S. Kumaran

Extrusion and precipitation hardening behavior of AZ91 magnesium alloy

Abstract The as-cast AZ91 magnesium alloy was extruded at 250 °C, 300 °C and 350 °C, respectively with extrusion ratio of 4:1 followed by precipitation hardening treatment (T6). The coarse and segregated Mg 17 Al 12 intermetallic precipitate in AZ91 magnesium alloy was refined and distributed uniformly in α -magnesium matrix by direct hot extrusion combined with precipitation hardening treatment. The dynamic recrystallization phenomenon, partial to full, was observed in this alloy with respect to the extrusion temperature. Ultimate tensile strength of this alloy is increased from 190 MPa (as cast) to 570 MPa after extrusion. The age hardening characteristics of AZ91 magnesium alloy is more dependent on the grain size. The peak hardnesses are obtained in the aging time of 35, 30 and 20 h for alloys extruded with ratio of 4:1 at three different temperatures. Uniform and fine precipitates of Mg 17 Al 12 in AZ91 Mg matrix were confirmed by scanning electron microscope (SEM).

Structure – property correlation of precipitation hardened AZ91 magnesium alloy

Abstract The microstructure and mechanical properties of AZ91 magnesium alloy with composition Mg – 8.8%Al – 0.78%Zn (wt-%) has been investigated to assess the possibility of improving the properties through precipitation hardening. Initially, both solutionising temperature and soaking time were optimised at 693 K and 8 h respectively. After solutionising, the AZ91 samples were subjected to artificial aging at 423 K, 448 K, and 473 K for a period of 12 to 120 h. The sample age hardened at 473 K attained peak hardness after a shorter time than the other two aging temperatures. Structural changes and corresponding mechanical properties were studied. As cast, solutionised and age hardened AZ91 samples were also subjected to corrosion studies to observe potentiodynamic polarisation behaviour. Fine precipitate formation throughout the matrix was observed following solutionising and then artificial aging. Microstructural modifications improved both tensile strength and elongation, however, the corrosion rate was not significantly influenced.

Nanocrystalline and amorphous structure formation in Ti-Al system during high energy ball milling

Abstract The aim of the present work was to study the structural evolution of Ti–48Al (at.-%) powder blend during mechanical alloying. Because milling parameters play a vital role in achieving the desired structure/phase, milling was carried out with optimised parameters. Both powder handling and milling operations were performed under high pure argon atmosphere to prevent oxidation of the powder blend. Mechanically alloyed powder blend was then characterised by scanning electron microscope (SEM), X-ray diffraction (XRD) and differential thermal analyzer (DTA). Mechanical alloying induced severe plastic deformation resulting in cold welding, powder particle refinement and narrowing of powder particle size distribution as is evidenced from SEM micrographs. XRD analysis indicated complete dissolution of aluminium in titanium at 20 h of milling and achieving nanostructure before amorphous phase formation. DTA scan indicated the disappearance of one of the exothermic peaks, and a gradual drop in crystallisation temperature with increasing milling time.

Fabrication of Al–Si/graphite composites and their structure–property correlation

In this study, mechanical and wear behaviors of aluminum silicon alloy and its composites reinforced with graphite (3 wt.%) particulates have been investigated. In order to improve the wettability of graphite with matrix alloy, graphite particulates were ball milled with Cu and Ni by weight percent (1% each) and preheated to 400°C and then incorporated into the molten metal. Tensile properties of composites were better than those of the matrix alloys. Wear rate of composites was lower than that of matrix alloys for given applied stress and varying sliding distance. The wear mechanism was understood from the scanning electron microscopic images of worn-out surfaces and wear debris.

Densification of Al 5083 Mechanically Alloyed Powder by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP), one of the most important methods in SPD, is used for the consolidation of mechanically alloyed Al 5083 powder. This paper mainly focuses on the densification of Al 5083 mechanically alloyed powder by ECAP with and without application of back pressure up to three passes with four different routes at room temperature. Aluminum can is used to encapsulate the powder. The particle size, crystallite size, microstructure and density were evaluated by scanning electron microscope and X-ray diffraction peak profile analysis. The crystallite size was measured by Williamson Hall analysis. Density and hardness were increased with increasing number of passes and upon sintering after ECAP. Good densification as well as good powder bonding was observed after three passes of ECAP.

