Effect of Various Processing Conditions on the Tensile Properties and Structural Developments of EN AW 2014 Aluminium Alloy

Jana Bidulskä1, Tibor Kvaökaj1*, Robert Bidulsky2, Marco Actis Grande2

1 Technical University in Kosice, Faculty of Metallurgy, Letnä 9, 04200 Kos ice, Slovakia 2 Politecnico Torino-Alessandria Campus, Viale T. Michel 5, 15100 Alessandria, Italy

(Received August 6, 2008: final form August 18, 2008.)

ABSTRACT applications in the civil, automotive and aircraft industries / l, 21. The paper deals with the influence of processing Demands of industry producers are to find new conditions on tensile properties, hardness and structure forms and facilities for appropriate properties of development of the aluminium alloy EN AW 2014, structural parts. Conventional forming methods are depending on various processing conditions (as-rolling, ineffective in achieving favorable properties for quenching, severe plastic deformation - SPD by equal produced parts, adequate to structural properties; channel angular pressing - ECAP and ageing). The moreover, through them only limited levels of structural evaluated tensile properties showed that ECAP has the and strength-plastic characteristics can be obtained. The highest effect on the material properties of aluminium solution may be non-conventional forming methods /3, alloy EN AW 2014. Severe plastic deformation by 4/, as well as SPD (equal channel angular pressing - means of ECAP caused rapid increase of strength and ECAP and equal channel angular rolling - ECAR only partial decrease of ductility was achieved. technologies) /5-8/ to obtain results structured at the nm Strengthening of material is caused by grains refinement level. A combination of high strength and ductility of and strain hardening of solid solution. Based on the ultrafine polycrystalline metals, prepared by severe results above, the tensile properties, hardness and plastic deformation, is unique and it indeed represents structure development of the EN AW 2014 alloy are interesting cases from the point of view of mechanical discussed. properties, as confirmed in /9-11/. The present paper deals with the tensile properties as Key words: aluminium alloy, SPD, ECAP, ageing functions of the processing conditions of the aluminium and mechanical properties alloy EN AW 2014. Based on the results above, the tensile properties, hardness and structure development of the EN AW 2014 alloy are discussed. 1. INTRODUCTION

Al-Cu-Si-Mg alloys have many excellent attributes, 2. MATERIAL AND EXPERIMENTAL such as high intensity, high elastic modulus, good METHODS corrosion resistance and high temperature properties, suitable for different miscellaneous structural The material used in this experiment was aluminium

' Corresponding author: T. KvaCkaj, T/F: + + 421-55-6024198, E-mail: [email protected]

203 Vol. 27, No. 3, 2008 Effect of Various Processing Conditions on the Tensile Properties And Structural Developments of EN AW 2014 Aluminium Alloy alloy EN AW 2014. The chemical composition of the Tensile specimens were prepared after each experimental material is shown in Table 1. processing treatments. Tensile test on the short specimens do χ lo = 5 χ 10 mm was made. The tensile Table 1 testing was done on a FP 100/1 machine with 0,15 Chemical composition of aluminium alloy mm min'1 cross-head speed (deformation rate of2,510"4 EN AW 2014 [mass %] s'1). Subsequently, characteristics of the strength (yield strength: 0,2% YS; ultimate tensile strength: UTS), Al elongation (El.) and reduction in area (Re.) were Al Cu Μη Si Mg Fe Zr Ti alloy determined. The tensile results under various processing EN conditions are summarized in Table 2. AW Bai. 4.32 0.77 0.68 0.49 0.29 0.12 0.03 2014 Table 2 Mechanical properties of investigated EN AW 2014 Hot rolling was carried out by rolling-mill DUO 210 at temperature of 733 Κ (rolled state). Solution Mechanical properties annealing after rolling was performed at temperature of Processing 793 Κ (holding time 9 000 s) and cooled to the room 0,2% YS UTS EL Re. temperature by water quenching (quenched state). The (MPa) (MPa) (%) (%) quenched specimens (d = 10 mm, 1 = 70 mm) were 0 0 As-rolled 235 381 22,3 27,8 subjected to deformation in an ECAP die with channels angle Φ = 90° at rate of 1 mm s"1 (ECAPed state). The Quenched 157 394 32,8 34,4 ECAP was realized by hydraulic equipment at room temperature, which makes it possible to produce the ECAPed 511 593 17,1 18,0 maximal force of the value of 1 MN. After one ECAP pass, the specimens were processed to natural or ECAPed + aged 515 541 14,4 14,0 artificial ageing at 373 Κ for 720 000 s (ECAPed + aged state). The experimental schemes are summarized in Fig. 1. For light optical microscopy (LOM), samples were individually mounted, mechanically ground and polished, and finally etched using a Keller's reagent. Transmission electron microscopy (TEM) analysis was made on thin foils. The foils for TEM were

prepared using a solution of 25% HN03 and 75%

CH3OH at a temperature 243 K. TEM was conducted at an accelerating voltage of 200 kV. Fractographical examinations of the fracture surface of the materials after a conventional tensile strength test was carried out using SEM JEOL 7000F.

