injection piston 10) the preparation chamber 9 is delimited by a counterjet 11 which is obtained in the movable die 4 of the diecasting mold 2 and is facing and opposite the injection piston 10. On the opposite side of the diecasting mold 2 (i.e. outside the support surface 7), the injection container 8 has a feed opening 12 which is formed passing through an upper wall of the injection container 8 and in use is used to feed the molten metal inside the preparation chamber 9. In correspondence with the counter-jet 11 there is a channel 13 for feeding the jet which puts an upper portion of the preparation chamber 9 in communication with a lower portion of the molding chamber 3.

An ultrasonic device 14 is provided, which is coupled to the injection container 8 of the diecasting machine 1 and is able to generate very high frequency mechanical vibrations in the injection container 8 and therefore in the molten metal contained in the chamber. 9 of preparation; in particular, the ultrasound device 14 generates ultrasounds which are channeled towards the injection container 8. The ultrasound device 14 comprises a piezoelectric transducer 15 which constitutes the source of the ultrasounds and a sonotrode 16 which conveys the ultrasounds towards the injection container 8. The piezoelectric transducer 15 is driven by an electronic controller 17 which sends to the piezoelectric transducer 15 the electrical driving waves which the piezoelectric transducer 15 transforms into mechanical waves. Preferably, the ultrasonic device 14 also comprises a sensor 18 (typically an accelerometer), which is mechanically coupled to the injection container 8, measures the vibrations of the injection container 8 and communicates with the electronic controller 17.

In use, the electronic controller 17 generates the electrical driving signals of the piezoelectric transducer 15 with a feedback logic based on the measurements received from the sensor 18: essentially, the electronic controller 17 varies the frequency, amplitude and / or shape of the electrical driving signals. of the piezoelectric transducer 15 until the optimal combination is found which allows to maximize the transmission of the ultrasounds to the injection container 8.

By way of example, the ultrasound device 14 could be made and controlled according to what is described in patent application EP1238715A1.

Preferably, the piezoelectric transducer 15 is arranged outside the injection container 8 and the diecasting mold 2 and the sonotrode 16 is arranged in a hole formed through the fixed matrix 5 of the diecasting mold 2. Eventually, the injection container 8 could also have a small blind hole to bring the tip of the sonotrode 16 closer to the preparation chamber 9. It is important to note that the ultrasound device 14 is arranged in correspondence with one end of the injection container 8 arranged in proximity to the jet feed channel 13.

According to a preferred embodiment, a heating device 19 (typically a thermoresistance) is provided which is arranged in proximity to the preparation chamber 9 and is able to be activated to heat the molten metal contained in the preparation chamber 9. In the embodiment illustrated in the attached figures, the heating device 19 is embedded inside the injection container 8 (for example the heating device 19 could have an annular shape and could therefore be arranged all around the preparation chamber 9) .

Finally, the semisolid diecasting machine 1 comprises a control unit 20 which supervises the operation of the semisolid diecasting machine 1 itself.

The operation of the semi-solid diecasting machine 1 for the production of a mechanical part 21 is described below (illustrated in Figure 5). Purely by way of example, the metal used to make the mechanical part 21 could be an aluminum and silicon alloy, for example alloy A356 having a liquidus temperature of 617 ° C.

Initially and as illustrated in Figure 1, the semi-solid diecasting machine 1 is completely empty (i.e. it is free of molten metal) and the injection piston 10 is completely retracted to give the preparation chamber 9 the maximum size.

Subsequently and as illustrated in Figure 2, molten metal 22 in a completely liquid state is fed into the preparation chamber 9 through the feed opening 12. In particular, the molten metal 22 is taken in a completely liquid state from a holding oven at a temperature of about 650 ° C and is poured by means of a generic pouring cup into the injection container 8 through the opening 12 of Power supply.

Subsequently and as illustrated in Figure 3, the injection piston 10 is translated at low speed and axially along the preparation chamber 9 to reduce the axial dimension of the preparation chamber 9 and therefore concentrate the molten metal 22 in proximity to the counterjet 11, or in the vicinity of the jet feed channel 13. Basically, the injection piston 10 advances at low speed to reduce the volume of the preparation chamber 9 until the molten metal 22 occupies substantially the entire useful volume of the preparation chamber 9 itself.

Once the molten metal 22 has been concentrated in the vicinity of the counterjet 11, that is, in the vicinity of the jet feed channel 13 and as illustrated in Figure 3, a partial solidification time interval is expected (lasting a few tens of seconds, for example example 42-52 seconds) during which a part of the molten metal 22 solidifies inside the preparation chamber 9. The partial solidification of the molten metal 22 occurs due to the cooling induced by the contact with the walls of the preparation chamber 9 (i.e. with the walls of the injection container 8), since the injection container 8 has a much lower temperature than the temperature. of the molten metal 22. During the entire partial solidification time interval, the ultrasonic device 14 is kept active to generate very high frequency mechanical vibrations in the injection container 8 and therefore in the molten metal 22 contained in the preparation chamber 9; these mechanical vibrations cause a continuous whirling mixing of the molten metal 22. In other words, the ultrasonic device 14 induces strong mechanical oscillations inside the molten metal 22 which cause a continuous whirling of the molten metal 22 itself.

