EP2552630A1

EP2552630A1 - Method for producing shaped bodies from aluminium alloys

Info

Publication number: EP2552630A1
Application number: EP11720714A
Authority: EP
Inventors: Herbert Danninger; Christian Gierl; Branislav Zlatkov; Johan Ter Maat
Original assignee: Technische Universitaet Wien; Rupert Fertinger GmbH; BASF SE
Current assignee: Technische Universitaet Wien; Rupert Fertinger GmbH; BASF SE
Priority date: 2010-04-01
Filing date: 2011-03-31
Publication date: 2013-02-06
Anticipated expiration: 2031-03-31
Also published as: HUE035814T2; WO2011120066A1; JP5956419B2; CN103038006A; ES2639134T3; AT509613A1; AT509613B1; EP2552630B1; JP2013524006A; KR20130079373A; DK2552630T3; SG184423A1; PL2552630T3; US20130101456A1

Abstract

The invention relates to a method for producing shaped bodies based on aluminium alloys by metal powder injection moulding, comprising the following steps: a) preparing a feedstock by blending the metals contained in the desired alloy in the form of metal powders and/or one or more metal alloy powders with a binder; b) preparing a green compact by injection moulding the feedstock; c) preparing a brown compact by at least partially removing the binder from the green compact by catalytic and/or solvent and/or thermal debinding; d) sintering the at least partially de-bindered brown compact to obtain the desired shaped body. The invention is characterised in that in step c) the binder is removed completely, wherein, optionally after carrying out one or more preceding de-binding stages, a thermal debinding in order to remove the (residual) binder is effected and is carried out in an atmosphere containing at least 0.5% by volume of oxygen, after which the resulting completely de-bindered brown compact is sintered.

Description

Process for the production of moldings from aluminum alloys

The technology of metal powder injection molding has taken a tremendous boost in recent years and is ready for production! intricately shaped small parts have introduced a technology with a worldwide annual turnover of about € 1 billion. The possibility of combining the molding technology of plastic injection molding with the material diversity of powder technology has opened up interesting markets for many materials. The manufacturing process consists essentially of the process steps described below. First, a feedstock is prepared in the form of a sprayable granulate of metal powder and a plastic component comprising at least two intensively mixed polymer components. This feedstock is then sprayed into molded parts in plastic injection molding machines. This so-called "green body" or "green body" usually contains about 40% by volume of plastic binder, which is removed in the subsequent step, the so-called debindering, for the most part. There remains only a residual component of the binder, the so-called. "Backbone", which ensures the residual strength of the unbent body. Debinding can be done in a variety of ways, e.g. thermally, by solvents, catalytically, etc., whereby it must be very well matched to the plastic binder used. The debinded body, the so-called "brown part" or "browning", is now subjected to a sintering process, in the first stage of which the "backbone" residual binder is normally removed thermally and the body is then sintered with appropriate shrinkage to form an approximately dense metallic component. The technology is currently used for high and low alloy steels, precious metals, hard metals, but also for ceramics.

Although several related patents exist, metal powder injection molding for aluminum materials has not been successfully introduced industrially since the mechanisms of sintering aluminum alloys are very different from those of the above-mentioned materials. The presence of non-reducible oxides on the surface of aluminum powders hampers massively the sintering tion. For this reason, the literature also consistently describes an oxygen-free atmosphere.

A particular difficulty in the processing of aluminum in the manner described above is also the relatively low melting point of aluminum (660 ° C), which is enhanced by the addition of alloying elements, such as aluminum. Tin, still lowered. The resulting problem is that the debindering of the plastic component must be completed at very low temperatures, which often makes the available process window too small to ensure complete removal. However, if this fails, undesirable reactions of residual organic constituents with the metallic components can occur, hindering sintering and thus impairing the mechanical properties that can be achieved. For example, Liu et al. in Powder Metallurgy 51, 78-83 (2008) discloses a method of adding tin as an alloying metal as well as magnesium blocks, wherein the magnesium is referred to as a "sacrificial metal", i. as an oxygen and moisture scavenger. Against this background, the object of the invention was the development of a metal powder injection molding process by means of which molded articles made of aluminum materials with good mechanical properties can be produced in a simpler and reproducible manner. DISCLOSURE OF THE INVENTION

