WO2015055654A1

WO2015055654A1 - Process and casting machine for casting metal parts

Info

Publication number: WO2015055654A1
Application number: PCT/EP2014/072023
Authority: WO
Inventors: Walter DEVIETTI; Roberto Dutto
Original assignee: Nemak Europe GmbH
Current assignee: Nemak Europe GmbH
Priority date: 2013-10-14
Filing date: 2014-10-14
Publication date: 2015-04-23
Anticipated expiration: 2016-04-14
Also published as: DE112014004716T5

Abstract

The invention provides a process and a machine for casting metal parts in a mould (3, 4) comprising a mould cavity (10, 11) formed by a surrounding wall and internal cores defining said mould cavity (10, 11) with the shape of said cast part, a system of sprues, runners or ingates defining a flow channel (7) through which said molten metal (M) flows to fill said mould cavity (10, 11) from the bottom (16) up; said mould (3, 4) further comprising a riser cavity (8) wherein a certain amount of molten metal is maintained in fluid communication with said mould cavity (10, 11) while said molten metal (M) in said mould cavity (10, 11) solidifies for supplying more molten metal (M) according to the shrinkage effect of solidification of said metal in said mould cavity (10, 11). According to the invention said process comprises two sequencing pouring steps: - a first pouring step wherein a first portion of molten metal (M) is fed to said flow channel (7) to fill said mould cavity (10, 11) and - a second pouring step wherein another portion of molten metal (Μ') is poured directly to the riser cavity (8). By these measures porosity defects are avoided which otherwise could be effected due to the shrinkage of the cast part by the metal solidification.

Description

Process and casting machine for casting metal parts

The present invention refers to a metal casting process and a casting machine for casting metal parts.

One focus of the invention is to improve the yield of the SPM process ("SPM" = Semi-Permanent Mould) such that it corresponds to the HPDC process ("HPDC" = High Pressure Die Casting).

The Semi-Permanent Mould Casting process uses the same general procedures as Permanent Mould Casting but in the SPM-process expendable cores of sand or other materials are added to the moulding process to create a desired shape or an internal passage. In Semi-Permanent Mould Castings a p re-formed core is inserted into the permanent mould cavity. The metal flows around the insert and creates the desired shape or passageway. Use of a sand core allows the easy removal of the insert to create the desired effect.

It is known in the casting industry that filling a mould cavity with molten metal to form a good quality cast part requires precision in performing all the sequenced stages of the casting process so that the molten metal solidifies properly and produces cast parts with the desired mechanical properties and without surface defects, inclusions of other materials and gases, as well as geometric dimensional integrity.

A number of factors affect the good quality of the cast parts. For example, for cast parts of complex geometry and thin-walled forms, the molten metal must be overheated for obtaining good fluidity which ensures that the mould cavity is fully filled up. However, the higher the temperature of the molten metal, the longer it takes for cooling and solidifying the cast part.

For sand moulds for example, it needs additional efforts to remove the heat of the - liquid metal to ensure a sufficiently quick solidification. If solidification occurs at a cooling rate which is too slow, the microstructure of the solid metal is not adequate for obtaining the desired mechanical properties such as tensile strength and elongation.

It is a widely-spread practice in the casting industry to fill the moulds from the bottom up with the liquid metal. The purpose of this measure is to ensure that the molten metal flows in a non-turbulent and quiescent flow into the mould so that it does not entrap any air or other gases which may form voids in the cast part.

To this end, in common practice a system of sprues, runners and ingates is provided which allows to pour the molten metal being provided in a ladle into the channel system of the mould so that it fills the mould cavity in which the respective cast part is formed and enters into a riser being arranged above the top level of the cast part. The liquid metal being present in the riser provides the amount metal which needed to fill any gap that might form due to the volume contraction involved in the solidification of the molten metal.

An example for a machine and method for casting metal parts by directly pouring molten metal from a bottom pour ladle into the mould is disclosed in US 7,140,415.

Against the background of the prior art, the object of the invention was to provide a process and a casting machine that minimize the porosity defects due to the shrinkage of the cast part by the metal solidification.

With regard to the casting process, this problem is solved as per the invention in that it has the features indicated in claim 1.

