THERMOMECHANICAL AGEING OF 2014 ALUMINIUM ALLOY

Abstract

The 2014 Al•-alloy has a nominal composition: 4.40 Cu, 0.40 Mg, 0.80 Si, 0.80 Mn (Wt.pct.) and Fe as impurity element_ Although this alloy has its own importance in the aircraft industry due, to high hardness and high strength, its effective application is significantly hampered because of the poor secondary properties like fatigue strength and fracture toughness. The results reported in the literature on the influence of thermomechanical ageing (TMA) on fatigue properties of this alloy are not consistent, As far as the fracture toughness is concerned the presence of dispersoids/inclusions is reported to be detrimental. Combined influence of dispersoids and the microstructure developed as a result of TNA, however, has not been studied systematically todate. The present investigation has, there-fore, been undertaken to explore the best combination of thermal and mechanical treatments, which may eliminate or minimise the density of dispersoids and modify suitably the microstructure so as to cause on overall improvement of primary as well as secondary mechanical properties. The 2014 Al-alloy of commercial purity was prepared, homogenised at 50000 and hot rolled at 425 oC to desired cross sections. Various properties were measured in the foll-owing conditions: (i) As Quenched (AQ): solution treatment at 5000C, followed thing. (ii) Peak ageing (PA): artificial ageing of as quenched (AQ) alloy at 1600C to peak hardness level. (iii TMA I a : 25 pct. preageing (ageing for a period corresponding to a hardness which is 25 pct. of the peak hardness obtained in PA treatment), followed by 10 pct. warm rolling (i.e. 10 pct. reduction in thickness`, followed by further ageing upto peak hardness value. b : 25 pct. preageing, followed by 20 pct. warm rolling, followed by further ageing upto peak hardness value. (iv} TMA II a : 50 pct. preageing, followed by 10 pct. warm rolling, followed by further ageing upto peak hardness value. b 50 pct. preageing, followed by 20 pct. warm rolling, iollowed.by further ageing upto peak hardness value. In all the above treatments, warm rolling and ageing were carried out at 1600C. Tests were conducted to determine hardness, tensile properties, fatigue properties and fracture toughness after various treatments. Optical and transmission electron microscopic (TEM) studies were conducted to study the effect of various thezmomechanical ageing treatments on the structural changes and the density of dispersoids. Scanning electron microscopy (SEM) was used to study the character-istics of fracture surfaces after fatigue failure and the -V- role played by the dispersoids on the crack initiation during fatigue of samples after different TMA. treatments. TEM studies reveal the formation of e'and el during peakageing. Formation of e"is suppressed by the TMA treatments. optical microscopy and electron probe micro-analysis (EPMA) show two types of dispersoids (I) dark etching, irregular shaped Al12(Fe,Mn) Si and (ii) light etching nearly rounded Al4CuMg5Si4. The TMA treatments reduce the density of these dispersoids. A significant improvement in the various mechanical properties is observed after various thermomechanical ageing treatments. It is observed, for both 10 and 20 pct. deformations, that the peak hardness increases with pre-ageing, attains maximum at 50 pct. preageing. At 75 pct. preageing it again decreases. The effect of thermomechanical treatment on mechanical properties has, therefore, been studied only for 25 pct. preaged (TMA Ia and TMA Ib) and 50 pct. preaged (TMA IIa and TMA IIb) conditions in the present investigation. Among all the TMA treatments the maximum improvement in the hardness, tensile strength and fracture toughness has been achieved through TMA Iib treatment. It is also seen that the TMA IIb treatment not only imparts maximum improvorent but also provides maximum stability to tensile properties when exposed to higher wo-rking temperatures. The fatigue properties for as quenched, peak aged -vi- and thermomechanically aged samples have been studied for various stress levels ranging from 98 Nimm2 to 177 N/mm2. It is observed that at any stress level the alloy in the PA condition has lower fatigue life than in AQ condition. Further it is also observed that at any stress level the fatigue life is higher for thermomechanically aged specimen as compared to AQ and PA specimens. The best fatigue properties are observed in the alloy after TMA Ib treatment. It is also seen that for any value of fatigue life the corresponding endurance limit after TMA Ib treatment is constantly higher by a margin of 25 to 35 N/mm2 from the values obtained in the peak aged condition, throughout the range of investigation. It is observed that the IMA Ib and 'TIA lib treatments enhance the KIc value from peak aged corziition by about 30 pct. In case of V-notch charpy test for dynamic fracture toughness (K1 d', however, the TMA IIb treatment is more effective than TMA Ib. The Kid value obtained after TIM Ib is 1060.00 N/mm'/2, whereas after TMA Ilb treatment it is 1168.00 N/mm3/2. The effect of the amount of deformation on Kjc and 'Id values is almost similar; but in general the 'Id values are higher than KIc values after any TMA treat-ment. The ILIA treatments have been observed to affect subs-tantially the ageing characteristics and the resultant microscopic structure of the alloy. The maximum effect on A' nucleation appears to be in the TMA lb treatment which yields finest 9' needles having the longitudinal dimension 0 of approximately 400 A. The TMA IIb treatment yields 9 0 of intermediate fineness (600 A). It is also observed that all the TMA treatments suppress 1~ ppress the formation of 9 platelets.