This object has been achieved by the inventors by providing a process for the production of moldings based on aluminum alloys by metal injection molding, comprising the following steps:

a) production of a feedstock by mixing the metals contained in the desired alloy in the form of metal powders and / or one or more metal alloy powders with a binder;

b) production of a green compact by injection molding of the feedstock; c) preparation of a browning by at least partial removal of the binder from the green compact by catalytic and / or solvent and / or thermal debinding;

d) sintering the at least partially debinded browning to obtain the desired shaped article;

wherein the inventive method is characterized in that in step c) the binder is completely removed, wherein, optionally after performing one or more preceding debindering stages, a thermal Entbin- tion takes place to remove the (residual) binder in at least one , 5 vol .-% oxygen-containing atmosphere is carried out, after which the resulting completely unbonded Braunling is sintered.

As a result of the complete removal of the binder in step c), no undesired reactions of the plastic with the alloying metals occur due to this process. This complete removal of the binder succeeds - even at relatively low temperatures - due to the presence of oxygen in the atmosphere. Contrary to the prevailing theory that oxygen must be avoided at all costs, the inventors have found that a small proportion of at least 0.5% by volume does not appreciably promote the oxidation of the aluminum, but contributes to rapid and complete debindering. Depending on the composition of the powder mixture and the temperature conditions, an oxygen content of between 20 and 100% by volume is used, for example. it can even be used pure (VGas.

The aluminum alloy contains, besides aluminum, one or more other metals which are not specifically limited. Preferably, the alloying partners are selected from the group consisting of magnesium, copper, silicon and manganese, and more preferably are contained in a respective proportion of 0.5 to 25% by weight to obtain moldings having desirable properties. Significantly lower melting metals, such as bismuth, tin, lead, indium or even zinc, or alloys such as Wood's metal, sometimes as sintering aids for the Ernie- Tempering serve the temperature of the onset of melting, are not required according to the present invention, but can still be added as an alloying partner, if desired, to obtain sintered bodies of the corresponding alloys. In a particularly advantageous manner, the other metals are used as alloys with aluminum, ie as master alloy or so-called master alloy powder.

According to the present invention, preference is given to using binders which are known to be removable at low temperatures, more preferably polyacetal-based binders, for example polyoxymethylene (POM) binders, for example those described by BASF in EP 413,231, WO 94/25205 and WO 92/25205 especially EP 446708 disclosed and also sold under the brand name Catamold ^®. In order to promote rapid and complete removability at low temperatures and in the presence of oxygen, a high polyacetal content is desirable in the binder, for which reason the binder is preferably 50 to 95%, more preferably 80 to 90% polyacetal , Alternatively, binder systems can be used which are based on wax polymer-based and in which the main component wax by previous Lösungsentbinderung, ie prior to the inventive implementation of the thermal debinding in the presence of oxygen, is removed.

Debinding in step c) of the process of the invention may involve a single step of thermal debinding in the presence of oxygen, in which the entire binder is removed. Alternatively, one or more preceding debinding steps may be performed to remove the bulk of the binder, followed by the thermal debinding step of the present invention to remove the residual binder in the presence of oxygen. Thus, a previous debinding step may also be a thermal debindering - in the absence or also in the presence of oxygen. This means that debranking can also involve multi-stage thermal debindering at different process parameters, for example different temperature or atmospheric pressure. re, eg with and without oxygen or with air and pure oxygen etc.

In preferred embodiments of the invention, a catalytic debinding and / or a solution debinding is carried out in step c) before the thermal debindering in order to remove the residual binder in the presence of oxygen. In this case, the bulk of the binder is already removed from the composition, so that in the subsequent thermal debinding preferably only the "backbone" component needs to be removed.

The catalytic debinding is carried out preferably in the presence of at least one acid selected from nitric acid, oxalic acid, formic acid and acetic acid, since these acids accelerate the complete removal of the preferred polyacetal binder by acidolysis, without leading to undesirable side reactions with the alloying partners. In the case of solution debonding, on the other hand, the bulk of the binder is obtained by extraction with a suitable solvent or solvent mixture, e.g. Acetone, n-heptane, water etc., removed. Particularly preferred according to the present invention is a catalytic debindering with sublimed oxalic acid.

