Uday Chakkingal

Analysis of forming loads, microstructure development and mechanical property evolution during equal channel angular extrusion of a commercial grade aluminum alloy

Abstract The process of equal channel angular extrusion (ECAE) is being widely investigated because of its potential for producing ultrafine grained structures. In the present study, a commercial grade aluminum alloy (AA 6063) was subjected to ECAE for three passes using two different processing routes: Route A where the specimen orientation is kept constant between passes; Route B where the specimen is rotated by 180° about its longitudinal axis between passes. Each extrusion pass imparts an equivalent true strain of 0.67 to the specimen. Analysis of the force–stroke diagram was carried out and compared with theoretical predictions. The mechanical properties and microstructure were investigated. In order to facilitate comparison of these parameters with those of conventional forming techniques, conventional cold extrusion studies were also carried out using an extrusion die that imparts an equivalent true strain of 0.65 to the material. The calculated extrusion loads were in the same range as the values experimentally observed for the first pass. The extrusion pressures increased slightly for processing by Route A but decreased for Route B with increasing number of passes. By processing through Route A, the tensile strength increased and elongation to failure (%) decreased as expected, but the elongation to failure (%) increased from the second to third pass when processed through Route B. After conventional cold extrusion, the mechanical properties were observed to be more or less similar but much higher extrusion loads were required. This implies that the material can be subjected to the same amount of plastic deformation by ECAE but with lower press loads to refine grain size and improve strength.

Friction stir processing of magnesium-nanohydroxyapatite composites with controlled in vitro degradation behavior.

Nano-hydroxyapatite (nHA) reinforced magnesium composite (Mg-nHA) was fabricated by friction stir processing (FSP). The effect of smaller grain size and the presence of nHA particles on controlling the degradation of magnesium were investigated. Grain refinement from 1500μm to ≈3.5μm was observed after FSP. In vitro bioactivity studies by immersing the samples in supersaturated simulated body fluid (SBF 5×) indicate that the increased hydrophilicity and pronounced biomineralization are due to grain refinement and the presence of nHA in the composite respectively. Electrochemical test to assess the corrosion behavior also clearly showed the improved corrosion resistance due to grain refinement and enhanced biomineralization. Using MTT colorimetric assay, cytotoxicity study of the samples with rat skeletal muscle (L6) cells indicate marginal increase in cell viability of the FSP-Mg-nHA sample. The composite also showed good cell adhesion.

Role of biomineralization on the degradation of fine grained AZ31 magnesium alloy processed by groove pressing

Groove pressing (GP) has been successfully adopted to achieve fine grain size up to 7 μm in AZ31 magnesium alloy with an initial grain size of 55 μm. The effect of microstructural evolution and surface features on wettability, corrosion resistance, bioactivity and cell adhesion were investigated with an emphasis to study the influence of deposited phases when the samples were immersed in simulated body fluid (SBF 5×). The role of microstructure was also evaluated without any surface treatments or coatings on the material. GPed samples exhibit improved hydrophilicity compared to the annealed sample. After immersion in SBF, specimens were characterized using scanning electron microscopy (SEM), energy dispersive X-ray (EDAX) analysis and X-ray diffraction (XRD) methods. More amount of white precipitates composed of hydroxyapatite and magnesium phosphate along with magnesium hydroxide was observed on the surfaces of groove pressed specimens as compared to the annealed specimens with an increase in immersion time in SBF. Corrosion behavior of the samples estimated using potentiodynamic polarization curves indicate good corrosion resistance for GPed samples before and after immersion in SBF. The MTT assay using rat skeletal muscle (L6) cells revealed that both the processed and unprocessed samples are nontoxic and cell adhesion was promising for GPed sample.

Processing and mechanical behavior of lamellar structured degradable magnesium-hydroxyapatite implants.

Multilayered (laminated) composites exhibit tunable mechanical behavior compared to bulk materials due to the presence of more interfaces and therefore magnesium based composites are gaining wide popularity as biodegradable materials targeted for temporary implant applications. The objective of the present work is to fabricate magnesium based lamellar metal matrix composites (MMCs) for degradable implant applications. Nano-hydroxyapatite (HA) powder was selected as the secondary phase and lamellar structured magnesium-nano-hydroxyapatite (Mg-HA) composites of 8, 10 and 15wt% HA were fabricated by ball milling and spark plasma sintering. It was found that HA particles were coated on the Mg flakes after 20h of ball milling carried out using tungsten carbide (WC) as the milling media. Spark plasma sintering of the milled powders resulted in the formation of lamellar structure of Mg with the presence of HA and magnesium oxide (MgO) at the inter-lamellar sites of the composites. Phase analysis of the milled powder by an X-ray diffraction (XRD) method confirms the presence of HA and MgO along with Mg after sintering. Corrosion behavior of the composites investigated by potentiodynamic polarization tests shows a reduction in the inter-lamellar corrosion with increase in HA content and the best corrosion resistance is found for the Mg-10% HA composite. This composite also exhibits maximum Vickers hardness. Young׳s modulus and fracture toughness measured by nano-indentation method were higher for the Mg-8% HA composite. The results thus suggest that lamellar structured Mg composites with 8% and 10% HA show promise for temporary degradable orthopedic implant applications because of their improved corrosion resistance and superior mechanical properties.

In vitro and in vivo studies of biodegradable fine grained AZ31 magnesium alloy produced by equal channel angular pressing.

