It has been hypothesized that one of the criteria for a successful CDS process is for Point B to be below the liquidus temperature of Alloy 1, TL1
a typical thermal data obtained during the CDS process. Alloy 2 acts as a heat sink for Alloy 1 and during mixing, the controlled natural convection facilitates copious nucle- ation events of the primary Al phase from Alloy 1; and this process is represented by the segment AB in Figure 1. In Figure 1, segment BC denotes the time period during which thermal homogenization of the mixture takes place; and seg- ment CD is when the solute homogenization (lag- ging behind thermal homogenization) takes place leading to the final bulk nucleation event at around Point D. A more detailed explanation of the steps involved in the mechanism has been presented in Khalaf et al.2
. The thermal homogenization at Point C, prior to grain growth, presents a favorable en- vironment for a nearly planar solid/liquid interface during growth resulting in a non-dendritic micro- structure of the cast component.
The critical process parameters in the CDS pro- cess2
are the compositions of the two precursor al-
loy 1 and Alloy 2 respectively. The compositions and melt superheat temperatures of Alloy 1 and Alloy 2 define the mass ratio, respective liquidus
loys, melt superheat temperatures of two precursor alloys, rate of mixing of the two alloys and mass ratio (m1
/m2 ) where m1 and m2 are the mass of Al-
Figure 1. A schematic of a typical thermal data obtained during the CDS process beginning with the mixing of the alloys through to the end of solidification.
temperatures of the two alloys and the temperature differ- ence (difference in thermal mass) between the two precur- sor alloys. There are many possible combinations of the two precursor alloys with defined composition and melt temper- atures to yield one specific resultant alloy. In this publica- tion we shall demonstrate the feasibility to shape cast three Al wrought alloys, namely, 2024, 6082 and 7075, into test
Figure 2. Flow chart showing several steps to design CDS procedure for each alloy. The two stages in the design are the optimization of precursor alloy compositions and optimization of the precursor alloy initial temperatures.
44 International Journal of Metalcasting/Spring 11
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 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88