As already mentioned, the thermal debinding to remove the residual binder in step c) is carried out at a relatively low temperature in order to suppress oxidation reactions, especially of the aluminum in the powder mixture. By a relatively low temperature herein is meant a temperature well below the melting point of aluminum, preferably below 500 ° C, more preferably between 100 and 420 ° C. In particular, an empirically optimized temperature profile for the respective powder mixture is set, which preferably provides a heating rate of not more than 5 K / min, more preferably not more than 1 to 2 K / min. As a result, the mixture to be debindered is heated in a gentle, uniform manner. The sintering step d) of the process of the present invention is not specifically limited except for the requirement that the binder must be completely removed beforehand. Preferably, however, is sintered to form a liquid phase, as will be explained in more detail below.

The hitherto known technology of powder metallurgical molding of aluminum alloys by means of compression molding is based on the theoretical approach that the surface of the aluminum particles coated with an aluminum oxide layer is mechanically damaged by the pressing process in the matrix, whereby a metallurgical reaction is made possible in the first place. In the case of a (completely) bindered brown body from powder injection molding, however, it is de facto a metal powder spill, with the oxide skins of the metals not being exposed to any mechanical stress and therefore not subject to this known mechanism. That is, there are no direct metal-to-metal contacts between the powder particles. Nevertheless, it is possible in the process according to the invention, by suitable choice of the sintering conditions to achieve the required shrinkage, by means of which manifests the compression of the sintered body, and thus to obtain largely dense components. Embodiments according to the invention are therefore preferred in which the completely debinded browning material is sintered in step d) to form a liquid phase. This liquid phase, which according to the inventors - without wishing to be bound by any particular theory - is present partly intermediate, but predominantly stationary, ie in thermodynamic equilibrium with the solid Al phase, via microcracks, pores or similar "openings" in the oxide skins of the metal powder particles and infiltration of the oxide skins the required contact between the metals in the powder mixture ago and thus supports the formation of a high-density sintered body from the completely unbonded Bräunling. Particularly preferably, the sintering in step d) is carried out at a temperature between the solidus and the liquidus temperature of the respective aluminum alloy, so that at any time during the sintering process only a controllable by the choice of a corresponding temperature profile proportion of alloying metals in liquid ger phase, which effectively prevents loss of dimensional and dimensional stability.

The composition of the particular atmosphere in the individual steps of the process according to the invention is not particularly limited, except for the presence of the oxygen in the thermal debindering in step c), and the person skilled in the art can select the most suitable for each powder mixture in each individual step Choose atmosphere, whereby also vacuum is possible. However, the sintering step d) is preferably carried out in an extremely dry nitrogen-containing atmosphere, i. in pure nitrogen, under normal pressure or reduced pressure ("partial pressure sintering"), or in a mixture of nitrogen and pure inert gas (helium, argon), preferably with a dew point <-40 ° C, since the presence of nitrogen with the wettability of the powder the resulting molten metal significantly supported.

If appropriate, sintering may be followed by a suitable after-treatment, by means of which the finished molded parts are obtained in the desired shape. For example, the known method of hot isostatic pressing (HIP) can be used to bring the moldings to the desired final density. In this case, residual pores remaining after sintering are pressed by the simultaneous action of external gas pressure and temperature, and the pore walls are welded together.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a photograph of the green compact (above) and the sintered body (bottom) obtained therefrom of Example 9.

Fig. 2 is a photograph of the green compact (left) and the sintered body (right) thereof obtained in Example 10.

The invention will be described below with reference to non-limiting specific embodiments. EXAMPLES

All feedstocks prepared in the examples below were homogenized in a heated kneader at 190 ° C. From these feedstocks, tensile test bars or hollow cylinders were molded by means of injection molding in accordance with ISO 2740, the method according to the invention being used as follows. To produce the green parts, a hydraulic injection molding machine (Battenfeld HM 600/130) with PIM equipment was used.

