Feeding Mechanisms in High-Pressure Die Castings
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TRODUCTION
HIGH-PRESSURE die casting (HPDC) is an efficient and economical shaped casting process for producing large numbers of complex thin-walled components requiring strict dimensional tolerance and good surface finish. A short fill time, typically 10 to 150 ms,[1] is essential to ensure complete cavity filling and good surface quality.[1] A high gate speed, ~18 to 40 ms 1 for Al alloys,[1] is commonly used to obtain the atomized filling pattern, which is desirable because entrapped air porosity in the casting is homogeneously distributed. Relatively thin gates, commonly 0.5- to 3-mm thickness,[2] are required to achieve a high gate velocity, and avoid part distortion or breakout after gate trimming.[3] However, the use of thin gates can have a detrimental impact on the casting quality if not properly designed. Thin gates often solidify earlier than the casting,[3] and during the HPDC process, partial solidification of the gate can inhibit filling and feeding of material to the casting.[3,4] Once the cavity is filled, if the casting is not properly fed, high levels of porosity can form, which has an adverse effect on mechanical properties and pressure tightness of components when used in the as-cast condition. Furthermore, the expansion of entrapped gases in the pores during heat treatment can result in surface blistering and dimensional change.[5] S. OTARAWANNA, Researcher, is with National Metal and Materials Technology Center, Pathumthani 12120, Thailand. Contact e-mail: [email protected] H.I. LAUKLI, Head of Research & Technology Development, is with Research & Technology Development, Hydro Aluminium, N-6601 Sunndalsøra, Norway. C.M. GOURLAY, Royal Academy of Engineering and EPSRC Research Fellow, is with the Department of Materials, Imperial College, London SW7 2AZ, United Kingdom. A.K. DAHLE, Professor, is with the ARC CoE for Design in Light Metals, Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Manuscript submitted August 10, 2009. Article published online April 27, 2010 1836—VOLUME 41A, JULY 2010
Intensification pressure (IP) is generally applied as the final step in HPDC to assist feeding once the cavity is completely filled. The effectiveness of IP is one of the governing factors determining the amount of porosity in the HPDC component. Using a higher IP generally reduces the level of porosity and therefore produces better mechanical properties.[6–10] However, the use of a high IP requires a high capacity hydraulic system to produce such high pressure and to clamp the die sections. Furthermore, at a higher IP, the wear and tear of the die and the machine occurs at a higher rate due to higher mechanical and thermal loads. Using the lowest IP to manufacture components with an acceptable porosity level and surface quality is therefore a common trade-off. Although the influence of IP on porosity level in HPDC has been reported,[6–10] the mechanisms by which material is forced through the gate and into the die cavity during the intensification stage have not been studied in detail. I
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