Production of Metallic Shaped Bodies by Injection Molding

The invention relates to a process for producing metallic shaped bodies and to an injection-molding composition which can be used for producing such shaped bodies. In particular, metallic shaped bodies containing oxidation-sensitive metals are to be produced.

Metallic shaped bodies can be produced by shaping a compound, removing the binder and sintering. In powder injection molding, an injection-molding composition is injected into a metal mold and after shaping has the binder removed and is sintered. The injection molding composition has to satisfy certain requirements in terms of morphology and particle size. Particles having spherical geometry show good flow properties and are therefore particularly readily processed in the injection-molding process. Fine powders are sinteractive and lead to a particularly homogeneous alloy having good mechanical properties.

Carbonyl metal powders, ie. powders which are prepared by the carbonyl process by decomposition of the corresponding metal carbonyl, are, owing to their finely divided nature and their spherical particle shape, well suited to producing metallic shaped bodies in an injection-molding process. A disadvantage is that carbonyl powders are obtainable for only a few metals. "Atomized powders", which are prepared by atomization of a metal melt in a jet of gas or water, are also suitable. However, atomization is not possible in the case of high-melting or reactive metals or in the case of alloys which demix on melting. Gas-atomized powders are free flowing since they have a spherical particle structure; but atomized finished-alloy powders are coarse-grained and therefore have little sinteractivity.

Zhang and German (The International Journal of Powder Metallurgy, Vol. 27, No. 3, 1991, pages 249 to 254) describe the use of an injection-molding composition using elemental nickel powder based on a mixture of carbonyl iron and carbonyl nickel powders. U.S. Pat. No. 5,055,128 discloses the use of a cobalt element powder for producing soft magnetic alloys. However, in both cases the powders used are of elements having little oxidation sensitivity.

It is generally considered that homogeneous alloys having high proportions by weight of oxidation-sensitive metals can only be produced using finished-alloy powders. Otherwise, the oxide skins which form would prevent the fine distribution of the metal phase added in elemental form. Impaired properties would result.

It is an object of the present invention to provide a simple process and a simple-to-produce injection-molding composition for producing metallic shaped bodies containing oxidation-sensitive metals. In particular, high-alloy steels containing oxidation-sensitive metals are to be produced.

We have found that this object is achieved by means of the process described in the claims. Here, an injection-molding composition comprising at least one carbonyl metal powder and at least one element powder of metals from the group Cr, Mn, V, Si, Ti or of other metals which are at least as oxidation-sensitive is shaped, the binder is removed and the body is sintered. The object is also achieved by a process in which an injection-molding composition comprising at least one carbonyl metal powder and at least one alloy powder is shaped, the binder is removed and the body is sintered. The alloy powder comprises at least one metal of the group Cr, Mn, V, Si, Ti or/and at least one other metal which is at least as oxidation-sensitive. The use of the inexpensive carbonyl metal powder here leads to a significant price advantage in the production costs. The process claimed also allows the production of alloys from which finished-alloy powders cannot be produced owing to their high melting point or owing to demixing effects occurring in the melt.

The carbonyl metal powders are preferably present in the injection-molding composition in an amount of at least 30% by weight. Further preference is given to the use of carbonyl metal powders produced from metals of the iron group. Preference is given to using carbonyl iron powder as carbonyl metal powder. The ratio of the mean particle diameter of the carbonyl metal powders to the element and alloy powders is preferably at most 1:2. The alloying metals are preferably present in the metallic shaped body in an amount of at least 5% by weight. Alloying metals are here those metals which have been mixed in by means of element or alloy powders. Preference is given to a sintering process under reduced pressure or in a reducing protective gas atmosphere, in particular in hydrogen, hydrogen/argon or hydrogen/nitrogen, or in an inert protective gas atmosphere, in particular in nitrogen or argon.

The object of the invention is also achieved by means of an injection-molding composition as described in the claims. It comprises at least one carbonyl metal powder and at least one element powder of metals from the group Cr, Mn, V, Si, Ti or of other metals which are at least as oxidation-sensitive. In place of an element powder, the composition can also contain an alloy powder comprising at least one metal of the group Cr, Mn, V, Si, Ti or/and at least one metal which is as oxidation-sensitive.

The injection-molding composition preferably contains a proportion of carbonyl metal powders of at least 30% by weight. The injection-molding composition preferably contains carbonyl metal powders of metals of the iron group, more preferably carbonyl iron powder. The ratio of the mean particle diameter of the carbonyl metal powder to the element and alloy powders is preferably at most 1:2.

