CN1708372A - Metal powder injection molding material and metal powder injection molding method - Google Patents

Abstract

This invention discloses a novel metal injection molding material comprising: a) 40 to 70 vol% metal powder, including at least 50 wt% iron-containing powder of the total metal content, wherein at least 90 wt% of the iron-containing powder particles have an effective diameter of at least 40 micrometers; b) 30 to 60 vol% thermoplastic binder; and c) 0 to 5 vol% dispersing agent and/or other optional additives. The injection molding material is formed by injection molding, the binder is removed from the molded workpiece, and the molded workpiece from which the binder has been removed is sintered.

Description

Metal injection molding material and metal injection molding method

The present invention relates to a metal injection molding method.

Metal Injection Molding (MIM, also commonly known as Powder Injection Molding, PIM) is a powder metallurgy process for producing molded articles by injection molding a thermoplastic injection molding material comprising metal powder and usually at least 30% by volume of thermoplastic binder, which is then removed from the molding and sintering the molding to obtain the final workpiece. Metal injection molding combines the advantages of forming by injection molding known from plastics technology with those of conventional powder metallurgy. In traditional powder metallurgy (commonly referred to as P/M), metal powder (to which typically up to 10% by volume of a lubricant, such as oil or wax) is formed by compression molding into the desired shape, and then sinter the molding. The advantage of the powder metallurgy method is the free choice of materials. In powder metallurgy, by sintering metal powder mixtures, materials that cannot be produced by pyrometallurgical methods can be produced. A significant drawback of conventional powder metallurgy by compression molding and sintering is that it is not suitable for producing workpieces with relatively complex geometries. For example, shapes with undercuts (ie depressions transverse to the direction of compression) cannot be produced by compression molding and sintering. In contrast, in injection molding, almost any desired shape can be produced. On the other hand, metal injection molding has the disadvantage that anisotropy sometimes occurs in the case of larger workpieces and that a separate step has to be carried out to remove the binder. Metal injection molding is therefore mainly used for smaller workpieces with complex shapes.

An important parameter of powder metallurgy technology is the particle size of the metal powder used or the composition of the metal powder mixture used. Generally, d90 values in microns are described herein. It means that 90% by weight of the relevant powder is present in the form of particles whose particle size does not exceed this d90 value. Occasionally similar d10 or d50 values are described (occasionally, a capital D is also used, thus denoting the value as D10, D50 or D90). In the case of spherical particles, the measured particle diameter is equivalent to the diameter of a sphere; in the case of non-spherical particles, the measurement method (usually laser diffraction method) must measure the effective diameter of the particle equivalent to the diameter of spherical particles of the same volume.

In metal injection molding of ferrous materials, relatively fine metal particles, especially iron or steel particles, are always used. Although the fine metal particles are relatively expensive and difficult to handle due to their tendency to agglomerate and their pyrophoric properties, they have better sintering properties. This is particularly important in the case of low-alloy steels (in the context of the present invention low-alloy steels are understood to mean steels with an iron content of at least 90% by weight, i.e. a content of alloying elements not exceeding 10% by weight), because High-alloy steels are generally easier to sinter very well than low-alloy steels, ie a homogeneous and densely sintered workpiece results. Therefore in the case of metal injection molding, especially in the production of sintered shaped products from low-alloy steels, iron or steel powders with a d90 value of 0.5 to 20 microns are always used, and only in very rare cases do they have a maximum A d90 value of not more than about 30 microns. The pyrophoric behavior of fine metal particles in the powder molding compound can be controlled due to the relatively high binder content of the ready-to-use metal molding compound, which prevents the contact of individual metal particles with atmospheric oxygen. On the other hand, in conventional powder metallurgy, fine powders with a tendency to agglomerate often lead to uneven filling of moldings, and the pyrophoric properties of metal powders are intolerable. Therefore, in conventional powder metallurgy by compression molding and sintering, relatively coarse particles with a d90 value of 40 microns or more are always used.

