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increasingly concentrated from intact mold to flat-notch mold to V-notch mold, which eventually resulted in cracking. Te casting simulation results confirmed the locations of stress concentration and demonstrated excellent accu- racy with the actual temperature profiles and cracking locations. Te established material model for the sand mold, based on the actual mechanical test data, can be used to predict casting defects related to cracking, such as vein- ing defects.


PRESENTATION


Comparing the Mechanical Properties inside the Same Cast Frame Produced in Alloy A356-T6 and in Alloy 206-T4


AUTHORS


Franco Chiesa, David Levasseur, Gheorghe Marin and Bernard Duch- esne, Centre de Métallurgie du Québec (Trois-Rivières, Quebec, Canada)


BACKGROUND Te mechanical properties of an


industrial casting weighing 31 lbs. (14kg) and measuring 23 x 17 x 10 in. (560 x 410 x 250 mm) was poured, first in aluminum A356 (AlSi7Mg03), then in aluminum 206 (AlCu4). ASTM E7 sub-size tensile specimens were excised from the castings and pulled in the T6 condition for alloy A356 and in T4 condition for alloy 206. Te degassing procedure was identical for the two alloys and the pouring temperature was 1,328F (720C) for alloy A356 and 1,364F (740C) for alloy 206. Six regions of the casting were investigated (19 tensile samples per casting) with solidification times varying from 2.8-7 minutes. Te tensile properties in the castings also were compared to those of standard ASTM B26 test bars, of which properties were always superior to those measured inside the cast- ings. In the 206-T4 casting, the tensile strength was about 30% higher and elongation 3-5 times than measured in the A356-T6 casting. For the identical degassing condition, the percentage surface area of porosity was higher in alloy 206; however, the maximum length of porosities was greater in the A356 alloy due to their inter-dendritic


This picture shows a surface hardness scratch on a three-point bend- ing sample after test.


nature. An important segregation of copper was observed in alloy 206 which was not found to be overly det- rimental to the tensile properties. Since the increase of the use of aluminum castings from the 1970s, the 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 field because of the brittleness caused by the high iron content of the alloys used and the dif- ficulty 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 cast- ing, high ductility castings can be produced by the high-pres- sure 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. Te reason is their susceptibility to hot tearing,


the segregation of copper, and their sensitivity to stress corrosion cracking. 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 balance be- tween the expected difficulties in pour- ing Al-Cu alloys and the far superior mechanical properties they provide? In sand casting, exceptionally strong, sound Al-Cu castings can be rou- tinely obtained through current “good foundry practices” by implementing process control tools readily available. Depending on the casting geometry, in the T4 condition, the important inter- nal stresses generated during quench- ing might make dimensional accuracy problematic as distortion is bound to occur at each machining passes. Tis 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


This is an aluminum 206-T4 housing substituting for ferritic ductile iron.


gains are important, aluminum 206 can substitute for ductile iron. Alloy 206 is always used in a heat-treated condi- tion, the T4 (solutionized, quenched and naturally aged) being preferred when ducility and endurance are sought, rather than tensile strength. Te T7 condition is obtained by aging the T4 casting for five hours at 374F (190C), resulting in an increased tensile strength to the detriment of elongation. Typically, aging a T4 treated 206 casting to a T7 temper in- creases its yield strength by 60% while dividing its elongation by 3. Tis T7


May 2017 MODERN CASTING | 49


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