With regard to the casting machine, this problem is solved as per the invention in that it has the features indicated in claim 9.

Advantageous embodiments and variations of the invention are indicated in the dependent claims and are explained in detail below and in the enclosed drawings as is the general inventive concept. - -

According to the invention a process for casting metal parts in a mould comprises a mould cavity formed by a surrounding wall and internal cores. The mould defines said mould cavity with the shape of said cast part. The mould also includes a system of sprues, runners or ingates defining a flow channel through which said molten metal flows in the course of the pouring of the molten metal to fill said mould cavity from the bottom up. The mould further comprises a riser cavity in which during the casting a certain amount of molten metal is maintained in fluid communication with said mould cavity while said molten metal in said mould cavity solidifies. By this molten metal is supplied corresponding to the shrinkage effect of solidification of said metal in said mould cavity.

According to the invention the pouring process is performed in two sequencing pouring steps:

- a first pouring step wherein a first portion of molten metal is fed to said flow channel to fill said mould cavity and

- a second pouring step wherein another portion of molten metal is poured directly to the riser cavity.

Correspondingly, a casting machine for casting metal parts in a mould comprises, in accordance with the invention, a mould cavity formed by a surrounding wall and internal cores defining said mould cavity with the shape of said cast part.

Furthermore the mould comprises a system of sprues, runners or ingates defining a flow channel through which in the course of pouring the molten metal into the mould said molten metal flows to fill said mould cavity from the bottom up, said mould further comprising a riser cavity wherein a certain amount of molten metal is maintained in fluid communication with said mould cavity while said molten metal in said mould cavity is solidifying, so that more molten metal is supplied

corresponding to the shrinkage effected by the solidification of said metal in said mould cavity. According to the invention the casting machine also comprises - -

- a first opening which is in fluid communication with said mould cavity so that a first portion of molten metal can be poured into the mould to fill said mould cavity from its bottom up, and

- a second opening in said riser cavity for pouring a second portion of said molten metal into said riser cavity.

The invention is based on the finding that by dividing the filling of the mould in two filling sequences which temporary may overlap but preferably are performed in a consecutive manner the productivity of the casting is increased and the porosity and shrinkage defects of the obtained cast parts are significantly decreased.

For this purpose according the invention in a first working step the molten cast metal is fed by way of gravity pouring into the mould via the first mould opening such that the mould forming the cast part is filled starting from its bottom up to its top.

In a second working step molten cast metal is filled separately from the first flow into a riser which is also connected with the mould cavity via a suitable conduct. The riser is arranged in the region of the top of the mould in a common manner such that molten metal advances into the riser in the course of the filling of the mould cavity and such that molten metal being present in the riser can flow back to the mould cavity vice versa, in case the volume of the metal being present in the mould cavity shrinks in the course of its solidification.

In this regard the invention has the important advantage that it allows to reduce the volume of the riser. The reduction of the volume is preferably done by increasing the height and reducing the width of the riser at the same time. The width reduction should overcompensate the height increase in order to achieve a minimized volume of the riser while keeping the height of the melt volume filled in the riser at a certain level.

For pouring the molten metal a ladle can be used as in common practice. - -

To simplify the filling of the mould in the first pouring step the first portion of the molten metal can be poured into a feeder cavity which is connected to the flow channel via which the molten metal flows into the cavity of the mould. For this purpose a machine designed in accordance with the invention can comprise a feeder cavity which is connected to the flow channel for receiving the molten metal and for feeding the molten metal in the flow channel. Preferably the feeder cavity is arranged above the level of the riser cavity to ensure that in the course of the first pouring step the molten melt advancing indirectly from the mould cavity reaches the riser cavity before solidification of the melt.

As mentioned before, the second step of the pouring performed in accordance with the invention processes filling should preferably start after the molten metal welling from the mould cavity has entered the riser cavity. This avoids turbulences in the molten metal present in the mould cavity when in the second pouring step the molten metal is directly poured into the riser. In practice it might be advantageous to start the second pouring step only after at least 20 % of the height of the riser cavity is filled with molten metal welling up from the cavity of the mould. To avoid any negative effects of the molten metal being poured directly into the riser cavity the start of the second pouring step can postponed until at least one third of the height of the riser cavity is indirectly filled with molten metal coming from the mould cavity.

