B. Ratna Sunil

Repetitive Corrugation and Straightening of Sheet Metals

Recently, severe plastic deformation (SPD) techniques have been gaining wide popularity in developing nano/ultrafine grained (UFG) structured materials for a wide variety of applications. Among SPD techniques, there are a few techniques that are specially used to process metallic sheets and plates. Repetitive corrugation and straightening (RCS) is one such promising technique, which can produce fine grained structures in metallic sheets or plates in bulk. The process was introduced to develop UFG metallic sheets and plates nearly a decade ago and is now gaining great interest in the material processing field. The aim of the present review is to give a comprehensive summary of the state-of-the-art of the process in developing fine grained structured sheets. Emphasis has been given to discuss different material systems processed by RCS. The mechanism behind the grain refinement during RCS, promising applications, and future perspectives in developing UFG structured sheets or plates by RCS are also discussed.

Nano and ultra fine grained metallic biomaterials by severe plastic deformation techniques

Metallic materials are widely studied for load-bearing applications such as orthopaedic implants. Titanium and its alloys find applications for load-bearing medical implants due to their biocompatibility, good corrosion resistance, high specific strength and good bioadhesion. However, the bioactivity of titanium which can be defined as the ability to form a hydroxyapatite (HA) layer, which is similar to the mineral phase of the bone, on its surface when in contact with the biological environment is poor. On the other hand, magnesium and its alloys are becoming the prime choice for degradable biomaterials targeted for temporary applications in cardiac and orthopaedic fields. However, controlling the degradation rate is the essential issue in developing magnesium-based biomaterials. Synthesis of nano/ultra fine grain materials to enhance the biofunctionalisation of orthopaedic implants is of considerable interest as cells live in a nano-featured environment consisting of a complex mixture of pores and fibres of the extracellular matrix. Recently severe plastic deformation (SPD) processes which can achieve considerable grain refinement, typically to the submicrometre or nanometre level, have gained significant attention in materials research. Therefore, using SPD processes to develop grain-refined titanium and magnesium-based materials for implant applications has become a promising strategy in developing new-generation medical materials. Particularly for titanium, nanostructuring results in improved mechanical properties and increased bioactivity. Whereas for magnesium, grain refinement results in controlled degradation due to higher biomineralisation with enhanced tissue response. The present review aims to provide a comprehensive summary of the progress achieved using SPD processes in developing nano/ultra fine grain structured titanium and magnesium for implant applications. Role of smaller grain size on enhancing bioproperties is also discussed including the challenges involved in processing to achieve the grain refinement up to nano/ultra fine grain level.

Different strategies of secondary phase incorporation into metallic sheets by friction stir processing in developing surface composites

Friction stir processing (FSP) is a solid-state processing method, which recently gained wide popularity to modify the microstructure of metallic surfaces and to produce surface composites. For the past decade, composites of different materials such as aluminum, copper, magnesium, titanium, and their alloys were successfully produced by FSP. The amount of secondary phase that is dispersed at the surface of the workpiece during FSP and the level of dispersion depend on many factors such as tool design, processing parameters, and type of material. Recently, the method of secondary phase incorporation into the surface metals was also considered as an important factor in developing surface composites by FSP. A few strategies such as groove filling, holes filling, sandwich method, direct method, and surface coating followed by FSP methods have been developed as promising ways of secondary phase incorporation into the surface of the materials during FSP. The aim of this review paper is to give a comprehensive summary of different methods developed to disperse the secondary phase into the surface of the workpiece during FSP to produce surface composites. The strategies have been explained, compared, and discussed to suggest an appropriate method based on the requirement to adopt in developing surface composites.

Machining characteristics of fine grained AZ91 Mg alloy processed by friction stir processing

Abstract AZ91 Mg alloy was considered and friction stir processing (FSP) was adopted to achieve grain refinement to investigate the effect of grain size and secondary phase on machining characteristics during drilling at various speeds and feeds. Super saturated AZ91 Mg alloy was obtained after FSP and the grain refinement was achieved from (166.5±8.7) µm to (21.7±13.5) µm. Surprisingly, hardness reduced for FSP AZ91 Mg alloy (88.95±6.1) compared with AZ91 alloy (108.2±15.6), which was attributed to the reduced secondary phase. However, the mean cutting force for FSP-treated (FSPed) AZ91 Mg alloy was marginally increased. The edge damage of the drilled holes was lower for FSPed AZ91 Mg alloy compared with unprocessed AZ91 Mg alloy. Hence, it can be understood that the grain refinement may slightly increase the cutting forces during drilling but better edge finishing can be achieved in machining of AZ91 Mg alloy.

Design and simulation of polymethyl methacrylate-titanium composite bone fixing plates using finite element analysis: Optimizing the composition to minimize the stress shielding effect

Stress shielding is a mechanical phenomenon usually found in load-bearing bone implants. Difference in mechanical properties between the natural bone and the artificial implant leads to stress shielding problem. In the present work, polymethyl methacrylate and commercial pure titanium were selected to design laminate and particulate composites. Optimum composition was theoretically obtained that exhibits mechanical properties close to that of natural human bone. Bone fixing plate was designed for femur bone using computer-aided design. Finite element analysis was adopted to analyze the stress distribution in the bone and implant under static load conditions. Fixing plate with three screws was modeled and simulated using finite element analysis to investigate the stress distribution. Simulation was also done considering 316 L stainless steel as fixing implant and compared with the present optimized composition. Laminate composite with 0.3 volume fraction of titanium has shown mechanical properties close to the bone compared with other combinations. The results have clearly shown that the von-Mises stress induced in the bone with polymethyl methacrylate-titanium laminate composite plates was increased compared with the bone implanted with 316 L steel. Interestingly, laminate composites exhibited higher stresses in the bone compared with particulate composites. From the present design and simulation, it is clearly demonstrated that the laminate composites of polymethyl methacrylate–30% titanium can be an optimum choice for load-bearing implant materials with reduced stress shielding effect.

Corrosion behavior of friction stir welded AZ31B Mg alloy - Al6063 alloy joint

AbstractIn the present work, AZ31B Mg alloy and Al6063 alloy-rolled sheets were successfully joined by friction stir welding. Microstructural studies revealed a sound joint with good mechanical mixing of both the alloys at the nugget zone. Corrosion performance of the joint was assessed by immersing in 3.5% NaCl solution for different intervals of time and the corrosion rate was calculated. The joint has undergone severe corrosion attack compared with both the base materials (AZ31B and Al6063 alloys). The predominant corrosion mechanism behind the high corrosion rate of the joint was found to be high galvanic corrosion. From the results, it can be suggested that the severe corrosion of dissimilar Mg–Al joints must be considered as a valid input while designing structures intended to work in corroding environment.Abstract In the present work, AZ31B Mg alloy and Al6063 alloy-rolled sheets were successfully joined by friction stir welding. Microstructural studies revealed a sound joint with good mechanical mixing of both the alloys at the nugget zone. Corrosion performance of the joint was assessed by immersing in 3.5% NaCl solution for different intervals of time and the corrosion rate was calculated. The joint has undergone severe corrosion attack compared with both the base materials (AZ31B and Al6063 alloys). The predominant corrosion mechanism behind the high corrosion rate of the joint was found to be high galvanic corrosion. From the results, it can be suggested that the severe corrosion of dissimilar Mg–Al joints must be considered as a valid input while designing structures intended to work in corroding environment.