Basically, during the partial solidification time interval, the molten metal 22 contained in the preparation chamber 9 is subjected to both a high cooling rate determined by contact with the metal walls of the injection container 8, and to a continuous swirling mixing and therefore the two necessary and sufficient conditions are achieved to obtain a material with a globular microstructure: mixing and rapid cooling. This globular microstructure will therefore be able to give the molten metal 22 marked rheological characteristics which allow it to be injected in a pasty state at temperatures within the solidification range.

Under the action of the ultrasonic field induced by the sonotrode 16 mounted on the lower part of the injection container 8 and due to the cooling effect caused by the contact with the walls of the injection container 8 itself, the molten metal 22 is subjected to the double action of mixing and partial solidification until reaching a temperature included in the solidification range (for example 585 ° C with a solid fraction equal to 50%). Obviously, the heating device 19 can be used to control the temperature of the molten metal 22 during all the time required by the ultrasounds to exert their action on the microstructure of the metal charge.

Once the partial solidification phase of the molten metal 22 has been completed to obtain a semi-solid state, that is, at the end of the partial solidification time interval, the injection piston 10 is further axially translated along the preparation chamber 9 (as illustrated in figure 4) to further reduce the axial dimension of the preparation chamber 9 and then push (inject) the metal 22 in the semi-solid state through the jet feed channel 13 and then into the molding chamber 3.

Once the filling of the molding chamber 3 with the metal 22 in the semi-solid state has been completed and an adequate cooling time has been waited to allow the molten metal 22 to cool down enough to assume the solid state (i.e. to assume its own shape no longer modifiable), the diecasting mold 2 is opened by translating the movable die 4 to extract the finished mechanical piece 21 (as illustrated in Figure 5).

It is important to note that the ultrasonic device 14 does not need to be immersed inside the molten metal (ie the liquid metal bath) to induce the propagation of ultrasonic energy; in fact, by means of the ultrasonic device 14 the multi-frequency ultrasonic power is able to overcome thick metal barriers, such as the walls of the injection container 8, and the ultrasounds are introduced into the molten metal through the resonance of the walls of the container 8 of injection capable of creating an acoustic pressure gradient. Cavitation and acoustic streaming phenomena therefore induce those phenomena of mixing in the molten metal 22 necessary to give globular shape to the solid particles being formed; cavitation, in particular, also promotes the heterogeneous nucleation process with the additional result of a refinement of the microstructure.

According to a possible embodiment, the counterjet 11 is movably mounted to move between an injection position (illustrated in Figures 1-4) in which the counterjet 11 does not interfere with the jet supply channel 13, and a treatment position (illustrated in Figure 6) in which the counterjet 11 closes the access to the jet feed channel 13 (ie it prevents the molten metal 22 present in the preparation chamber 9 from entering the jet feed channel 13). The counterjet 11 is placed in the treatment position (illustrated in Figure 6) when the molten metal 22 is fed into the preparation chamber 9 and until the partial solidification process of the molten metal 22 is completed; at the end of the partial solidification process of the molten metal 22, the counterjet 11 is placed in the injection position (illustrated in Figures 1-4) to allow the injection of the molten metal 22 into the molding chamber 3.

The semi-solid die casting machine 1 described above has numerous advantages.

In the first place, the semi-solid diecasting machine 1 described above is extremely economical to manufacture, as it is completely identical to a commercial diecasting machine 1 with the sole addition of the ultrasonic device 14 and, possibly, of the device 19 heater. Consequently, to make the semi-solid die casting machine 1 described above, it is possible to start from a commercial die casting machine 1 to which the missing parts are added (the ultrasonic device 14 and the heater device 19). It is therefore evident that even the â € ™ upgradeâ € ™ of an existing diecasting machine 1 directly in the production plant is simple and economical.

Furthermore, the semi-solid diecasting machine 1 described above allows to proceed directly with the injection of the metal 22 in the semi-solid state inside the diecasting mold 2 without the need for any additional transfer since, as described, the metal 22 at the the semisolid state is prepared directly in the injection container 8 of the semisolid diecasting machine 1. In this way, the mixing induced by the ultrasounds can continue inside the injection container 8 for all the time necessary to reach high fractions of solid (even up to 52%) which would be difficult to reach with different operating modes. The known processes, in fact, require the mixing phase to be carried out inside dedicated containers or equipment and it is only at the end of this operation that the pasty material can be poured into the injection container: consequently the solid fraction cannot exceed values of the order of 10-15% otherwise the high viscosity of the semi-solid could probably prevent the pouring operation itself.

Finally, in the semisolid diecasting machine 1 described above, the piezoelectric transducer 15 of the ultrasonic device 14 is relatively spaced from the diecasting mold 2 and from the injection container 8, i.e. it is relatively far from the hottest parts that come into contact with the molten metal 22; in this way it is possible to keep the piezoelectric transducer 15 of the ultrasound device 14 at a low working temperature (not higher than 100-120 °) which does not affect its operation or its useful life.