The flow rates with which the molten metal enter into the mould can be controlled in a known manner by the design of the gating provided in the mould (s. for example Friedrich Nielsen: GieB- und Anschnitttechnik-Grundlagen und

Anwendung einer Methode; Giesserei-Verlag GmbH Diisseldorf). In the first stage the flow rate should not exceed a critical value, because higher flow rates could lead to unwanted turbulences in the melt. For a cylinder head of about 15 kg casting weight the typical flow rate is about 1 kg/s.

For the second stage the flow rate can be adjusted in a more liberal manner because no sensitive cavity sections of the mould cavity, in which a proper venting of the air has to be ensured during the filling process, have to be filled by the molten metal poured in the riser. Accordingly, is possible to cast the melt in the - second pouring step with higher flow rates compared to the first stage. The upper limit of the flow rate adjusted in the second pouring step is reached when the melt starts splashing out of the ladle instead of flowing in a laminar manner.

The occurrence of turbulences or any other negative effects which could be effected by molten metal flowing back into the flow channel system when the second pouring step starts can be avoided in that the metal being present in the sprues, runners or ingates of the flow channel is at least partially solidified before the second pouring step starts. It showed to be advantageous that the cooling in particular in the area of the downsprue and ingates starts earlier compared to the standard process. This ensures that the metal freezes early in these sections so that the melt is prevented from flowing back from the mould cavity into the gating system once the second stage pouring has started. This allows that the final metal level in feeder head to be higher than the pouring cup. A high level in the feeder head is favorable because it increases the metallostatic pressure which reduces the porosity generation during solidification of the metal in the mould cavity.

The first pouring step and the second pouring step might temporarily overlap. However, to avoid that turbulences occur as the result of a collision between the molten melt welling up from the mould cavity and the melt which is directly poured into the riser, it is preferable, that the second pouring step starts only after the first pouring step is finished.

The design of the riser cavity or the feeder cavity should preferably provide a horizontal or inclined plane which can be shaped by an outer sand core which defines the respective cavity. When pouring the melt against such a horizontal or inclined plane the kinetic energy of the melt is reduced and mixing of hot melt and colder, partly solidified metal in the cavity is restricted to the lowest possible degree. Also this measure contributes to an optimized quality of the cast part produced in accordance with the invention. For the same purpose it can be advantageously to provide in the mould a sector of the flow channel which is inclined such that longitudinal axis of the said sector incloses an acute angle with the direction of gravity. To simplify its integration in an automated production process the casting machine according to the invention may comprise a programmable controller for controlling the operation of robot means for performing the two sequencing pouring steps in accordance with the invention.

The main purpose of the invention is providing hot metal to the riser in castings produced with SPM-P (semi-permanent mould process). The higher riser temperature which is effected by directly filling the riser with molten metal according to the invention creates better directional solidification over the cross section of the casting and helps to reduce riser volume while the lower amount of metal passing in the bottom areas of the casting can improve mechanical characteristics in areas of deck face.

Since the moulds needed for the implementation of the invention can be designed in a common manner, the invention can be carried out without any difficulty on every production line established in the casting industry.

The invention is suited to cast parts for combustion engines, such as blocks and cylinder heads from light metal alloys, especially aluminium alloys.

The invention is explained in more detail below using a drawing showing exemplary embodiments. The Figures of the drawing respectively show schematically in a cross-sectional view:

Figures 1a - 1d a casting machine for casting an engine block for a combustion engine during four different stages of the casting process;

Figures 2a - 2d a casting machine for casting a cylinder head for a combustion engine during four different stages of the casting process;

Figure 3 a riser of the casting machine shown in Fig. 1.

The casting machines 1 ,2 shown in Figures 1a to 2d are essentially identical with the exception that the mould 3 of the casting machine 1 is provided for the casting - - of an engine block whereas the mould 4 of the casting machine 2 is provided for the casting of a cylinder head.

Accordingly, the casting machines 1 and 2 each comprises the respective mould 3,4, a conventional ladle 5 for pouring molten metal into the respective mould 3,4, a feeder cavity 6, a flow channel 7 and a riser cavity 8.

