Melting temperature of Cu is about 1083ºC and for Al is 660ºC, but why Al alloys have more fluidity than Cu alloys?
Fluidity depends on alloying element &Base metal
You should find the phase-diagram you want to compared
All aluminum alloys do not have the same level of fluidity. Al-Si alloys have excellent fluidity compared to other grades. Please refer the attachment.
Case:Pure Element
Fluidity has two basic factors: (1) characteristic molten metal and (2) casting parameters. More precisely:
a) Viscosity --> Inverse Relation
b) Surface tension --> Inverse Relation. Means, high surface tension reduces fluidity. Surface tension triples in molten Al forms due to formation of oxide films
c) Inclusions --> Inverse relation due to negligible solubility
Case: Alloy
d) Solidification (solid wt.% vs Temp) --> Inverse relation. Shorter the range the better.
Test Method
Molten metal is allowed to pass through circular flow, eventually solidify. Finally measure distance where flow stops at room temperature.
For me, you can find the answer in Syed Abbas comments. The viscosity may be the principal reason. If you want, you can use the inclined plane test to know the relative difference between viscosities.
Attach there are more information, but excuse me, because is in spanish.
Dear Nima Tabatabaei,
Limiting the discussion to pure metals, so excluding the effects due to eutectics, it can be said that in a first Grüneisen approximation the metal melting temperatures are proportional to the elastic constants (Ref. Section 2.2.4) . This implies that for Cu the binding forces are larger than for Al. This might explain the difference in viscosity or fluidity as you mentioned.
Reference
Materials Science in Design and Engineering, Delft NL, 2012
http://www.delftacademicpress.nl/m017.php
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