EP0254680A1 - Low pressure die casting method - Google Patents

Low pressure die casting method Download PDF

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Publication number
EP0254680A1
EP0254680A1 EP87810395A EP87810395A EP0254680A1 EP 0254680 A1 EP0254680 A1 EP 0254680A1 EP 87810395 A EP87810395 A EP 87810395A EP 87810395 A EP87810395 A EP 87810395A EP 0254680 A1 EP0254680 A1 EP 0254680A1
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EP
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Prior art keywords
casting
pressure
solidification
time
formation
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Application number
EP87810395A
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German (de)
French (fr)
Inventor
Gunther Wulff
Jeanpierre Gabathuler
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Alcan Holdings Switzerland AG
Original Assignee
Alusuisse Holdings AG
Schweizerische Aluminium AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
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Application filed by Alusuisse Holdings AG, Schweizerische Aluminium AG filed Critical Alusuisse Holdings AG
Publication of EP0254680A1 publication Critical patent/EP0254680A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/09—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting by using pressure

Definitions

  • the invention relates to a low-pressure mold casting process for metals, in which the casting is pressurized with a gas pressure at least until it has completely solidified in the casting mold.
  • the molten metal is lifted from a melt container through a riser pipe into the mold cavity by a relatively low gas pressure. After the mold has been filled, the pressure is maintained until the solidification beginning at the highest point has progressed to the opening of the riser on the mold. During the entire solidification process, further metals melt is fed from below for refilling. After the casting has solidified, the melt container is vented and the molten metal in the riser pipe flows back into the melt container.
  • the mold is filled and solidified with counter-pressure - die casting under higher pressure, up to 10 bar.
  • the casting mold is located in a pressure chamber. Despite maintaining the pressure until the casting has completely solidified, this process does not succeed in avoiding the formation of cavities.
  • the pressure is applied only after the surface of the casting has solidified.
  • the pressure is preferably applied after the start of the solidification of the surface of that part of the casting in which the formation of voids is to be prevented.
  • the application of the method according to the invention - namely the effect of an increased gas pressure on the casting after the formation of a solidified edge zone - has the result that the voids and pores in the development phase expose to the atmosphere not in communication outside of the casting.
  • a relatively low compression pressure is therefore sufficient to effectively prevent the formation of voids and pores.
  • the optimal time for pressurization is from the start of solidification, which is 20 to 70% of the solidification time.
  • the solidification time is defined as the time span between the start of solidification and the complete solidification of a part of the casting and can be found empirically for any casting mold or derived from heat balance calculations.
  • a simple casting mold with essentially the same cross-sectional deposits leads to uniform solidification.
  • the solidification occurs practically simultaneously on the entire surface of the casting and ends everywhere in the casting, i.e. the casting has a single defined solidification time.
  • the solidification on the surface of the casting begins at different times. Solidification times of different lengths also result for each cross-section.
  • the solidification time of the most vulnerable section of the casting is preferred - or the solidification time of the section in which the void formation e.g. should be prevented for reasons of strength - used.
  • the time to pressurization is usually counted from the time the mold is filled. In this case, the time period from the completed mold filling to the beginning solidification of the time defined according to the invention must simply be added.
  • the solidification time includes depending on the molten metal to be cast, the type and size of the casting mold and the properties of the size and is normally in a range from a few seconds to a few minutes. Since working under higher pressures inevitably requires more equipment, efforts are made to keep the pressure as low as possible.
  • the gas pressure to be applied depends on the degree of solidification at the time the pressure is applied. In the preferred range of 20 to 70% of the solidification time, a pressure of 2 to 10 bar is normally sufficient. As the solidification progresses outside the preferred range, a correspondingly higher gas pressure is required. The gas pressure is maintained at least until the casting has completely solidified.
  • Air or nitrogen is usually used as the compressed gas. But it can - taking into account the response behavior of the to ver g iessenden material - other gases are used.
  • the process is particularly suitable for casting aluminum, magnesium, copper, zinc and their alloys.
  • the implementation of the method is not tied to a specific device.
  • the process can be implemented in a simple manner on a conventional counter-pressure die-casting system without major retrofitting.
  • the method according to the invention is demonstrated below using a model test with a material with a strong tendency to form voids or pores.
  • Spheres of 2 cm in diameter were cast from pure aluminum (Al 99.99). Under the given test conditions, the solidification time of the balls was 11 seconds. Until the complete solidification, the balls in the casting mold were exposed to pressures of different heights in an autoclave, the pressurized air being applied at different intervals after the start of solidification. The pore volume of the balls was then determined. The results are summarized in the table below. The pressures given are understood as excess pressure against atmospheric pressure. The values for the pore volumes correspond to mean values from 10 casting tests each.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Molds, Cores, And Manufacturing Methods Thereof (AREA)

