EP1381484A2

EP1381484A2 - Production of component parts by metal injection moulding (mim)

Info

Publication number: EP1381484A2
Application number: EP02740268A
Authority: EP
Inventors: Heinz Nelles; Volker Bader; Hans-Peter Buchkremer; Ariane Marjam Ahmad-Khanlou; Martin Bram; Detlev STÖVER
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2001-04-24
Filing date: 2002-04-20
Publication date: 2004-01-21
Also published as: WO2002085561A3; WO2002085561A2; DE10120172C1; JP2004525264A; US20040146424A1

Abstract

The invention relates to a method for producing component parts with a high degree of dimensional precision, made of NiTi-shape memory alloys. The invention makes it possible to apply a known metal injection moulding method to NiTi-shape memory alloys by adapting the binding system, temperature control and reducing the time necessary for debinding.

Description

description

Manufacture of components by metal injection molding (MIM)

The invention relates to a method for the production of components, in particular a method for the production of components close to the final dimension with the aid of metal injection molding.

State of the art

Metal powder injection molding (MIM), also known as metal powder spraying, is known as a process for the mass production of metallic components, especially for the near net shape (NNS) production of such components (RM German, A. Böse (ed. ): "Injection Molding of Metals and Ceramics", New Jersey, Metal Powder Industries Federation MPIF (1997)).

The MIM process enables small to medium-sized, complex-shaped parts to be produced inexpensively and automatically in large quantities. The MIM process provides components with a density of 95 - 98% of the theoretical density, and subsequent hot isostatic pressing of the body (without capsule material) enables a density of 100% to be achieved.

The process comprises plasticizing metal powders with spherical or irregular morphology (particle sizes of the powder 5 - 300 μm) using a binder. dersystems to a so-called feedstock. The feedstock is homogenized in a kneader. The feedstock is then filled into the injection molding machine. Parts of the binder system (e.g. suitable waxes) are melted in a heated zone. A worm conveys the thermoplastic mass into the divisible mold. After filling the mold, the liquid phase solidifies again and enables the component to be removed from the mold. The binder system is removed by a debinding step upstream of the sintering. Depending on the binder system, the additives are removed from the component in different ways.

A distinction is made between thermal debinding processes (melting out or decomposition via the gas phase), solvent extraction and catalytic debinding. This is followed by the sintering process, in which the component is compressed to up to 98% of the theoretical density using diffusion processes. Due to the high binder content, large shrinkages (15-20%) occur during sintering. The control of the shrinkage behavior is one of the essential requirements in the manufacture of near-net-shape components.

Typical suitable materials for the metallic component in metal powder spraying are stainless steel, carbon steel, tool steel or alloy steel, but also ferrite, tungsten carbide and mixtures of copper / bronze, cobalt / chromium or also tungsten / copper. Shape memory alloys are metals that, after being deformed, return to their original shape when heated to a certain temperature. In doing so, they can develop considerable powers. (WJ Buehler, JV Gilfrich, RC Wiley: Ocean Eng. 1

(1968), 105). Perhaps the best known are nickel-titanium alloys.

Possible areas of application for shape memory alloys are micromanipulators and robot actuators that can mimic the flowing movements of human muscles. In the case of reinforced concrete structures, sensors made of shape memory alloys could be used, which, for example, detect cracks in the concrete or corrosion in the steel reinforcing bars and counteract the stresses occurring inside.

So far, shape memory alloys based on the intermetallic NiTi phase have preferably been produced by melt metallurgy. The conventional shaping by mechanical processing of semi-finished products made of NiTi materials is limited, however, because the alloys have a high elongation at break in the martensitic state and are therefore difficult to machine. As a result, post-processing of melt-metallurgically manufactured components is only possible with a great deal of time and tool wear.

The use of the metal injection molding process also for

NiTi shape memory alloys have so far failed due to the fact that in addition to economical production all of the low contamination levels in the end product necessary for the shape memory properties must be guaranteed. The resulting difficulty in NiTi component production via metal powder injection molding (MIM) is the oxygen and

To keep carbon as low as possible. High levels of impurities lead to reduced sintering activity of the metal powder and deteriorate the material properties in the sintered component (e.g. due to embrittlement).

Another important prerequisite for the production of NiTi shape memory alloys is the precise and reproducible setting of the alloy composition. Even slight deviations in the composition lead to significant deviations in the characteristic properties (e.g. the transition temperatures).

Task and solution

The object of the invention is to provide an effective and inexpensive method for the near-net-shape production of components from a pre-alloyed NiTi shape memory alloy. Furthermore, it is an object of the invention to provide the materials necessary for the method.

