Pressure diecasting is a process that induces extremely rapid freezing. This induces an extremely fine metallurgical structure that enhances most of the mechanical properties of the material. In consequence, parts cast from the same alloy but utilising gravity processes, which give rise to much slower freezing, usually have lower mechanical properties. (A notable exception to this rule is creep resistance, which is generally enhanced by increasing grain size).
A consequence of the above observation is that if a prototype is to be subject to mechanical tests it is usual to make it out of a different alloy from that which is proposed for the production diecasting. Most frequently the prototype alloy will be ZL12 because its tensile strength in the gravity cast condition most closely approaches ZP3 and ZP5 in the pressure diecast condition. However there are very significant differences between other properties of gravity cast ZL12 and pressure diecast ZP3 and ZP5. The ductility and toughness of the ZL12 prototype will be much lower but conversely its wear resistance and creep resistance significantly higher. If it is desired to simulate the ductility and toughness of a ZP3 or ZP5 pressure diecasting a prototype can be cast or machined from ZL27 and heat-treated (320°C for 3 hours and furnace cool) to reduce its tensile strength and increase its ductility to equivalent levels. (Note that for prototypes made in ZL27, this alloy must not be tested in situations where a non-sparking material is deemed essential.)
If the tests to be undertaken on the prototype require equivalent levels of physical properties, eg when checking the heat dissipation characteristics of a component, then the same alloy as that intended for production should be used for the prototype. Likewise when determining the effects of long-term moderate stress at elevated temperatures.
In situations where it is wished to perform mechanical strength testing but it is important that the part has similar density to the pressure diecasting, a gravity cast ZP8 prototype is a reasonable compromise.
If more than one type of test is to be made on a prototype it may be desirable to produce a number of components in different alloys to suit the differing test conditions.
The mechanical and physical properties of ZL8, ZL12 and ZL27 in the gravity cast condition are given in the table below.
Properties of Sand Cast Zinc/Aluminium/Copper Alloys Commonly Used for Prototypes
|Alloy Short Designation||ZL2||ZL8||ZL12||ZL27||ZL27
|Yield Stress (0.2% offset) MPa (1)||177||200||207||372||255|
|Poisson’s Ratio (1)||0.3||0.32||0.32|
|Shear Strength MPa||227||255||290||228|
|Hardness Brinell (500-10-30)||100||80-90||92-96||110-120||90-100|
|Impact Energy Joules
(6.3mm x 6.3mm bar)
|Youngs Modulus GPa||83.4||85||83||80||80|
|Compressive Yield Stress
|Fatigue Strength, 108 cycles,MPa||103||172||103|
|Density at 21oC,kg/m3||6600||6300||6030||5000||5000|
at 20-100oC, µm/mm/oC
at 20oC, W/m/hr/m2
|Specific Heat Capacity J/kg/oC
|Electrical Conductivity % IACS||24.7||27.7||28.3||29.7||29.7|
µohm-cm at 20oC
|Melting Temperature Range oC||379-
*HT = Heat Treated 3 hours at 320oC and furnace cooled