Billet-Type Indirect Semi-Solid Metalworking

74 SEMI-SOLID METALWORKING Figure 5.5 Microstructure of an aluminum component produced with an in- direct semi-solid metalworking process. nent is ejected and extracted from the die using automation that then loads the component into a saw or trim press for removal of the runner system and flash. Systems are commercially available for sawing semi-solid feed- stock into billets as well as robotic arms, conveyors, and trim presses. Induction systems for heating semi-solid billets are sim- ilar to those used by the forging industry. Induction systems used in semi-solid metalworking are typically of the rotary type with at least seven heating stations. Five of these stations are used to preheat the billet. The remaining two are used to bring the billet to the semi-solid state. Control systems in induction heating sys- tems often include contact thermocouples. Some systems utilize a plunge thermocouple that penetrates into the heated semi-solid billet just prior to loading into the shot sleeve. Both horizontal and vertical conventional die casting machines can be used in conjunction with indirect semi-solid metalworking. As with any die casting process, shot control is essential. Often

5.5 SEMI-SOLID METALWORKING EQUIPMENT

75 Scrap Finished Parts Bin Trim Press or Saw Toggles Die Short End Horizontal SSM Machine Saw Cut Billet Conveyor SSM Bar Stoc k Rotary Induction Heating Table Figure 5.6 Graphical representation of a typical indirect semi-solid metal- working manufacturing cell. the shot control systems currently available on conventional die casting machines may be used with the semi-solid metalworking process. Process parameters, however, must be adjusted to allow for slower fill of the die cavity. Although semi-solid metalworking has been utilized for many years to manufacture production components, a consistent die de- sign methodology has not been documented in the technical lit- erature. Semi-solid metalworking die designs are viewed as a trade secret by most component producers. As such, very little infor- mation has been disclosed regarding die design philosophies. Nonetheless, several qualitative characteristics are known about semi-solid metalworking dies. In comparison to conventional die casting, semi-solid metal- working dies have larger gate areas. Gates are no less than 3 mm in thickness to avoid premature solidification during intensifica- tion. Some manufacturers utilize classical fan gating such as that used in conventional die casting. Other producers have found large single-point gates ideal. Single-point gates, however, may lead to phase separation. This phenomenon is discussed in Chapter 11. 76 SEMI-SOLID METALWORKING Figure 5.7 Continuously cast semi-solid metalworking feedstock. Courtesy of Formcast, Inc. As components produced using semi-solid metalworking have thick gates, trimming is not a viable option for removing the run- ner systems. Sawing is typically required. Automated sawing sys- tems with custom fixtures are available for high volume production. Although sawing is required for removing compo- nents from their respective runner systems, trimming is not avoided. The removal of overflows and flash is still accomplished using traditional trimming techniques. Semi-solid feedstock such as that shown in Figure 5.7 is man- ufactured using two primary methods: continuous casting and ex- trusion. The structure and method of handling semi-solid billets vary depending on the way in which the feedstock is manufac- tured. Magneto-hydrodynamic technology is utilized when continu- ously casting semi-solid feedstock as a means of stirring the liquid metal as it solidifies. This stirring action creates a fine equiaxed structure within the core of the feedstock. Regardless of the stir- ring induced by the electromagnetic field, a dendritic structure forms on the outer surface of the continuously cast feedstock. This dendritic case forms as a result of the substantial temperature gra- dient present at the liquid metal–mold interface. The dendritic