Each mould 3,4 is made of sand cores which are formed and assembled in a common manner. In the same common way additional cores 9 are placed in the respective mould cavity 10,11 of the moulds 3,4 to form channels, cavities and other functional form elements of the part to be cast in the respective mould 3,4.

The feeder cavity 6 is placed sidewise of the mould and on a level above and distances to the top of the mould cavity 10,11 of the respective mould 3,4. The feeder cavity 6 is formed in a known manner by sand cores and has in its bottom an opening to which the entrance of the flow channel 7 is connected.

The feeder is connected via the flow channel 7 with the respective mould cavity 10,11 , the flow channel 7 ending at the bottom 16 of the mould cavity 10,11. The flow channel 7 is formed by a system of sprues, runners or ingates which are formed in a known manner but are not shown here for clarity reasons. To ensure that the melt being fed through the flow channel 7 flows in a smooth as possible manner the flow channel sector 12 which extend between the feeder cavity 6 and that part 13 of the flow channel 7 which is placed below the bottom 16 of the respective mould cavity 10,11 , is inclined such that the longitudinal axis of the sector 12 incloses an acute angle β with the direction G of gravity. In this way the feeder cavity 6 stands via the flow channel 7 in fluid communication with the respective mould cavity 10,11.

The riser cavity 8 is formed in the cover core 14 which forms the head of the respective mould 3,4. It is connected with the respective mould cavity 10,11 via openings 15 formed in the bottom of the riser cavity 8. Accordingly, the openings 15 form a flow channel via which the riser cavity 8 is connected to the mould 10,11 of the respective mould 3,4. In the first process step shown in Figures 1a, 2a a first portion of an aluminium melt M is poured into feeder cavity 6.

To ensure a smooth filling of the mould cavity the pouring of the aluminium melt is performed in the first pouring step such that the melt M enters the respective cavity 10,11 with a flow rate Q of less than 1 ,0 kg/sec (Figures 1 b,2b).

The melt M flowing through the flow channel 7 enters the respective mould cavity 10,11 through the openings formed in the bottom 16 of the respective mould 3,4 so that the respective cavity 10,11 is filled from the bottom 15 up. The melt M being fed in the mould cavity 10,11 advances against the direction of gravity and wells up until it passes the openings 15 formed in the bottom of the riser 8. The feeding of melt M into the feeder cavity 6 is stopped as soon as the melt portion M being fed in the respective mould 3,4 is sufficient to fill the respective mould 3,4 such that its respective mould cavity 10,11 is completely filled and the level of the melt M entering indirectly via the openings 15 into the riser cavity 8 takes about one third of the height H of the riser cavity 8.

Now molten aluminium melt M by using the ladle 5 is additionally filled directly into the riser cavity 8 so that the melt M which is already present in the riser cavity 8 is combined with fresh hot molten melt M'. Accordingly, the temperature of the melt portion M+M' being present in the riser cavity 8 is considerably higher than it would be if only the melt M coming from the mould cavity would be present in the riser 8. Due to its higher temperature the melt portion M+M' has an optimized flow behaviour which ensures that the volume loss of the melt in the mould cavity 10,11 is reliably equalized said volume loss being effected by the shrinkage which occurs in the course of the solidification of the aluminium alloy melt M.

A programmable controller 17 is provided for controlling the operation of robot means 18 for moving the ladle 5 to pour the respective portion of molten metal Μ,Μ' in the feeder cavity 6 (first pouring step) and the riser cavity 8 (second pouring step). - -

Reference signs

1 ,2 casting machines

3 mould of the casting machine 1

4 mould of the casting machine 2

5 ladle

6 feeder cavity

7 flow channel

8 riser cavity

9 cores

10 mould cavity of mould 3

1 mould cavity of mould 4

12 sector of flow channel 7

13 part of flow channel 7 which is placed below the bottom 16 the mould cavity 10,11

14 cover core of the respective mould 3,4

15 openings formed in the bottom of the riser cavity 8

16 bottom of the respective mould cavity 10,11

17 controller

18 robot means for moving the ladle 5 β angle

G direction of gravity

H height of the riser cavity 8

H1/3 one third of the height of the riser cavity

M first melt portion

M' second melt portion

Claims

1. A process for casting metal parts in a mould (3,4) comprising

- a mould cavity (10,11) formed by a surrounding wall and internal cores defining said mould cavity (10,11) with the shape of said cast part;