A.R.Erickson and R.E.Wiech in Injection Molding (Injection Molding), ASMHandbook (ASM Handbook), Volume 7, Powder Metallurgy (Powder Metallurgy), American Society for Metals (American Society for Metals), 1993 Edition (ISBN0-87170-013 -1) The metal injection molding process is reviewed. R.M.German and A.Bose in Injection Molding of Metals and Ceramics, Metal Powder Industries Federation, Princeton, New Jersey, 1997 edition (ISBN 1-878-954-61- Powder injection molding techniques (metals and ceramics) are summarized in X) and in particular powders for powder injection molding are reviewed in Chapter 3. L.F.Pease III and V.C.Potter in Mechanical Properties of PIM Materials disclose typical alloys in powder metallurgy processes and the achievable properties of workpieces produced therefrom.

EP 446 708 A2 (patent family: US 5,198,489), EP 465 940 A2 (patent family: US 5,362,791), EP 710 516 A2 (patent family: US 5,802,437) and WO 94/25205 (patent family: US 5,611,978) disclose various An injection molding material for a metal injection molding process, and a metal injection molding process in which a binder is catalytically removed from an injection molded workpiece, which is then sintered. EP 582 209 A1 (patent family: US 5,424,445) discloses certain dispersants used as auxiliary agents in powder injection molding materials. WO 01/81 467 A1 discloses binder systems for metal injection molding. On the other hand, WO 96/08 328 A1 discloses a typical composition for conventional powder metallurgy by compression molding and sintering, using up to 10% by weight of polyether wax as lubricant.

There remains a need for more widely applicable and exceptionally economical injection molding materials and injection molding methods. It is an object of the present invention to provide an economical and widely applicable method of metal injection molding and an injection molding material for this purpose.

It has been found that this object is achieved by a metal injection molding material comprising:

a) 40 to 70% by volume of metal powder comprising at least 50% by weight, based on the total amount of metal, of iron-containing powder, and based on the amount of the iron-containing powder, at least 90% by weight of the particles of the iron-containing powder have a particle size of at least 40 microns effective diameter,

b) 30 to 60% by volume of thermoplastic binder, and

c) 0 to 5% by volume of dispersants and/or other auxiliaries.

Furthermore, we have discovered a metal injection molding method in which the injection molding material is formed by injection molding, the binder is removed from the injection molded workpiece, and the binder-free workpiece is sintered.

The new metal injection molding material comprises relatively extremely coarse iron or iron alloy powders. The present invention is based on the realization that, although contrary to the opinion of some persons skilled in the art, such coarse metal powders are also believed to give satisfactory results in metal injection molding, and indeed do, in particular In the production of sintered shaped articles from low alloy steels. The coarse metal powder results in a significant reduction in the cost of metal injection molding materials and makes their handling easier. Sintered shaped articles produced by this new method have properties at least as good as sintered articles produced by conventional powder metallurgy, and can also be produced with very complex geometries.

The new metal injection molding compound generally comprises at least 40 vol %, preferably at least 45 vol % and generally not more than 70 vol %, preferably not more than 60 vol % metal powder, respectively based on the total volume of the injection molding material. As is common in powder metallurgy, the metal powder may be a single pure metal powder, a mixture of different pure metal powders, a pure powder of a metal alloy, a mixture of different metal alloy powders, or one or more pure metal powders Mixture with one or more metal alloy powders. The overall composition of the powder determines the overall composition of the final sintered shaped article and is selected according to the desired composition and also, as is common in powder metallurgy, the carbon required for the final sintered shaped article can be determined during sintering , oxygen and/or nitrogen content.

At least one of the metal powders contained in the novel injection molding compound contains iron. The iron-containing powder is preferably low alloy steel or pure iron. In one embodiment, the metal powder in the novel powder injection molding material consists entirely of iron, optionally also having a carbon content of 0 to 0.9% by weight. In another embodiment, the metal powder consists of low alloy steel comprising 0 to 0.9% by weight carbon, 0 to 10% by weight nickel, 0 to 6% by weight molybdenum, 0 to 11% by weight Copper, 0 to 5% by weight of chromium, 0 to 1% by weight of manganese, 0 to 1% by weight of silicon, 0 to 1% by weight of vanadium and 0 to 1% by weight of cobalt, the rest is iron, and except iron The total amount of all elements present in the outer layer does not exceed 10% by weight. In this case, the metal powder contained in the novel metal injection molding compound preferably contains at least 90% by weight of iron.

