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by: P.H. Mani
Introduction: Nearly all metals and alloys of commercial importance solidify dendritically, either with a columnar or with an equiaxed dendritic structure.
When an alloy that normally solidifies dendritically is vigorously stirred during solidification, the dendritic structure can be broken up and replaced by the more or less spherical structure. The resulting semi-solid structure deforms homogeneously and can be formed into shapes by several methods.
Semi-solid forming is the generic term applied to a process in which a mixture of solid and liquid phase metal is introduced into a mold or a die for net shape forming. One might think of the process as a hybrid between casting and forging and because the equipment used more closely resembles the die casting process. Semi-solid manufacturing seems to have fallen into the domain of metal casters.
Semi-solid processing of non-ferrous alloys, especially Aluminum alloys are in production phase for many critical automotive components. They have taken a bite on the traditional market share of ferrous components, especially ductile iron.
However, the possibility of near net shaping of high melting point alloys in the semi-solid state has already been demonstrated.
This paper presents the results of the possibilities of making semi-solid processed ductile iron components and the potential applications for such components.
Semi-solid processing of 'as cast' ductile iron slugs:
It was decided to manufacture a series of ductile iron gears (cog wheels) using the semi-solid forming technique. Fig 1 shows the drawing of the proposed gear.
| Figure 1. Gear (cog wheel)
Material: Ferritic Ductile Iron
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The chemistry of the slugs is as follows:
| Element |
% |
Element |
% |
| Carbon |
3.65 |
Silicon |
2.63 |
| Manganese |
0.29 |
Chromium |
0.03 |
| Aluminum |
0.01 |
Sulfur |
0.007 |
| Phos |
0.013 |
Copper |
0.140 |
| Nickel |
0.02 |
Magnesium |
0.045 |
| Moly |
less than 0.01 |
Tin |
less than 0.01 |
| Titanium |
less than 0.01 |
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The slugs were machined to a dimension of 58mm diameter by 80mm height.
A graphite die was machined to the shape of the gear. A D2 Steel die or any other metallic die would be used in production. The machined slugs were placed inside the induction coil with the thermocouple inserted in position. See Figs 2,3,4 for the set up.
Figure 2.
Experimental arrangement for heating trials of ductile iron slug.
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| Figure 3
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| Figure 4. Hot slug
traveling towards die cavity
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Nitrogen was used as an inert gas to provide the inert atmosphere inside the coil. By having the slugs in the center of the induction coil and having the top and bottom of the slugs insulated with insulating pads after the 'soaking' period the radial temperature difference was around
12-15oF (approximately 5oC) and the axial temperature difference was around
25-30oF (12-15oC). This temperature difference appears to be sufficient to allow semi-solid forming of the ductile iron slugs within the processing window. The as-cast slug was first heated to a temperature of
2120oF (1160oC) for about 170 seconds and then further heated to a temperature of
2138oF(1170oC) and held at this temperature for another 90 seconds.
The dwell time when the forging load was applied was set to 30 seconds.
The slug was injected into the die in this condition to produce the gears.
Fig 5&6 shows the gears made by this process.
| Figures 5 and
6. S.S.M. (Thixoformed) ductile iron cog wheels
(gears) |
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The presence of carbides in the microstructure due to rapid cooling of the liquid fraction of the slug during the semi solid forming is eliminated by 'synchronized' annealing above the critical temperature.
Semi solid formed components can be austempered in 'tandem' with this process to obtain high strength, high toughness properties.
In addition to ductile iron, compacted graphite iron can be semi-solid formed to near net
shapes.
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