The objective of the present work is to investigate the role of different grain sizes produced by equal channel angular pressing (ECAP) on the degradation behavior of magnesium alloy using in vitro and in vivo studies. Commercially available AZ31 magnesium alloy was selected and processed by ECAP at 300°C for up to four passes using route Bc. Grain refinement from a starting size of 46μm to a grain size distribution of 1-5μm was successfully achieved after the 4th pass. Wettability of ECAPed samples assessed by contact angle measurements was found to increase due to the fine grain structure. In vitro degradation and bioactivity of the samples studied by immersing in super saturated simulated body fluid (SBF 5×) showed rapid mineralization within 24h due to the increased wettability in fine grained AZ31 Mg alloy. Corrosion behavior of the samples assessed by weight loss and electrochemical tests conducted in SBF 5× clearly showed the prominent role of enhanced mineral deposition on ECAPed AZ31 Mg in controlling the abnormal degradation. Cytotoxicity studies by MTT colorimetric assay showed that all the samples are viable. Additionally, cell adhesion was excellent for ECAPed samples particularly for the 3rd and 4th pass samples. In vivo experiments conducted using New Zealand White rabbits clearly showed lower degradation rate for ECAPed sample compared with annealed AZ31 Mg alloy and all the samples showed biocompatibility and no health abnormalities were noticed in the animals after 60days of in vivo studies. These results suggest that the grain size plays an important role in degradation management of magnesium alloys and ECAP technique can be adopted to achieve fine grain structures for developing degradable magnesium alloys for biomedical applications.

Equal Channel Angular Extrusion of Tubular Aluminum Alloy Specimens—Analysis of Extrusion Pressures and Mechanical Properties

The Equal Channel Angular Extrusion (ECAE) process is a promising technique for imparting large plastic deformation to materials without a resultant decrease in cross-sectional area. The die consists of two channels of equal cross section intersecting at an angle; the workpiece is placed in one channel and extruded into the other using a punch. In the present study, the suitability of this technique for processing of tubular specimen geometries has been investigated. Tubular specimens of an aluminum alloy were extruded to three passes through two processing routes using sand as a mandrel. The pressures required for extrusion were measured, and the mechanical properties of the extruded material were evaluated. The low extrusion pressures during ECAE of tubular specimens are due to the movement of the mandrel (sand) along with the specimen (drag friction acts in the same direction as the main punch force). On processing to three passes of ECAE (by inducing a strain of 0.9), the tensile strength, yield strength, and hardness are improved, and elongation to failure (percent) decreased as expected. The process requires low forming loads while ensuring retention of specimen shape. It is also possible to impart further deformation to the specimen using the same die. It is concluded that ECAE is a promising technique for improving properties of tubular specimens.

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.

Microstructure and mechanical properties of commercial purity copper resulting from repeated groove pressing followed by cold rolling

Groove pressing (GP) is a severe plastic deformation technique for producing ultra fine grain sized microstructures in metals and alloys. In the present study, groove pressing and a two-step process of groove pressing followed by cold rolling was used to investigate the potential of these processes to produce ultra fine grained copper with significantly enhanced strength. Mechanical and microstructure properties were evaluated after groove pressing and after groove pressing followed by cold rolling. The advantages conferred by groove pressing prior to cold rolling on producing copper with enhanced properties has been investigated.

Making ceramic- metal composite material by friction stir processing

An innovative method to add ceramic particles in the metal matrix to make ceramic metal matrix composite was experimented and proved with alumina powder as particles and AE42 magnesium alloy as matrix. The alloy was subjected to friction stir processing and alumina particles were added through the processing tool. AE42 magnesium alloy has primary α-Mg phase of 100-150 micron grain size and secondary phase of 10-50 micron size as precipitates. Al2RE, Al11RE3 and Al17Mg12 are main secondary phases in the form of precipitates. Alumina powder was selected with average particles size of 5 micron. For processing parameters of 300-400 rpm tool speed, 15-20 mm/minute traverse speed and a threaded pin geometry; composites with fine distribution of second phase precipitates and alumina particles in the matrix were observed. Mechanical and microstructural characterization revealed uniform properties in longitudinal and transverse directions. Composite material has superior mechanical properties than the magnesium alloy. Distribution of particles was up to the length of tool pin. Tool pin geometry, feed rate and volume percentage of alumina particles, processing speed and tool rpm on the effect of mechanical and micro-structural properties were analyzed in detail.

Influence of Groove Pressing Process on the Drawability (R Value) of Aluminium Alloy AA 5052 Sheets

Aluminium alloy sheets have poor drawability compared to steel sheets as indicated by the values of the plastic strain ratio or the R value. Because of the textures developed during commercial annealing and cold rolling processes, the R value for aluminium alloys is typically less than 1. Since the R value is heavily influenced by the crystallographic texture in the sheet, processes that develop a favourable texture can be utilised to improve the R value. In this study, a severe plastic deformation process called groove pressing has been used to repeatedly deform sheet specimens of aluminium alloy AA 5052. The R values of groove pressed specimens were experimentally determined. X-ray diffraction scans of the groove pressed specimens were carried out to measure the relative intensities of (111) and (002) peaks in the pattern. The largest increase in the R value was for specimens cut at 90° to the rolling direction and groove pressed to four passes. XRD data indicate that the groove pressing process is capable of introducing a favourable shear texture.