Abstract

A low-pressure dyecasting method for metals is improved with a view to reducing the formation of voids. This is achieved by applying gas pressure to the casting in the mould, at least until it solidifies completely, pressure application taking place after a time, starting from the beginning of solidification, which corresponds to 20 to 70% of the solidification time of that portion of the casting in which the formation of voids is to be prevented. Choosing the optimum time for applying pressure is very important, whereas the level of pressure has a rather more secondary influence on the formation of voids.

Description

Die Erfindung betrifft ein Niederdruck-Kokillengiessverfahren für Metalle, bei dem das Gussstück zumindest bis zur vollständigen Erstarrung in der Giessform mit einem Gasdruck beaufschlagt wird.The invention relates to a low-pressure mold casting process for metals, in which the casting is pressurized with a gas pressure at least until it has completely solidified in the casting mold.

Beim Niederdruck-Kokillengiessen wird die Metallschmelze, durch einen verhältnismässig niedrigen Gasdruck von einem Schmelzebehälter über ein Steigrohr in den Formhohlraum einer Kokille gehoben. Nach erfolgter Füllung der Kokille wird der Druck so lange aufrecht erhalten, bis die an der höchsten Stelle beginnende Erstarrung bis zur Oeffnung des Steigrohrs an der Kokille fortgeschritten ist. Während des gesamten Erstarrungsablaufs wird somit weitere Metalle schmelze zum Nachspeisen von unten zugeführt. Nach der Erstarrung des Gussstücks wird der Schmelzebehälter entlüftet und die im Steigrohr befindliche Metallschmelze fliesst in den Schmelzebehälter zurück.In low-pressure mold casting, the molten metal is lifted from a melt container through a riser pipe into the mold cavity by a relatively low gas pressure. After the mold has been filled, the pressure is maintained until the solidification beginning at the highest point has progressed to the opening of the riser on the mold. During the entire solidification process, further metals melt is fed from below for refilling. After the casting has solidified, the melt container is vented and the molten metal in the riser pipe flows back into the melt container.

Im Gegensatz zum herkömmlichen Niederdruck-Kokillengiessverfahren erfolgt die Formfüllung und Erstarrung beim Gegendruck - Kokillengiessen unter höherem Druck, bis zu 10 bar. Die Giessform befindet sich hierbei in einer Druckkammer. Trotz Aufrechterhaltung des Drucks bis zum vollständigen Erstarren des Gussstücks gelingt es jedoch mit diesem Verfahren nicht, die Bildung von Lunkern zu vermeiden.In contrast to the conventional low-pressure die-casting process, the mold is filled and solidified with counter-pressure - die casting under higher pressure, up to 10 bar. The casting mold is located in a pressure chamber. Despite maintaining the pressure until the casting has completely solidified, this process does not succeed in avoiding the formation of cavities.

Angesichts dieser Gegebenheiten haben sich die Erfinder das Ziel gesetzt, ein Verfahren der eingangs erwähnten Art im Hinblick auf die Verminderung der Lunkerbildung zu verbessern.In view of these circumstances, the inventors have set themselves the goal of a method of the type mentioned in the opening paragraph With a view to reducing the formation of voids.

Zur Lösung der gestellten Aufgabe führt, dass die Druckbeaufschlagung erst nach Beginn der Erstarrung der Oberfläche des Gussstücks erfolgt.To achieve the object, the pressure is applied only after the surface of the casting has solidified.

Die Druckbeaufschlagung erfolgt bevorzugt nach Beginn der Erstarrung der Oberfläche desjenigen Teilbereichs des Gussstücks, in welchem die Bildung von Lunkern verhindert werden soll.The pressure is preferably applied after the start of the solidification of the surface of that part of the casting in which the formation of voids is to be prevented.

