AT509613B1 - METHOD FOR PRODUCING MOLDINGS FROM ALUMINUM ALLOYS - Google Patents

Abstract

The invention relates to 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; characterized in that the binder is completely removed in step c), wherein, if appropriate after carrying out one or more preceding debinding stages, a thermal debinding takes place to remove the (residual) binder, which is at least 0.5% by volume Oxygen-containing atmosphere is carried out, after which the resulting completely unbonded Braunling is sintered.

Description

Description [0001] The technology of metal powder injection molding has gained a tremendous upswing in recent years and is an established technology with a global annual turnover of about € 1 billion for the production of intricately shaped small parts. 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 solvent, catalytically, etc., where 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 yet been industrially successfully introduced because 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 massively hinders sintering. For this reason, the literature also consistently describes an oxygen-free atmosphere.

A particular difficulty in processing aluminum in the manner described above is also the relatively low melting point of aluminum (660 ° C) produced by the addition of alloying elements, e.g. Tin, still lowered. The resulting problem is that the debinding 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.

In US 2003/220424 A1, a specific binder composition is disclosed which can be used in powder injection molding "(PIM) or" Press & Sintering process can be used and requires a defined number of debindering steps at different temperatures. "Inorganic powders" that can be processed in this way include, inter alia, aluminum oxide and nitride.

The aim of the invention was against this background, the development of a metal powder injection molding process, can be prepared by the moldings of aluminum materials with good mechanical properties 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 debindering; D) sintering the at least partially debinded Bräunlings to obtain the desired

Shaped body; wherein the inventive method is characterized in that in step c) the binder is completely removed, wherein, optionally after performing one or more preceding debinding steps, a thermal debinding takes place to remove the (residual) binder, which is in at least 0.5 Vol .-% oxygen-containing atmosphere is carried out, after which the resulting completely unbonded Braunling is sintered.

By this method, high-purity moldings are obtained from aluminum alloys, since there is no unwanted reactions of the plastic with the alloy metals due to the complete removal of the binder in step c). 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. even pure 02 gas can be used.

The aluminum alloy contains, besides aluminum, one or more other metals which are not particularly limited. Preferably, the alloying partners are selected from the group consisting of magnesium, copper, silicon and manganese, and are more preferably contained in a respective proportion of 0.5 to 25% by weight to obtain molded articles having desirable properties. Significantly lower melting metals, e.g. Bismuth, tin, lead, indium, or even zinc, or alloys such as Wood's metal, sometimes serving as sintering aids to lower the onset of melting temperature, are not required in the present invention, but may be added as alloying partners to sintered bodies if desired to obtain the corresponding alloys. In a particularly advantageous manner, the metals other than alloys with aluminum, i. as master alloy or so-called Masteralloy powder used.

In the present invention it is preferred to use binders which are known to be removable at low temperatures, more preferably polyacetal-based binders, e.g. Polyoxymethylene (POM) binders, for example those disclosed by BASF in EP 413,231 A2, WO 94/25205 A1 and especially EP 446.708 A2 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, and therefore, the binder is preferably 50 to 95%, more preferably 80 to 90%, of polyacetal. Alternatively, binder systems based on wax-polymer-based and in which the main component wax by prior Lösungsentbinderung, i. E. before carrying out the thermal debinding in the presence of oxygen according to the invention is removed.

The binder removal in step c) of the process according to the invention may comprise 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. That is, as debindering can also be a multi-stage thermal debindering at different process parameters, such as different temperature or atmosphere, e.g. without and with oxygen or with air and pure oxygen etc.

In preferred embodiments of the invention, first a catalytic debinding and / or a Lösungsentbinderung performed in step c) before the thermal debinding 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 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 method according to the invention is not particularly limited except for the requirement that the binder must have been previously completely removed. Preferably, however, is sintered to form a liquid phase, as will be explained in more detail below.