Based on the total amount of metal powder, at least 50% by weight of the metal powder in the novel injection molding material comprises iron-containing powder. Preferably, based on the total amount of metal powder, at least 60% by weight, particularly preferably at least 80% by weight, of the metal powder in the novel powder injection molding material comprises iron-containing powder. In one embodiment of the novel powder injection molding material, only the iron-containing powder is used as metal powder.

Instead of the iron-containing powder, however, it is also possible to use other metal powders which either contain iron as well as other elements or even consist of iron. For example, low-alloy steels are produced by master alloying techniques from iron powders and powders of iron-free alloys containing the desired alloying elements, or from corresponding high-alloy steels or corresponding mixtures (pre-alloyed or partially fused powders). These techniques are known. All that is decisive for the invention is that at least 50% by weight of the metal powder present in the powder injection molding material comprises iron-containing powder and that, based on the amount of this iron-containing powder, at least 90% by weight of the particles of the iron-containing powder have a particle size of at least 40 micron effective diameter. In other words, the metal powder in the novel metal injection molding compound comprises at least 50% by weight of iron-containing powder having a particle size of at least 40 microns (expressed as a d90 value). The portion of the metal powder not formed from the iron-containing powder is any desired metal powder or metal powder mixture suitable for metal injection molding and is selected according to the desired final composition of the sintered shaped article to be produced.

The iron-containing powder in the novel injection molding material consists of particles at least 90% by weight, based on the amount of the iron-containing powder, having an effective diameter of at least 40 micrometers. Preferably, the effective diameter is at least 50 microns, particularly preferably at least 60 microns. In other words, the iron-containing powder has a d90 value of at least 40, preferably at least 50, particularly preferably at least 60. A suitable d90 value is, for example, 70. The d90 value is determined by laser diffraction method according to ISO/DIS standard 13320 "Guidelines for Laser Diffraction Particle Size Analysis".

The metal powders used in the new injection molding materials are conventional commercial products.

The novel metal injection molding materials generally comprise at least 30% by volume, preferably at least 40% by volume and generally not more than 60% by volume, preferably not more than 55% by volume, of thermoplastic binders, respectively, based on the total volume of the injection molding material. The essential purpose of binders is to impart thermoplastic properties to powder injection molding materials, and an important criterion for the suitability of a certain thermoplastic as a binder is the possibility to remove it after injection molding. Various binders and methods of removing binders from powder injection molded articles are known, such as thermal removal of binders by pyrolysis of thermoplastics, removal of binders by use of solvents, or catalytic decomposition of thermoplastics to Catalytic removal of binders. As the thermoplastic binder for the novel powder injection molding material, any known thermoplastic binder for powder injection molding can be selected.

Binders that can be removed catalytically are preferably used. Such binder systems are generally based on polyoxymethylene as a thermoplastic material. Polyoxymethylene depolymerizes under acid catalysis so that it can be removed quickly and at relatively low temperatures from injection molded parts. The thermoplastic binder preferably consists of 50 to 100% by weight of a polyoxymethylene homopolymer or copolymer and 0 to 50% by weight of a polyoxymethylene homopolymer or copolymer that is not miscible and can be removed thermally. There are no residues of polymers, or mixtures of such polymers. Such binders are disclosed, for example, in EP 446 708 A2, EP 465 940 A2 and WO 01/81467 A1, which are hereby incorporated by reference.

The novel powder injection molding materials can also contain dispersants and/or other auxiliaries in amounts of up to 5% by volume. Preferably, it contains at least 1% by weight of dispersants and/or other auxiliaries. Dispersants are used to prevent separation and are disclosed, for example, in the documents cited above and in EP 582 209 A1 (which is likewise incorporated herein by reference). Other additives are usually added to influence the rheological properties of the powder injection molding material. Sometimes carbon is also added, usually in the form of graphite or pyrolyzable polymers, in order to determine the carbon content in the sintered shaped product during sintering. These measures are known, for example, from the publications cited above.