Der überraschende Effekt, dass die Lunkerbildung erst durch das Aufbringen eines Gasdrucks nach der Bildung einer erstarrten Randzone am Gussstück wirkungsvoll und reproduzierbar vermindert werden kann, lässt sich mit dem unterschiedlichen Gasdruck in den Lunkern und ausserhalb des Gussstücks erklären. Erfolgt die Druckbeaufschlagung bereits während der Formfüllphase, bzw. unmittelbar danach, so stehen die sich in der erstarrenden Metallschmelze bildenden Lunker und Poren über feine, an die Oberfläche des Gussstücks dringende Kanäle mit der das Gussstück bzw. die Giessform umgebenden Atmosphäre in Verbindung. Eine gewisse Anzahl Lunker ist dadurch zumindest zeitweise offenporig und steht demzufolge ebenfalls unter dem erhöhten Gasdruck. Diese Bedingungen wirken nun dem Kompressionseffekt der Druckbeaufschlagung entgegen, so dass ein Zusammenpressen der Lunker und Poren nicht erfolgen kann. Die Anwendung des erfindungsgemässen Verfahrens - nämlich die Einwirkung eines erhöhten Gasdrucks auf das Gussstück nach Ausbildung einer erstarrten Randzone - hat zur Folge, dass die Lunker und Poren in ihrer Entstehungsphase mit der Atmosphäre ausserhalb des Gussstücks nicht in Verbindung stehen. Ein verhältnismässig geringer Kompressionsdruck ist deshalb ausreichend, um die Bildung von Lunkern und Poren wirksam zu unterbinden.The surprising effect that the formation of voids can only be effectively and reproducibly reduced by applying a gas pressure after the formation of a solidified edge zone on the casting can be explained by the different gas pressure in the voids and outside of the casting. If the pressure is already applied during the mold filling phase or immediately afterwards, the voids and pores that form in the solidifying metal melt are connected to the atmosphere surrounding the casting or the casting mold via fine channels that penetrate to the surface of the casting. As a result, a certain number of blowholes are at least temporarily porous and are therefore also under the increased gas pressure. These conditions now counteract the compression effect of the pressurization, so that the voids and pores cannot be pressed together. The application of the method according to the invention - namely the effect of an increased gas pressure on the casting after the formation of a solidified edge zone - has the result that the voids and pores in the development phase expose to the atmosphere not in communication outside of the casting. A relatively low compression pressure is therefore sufficient to effectively prevent the formation of voids and pores.

Der optimale Zeitpunkt für die Druckbeaufschlagung liegt bei einer vom Erstarrungsbeginn an gerechneten Zeit, welche 20 bis 70 % der Erstarrungszeit entspricht. Die Erstarrungszeit ist hierbei definiert als Zeitspanne zwischen Erstarrungsbeginn und vollständiger Erstarrung eines Teilbereichs des Gussstücks und kann für jede beliebige Giessform empirisch gefunden oder aber aus Wärmehaushaltsberechnungen abgeleitet werden.The optimal time for pressurization is from the start of solidification, which is 20 to 70% of the solidification time. The solidification time is defined as the time span between the start of solidification and the complete solidification of a part of the casting and can be found empirically for any casting mold or derived from heat balance calculations.

Eine einfache Giessform mit im wesentlichen gleichen Querschnittsablagerungen führt zu einer einheitlichen Erstarrung. Die Erstarrung setzt an der gesamten Oberfläche des Gussstücks praktisch gleichzeitig ein und endet überall im Gussstück nach gleicher Zeit, d.h. das Gussstück hat eine einzige definierte Erstarrungszeit. Bei einer komplizierten Giessform mit örtlich unterschiedlichen Querschnitten setzt die Erstarrung an der Oberfläche des Gussstücks zu entsprechend verschiedenen Zeitpunkten ein. Ebenso ergeben sich für jeden Querschnitt unterschiedlich lange Erstarrungszeiten. Zur Festlegung des Zeitpunkts der Druckbeaufschlagung wird demzufolge bevorzugt die Erstarrungszeit des am stärksten lunkergefährdeten Teilbereichs des Gussstücks - bzw. die Erstarrungszeit desjenigen Teilbereichs, in welchem die Lunkerbildung z.B. aus Festigkeitsgründen verhindert werden soll - herangezogen.A simple casting mold with essentially the same cross-sectional deposits leads to uniform solidification. The solidification occurs practically simultaneously on the entire surface of the casting and ends everywhere in the casting, i.e. the casting has a single defined solidification time. In the case of a complicated casting mold with locally different cross sections, the solidification on the surface of the casting begins at different times. Solidification times of different lengths also result for each cross-section. In order to determine the point in time at which pressure is applied, the solidification time of the most vulnerable section of the casting is preferred - or the solidification time of the section in which the void formation e.g. should be prevented for reasons of strength - used.

