Mechanical Property of Magnesium
Table1: Result of tensile test on various magnesium alloys
* The values for die casting are taken from the catalogue
Table1 shows the result of tensile test obtained from thixomolded bar-type specimens at room temperature. As a reference, catalogue values and actual data obtained with a cold chamber type die-cast process are included. As can be seen clearly, thixomolded specimens show properties superior to die cast ones. As the most of actual parts have rather complicated shapes,it is advisable to choose an adequate safety factors at the designing.
The relationship between process parameters and tensile properties of thixomolded AM50A is illustrated in Fig.1. The higher the barrel temperature,the better the tensile properties. Nevertheless, tensile properties tend to saturate above the melting point of AM50A. Injection velocity improves the properties especially at lower barrel temperatures.
Fig.2 demonstrates the relationship between the barrel temperatures and volume fraction of solid phase on AM series alloys. Volume fraction of solid phase was determined by an image analyzer on cross-section of the fractured specimens. The higher barrel temperatures leads less volume fraction of solid phase. The calculated valued from the equiribrium phase diagram of Mg-Al system is also plotted.
Why do the tensile properties decrease with increasing volume fraction of solid phase?
The reason can be explained by the observation of the microstructure.
This photo shows the typical microstructure at a cross-section of the fractured specimen of thixomolded AM50A after tensile test. White rounded particles are the solid phase before shot and the rest of them are the particles solidified rapidly from the liquid phase after the shot. It seems that the crack propagates along the interface of solid phase and residual liquid phase. Therefore, the presence of such interfaces may provide the crack routes. However, in the actual operation, process with higher solid phase ration(lower barrel temperature)is preferable for following reasons.
1. Avoid the trapped air in the products.
2. Fewer solidification shrinkage.
3. Shorter cooling time.
4. Longer machine and die life.