MECHANICAL BEHAVIOUR OF ULTRAFINE GRAINED 5083 Al ALLOY PROCESSED BY CRYO ROLLING

Abstract

Aluminium 5083 alloy (Al-4.4Mg-0.7Mn-0.08Cr) is a medium strength, non-heat treatable alloy known for its excellent corrosion resistance. The unique mechanical properties of the alloys, greater design efficiency, better welding characteristics, good formability, excellent resistance to corrosion, and relatively high strength-to-weight ratio coupled with low cost makes it suitable for structural applications. Specifically, 5083 Al alloy is attractive for ships and marine applications due to the above mentioned properties. With the development of super plastic and quick plastic forming techniques, its application is extended to automobile and aerospace structural components. The extensive use of the alloy demands for the improvement in strength and formability of the alloy for ensuring long term performance of the alloy. Strengthening in polycrystalline materials is related to refining the grain structure to ultrafine regime (100 nm-1000 nm) through severe plastic deformation processing of the materials. Several plastic deformation techniques have been explored to develop homogeneous and equiaxed material with ultrafine grain sizes (UFG) in the bulk materials. It requires exceptionally high strain to induce high dislocation density for the formation of ultrafine grain structures in the alloy. In spite of enormous efforts over the decade, maintaining high strength with reasonable ductility is a challenging task. UFG materials possess superior properties such as high strength, excellent fatigue life, high toughness and low temperature super plasticity, improved corrosion resistance, enhanced electrical conductivity, wear characteristics, higher specific heat, enhanced thermal coefficient, and superior magnetic properties. The stringent demand of exceptional material properties requirements has led to a considerable interest in the development of ultrafine-grained/ nanomaterials worldwide. Non-heat treatable aluminium alloys found their application in the form of rolled products (thin sheets and foils). Therefore, there is increasing demand to develop rolling technologies to fabricate sheets ensuring its long time performance. It is identified that cold rolling can result in significant refinement in microstructure from its bulk alloys at very low temperature (Liquid nitrogen temperature) called as cryorolling. It is widely used to produce UFG structure in different metals and their alloys. However, it produces cellular type substructure consisting of boundaries with low angle misorientation. Cryorolling followed by warm rolling and asymmetric cryorolling is also gaining popularity to produce ultrafine grained aluminium alloys. The effect of cryorolling on microstructural evolution and mechanical behaviour of pure metals and heat treatable iv aluminium alloys (6xxx, 7xxx) has been reported in literature. On the other hand, there is scarcity of literature on non-heat treatable alloys, especially ultrafine grained 5xxx (5083 Al) alloys, where the prime mechanism of strengthening is solid solution strengthening and work hardening. Therefore, the present work has been envisaged to develop high strength 5083 Al alloy by grain refinement processed through cryorolling and cryorolling followed by warm rolling. The microstructure of deformed Al alloys was characterized by Optical microscopy, TEM, and FE-SEM/EBSD to substantiate their influence on mechanical properties. Optimization of annealing conditions for the deformed Al alloy so as to achieve best combination of strength and ductility with impact strength has been carried out in the present work. The objectives of present research work are: (i) To investigate the mechanical and microstructural behaviour of 5083 Al alloy deformed through cryorolling; (ii) To study microstructures and impact toughness behaviour of 5083 Al alloy processed by cryorolling and annealing; (iii) To study the effect of deformation temperature on mechanical properties of ultrafine grained Al-Mg alloys processed by rolling; (iv) To study the high cycle fatigue of ultrafine grained Al-Mg alloy processed by cryorolling and warm rolling; (v) To study the corrosion behaviour of ultrafine grained 5083 Al aluminium alloy developed by cryorolling. A detailed description of the present work is presented in five chapters and it is briefly discussed below. Chapter 1 gives an overview of aluminium alloys, their applications, worldwide production and consumption in different sectors. Explanation for non heat treatable aluminium alloys and their applications are also discussed. Chapter 2 presents the details of aluminium and their alloys, their classification, and different strengthening mechanisms. Development of ultrafine grained aluminium alloys through different techniques and their characteristics are included. Literature review pertaining to different severe plastic deformation techniques and cryorolling used for producing ultrafine grain microstructure in aluminium and other materials are also discussed. Based on literature review, the formulation of problem for the present research work has been made. The scope, objectives, and outline of the dissertation work are discussed in this chapter. Chapter 3 describes the selection of material and its sampling for different experimental technique adopted for development of ultrafine grained materials. The methodology of various v characterization techniques and mechanical testing employed in dissertation work has been discussed. Chapter 4 describes the results and discussion of different experimental investigations performed on 5083 Al alloy processed through cryorolling, and cryorolling followed by warm rolling. Section 4.1 investigates the effect of rolling at very low temperature on microstructure and mechanical behaviour of 5083 Al alloy. The alloy was subjected to rolling up to different strain levels at room temperature and immediate quenching at liquid nitrogen temperature. The effect of second phase particles in the alloy was investigated, especially on the evolution of microstructure and grain