Development and Strengthening of 2219 Aluminium Alloy by Mechanical Working and Heat Treatment

RESULTS AND DISCUSSION

Chemical analysis: Using the optical emission spectroscopy (Emission GNR ITALY metal lab 75-80 J) checked the composition of the prepared alloy. The achieved composition of the alloy is shown in Table 2. The result showed that the composition of fabricated alloy is according to the standard.

Microhardness and tensile strength: The test samples were characterized for hardness and tensile strength at 210, 230 and 240°C, the obtained results are mentioned in Table 3.

Results of tensile testing of the alloy aged at different temperatures in the range of 210 to 240°C are graphically represented in Fig. 1.

The ultimate tensile strength versus time at aging temperature of 210°C shows that the maximum ultimate tensile strength of 410.9 MPa is achieved after aging for 4 h, after which the strength reduces to 352 MPa. Test samples aged at 230°C showed the maximum ultimate tensile strength of 370.7 MPa after 3.5 h aging and at 240°C the maximum ultimate tensile strength of 368.12 MPa is obtained after much less aging time of 1.5 h.

Fig. 1: Graphical representation of UTS versus time at temperature 210, 230 and 240°C
Table 2: Achieved composition of the 2219 Al-alloy
Table 3: Hardness and tensile strength of 2219 Al alloy at different level of temperature

From these results it may be seen that the ultimate tensile strength initially increases with aging time reaching a maximum value and then decreases as expected in such aging heat treatment. The strength increased initially because the formation of precipitates. After achieving maximum strength it gradually fall, its all because of over aging (Raghavan, 1998). It is also observed hat with the increase in aging temperature respond the maximum strength in less time. However, the maximum value of ultimate tensile strength obtained at 210°C is higher than those observed at 230 and 240°C which may be explained on the basis of the possibility of not using an aging time which could provide a value higher than these. Increase in strength observed from these graph may be related to the beginning of precipitation process leading to the formation of GP (Guinier Preston) zones (Smith, 1993) which hinder the movement of dislocation within alloy. The decrease in ultimate tensile strength values after reaching a maximum is termed as over aging and is mostly related with the formation of coherent second phase particles which increases in size with time and reduce the hinderness to the dislocation moment to a certain extent.

Similar behavior was observed for hardness versus time graph at temperatures 210, 230 and 240°C. The Fig. 2 represents the maximum hardness values versus temperature. The maximum hardness values obtain 113.76 HV at temperature 210°C for aging time of 4 h.

In a previous work on the same alloy (Jabeen and Ahmad, 2007) the aging was carried out at 190, 210 and 230°C and maximum ultimate tensile strength of 370 Mpa was obtained at aging temperature 210°C after 6 h where as in the present work the maximum ultimate tensile strength of 410.9 MPa was obtained at same temperature after 4 aging.

Fig. 2: Graphical representation of vickers hardness versus time at temperature 210, 230 and 240°C
Fig. 3: Precipitate formation in microstructure of the specimen aged at 210°C for 4 h: 200X
Fig. 4: Precipitate formation in microstructure of the specimen aged at 240°C for 1.5 h: 200X

Also the ultimate tensile strength values of 370.7 and 368.12 MPa were obtained at 230 and 240°C, respectively after much less aging time.

Figure 3 and 4 showed micro structure of representative samples which were aged at 210°C for 4 h and 240°C for 1.5 h, respectively. From these microstructures few intermetallic precipitates may be seen distributed in the matrix however, the finer precipitates such as (CuFe) Al6, Cu2FeAl17, (CuFeMn) Al6 and Cu2Mn3Al20, CuAl2 etc are expected to form which are not clearly seen in optical micrograph (Mondolfo, 1976; Van Horn, 1967).