The previously known technology of powder metallurgical molding of aluminum alloys by means of molding based on the theoretical approach that the surface of the aluminum oxide coated with an aluminum oxide particles is mechanically injured by the pressing process in the die, whereby a metallurgical reaction is made possible. 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, in the inventors' view, without wishing to be bound by any particular theory, is to some extent intermediate, but predominantly stationary, i. in thermodynamic equilibrium with the solid Al phase, provides via microcracks, pores or similar "openings" in the oxide skins of the metal powder particles and down migration of the oxide skins the required contact between the metals in the powder mixture forth and thus supports the formation of a high-density sintered body the completely unbound 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 there is only one controllable by the choice of a corresponding temperature profile of the alloying metals in the liquid phase at any time during the sintering process , which effectively prevents loss of dimensional and dimensional stability.

The composition of the respective atmosphere in the individual steps of the inventive method is not particularly limited, except for the presence of the oxygen in the thermal debindering in step c), and the relevant expert can in each step for each powder mixture choose the most suitable atmosphere, with vacuum is also 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 noble gas (flelium, 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.

The sintering may optionally be followed by a suitable aftertreatment, 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 resulting sintered body (bottom) of Example 9.

Fig. 2 is a photograph of the green compact (left) and the sintered body (right) obtained therefrom from 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 funnel of the injection molding machine. The powder injection molding to produce the green parts was carried out in the following steps: The treated feed was plasticized and pre-dosed by a heated injection cylinder in which a screw rotates according to preset setting parameters (such as revolution speed, metering volume, back pressure, etc.). Subsequently, the pre-dosed quantity was injected into a suitably 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 ejected from the tool and removed with a handling.

EXAMPLE 1 - PULLERS: SOLVENT DISTRIBUTION / THERMAL DISTRIBUTION

A commercially available metal powder mixture (Alumix® 231 from Ecka), consisting of aluminum with 14 wt .-% silicon, 2.5 wt .-% copper and 0.6 wt .-% magnesium, was coated with a wax / Thermoplastics existing solvent binder carefully mixed into a feedstock.

Feedstock component proportion (% by weight)

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 silicon, 2.5% by weight copper and 0.6% by weight magnesium (from Ecka)

Debinding and Sintering of the Tensile Rods This feedstock was first debindered by means of solvent extraction in a 60 liter oven with acetone at a temperature of 45 ° C. within 12 hours.

The Bräunling thus obtained contained a residual binder content of about 14.5 wt .-%, which is then by thermal debinding according to the invention by means of a temperature profile of 150 ° C to 320 'O for 1 h and then from 320 to 420 ^ 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 rod diameter: 12.25% Sintered density: 2.36 g / cm3

EXAMPLE 2 - PULLERS: THERMAL DISTRIBUTION IN ONE STEP

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder * 4.3 POM binder 25.8

Lucryl G55 ** 2.8 100.0 * Pre-alloy of aluminum and magnesium in a ratio of 50:50 ** Commercially available polymethyl methacrylate (PMMA, ex BASF)

Debinding and sintering of the tensile bars Here, a complete thermal debindering 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. at a heating rate of 2 K / min

- 4h hold at 130 ° C.

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

- 5 h holding time at 200 ° C.

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

- 4h hold at 420 ° C.

The weight loss after the thermal debindering was 24.2%.

Subsequently, the sintering was carried out at a Ofeneinstelltemperatur 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 rod diameter: 14.52% Sintered density: 2.46 g / cm3

EXAMPLE 3 - TOW BARS: 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 C13-C15-oxoalcohol with 7 EO units Debinding and sintering of the tensile bars First, a first thermal debindering in a 50-liter oven was carried out in 500 l / h of air at 180 ° 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 at a Ofeneinstelltemperatur of 665 ° C for 1 h under nitrogen.

