JPH031103B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for casting easily oxidizable alloys such as aluminum alloys. [Prior art and problems] Aluminum alloys, such as Al-Mn alloys,
Since both Mn and Mn are easily oxidizable elements, cast products often contain many nonmetallic inclusions and have hot water wrinkles in the surface layer. Figure 6 []
Figure 1 shows nonmetallic inclusions a scattered in the center of a billet of 30I-700Mn alloy cast by a die, and the same figure [ ] shows mold wrinkles b in the surface layer of the same billet. The nonmetallic inclusion a is Al,
These are oxide particles such as Mn, and the melt wrinkles b are caused by a solidified film of the molten metal formed at the contact interface between the mold and the molten alloy, peeled off from the mold surface and rolled into the molten metal. In particular, in the case of mold casting, the solidification rate of the cast molten metal is fast, so non-metallic inclusions and oxide films in the molten metal are prevented from floating and separating into the feeder. A large amount remains in the cast product. Moreover, the solidification shrinkage rate of Al-Mn alloy is approximately 3%.
(for cast iron, it is about 0.9%) and has poor deformability (the elongation rate and reduction rate are almost zero at temperatures below about 600℃), so it is highly susceptible to cracking during the solidification process. Therefore, as mentioned above, if nonmetallic inclusions and oxide films that cause molten metal wrinkles are present in the molten metal, the occurrence of cracks in the cast product during the cooling and solidification process is significantly promoted. In addition, even if no cracks occur during the cooling and solidification process, cast billets that contain many nonmetallic inclusions and wrinkles are susceptible to cracking during subsequent plastic working and heat treatment, and are susceptible to mechanical damage. It is also inferior in properties etc. As a countermeasure for this, when casting an Al-Mn alloy with a mold, as shown in Figure 4, a mold 1 with a large feeder section 11 is used, and the amount of molten metal poured is increased to increase the heat content of the molten metal in the mold. and 300 to 500 molds in total.
By preheating to ℃, the solidification of the molten metal is delayed and nonmetallic inclusions, oxide films, etc. are easily floated to the feeder section. However, since the feeder section is made large (its size is about the same as or larger than the product section 12) and the amount of casting is increased, the casting yield is usually extremely low, about 40 to 50%. Moreover, even if such measures are taken, the effect of reducing and preventing nonmetallic inclusions and hot water wrinkles is not sufficient, and after casting, machining costs required to remove hot water wrinkles from the surface layer are large. The reality is that machining requires a large amount of cost. Other countermeasures include using a lost wax mold instead of a mold and preheating the mold to a high temperature of 600 to 1200°C, or placing a straight (metal or refractory mesh) on top of the mold. Methods have been adopted, such as passing the molten metal through a strainer to filter out non-metallic inclusions, and then casting it into a mold, but neither method has a satisfactory effect. The present invention aims to provide an improved casting method to solve the above problems. [Means and effects for solving the problem] The method for casting an easily oxidizable alloy of the present invention includes disposing an electromagnetic stirring device around the mold to form a rotating magnetic field with the vertical axis of the mold as the center of rotation. Then, while the molten metal poured into the mold is within a certain temperature range,
Due to the rotating magnetic field provided by the electromagnetic stirring device,
It is characterized by stirring the molten metal. FIG. 1 shows a basic example of an embodiment of the invention. An electromagnetic stirring device 2 is installed on the side of the mold 1, extending from the top to the bottom, and the molten metal M cast into the mold 1 is rotated as shown by the arrow by the rotating magnetic field given by the electromagnetic stirring device 2. It is beginning to cause movement. Figures 2 and 3 show other embodiments. In the example shown in FIG. 2, the electromagnetic stirring device 2 is placed at the bottom of the mold so that it can move up and down along the mold 1, and the electromagnetic stirring device 2 follows the pouring of the molten metal M into the mold 1. By moving the mold upward, the molten metal in the mold is given a rotational motion in the direction of an arrow from the bottom upwards. Also,
In the example shown in FIG. 3, two electromagnetic stirring devices 2, 2 are arranged on the side of the mold 1 so as to move up and down independently, and during or after pouring into the mold 1, The electromagnetic stirring devices 2, 2 are moved to the position to be stirred, and the molten metal in that area is given a rotational motion in the direction of an arrow. In order to make the stirring action effective, stirring of the molten metal in the mold in the method of the present invention is started in the temperature range of the freezing point (mp) of the molten metal (MP) + 160 to 180°C, and the temperature of the molten metal is lowered to mp + 40 to 60°C. It is necessary to carry out the process for a period of up to 30 minutes and then stop in that temperature range. The rotation speed and magnetic field strength of the rotating magnetic field depend on the shape of the target cast product, alloy properties, etc., but for example, the magnetic field strength at the center axis of the mold is 320G.
By doing so, good results can be obtained. Needless to say, the mold used may be a sand mold, a graphite mold, a ceramic shell mold, etc. in addition to a non-magnetic mold. In addition, if the mold rotates and oscillates as a reaction to the rotational movement within the mold 1, it may cause defects such as creases, so install an anti-rotation guard at an appropriate location (for example, the bottom) of the mold. It is preferable to attach a tool. [Function] The molten metal poured into the mold is stirred by the rotating magnetic field, which promotes the floating and separation of nonmetallic inclusions and entangled oxide films to the feeder section. Furthermore, by stirring the molten metal, its solidification rate is delayed and the fluid state is maintained for a longer period of time, so that nonmetallic inclusions are promoted to float and separate from the oxide film. Furthermore, when the molten metal is stirred, the temperature difference between the inside and outside of the metal is relaxed, the temperature gradient becomes gentler, and the solidification rate slows down as described above, resulting in the occurrence of shrinkage cavities within the cast product. suppressed. Moreover, even in the case of die casting, the formation and growth of columnar crystals from the side wall surfaces of the mold is inhibited, so the resulting cast product has a macrostructure with developed columnar crystals, unlike conventional macrostructures. It has a uniform granular structure both inside and outside. In addition, in the method of the present invention, the reason why the molten metal temperature during stirring of the molten metal by the rotating magnetic field is specified as described above is to ensure the above-mentioned effects of stirring, and the reason for this is to ensure the above-mentioned effects of stirring. Alternatively, if the timing of stopping is lost, the stirring effect may become insufficient, and the casting quality may even deteriorate due to the stirring action. [Example] In the casting apparatus shown in Fig. 1, 30Al-70Mn
The molten alloy (solidification point: 1230℃) is put into a heat-resistant steel mold (inner diameter: 60φ, depth: 150l, side wall thickness: ^35t (mm)).
A cast product was obtained by stirring the molten metal using an electromagnetic stirring device (number of poles: 2, power frequency: 60 Hz). Table 1 shows the casting conditions and properties of the cast product. In the table, "magnetic field strength" is the magnetic flux density at the center of rotation of the magnetic field, and "stirring start temperature" and "stirring stop temperature" indicate the temperature relative to the freezing point of the molten metal. Trial numbers (No.) 1 to 30 are invention examples, No. 10 to 14 are comparative examples,
Comparative Example No. 14 is a conventional example in which electromagnetic stirring was not performed and the mold shown in FIG. 4 was used. Figure 5 [] and [] are trial number 2 of the present invention example.
This is the microstructure of the cast product obtained by the method, and a in the figure shows nonmetallic inclusions in the center of the product;
b in [ ] indicates hot water wrinkles on the surface layer of the product. Moreover, FIGS. 6 [] and [] show the microstructure of a cast product made by Comparative Example No. 14 (conventional method, no electromagnetic stirring). As shown in Table 1, the cast products in the examples of the present invention have sound quality, and as shown in Figure 5 [] []
As is clear from the comparison between (inventive example) and Fig. 6 [] [] (conventional example), both non-metallic inclusions a and mold wrinkles b are extremely slight, and the casting quality is comparable to that of the conventional method. A clear improvement effect can be seen during this period. The casting yields of Invention Examples Nos. 1 to 3 are 55 to 58%, which is about 10% higher than the casting yield of the conventional method (about 45%). Furthermore, it was observed that the cast products of Invention Examples Nos. 1 to 3 had almost no formation or growth of columnar crystals, and had a homogeneous granular crystal structure both inside and outside. On the other hand, in Comparative Examples Nos. 10 to 13, although electromagnetic stirring was performed, the timing of starting and stopping was inappropriate, resulting in casting defects such as shrinkage cavities and creases, resulting in poor quality. It is not as good as the invention examples (Nos. 1 to 3). In addition, in Comparative Examples No. 10 and 11, the temperature of the molten metal when stirring is stopped is close to the solidification temperature, so unless measures are taken such as reducing the rotation speed of the molten metal, damage will occur not only in the feeder section but also in the feeder section. Significant hollow shrinkage defects occur in the product. In addition, for Comparative Examples No. 12 and 13, when the stirring was stopped while the molten metal temperature was high, the results were similar to those of the general pour casting method.
The slag, etc. that were floating due to the rotational force will be cooled and solidified while being entrapped, making it impossible to achieve the intended purpose.

