SEMI-SOLID PROCESSING OF ALUMINIUM ALLOYS

Abstract: A method for bringing aluminium alloy blocks, such as ingots, billets and others to a semi-solid and semi-liquid state which allows them to be moulded, according to which there is a first phase for refining the said aluminium alloy by adding a silicon-boron alloy to the said aluminium alloy when it is melted, a second phase for heating the said ambient temperature blocks until a temperature is reached in which there is a balance between its solid and liquid states and in which the phases of eutectic silicon are liquid and the phases of alpha -aluminium are solid and a third operating phase for using the said alloy blocks obtained in the previous phases inside the moulding system in the semi-solid and semi-liquid state, through which the resulting semi-finished products are obtained.

Description: SEMI-SOLID PROCESSING OF ALUMINIUM ALLOYS Technical Field The present invention relates to a method for bringing aluminium alloy blocks, such as ingots, billets and others to a semi-solid and semi-liquid state which allows them to be moulded when this state has been reached. Background Art A new type of moulding for metal alloys has been recently introduced, for example, moulding, pressure injection, forging and similar operations, in which the metal alloy forming the blocks (ingots or billets, for example) to be moulded are in a semi-solid or semi-liquid state and have a special homogeneous structure consisting of solid globules or granules immersed in a liquid phase. This means that this special type of moulding requires the either partial or total fusion of the alloy phases at the lowest fusion point and the maintenance of the globular phases, which ensure that the moulding phase is reached. In practise, this structure of these metal alloys homogeneously consists of solid globules immersed in a liquid phase. Therefore, the structure has no dendrites, that is, it has no crystals growing around the crystallisation nuclei in a particular direction. Therefore, these structures, known to experts in the field as globular structures, have the so-called thixotropic property. Figure 1, which is a schematic diagram of the state of a metal alloy suitable for moulding as described above, shows that to the left of point A there is only material in a liquid state while to the right of point C, there is only material in a solid state. In the area between points A and C, there is material in a semi-solid or semi-liquid state. To be more specific, in the area between points B and C, there is a material in which there are solid granules or globules immersed in a liquid phase consisting of the eutectic alloy. Moving from point C towards point B, the percentage of eutectic alloy found in the liquid state compared to the solid crystals increases from 0 to 100. Moving from point B to point A, on the other hand, increases the percentage of solid solution crystals which pass to the liquid state from 0 to 100. The area of interest in relation to the thixotropic alloys is normally the area between points B and C in which there are solid crystals immersed in eutectic alloy in the liquid state and part of the area between points B and A depending on the liquid fraction required. A block of this alloy behaves like a solid element when it is handled for transfer and similar operations but behaves like a liquid element when it is subjected to any moulding operations. As previously described, a block found in this state is without dendrites which would compromise its homogeneity and its mechanical resistance. This is well known to experts in the field. Figure 1, and in particular the area between points B and C, also shows that heating alone is not sufficient to bring the material to the required semi-solid or semi-liquid state. It is necessary to maintain this temperature for a given period of time. Currently, in relation to this, in order to produce metal alloy blocks, in particular, aluminium alloy blocks, which have the said globular structure (previously obtained) ideal for use in the said moulding operations, there are, substantially, three methods of preparation or globulisation of the alloys: electro-magnetic or mechanical stirring treatment, heat conversion and the super-refining of the alloy grain with heat conversion. Hereafter, Su will refer to the physical state of the alloy block, meaning the globular structure of the block ideal for use in subsequent moulding to obtain the required semi-finished item. The first treatment of the aluminium alloys envisages, therefore, a casting phase starting at a temperature of around 700°C and progressive cooling of the aluminium alloy, using suitable time intervals, until ambient temperature is reached. This allows the alloy to be formed into ingots and billets of the dimensions required. During the cooling phase, which follows a pattern of the type illustrated in Figure 1, there is the above-mentioned stirring treatment. In practise, the stirring treatment of the alloy is carried out when the alloy passes between the liquid temperature (around 610/620°C, point A of the curve in Figure 1) and the solid temperature (around 577°C, point C of the curve in Figure 1) . In this area, the alloy is agitated in such a way that it forms these globular structures ideal for bringing the alloy to its Su state and, therefore, for use in the subsequent heating phases which prepare it for its moulding phase. The second, so-called heat conversion treatment, has