TAILIEUCHUNG - Foseco Non-Ferrous Foundryman’s Handbook Part 6

Tham khảo tài liệu 'foseco non-ferrous foundryman’s handbook part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 88 Foseco Non-Ferrous Foundryman s Handbook The commonly used alloys contain 8-10 Si and 2-3 Cu with around 1 Fe. The alloys are usually bulk melted and transferred to holding furnaces at each casting machine from which metal is dispensed into the die by an automatic ladle or in manual operations by baling out. The holding furnaces are topped up from time to time from the melting unit. In the past it was not considered worthwhile to degas the alloy before casting because some gas porosity was accepted as inevitable . Furthermore the cooling rate in the die is so fast that grain refinement and modification were also not considered necessary. However with improvements in the practice of pressure diecasting see Chapter 9 it is found that treatment of the liquid alloy by Rotary Degassing is effective both in degassing and removing non-metallic inclusions so some diecasters degas in the transfer ladle for this reason. Strontium modification is also found to be beneficial. Bulk melting is carried out under a covering drossing flux to ensure minimum metal loss. COVERAL GR2220 granular flux at square metre is used or COVERAL 5 powder flux at of metal weight or 1 kg square metre of melt area forming a complete cover adding half early and the rest at final melt down. In the holding furnace COVERAL 75 low temperature flux can be used as a cover to reduce metal loss. Scatter onto the metal surface and rabble gently until the exotherm develops. Push aside or remove before taking ladles. See Chapter 9 for further details. Chapter 7 Running gating and feeding aluminium castings Introduction The primary function of a gating system is to introduce clean dross-free metal from the pouring ladle to the mould cavity and to do so in a manner which will not cause subsequent reoxidation and gas pick-up. Aluminium alloys are all subject to dross formation a film of oxide forms immediately on any metal surface exposed to air see Table Oxide films form on the surface

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