〔Effect of the invention〕

According to the method of the present invention, non-metallic inclusions generated and mixed in the molten metal promote the flotation and separation of oxide films, etc. In casting an alloy, it is possible to suppress the occurrence of casting defects due to the impurities and obtain a cast product with excellent soundness. Therefore, problems such as the occurrence of cracks during plastic working and heat treatment of cast billets are also solved. In addition, shrinkage cavities are less likely to occur, improving casting yield, and machining costs for removing defects such as mold wrinkles are reduced, resulting in a reduction in manufacturing costs. In addition, when a cast product obtained by the conventional method has a structure in which columnar crystals grow and develop in a direction perpendicular to the mold wall surface, when plastic working is performed in a direction perpendicular to the direction of the columnar crystals, the grains Although cracks are likely to occur along the boundaries, the cast products obtained by the method of the present invention have a homogeneous granular crystal structure, which eliminates such plastic working problems and improves mechanical properties. Improvements can also be expected.

[Brief explanation of the drawing]

1 to 3 are schematic cross-sectional explanatory diagrams showing an example of the casting method of the present invention, FIG. 4 is a schematic cross-sectional explanatory diagram showing a conventional casting method, and FIGS. 5 and 6 are These are micrographs (all ×50) that are used as drawings to show the structure of the metal. 1: Mold, 2: Electromagnetic stirring device, M: Molten metal, a:
Nonmetallic inclusions, b: hot water wrinkles.

Claims (1)

[Claims] 1. A molten easily oxidizable alloy is poured into a mold around which an electromagnetic stirring device is placed, and the molten metal is stirred by a rotating magnetic field centered on the vertical axis of the mold until it reaches the freezing point +
Starts at a temperature range of 160-180℃, freezing point +40-60
A method for casting easily oxidizable alloys, characterized in that the method continues until the temperature drops to ℃.