Unter gleichbleibenden Giessbedingungen wird die Zeit bis zur Druckbeaufschlagung üblicherweise vom Zeitpunkt der beendeten Formfüllung an gerechnet. Hierbei muss einfach die Zeitspanne von der beendeten Formfüllung bis zur beginnenden Erstarrung der erfindungsgemäss definierten Zeit hinzugerechnet werden.Under constant casting conditions, the time to pressurization is usually counted from the time the mold is filled. In this case, the time period from the completed mold filling to the beginning solidification of the time defined according to the invention must simply be added.

Die Erstarrungszeit ist u.a. abhängig von der zu vergiessenden Metallschmelze, von der Art und Grösse der Giessform sowie den Eigenschaften der Schlichte und liegt normalerweise in einem Bereich von wenigen Sekunden bis zu einigen Minuten. Da ein Arbeiten unter höheren Drücken zwangsläufig einen grösseren apparativen Aufwand erfordert, ist man bestrebt, den Druck so niedrig wie möglich zu halten. Der aufzubringende Gasdruck richtet sich nach dem Ausmass der Erstarrung im Zeitpunkt der Druckbeaufschlagung. Im bevorzugten Bereich von 20 bis 70 % der Erstarrungszeit ist ein Druck von 2 bis 10 bar normalerweise ausreichend. Mit fortschreitender Erstarrung ausserhalb des bevorzugten Bereichs ist ein entsprechend höherer Gasdruck erforderlich. Der Gasdruck wird zumindest bis zur vollständigen Erstarrung des Gussstücks aufrecht erhalten.The solidification time includes depending on the molten metal to be cast, the type and size of the casting mold and the properties of the size and is normally in a range from a few seconds to a few minutes. Since working under higher pressures inevitably requires more equipment, efforts are made to keep the pressure as low as possible. The gas pressure to be applied depends on the degree of solidification at the time the pressure is applied. In the preferred range of 20 to 70% of the solidification time, a pressure of 2 to 10 bar is normally sufficient. As the solidification progresses outside the preferred range, a correspondingly higher gas pressure is required. The gas pressure is maintained at least until the casting has completely solidified.

Als Druckgas wird üblicherweise Luft oder Stickstoff verwendet. Es können aber - unter Berücksichtigung des Reaktionsverhaltens des zu vergiessenden Werkstoffs - auch andere Gase eingesetzt werden.Air or nitrogen is usually used as the compressed gas. But it can - taking into account the response behavior of the to ver g iessenden material - other gases are used.

Das Verfahren eignet sich insbesondere zum Giessen von Aluminium, Magnesium, Kupfer, Zink sowie deren Legierungen. Die Durchführung des Verfahrens ist nicht an eine bestimmte Vorrichtung gebunden. Auf einfache Weise lässt sich das Verfahren auf einer herkömmlichen Gegendruck-Kokillengiessanlage ohne wesentliche Umrüstung realisieren.The process is particularly suitable for casting aluminum, magnesium, copper, zinc and their alloys. The implementation of the method is not tied to a specific device. The process can be implemented in a simple manner on a conventional counter-pressure die-casting system without major retrofitting.

Das erfindungsgemässe Verfahren wird nachstehend anhand eines Modellversuchs mit einem Werkstoff mit starker Neigung zur Lunker- bzw. Porenbildung demonstriert.The method according to the invention is demonstrated below using a model test with a material with a strong tendency to form voids or pores.

Aus Reinstaluminium (Al 99.99) wurden Kugeln von 2 cm Durchmesser gegossen. Unter den gegebenen Versuchsbedingungen lag die Erstarrungszeit der Kugeln bei 11 Sekunden. Bis zur vollständigen Erstarrung wurden die Kugeln in der Giessform in einem Autoklaven unterschiedlich hohen Drücken ausgesetzt, wobei die Beaufschlagung mit Druckluft in unterschiedlichen Zeitabständen nach Erstarrungsbeginn erfolgte. Anschliessend wurde das Porenvolumen der Kugeln ermittelt. Die Ergebnisse sind in der nachstehenden Tabelle zusammengefasst. Die angegebenen Drücke verstehen sich als Ueberdruck gegen Atmosphärendruck. Die Werte für die Porenvolumen entsprechen Mittelwerten aus je 10 Giessversuchen.Spheres of 2 cm in diameter were cast from pure aluminum (Al 99.99). Under the given test conditions, the solidification time of the balls was 11 seconds. Until the complete solidification, the balls in the casting mold were exposed to pressures of different heights in an autoclave, the pressurized air being applied at different intervals after the start of solidification. The pore volume of the balls was then determined. The results are summarized in the table below. The pressures given are understood as excess pressure against atmospheric pressure. The values for the pore volumes correspond to mean values from 10 casting tests each.

