Semi-solid metal casting
Thixoforming, also known as Thixoverfahren and Semi -Solid Metal Casting ( SSM), is a molding method in metalworking, which combines the advantages of casting and forging.
The to be processed alloy is heated to transition temperature between solid and liquid. At this threshold, which is only a few tenths of degrees Kelvin, depending on the material, the thixotropic state has been achieved. The goal is a microstructure embedded in the finely divided crystalline components into cohesive melting ranges. In addition to the precise temperature control and the electromagnetic stirring chemical Kornfeinungsverfahren be used in the industry usually.
In the thixotropic state, the viscosity of the material is reduced under the action of shear forces. The knetmasseartige metal can be pressed so with low pressures very precise shapes.
Method
During thixomolding is a magnesium alloy with a screw in the injection molding process is transported to a mold. In contrast to die-casting can be used in a hot runner mold, whereby cycle time of material and the flow path is reduced and part quality and performance is improved. Also, two times hot runner systems are already in use to reduce the size of the machine required in particular major structural components.
Limits of the method consist in the application of machine size, especially in the size of the screw, in which the granules are plasticized. The limitation is given by the limited heat transfer through the heater bands. The largest machines are on the order of 10 mega Newton ( MN) closing force for about 1800 cm ² casting mass ( 3.2 kg shot weight ).
When thixoforging the manipulable blank is pressed in a die into its final form, the force is applied over the entire mold surface. It also processes Thixoquerfließpressen and Thixostrangpressen be applied.
Compared with traditional casting process thixotropic melts are homogeneous. The finished parts are characterized by lower shrinkage with simultaneously higher toughness. Comparable properties can indeed achieve by forging, however, be possible only in comparatively coarse forms. One advantage lies in the fact that significantly lower wall thicknesses can be achieved: 1 mm for smaller components compared to 2-3 mm in die casting process.
Applications
Thixoforming is particularly common in the processing of aluminum and magnesium alloys, but also gains in steel processing increasingly important. Primary fields of application are far in the production of chassis parts (for example, suspensions, and the cylinder cover ), and pressure-sealed parts. Thixoverfahren are ideal for lightweight construction. Since the 1990s, the method is used primarily for Asian producers in the production of mobile shells or camera housings Application. World's largest manufacturer of Thixoforming machines is Japan Steel Works. With the increasing demand for consistent lightweight construction in the automotive industry, especially the magnesium Thixomolding gaining in importance. In Europe there are only a few providers that specialize in the procedure for the automotive industry. The following components are potential applications for Thixomolding:
- Seat components ( backrest, seat frame)
- Steering wheels and steering column recording
- Elements of convertible tops
- Electronics housing and the display housing
- Cylinder head cover
- Mittelkonsolträger and instrument panel.
Since the mechanical properties of thixomolding components are superior to those of die cast parts, new applications of magnesium are possible.