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The aim of the present work is to study the influence of ox- ide additions on the porosity development and the mechani- cal properties of gravity diecast A356 aluminium alloys. The hydrogen concentration of the melt is chosen to be constant while the oxide content of the melt is varied by the addition of turning chips, a highly oxidized waste product.


experimental


The material used for the investigations was a standard A356 aluminium alloy. The chemical composition is given in Table 1.


experimental Procedure


Three separate melts of A356 alloy (60kg) were prepared by adding alloy turning chips (10, 25 & 50 wt%) to a primary A356 alloy melt in an electric resistance furnace. Turning chips were melted and diluted with small amounts of A356 alloy and cast into ingots. These enriched ingots were then added to the alloy melt. A hydrogen level of 0.17 ml/100g was kept constant during the three casting series by either de- gassing with nitrogen or upgassing with argon-10% hydro- gen mixture with a graphite lance. At the beginning of each casting trial, the die was preheated to 320C (608F) using heat exchange oil that was circulated through channels in both die halves (Figure 1). The oil circulation was used to keep the die temperature distribution constant. In all casting series, the first three cast- ings were scrapped to ensure that the die had reached a quasi-steady state temperature distribution. The follow- ing six castings from each trial were used for the subsequent investiga- tions. The hydrogen content of each melt was measured with an ALSPEK- H probe. This device can be used for in-situ hydrogen measurements both on a laboratory and industrial scale. More detailed descriptions of this probe and its measuring principles have been published.13-14


The melt


cleanliness was assessed using RPT and PoDFA. RPT samples were taken continuously throughout the casting trial. RPT samples were used to mea- sure the bifilm index, a parameter de- fined as an indicator of the melt qual- ity (Equation 1).


Equation 1


For each melt, PoDFA samples were poured before the start of the cast- ing trial, after six castings and at the end of a series. The PoDFA residues


42


were investigated in terms of number and type of inclusions, and aluminium oxide film concentration.


die and Position of thermocouples


A step mould die was used in the gravity die casting trials and the die dimensions are presented in Figs. 1 and 2. The assembled die was 180 mm (18 cm) wide, 150 mm (15 cm) high and 445 mm (44.5 cm) long. The die cavity had a stair- like shape and the thicknesses of the five steps were 30, 20, 15, 10 and 5 mm (3, 2, 1.5, 1 and 0.5 cm). There were eight thermocouples placed in the die to ascertain that a reproduc- ible die temperature distribution was achieved in all castings trials. A more detailed description of the thermocouple posi- tioning has been presented by Akthar.15


The bithermocouple


readings were recorded and an additional thermocouple la- belled TC9 was used in the ladle to control the pouring tem- perature [720C (1328F)].


Porosity characterization and mechanical testing


The sectioning plan for porosity and mechanical properties characterization as well as the dimensions of the step cast- ing are given in Figure 2. Sections from Sample B with di- mensions of 15 x 15 mm (1.5 x 1.5 cm) have been used for the porosity characterization. Two tensile specimens were


Table 1. Chemical Composition of A356 Alloy


Figure 1. Redraft15 for gravity casting.


of the thermocouple positioning in the step mould die used


International Journal of Metalcasting/Spring 2012


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