Improving Performance - Billet Casting Improving ... - Pyrotek

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Improving Performance - Billet Casting Jonathan Prebble, Manager of Aluminium Process Technology Pyrotek has the expertise, technology, experience and the global resources to maximize the performance of your billet casting systems. Page 2 LATEST BiLLET CASTing TECHnOLOgY in THE MODERn CASTHOUSE improving Performance For The Customer This year, Aluminium international Today (AiT) published a series of Pyrotek articles on the processes, techniques and equipment used in Furnace Building and Repair, Furnace Melt Treatment, Degassing, Filtration, Slab and Continuous Sheet Casting. We have detailed the metal process path from the furnace to the casting process in order to demonstrate clearly why each of the various stages is necessary to achieve a final, desired metal quality designed to meet or exceed the end-customer’s expectations. This article is the last one in the series, addressing the casting of Billet in a Vertical Direct Chill (VDC) Pit. The in-line processing work which has been already undertaken (please refer to the previous articles in this AIT series) upstream of the casting pit has been required to provide the billet with the correct levels of metal cleanliness, chemistry and hydrogen content. Billet casting covers a range of alloys in many diameters for extrusion or forging applications. 1xxx, 3xxx, 6xxx, and 7xxx alloys are all cast into billet form for applications that require them to be either extruded to shape or cut to blanks and forged. It is worth remembering at this point that the 6xxx series alloys are capable of being solution heat-treated as well as being quenched and aged to different tempers after extrusion. This necessitates homogenising furnaces in the casthouse area and aging ovens in the extrusion plant. Furthermore, ultrasonic non-destructive testing is usually carried out on all cast billet to ensure that there are no internal cracks or major defects present. Billet casting needs to be done correctly, requiring high investment in equipment and skill, in order to earn the premium it commands in the market. pyrotek supplement iMPROVing PERFORMAnCE in THE BiLLET CASTing PROCESS In general, most aluminium casthouses would deal with casting 6xxx series.The other alloy series are used for more specialised or exotic products for aerospace, automotive, transportation, etc. However, all billet casting requires the same basic approach to metal distribution and solidification, best performed in a VDC Pit. These pits are becoming larger and deeper to allow for more, longer cast billets because this reduces process scrap and production downtime between casts. The new casting challenges are due to operating VDC billet tables on a vast scale: more than 100 strands of billet per cast is increasingly commonplace! The upstream metal treatment, which we have focused on previously, has been designed to reduce inclusions, alkali metals and hydrogen – all or some of which can cause problems during the subsequent extrusion or forging processes. The conditions in the actual billet casting mould, however, can prove to be just as troublesome during downstream processing if the casting parameters are not well inside the “envelope”, which will produce acceptable finished quality. Here we are talking about effects such as grain-size, segregation, shell zone, cracking, surface defects including drag marks, tears, bleeds, etc., and diameter tolerances. These factors generally result from the conditions inside the actual billet casting mould - where no amount of upstream processing can assist. A good understanding of the billet casting technology being used is vitally important as this permits the right methods to be selected for the best possible productivity and performance. Most casting recipes are a compromise: a balance between the best repeatable quality achievable for a particular casting technology and practical, safe, dependable casting which can be relied upon to produce good product consistently. In this article, we will first review the various types of billet casting technology in general use today. We will also mention the importance of casting lubricant selection and how this can both improve cast surface
Improving Performance - Billet Casting Figure 1a - Horizontal Direct Chill Casting Figure 1b - Verticle Direct Chill Casting (picture courtesy of Wagstaff) overall consumption and greatly reduce the need for expensive chemical treatment of closed cooling water systems for the casting pits. Several aluminium casthouses, for a variety of reasons, are still confidently casting billet using the traditional conventional mould set-up, requiring a distributor basin, spouts, floats and pins. As I have mentioned before, billet casting needs to produce a product that meets or exceeds the customer’s expectations. This does not mean that the latest casting technology is either essential or even applicable to all billet casting operations. For these customers, Pyrotek supplies a variety of machined parts for spouts, pins and floats. Usually each of these is designed to fit the customer’s particular billet diameters and metal pouring conditions. Please refer to Pyrotek’s web-site for further details of our entire range of billet casting products and technical support staff who are available to assist – www.pyrotek.info. HiSTORY OF BiLLET CASTing Aluminium VDC billet casting is the process whereby molten aluminium is converted into a round ingot or billet. Although the process itself is rather straightforward, there are many variables during casting affecting the quality of the billet. Today there are two basic processes for converting molten aluminium into billet: Horizontal Direct Chill (HDC) and Vertical Direct Chill (VDC) (see Figures 1a, 1b). Each of these processes has it’s own distinct advantages. Since the output of the VDC process is by far the most widely utilised, it is therefore the focus of this article. Before the VDC process was developed, aluminium wrought billets were cast using the tilt mould ingot process. The molten aluminium was poured from a crucible into a watercooled book-mould tilted at an anglefromthehorizontal.When the aluminium in the mould reached the upper lip, it was tilted at a controlled rate until it was vertical. The tilt mould ingot was not suitable, however, for large diameter production and was limited to non heat-treatable alloys such as 1100 and 3003 pyrotek supplement Alan Sale global Billet Casting Specialist Billet casting needs to produce a product that meets or exceeds the customer’s expectations. For these customers, Pyrotek supplies a variety of machined parts for spouts, pins and floats. (see Figure 2). As demand Figure 2 - Casting Aluminium in a Tilt Mould for higher productivity coupled with the need for larger billets and high strength alloys increased, the VDC casting process replaced tilt mould casting in the 1930’s. Page 3
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