EP0035601A1 - Process for making a memory alloy - Google Patents

Abstract

Process-related improvement of the mechanical properties of Cu / Al and Cu / Al / Ni memory alloys by powder metallurgical production from powder mixtures, some of which in turn consist of pre-alloyed and / or premixed starting powders. Isostatic pressing and sintering of the powder mixture in an inert gas atmosphere with subsequent multi-stage thermomechanical processing, each interrupted by intermediate annealing for homogenization. Final annealing in the beta mixed crystal area and quenching in water.

Description

The invention is based on a method for producing a memory alloy according to the preamble of claim 1.

Memory alloys based on the Cu / Al / Ni system are known and have been described in various publications (e.g. U.S. Patent No. 3,783,037). Such memory alloys, which belong to the general type with the β high-temperature phase, are generally produced by melt metallurgy.

When these alloys are cast, a coarse-grained structure is usually obtained, which is further coarsened by grain growth due to the subsequent annealing in the region of the β mixed crystals and cannot be reversed by thermomechanical treatment. Accordingly, the mechanical properties, especially the elongation and notch toughness of memory alloys produced in this way are relatively poor and their field of application is limited.

There is therefore a need to improve these memory alloys in terms of process technology in such a way that further practical fields of application can be opened up for them.

It has already been proposed to produce memory alloys of the Cu / Zn / Al type by powder metallurgy, starting from finished starting alloys corresponding to the final composition (e.g. M. Follon, E. Aernoudt, _P ow. Der-metallurgically processed shape-memory alloys, 5th European Symposium on Powder Metallurgy, Stockholm 1978, pp. 275-281). In this case, the finished powder is apselt einge- _k, cold compacted, warm compacted undstranggepresst.

However, this method is unsuitable for the production of compact and dense finished parts made of Cu / Al / Ni because the powder cannot be compacted and disintegrates again.

The invention has for its object to provide a manufacturing method for memory alloys based on copper, aluminum and nickel, which leads to dense, compact bodies with good mechanical properties and at the same time to exactly reproducible values of the transition temperature and other variables related to the memory effect.

According to the invention, this object is achieved by the features of claim 1.

The essence of the new process is not to start from elementary powders or from a starting powder corresponding to the final alloy, but to use a mixture of pre-alloyed powders and specially composed powder mixtures. This allows the required ductility to be optimally adapted to the processing process with extensive freedom in terms of composition.

The grain size of the crystallite of the finished body can largely be predetermined. Grain growth is not to be feared. Coherent oxide skins that prevent homogenization and impair mechanical properties are avoided. If a certain small percentage is present, the metal oxides are present in finely divided form as dispersoids and have a beneficial effect on the mechanical properties of the end product, preventing grain growth.

• Ausführungsbeispiel I:
  • Es wurde ein Rundstab aus einer Gedächtnislegierung folgender Endzusammensetzung hergestellt:
    
    Als Ausgangsmaterialien wurden folgende Pulver verwendet:
    • Pulver A: Cupro-Aluminium: 93 Gew.-% Cu; 7 Gew.-% Al, erschmolzen, atomisiert; Korngrösse 40 - 100 µ. Hersteller Baudier.
    • Pulver B: Aluminium premixe 202 AC: 96 Gew.-% Al; .4 Gew.-% Cu, Korngrösse 23 - 28 µ. Hersteller Alcoa.
    • Pulver C: Reinnickel: 100 Gew.-% Ni Korngrösse 44 µ. Mond-Nickel (z.B. Int. Nickel Co.)

The invention is described using the following exemplary embodiments:

• Embodiment I:
  • A round rod was made from a memory alloy with the following final composition:
    
    The following powders were used as starting materials:
    • Powder A: Cupro aluminum: 93% by weight Cu; 7% by weight Al, melted, atomized; Grain size 40 - 100 µ. Manufacturer Baudier.
    • Powder B: aluminum premixe 202 AC: 96% by weight Al; .4 wt .-% Cu, grain size 23 - 28 µ. Manufacturer Alcoa.
    • Powder C: Pure nickel: 100% by weight Ni grain size 44 µ. Moon nickel (e.g. Int.Nickel Co.)

