aging after as-cast condition) and T6 (artificial aging after solution and water quench) were experimented. An aging temperature of 232C (450F) was selected for AM alloys as recommended by the ASM Handbook11
aging condition of 16 hours at 168C (335F)13
and the standard was used
for AZ91 specimens. Several AM80 tophat castings were heat-treated to T5 (5 hours at 232C (450F) for mechanical testing and microstructure analysis.
Mechanical Testing and Microstructure Analysis
. For each condition, at least three specimens were tested and the average properties reported.
Tensile testing was conducted at room temperature accord- ing to ASTM E21-92 procedures at an initial strain rate of 0.001 s-1
The microstructure of the alloy specimens was analyzed using optical microscopy. Specimens were sectioned from the center of the tensile bars, and then polished and etched using a freshly prepared solution of picric acid, glacial acetic acid and methanol (50 ml picric acid, 20 ml glacial acetic acid, 10 ml methanol and 10 ml distilled water). Optical metallography was carried out using a Nikon Epiphot microscope.
Results and Discussion Mechanical Properties Gravity Casting Samples
Fig. 2 illustrates the effect of aluminum content on the ten- sile properties of Mg-Al-Mn alloys in the as-cast condition (F temper) using the GPMC process. As shown in Fig. 2, the ultimate tensile strength (UTS) and yield strength in- crease with Al content while the elongation (i.e. ductility) decreases. The results suggest that an addition of 7-8% Al should provide a good balance of strength and ductility in the as-cast condition.
Fig. 3 plots the age-hardening curves for the Mg-Al-Mn al- loys at 232C (450F). It should be noted that different hard- ness scales were used: HRH for AM30 to AM80 alloys, and HRF for AM90 and AM100; since it was not possible to cover the wide range of hardness values of these alloys in one reading scale. Therefore, it is not intended to com- pare the initial hardness values between the two scales, but to identify the hardness increases of each alloy during ag- ing treatment. It is evident that AM alloys containing less than 8% Al have very limited age-hardening effect, reaching peak hardness at about 3-5 hours. On the other hand, AM90 and AM100 show considerable age-hardening, with about 30% increase in hardness in 3-5 hours of aging. Therefore, 5 hours at 232C (450F) was chosen as the artificial aging condition for all AM alloys.
Fig. 4 shows the tensile properties of the AM alloys in the T5 (artificial aging after as-cast condition) and T6 (artificial aging after solution treatment and water quench) conditions. The effect of aluminum content on the properties is similar to the observation in the as-cast condition (Fig. 2). Fig. 5 highlights the effect of heat treatment (T5 and T6) on the tensile yield strength of the AM alloys, which confirms that the improve- ment in yield strength with heat treatment is more significant at higher aluminum contents, especially for alloys containing more than 7% Al. Another important observation is that T5 and T6 heat treatments provide essentially the same strength-
Figure 2. Tensile properties of Mg-Al-Mn alloys in as-cast condition (GPMC).
International Journal of Metalcasting/Fall 10
Figure 3.
Age-hardening curves for Mg-Al-Mn alloy castings (GPMC) at 232C (450F). 53
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85