MECHANICAL BEHAVIOUR OF ULTRAFINE GRAINED 5052 Al ALLOY PROCESSED BY SPD

Abstract

5052 Al alloy is a non-heat treatable alloy and it possesses good weldability and cold formability along with medium to high fatigue strength. It exhibits high corrosion resistance to sea water and industrial atmosphere. Hence, it finds applications in marine, automobile and aerospace industries. The extensive use of this alloy demands for improving both monotonic and fatigue strength further to ensure long service life of components used in structural applications. Owing to this reason, the present work has been focussed on processing Al 5052 alloy through various thermo mechanical treatments for producing ultrafine grains and correlating it with structure-property relations. The mechanical properties of the starting as well as processed samples were evaluated through tensile, hardness, three points bend test, high cycle fatigue test and fatigue crack growth rate test. These properties were correlated with their microstructures characterised through various techniques such as X-ray diffraction (XRD), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Electron back scattered diffraction (EBSD) and to ensure the formation of UFG structure. To investigate the combined effect of compressive as well as shear stress induced during cryogenic rolling process on grain refinement, a comparative study on tensile and fracture behaviour of 5052 Al alloy processed through cryorolling and cryo groove rolling has been carried out. The solution treated Al alloys were cryorolled (CR) and cryo groove rolled (CGR) to different true strains (0.69, 1.38 and 2.3). Furthermore, the post deformation annealing process of the deformed samples was performed between the temperature 180 and 300oC. The microstructure of the samples (ST, deformed and postannealed) was characterized by optical microscopy, X-ray diffraction, and TEM to substantiate the mechanisms of grain refinement and its influence on tensile and fracture properties. Fractography of the tensile as well as three point bending test samples was carried out using a scanning electron microscope (SEM) to reveal the fracture mode. The deformed samples (90% thickness reduction) exhibit significant improvement in strength (291 MPa) and hardness (110 HV) in cryorolled samples and 313 MPa and 122 HV in cryo groove rolled samples as compared ST condition (175 MPa and 55 HV), which is due to high dislocation density and grain size reduction. Post annealing of the deformed samples ii (true strain 2.3) led to improvement in its ductility as well as fracture toughness, with slight decrease in strength. The cryo groove rolled samples and post-annealed samples have shown better fracture toughness (142 kJ/m2) as compared to cryo rolled samples (29 kJ/m2) due to the relatively larger grain and dimples as observed from TEM and fractography studies. In order to investigate the effect of warm rolling on cryo deformed alloys, cryo groove rolling followed by warm rolling (CGW) of 5052 Al alloy has been carried out. The solution treated Al alloys were subjected to cryo groove rolling followed by warm rolling to a true strain of 2.3 at different temperatures ranging between 175 and 250oC. The CGR samples rolled at 175oC shows improved strength (328 MPa) and hardness (131 Hv) with 4.1% ductility, as compared to ST alloy (175 MPa, 55 HV). It was due to the formation of duplex microstructure consisting of both elongated as well as equiaxed subgrains. Apart from this, the formation of fine precipitates along with deformed or broken impurity phase particles is also responsible for enhancement in tensile strength in CGW samples. Further, the deformed (CGW) material was subjected to post annealing treatment between temperature 180 and 300oC for one hour to study its effect on mechanical behaviour of the alloy. The post deformation annealed samples composed of coarse grains along with ultrafine grains are responsible for increasing ductility (24.7%) and fracture toughness (115.8 kJ/m2). The microstructural characterization of the deformed material was performed through optical microscopy, X-ray diffraction, SEM fractography and TEM and it was correlated with the tensile and fracture characteristics of the alloy. To investigate fatigue properties of the UFG 5052 Al alloy processed through different cryo rolling methods, the solution treated 5052 Al-Mg alloys, which were deformed through different methods such as cryorolling (CR), cryo groove rolling (CGR) and cryo groove rolling followed by warm rolling (CGW), up to 75% thickness reduction, were subjected to mechanical testing such as hardness, tensile and high cycle fatigue (HCF) test at stress control mode. The CGW samples exhibit better ultimate tensile strength (321MPa) and