substantially the same procedure of cooling the alloy until it reaches the ambient temperature so that it can be worked as described above. At this point, the billet (or ingot), following a short period of time at the ambient temperature, is heated again, preferably using a temperature gradient between 20 and 30°C per minute, until it reaches a temperature between the liquid temperature and the solid temperature (See Figure 1) . When this temperature is reached, there is an operating phase for maintaining the billet at this temperature for a number of hours (2 hours, for example) and then bringing the billet back to ambient temperature in which its Su physical state is obtained. This type of treatment, therefore, includes the permanence of the alloy at a temperature between the liquid and solid temperatures for the time necessary for the globulisation of the alpha-aluminium solid solution phase. In relation to the third type of treatment, which is substantially the same as the second, it has recently been discovered that the time for which the billet remains at the semi-solid and semi-liquid state is less than around five minutes if the starting alloy structure has been previously super-refined with the master alloy Al-Ti-B. The quantities of the components must be such that the Ti content is at least 0.26% of the weight. To summarise, the treatment systems currently used for bringing the alloys, and therefore the billets made in this alloy, to the special physical state described above and referred to as Su, in which state it is possible to use these billets to give them thixotropic moulding, envisage the following: the casting of the billet with a magnetic and mechanical stirring phase during the solidification phase in order to keep the alloy agitated and obtain the ideal globulisation of the alpha-aluminium phase (in this particular case) and, as an alternative to this, the heat conversion treatment system, in which the billets become globular only if they are maintained for a given period of time at a temperature between the solid temperature and the liquid temperature, which allows the dendrites to gradually become spherical. It is obvious that the stirring treatment requires the use of sophisticated and complex equipment which can carry out the said operating phases before the billet reaches its Su physical state. Moreover, this kind of equipment requires very precise and efficient control systems which ensure the control of the various parameters involved (temperature, time, etc.) during the operating phases of casting, cooling and stirring before the billets, obtained in this way, can then be inserted in the casts for moulding. In the case of heat conversion treatment, the operations which characterise this treatment are extremely complicated. In fact, we have seen that it is necessary to maintain the billet at the said semi-solid and semi-liquid temperature for a given time. In this situation, the alloy is extremely unstable. In fact, a small temperature change is sufficient to liquefy the entire alloy or to solidify it too much, thereby rendering the effect .of the treatment useless or insufficient. It is also obvious that this type of treatment must be continuously controlled and is extremely costly because of the large amounts of energy which the manufacturer must produce and sustain. In this regard, it should be noticed that, in theory, in the case of conversion treatment, the phase for the cooling of the billets can be avoided and the billets, previously maintained at the temperature between the solid temperature and the liquid temperature for a given period of time, can be inserted inside the cast or the press for immediate moulding. This, however, is not applicable in practice since, if the billets must be maintained for two hours at the said temperature and then inserted in the press, one per minute (the time necessary for their moulding), it is necessary to have a heating furnace designed to contain 120 billets at this temperature. Disclosure of the Invention The object of the present invention is to provide a method for bringing the billets, ingots and similar objects to the semi-solid and semi-liquid state which allows them to be moulded, avoiding the need for these treatments described above for bringing the said aluminium alloy billets to the Su state essential for carrying out the subsequent moulding phase to obtain the semi-finished items and which has none of the disadvantages described with reference to the known prior art. The present invention discloses a method for bringing aluminium alloy blocks such as ingots, billets and others to the semi-solid and semi-liquid state which allows them to be moulded when this state has been reached, characterised in that it envisages the following operating phases: - a first phase for refining the grain of the said aluminium alloy, consisting of blocks, by adding to the said aluminium alloy, when it is melted, a silicon-boron alloy which produces a refined alloy consisting mainly of α-aluminium and eutectic silicon phases; - a second phase for heating the said blocks of this refined alloy, starting from a condition in which the temperature is substantially the ambient temperature, to bring the said refined alloy to a temperature in which there is a balance between its solid state and its liquid state and in which the eutectic silicon phases are liquid and the α-aluminium phases are solid; - a third phase for using the said alloy blocks, obtained