240 g dieser Pulvermischung wurden in einen Gummischlauch von 20 mm Innendurchmesser abgefüllt und bei einem Druck von 8000 bar isostatisch zu einem Zylinder von 18 mm Durchmesser und 220 mm Höhe verpresst. Der Grünling wurde im Wasserstoffstrom bei einer Temperatur von 950°C während 1 h reduziert und vorgesintert und anschliessend im Argonstrom bei einer Temperatur von 950°C während 19 h fertiggesintert. Der rohe Sinterkörper wurde auf einen Durchmesser von 17 mm abgedrehte in ein weichgeglühtes Kupferrohr von 20 mm Aussendurchmesser eingeführt und durch Abdecken der Enden mittels Stöpsel und Verlöten unter Argonatmosphäre vollständig eingekapselt. Das derart gebildete Werkstück wurde nun abwechslungsweise einer thermomechanischen Bearbeitung und einer Homogenisierungsglühung im Argonstrom während je 1 h bei 950°C unterworfen. Im vorliegenden Fall bestand die thermomechanische Bearbeitung in einem Rundhämmern bei 950°C, wobei im 1. Rundhämmerstich der Durchmesser des Stabes auf 18 mm und bei jedem weiteren Stich um je 2 mm reduziert wurde. Dabei wurde so vorgegangen, dass auf je 2 thermomechanische Operationen eine Homogenisierungsglühung folgte. Der auf 8 mm Durchmesser heruntergehämmerte Stab wurde schliesslich einer abschliessenden Glühung im Argonstrom während 15 min bei einer Temperatur von 950°C unterworfen und unmittelbar daraufhin in Wasser abgeschreckt. Die Prüfung ergab für das Werkstück eine Dichte von 99,5 - 99,8 % des theoretischen Wertes.The following sample was mixed in a tumble mixer for 10 min:

240 g of this powder mixture were filled into a rubber tube with an inner diameter of 20 mm and pressed isostatically at a pressure of 8000 bar to a cylinder with a diameter of 18 mm and a height of 220 mm. The green compact was reduced and presintered in a hydrogen stream at a temperature of 950 ° C. for 1 h and then sintered in a stream of argon at a temperature of 950 ° C. for 19 h. The raw sintered body was turned to a diameter of 17 mm and introduced into a soft-annealed copper tube with an outer diameter of 20 mm and completely encapsulated by covering the ends with plugs and soldering under an argon atmosphere. The workpiece formed in this way was then alternately subjected to thermomechanical processing and homogenization annealing in a stream of argon for 1 h at 950 ° C. In the present case, the thermomechanical processing consisted of round hammering at 950 ° C, with the diameter of the rod being 18 mm in the first round hammering stitch and every subsequent one Stitch was reduced by 2 mm each. The procedure was such that 2 thermomechanical operations were followed by homogenization annealing. The rod hammered down to 8 mm in diameter was finally subjected to a final annealing in a stream of argon for 15 minutes at a temperature of 950 ° C. and immediately quenched in water. The test showed a density of 99.5 - 99.8% of the theoretical value for the workpiece.

Of course, the cycle of thermomechanical machining / homogenization can be continued for as long as required until the final shape of the workpiece is reached. When the theoretical density is reached, further homogenization is generally no longer necessary.

• Es wurde ein Rundstab aus einer Gedächtnislegierung folgender Endzusammensetzung hergestellt:
  

Working example II:

• A round rod was made from a memory alloy with the following final composition:
  

