1


fading of the modification through holding the melt at the high tem- perature and through remelting have all been evaluated in this study. The composition of the alloy used is presented in Table 1. For strontium additions, Al-10%Sr master alloy was used. The barium and calcium addi-


2


tions were done using a Ca-Si-Ba for which the composition is pre- sented in Table 2. The additions of calcium and barium for modification purposes were calculated using a specific barium concentration. The first step of the study was to


evaluate the optimal concentration of barium and calcium in order to obtain complete modification and optimal mechanical properties. The effect of the phosphorus content on


Background A356 alloy contains


7% weight of silicon and 0.3% weight of magnesium. During solidification, the


aluminum phase forms first and the silicon is rejected in the liquid ahead of the dendritic solidifica- tion front. The composition of the interdendritic zone is then close to the eutectic composition and the resulting microstructure is eutectic. When solidification rates are rela- tively slow, the silicon in the eutectic possesses a coarse acicular morphol- ogy. When faster cooling rates are achieved, the eutectic silicon has a


fibrous morphology, in which the structure is smaller and less acicular. The acicular morphology is


considered to have a negative effect on mechanical properties. The elongated and angular shape of the silicon acts as crack initiator, which in turn decreases the elongation and ultimate strength of cast parts. The fibrous morphology allows improved ductility and strength in cast parts. In order to obtain the modified structure without affect- ing the solidification rates, small additions of alloying elements are made. This chemical modification is used for Al-Si hypoeutectic alloys.


Phosphorus presence in the melt negates the modification as it reacts with strontium and sodium to form phosphates. The modifying elements are thus rendered nonavailable for the change of the eutectic silicon. Strontium is the most commonly used modifying element. Additions of 80 to 200 ppm usually are used to obtain a modified eutectic silicon structure. Even though the fading phenomenon is not as important with strontium as it is with sodium, it will still decrease in concentration due to oxidation and the modifying effect will disappear with time and re-melting.


Procedure The optimal barium


and calcium concentration, the effect of phosphorus on such addition, and the


the modification using barium and calcium also was evaluated. Moreover, A356.2 with strontium was cast to compare the properties of the parts. Finally, A356.2 with barium and calcium was remelted and strontium was added to evalu- ate their interaction. For this set of experiments,


loads of 187.4 lbs. (85 kg) were melted in a resistance furnace. The metal was held at 1,391F (755C). The melt was degassed using


dry argon and a rotary impeller. A reduced pressure test was per- formed to validate the effective- ness of degassing. Once satisfac- tory, the modification elements of Ca-Si-Ba, phosphorus or stron- tium were added. The casting was in a modified


ASTM B108 mold. This modified mold was designed to reduce the variability of the casting operator (Fig.1). For each composition, 20


Table 1. Chemical Composition of A356.2 Alloy Used for Experiments Elements


Si Wt % 7.24


Mg 0.39


Si Fe 0.12 Ca Cu 0.06


Table 2. Chemical Composition of Ca-Si-Ba Compound Elements


Wt % 51.97 34 | MODERN CASTING August 2016 15.95 Ba 14.24 Mn 0.007


ASTM B108 test bars were cast. To measure the calcium and barium concentration, optical emission spectrometry disks were cast right before the first, the fifth and the last casting. A metallography specimen was cut from tensile specimens from the first, fifth and 10th castings.


Ti 0.12 Fe 16.64


Al Bal. Al 1.2


Fig. 1 Here is the modified ASTM B108 mold design used in the study.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60