Thixoforming : Semi-solid Metal Processing

Table 4.1 Essential requirements for thixoforming materials.
Target value a
Requirement (determination method) Parameter Formula a
Small grain size (metallography) Mean grain diameter dm ¼ f T .
; c dm < 100 mm
f > 0.6
Globulitic structure (metallography) Form factor f ¼ 4pA
U2 0.4 < C fS < 0.6
s 2SSS
2SSS þ SSL Small cross-linking of dendrites (metallography) Contiguity C fS ¼ f
0.1 < V c < 0.3
Small cross-linking volume of dendrites (metallography) Contiguity volume Vc ¼ VsCs
4.2 Chemical Grain Refinement of Commercial Thixoalloysj109
Vc ¼ VsCs
Low viscosity of the melt under shear load (rheometer tests) Viscosity under shear load h ¼ kg_ n 1 Z 1–10 cP
1
1 p
40–60 mass%
TM TL
T M T
Defined solid–liquid phase ratio (DTA, simulation, metallography) Fliq f S ¼ 1
Large melting interval (DTA, simulation) dT ¼ (Tliq Tsol) dT¼fc; T .
; p max. 130 K,
min. 70 K
Temperature sensitivity (DTA, simulation) Stop at 50% liquid portion df l =dT 10 K
a
h ¼ viscosity; k ¼ constant; g_ ¼ shear velocity; n ¼ Oswald–de Waele exponent; dm ¼ mean grain diameter; f ¼ forming factor; A ¼ grain surface area; U ¼ circumference of a
grain; S SS ¼ the grain boundary surface area between the solid phase, that is, the surface area between the cohering grains not separated melt; S SL ¼ phase boundary surface
area between solid phase and melt; dT ¼melt range; f(c; T; p) ¼ function of (alloying element content; rate of cooling; pressure); fs ¼ proportion of solid material; TM ¼ melt
temperature of a component; TL ¼ liquidus temperature; p ¼ exponent defined by the phase state equilibria.