In a first step, the feedstock was first filled into a hopper of the injection molding machine. The powder injection molding for the production of the green parts was carried out in the following steps: The prepared feed material was plasticized and pre-dosed by means of a heated injection cylinder in which a screw rotates according to preset setting parameters (such as, for example, rotational speed, metering volume, dynamic pressure, etc.). Subsequently, the pre-metered quantity was injected into a correspondingly tempered tool. Depending on the feedstock or binder used, the plasticizing temperature in the injection cylinder was between 120 and 220 ° C, while in the tool between 25 and 140 ° C prevailed. After sufficient cooling time, the injection mold was opened and the green part was ejected from the mold and removed with a handling.

Example 1 - Ingredients: Solution Disinfectant / Thermal Debinder

A commercially available metal powder mixture (Alumix ^® 231 of Ecka) consisting of aluminum, with 14 wt .-% of silicon, 2.5 wt .-% copper and 0.6 wt .-% magnesium, was with a group consisting of wax / thermoplastic Solvent binder is carefully mixed into a feedstock.

Feedstock component proportion (wt

Alumix 231 powder * 74.8

Solvent binder: wax content 14.8

Solvent binder: thermoplastic content 8.2

Stearic acid 2,2

100.0

* Commercially available metal powder mixture of aluminum with 14% by weight of silicon, 2.5% by weight of copper and 0.6% by weight of magnesium (from Ecka)

Debinding and sintering of the tension rods

This feedstock was first debinded by solvent extraction in a 60 L oven with acetone at a temperature of 45 ° C within 12 h.

The Bräunling thus obtained contained a residual binder content of about 14.5 wt .-%, which then by thermal debinding according to the invention by means of a temperature profile of 150 ° C to 320 ° C for 1 h and then from 320 to 420 ° C for 1, 5 h was removed at a heating rate of 3 K / min under a pure oxygen-containing atmosphere. The thus completely unbonded Bräunling was then sintered at 560 ° C within 1 h in pure nitrogen (dew point: -50 ° C).

Results

Length shrinkage: 11, 6%

Shrinkage of the bar diameter: 12.25%

Sintered density: 2.36 g / cm ³ Example 2 - Zuqstäbe: thermal debindering in one step

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder ^* 4.3

POM binder 25.8

Lucryl G55 **

100.0 ^* Aluminum and magnesium master alloy in the ratio 50:50

** Commercially available polymethylmethacrylate (PMMA, from BASF)

Debinding and sintering of the tension rods

Here, a complete thermal debinding was carried out in a 40-liter oven with 200 l / h of pure oxygen according to the following debindering profile:

- Heating to 130 ° C with a heating rate of 2 K / min

- 4 h hold time at 130 ° C

- Heating to 200 ° C with a heating rate of 2 K / min

- 5 h hold time at 200 ° C

- Heating to 420 ° C with a heating rate of 2 K / min

- 4 hours hold at 420 ° C

Weight loss after thermal debinding was 24.2%.

Subsequently, the sintering was carried out at a furnace setting temperature of 665 ° C, which corresponds to a temperature within the furnace of about 630 ° C, during 1 h in pure nitrogen.

Results

Length shrinkage: 12.27%

Shrinkage of the bar diameter: 14.52%

Sintered density: 2.46 g / cm ³ Example 3 - Tension rods: double thermal debinding

Feedstock component proportion (% by weight)

Aluminum powder 70.1

Magnesium powder 2.2

POM binder 24.0

Surfactant ^* 3.7

100.0 * Ethoxylated C ₃ -C ₅ -oxoalcohol with 7 EO units debinding and sintering of the tensile bars

First, a first thermal debinding was carried out in a 50-liter oven in 500 l / h of air at 80 ° C for 14 h. Weight loss: 27.0%.

This was followed by a second thermal debindering to 420 ° C under pure oxygen within 1 h, which in turn was sintered under nitrogen at a Ofeneinstelltemperatur of 665 ° C for 1 h. Results

Length shrinkage: 9.5%

Shrinkage of the bar diameter: 11, 4%

Sintered density: 2.13 g / cm ³

Example 4 - Supplies: catalytic / thermal debinding

Feedstock component proportion (% by weight)

Aluminum powder 70.1

Magnesium powder 2.2

POM binder 24.0

Surfactant ^* __3

100.0 * Ethoxylated Ci ₃ -Ci ₅ -Oxoalkohol with 7 EO units debinding and sintering of the tensile bars

First, a catalytic debinding in a 50-liter oven with 2% by volume of HNO ₃ in 500 l / h of nitrogen (technically pure) at 140 ° C for 10 h. Weight loss: 22.1%. There were pearl-like outgrowths on the surface, which had probably been formed by reaction of Mg with HNO3.