The object is achieved by a method with all the features of the main claim. Advantageous embodiments of the method and the materials required for this can be found in the subclaims which refer back to it. Subject of the invention

The method according to the invention for the near-net-shape production of components from pre-alloyed NiTi shape memory alloys comprises the following steps. Pre-alloyed NiTi powder is mixed with a thermoplastic binder system to form a feedstock. The feedstock is brought into the shape of the component by a spraying process. The component is exposed to a capillary-active substance, debindered and then subjected to a thermal debinding process. This is followed by sintering to the finished component.

An advantageous process variant provides for the use of a two-component binder based on wax, in particular a two-component binder consisting of an amide wax and a polyolefin wax in a suitable composition. The amide wax influences the flowability of the feedstock and serves for plasticization, while the polyolefin contributes to stabilizing the feedstock and the component obtained from it.

Injection molding compounds with viscosities of approx. 5 to 30 Pa-s can be injected in this way. The very good flowability of the mass enables rapid injection and a void-free and flawless filling of even complex cavities with undercuts.

Another advantage is the high material yield, since all spray by-products can be returned to the manufacturing process without loss of quality. This increases the economic efficiency especially with regard to the processing of expensive materials.

The debinding process is to be regarded as a further process step. Here, the plasticizing component (amide wax) is pre-released in a capillary-active substance at 120 to 150 ° C. This leads to a partial debinding of the entire green body. This process takes about 2 hours. This is followed by thermal debinding for the components at approx. 480 ° C for 10 hours. Depending on the material, the capillary-active substances can be sands or ceramic powders (e.g. Zr0 ₂ , Si0 ₂ , SiC, Si ₃ N ₄ or A1 ₂ 0 ₃ ). The separating agent between the capillary-active material and the component serves as filter paper, which ashes without residue during the debinding process.

The advantage of the production method according to the invention is that the proportions of carbon and oxygen are kept so low that they meet the requirements for good shape memory properties.

In experiments, for example, carbon fractions of 1400 ppm and oxygen fractions of 2300 ppm (Table 1) could be obtained. At these contamination levels, the reversible austenite-martensite conversion, which is important for the shape memory properties, could be demonstrated by DSC measurements (differential scanning calometry). Through the inventive According to current processes, Ni-Ti components with a density of <95% can be produced reproducibly.

A special part of the description, Metal Powder Injection Molding (MIM), also known as metal powder injection molding, is a powder metallurgical molding process that enables cost-effective and near-net-shape production of very complicated molded parts in a rational and automated manner. In addition, powder spraying is characterized by a high material yield, which ensures efficient use of the currently very expensive pre-alloyed NiTi powder, since the sprues and scrap parts are returned to the process. The process combines the advantages of injection molding of thermoplastics (known from plastics processing) with the metallurgical possibilities of powder metallurgy.

1.) Powder and feedstock production:

Several factors are decisive for the suitability of a powder. The basic requirement is a high purity of the starting powder, since an increase in impurities such as oxygen, carbon and nitrogen in the powder is inevitable during the course of the process. When processing Ti alloys that have a high affinity for these elements, these lead to the formation of oxides, carbides and nitrides. These in turn influence the shape memory properties and lead to embrittlement of the material. Normally, powders with a grain size (KG) <25μm should be used. Coarser powders would result in lower final dense lead. The grain shape also plays a role, since e.g. B. spherical powders enable a higher packing density and thus smaller amounts of binder are required, which causes less shrinkage of the component. The powders are processed with heatable kneaders with the addition of the respective binder system to the feedstock. The binder takes on the task of giving the powder the right rheological properties for the spraying process and ensuring a manageable and stable green body. However, a residue-free removal of the binder is an important requirement for the properties of the component. The choice of binder is decisive for the quality of the product, so the correct binder composition is to be regarded as "know-how". Since the requirements for binder systems cannot be met by one system alone, a multi-component system is generally used to to ensure the necessary viscosity when spraying and on the other hand to ensure sufficient stability of the green body during debinding or sintering to prevent solidification cracks due to the different thermal expansion of the binder and the powder during cooling and solidification of the samples. This can be achieved through process control but also through the selection of a suitable binder system No chemical reac- tion between powder and binder, and there should be good wettability.

In general, the following binder systems can be described as known, which essentially differ in the debinding process:

- thermoplastic binders;

- polymerization binder;

- gel-forming binders;

- extraction binder; - thermosets.

The use of thermoplastic binders is currently the most widespread. These include commercially available polymers such as polyethylenes, polystyrenes, polypropylenes and waxes. Waxes or low molecular weight polymers are mostly used as the main component.

In the starting powder used in the process, for example, a commercially available, melted NiTi powder with a KG <25 μm and spherical particles can be used. The thermoplastics amide wax (as plasticizer) and polyolefin (as stabilizer) have proven to be advantageous binder components. The resultant advantage lies in the possibility of a purely thermal debinding, which simplifies the process and shortens the time and thus saves costs. This avoids the effort of an intermediate step such as catalytic debinding or solvent extraction. 2.) Childbirth

The debinding process is an important process step for achieving the quality of the finished parts. The time required for the process influences the profitability of the overall process. The debinding process depends on the binder system used.

