Table 1. Surface Finishes of Various Casting Processes Casting Method
bulk of structural castings have been produced in Al-Si-Mg alloys (A356, 357), whether poured in sand or in permanent mold. Until recently, high pressure diecast (HPDC) parts could not compete in this fi eld because of the brittleness caused by the high iron content of the alloys used and the dif- fi culty in heat treating or welding these parts due to their high porosity content. Since the mid-1990s, the development of low iron HPDC alloys, along with the high-pressure vacuum die casting, high ductility castings can be produced by the high-pressure process which can match the strength and ductility or permanent mold cast parts. On the other hand, despite their outstanding mechanical properties, aluminum-copper alloys of the 2xx series are still seldom used. T e reason is their susceptibility to hot tearing, the segregation of copper, and their sensitivity to stress corrosion crack- ing. However, the question arises: is the prevalence of Al-Si-Mg alloys due to a tradition established over the years rather than to a rational bal- ance between the expected diffi culties in pouring Al-Cu alloys and the far superior mechanical properties they provide? In sand casting, exception- ally strong, sound Al-Cu castings can be routinely obtained through current “good foundry practices” by imple- menting process control tools readily available. Depending on the casting geometry, in the T4 condition, the important internal stresses generated during quenching might make dimen- sional accuracy problematic as distor- tion is bound to occur at each ma- chining passes. T is can be alleviated by practicing a stress relief treatment (T43 per ASTM B917) consisting in heating the T4 casting for one hour at 320F (160C). Caution should be taken to not overdo this treatment as it might make the casting vulnerable to stress corrosion. For structural castings where weight
gains are important, aluminum 206 can substitute for ductile iron. Alloy 206 is always used in a heat-treated condition, the T4 (solutionized, quenched and naturally aged) being preferred when ducility and endurance are sought, rather than tensile strength. T e T7
Die
Investment Shell Mold
Centrifugal-Perm Mold Normal Non-Ferrous Sand Normal Ferrous Green Sand 3-D Printed Molds
RMS Surface Finish 20-120 60-200 120-300 20-300
200-400 560-900 560-900+
condition is obtained by aging the T4 casting for fi ve hours at 374F (190C), resulting in an increased yield strength to the detriment of elongation. Typi- cally, aging a T4 treated 206 casting to a T7 temper increases its yield strength by 60% while dividing its elongation by 3. T is T7 treatment provides tensile properties which far exceed those of gray iron, approaching those of ferritic ductile iron.
CONCLUSION T e metallurgical study at six loca-
tions inside two identical 30.9 lb. (14 kg) aluminum A356-T6, 206- T4 and 206-T7 castings with local solidifi cation times ranging between 2.8-13 minutes brought the following conclusions: • For an equivalent degassing proce- dure, the surface area of microvoids is higher in alloy 206 (1.66% max versus 1.26% max for alloy A356); this may be attributed to its wider freezing range (70C vs 50C for unmodifi ed alloy A356).
• T e maximum length of microvoids are higher in alloy A356 (highest value of 780µm versus 450 µm in alloy 206) due to the interdendritic nature of the voids in alloy A356.
“
• Within the 2.8-13 minute local solidifi cation time bracket, the grain size varies from 126-246µm in al- loy 206 while the A356 secondary dendrite arm spacing (DAS) lies between 52 to 88µm.
• The yield stress is almost in- dependent of the metallurgical quality (i.e. the structure fine- ness and the microvoid level); it depends on the heat treat- ment applied and the amount of hardening elements (Cu, Mg). Consequently, yield stress values inside the casting are similar to that of the separately cast ASTM B26 test bar.
• Elongation and ultimate tensile strength are closely correlated to the metallurgical quality of the alloy; their variation inside the cast- ing are greater in A356-T6 than in 206-T4, despite the important inverse segregation of copper ob- served in the 206 casting.
• Alloy 206 tensile properties are far superior to those of alloy A356-T6. Elongation values in the 206-T4 casting were three to fi ve times those measured in the A356-T6 casting while its yield strength was 10% higher.
Simulation technology has made remarkable progress in casting design and process optimization prior to actual manufacturing, which reduces the time and cost of conventional trial-and-error methods.”
May/Jun 2017 | METAL CASTING DESIGN & PURCHASING | 43
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