Aus der Tabelle geht deutlich hervor, dass eine wirkungsvolle Verminderung des Porenvolumens bzw. der Anzahl und Grösse der Lunker nur dann erzielt werden kann, wenn die Druckbeaufschlagung des Gussstücks erst nach einer gewissen Zeit nach Erstarrungsbeginn erfolgt. Ebenso ist ersichtlich, dass der Wahl des optimalen Zeitpunkts für die Druckbeaufschlagung ganz wesentliche Bedeutung zukommt, währenddem die Höhe des Drucks im bevorzugten erfindungsgemässen Zeitbereich auf das Porenvolumen einen nur untergeordneten Einfluss hat.

Figure imgb0001
The table clearly shows that an effective reduction in the pore volume or the number and size of the blow holes can only be achieved if the casting is pressurized only after a certain time after the start of solidification. It can also be seen that the selection of the optimum time for the pressurization is of very great importance, while the pressure in the preferred time range according to the invention has only a minor influence on the pore volume.
Figure imgb0001

Tabelle Porenvolumen (in Prozenten) in Abhängigkeit des Gasdrucks und des Zeitpunkts der Druckbeaufschlagung.Table pore volume (in percent) depending on the gas pressure and the time of the pressurization.

Claims (4)

1. Niederdruck-Kokillengiessverfahren für Metalle, bei dem das Gussstück zumindest bis zur vollständigen Erstarrung in der Giessform mit einem Gasdruck beaufschlagt wird,
dadurch gekennzeichnet,
dass die Druckbeaufschlagung erst nach Beginn der Erstarrung der Oberfläche des Gussstücks erfolgt.1. Low-pressure mold casting process for metals, in which the casting is pressurized with a gas pressure at least until it completely solidifies in the casting mold,
characterized,
that the pressure is applied only after the surface of the casting has solidified. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Druckbeaufschlagung nach Beginn der Erstarrung der Oberfläche desjenigen Teilbereichs des Gussstücks erfolgt, in welchem die Bildung von Lunkern verhindert werden soll.2. The method according to claim 1, characterized in that the pressure is applied after the start of solidification of the surface of that portion of the casting in which the formation of cavities is to be prevented. 3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass die Druckbeaufschlagung nach einer Zeit, ausgehend vom Erstarrungsbeginn, erfolgt, welche 20 bis 70 % der Erstarrungszeit desjenigen Teilbereichs des Gussstücks entspricht, in welchem die Lunkerbildung verhindert werden soll.3. The method according to claim 1 or 2, characterized in that the pressure is applied after a time, starting from the start of solidification, which corresponds to 20 to 70% of the solidification time of that part of the casting in which the formation of voids is to be prevented. 4. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass der Gasdruck 2 bis 10 bar beträgt.4. The method according to any one of claims 1 to 3, characterized in that the gas pressure is 2 to 10 bar.
EP87810395A 1986-07-21 1987-07-14 Low pressure die casting method Withdrawn EP0254680A1 (en)

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CH2909/86 1986-07-21
CH290986 1986-07-21

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AU592905B2 (en) * 1986-11-17 1990-01-25 Aluminium Pechiney A lost foam casting process for the casting of metal objects

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE340402A (en) *
EP0005239A1 (en) * 1978-04-27 1979-11-14 Leibfried, Dieter Low-pressure casting method for metals, especially non-iron metals, and apparatus for carrying out this method

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE340402A (en) *
EP0005239A1 (en) * 1978-04-27 1979-11-14 Leibfried, Dieter Low-pressure casting method for metals, especially non-iron metals, and apparatus for carrying out this method

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN, Band 8, Nr. 136 (M-304)[1573], 23. Juni 1984; & JP-A-59 35 874 (TOYOTA JIDOSHA KOGYO K.K.) 27-02-1984 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AU592905B2 (en) * 1986-11-17 1990-01-25 Aluminium Pechiney A lost foam casting process for the casting of metal objects

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