Microstructural refinement and mechanical properties of direct extruded ZM21 magnesium alloys

Cast ZM21 magnesium alloys were subjected to symmetric extrusion at four different temperatures (200, 250, 300 and 350 °C) with three extrusion ratios of 4:1, 9:1 and 16:1, respectively. The effects of extrusion parameters such as temperature and extrusion ratio were studied by optical microscopy, X-ray diffraction (XRD) and tensile test. The optical micrographs exhibited various stages of recrystallization, i.e., partial to full recrystallization influencing mechanical properties to good extent. Higher extrusion temperature resulted in coarse grains, whereas finer grains were obtained at higher extrusion ratios. Ultimate tensile strength of this alloy was increased from 160 MPa to 316 MPa after extrusion at 250 °C with an extrusion ratio of 9:1.

Microstructure and mechanical properties of 2014 Al alloy processed by equal channel angular pressing (ECAP)

The present work aims to investigate the microstructural evolution and mechanical behaviour of 2014 Al alloy upon equal channel angular pressing (ECAP) at ambient temperature. ECAP was carried using a 90° die through route A up to five passes. Microstructural and mechanical properties were compared for three different conditions namely: 1) as received; 2) homogenised at 768 K for 1 hr; 3) homogenised at 768 K for 1 hr + aged at 468 K for 5 hr. The improved strength and hardness of ECAPed 2014 Al homogenised at 768 K for 1 hr + aged at 468 K for 5 hr alloy was due to the precipitation hardening, grain refining and higher dislocation density. It was found that the grain size gradually decreased with number of passes and the average grain size was about 0.252 μm after five ECAP passes. The hardness and tensile strength of 2014 Al alloy remarkably increased up to 165 HV and 468 MPa, respectively. Correlation between tensile strength and hardness for the processing route A was also reported.

Comparison study of fly ash reinforced AA6061 composites using press sinter extrusion and press extrusion approaches

Abstract This paper presents the preliminary results obtained in developing a powder metallurgy process involving an approach based on the use of press (compaction) and sintering together with subsequent extrusion on one hand and press and extrusion on the other hand. Two systems have been compared: an unreinforced AA6061 alloy (matrix) and AA6061-2 wt.% fly ash particulate composite. Mechanical mixing of the matrix powder, obtained from elemental powders, and particulate reinforcement was followed by compaction. Pressurisation was carried out at 345 MPa with zinc stearate as die wall lubricant. Some green compacts were extruded subsequently at 5–10 mm min−1 rate with a reduction ratio of 16:1 at 500°C. And some were sintered at 620°C for 4 h before extrusion. From XRD, SEM EDAX and mechanical testing studies it was observed that the press extruded samples were showing improvement in density, hardness, 0·2% proof stress and tensile strength over press sinter extruded samples.

Effect of Equal Channel Angular Pressing on Densification Behavior of Al 5083 Alloy Powder

In this present study Al-5083 alloy powders were prepared from elemental powders using high energy ball milling under optimized milling parameters. Various properties such as crystalline size, particle size and morphology have been studied using X-Ray diffraction analysis and Scanning Electron Microscopy. It was found that Al-5083 alloy was formed and nanocrystalline size particles were achieved. These nanocrystalline Al-5083 alloy powders were consolidated using equal channel angular pressing with and without application of back pressure. Physical and mechanical properties such as density and hardness are studied.

Microstructural Transformation of Titanium-Boron Carbide (B4C) Powder Mixture during Spark Plasma Sintering

Present study investigates the microstructural evolution and in-situ formation of Ti/(TiB+TiC) composite. The sintered compacts are having near theoretical density. The relative density of Ti/(TiB+TiC) composites decreases with increasing TiB and TiC content.The phase evolutions of TiB and TiC according to the in-situ reactions are analyzed by X-Ray diffraction technique (XRD). Optical microscope (OM) and Energy dispersive spectroscope (EDS) observations of the Ti/(TiB+TiC) composites shows the presence of TiC and TiB reinforcements as equiaxed and needle like structures respectively.