5.5 SEMI-SOLID METALWORKING EQUIPMENT

77 Spheroidal Core Dendritic Case b a Figure 5.8 Anatomy of a continuously cast semi-solid metalworking billet a before heating and b after heating. case is shown in Figure 5.8, which illustrates the anatomy of a continuously cast semi-solid metalworking billet before heating and after heating. After a billet is heated to the semi-solid state, the liquid fraction of the billet has a tendency to rise to the top portion of the billet as the solid fraction has a tendency to settle to the bottom. This often results in bulge at the base of heated billets. The dendritic case, however, helps support the heated billet while it is transferred to the shot sleeve. Although the dendritic case helps hold the billet together while heating, this portion of the feed material does not flow well within the die. Methods have been developed to capture the dendritic case with the biscuit when forming a component, as shown in Figure 5.9. The plunger tip used with continuously cast billet material has a blunt point that pushes the bulk material out beyond the dendritic case. The main runner can also be used to capture the dendritic case in the biscuit if it is smaller than the diameter of the billet. Semi-solid feedstock can also be manufactured by extruding grain refined material. This feedstock has a fine equiaxed structure without a dendritic case. This may pose a problem during heating, as shown in Figure 5.10. As the liquid fraction rises, the extruded 78 SEMI-SOLID METALWORKING Blunt Point on Plunger Tip Runner Smaller than Biscuit Diameter Figure 5.9 Plunger tip and die design for capturing the dendritic case of a continuously cast semi-solid metalworking billet. a b Billet Cannot Support Itself During Heating No Dendritic Case Figure 5.10 Anatomy of an extruded semi-solid metalworking billet a before heating and b after heating.

5.5 SEMI-SOLID METALWORKING EQUIPMENT

79 Figure 5.11 Schematic of Thixomolding威 machine use in direct semi-solid metalworking. Courtesy of Thixomat. billet will sag. When utilizing extruded billet material, a crucible or other holding container must be used to contain the semi-solid charge.

5.5.2 Thixomolding威 Direct Semi-Solid Metalworking

Direct semi-solid metalworking processes avoid reheating of stock material. These processes produce the semi-solid metal mixture on demand just prior to metal injection into the die. Numerous variants of this process exist. However, Thixomolding威 is among the most efficient of all direct semi-solid metalworking processes, and for this reason, it will be the only direct semi-solid metal- working process discussed in this section. A Thixomolding威 machine schematic is presented in Figure 5.11. The shot end of the machine is very different from those used in all other die casting processes. A screw surrounded by heating bands, similar to those used for injection molding, is used to inject metal into the die. Moreover, the rotating action of the screw mechanically shears the heated metal creating a semi-solid mixture. Controlled quantities of pellet or chipped feedstock are metered into the screw in an inert atmosphere after each cycle. A typical manufacturing cell is pictured in Figure 5.12. Thix- omolding威 begins by blowing feedstock into the feed bin mounted 80 SEMI-SOLID METALWORKING Scrap Conveyor Grinder Trim Press or Saw Finished Parts Bin Die Screw Feed Bin Thixomolding Machine Blo w T ube Feedstock Virgin or Regrind TogglesPiston Figure 5.12 Graphical representation of a typical manufacturing cell. above the molding machine. The feedstock is metered into the screw, heated, sheared, and injected into the die cavity. The solid- ified component is ejected and extracted from the die using au- tomation that then loads the component into a saw or trim press for removal of the runner system and flash. Scrap is conveyed to a grinder, which chips the off-fall into usable feedstock. In comparison to conventional die casting, dies used in Thix- omolding威 components have one major difference. After metal injection is complete, the end of the screw freezes shut. The plug that forms Figure 5.13 keeps the semi-solid mixture from leak- ing out of the screw. The die must be designed to capture this plug during metal injection, as shown in Figure 5.14. At the start of metal injection, the plug is shot into a ‘‘catch’’ that captures the plug. The cone-shaped feature around the catch feeds the semi- solid metal into the die cavity. The Thixomolding威 process is very flexible. Since the semi- solid metal is completely contained within the screw, the liquid– solid fraction can be varied without creating complications in handling. As such, multiple microstructures may be obtained by varying the percent solid, as shown in Figure 5.15. In extreme cases, the metal may be heated to a completely liquid state. This, in essence, turns the Thixomolding威 machine into a conventional die casting machine.