Thixoforming
Thixoforming , also known as the thixo process , thixocasting and semi-solid metal casting (SSM) , is a primary forming process in metal processing that combines the advantages of casting and forging .
The alloy to be processed is heated up to the transition temperature between solid and liquid. At this threshold, which is only a few tenths of a Kelvin depending on the material, the thixotropic state is reached. The aim is a microstructure in which finely distributed, crystallized components are embedded in coherent melting areas. In addition to precise temperature control, chemical grain refinement processes and electromagnetic stirring are mostly used in industry .
In the thixotropic state, the viscosity of the material decreases under the action of shear forces . The putty-like metal can be pressed very precisely into shapes with low pressure.
Procedure
When Thixomolding a magnesium or aluminum alloy with a screw in the injection molding process carried in a mold. In contrast to die casting , a hot runner can be used in the tool, which reduces cycle time, material usage and flow paths and improves component quality and properties. Double hot runner systems are also already in use in order to reduce the required machine size, especially for larger structural components.
The limits of the process application are the size of the machine, especially the size of the screw in which the granulate is plasticized. The restriction is given by the limited heat supply via the heating bands. The largest machines are in the order of magnitude of 10 meganewtons (MN) clamping force for around 1800 cm³ of casting compound (3.2 kg shot weight ).
In thixo-forging , the manipulable raw part is pressed into its final shape in a die , and the force is introduced over the entire surface of the mold. The thixo cross extrusion and thixo extrusion processes are also used.
Compared to traditional casting processes, thixotropic melts are more homogeneous. The manufactured parts are characterized by lower shrinkage and higher toughness at the same time. Comparable properties can be achieved by forging, but only in comparatively coarse forms. Another advantage is that significantly smaller wall thicknesses can be achieved: 1 mm for smaller components compared to 2–3 mm for die-casting processes.
Applications
Thixoforming is mainly used in the processing of aluminum and magnesium alloys, but is also becoming increasingly important in steel processing. The primary areas of application have so far been in the manufacture of chassis parts (e.g. wheel suspensions and cylinder covers) and pressure-tight parts. Thixo methods are particularly suitable for lightweight construction . Since the 1990s, the process has been used primarily by Asian manufacturers in the manufacture of cell phone cases or camera housings. Japan Steel Works is the world's largest manufacturer of thixoforming machines . With the increasing demand for consistent lightweight construction in the automotive industry, magnesium thixomolding in particular is gaining in importance. In Europe there are so far only a few suppliers who have specialized in the process for the automotive industry. The following components are potential applications for thixomolding:
- Seat components (backrest, seat frame)
- Steering wheels and steering column mount
- Elements of convertible tops
- Electronics housing and display housing
- Cylinder head cover
- Center console bracket and instrument bracket.
Since the mechanical properties of thixomolding components are superior to those of die-cast parts, new areas of application for magnesium are possible.
Web links
- Video of an automated forging cell for Thixo forging (MPG file, 1.74 MB)
- 4th International Thixomoulding Magnesium Conference
- Application of the thixoforming process - "SAG Thixalloy® Components"
Individual evidence
- ↑ New materials Fürth: New developments in thixotropic injection molding of magnesium alloys ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , August 16, 2008.
- ↑ See thixomolding with die casting: G-MAG website (PDF; 48 kB) , August 16, 2008.
