Direct-chill casting

Direct-chill casting of aluminium

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Explore the direct chill aluminium casting method used to produce ingots for rolling and extrusion in the aluminium industry.

DC casting

What is DC casting?

Direct-chill casting of aluminium alloys is used in the aluminium industry’s primary and secondary aluminium smelter cast houses to produce ingots for rolling and extrusion. During DC casting molten metal is poured into a water-cooled, bottomless mould. The melt is fed into the mould from the top, and the solidified part was withdrawn from the bottom. This is clearly seen in Figure 1.

DC casting is so called because the produced billets are directly chilled with cooling water (Fig. 2) [1]:

primary cooling at the rear face of the mould
secondary cooling onto the billet surface as it extended from the mould.

Figure 1 – A DC casting method patented in the 1940s.
(Taylor & Francis/CRC Press) [1]

Figure 2 – The modern technology:
the heat flow in the mould (primary cooling) and
below the mould (air gap and secondary cooling) [2]

Mould tables

Molds are installed on the mould table. The mould table provides the distribution channels for cooling water to moulds.

There are two types of mould table (Fig. 3):

a table wich rolls away from cast position on rails (Fig. 3(a) and also Fig. 4)
a tilting table which is raised and lowered by hydraulics (Fig. 3(b)).

On small and old DC casting machines, there are also may be a mould table, which are placed and removed with a crane.

For all types accurate and rigid location relative to the platen is important as the mould and starter blocks must be in alignment to within 0,1 mm [2].

Figure 3 – General arrangement of two forms of mould table:
(a) shows the “roll-aside” arrangement,
and (b) shows a tilting table [2].

DC casting systems

There two main of DC casting systems:

Open top casting (float casting)
Closed casting (hot top casting). It has several modifications and is widely used.

Open top casting

The method

The DC open top casting systems (DC float casting) is an old proven technology that was widely used as far back as the 1980-90s. Now it have been replaced in Europe and North America by Hot Top casting systems, but they are still widely used in Asia and South America.

The advantage of the float casting system is that the operator sees the molten aluminum at all stages of its movement into the mould. In the event of a problem with a mould, the operator shuts off the corresponding downspout and casting continues without that mould.

For a 152 mm diameter billet a casting speed of approximately 110 to 130 mm per minute is used [4].

The limitations

Lubrication of the molds of the float casting system is carried out manually before casting. The grease has a thick consistency and a special composition that can withstand casting conditions. In principle, such a lubricant can be enough for casting poles up to 6 meters long. However, there is a problem of lubrication uniformity at the beginning and at the end of the casting. Therefore, with a single lubrication of the molds, ingots with a length of 4-5 meters are usually cast.

In addition, the number of moulds on the mould table is naturally limited, so it is difficult to keep track of two or three dozen downspouts and floats.

Figure 4 – Schematic of open top vertical direct chill casting of aluminium.
Liquid is supplied to the mould via a refractory downspout.
The level may be controlled by a refractory float at the tip of the downspout or
by an actuated pin in the downspout
which is adjusted according to the measured metal level [3].

Figure 5 – Conventional float and downspout mould assembly for direct-chill casting of aluminium [4]

This simple and well proven system is yet in common use throughout thr world. It requires no involved maintenance and inexperienced crews quickly become proficient [4].

Hot-Top casting

The method

In present time closed head (or HotTop) casting is the standard. This has a common metal level in the launder with channel insted a basin distributing the metal into each mould as it is done in a float casting system. The distribution head is fixed directly into the mould. Hence the term “closed head” [4] (Fig. 6-7).

The moulds are lubricated from a continuous oil supply system. It feeds oil to and through a porous graphite ring in the mould (Fig. 6 and also Fig. 10).

The advantages

The advantages of Hot-Top casting system are [4]:

an improvement in grain size
a reduction in inverse segregation of the alloying elements
increase in the billet cast speed
higher density mould tables on large casting machines (up 50 to 100 billets)
fewer operators
better surface finish.

Figure 6 – Cross section of a Hot-Top mould
together with cast product and
the molten metal feed into the mould,
plus water cooling supply to the mould [2]

Figure 7 – Hot-Top mould assembly during casting [4]

Ingot casting cycle

Briefly and schematically, the ingot casting process is as follows (Fig. 8).