- a system of sprues, runners or ingates defining a flow channel (7) through which said molten metal (M) flows to fill said mould cavity (10,11) from the bottom (16) up;

- said mould (3,4) further comprising a riser cavity (8) wherein a certain

amount of molten metal is maintained in fluid communication with said mould cavity (10,11) while said molten metal (M) in said mould cavity (10,11) solidifies for supplying more molten metal (M) according to the shrinkage effect of solidification of said metal in said mould cavity (10,11),

-characterized in that said process comprises two sequencing pouring steps:

- a first pouring step wherein a first portion of molten metal (M) is fed to said flow channel (7) to fill said mould cavity (10,11) and

- a second pouring step wherein another portion of molten metal (Μ') is poured directly to the riser cavity (8).

2. The process according to claim ^characterized in that in the first pouring step the first portion of molten metal (M) is poured into a feeder cavity (6) which is connected to the flow channel (7).

3. The process according to claim 1 or2, characterized in that the second pouring step starts after the molten metal (M) welling up from the cavity (10,11) of the mould (3,4) has entered into the riser cavity (8). -2-

4. The process according to claim 3, characterized in that the

second pouring step starts after at least 20 % of the height (H) of the riser cavity (8) is filled with molten metal (M) welling up from the mould cavity (10,11) into the riser cavity (8).

5. The process according to claim 4, characterized in that the

second pouring step starts after at least one third (H1/3) of the height (H) of the riser cavity (8) is filled with molten metal (M) welling up from the cavity of the mould (3,4) into the riser cavity (8).

6. The process according to any of the claims 1 to 5, characterized in that in the second pouring step the flow rate of the molten metal (Μ') is higher than the flow rate of the molten metal (M) in the first pouring step.

7. The process according to any of the claims 1 to 6, characterized in that the metal being present in the sprues, runners or ingates of the flow channel (7) is at least partially solidified before the second pouring step starts.

8. The process according to any of the claims 1 to7, characterized in that the first pouring step is finished once the second pouring step is started.

9. A casting machine for casting metal parts in a mould comprising a mould

cavity (10,11) formed by a surrounding wall and internal cores (9) defining said mould cavity (10,11) with the shape of said cast part; a system of sprues, runners or ingates defining a flow channel (7) through which said molten metal (M) flows to fill said mould cavity (10,1 ) from the bottom up; said mould (3,4) further comprising a riser cavity (8) wherein a certain amount of molten metal (M) is maintained in fluid communication with said mould cavity (10,11) while -3- said molten metal (M) in said mould cavity (10,11) solidifies for supplying more molten metal (M) according to the shrinkage effect of solidification of said metal (M) in said mould cavity (10,1 ), characterized in that said casting machine (1,2) comprises a first opening (15) in fluid

communication with said mould cavity (10,11) for pouring a first portion (M) of molten metal to fill said mould cavity (10,11) from its bottom up, and a second opening (15) in said riser cavity (8) for pouring a second portion (Μ') of said molten metal into said riser cavity (8).

10. The casting machine according to claim 9, characterized in that it comprises a programmable controller (17) for controlling the operation of robot means (18) for pouring molten metal (Μ,Μ') in the two sequencing pouring steps.

11. The casting machine according to any of the claims 9 or 10,

characterized in that it comprises a feeder cavity (6) which is connected to the flow channel (7) for receiving the molten metal (M) and for feeding the molten metal (M) in the flow channel (7).

12. The casting machine according to claim 11, characterized in that the feeder cavity (6) is arranged above the level of the riser cavity (8).

13. The casting machine according to claim 11 or 12, characterized i n t h a t a horizontally aligned or inclined plane is arranged in the feeder (6) or the riser cavity (8) such that molten metal (Μ,Μ') being cast into the feeder (6) or the riser cavity (8) impinges on said plane. The casting machine according to any of the claims 9 to 13,

characterized in that the flow channel (7) comprises a sector (12) which is inclined such that longitudinal axis of the said sector (12) incloses an acute angle (β) with the direction of gravity (G).