Ensuring quality prior to pouring: of the various aspects of the aluminum casting production process, the quality of the molten metal is perhaps the most controllable.

Metalcasting facilities must maintain considerable control over their processes to ship high-integrity castings reliably and on time. But, many of those processes are highly influenced by external factors.

In any product, five main factors influence casting quality--four of them are often out of the metalcasting facility's control. First, the casting requirements must be set by the end-user, the product designers. In the best of circumstances, this is a cooperative effort between the customer and the caster, but often it is not. Second, the proper metal alloy must be selected--again not necessarily in the control of the casting producer--that will meet the engineering requirements of the casting. Third, the casting process is selected, which controls the fourth tenant in this series--the microstructural response of the casting--which in turn plays a vital role in the product's capability.

However, before this fourth tenant, which involves the actual casting solidification process, comes into play, a fifth tenant--the quality of the molten metal delivered to the casting mold--has an influence. This fifth tenant, melt quality, is firmly in the control of the casting facility.

Melt quality involves control of two basic impurities common to most aluminum melting/casting processes--hydrogen content and non-metallic or intermetallic inclusions. Both are readily present in all processes. Melting in air, high temperatures, fossil fuel decomposition, humidity, oxide-covered charge materials, returns, trim scrap, gates and runners, and sand system debris all contribute to the presence of melt impurities. These impurities are extremely damaging to high integrity casting performance if not removed. In addition to common sense and conscientious melt handling, melt treatments such as degassing, flux injection, and filtration are typically the in-process steps employed to control and remove these impurities.

Combining Degassing and Flux Injection

Degassing is a standard melt treatment practice employed in many metalcasting facilities. Straight lance degassing produces large purge gas bubbles, which are not very efficient for degassing. The advent of rotor degassing proved to be much more efficient, as the rotary action produces smaller bubbles. Manual fluxing with solid flux chemical compounds has its merits, but environmental and personal health concerns have greatly impacted flux usage and practice, and flux injection processes are now more commonly utilized.

By combining flux injection with rotor degassing, complementary benefits arise. Injecting flux chemicals in a controlled, measured amount below the melt surface provides for more environmentally friendly, efficient usage. The rotor degassing action not only achieves small gas bubble formation through the gas which serves as a carrier for the flux, but it also achieves better mixing in the ladle between the flux and the metal. This provides more comprehensive treatment coverage of the full melt volume contained within the ladle and usually shortens the treatment time, thereby increasing productivity.

In addition to process validation and ongoing verification, it is necessary for a metalcasting facility to take into consideration three factors--temperature, metal transfers and ladle condition--in order to achieve a successful trial result when evaluating the merits of a new melt treatment such as the flux injection process.

Excessively high temperatures in melt processing mean a greater tendency for hydrogen absorption and oxide formation. However, some superheat is necessary in most processes to accommodate temperature loss during a degassing/flux injection process, as well as the time of transport into a transfer ladle or into one or more casting units/holding furnaces.

Minimizing turbulence during metal transfers is necessary in all circumstances. "Cascading" of metal must be avoided with tapout or pump transfer into a transfer ladle. Additionally, when transferring metal from a melting furnace into a ladle or holder, care must be taken to avoid surface dross entrainment or bottom sludge.

If a ladle has not been thoroughly cleaned, initial sampling from a degassing and/or flux injection process will show erratic results. The action of the degas/flux mixing tends to provide self-cleaning action, and these loosened accretions from the ladle surfaces will usually foul the positive result desired from the new melt treatment.

In-Furnace Filtration

In many metalcasting operations, it is neither practical nor economical to employ an in-mold, single-use filter. In these instances, the application of filters within the casting furnace itself is beneficial. Typically, the type of filter employed in this application is a robust, silicon-carbide bonded particulate filter. Bonded particle filters have the advantages of higher strength at molten aluminum temperatures, chemical stability for long-term, sustained usage, high thermal conductivity for minimal temperature gradients and enhanced filtration efficiency by means of a low-porosity, complex internal structure. Filtration in general, and the bonded particle filter in particular, achieves improved melt quality by removing inclusions, which results in reduced scrap, better machinability, elimination of hardspots, improved surface finish processes and improved mechanical properties, especially elongation.

It is well known that inclusions will induce porosity formation during the casting solidification process, as these inclusions constitute a free surface upon which the hydrogen atoms can nucleate and form a hydrogen molecule. Enough of these hydrogen molecules getting together with no place to go will indeed result in pores forming within the casting. In a critical section of the casting, porosity can have very detrimental results. Fig. 1 shows how filtration--following a degassing--can provide a boost in specific gravity results as measured with a reduced pressure test. Filtered metal with fewer inclusions results in lesser porosity formed during the test and hence a higher specific gravity.

[FIGURE 1 OMITTED]

With proper casting design parameters, gating and risering techniques, and overall solidification characteristics, a porosity-free and inclusion-free cast product can result when molten metal quality is carefully controlled.

This article was adapted from a paper presented at the 2005 AFS International Conference on High Integrity Light Metals Castings, Indianapolis.

Results of Flux Injection Plant Evaluations

Several flux injection evaluation trials were performed at diecasting, permanent mold and sand casting plants to demonstrate improvements in melt quality. Evaluations have been made, providing a real-time flow curve and subsequent laboratory analysis of inclusions concentrated on a test filter. using metallographic preparation and image/microscopic analysis.

Plant A

In the first example, Plant A produces 383 alloy die castings. The curves shown in Fig. A depict the difference in metal cleanliness--as evidenced by the greater flow volume as a function of time--for three conditions: before treatment, after degassing only, and after degassing coupled with flux injection. An improvement is observed with the flux injection. The inclusion removal results over several tests showed an 89.5% removal.

Another benefit of flux injection is the amount and nature of the dross. When rotor degassing is employed alone, considerable dross can be generated from the process, as shown in Fig. B. This is due to inclusion flotation, as well as the turbulence created by the shaft/rotor action itself. However, when flux injection is part of the process, the dross becomes treated in-situ with an appropriate drossing flux composition, such that the amount of dross becomes substantially lower (Fig. C). Additionally, note the differences between Figs. D and E, which show the appearance in cooled dross between these two conditions. In the fluxed case. the dross appears more powdery and dull, "dry" and non-shiny, indicative of its lower aluminum content.

Plant B

The second case, Plant B. produces pistons by permanent mold casting using hypereutectic aluminum-silicon alloys. The curves shown in Fig. F indicate a small but measurable improvement afforded by the flux injection process. When examining inclusion removal, however, there is a much more discernible effect. An average of 85.5% inclusion removal was achieved with flux injection.

Plant C

in this plant, A356 automotive components are produced by pressure diecasting. Fig. G shows a marked improvement in the result with flux injection. Inclusion removal is also substantial.

For More Information

"Analyzing Aluminum Melt Quality from Furnace to Mold," D.P.K. Singh and D.J. Mitchell, MODERN CASTING, May 2001, p. 44-46.

Dr. David Neff is the director of molten metal treatment, metaullics systems division, Pyrotek Inc.