FR2553015A1 - MECHANICAL PIECE OF ALUMINUM ALLOY POWDER AND PROCESS FOR OBTAINING THE SAME - Google Patents

Abstract

MECHANICAL PART IN PREALLYED ALUMINUM ALLOY POWDER CHARACTERIZED BY THE PRESENCE OF A HOMOGENEOUS ALUMINUM ALLOY MATRIX WITH A VERY FINE STRUCTURE WHOSE GRAIN SIZE IS OF THE SAME ORDER OF SIZE AS THAT OF THE INITIAL GRAINS OF POWDER, AND WHOSE INTERFACES ARE MARKED BY A VERY THIN NETWORK OF PRECIPITATES, THE SAID PART HAVING A DENSITY EQUAL TO 100 OF THE THEORETICAL DENSITY, WITHOUT RESIDUAL POROSITY AND WITHOUT COARSE PRECIPITATION. MAIN APPLICATION: ALL MECHANICAL PARTS OBTAINED BY POWDER METALLURGY.

Description

MECHANICAL PIECE IN ALUMINUM ALLOY POWDER AND

PROCESS FOR OBTAINING

  The present invention relates to a process for obtaining mechanical parts made 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 forging and foundry techniques, powder metallurgy is likely to bring about a reduction in material losses between the blank and the finished part (reduction in the weighting of the material) and a reduction in the costs of 'machining. Several ways are possible to pass from the powder 20 to the mechanical part; the simplest so far is to use the alloys in the form of aluminum powder mixed with powders of alloy elements for example copper, magnesium, silicon, zinc; this method imposes a sintering step, that is to say a heat treatment which gives its cohesion to the

cold pressed piece.

  The treatment is necessary because the aluminum particles are covered with a film of alumina of about 0, there 0.01m, adherent, stable and hardly reducible, thus diffusion inhibitor, and which makes it impossible to sinter by solid phase in air 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 2,000 family (family of copper aluminum alloys), the eutectic liquid phase is formed around 550 ° C .; the liquid wets and dissolves the alumina film, penetrates the cracks formed during compression of the powder and diffuses into the metal particles; this process imposes a precise treatment temperature to +3 "C, the alloys thus obtained have a density of 90 to 96% of the theoretical density, with porosities and heterogeneity of the structure The mechanical properties obtained for a 2014 alloy d After the powder manufacturers are as follows: Resistance to fracture R = 330 M Pa elastic limit at 0.2% RE 2 320 M a RE 0.2 = 320 M Pa Elongation in% A% = 2% limit endurance 55 M Pa20 for the treated state T 6 (quenching with water at 500 ° C and

  income 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 with respect to nitrogen (review "POWER METALLURGY" No. 3 of 1976 page 171). This method thus has the disadvantage of obtaining a mechanical part with low characteristics.

  In addition, this process requires heat treatment with extremely precise temperature control.

  -3 A significant improvement of the characteristics passes by the suppression of the porosities; this can be obtained by hot forging tablets Messrs KE BUCHOVECKY and MR REARICK have used the following range: cold compression of mixed powders, sintering with nitrogen, forging at different rates of wrought (work "FORGING OF POWDER METALLURGY PREFORM" The results are then greatly improved and for an average cure rate resistance of 460 M Pa with 4% elongation is achieved. This method has the disadvantage of still requiring a heat treatment with

  an extremely precise temperature setting.

  Another method consists of starting from a prealloyed powder, that is to say in which each grain of powder is homogeneous in composition of the alloy; the alumina film then remains and can not be removed since it is no longer possible to sinter in the presence of a liquid phase; it is then necessary to encapsulate the powder before forging, then to remove the capsule; a strong grooming breaks the alumina films and allows the grains of powder to weld together; it is then possible to reach properties close to that of the rolled and forged metal; this technique remains relatively complex,

  expensive and applicable only in small series.

  The method according to the invention aims to obtain mechanical parts of high characteristics, by a simple and inexpensive route, capable of manufacture.

in large series.