in this way by the previous operating phases, directly inside the moulding system in their semi-solid and semi-liquid state to obtain the resulting semi-finished products. The advantages and the characteristics of the present invention are best described by the detailed description that follows made with reference to the accompanying drawings, in which: - Figure 1 is the temperature-time-state graph for a possible thixotropic type metal alloy, equivalent to the aluminium alloy which is the subject of the present invention; - Figure 2 is the temperature-time-state graph for the phase for heating from ambient temperature to the temperature between solid and liquid which is obtained so that the billet can be moulded. The present invention, therefore, relates to a method for bringing aluminium alloy blocks, such as ingots, billets and others to a semi-solid and semi-liquid state which allows them to be moulded and therefore allows the manufacturing of a semi-finished product, car wheels, for example. In its most general form, the method envisages a first phase for refining the grain of the aluminium alloy which forms the blocks. In this respect, this degree of refinement is reached by adding a silicon-boron alloy to the aluminium alloy, when it is melted. In this way, a refined alloy is obtained which consists primarily of α-aluminium and eutectic silicon phases. With this refining phase, the aluminium alloy and, therefore, the aluminium alloy billets are brought to their Su state, as seen in Figure 2. Again with reference to Figure 2, the second operating phase envisages the heating of the billets or ingots made using the refined alloy, starting from the Su physical condition in which its temperature is substantially the ambient temperature ta, to bring the said refined alloy to a temperature in which there is a balance between its solid state and its liquid state and in which the phases of eutectic silicon are liquid and the phases of α- aluminium are solid. The temperature tf is reached in a suitable time which is shown in Figure 2 from TO to TF. The method then envisages a third operating phase for using the alloy billets, obtained by the previous operating phases, directly inside the moulding system in their semi- solid and semi-liquid state to obtain the resulting semi¬ finished products. In its most common form, the method according to the present invention envisages the substitution of the first phase for the refinement of the grain of the aluminium alloy described above with the use of already refined aluminium alloy blocks. In particular, the alloy used in place of the said first operating phase is a refined aluminium alloy obtained by adding a silicon-boron alloy to the aluminium alloy, when it is melted. This silicon-boron alloy has a boron weight of between 0.01 and 4.0% and is of a quantity which ensures that the resulting fusion of the aluminium alloy contains at least 50 p.p.m. of boron. This type of alloy is obtained with a special method which was the subject of an application for a European patent with the number EP 553 533 owned by ELKEM ALLUMINIUM ANS. In this regard, the innovation of the Applicant is to use already refined ELKEM ALLUMINIUM blocks, subjecting these blocks directly to a first heating phase, starting from a condition in which their temperature is substantially the ambient temperature, to bring them to a temperature in which there is a balance between their solid state and their liquid state and in which the silicon phases are liquid and the α- aluminium phases are solid. At this point, the blocks are ready for the second operating phase, that is, for direct use inside the moulding system in their semi-solid and semi-liquid state to obtain resulting semi-finished products. One of the conditions which must be satisfied is that the said second phase of heating the already refined blocks envisages the application of heat gradients to these blocks which keeps the temperature constant in all the areas of the blocks, both internally and externally. Therefore, it is envisaged that the said heating phase subjects the blocks to a temperature gradient with a high temperature increase co-efficient until the fusion or eutectic point of the refined aluminium-silicon-boron alloy is reached so that the blocks can then be subjected to a heat gradient with controlled co-efficients proportional to the requirements for keeping the temperature constant in all parts of the blocks. In particular, as shown in Figure 2, the first part of the curve has a steep heat co-efficient while the second part which approaches the temperature between solid and liquid (585°C), used for the moulding of the billets, has a low heat co-efficient. In other words, during the heating phase, the billets are subjected to a low temperature increase gradient until the fusion or eutectic point of the refined aluminium- silicon-boron alloy is reached so that the billets can then be subjected to a heat gradient with controlled co-efficients proportional to the requirements for keeping the temperature constant in all parts of the billets. It is evident that the present invention achieves the pre-defined object and allows moulding of the said billets without using the treatments described above. The invention described can be subject to modifications and variations without thereby departing from the scope of the inventive concept. Moreover, all the details of the invention may be substituted by technically equivalent elements.

