EP0142405A1 - Work pieces made from aluminium alby powder, and process for manufacturing it - Google Patents

Abstract

Mechanical component made of prealloyed aluminium alloy characterised by the presence of a homogeneous matrix of aluminium alloy of very fine structure in which the grain size is of the same order of magnitude as that of the initial powder grains and whose interfaces are marked by a very fine network of precipitates, the said component having a density of 100% of the theoretical density, without residual porosity and without coarse precipitation. Main application: all mechanical components obtained by powder metallurgy.

Description

The present invention relates to a process for obtaining mechanical parts produced from aluminum alloy powders and undergoing a forging operation.

Aluminum alloys are of interest in mechanical parts for their low density, their better machinability, their better thermal conductivity; compared to the usual techniques of forging and foundry, powder metallurgy is likely to bring about a reduction in material losses between the raw part _f and the finished part (reduction in the weight per thousand of material) and a reduction in costs machining.

Several ways are possible to pass from the powder to the mechanical part; the simplest until now consists in using the alloys in the form of aluminum powder mixed with powders of the elements of alloys for example copper, magnesium, silicon, zinc; this process requires a sintering step, that is to say a hot treatment which gives its cohesion to the cold pressed part.

• - Résistance à la rupture R = 330 M Pa
• - limite élastique à 0,2 % R_E 0,2 = 320 ^{M P}a
• - Allongement en % A % = 2 %
• - limite d'endurance 55 M Pa

pour l'état traité T6 (trempe à l'eau à 500 °C et revenu pendant 20 heures à 160°C).The treatment is necessary because the aluminum particles are covered with an alumina film of approximately 0.1 to 0.01 μm adherent, stable and difficult to reduce, therefore inhibiting diffusion, and which makes phase sintering impossible. air solid in an industrial oven; the role of sintering is to form a liquid phase from the metallic elements; thus in the case of alloys of the family 2000 (family of alloys aluminum copper), the eutectic liquid phase forms around 550 ° C; the liquid wets and dissolves the alumina film, penetrates into the cracks formed during the compression of the powder and diffuses into the metallic particles; this method requires a precise processing temperature at + 3 ° C of the resulting alloys have a density of 90 to 96% of theoretical density, with porosity and heterogeneity of the struc _- ture. The mechanical properties obtained for a 2014 alloy according to the powder manufacturers are as follows

• - Tensile strength R = 330 M Pa
• - elastic limit at 0.2% R _E 0.2 = 320 ^MP a
• - Elongation in% A% = 2%
• - endurance limit 55 M Pa

for the treated state T6 (water quenching at 500 ° C and tempered for 20 hours at 160 ° C).

The sintering atmosphere is an important parameter of the process, Messrs. AMATO, CORSO and SGAMBETTERRA have shown that sintering in pure nitrogen is better than in air; vacuum sintering does not improve the results compared to nitrogen (review "POWER METALLURGY" n ° 3 of 1976 page 171).

This process thus has the drawback of obtaining a mechanical part with low characteristics and a low final density. In addition, this process requires heat treatment with extremely precise temperature control.

A significant improvement in the characteristics requires the elimination of the porosities; this can be achieved by hot forging the tablets. Messrs. K.E. BUCHOVECKY and M.R. REARICK used the following range: cold compression of mixed powders, sintering with nitrogen, forging at different rates of wrought (work "FORGING OF POWDER METALLURGY PREFORM" by HAUSNER - 1973 editor M. P. I. F.). The results are then very improved and for an average rate of wrought we reach resistances of 460 M Pa with 4% elongation. This process has the drawback of still requiring heat treatment with extremely precise temperature control.

Another method consists in starting with a pre-alloyed powder, that is to say in which each grain of powder is homogeneous in composition of the alloy; the alumina film then remains and cannot be removed since it is no longer possible to sinter in the presence of a liquid phase; we are then obliged to encapsulate the powder before forging, then to eliminate the capsule; a strong wrought breaks the alumina films and allows the powder grains to weld; it is then possible to achieve properties close to that of rolled and forged metal; this technique remains relatively complex, expensive and applicable only in small series.

The purpose of the process according to the invention is to obtain mechanical parts with high characteristics, by a simple and inexpensive route, capable of mass production.

• - utilisation d'une poudre préalliée ;
• - compactage à froid d'une ébauche de forme simple ;
• - pesée de l'ébauche automatique afin de contrôler que le poids de l'ébauche est à + 0,5 % ;
• - mise en température de ladite ébauche en atmosphère d'azote pur ;
• - forgeage en température en matrice fermée.