In the thermal debinding according to the invention, the binder system is liquefied by the heating. The liquid binder is removed from the mold surface, creating a concentration gradient from the inside of the molded part to the surface. With the help of the capillary forces, binder is constantly transported from the inside to the surface, which leads to continuous debinding.

A very effective method of removing the wax has proven itself to be the capillary force-induced sucking-up (puckering) of the wax in the temperature range between 120 ° C and 150 ° C. Here, a powder bed is used, the KG distribution of which is significantly below that of the starting material. Embedding the molded part considerably reduces the debinding time and also has a stabilizing effect on the component. The melted wax is drawn out of the component by the capillary forces. The use of filter paper supports this effect and protects the components against adhesion of the wicking material used. The binder system is constructed in such a way that after the debinding process the plasticizing part is expelled or evaporated. A small amount of residual binder between see individual particles ensures the dimensional stability of the green body until it is driven out via the combined debinding / sintering step. The component is still manageable after the wicking process and can be removed from the sand bed for sintering and placed on Zr0 ₂ sintered underlays for the sintering process.

3.) Sintering In the initial phase of sintering, the residual binder still present in the green body must be baked out. For this purpose, the molded part is removed from the wicksand, placed on a sintering pad and slowly heated to the debinding temperature and held for 1-2 hours. The sintering then takes place in the same process step. Sintering takes place under protective gas or in a vacuum. The sintered densities achieved are reproducibly in the range of ~ 95% of the theoretical density. The theoretical density can be achieved by an additional HIP process.

Examples: Example 1:

1400 g NiTi powder 64.4 g amide wax 42.9 g polyolefin Total binder content: 7.12% by weight = 34% by volume

Example 2: 1800 g NiTi powder

64.4 g amide wax

42.9 g polyolefin Total binder content: 5.63% by weight = 28% by volume

Winding parameters:

- in Zr0 ₂ sand with KG <10 μm using filter paper

- flowing argon atmosphere or vacuum

- at 120 - 150 ° C up to 4 hours.

The samples are then removed from the wicksand.

Debinding / sintering parameters (sinter pad Zr0 ₂ ):

- heating from RT to 450 ° C under a flowing argon atmosphere at a rate of 1 K / minute,

- holding time at 450 ° C: 2 hours,

- heat up to 1250 ° C at a rate of 5 K / minute,

- holding time 5 hours,

- Everything under protective gas (argon, 1270 mbar) Sintered density corresponds to 95% of the theoretical density.

Tab. 1: Contamination of carbon, nitrogen and oxygen in the starting material and after debinding or sintering of the components.

Figures Figure 1 Principle of the winding process Figure 2 molded parts after the injection process Figure 3 component after the final sintering (different geometry than in Figure 2

Claims

claims

1. Method for the production of components from pre-alloyed NiTi shape memory alloy using metal injection molding, with the steps - pre-alloyed NiTi powder is mixed with a thermoplastic binder to form a feedstock,

the feedstock thus formed is brought into the shape of the component by a spraying process,

- the component is made of a capillary-active substance and pre-released,

the component is then subjected to a thermal debinding process,

- The component freed from the binder is sintered.

2. The method according to the preceding claim, characterized by the use of a two-component binder based on wax.

3. The method according to any one of the preceding claims, characterized by a binder comprising polyolefin.

4. The method according to any one of the preceding claims, characterized by a binder comprising 50 to 65 wt .-% amide wax and 35 to 50 wt .-% polyolefin.

5. The method according to any one of the preceding claims, characterized by a binder with a content of 64.4 wt .-% amide wax and 42.9 wt .-% polyolefin.

6. The method according to any one of the preceding claims, characterized by a feedstock with a viscosity between 5 and 30 Pa-s.

7. The method according to any one of the preceding claims, characterized by a partial debinding process in which an amide wax is first removed from the interior of the green part.

8. The method according to the preceding claim, in which the amide wax is caused by capillary force-induced suction with the aid of a powder bed.

9. The method according to the preceding claim, in which the powder bed has a grain size which is smaller than the grain size of the NiTi powder used.

10. The method according to any one of the preceding claims 7 to 9, in which a filter paper is used between the green part and the powder filling.

11. The method according to any one of the preceding claims 7 to 10 using a Zr0 ₂ powder spill.

12. The method according to any one of the preceding claims 7 to 11, wherein the powder bed has a grain size that is smaller than the grain size of the metal powder used, in particular with particle sizes in the range from 0.01 to 1 μm.

13. The method according to any one of the preceding claims 7 to 12, in which the debinding process is carried out at temperatures between 120 and 480 ° C.