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Better casting technology for better castings
John Campbell , Emeritus Professor of Casting Technology , Department of Metallurgy and Materials at the University of Birmingham delivered the following keynote presentation at the 5th Foundry Workshop last April .
For foundries wanting the best quality possible , at the lowest cost , the best solution is almost certainly a good variety of counter-gravity casting . This is why it should be in the next five year plan for every Al alloy foundry . Copper-base alloy foundries could also be added to the list , as well as those casting cast irons and steels up to perhaps a few tons in weight . All such foundries would benefit enormously . When installed correctly , the process is capable of delivering an ultimate quality . However , in the interests of keeping the business going for the next five years , a useful intermediate step can be taken , once again simultaneously reducing costs by reducing scrap and repair costs .
The problem The central challenge for all casting processes is to avoid the entrainment of air and the surface oxide film during melting and casting . These lead to the two major problems for castings , both of which are entrainment defects ; air bubbles and oxide bifilms .
• The bubbles may or may not float out prior to the freezing of the casting but even if they do , they bequeath a long oxide tube behind them , a bubble trail . It is a highly efficient leak path , while at the same time acting as a strangely long crack .
• The oxide is always incorporated as a double oxide , dry face to dry face and so acts as a crack in the liquid and is subsequently frozen into the solid as a crack . The presence of millions of small cracks will reduce properties but a handful of large cracks may scrap the casting . Most castings are a dense mix of the two populations . The pouring geometry , ie the filling system design is extremely important as a major source of entrained bubbles and bifilms . However , most critical is the conical pouring basin . Regrettably , however , as everyone knows , the conical basin is almost universal . It is no wonder the casting industry has problems !
Figure 1 . An offset step basin , effective at detraining bubbles but not bifilms .
People worry about the swirl introduced by a conical basin , forming a vortex and go to some trouble to control this effect by adding flat sides . However , frankly , this precaution is a waste of time compared to the major threat from the conical basin . The major issue is that the cone shape acts as a venturi pump , entraining masses of air into the falling metal . This may not be too serious with a lip-poured ladle held close to the basin but for those castings ( mainly steels ) poured from a bottom-teemed ladle , the air content of the metal can reach 80 %; it is mostly air going through the running system and the metal becomes crammed with oxide bifilms to degrade the melt and possibly scrap the casting . This mix of up to 80 % air bubbles and 20 % steel enters the mould cavity in some chaos and has to unscramble in an effort to yield a casting . It is amazing that any good steel castings are ever produced . What is certain is that all are faulty and all much less good than they could be . As a cautionary note , some of the systems described here , particularly the offset step basin and various forms of contact pouring , have been tried , often repeatedly , years before . However , the systems have been found to be ineffective and have generally been abandoned . This repeated experience reflects the general awfulness of the remainder of filling system designs , which have been so bad that the great benefits of the improved basin or the contact pour have been obliterated . To achieve the benefits that are offered by these systems , the whole filling system has to be good . In particular , one of the best designs to date is the author ’ s ‘ naturally pressurised ’ design . It is strongly recommended .
The solutions What solutions are available ? Any potential solution has to eliminate the conical basin . After that , there are three main choices :
• Use an offset step basin ( Figure 1 ) to detrain at least 90 % of the bubbles . A properly designed offset basin can detrain bubbles by encouraging them to float out in the basin , prior to the metal entering the downsprue . Unfortunately , bifilms created by the pour into the basin need minutes or hours to detrain and the time available in the basin is usually only one or two seconds . Thus the bifilms generated by the pour into the basin are taken down into the casting . Even so , the basin is perhaps between 10 and 100 times better than the conical basin from the point of view of reducing casting defects .
• Use contact pouring ( Figure 2 ), a technique that eliminates all pouring basins of every type . The nozzle at the base of a bottomteemed ladle is placed directly in contact with the entrance to the downsprue . Any gap between the two can be sealed with a
Figure 2 . Conventional ingot pouring and contact pouring of an ingot .
gasket of ceramic fibre paper or blanket to avoid the ingress of air . This simple technique eliminates not only the additional cost of a pouring basin and the cost of any undrained metal in the basin but it performs excellently , eliminating both bubbles and bifilms . The effect on foundry quality and scrap reduction is usually instantly beneficial . A further natural benefit of contact pouring is the flotation of oxides to the top of the melt , so that the metal taken from the base of the ladle into the casting is the cleanest . This is clearly important for all the dense metals such as copper-base alloys , nickel-base alloys , cast irons and steels . ( It is less true for Al , where the oxide is slightly heavier but the entrained air trapped between the double films tends to make the oxides neutrally buoyant . It is also not true for Mg , in which MgO sediments rapidly .) To be really effective , contact pouring should be linked to a nicely designed , naturally pressurised filling system . If linked in this way , most foundry quality problems would be solved at a stroke .
• Convert completely to counter-gravity filling . This needs to be carried out well and of course , is a non-trivial change of practice . The foundry might need to be re-built !
One counter-gravity technique widely used for Al alloy castings but which is not recommended , is low pressure diecasting . Even though its disadvantages can be eliminated by more sophisticated varieties of the process , in its usual form the process has a number of significant faults . These include the fall of metal into the pressurised furnace and the fall down the riser tube etc . For these and other reasons , LPDC cannot normally be recommended . Aerospace may have to content itself with the less-thansatisfactory aspects of this batch interpretation of counter-gravity simply because of the wide variety of alloys used and because of its archaic and misguided devotion to batch melting as apparently fulfilling the traceability system . In the author ’ s opinion , for high volume Al alloy casting , the best option is to use a
Cast Metal & Diecasting Times July / August 2016