Claims: Claims 1) A method for bringing aluminium alloy blocks, such as ingots, billets and others to a semi-solid and semi-liquid state which allows them to be moulded when this state has been reached, characterised in that it envisages the following operating phases: - a first phase for refining the grain of the said aluminium alloy, consisting of blocks, by adding to the said aluminium alloy, when it is melted, a silicon-boron alloy which produces a refined alloy consisting mainly of α-aluminium and eutectic silicon phases; - a second phase for heating the said blocks of this refined alloy, starting from a condition in which the temperature is substantially the ambient temperature to bring the said refined alloy to a temperature in which there is a balance between its solid state and its liquid state and in which the eutectic silicon phases are liquid and the α-aluminium phases are solid; - a third phase for using the said alloy blocks, obtained in this way by the previous operating phases, directly inside the moulding system in their semi-solid and semi-liquid state to obtain the resulting semi-finished products. 2) The method, according to claim 1, characterised in that the said first operating phase for refining the grain of the said aluminium alloy is substituted by the use of already refined aluminium alloy blocks. 3) The method, according to claim 1, in which the said aluminium alloy blocks are refined aluminium alloy blocks, obtained by adding to the aluminium alloy, when it is melted, a silicon-boron alloy containing a boron weight of 0.01 to 4.0% in a quantity which ensures that the resulting fusion of the aluminium alloy contains at least 50 p.p.m. of boron characterised in that it envisages: - a first phase for heating the said refined alloy blocks, starting from a condition in which their temperature is substantially the ambient temperature, to bring the said refined alloy to a temperature in which there is a balance between its solid state and its liquid state and in which the silicon phases are liquid and the α-aluminium phases are solid; - a second operating phase for using the said alloy blocks, obtained in this way by the previous operating phases, directly inside the moulding system in their semi-solid and semi-liquid state to obtain resulting semi-finished products. 4) The method, according to any of the previous claims, characterised in that the said second phase for heating the said blocks envisages subjecting the blocks to heat gradients which can keep the temperature constant in all internal and external areas of the blocks. 5) The method, according to claim 4, characterised in that the said heating phase subjects the said blocks to a temperature gradient with a high temperature increase co¬ efficient until the fusion or eutectic point of the refined aluminium-silicon-boron alloy is reached so that the said blocks can then be subjected to a heat gradient with controlled co-efficients proportional to the requirements for keeping the temperature constant in all parts of the said blocks . 6) The method, according to claim 4, characterised in that the said heating phase subjects the said blocks to a temperature gradient with a low temperature increase co¬ efficient until the fusion or eutectic point of the refined aluminium-silicon-boron alloy is reached so that the said blocks can then be subjected to a heat gradient with controlled co-efficients proportional to the requirements for keeping the temperature constant in all parts of the said blocks. 7) The method, according to any of the previous claims, characterised in that the said aluminium-silicon alloys are hypo-eutectic alloys. 8) The method, according to the previous claims and as described and illustrated herein with reference to the accompanying drawings and for the objects stated herein.

Inventors: MUNERATTI GIORGIO, MUNERATTI, GIORGIO
Applicants: MUNERATTI GIORGIO, REYNOLDS WHEELS SPA, MUNERATTI, GIORGIO, REYNOLDS WHEELS S.P.A.
Application year: 1996
Main classification: C22C1
IPC codes: B22D1, C22C1, C22C21, C22F1