Thin plates (Fig. 1) are produced by a horizontal cold chamber
die casting machine. Die casting parameters adopted in this experiment
are optimized and constant. The 12 kg foundry ingots are
melted by electric resistance heating furnace and the required
amount of pure magnesium is added into the melt when the temperature
is around 650 C. The percentage of weight loss of magnesium
(10%) during the melting process is also taken into account.
The alloy melt is degassed for 15 min with argon gas through a
graphite lance and then poured into the shot chamber at 700–
710 C. In addition, the die is preheated to 200 C and the injection
pressure applied in this experiment is 115 MPa.
Schematics of tensile and fatigue specimens are shown in Fig. 2.
All specimens are electro-discharge machined from the thin plates
and fatigue samples are grinded on abrasive papers with increasing
number and polished to make sure that all remaining polishing
marks are in the longitudinal direction to minimize the influence
of imperfections produced by machining.
Tensile tests are carried out by electronic universal testing machine
under ambient temperature (20 C) and loading rate is
5 mm/min. Elongation and Young’s modules are obtained by a
knife-edge extensometer fixed in the range of gauge length
(50 mm). Six tensile specimens are tested and the average value
of the results is adopted to evaluate their strength and ductility.
Meanwhile, the hardness is tested by BH3000 Brinell Hardness tester
under the test load of 1000 kgf.
Fatigue tests are carried out by electro-hydraulic servo fatigue
testing machine. Testing frequency is 10 Hz and stress ratio R = 0.
Stress amplitudes applied in this paper are ranged from 57 MPa
to 125 MPa. Multiple samples are loaded to fracture at the given
stress levels and the number of cycles to failure is recorded by data
acquisition system.
Fracture surface and microstructure of those alloys are observed
by SEM and OM. The true chemical composition of the alloys (listed
in Table 1) is determined by the optical emission spectroscopy
analysis. In addition, phase composition is defined by phase diagrams
[14] and XRD. Finally, a part of AlMg5.5Si2Mn samples received
artificial age treatment (250 C/1 h).
3.