Cast start

For all casting systems, before the start of each casting, the so-called “starter” or “starting head” (Fig. 7 and Fig. 9) enters the bottom of each mold to a predetermined depth.
Cooling water is supplied with the set start flow (Fig. 8a).
Molten metal is fed through the launder to the casting machine.
The refractory dams open and the metal begins to fill the molds (Fig. 10).
Then, after a pause of a few seconds, the starters, together with the platform, begin to move down (Fig. 8a).
Water consumption increases (Fig. 8a).
After about 1 minute, freshly formed ingots emerge from the molds.
Further, this process continues continuously until the specified length of the ingots is reached (Fig. 8a).

Cast end

When the predetermined length of the ingots is reached, the metal supply stops, the casting speed gradually decreases, the water supply decreases and stops ( Fig. 8b).
The platform slowly descends and brings the ingot heads down from the molds (Fig. 8b).
The mould table moves or tilts out of the casting position.
Ingots are removed from the casting pit with a crane and stored on a special site.
The mould table undergoes appropriate maintenance and is placed back in the casting position.
The platen slowly rises and introduces the starters into the molds to a predetermined depth.
The casting table is ready for a new casting.

Figure 8 – Typical control sequence chart used for programming control systems.
The two end sections of the chart are shown:
(a) cast start and
(b) cast end [2]

Figure 9 – Cross section of billet mould with starting head
located in cast start position
showing starting head/mould gap and
starting head alignment system [2]

Figure 10 – Section of molten metal feed system for multi-strand billet system.
At cast start the dam is closed, blocking entry to the group of moulds.
When the molten metal level in the launder reaches a set point,
the dam is actuated and metal is permitted to flow to the group of moulds,
with cast start then commenced
when the height of the metal in the mould area reaches a second set point.
Each group of moulds on a multi-strand table has a control dam [2].

Mould lubrication systems

Interaction of the molten metal with the mould

Direct contact of the molten metal with the mould wall results in the possibility of interaction of the molten metal with the solid mould, particularly if the latter is made of an aluminium alloy. It has therefore been a general practice to use some form of lubrication interposed between the molten metal and the mould wall as a simple means to overcome the problems encountered with dry moulds.

Manual lubrication

Original lubricants were often solid pig or goose fat, or similar greases wiped onto the mould wall prior to commencement of casting. These lubricant manually applied to the mould wall prior to casting.

In-built lubrication

All modern metal mould designs have an in-built liquid lubricant feed system that distributes the oil around the perimeter of the mould wall at the top of the mould, with the oil forming a thin film over the casting face of the mould. The graphite used for any of these technologies has a natural lubricity which does permit acceptable casting for a short time without any injected lubricant (Figure 11).
Figure 11 – Schematic of gas-pressurised and lubricated mould [2]

Gas-pressurised mould

To achieve a good cast surface, the gas-pressurised mould must achieve an air pressure sufficient to balance the metallostatic head pressure (see Figure 11). In this instance, the meniscus reaches a stable position where the outflow of air down the mould wall equals the incoming air flow and the pressure equilibrates at the metallostatic head pressure. If too high a flow rate is set, then the inflow exceeds the outflow down the mould, and the pressure rises above the metallostatic head pressure, causing bubbling of gas up into the liquid metal. If the air pressure is too low, then the meniscus equilibrates too high in the mould, and the shell begins to form too soon and then reheats, causing surface segregation.

Low Pressure DC Casting

The basic idea of the Low Pressure Catsting (LPC) technology is to avoid metallostatic pressure in the mould cavity, and thus eliminating the driving force for exudation.

Exudation is residual melt enriched on alloying elements who is squeezed through the partly solidified ingot surface during casting. The exudated material will exit the original surface of the billet below the air gap position. Exudation will give a solute enriched layer on the ingot surface and a solute depleted zone near the surface. The driving force for exudation is the pressure in the melt due to melt head (Figure 12).

Figure 12 – Conventional DC casting [5]

Figure 13 shows the LPC principle. To ensure feeding to each mould the LPC technology utilizes siphon filling to each mould by applying a under-pressure in the basin above the moulds. An ejector is used to generate and control the underpressure so that the position of the metal in the basin is stable during casting. The mould is ventilated towards the casthouse atmosphere. This ensures that the pressure abovethe metal in the mould cavity is the same as pressure on the outer surface of the billet below the air gap position – the point at which the billet moves away from the mould wall and an air gap forms between the billet and the mould. In this way there is no driving force for exudation since the pressure in the residual melt between the grains just below the air gap is virtually the same as the air pressure outside the billet surface [5].

Figure 13 – Low Pressure Casting (LPC) [5]