240 g dieser Pulvermischung wurden in ein weichgeglühtes Kupferrohr von 18 mm Innendurchmesser und 2 mm Wandstärke abgefüllt und durch Abdecken der Enden und Verlöten unter Argonatmosphäre vollständig eingekapselt. Hierauf wurde das Rohr samt Pulver bei einem Druck von 10 000 bar isostatisch gepresst und der Pressling im Wasserstoff/Stickstoff-Strom bei einer Temperatur von 750°C während 2 h reduziert und vorgesintert und anschliessend im Argonstrom bei einer Temperatur von 800°C während 25 h fertiggesintert. Daraufhin wurde das Werkstück abwechslungsweise je 2 Rundhämmeroperationen und einer Homogenisierungsglühung bei je 900°C ähnlich Beispiel I unterworfen. Der auf 6 mm heruntergehämmerte Stab wurde einer abschliessenden Glühung bei 1000°C während 10 min im Argonstrom unterzogen und in Wasser abgeschreckt. Die Dichte betrug 99,5 des theoretischen Wertes.The powders given in Example I were weighed out as follows and mixed in a tumble mixer for 15 minutes:

240 g of this powder mixture were filled into a soft annealed copper tube with an inner diameter of 18 mm and a wall thickness of 2 mm and completely encapsulated by covering the ends and soldering under an argon atmosphere. The tube and powder were then isostatically pressed at a pressure of 10,000 bar, and the compact was reduced and presintered in a hydrogen / nitrogen stream at a temperature of 750 ° C. for 2 h and then in a stream of argon at a temperature of 800 ° C. for 25 h sintered. The workpiece was then alternately subjected to 2 round hammer operations and a homogenization anneal at 900 ° C each, similar to Example I. The rod hammered down to 6 mm was subjected to a final annealing at 1000 ° C. for 10 minutes in a stream of argon and quenched in water. The density was 99.5% of the theoretical value.

• Es wurde ein Band aus einer Gedächtnislegierung folgender Endzusammensetzung hergestellt:
  

Working example III:

• A tape was made from a memory alloy of the following final composition:
  

240 g dieser Pulvermischung wurden in ein weichgeglühtes Tombakrohr von 20 mm Innendurchmesser und 1,6 mm Wandstärke abgefüllt und durch Abdecken der Enden und Verlöten unter Argonatmosphäre vollständig eingekapselt. Hierauf wurde das Rohr samt Pulver bei einem Druck von 12 000 bar isostatisch gepresst und der Pressling im Wasserstoffstrom bei einer Temperatur von 850°C während 1 Y2 h reduziert und vorgesintert und anschliessend im Argonstrom bei einer Temperatur von 820°C während 22 h fertiggesintert. Daraufhin wurde das Werkstück in 2 Rundhämmerstichen bei einer Temperatur von 900 C auf 18 bzw. 16 mm Durchmesser reduziert und während 1 h im Argonstrom bei 920°C homogenisiert. Es folgten nochmals 2 Rundhämmerstiche bei 900°C, so dass der Stab schliesslich einen Durchmesser von 13 mm hatte. Nach abermaliger Homogenisierung während 1 h bei 920°C wurde der Stab in mehreren aufeinanderfolgenden Warmwalzoperationen mit jeweils 20 - 25 % Querschnitssabnahme zu einem Band von 1,5 mm Dicke und 20 mm Breite heruntergewalzt. Nach einer abschliessenden Glühung bei 950°C während 12 min wurde das Band in Wasser abgeschreckt. Die Dichte des fertigen Bandes betrug 99,7 %.The powders given in Example I were weighed out as follows and mixed in a tumble mixer for 12 minutes:

240 g of this powder mixture were filled into a soft annealed tombac tube with an inner diameter of 20 mm and a wall thickness of 1.6 mm and completely encapsulated by covering the ends and soldering under an argon atmosphere. The tube and powder were then isostatically pressed at a pressure of 12,000 bar and the compact was reduced and presintered in a hydrogen stream at a temperature of 850 ° C. for 1 Y2 h and then sintered in a stream of argon at a temperature of 820 ° C. for 22 h. The workpiece was then reduced in 2 round hammer passes at a temperature of 900 C to 18 or 16 mm in diameter and homogenized in a stream of argon at 920 ° C for 1 h. This was followed by two round hammer stitches at 900 ° C, so that the rod finally had a diameter of 13 mm. After repeated homogenization for 1 h at 920 ° C, the rod was rolled down in several successive hot rolling operations, each with 20-25% reduction in cross section, to form a strip 1.5 mm thick and 20 mm wide. After a final annealing at 950 ° C. for 12 minutes, the strip was quenched in water. The density of the finished tape was 99.7%.