refinement in terms of pinning and driving forces for recrystallization. It is observed that there is a significant improvement in yield strength (103%) and ultimate tensile strength (40.8%) with CR 90% sample. It is due to the effective suppression of dynamic recovery and accumulation of high dislocations density during low temperature deformation. A homogeneous ultrafine grained structure with an average grains size of 300 nm was achieved after short annealing treatment at 300 ºC for 6 min of CR 90 % thickness reduction sample. The volume fraction of high angle grain boundaries has increased after short annealing treatment due to formation of recrystallized grains, which is due to the combined effect of static recovery and recrystallization. After investigating microstructure and mechanical behaviour of cryorolled material, the energy absorption capability of cryorolled 5083 Al alloy and effect of subsequent annealing treatment on impact and tensile properties of 5083 Al alloy are reported in Section 4.2. The microstructure of the CR 5083 Al alloy and CR and annealed 5083 Al alloy were characterized. The fractography of impact test samples were characterized by FE-SEM to reveal the mode of failure in the CR 5083 Al alloy. The Charpy impact test results have shown that cryorolling significantly decreases impact toughness of the alloy (ST - 32 J to CR 30% - 10 J) and that further decreases with increasing % of cryorolling (CR 30% - 10 J to CR 50% - 7.5 J). The loss in impact energy after cryorolling is due to lack of plastic deformation in the sample due to increase in yield strength, and brittle fracture as observed from SEM micrographs. Annealing at 350 °C for 1 hr, CR 50% sample shows an average grain size of 14 μm, whereas in CR 30%, the average grain size is 25 μm. The difference in the size is due to more nuclei generated during 50% CR resulting in smaller size of the grains. A comparative study of impact toughness behaviour of starting solution treated material with 50% cryorolled sample after annealing shows that the loss in impact vi toughness due to cryorolling has been compensated by occurrence of recrystallization after annealing treatment. Section 4.3 describes the effect of rolling performed at room temperature, cryogenic temperature and cryorolling followed by warm rolling for establishing the optimum rolling condition. Rolling was carried out up to 90% reduction (~2.3 true strain) at room temperature, cryogenic temperature and warm rolled at different temperatures of 100, 145, 175, 200 °C on 50% cryorolled samples up to 90% reduction. On the basis of hardness values for different conditions, cryorolling followed by warm rolling at 175 °C was found to be optimum condition (HV-146) for further investigation and compared with cryorolled alloy alone (HV-127). Annealing was also performed to observe thermal stability of the material. The significant improvement in mechanical properties (UTS-539 MPa, YTS-522 MPa, and ductility 6.8 %) was observed in the samples processed by cryorolling followed by warm rolling. The increase in strength and ductility may be attributed to formation of fine precipitates and dynamic recovery effect. It was evident from microstructural characterization that cryorolling and cryorolling followed by warm rolling samples exhibit high density of dislocations and substructures, high fraction of high angle grain boundaries. An attempt has also been made to investigate the effect of rolling condition on high cycle fatigue (HCF) in Section 4.4. The 5083 Al alloy was rolled for different thickness reductions of 50% and 85% at cryogenic (liquid nitrogen) temperature. The CR 50% sample was subjected to warm rolling upto 85% total reduction. Their hardness, tensile strength, and fatigue life using constant amplitude stress controlled fatigue were investigated. The cryorolled Al alloy after 85% thickness reduction exhibits high dislocation density due to suppression of dynamic recovery. Whereas, cryorolling followed by warm rolling material has shown ultrafine grain (UFG) structure associated with dynamic recovery during rolling. The deformed 5083 Al alloy was cyclically deformed over a range of stress amplitude at ambient temperature and observed the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy. Cryorolling followed by warm rolling sample has shown subgrain structure with high angle grain boundaries. It shows occurrence of both high angle and low angle grain boundaries, and very fine grains of 100- 200nm were observed, which contributes for HCF life of the material. Apart from investigating mechanical properties of the ultrafine grained materials, understanding the corrosion behaviour is essential for structural applications as discussed in Section 4.5. Since 5083 Al alloy is potential material for marine applications, the corrosion vii behaviour of UFG 5083 Al alloy in chloride environment is also investigated for cryorolling, cryorolling followed by warm rolling and compared with as received and solution treated material. It was observed from polarization test that cryorolling followed by warm rolling samples has shown corrosion current density (Icorr- 2.37 μA/cm2) compared to cryorolled alone (Icorr-2.05 μA/cm2) and ST material (Icorr-1.82 μA/cm2), but showing higher Epit - Ecorr value (119 mV), which indicates more resistance to pitting corrosion. Solution treated material has undergone pitting corrosion as observed in immersion test and SEM microscopy. Whereas, cryorolling and cryorolling followed by warm rolling (WR) materials has shown resistance to pitting corrosion. These samples have also shown high resistance to IGC, attributed to the combination of various effects, which reduced the presence of active precipitates at the grain boundaries. In cryorolling followed by warm rolling material processing temperature did not sensitize the material and formation of low angle grain boundaries which are less prone to intergranular corrosion, leads to increased IGC resistance. Chapter 5 presents the summary and conclusion of entire work presented in this thesis as well as future scope of the work in 5083 Al alloys