New powder injection molding materials are usually prepared by mixing their components. Preference is given to preparation by intensive mixing in molten or at least pasty form. All devices which can thoroughly mix pasty to liquid substances are suitable, eg heatable kneaders. The new powder injection molding materials are manufactured in granular form suitable for feeding into conventional injection molding machines, such as strands, extrudates, pellets or comminuted kneaded material.

The new powder injection molding method is performed in the same manner as the conventional powder injection molding method. For this purpose, a new injection molding material (i.e. raw material) is shaped by injection molding to obtain a green compact, the binder is removed from the injection molded workpiece (i.e. binder removal) and intermediate compacts are produced from said green compact. A brown compact, and sintering the intermediate compact to obtain a final sintered shaped article.

Molding of the stock is carried out in a conventional manner using a conventional injection molding machine. The thermoplastic binder is removed from the moldings in a customary manner, for example by pyrolysis or by solvent treatment. The binder is catalytically removed from the preferably novel injection molding materials comprising a polyoxymethylene-based binder, preferably by thermally treating the green body in a known manner with an atmosphere containing a gaseous acid. The atmosphere is prepared by evaporating the acid to a sufficient vapor pressure, or more conveniently by passing a carrier gas (especially nitrogen) into the container containing the acid (preferably nitric acid) and then passing the acid-containing gas through the bonded agent removal oven. The optimum acid concentration in the binder removal oven depends on the desired steel composition and workpiece dimensions and is determined in each case by routine trials. Usually, a treatment at 20 to 180° C. for 10 minutes to 24 hours in this atmosphere is sufficient to remove the binder. Any residues of thermoplastic binder and/or additives still present after removal of the binder are pyrolyzed during heating up to the sintering temperature and are thus completely removed.

After shaping and subsequent removal of the binder, the molded article is sintered in a sintering furnace to obtain a sintered shaped article. Sintering is performed by known methods. Depending on the desired result, for example, sintering can be carried out under air, hydrogen, nitrogen or gas mixtures or under reduced pressure.

The composition, pressure and optimum temperature range of the furnace atmosphere most suitable for sintering depend on the exact chemical composition of the steel used or to be prepared and are known or, in individual cases, can be determined by several routine procedures. Tests are easily determined.

The optimum heating rate can be easily determined by several routine tests and is generally at least 1°C, preferably at least 2°C, particularly preferably at least 3°C per minute. Economical considerations usually require very high heating rates. However, to avoid negative effects on the quality of the sinter, the heating rate is usually set at 20°C per minute. In some cases it may be advantageous to maintain a waiting period at a temperature below the sintering temperature during heating to the sintering temperature, for example at a temperature of 500 to 700°C, for example 600°C for 30 minutes to 2 hours, For example 1 hour.

The set sintering duration, ie the holding time at the sintering temperature, is generally such that the sintered shaped article is sufficiently densely sintered. At conventional sintering temperatures and shaped product dimensions, the duration of sintering is generally at least 15 minutes, preferably at least 30 minutes. The total duration of the sintering treatment basically determines the production rate, and thus from an economical point of view, the sintering is preferably performed such that the sintering treatment does not take an unsatisfactorily long time. Typically, the sintering process (including the heating phase but not the cooling phase) can be completed after at most 14 hours.

The sintering process is terminated by cooling the sintered shaped article. Depending on the composition of the steel, specific cooling processes, such as very rapid cooling, may be required in order to obtain high temperature phases or to prevent segregation of the steel components. For economic reasons, very rapid cooling is often also required to achieve high production rates. The upper limit of the cooling rate is reached if the occurrence of sintered formed articles having defects such as cracks, fractures or deformations due to excessive cooling occurs in an economically unsatisfactory amount. Therefore, the optimal cooling rate can be easily determined in a few routine tests.

After sintering, any desired post-treatments such as sinter hardening, austenitizing, annealing, hardening, heat treatment, carburizing, surface hardening, carbonitriding, nitriding, steam treatment, solution flow can be performed on the sintered shaped article Heat treatment, quenching in water or oil and/or hot isostatic pressing, or a combination of these steps. It is also possible to carry out some of these treatment steps in a known manner during sintering, for example sinter hardening, nitriding or carbonitriding.