• Es wurde ein Vierkantstab aus einer Gedächtnislegierung folgender Endzusammensetzung hergestellt:
  

Working example IV:

• A square bar was made from a memory alloy of the following final composition:
  

250 g dieser Pulvermischung wurden in einen Gummischlauch von 35 mm Innendurchmesser abgefüllt und bei einem.Druck von 12 000 bar isostatisch zu einem Zylinder von 31 mm Durchmesser und 80 mm Höhe verpresst. Der Grünling wurde im Wasserstoffstrom bei einer Temperatur von 920°C während-1 h reduziert und vorgesintert und anschliessend im Argonstrom bei einer Temperatur von 950°C während 20 h fertiggesintert. Der rohe Sinterkörper wurde auf einen Durchmesser von 30 mm abgedreht, in den Rezipienten einer Strangpresse eingesetzt und bei einer Temperatur von 780°C zu einem Vierkantstab quadratischen Querschnitts von 8 mm Kantenlänge verpresst. Das Reduktionsverhältnis (Querschnittsabnahme) betrug dabei 11 : 1. Daraufhin wurde der Stab bei einer Temperatur von 920°C während 30 min homogenisiert und anschliessend in 2 Stichen auf einer Warmziehbank bei einer Temperatur von 750°C auf eine Kantenlänge von 6 mm heruntergezogen. Nach der abschliessenden Glühung bei 900°C während 15 min im Argonstrom wurde der Stab in Wasser abgeschreckt. Die Dichte des fertigen Stabes betrug 99,8 % des theoretischen Wertes.The powders given in Example I were weighed out as follows and mixed in a tumble mixer for 10 min.

250 g of this powder mixture were filled into a rubber tube with an inner diameter of 35 mm and pressed isostatically at a pressure of 12,000 bar to a cylinder with a diameter of 31 mm and a height of 80 mm. The green compact was reduced in a hydrogen stream at a temperature of 920 ° C. for 1 h and presintered and then sintered in a stream of argon at a temperature of 950 ° C. for 20 h. The raw sintered body was turned to a diameter of 30 mm, inserted in the recipient of an extrusion press and pressed at a temperature of 780 ° C. to a square bar with a square cross section and an edge length of 8 mm. The reduction ratio (decrease in cross-section) was 11: 1. The rod was then homogenized at a temperature of 920 ° C. for 30 minutes and then pulled down in 2 passes on a warming bench at a temperature of 750 ° C. to an edge length of 6 mm. After the final one Annealing at 900 ° C for 15 min in a stream of argon, the rod was quenched in water. The density of the finished rod was 99.8% of the theoretical value.

• Es wurde eine runde Platte aus einer Gedächtnislegierung folgender Endzusanmensetzung hergestellt:
  

Embodiment V:

• A round plate was made from a memory alloy of the following final composition:
  