According to one embodiment of the invention, the process for obtaining mechanical parts is carried out from aluminum alloy powders. Said method comprises the following steps:

• - use of a pre-alloyed powder;
• - cold compaction of a blank of simple shape;
• - weighing the automatic blank to check that the weight of the blank is + 0.5%;
• - temperature setting said blank in an atmosphere of pure nitrogen;
• - temperature forging in a closed matrix.

According to one embodiment of the invention, the mechanical part in powder of pre-alloyed aluminum alloy is characterized by the presence of a homogeneous matrix of aluminum alloy with a very fine structure whose grain size is the same order of magnitude as that of the initial powder grains, and the interfaces of which are marked by a very fine network of precipitates, said part having a density equal to 100% of the theoretical density, without residual porosity, and without coarse precipitation.

The method according to the invention thus has the advantage of obtaining mechanical parts with high characteristics and a correct density. In addition, the method used according to the invention offers the additional advantage of being inexpensive and applicable for mass production.

The most homogeneous structure is obtained with a pre-alloyed powder; it also has the advantage by compared to mixed powders to avoid mixing and to limit the handling of powders to the possible addition of a compacting lubricant. The following operation, cold compaction, is carried out on an industrial press to obtain a blank of moderate density 2.0 to 2.5 of simple shape, therefore of economical realization the mass of powder must be precise to within 1% (i.e. at + 0.5% of the correct weight of the blank), the weighing of the blank is carried out automatically on a balance at the press outlet.

The blank is brought to temperature in an oven in a neutral atmosphere (for example pure nitrogen) with dew point <- 30 ° C; the forging temperature is between 470 and 500 ° C (for an alloy 2214); it does not need to be specific.

The blank is finally quickly removed from the oven and forged in a period of less than ten seconds in a closed die, without thick burrs, which is made possible by the weight precision of the blank; this absence of burrs is an advantage of the process, since they constitute a loss of metal and their formation imposes a high forging pressure which places a heavy load on the press and constitutes an important wear factor of the tool. The final densification at 100% without porosity takes place at the same time as the flow in the solid state of the metal in the cavity; moreover, a resistance to deformation of the pieces forged by the method of the invention from powder was measured, lower than that of the pieces forged from rolled metal.

The product obtained is at 100% of the theoretical density without residual porosity. It is characterized by a structure with a very fine homogeneous matrix whose grain size is of the same order of magnitude as that of the initial powder grains, in which there remains a fine network of intergranular precipitates.

This structure is presented on the micrographic reproduction at magnification 500 of FIG. 1. This FIG. 1 relates to a part obtained with a pre-alloyed powder 2214 (AU 4 SG), forged at 500 ° C. and followed by a T6 treatment (quenching at 1 water at 500 ° C and returned for 20 hours at 160 ° C). The intergranular network is very fine and apparent.

The mechanical characteristics of the products obtained for a low degree of working are high (see table below) and even fatigue strengths in rotary bending of 160 M Pa for 10 ⁸ cycles at room temperature are reached.

Mechanical product characteristics obtained according to the process of the invention for the AU 4 SG alloy:

• - pour T4 : trempe à l'eau à 500 °C et maturation pendant 4 jours ;
• - pour T6 : trempe à l'eau à 500 °C et revenu. pendant 20 heures à 160 °C.

The T4 and T6 heat treatments consist of:

• - for T4: water quenching at 500 ° C and maturation for 4 days;
• - for T6: water quenching at 500 ° C and tempered. for 20 hours at 160 ° C.

A characteristic of the rupture of a tensile test piece shown in FIG. 3 is the absence of necking; the reduction in section being distributed over the entire length of the test piece (dotted lines of the test piece), unlike a conventional tensile test piece shown in FIG. 2 with a necking. This unexpected phenomenon characterizes the product obtained according to the process of the invention.

Claims (2)

1. Method for obtaining mechanical parts produced from aluminum alloy powders, characterized in that said method comprises the following steps: - use of a pre-alloyed powder; - cold compaction of a blank of simple shape; - weighing the automatic blank to check that the weight of the blank is + 0.5%; - temperature setting said blank in an atmosphere of pure nitrogen; - temperature forging in a closed matrix. 2. Mechanical part made of pre-alloyed aluminum alloy powder obtained by the process of claim 1, characterized by the presence of a homogeneous matrix of aluminum alloy with a very fine structure whose grain size is of the same order larger than that of the initial powder grains, and the interfaces of which are marked by a very fine network of precipitates, said part having a density equal to 100% of the theoretical density, without residual porosity, and without coarse precipitation.

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