Results Length shrinkage: 9.5% Shrinkage of the bar diameter: 11.4% Sintered density: 2.13 g / cm3

EXAMPLE 4 - TOW BARS: CATALYTIC / THERMAL DISPERSION

Feedstock component proportion (% by weight)

Aluminum powder 70.1

Magnesium powder 2.2 POM binder 24.0

Tenside * 3.7 100.0 * Ethoxylated C13-C15-oxoalcohol with 7 EO units Debinding and sintering of the tensile bars First, a catalytic debinding was carried out in a 50-liter oven with 2% by volume of HNO3 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 were probably formed by the 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 rod diameter: 14.65% Sintered density: 2.36 g / cm3

EXAMPLE 5 - PULLERS: CATALYTIC / THERMAL DISPERSION

Feedstock component proportion (% by weight)

Aluminum powder 70.1

Magnesium powder 2.2 POM binder 24.0

Surfactant * 3.7 100.0 * Ethoxylated C13-C15-oxoalcohol with 7 EO units debindering and sintering of the tensile bars First, catalytic debinding was carried out analogously to Example 4, but using 80 g of anhydrous oxalic acid a Sublimierschale instead of HN03 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 rod diameter: 15.68% Sintered density: 2.42 g / cm3

EXAMPLE 6 - TUBES: CATALYTIC / THERMAL DISPERSION

Feedstock component proportion (% by weight)

Alumix 231 powder * 70.8 POM binder * 25.6

Surfactant ** 3.6 100.0 * Commercially available metal powder mixture of aluminum with 14% by weight silicon, 2.5% by weight copper and 0.6% by weight magnesium (from Ecka) [0069 ] ** Ethoxylated Ci3-Ci5-oxo-alcohol with 7 EO units Debinding and sintering of the tensile bars. 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 - TUBES: CATALYTIC / THERMAL DISPERSION

Feedstock component proportion (% by weight)

Aluminum powder 68.0

Masteralloy powder * 4.3 POM binder 24.0

Surfactant ** 3.7 100.0 * Pre-alloy of aluminum and magnesium in a ratio of 50:50 ** Ethoxylated Ci3-Ci5-oxo-alcohol with 7 EO units

Debinding and sintering of the tensile bars 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 bar diameter: 13.25% Sintered density: 2.56 g / cm 3

Example 8 - Hollow Cylinders: Catalytic / Thermal Decontamination

Feedstock component proportion (% by weight)

Aluminum powder 68.0

Masteralloy powder * 4.3 POM binder 24.0

Surfactant ** 3.7 100.0 * Pre-alloy of aluminum and magnesium in the ratio 50:50 ** Ethoxylated Ci3-Ci5-oxo-alcohol with 7 EO units

Debinding and sintering of the hollow cylinders. 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 diameter: 14.48% Sintered density: 2.59 g / cm3

EXAMPLE 9 - TOW BARS: CATALYTIC / THERMAL DISPERSION

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder * 4.3 POM binder 25.8

Lucryl G55 ** 2.8 100.0 * Pre-alloy of aluminum and magnesium in a ratio of 50:50 ** Commercially available polymethyl methacrylate (PMMA, ex BASF)

Debinding and sintering of the tensile bars 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 rod diameter: 19.55% Sintered density: 2.59 g / cm3

Example 10 - Hollow Cylinders: Catalytic / Thermal Decontamination

Feedstock component proportion (% by weight)

Aluminum powder 67.1

Masteralloy powder * 4.3 POM binder 25.8

Lucryl G55 ** 2.8 100.0 * Pre-alloy of aluminum and magnesium in a ratio of 50:50 ** Commercially available polymethyl methacrylate (PMMA, ex BASF)

Debinding and sintering of the hollow cylinders 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 diameter: 14.48% Sintered density: 2.56 g / cm 3 Thus, by the method of the present invention, sintered bodies of aluminum alloys by injection molding can be provided for practical use in many fields, eg 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 (20)

claims 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; characterized in that in step c) the binder is completely removed, wherein, optionally after performing one or more preceding debinding steps, a thermal debinding to remove the (residual) binder is carried out in an at least 0.5 vol .-% oxygen atmosphere is carried out, after which the resulting completely unbonded Bräunling is sintered. 2. The method according to claim 1, characterized in that the .Aluminiumlegierung 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. 15. 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. For this 2 sheets of drawings