Furthermore, there is provided a sintered metallic shaped body which is produced by shaping an injection-molding composition as claimed in any of the claims pertaining to the injection-molding composition, removing the binder and sintering, preferably using a process as claimed in any of the process claims. The proportion of alloying metals is preferably at least 5% by weight.

The shaped bodies produced in this way have lower surface roughness and higher surface gloss, which significantly reduces the expense of further machining.

FIG. 1 shows a comparison of the shrinkage behavior of alloys produced by different processes.

To produce shaped bodies of stainless steel of grade AISI 316L, a granulated material was prepared by mixing and compounding a powder mixture with binder materials in a heatable laboratory compounder.

The powder mixture consisted of 6900 g of carbonyl iron powder having a carbon content of 0.7% by weight and a mean particle size of 4 .mu.m and 3100 g of a gas-atomized prealloy of 55% by weight of Cr, 38% by weight of Ni and 7% by weight of Mo, with the mean particle size in the prealloy being below 25 .mu.m. The binder materials used were 952 g of polyoxynethylene and 104 g of polyethylene.

The granulated material obtained was processed in a screw injection-molding machine to give tensile test bars having a length of 85.5 mm and a diameter of 4 mm (in accordance with MPIF Standard 50, 1992).

For comparison, a granulated material was prepared from 8886 g of a finished-alloy powder of the alloy AISI 316L having a mean particle size of <25 .mu.m, 1003 g of polyoxymethylene and 116 g of polyethylene in the manner described and was processed to give injection-molded specimens.

For better comparability, both granulated materials thus contained 62% by volume of metal powder, based on the total granulated composition.

All injection-molded specimens were subjected to catalytic binder removal at 110.degree. C. in a stream of nitrogen of 500 l/h into which 20 ml/h of concentrated HNO.sub.3 were metered. The specimens were subsequently sintered in an electrically heated furnace in dry hydrogen having a residual moisture content corresponding to a dew point of -45.degree. C. For this purpose, they were brought to 1360.degree. C. at a heating rate of 5 K/min and held at this temperature for 1 hour.

The density of the sintered specimens, determined by the Archimedes method in water, was in both cases more than 7.7 g/cm.sup.3. In both cases, the optical microscopic examination of the polished sections indicated a uniform austenitic microstructure having a low residual porosity in the form of small, closed pores.

Table 1 shows the mechanical properties of the injection-molded parts produced by the different methods, and also their carbon, nitrogen and oxygen contents after sintering.

The comparison shows that injection-molded parts having comparable mechanical properties are obtained by both methods. The carbon, nitrogen and oxygen contents are in both cases below the maximum values required for good corrosion resistance. However, the injection-molded parts produced by the process of the invention have a significantly better surface quality.

FIG. 1 shows a comparison of the shrinkage behavior of the alloys produced by the different processes. For this purpose, the injection-molded green parts were, after binder removal, sintered in a dilatometer.

The relative length change of the cylindrical injection-molded green parts is plotted over the duration of sintering. The associated sintering temperature is given by the temperature curve T(.degree.C.) together with the temperature axis.

Since the granulated materials used in the different processes have the same metal content by volume, the densification of the injection-molded parts can be concluded directly from the length change. It can therefore be seen from FIG. 1 that the injection-molded parts produced by the two different processes achieve about the same final density after sintering. In the case of the specimens produced by the process claimed, shrinkage commences at as low as 600.degree. C. This gives the injection-molded green parts increased strength from this temperature upwards. In contrast, the comparative specimens showed discernable shrinkage only at 1150.degree. C.

Therefore, it is also possible to sinter thin-walled injection-molded parts having complicated shapes without support, without resulting in distortion of the sintered body. The susceptibility of the injection-molded parts to mechanical shocks, as can occur in continuous sintering furnaces, is also reduced.

It was also surprisingly found that the reproduction accuracy of the shaped bodies produced is significantly better than when using atomized powders. This advantage is of particular importance in the case of shapes having long flow paths and thin channels, ie. a high flow path/wall thickness ratio.

Tensile bars with the binder removed were produced as described in Example 1. In contrast to Example 1, the sintering cycle was interrupted at 600.degree. C. or 1000.degree. C. The flexural strength of the cylindrical specimens thus obtained was determined in a 3-point bend test with a span of 30 mm. The results are shown in Table 2.

It can be seen that the flexural strength of the alloy produced by the process of the invention from a carbonyl iron powder and a CrNiMo prealloy is significantly higher than for the alloy sintered from a finished-alloy powder in the comparative process. This property is particularly advantageous for industrial manufacture, since the injection-molded parts are less sensitive to mechanical shocks. This also makes the storage of large injection-molded parts of complicated shape simpler.