1000 g dieser Pulvermischung wurden in einen Kunststoffschlauchvon 66 mm Innendurchmesser abgefüllt und bei einem Druck von 12 000 bar isostatisch zu einem Zylinder von 60 mm Durchmesser und 80 mm Höhe zusammengepresst. Der Grünling wurde im Wasserstoff/Stickstoff-Strom bei einer Temperatur von 880°C während 1 h reduziert und vorgesintert und anschliessend im Argonstrom bei einer Temperatur von 930°C während 25 h fertiggesintert. Der rohe Sinterkörper wurde auf einen Durchmesser von 58 mm abgedreht, in eine weichgeglühte Dose aus Weicheisen von 64 mm Aussendurchmesser eingeführt und durch Aufsetzen des Deckels und Verlöten in Argonatmosphäre vollständig eingekapselt. Das derart hergestellte Werkstück wurde einer thermomechanischen Bearbeitung unter einer Warmpresse, unterbrochen von Homogenisierungsglühungen, unterzogen. Durch abwechslungsweises Stauchen und Glühen bei 900°C wurde die Höhe des Zylinders sukzessive auf ca. 32 mm reduziert, wobei sich das Material bis auf ca 95 % der theoretischen Dichte verdichtete und nun einen dem Gesenk . entsprechenden Durchmesser von 70 mm aufwies. Nach einer zusätzlichen Homogenisierungsglühung bei 950⁰C während 1 h wurde die vorgeformte kreisrunde Platte mit parallelen ebenen Stirnflächen in ein in ihren Durchmessern abgesetztes Schmiede-. gesenk eingesetzt und in mehreren Arbeitsgängen, welche durch Zwischenglühungen unterbrochen waren, bei Temperaturen zwischen 1000°C und 750°C auf die fertige Form heruntergeschmiedet. Die 20 mm dicke Platte wies bei einem maximalen Aussendurchmesser von 90 mm auf der oberen Seite einen radialen Wulst von 5 x 5 mm und auf der unteren Seite eine zentrale Ausnehmung von 20 mm Durchmesser und 5 mm axialer Tiefe auf. Nach einer abschliessenden Glühung bei 980°C während 15 min wurde die Platte in Wasser abgeschreckt. Die Dichte betrug 99,2 - 99,5 % des theoretischen Wertes.The powders given in Example 1 were weighed out as follows and mixed in a tumble mixer for 15 minutes:

1000 g of this powder mixture were filled into a plastic tube with an inner diameter of 66 mm and pressed isostatically at a pressure of 12,000 bar into a cylinder with a diameter of 60 mm and a height of 80 mm. The green compact was reduced in a hydrogen / nitrogen stream at a temperature of 880 ° C. for 1 h and presintered and then sintered in a stream of argon at a temperature of 930 ° C. for 25 h. The raw sintered body was turned to a diameter of 58 mm, in a soft-annealed box made of soft iron with an outer diameter of 64 mm was inserted and completely encapsulated by placing the lid and soldering in an argon atmosphere. The workpiece produced in this way was subjected to thermomechanical processing under a hot press, interrupted by homogenization annealing. By alternately upsetting and annealing at 900 ° C, the height of the cylinder was successively reduced to approx. 32 mm, whereby the material condensed to approx. 95% of the theoretical density and now the die. had a corresponding diameter of 70 mm. After an additional homogenization annealing at 950 ^° C. for 1 h, the preformed circular plate with parallel flat end faces was turned into a forge with a reduced diameter. used lowered and forged in several work steps, which were interrupted by intermediate annealing, at temperatures between 1000 ° C and 750 ° C to the finished shape. With a maximum outer diameter of 90 mm, the 20 mm thick plate had a radial bead of 5 x 5 mm on the upper side and a central recess of 20 mm diameter and 5 mm axial depth on the lower side. After a final annealing at 980 ° C. for 15 minutes, the plate was quenched in water. The density was 99.2 - 99.5% of the theoretical value.

• Pulver A: Vorlegierung
  
• Pulver B: Vormischung und/oder Vorlegierung
  
• Pulver C: Reines Metall (mindestens eines vorhan-
  

The invention is not restricted to the sizes and values described in the examples. In general, the powder compositions and the particle sizes can be varied and substituted within the following limits:

• Powder A: master alloy
  
• Powder B: premix and / or master alloy
  
• Powder C: Pure metal (at least one available
  

The powder mixtures can be within the following limits:

Isotatic pressing preferably requires pressures of at least 8000 bar. The reduction and presintering of the compact can expediently take place in the temperature range from 700 to 1000 ° C. for at least 30 minutes in a stream of hydrogen or hydrogen / nitrogen. The sintering of the compact must be carried out above the temperature of the eutectoid transformation, ie at a temperature of at least 700 ° C. for 10 hours in an argon flow to achieve the most homogeneous structure possible. The thermomechanical processing, which can consist of hot pressing, hot extrusion, hot forging, hot rolling, hot drawing and / or hot round hammering, should be carried out at temperatures between 700 and 1000 ° C, as well as the intermediate homogenization in the inert gas flow (intermediate annealing) at at least 700 ° C for at least 30 min. The final annealing in a stream of argon is carried out at temperatures between 700 and 1050 ° C (β mixed crystal region) for 10 to 15 minutes and the workpiece is then immediately quenched in water.

