SQUEEZE CASTING DEFINED 1f handbook die casting proceses edwadrd j virnacik 2003

52 SQUEEZE CASTING a b c Figure 4.1 Schematic of the squeeze forming process. shown in Figure 4.1a. The die is closed Figure 4.1b and the metal flows within the die, filling the cavity. During solidification, an intensification pressure is applied to the metal by the dies. After solidification is complete, the component is ejected, as presented in Figure 4.1c. Squeeze casting process parameters are very similar to conven- tional die casting in that the liquid metal is pressurized during solidification. The major difference between squeeze casting and conventional die casting is with regards to the gate velocity. Shown in Figure 4.2 is a graph illustrating the respective process windows for numerous casting processes with respect to casting pressure and gate velocities. Gate velocities are often achieved during squeeze casting that are orders of magnitude slower than in conventional die casting. The gate velocities in squeeze casting can be as low as those characteristic to permanent-mold casting. Cycles times for squeeze casting are longer than those of con- ventional die casting. This is due to both the slower metal injec- tion speeds required to obtain the low gate velocities noted in Figure 4.2 and the longer solidification times. The resulting microstructures are much different. Figure 4.3 compares represen- tative microstructures for an aluminum alloy. The microstructure in the squeeze casting is not as fine as that observed in conven- tional die casting, and the dendrites are much more pronounced. The mechanical properties of squeeze castings are much improved due to reduced levels of porosity and the formation of micro- structures not possible in conventionally die cast components. The

4.2 MANAGING GASES IN THE DIE

53 Conventional High Pressure Die Casting Squeeze Casting Medium Pressure Die Casting Semi-Solid Metalworking Low Pressure Die Casting Gravity Permanent Mold Casting Casting Pressure bar Gate V elocity msec Figure 4.2 Comparisons of casting pressures to gate velocities for numerous die casting processes. viability of subsequent heat treating is also possible due to re- duced air entrapment, as discussed in the next section.

4.2 MANAGING GASES IN THE DIE

As discussed in Chapter 1, gas porosity is attributed to physical gas entrapment during die filling, to the gasification of decom- posing lubricants, and to gas dissolved in the liquid alloy, which evolves during solidification. The nature of the squeeze casting process minimizes gas entrapment in comparison to conventional die casting. By utilizing larger gate cross-sectional areas and slower shot speeds, atomized fill is avoided. In many case, planar fill can be achieved during squeeze casting. This comparison is shown in Figure 4.4. By avoiding liquid metal atomization, vents within the die re- main open throughout much of cavity fill. Slower shot speeds also allow more gases to escape from the die before compression oc- curs. 54 SQUEEZE CASTING Figure 4.3 Microstructural comparisons between conventional die casting and squeeze casting. Courtesy of UBE Machinery, Inc. Gas porosity can also originate from gases dissolved in the liquid metal. Although not a major factor in conventional die cast- ing due to the extremely high cycle times, the longer solidification durations associated with squeeze casting may allow dissolved gases to precipitate and form porosity. This source of porosity can be controlled using good melting and holding practices.

4.3 MANAGING SHRINKAGE IN THE DIE

High metal intensification pressures are maintained throughout so- lidification in conventional and vacuum die casting. Unfortunately, the small gates typically used in conventional die casting freeze quickly. Once solidified, the gates are a barrier that inhibits further pressurization within the die.