3. EXPERIMENTAL RESULTS AND DISCUSSION

The measurements of hardness vs ageing time are summarized in Fig. 2. Fig. 1 Experimental schemes

204 Bidulska el al. High Temperature Materials and Processes

190 MPa respectively, if compared to the as-rolled and 180 9 • · (k ΟοβΟ«ο°β quenched alloy. Values of yield strength of 511 MPa 170 > from 235 MPa and 157 MPa respectively, of the as- X 160 ECAPed + aged Μ rolled and quenched material were obtained. Strength «0 150 ECAPed Quenched and ductility are important mechanical properties of the C 140 Έ material but these properties typically have opposite «0 130 Ζ 120 characteristics. Results of tensile test show that ECAP

110 increased mechanical properties with only partial

100 acceptable decrease of ductility.

90 10 100 1000 10000 100000 OS 0- 600 Ageing time [s] 500 5Λ 400 Η Fig. 2 Hardness values vs ageing time » 300 (Λ > 200 ECAPed samples achieved 83 % of hardness growth 35 _ to the value of 179.2 HV10 in comparison to the as- N? 30 L rolled value of 96.6 HV10. Quenched state materials show an increase of hardness up to the value of 142 25,2 20 -i HV,o with increased ageing times (up to time of ω 15 360 000 s); then there is a plateau with constant •8 JS a. hardness values for increasing ageing times. < υ The stress-strain curves and tensile results are Id a. < summarized in Figs. 3 and 4. υ w

Fig. 4: Strength values as a function of processing conditions

Interaction of the slip bands and elongated grains, Fig. 5 and Fig. 6, is the most significant consequence of ECAP after only a single pass in ECAP die /ll, 12/.

As-rolled Quenched ECAPed + aged ECAPed

0.00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 Strain [-]

Fig. 3 Stress-strain curves

The difference in the strength values is basically due to the various processing modification. The reduction of strain hardening is the reason for the increasing of strength and ductility in the case of the as-rolled state in comparison to the quenched state, as well as for the strength increasing in ECAP. ECAP increased the Fig. 5: Interaction of the shearing bands and elongated strength value up to 593 MPa from 381 MPa and 394 grains, LOM

205 Vol. 27, No. 3, 2008 Effect of Various Processing Conditions on the Tensile Properties And Structural Developments of EN AW 2014 Aluminium Alloy

The effect of plastic deformation was revealed in particles cracking for the relevant materials. During plastic deformation, particles were cracked and/or particles were divided from interphase surface, which after that exhibited in the dimples. Detailed fractographical examinations are presented in /16/. The presented results showed the possibility of SPD. It is clear that the result of such grains refinement is first of all related to the improvement of mechanical properties; it also increases markedly the density of lattice defects in the solid solution of Al-based alloys and thus accelerates the precipitation process of strengthening particles during the subsequent ageing /9, 17/. Some other possibilities for an increase in strength Fig. 6: Interaction of the shearing bands and elongated properties, taking care of sufficient ductility, can be grains, TEM represented by a combined treatment by ECAP and subsequent processing conditions like solid solution The analogy found in the present study of the treatment, quenching and ageing /18-23/. This microstructure developments with the major features of combination led to an additional enhancement in hot rolling /13, 14/, suggests that the mechanisms of strength with acceptable ductility. microstructure formation are also similar in both cases. Finally, present results show that grain refinement The grain size diameter after rolling was 400 μηι. Next by ECAP can lead to a unique combination of strength microstructure refinement was obtained with SPD and ductility. The achieved mechanical properties by realized by ECAP. A single passage through the die ECAP and subsequent treatment can be useful for decreased the grain size diameter of 325 μπι; that means producing high strength and good ductility in 20% grain refinement was obtained. precipitation-hardened alloys. The fracture surfaces analyses of investigated materials showed dominant areas of transcrystalline ductile fracture /15/, Fig. 7. 4. CONCLUSION

Coupling the experimental results obtained and the literature analysis it is possible to the achieved following conclusions: 1. Hardness results showed that the stress increased with increasing strain conditions due to severe plastic deformation via ECAP. ECAPed samples achieved the 83 % of hardness growth in comparison to the as-rolled value. 2. Tensile test results showed that severe plastic deformation can be a solution for achieving attractive mechanical properties. ECAP increased the strength value up to 593 MPa from 381 MPa and 394 MPa respectively, if compared to the as-rolled Fig. 7: The typical fracture surfaces of investigated and quenched alloy; and values of yield strength of materials with dominant transcrystalline ductile 511 MPa from 235 MPa and 157 MPa respectively, fracture

206 Bidulska et al. High Temperature Materials and Processes

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