For most types of thermomechanical processing, it is advisable to encapsulate the material beforehand in a ductile metallic shell that does not chemically react with it, which is removed mechanically or chemically at the end of the shaping as a surface layer in most applications. Soft-annealed metals and alloys such as copper, copper alloys and soft iron are particularly suitable as materials for the casing. Encapsulation can take place immediately before the thermomechanical processing, in that the sintered body undergoes a mechanical surface treatment beforehand by turning, milling, planing, etc., or the powder can be filled directly into a suitable tube, a can, etc., instead of into a rubber or plastic tube will.

The powder metallurgical method according to the invention enables the production of workpieces from a memory alloy of the Cu / Al and Cu / Al / Ni type, which are produced in a conventional manner, ie by melt metallurgy Bodies have a fine-grained structure and optionally contain dispersoids in the form of finely divided oxide particles. The mechanical properties, in particular the elongation, notch toughness and the working capacity of such workpieces are significantly better than those of cast and / or hot-kneaded bodies. This opens up a further area of application for this type of alloy.

A comparison for the alloy with 13% by weight aluminum, 3% by weight nickel and 84% by weight copper serves to illustrate the differences mentioned above:

Claims (8)

1. A method for producing a memory alloy based on copper, aluminum and nickel, characterized by the following steps: a) Production of a powder A with a particle size of 10 to 200 .mu.m from a copper-rich master alloy with 84 to 99% by weight of Cu, remainder Al, production of a powder B with a particle size of 5 to 100 p by mixing and / or alloying 95 to 99.5% by weight aluminum powder with 0.5 to 5% by weight copper powder, production of a powder C with a particle size of 10 to 100 μ from at least one of the elements nickel, iron, manganese, cobalt; b) mixing 0.5 to 10% by weight of powder B and 0 to 6% by weight of powder C, remainder powder A, in a tumble mixer for at least 10 minutes; c) Isostatic pressing of the powder mixture in a plastic or rubber hose at a pressure of at least 8000 bar; d) reducing and presintering the compact produced under c) in a hydrogen or hydrogen / nitrogen stream at a temperature between 700 and 1000 ° C. for at least 30 min; e) sintering the reduced and presintered compact in a stream of argon at at least 700 ° C. for at least 10 h; f) alternating thermomechanical processing at a temperature between 700 and 1000 ° C and homogenization in the inert gas stream at a temperature of at least 700 ° C for at least 30 min; g) Final annealing in a stream of argon at a temperature between 700 and 1050 ° C. for 10 to 15 minutes and immediately followed by quenching in water. 2. The method according to claim 1, characterized in that the sintered workpiece undergoes a mechanical surface treatment before step f) and is then encapsulated in a jacket made of soft annealed copper, iron or a soft copper alloy. 3. The method according to claim 2, characterized in that the mechanical surface treatment consists in turning and the workpiece thus machined is inserted into a soft annealed copper tube and the latter by covering the ends by means of plugs and soldering is completely sealed under an argon atmosphere. 4. The method according to claim 1, characterized in that the isostatic pressing under c) takes place in a tube made of soft annealed copper, iron or a soft copper alloy and that the jacket formed thereby is removed mechanically or chemically only after process step g). 5. The method according to claim 1, characterized in that the thermomechanical processing of step f) consists in a hot pressing, hot extrusion, hot forging, hot rolling or hot drawing. 6. The method according to claim 1, characterized in that the thermomechanical processing of step f) consists of a rotary hammer. 7. The method according to claim 5, characterized in that the workpiece has a rod shape, and that alternately during process step f) 2 round hammer picks at 950 ° C is followed by a homogenization annealing at 950 ⁰ C, such that a total of 6 round hammer picks and 2 to 3 homogenization anneals be performed. 8. The method according to claim 1, characterized in that the cycle specified in step f) is of any length until the final shape of the workpiece is reached.