  According to one embodiment of the invention, the method for obtaining mechanical parts is made from aluminum alloy powders. Said method comprises the following steps: use of a prealloyed powder; cold compaction of a simple form blank weighing of the automatic blank to check that the weight of the blank is at + 0.5%; heating said blank in a pure nitrogen atmosphere; temperature forging in a closed matrix. According to one embodiment of the invention, the pre-alloyed aluminum alloy powdered mechanical part is characterized by the presence of a homogeneous matrix of very fine-grain aluminum alloy, the grain size of which 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, 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 process implemented according to the invention offers the additional advantage of being inexpensive and applicable.

  for manufacturing in very large series.

  The most homogeneous structure is obtained with a prealloyed powder; it also has the advantage over mixed powders of avoiding mixing and limiting the handling of powders upon addition.

  possible compacting lubricant.

  The next operation, cold compaction, is carried out on industrial press to obtain a draft of moderate density 2.0 to 2.5 of simple form, thus of economic realization; the powder mass must be accurate to less than 1% (ie + 0.5% of the correct weight of the blank), the weighing of the blank is carried out automatically on a weighing scale. press release. The temperature of the blank is carried out in a furnace in a neutral atmosphere (for example pure nitrogen) at a dew point <30 ° C .; the forging temperature is between 470 and 500 C (for an alloy 2214);

  she does not need to be precise.

  The blank is finally rapidly removed from the furnace and forged within a period of less than ten seconds in a closed die, without thick burr, which is made possible by the precision of the weight of the blank; this absence of burrs is an advantage of the process, because they constitute a loss of metal and their formation imposes a high forging pressure strongly stressing the press and constitutes a significant wear factor of the tooling The final densification to 100% without porosity is performed at the same time as the solid state flow of the metal in the cavity; in addition, the deformation resistance of the slugs forged by the process of the invention from powder was measured to be lower than that of the slabs forged from rolled metal. The product obtained is at 100% of the theoretical density without residual porosity It is characterized by a very fine homogeneous matrix structure 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 shown 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 T 6 treatment (quenching). the water at 500 C and returned for 20 hours at 160 C) The intergranular network is very thin and apparent. The mechanical characteristics of the products obtained for a low degree of wrought ironing are high (see table below) and even uniforms are obtained.

  rotational bending fatigue of 160 M Pa for 10 cycles at room temperature.

  Mechanical properties of products obtained according to the process of the invention for the alloy A U 4 S G: Granulo-I Density Tempera Treats Hardness

R IR 0.2

  A% measurement of the compacted blank forging C and duration

HB 2.5 / 62.5

  M Pa M Pa Mesnager da J / cm 2 Resilience l

15 20 25

T 6 123 443 380 7

  fine 2.35 480 lh T 4 107 443 282 17.5 1.45 500 lh T 6 138 485 415 8

T 4 122 441 286 20 1.35

480 3 hrs 6 129 455 390 5

T 4 106 453 290 18.7 1.10

  Average 2,48,480 lh T 6,133 469 394 7 not T 4 108 421 270 11.2 1.65 500 lh T 6 140 469 414 4

T 4 109 439 276 15 1.65

480 3 hrs 6 137 422 410 2

T 4 106 439 280 13.7 1.25

  The heat treatments T 4 and T 6 consist of: for T 4: quenching with water for 4 days;

at 500 C and maturation

  for T 6: quenching with water at 500 C and tempering

hours at 160 OC.

49398 1 1676

  A characteristic of the rupture of a tensile test piece shown in FIG. 3 is the absence of necking; the section decrease being distributed

  along the entire length of the test piece (marked lines in the test piece), unlike a conventional tensile test piece shown in Figure 2 with a necking This unexpected phenomenon characterizes the product obtained according to the method of the invention.

49398 11676

-9

Claims (3)

  Process for obtaining mechanical parts made from aluminum alloy powders, characterized in that said process comprises the following steps: use of a prealloyed powder; cold compaction of a simple form blank; Weighing of the automatic roughing to control that the weight of the blank is at + 0.5%; heating said blank in a pure nitrogen atmosphere;   temperature forging in a closed matrix.   2 mechanical part powder of pre-alloyed aluminum alloy obtained by the method 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 of the size of the initial powder grains, and whose intercepts 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. 49398 1 If