IE893715A1 - Improvement to the process for the lost-foam casting under¹pressure of metal articles - Google Patents

Abstract

The invention relates to a lost foam pressure casting process for metal pieces. This process consists, after having filled the mould with metal in the liquid state and before the solidified fraction of metal exceeds 40% by weight, in applying an isostatic gaseous pressure over the entire mould/metal, of which the value is between 1.5 and 10 mPa. The invention applies to the production of pieces, particularly made from aluminium alloys, having improved mechanical characteristics and, in particular, better resistance to fatigue.

Description

IMPROVEMENT TO THE PROCESS EOK THE LOST-EGAM CASTING, UNDER PRESSURE, of metal articles The invention relates to an improvement to the process for the lost foam casting, under pressure, of metal articles.

This process involves applying an isostatic gas pressure, having a value of between 1.5 and 10 MPa, to the mould/meta1 assembly before the mould has been filled with molten metal and before the solidified fraction of metal exceeds 40% by weight.

The invention is used for obtaining articles, in particular of aluminium alloys, having improved mechanical characteristics and, in particular, better resistance to fatigue.

PATENTS ACT 1964 3315 1' z i' Il ALUMINIUM PECHINEY, a Societe Anonyme organised and existing under the laws of France, of 23 Rue Balzac, 75008 Paris, France A IMPROVEMENT TO THE PROCESS FOR THE LOST-FOAM CASTING, UNDER PRESSURE of metal articles The present invention relates to improvement to the process for the lost—foam casting, under pressure, of metal articles, in particular of aluminium and alloys thereof, as described in the main application n° 3062/87 in Ireland on November 13, 1987.

It is known to a person skilled in the art, mainly from the teaching of USP No. 3 157 924, that patterns of polystyrene foam which are immersed in a mould forr.ed from dry sand containing no binder can be used for casting. In such a process, the metal to be cast, which has previously been melted, is brought into contact with the pattern by means of channels traversing the sand and is gradually substituted for said pattern by burning it and transforming it into vapour which escapes between the grains of sand.

This method has proven to be attractive on an industrial scale because it avoids the preliminary manufacture, by compacting and agglomeration of powdered refractory materials, of rigid moulds connected in a fairly complicated manner via channels to cores, and allows simply recovery of the castings and easy recycling of the casting materials.

However this method is handicapped by two factors: the relative slowness of solidification which promotes the formation of gassing pin-holes the relative weakness of the thermal gradients which can cause micro-shrinkage if the outline of the part complicates feeding thereof.

With aim of overcoming such drawbacks, the applicants have developed a lost-foam casting process which forms the subject of the [main patent application.

This application teaches that, after having filled the mould with the molten metal, that is to say when the pattern has been destroyed completely by the metal and the vapour given off by the foam has been evacuated, an isostatic gas pressure is exerted on the assembly of mould and metal, preferably before the metal begins solidifying. This pressure is applied with values which increase over time so as to avoid the phenomenon of metal penetration and so that the maximum value is attained in less than 15 seconds.

Under these conditions, the castings obtained have increased density which is manifested by an improvement in the mechanical characteristics, in particular with regard to the strength.

However, the applicants considered in this application that it was preferable to employ a maximum pressure of between 0.5 and 1.5 MPa and that it was unnecessary to exceed the latter limit.

In fact, it was noted after more advanced research that if the pressure were further increased, not only the mechanical characteristics such as the breaking strength Rm , the yield stress LE and the elongation A but also the resistance to fatigue F were improved.

Hence the present invention, which involves a process for the lost-foam casting, under pressure, of metal articles, in particular of aluminium and alloys thereof, in which an organic foam pattern of the article to be cast is immersed into a mould, the walls of which are defined by a bath of dry sand containing no binder, the mould is filled with the molten metal which is substituted for the foam and gradually solidifies, and an increasing isostatic gas pressure is applied simultaneously to the mould and to the metal at the earliest on completion of filling, characterised in that the pressure exerted rises to a value of between 1.5 and 10 MPa.

Therefore, the invention involves employing pressures of between 1.5 and 10 MPa and preferably between 5 and 10 MPa.

As in the main patent application, , the pressure can be exerted by means of a sealed box in which the mould is placed, said box being equipped with one or more nozzles conveniently distributed over its wall and connected to a source of gas under pressure.

In the selected pressure range, the applicants have found that the phenomena produced during the application of pressure were quite different from those according to the prior art.

In fact, between 0.5 and 1.5 MPa, the pressure serves mainly to accelerate the flow of molten metal between the dendrites of the solidifying metal and the effect stops when the solid network has reached a certain stage of development. In particular, this is how the low pressures enable the feeder effectively to prevent the phenomenon of shrinkage marks due to the contraction of the solidfying metal.