Claims (10)

CLAIMS 1) Semi-solid diecasting machine (1); the semi-solid diecasting machine (1) includes: a diecasting mold (2), inside which there is a molding chamber (3); an injection container (8) in which a preparation chamber (9) is formed which is adapted to receive molten metal (22) in the liquid state and at one end has a jet feed channel (13) communicating with the molding chamber (3); and an injection piston (10) which is mounted axially movable along the preparation chamber (9) and is arranged on the opposite side with respect to the jet feed channel (13); the semisolid diecasting machine (1) is characterized by the fact that it comprises an ultrasonic device (14), which is coupled to the injection container (8) and is able to generate very high frequency mechanical vibrations in the container (8) of injection and then into the molten metal (22) contained in the preparation chamber (9). 2) Semi-solid diecasting machine (1) according to claim 1, wherein: the ultrasound device (14) comprises a transducer (15) which constitutes the source of the ultrasounds and a sonotrode (16) which conveys the ultrasounds towards the injection container (8); and the transducer (15) is arranged outside the injection container (8) and the diecasting mold (2). 3) Semi-solid diecasting machine (1) according to claim 2, wherein the sonotrode (16) is arranged in a through hole made through a fixed die (5) of the diecasting mold (2). 4) Semi-solid diecasting machine (1) according to claim 2 or 3, in which the injection container (8) has a blind hole inside which the tip of the sonotrode (16) is arranged. 5) Semi-solid diecasting machine (1) according to claim 2, 3 or 4, wherein the ultrasonic device (14) comprises: a sensor (18), which is mechanically coupled to the injection container (8) and measures the vibrations of the injection container (8) itself; and an electronic controller (17) which drives the transducer (15) according to the reading of the sensor (18). 6) Semi-solid diecasting machine (1) according to one of claims 1 to 5, in which the ultrasonic device (14) is arranged at one end of the injection container (8) arranged near the channel (13 ) to feed the jet. 7) Semi-solid diecasting machine (1) according to one of claims 1 to 6 and comprising a counterjet (11) which delimits the preparation chamber (9) from the side opposite the injection piston (10) and is mounted movably for move between an injection position in which the counterjet (11) does not interfere with the jet supply channel (13), and a treatment position in which the counterjet (11) closes the access to the supply channel (13) of the jet. 8) Semi-solid diecasting machine (1) according to one of claims 1 to 7 and comprising a control unit (20) which: performs a first translation of the injection piston (10) along the preparation chamber (9) to reduce the axial dimension of the preparation chamber (9) and thus concentrate the molten metal (22) in proximity to the feed channel (13) of the jet; performs a second translation of the injection piston (10) along the preparation chamber (9) to further reduce the axial dimension of the preparation chamber (9) and then push (inject) the molten metal (22) through the channel (13) feeding the jet and towards the molding chamber (3); performs a semi-solidification of the molten metal (22) inside the preparation chamber (9) between the first translation of the injection piston (10) and the second translation of the injection piston (10), i.e. after performing the first translation of the injection piston (10) and before carrying out the second translation of the injection piston (10); And generates, during the semi-solidification of the molten metal (22) inside the preparation chamber (9), ultrasounds which are transmitted to the injection container (8) to determine a whirling mixing of the molten metal (22) with the ™ inside the preparation chamber (9). 9) Semi-solid diecasting method; the semi-solid diecasting method includes the steps of: to feed the molten metal (22) in a completely liquid state inside a preparation chamber (9) obtained in the injection container (8) and bounded at one end by an injection piston (10) ; perform a first translation of the injection piston (10) along the preparation chamber (9) to reduce the axial dimension of the preparation chamber (9) and therefore concentrate the molten metal (22) in proximity to a supply channel (13) of the jet arranged on the opposite side with respect to the injection piston (10); and perform a second translation of the injection piston (10) along the preparation chamber (9) to further reduce the axial dimension of the preparation chamber (9) and then push (inject) the molten metal (22) through the channel (13 ) feeding the casting and towards a molding chamber (3) obtained in a diecasting mold (2); the semi-solid diecasting method is characterized by the fact that it includes the further steps of: perform a semi-solidification of the molten metal (22) inside the preparation chamber (9) between the first translation of the injection piston (10) and the second translation of the injection piston (10), i.e. after performing the first translation of the injection piston (10) and before carrying out the second translation of the injection piston (10); And generate, during the semi-solidification of the molten metal (22) inside the preparation chamber (9), ultrasounds that are transmitted to the injection container (8) to determine a vortex mixing of the molten metal (22) with the ™ inside the preparation chamber (9). 10) Semi-solid die casting method according to claim 9, in which the partial solidification of the molten metal (22) inside the preparation chamber (9) determines the formation of a globular microstructure due to the cooling induced by contact with the walls of the injection container (8) and due to the vortex mixing induced by ultrasounds.