Example

Example 1: Production of moldings from Fe-Ni-C steel containing 2% by weight of nickel and 0.5% by weight of C:

In a heated laboratory kneader, 4400g iron powder (model ASC 300, HgansAB, 26383 Hgans, produced in Sweden, d50=30 microns, d90=70 microns, 0.01% by weight carbon), 90g of nickel powder (d90=26 micron) and 2.2g of graphite powder (d90=8 micron) and a binder comprising 500g of polyoxymethylene, 70g of polypropylene and 30g of dispersant are mixed by kneading and cooled When crushed to obtain particles. The granules were processed using a screw-type injection molding machine to obtain tensile test bars with a length of 85.5 mm and a diameter of 4 mm (according to MPIF Standard 50, 1992). The injection molded articles were subjected to catalytic binder removal in a chamber furnace at a temperature of 110° C. under a nitrogen atmosphere into which 25 ml/h of concentrated nitric acid were metered. The sample was then sintered in an electrically heated furnace in dry nitrogen at a rate of 5 K/min to 1360° C., maintained at this temperature for 1 hour, and cooled slowly in the furnace. The density of the sample exceeds 7.1 g/cm ³ . Metallographic studies of the cross-sections revealed a ferrite/pearlite structure with elongated pores. The carbon content of the sample was 0.5% by weight.

The samples were heat treated by austenitizing at 870°C, oil quenching and annealing at 200°C for 1 hour. Their hardness after this treatment was 43 HRC.

Example 2

Example 1 was repeated except that 30% by weight of the coarse iron powder was replaced by carbonyl iron powder (d90 = 10 microns). After sintering a density of 7.3 g/cm ³ and a carbon content of 0.5% by weight was achieved. Its structure is slightly more uniform than the sample in Example 1 and the proportion of elongated pores is smaller. After heat treatment, the hardness reaches 46HRC.

comparative example

Example 1 was repeated except that the coarse iron powder was completely replaced by carbonyl iron powder (d90 = 10 microns). A density of 7.6 g/cm ³ and a carbon content of 0.5% by weight were achieved after sintering. The apertures are all circular and smaller than Examples 1 and 2. After heat treatment, the hardness reaches 55HRC.

These examples show that even with relatively very coarse metal powders, sintered shaped articles can achieve properties that are not at all inferior to those typical of shaped articles made by compression molding and sintering, and barely inferior to those typical of conventional powder injection molded workpieces. performance performance.

Claims (6)

1. A metal injection molding material comprising: a) 40 to 70% by volume of metal powder comprising at least 50% by weight, based on the total amount of metal, of iron-containing powder, and based on the amount of the iron-containing powder, at least 90% by weight of the particles of the iron-containing powder have a particle size of at least 40 microns effective diameter, b) 30 to 60% by volume of thermoplastic binder, and c) 0 to 5% by volume of dispersants and/or other auxiliaries. 2. The metal injection molding material of claim 1, wherein at least 90% by weight of the particles of the iron-containing powder have an effective diameter of at least 50 microns, based on the amount of the iron-containing powder. 3. The metal injection molding material of claim 2, wherein at least 90% by weight of the particles of the ferrous powder have an effective diameter of at least 60 microns, based on the amount of the ferrous powder. 4. The metal injection molding material of claim 1, wherein the total amount of metal powder contained comprises at least 90% by weight iron. 5. The metal injection molding material according to claim 1, wherein the thermoplastic binder consists of 50 to 100% by weight of a polyoxymethylene homopolymer or copolymer and 0 to 50% by weight of a polyoxymethylene homopolymer or copolymers which are miscible and which can be removed thermally without residues, or mixtures of such polymers. 6. A method of metal injection molding which will comprise: a) 40 to 70% by volume of metal powder comprising at least 50% by weight, based on the total amount of metal, of iron-containing powder, and based on the amount of the iron-containing powder, at least 90% by weight of the particles of the iron-containing powder have a particle size of at least 40 microns effective diameter, b) 30 to 60% by volume of thermoplastic binder, and c) Metal injection molding materials with 0 to 5% by volume of dispersants and/or other additives are formed by injection molding, the binder is removed from the injection molded workpiece, and the binder-free workpiece is sintered .