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Acta Metall Sin  2014, Vol. 50 Issue (8): 921-929    DOI: 10.11900/0412.1961.2014.00013
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EFFECT OF WITHDRAWING RATE ON PORE MORPHOLOGY OF LOTUS-TYPE POROUS COPPER PRODUCED BY SINGLE-MOLD GASAR TECHNIQUE
ZHUO Weijia1, LIU Yuan1,2(), LI Yanxiang1,2
1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084
2 Key Laboratory for Advanced Materials Processing Technology (Ministry of Education), Department of
Mechanical Engineering, Tsinghua University, Beijing 100084
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ZHUO Weijia, LIU Yuan, LI Yanxiang. EFFECT OF WITHDRAWING RATE ON PORE MORPHOLOGY OF LOTUS-TYPE POROUS COPPER PRODUCED BY SINGLE-MOLD GASAR TECHNIQUE. Acta Metall Sin, 2014, 50(8): 921-929.

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Abstract  

A single mold Gasar process was developed to fabricate lotus-type porous copper with long and straight pores. The effects of withdrawing rate on the solidification front shape, pore morphology and average solidification rate of porous copper ingots were investigated through experimental study and Procast simulation. The results show that the solidification front shape evolves from convex to planar, then to concave with increasing withdrawing rate. In this work, 1.0 mm/s is an appropriate rate for planar solidification front. In this case, all of the gas pores grow along the axial direction (parallel to the withdrawing direction) and the pores′ straightness is the best. The average porosities of copper ingots are constant and independent of the withdrawing rate. But the average pore diameter and penetration ratio of gas pores decreased with increasing withdrawing rate.
Key words:  lotus-type porous copper      single mold Gasar process      withdrawing rate      pore morphology     
Received:  06 January 2014     
ZTFLH:  TG249.6  
  TG146  
Fund: Supported by National Natural Science Foundation of China (No.51271096) and Program for New Century Excellent Talents in Universities (No.NCET-12-0310)
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Weijia ZHUO
Yuan LIU
Yanxiang LI

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https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00013     OR     https://www.ams.org.cn/EN/Y2014/V50/I8/921

Fig.1  Schematics of single-mold Gasar process without melt pouring operation (R—withdrawing rate)

(a) melting and temperature holding (b) unidirectional solidification

Fig.2  Typical pore morphologies on the longitudinal sections of lotus-type porous copper ingots produced at different withdrawing rates (pressure of H2PH2=0.2 MPa, melt temperature T=1457 K)

(a) 0 mm/s (b) 0.5 mm/s (c) 1.0 mm/s (d) 2.5 mm/s (e) 5.0 mm/s

Fig.3  Schematic views on the growth direction of gas pores under different withdrawing rates condition

(a) lower withdrawing rate (b) medium withdrawing rate (c) higher withdrawing rate

Fig.4  Pore morphologies on the cross-sections at heights of 45 mm (a), 65 mm (b), 85 mm (c) and 105 mm (d) in lotus-type porous copper ingot produced at withdrawing rate of 5 mm/s
Fig.5  Schematic view on formation principle of solid skin layer and big pores close to the solid skin layer
Fig.6  Average porosities of porous copper ingots produced at different withdrawing rates (PH2=0.2 MPa,T=1457 K)
Fig.7  Average pore diameters of porous copper ingots fabricated at different withdrawing rates
Fig.8  Average penetration ratios of porous coppers fabricated at different withdrawing rates (the height of the sample is 20 mm)
Fig.9  Schematic view on the pore growth interruption and secondary nucleation during Gasar solidification
Fig.10  Gas temperature distribution in Gasar furnace (a), physical model for heat transfer (b) and boundary conditions (c)
Parameter Value Unit
Heat conductivity of graphite crucible, lgraphite 131.06-0.0425T W/(m·K)
Density of graphite crucible, rgraphite 2200 kg/m3
Specific heat of graphite crucible, Cpgraphite 0.71 kJ/(kg·K)
Emissivity of heater, eheater 0.98[23]
Emissivity of graphite crucible, ecrucible 0.98[23]
Emissivity of copper and chiller, ecopper 0.78[23]
Convection heat transfer coefficient, hcrucible/gas 200* W/(m2·K)
Convection heat transfer coefficient, hmelt/gas 200* W/(m2·K)
Convection heat transfer coefficient, hchiller/gas 200* W/(m2·K)
Interfacial heat transfer coefficient, hcrucible/solid copper 3000[24] W/(m2·K)
Interfacial heat transfer coefficient, hcrucible/copper melt 5000* W/(m2·K)
Interfacial heat transfer coefficient, hchiller/crucible 3000[24] W/(m2·K)
Interfacial heat transfer coefficient, hchiller/water 8000[25] W/(m2·K)
Table 1  Parameters for heat transfer simulation
Fig.11  Simulated solidification front evolution at different withdrawing rates

(a) R=0 mm/s (b) R=0.5 mm/s (c) R=1.0 mm/s (d) R=2.5 mm/s (e) R=5.0 mm/s

Fig.12  Simulated solidification rates at different withdrawing rates (a) and comparison between the predicted pore diameter and corresponding experimental results (b)
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