Mechanical Property of Magnesium

Table1: Result of tensile test on various magnesium alloys

Material Process Barrel
temperature
(K)
Injection
velocity
(m/s)
Y.S.
(Mpa)
T.S
(Mpa)
El.
(Mpa)
AZ91D Thixomolding™ 878 1.4 180 299 10
(Die casting) * 160 240 3
AM60B Thixomolding™ 893 1.4 148 278 19
Die casting 963 2.9 115 239 12
(Die casting) * 130 225 8
AM50A Thixomolding™ 898 1.4 140 269 20
Die casting 963 2.9 112 232 13
(Die casting) * 125 210 10
AS41B Thixomolding™ 903 1.7 157 249 9
(Die casting) * 140 215 6

* 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.

Fig.1: Relationship between process parameters and tensile properties (AM50A)
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Fig.1: Relationship between process parameters and tensile properties (AM50A)

 

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: Relationship between barrel temperatures and volume fraction of solid phase (AM series alloys)
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Fig.2: Relationship between barrel temperatures and volume fraction of solid phase (AM series alloys)

 

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.

 

Fig.3: Microstructure at cross-section of fractured specimen (AM50A)
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Fig.3: Microstructure at cross-section of fractured specimen (AM50A)

 

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.