ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2024 30 ADVANCES IN THIXOMOLDING Larger Thixomolders. Growing from the original 250-ton clamp capacity, 3000-ton thixomolders have been developed and marketed by the machine builder Japan Steel Works. This allows much larger parts up to 19.5 lb of Mg alloy net shape parts. Short Hot Sprue/Runners. Molded Magnesium Products has perfected the short hot sprue/runner shown in Fig. 2. This technology reduces the temperature drop in the screw and runner to enable longer flow distance in the mold and lower porosity. Most importantly, the yields are dramatically increased from 75% with conventional design of sprue and runner to 90% part yields, and 100% metal yields. The small sprue/runner seen in Fig. 2 is totally recyclable to new chips. Thixomolding and ThermoMechanical Processing (TTMP). Processing after thixomolding builds on the fine structure of thixomolded preforms to double the yield strength, without loss of ductility, see Table 2. This results from grain size reduction, maintaining low texture, remodeling of the β phase, and introduction of <20 nm transition microstructures[1] (Fig. 3). TTMP has been practiced commercially on medical and recreational applications, for example, by warm rolling. TTMP stock has high Erickson numbers and low r values compared to commercial AZ31 sheet. At 200° to 300°C, bulge testing of TTMP super- plastic stock of AZ61 compared well with Al alloy while requiring lower bulging pressures[2]. Also, hot pressing has proven to be a practical means of TTMP. An ideal machine includes a combination thixomolder/hot press to impart hot deformation to the molded part immediately after injection. Thus the energy, cost, and time of separate rolling would be saved. Net shape 3D TTMP parts would be enabled. Thixo-Blending. Special alloys can be blended by thixomolding using conventional alloy feed stock to minimize extra inventory of the special alloy feed stock. This has been shown Fig. 2 — Thixomolded child seat showing trimmed short hot sprue/runner. Courtesy of Molded Magnesium Products. TABLE 2 — EFFECT OF TTMP ON THIXOMOLDED AZ61 Condition Yield strength, MPa UTS, MPa Elongation, % r value As-thixomolded 150 250 10 As-rolled (TTMP) 297 363 13 1.48 TTMP + anneal 252 336 16 1.00 Fig. 1 — Overview of thixomolding. molding exceeded all initial expectations. The current market size of over 720 machines in Asia, Europe, and the U.S. is more than 20 times the original estimate of 30 machines. Today’s applications, including knee braces and laptops, are far afield from the original market target of auto steering wheels, see Table 1. The electronics market (Walkman, digital cameras, and DLP projectors) was the first application to take off in a time when attainment of < 20 nm transition microstructures was not imagined.
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