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



M. Rafi Raza, Faiz Ahmad, N. Ikram, R. Ahmad and A. Salam
 
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ABSTRACT

The wrought aluminum alloy 2219 was developed using an induction furnace followed by forging and hot rolling for the improvement of properties. Then the alloy was solution treated at 535°C for 48 min followed by water quenched and then aged at 210, 230 and 240°C for different time period. The maximum tensile strength of 411 MPa and hardness of 113.7 HV were achieved at 210°C after 4 h aging. The tensile strength of 368.12 MPa and hardness of 95.32 HV were obtained at 240°C after only 1.5 h aging time. The strength and hardness values of aged alloy specimens obtained in this work were reasonably higher (strength 51% and hardness 81%) than the specimens which were solution treated and quenched. Microstructures of the aged specimens also provided an evidence of dispersion of second phase precipitates particles in the single phase matrix.

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  How to cite this article:

M. Rafi Raza, Faiz Ahmad, N. Ikram, R. Ahmad and A. Salam, 2011. Development and Strengthening of 2219 Aluminium Alloy by Mechanical Working and Heat Treatment. Journal of Applied Sciences, 11: 1857-1861.

DOI: 10.3923/jas.2011.1857.1861

URL: https://scialert.net/abstract/?doi=jas.2011.1857.1861
 
Received: October 22, 2010; Accepted: February 26, 2011; Published: April 18, 2011



INTRODUCTION

Aluminium and its alloys have been in use since long in industrial applications, due to their many useful properties such as low density, good electrical and thermal conductivity and corrosion resistance in many environments (Dwight, 1999). Pure Aluminium with a tensile strength ranging from about 90 to 140 MPa depending upon the temper type but for structural use it has to be strengthened by alloying; the strongest alloys have a tensile strength of over 500 Mpa (Dwight, 1999; Shanmugasundaram et al., 2006). The optimum strengthening of Al is achieved by alloying and heat treatments (Taminger and Hafley, 2002). High strength aluminium alloys have been used in aircraft and spacecraft construction for many years, particularly Al-Cu from the 2000 series (According to the Designation of the American Association USA). Alloy 2219 is an Al-Cu binary alloy commonly used as an aerospace material due to its characteristics such as excellent weldability as well as weld strength. These properties extend its use to other applications (Starke and Staley, 1996; Smith, 1981).

In 2219 Al alloy, strain hardening of solution treated products accelerates the response to artificial aging and produces higher tensile strengths. This type of work of artificial aging response is used for certain Al-Cu-Mg alloys (Dudas and Thompson, 1970) but had not been used commercially for Al-Cu alloys before the development of 2219 alloy (Hatch, 1984). According to Taminger and Hafley Taminger and Hafley (2002) heat treated deposits of Al-2219 to the T6 temper, the microstructures of the deposits exhibited features more typical of cast material rather than these associated with wrought products (Kovacs et al., 1977). Many researchers have studied evaluation of microstructure, tensile strength and fracture toughness of precipitation hardened of 2219 Al alloy (Kovacs et al., 1977; Garrett and Knott, 1978; Melander and Persson, 1978; Srivatsan, 1992; Mukhopadhyay et al., 1994; Li et al., 1999; Ringer and Hono, 2000; Waterloo et al., 2001; Dumont et al., 2003). Rosen et al. (1982a) measured sound velocity, attenuation and hardness to study the precipitation hardening in 2219 alloy. Measurements as a function of aging time at various temperatures were found to exhibit prominent changes related to the formation of precipitates. Rosen et al. (1982b) also studied the influence of precipitation kinetics during ageing on electrical conductivity and hardness of 2024 alloy.

During the present research work the 2219 Al alloy was locally developed by using the locally available raw materials. The alloy was tested for mechanical properties. The results were verified by microstructural studies. The objective of this research work was to study the effect of aging time and temperature on mechanical properties of 2219 Al-alloy. The main emphasis was to find the minimum aging time at any investigated temperature to achieve the maximum UTS and hardness values.

MATERIALS AND METHODS

Materials: 2219 Al-alloy was prepared using locally available high purity constituents as shown in Table 1.

Methods: Al-ingot, Al-Si, Al-Mn, Al-Cu were heated together in an induction furnace at 750°C. An addition of cleansing fluxes was made at same temperature. Heating was continued and maintaining a layer of covering fluxes protected metal. When all of these metals were in molten state, Al-Mg and Zn were added and finally AlTiB5 was incorporated. The furnace heating was then stopped and fluxing layer was removed thoroughly. The degassing procedure was then adopted with the help of plunger by using nitrogen degassing tablets. A vigorous reaction took place and on completion of reaction plunger was taken out of the crucible and slag was removed. The molten metal was then poured at 720°C using pre-heated metallic mould.

Forging operation was performed with the open die and hammer forger of capacity 400 kg. Similarly, hot rolling was carried out upon the samples by using STANAT rolling mill.

The alloy was solution treated at 535°C in muffle type furnace for 48 min and then followed by the aging at 210, 230 and 240°C for different period of time.

Mechanical testing: Using SHIMADZU Autograph AG50K NISD-MS-Series carried out the tensile tests. The test samples were prepared according to the ASTM (E8M-00) standards. The Ultimate Tensile Strength (UTS) of each specimen was calculated from the graph by dividing the maximum load value by the original cross sectional area of the specimen. The hardness of each specimen was measured according to ASTM standard (E92) by using SHIMADZU Micro Hardness Tester HMV-2 Series.

Microstructure analysis: Optical micrographs were taken using Nikon Optiphot UFX-DX microscope. All specimens were cold mounted and hand ground on a wet emery paper of grade 240, 320, 400 and 600 grit, respectively.

Table 1: Composition of metals and master alloys used for the development of 2219 alloy

Polishing was carried out using cloth impregnated with 6, 3 and 1μm diamond paste lubricated with paraffin.

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).

CONCLUSION

Following conclusions may be drawn from the above work:

The maximum ultimate tensile strength (410.9 MPa) and hardness (113.76 HV) were achieved at 210°C in 2219 Al-alloy after aging for 4 h
It was noted that maximum strength and hardness generally decreased at higher temperatures with increasing aging time
The maximum ultimate tensile strength (368.12 MPa) and hardness (95.32 HV) were obtained in a much less time at the highest temperature of 240°C.
Some coarse particles of the second phase which are expected to precipitate out during aging were also seen in the photomicrographs of the alloy aged at 210 and 240°C

ACKNOWLEDGMENTS

The authors are thankful to Centre of Excellence in Solid State Physics and Faculty of Engineering and Technology, University of the Punjab, Quaid-e-Azam Campus, Lahore-54590 Punjab, Pakistan for providing necessary funds, experimental and characterization facilities.

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