On the other hand, the flowing effect of the molten metal, which is preponderant at the beginning of solidification, is gradually replaced by an effect of hot deformation of the already solidified metal network, under pressures higher than 1.5 MPa and, in particular, higher than 5 MPa, this phenomenon becoming dominant and then exclusive when the solidification rate reaches values of about 50 to 70%, depending on the type of alloy cast. The application of high pressures therefore produces a type of isostatic forging which affects the entire surface of the casting.

The accompanying Figure 1 is a micrograph of an A-S7G03 alloy cast according to the invention under a pressure of 7 MPa then heat treated. This micrograph shows the plastic deformation imposed on the dendritic network which has the effect of filling up the pores, and illustrates well the forging effect to which the metal is subjected in this process .

Under these conditions, it is found that the mechanical characteristics of the articles are significantly improved and, in particular, the resistance to fatigue. Pressures higher than 10 MPa only produce insignificant improvements.

This new pressure range is preferably applied before the quantity of solidified metal reaches 40% by weight so that the liquid flow can be acted upon.

It is also preferable for the maximum pressure applied to be attained before the quantity of solidi.fied metal exceeds 90% so as to benefit fully from the effect of deformation.

As in the main patent application, , it is preferable for the pressure to be applied by a gradual increase, in particular at the beginning of solidification, to prevent metal penetration, a phenomenon resulting-from a transitory imbalance between the pressure exerted directly on the metal and the pressure exerted on the metal by means of the sand bath. In fact, the bath causes a relatively great loss of charge in the transmission of pressure resulting, in the region of the metal which is in contact with the sand, in a tendency for this pressure to push the metal through the grains of sand and to deform the casting.

The invention can be illustrated by the following embodiment: hollow cylindrical bodies having an external diameter of 45 mm and a wall thickness of 4 mm and comprising adjacent ribs and bosses of 20 x 20 x 80 mm were cast by the earlier process and by the process according to the invention, that is to say an isostatic gas pressure corresponding to atmospheric pressure, to 1 MPa, to 5 MPa and to 10 MPa respectively was applied to the interior of the chamber containing the mould just before the beginning of solidification .

These bodies were produced from two types oi alloys having high mechanical characteristics: A-S7G03 having a composition in per cent by weight of Fe 0.20; Si 6.5-7. 5; Cu 0.10; Zn 0.10; Mg 0.25-0.40; Mn 0.10; Ni 0.05; Pb 0.05; Sn 0. 05 ; Ti 0.05-0.20; remainder Al; A-U5GT having a composition: Fe 5.00; Zn 0.10; Mg 0.15-0.35; Mn Sn 0.05; Ti 0.05-0.30; remainder 0.35; 0.10; Al.

Si 0.20; Cu 4.20Ni 0.05; Pb 0.05; The mechanical tests carried out on said bodies after standard Y23 heat treatments in the case of A-S7G03 and Y24 heat treatments in the case of A-U5GT enabled the following characteristics to be measured as a function of the pressures applied: In the A-S7G03, the quality index Q in MPa which corresponds to the formula Q = R + 1 50 log A, wherein R is the strength in MPa and A is the elongation in % on both the thick and thin regions of the articles.

In A-U5GT, the yield stresses LE in MPa, the strength R in MPa and the elongation A in %, also in both the thick and thin regions.

Furthermore, the resistance to fatigue F was measured in MPa for each of the alloys and each of the pressures applied from torsion tests on a sample machined at 10^ cycles by the staircase method. F applies both to the thick and thin regions because it does not depend on the rate of solidification but on the porosity and consequently on the pressure applied.

The results are given in the following table.

-S7G03 A-U5GT An improvement in nil the characteristics measured and, in particular, increased resistance to fatigue arc observed. '

Claims (7)

Claims

1.Improvement to the process for the lost- foam casting, under pressure, of metal articles, in particular of aluminium and of its alloys, in which an organic foam pattern of the article to be cast is immersed into a mould of which the walls are defined by a bath of dry sand containing no binder, the mould is filled with the molten metal which replaces the foam and gradually solidifies and an increasing isostatic gas pressure is applied simultaneously to the mould and to the metal at the earliest on completion of filling, characterised in that the pressure exerted rises to a value <>f between 1.5 and 10 MPa.

2. A process according to claim 1, characterised in that the pressure exerted rises to a value of between 5 and 10 MPa .

3. A process according to claim 1, characterised in that the pressure is applied at the latest when the quantity of solidified metal reaches 40% by weight.

4. A process according to claim 1, characterised in that the maximum pressure is attained before the quantity of solidified metal exceeeds 90% by weight.

5. A process for the lost-foam casting of metal articles substantially as hereinbefore described by way of Example.

6. A metal article whenever cast by a process as claimed in any of the preceding claims.

7. An aluminium or aluminium alloy article whenever cast by a.process as claimed in any of claims 1 to 5. Dated this 21st day of November, 1989. TOMKINS & CO., Applicant's Agents. (Signed) 5, Dartmouth Road, DUBLIN 6.