Classifications

• • 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)

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Citations (2)

• 1987
  • 1987-07-14 EP EP87810395A patent/EP0254680A1/fr not_active Withdrawn

Patent Citations (2)

Non-Patent Citations (1)

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