Subsequently, as in Example 3, a thermal debindering to 420 ° C under pure oxygen within 1 h, after which it was again sintered at a Ofeneinstelltemperatur of 665 ° C for 1 h under nitrogen.

Results

Length shrinkage: 10.7%

Shrinkage of the bar diameter: 14.65%

Sintered density: 2.36 g / cm ³

Example 5 - Supplies: catalytic / thermal debinding

Feedstock component proportion (% by weight)

Aluminum powder 70.1

Magnesium powder 2.2

POM binder 24.0

Surfactant ^* _3

First, a catalytic debinding was carried out analogously to Example 4, but using 80 g of anhydrous oxalic acid on a Sublimierschale instead of HNO ₃ at 140 ° C for 24 h. Weight loss: 23.0%. Due to the use of oxalic acid, no outgrowths appeared on the surface. Subsequently, thermal debinding and sintering were also carried out analogously to Example 4.

Results

Length shrinkage: 14.28%

Shrinkage of the bar diameter: 15.68%

Sintered density: 2.42 g / cm ³

Example 6 - Supplies: Catalytic / Thermal Debinding

Feedstock component proportion (% by weight)

Alumix 231 powder * 70.8

POM binder 25.6

Surfactant _* *

100.0

** Ethoxylated C ₁₃ -C ₁₅ oxo alcohol with 7 EO units

Debinding and sintering of the tension rods

First, a catalytic debinding was carried out analogously to Example 5. Weight loss: 25.2%. Subsequently, thermal debinding and sintering were carried out analogously to Example 4, but at a Ofeneinstelltemperatur of 560 ° C.

Results

Length shrinkage: 11, 2%

Shrinkage of the bar diameter: 13.2%

Sintered density: 2.45 g / cm ³

Example 7 - Ingredients: Catalytic / Thermal Debinding

Feedstock component proportion (% by weight)

Aluminum powder 68.0

Masteralloy powder ^* 4.3

POM binder 24.0

Surfactant ** 3,7

100.0 * Aluminum and magnesium master alloy in the ratio 50:50

** Ethoxylated Ci ₃ -Ci ₅ -Oxoalkohol with 7 EO units

Debinding and sintering of the Zuqstäbe

First, a catalytic debinding was carried out analogously to Example 5. Weight loss: 23.2%. Subsequently, thermal debinding and sintering were carried out analogously to Example 4.

Results

Length shrinkage: 12.6%

Shrinkage of the rod diameter: 13.25%

Sintered density: 2.56 g / cm ³

Example 8 - Hollow Cylinder: Catalytic / Thermal Debinding

Feedstock component proportion (% by weight)

Aluminum powder 68.0

Masteralloy powder ^* 4.3

POM binder 24.0

Surfactant **

100.0 * Aluminum and magnesium master alloy in the ratio 50:50

** Ethoxylated C ₁₃ -C ₅ -oxoalcohol with 7 EO units

Debinding and sintering of the hollow cylinder

First, a catalytic debinding was carried out analogously to Example 5. Weight loss: 23.7%. Subsequently, thermal debinding and sintering were carried out analogously to Example 4.

Results

Height shrinkage: 17.24%

Shrinkage of the diameter: 14.48%

Sintered density: 2.59 g / cm ³

Example 9 - Tension rods: catalytic / thermal debinding

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder ^* 4.3

POM binder * 25,8

Lucryl G55 ** 2,8

100.0 * Aluminum and magnesium master alloy in the ratio 50:50

** Commercially available polymethylmethacrylate (PMMA, from BASF)

Debinding and sintering of the tension rods

First, a catalytic debinding was carried out analogously to Example 5. Weight loss: 25.7%. Subsequently, thermal debinding and sintering were carried out analogously to Example 4.