fatigue strength (95 MPa) as compared to other conditions. The microstructure of the tested samples was characterized by optical microscopy, SEM fractography and TEM to understand the deformation behavior of deformed Al alloy. The improvement in fatigue life of CR and CGR samples is due to effective grain refinement, sub-grain formations, and high dislocation density observed in the heavily deformed iii samples at cryogenic condition as observed from SEM and TEM analysis. However, in case of CGW samples, formation of nano shear bands accommodates the applied strain during cyclic loading thereby facilitates dislocation accumulation along with sub-grain formations, leading to the high fatigue life. The deformed or broken impurity phase particles found in the deformed samples along with the precipitates that were formed during warm rolling also play a prominent role in enhancing the fatigue strength. These tiny particles hindered the dislocation movement by effectively pinning it at grain boundaries, thereby improving the resistance of crack propagation under cyclic load. To investigate the nature of fatigue crack growth in UFG 5052 Al alloy, the solution treated 5052 Al alloy was subjected to different cryogenic rolling methods such as CR, CGR and CGW. The fatigue crack growth studies of the deformed samples were conducted as per ASTM E647-08 standard. The CR, CGR and CGW processed samples exhibit threshold stress intensity value (ΔKth) of 4.88, 6.5 and 5.5 MPa m1/2, whereas ST sample possesses ΔKth of 3.75 MPa m1/2. The formation of UFG grains of size 125 nm to 200 nm as observed through the TEM images along with improved elastic strength of the cryo deformed samples are responsible for the improvement of ΔKth. The deformed or broken impurity phase particles present in the CGR and CGW samples which hinder the dislocation movement also play a prominent role in increasing elastic strength and thereby their ΔKth. The precipitates formed during warm rolling of CGW also play an important role in increasing the ΔKth by effectively pinning the dislocation movement. The ST samples with coarser grains have exhibited slow FCGR as the coarser grains in them are responsible for tortuous crack which delays the crack propagation rate in them. The fine striations along with bigger dimple surfaces observed through SEM at fractured surface corresponding to higher ΔK values of Paris regime depicts slow crack propagation, which in turn is responsible for improved stress intensity factor corresponding to catastrophic failure (ΔKc) in ST samples. On the other hand, the SEM micrographs at fractured surface corresponding to higher ΔK values of Paris regime in CGW samples exhibit bigger dimples as compared to other cryo deformed samples, revealing the improved ductility/fracture toughness, due to which their ΔKc (19.2 MPa m1/2) was observed to be better among them. The nano shear bands are high energy stored regions with large orientation gradient in sub grain and therefore favors sub grain growth during plastic strain. Hence, it is responsible for improved ΔKc in CGW samples than CR samples. iv The effect of combined severe plastic deformation (SPD) on fatigue and fracture behaviour of 5052 Al-Mg alloy has been investigated. The as received 5052 Al alloy was solution treated at temperature 540 °C for two hours and subjected to multi axial forging in liquid N2 temperature to a cumulative true strain of 4.2. Subsequently, these deformed samples were subjected to further deformation (compressive and shear) through cryo groove rolling followed by warm rolling process. The samples for fatigue test and three point bend test were prepared as per ASTM standards. The solution treated samples exhibited high cycle fatigue strength of 40 MPa, whereas it was found to be 105 MPa in case of combined deformed samples with a significant increase of 262 %. It was due to formation of mutually intersecting nano shear bands that are responsible for ultimate grain fragmentation resulting in ultrafine grains of size around 110 nm. The broken/deformed impurity phase particles along with fine precipitates that were formed during warm rolling have played a prominent role in increasing the fatigue strength of the samples by effectively hindering the dislocation movement. On the other hand, the fracture energy in terms of J of the combined deformed samples has decreased from 249 kJ/m2 to 74.1 kJ/m2, as crack propagation in case of ultrafine grained material is faster than coarse grained material. In case of post annealed samples, the high cycle fatigue strength of the material has decreased to 95 MPa, due to recovery and recrystallization phenomenon; whereas the fracture energy get increased to 220 kJ/m2, due to the formation of coarser recovered grains.