Results

Length shrinkage: 13.57%

Shrinkage of the bar diameter: 19.55%

Sintered density: 2.59 g / cm ³

Example 10 - Hollow Cylinder: Catalytic / Thermal Debinding

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder * 4.3

POM binder 25.8

Lucryl G55 ** Α

100.0 * Aluminum and magnesium master alloy in the ratio 50:50

** Commercially available polymethylmethacrylate (PMMA, from BASF)

Debinding and sintering of the hollow cylinder

First, a catalytic debinding was carried out analogously to Example 5. Weight loss: 25.6%. Subsequently, thermal debinding and sintering were carried out analogously to Example 4.

Results

Height shrinkage: 16.52%

Shrinkage of the diameter: 14.48%

Sintered density: 2.56 g / cm ³

Thus, by the method according to the invention sintered bodies of aluminum alloys can be provided by means of injection molding, which are suitable for practical use in many fields, e.g. in the transport sector, construction, mechanical engineering, packaging, iron and steel, electrical engineering, household appliances, etc., for example for heat dissipation in electronic devices ("heat sinks") or as components of air conditioning systems.

Claims

1. A process for the production of moldings based on aluminum alloys by metal injection molding, comprising the following steps:

a) Production of a feedstock by mixing in the desired

Alloy-containing metals in the form of metal powders and / or one or more metal alloy powders with a binder;

b) production of a green compact by injection molding of the feedstock;

c) preparation of a browning by at least partial removal of the binder from the green product by catalytic and / or solvent and / or thermal binder removal;

characterized in that in step c) the binder is completely removed, whereby, optionally after carrying out one or more preceding Entbin- mentation stages, a thermal debinding to remove the (residual) binder is carried out in at least 0.5 vol % Oxygen-containing atmosphere is carried out, after which the resulting completely unbonded Braunling is sintered.

2. The method according to claim 1, characterized in that the aluminum alloy in addition to aluminum one or more metals selected from magnesium, copper, silicon and manganese. 3. The method according to claim 1 or 2, characterized in that the aluminum alloy in addition to aluminum contains one or more metals in a respective proportion of 0.5 to 25 wt .-%.

4. The method according to any one of claims 1 to 3, characterized in that the or the metal (s) is used as a master alloy ("master alloy") powder or are.

Process according to any one of the preceding claims, characterized in that a polyacetal-based binder, e.g. Polyoxymethylene (POM) binder is used. 6. The method according to claim 5, characterized in that the binder consists of 50 to 95% of polyacetal.

7. The method according to claim 6, characterized in that the binder consists of 80 to 90% of polyacetal.

8. The method according to any one of claims 1 to 7, characterized in that in step c) only thermal debinding in the presence of oxygen, in one or more steps, is performed, in which the entire binder is removed.

9. The method according to any one of claims 1 to 7, characterized in that in step c) a Lösungsentbinderung to remove the main amount of the binder, followed by the thermal debinding to remove the residual binder are performed.

10. The method according to any one of claims 1 to 7, characterized in that in step c) a catalytic debinding to remove the main amount of the binder, followed by the thermal debinding to remove the residual binder are performed.

11. The method according to claim 10, characterized in that the catalytic debinding in the presence of at least one acid selected from nitric acid, oxalic acid, formic acid and acetic acid, is performed. 12. The method according to claim 11, characterized in that as acid sublimed oxalic acid is used.

13. The method according to any one of the preceding claims, characterized in that the thermal debinding to remove the residual binder is carried out at a temperature below 500 ° C. 14. The method according to claim 13, characterized in that the thermal debinding to remove the residual binder at a certain temperature profile between 100 and 420 ° C is performed. 5. The method according to claim 13 or 14, characterized in that during the thermal debinding to remove the residual binder, the heating rate is not more than 5 K / min.

16. The method according to claim 15, characterized in that the heating rate is not more than 1 to 2 K / min.

17. The method according to any one of the preceding claims, characterized in that the completely unbonded Bräunling is sintered in step d) to form a liquid phase. 18. The method according to claim 17, characterized in that the sintering takes place at a temperature between the solidus and the liquidus temperature of the respective aluminum alloy.

19. The method according to any one of the preceding claims, characterized in that after the thermal debinding to remove the residual binder, the heating rate to the sintering temperature 4 to 20 K / min.