Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2009"

Laser aided direct metal deposition of Inconel 625 superalloy: Microstructural evolution and thermal stability

Abstract: Direct metal deposition technology is an emerging laser aided manufacturing technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process that can be used to manufacture near net shape components from their CAD files. In the present study, direct metal deposition technology was successfully used to fabricate a series of samples of the Ni-based superalloy Inconel 625. A high power CO2 laser was used to create a molten pool on the Inconel 625 substrate into which an Inconel 625 powder stream was delivered to create a 3D object. The structure and properties of the deposits were investigated using optical and scanning electron microscopy, X-ray diffraction and microhardness test. The microstructure has been found to be columnar dendritic in nature, which grew epitaxially from the substrate. The thermal stability of the dendritic morphology was investigated in the temperature range 800–1200 °C. These studies demonstrate that Inconel 625 is an attractive material for laser deposition as all samples produced in this study are free from relevant defects such as cracks, bonding error and porosity.

Fabrication and properties of dispersed carbon nanotube–aluminum composites

Abstract: Powder metallurgy techniques have emerged as promising routes for the fabrication of carbon nanotube (CNT) reinforced metal matrix composites. In this work, planetary ball milling was used to disperse 2 wt% MWCNT in aluminum (Al) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, which may have implications on the final properties of the composite. Both un-annealed and annealed Al–2 wt% CNT composites were investigated. It was found that, ball-milled and extruded (un-annealed) samples of Al–2 wt% CNT demonstrated high notch-sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500 °C for 10 h prior to testing, exhibited more ductile behavior and no notch sensitivity. Under the present investigated processing conditions, ball milling for 3 h followed by hot extrusion and annealing at 500 °C resulted in enhancements of around 21% in tensile strength compared with pure aluminum with the same process history. The ball-milling conditions used were found to result in the creation of a nanostructure in all samples produced, as shown by XRD and TEM analysis. Such nanostructure was retained after prolonged exposures to temperatures up to 500 °C. The tensile testing fracture surfaces showed uniform dispersion and alignment of the CNTs in the aluminum matrix but also showed CNTs acting as nucleation sites for void formation during tensile testing. This has contributed to the observation of CNT pull-out due to the poor bond between the CNTs and the matrix.

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

Abstract: The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.

Development of high-performance A356/nano-Al2O3 composites

Abstract: In this study, a new method was used in stir casting to fabricate nano-Al2O3 particulate reinforced aluminum composites and avoid agglomeration and segregation of particles. Different volume fractions of nano-alumina particles were incorporated into the A356 aluminum alloy by a mechanical stirrer and then cylindrical specimens were cast and tested. The microstructural characterization of the composite samples showed uniform distribution of reinforcement, grain refinement of aluminum matrix, and presence of the minimal porosity. The effects of nano-Al2O3 particle content on the mechanical properties of the composites were investigated. Based on experiments, it was revealed that the presence of nano-Al2O3 reinforcement led to significant improvement in hardness, 0.2% yield strength, UTS and ductility. This combination of enhancement in UTS and ductility exhibited by nano-Al2O3 reinforced aluminum is due to uniform distribution of reinforcement and grain refinement of aluminum matrix.

Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel

Abstract: The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1523 K) and strain rates (10−3–102 s−1), were employed to develop constitutive equations in a Ti-modified austenitic stainless steel. The effects of temperature and strain rate on deformation behaviors were represented by Zener-Holloman parameter in an exponent type equation. The influence of strain was incorporated in the constitutive analysis by considering the effect of strain on material constants. The constitutive equation (considering the compensation of strain) could precisely predict the flow stress only at 0.1 and 1 s−1 strain rates. A modified constitutive equation (incorporating both the strain and strain rate compensation), on the other hand, could predict the flow stress throughout the entire temperatures and strain rates range except at 1123 K in 10 and 100 s−1. The breakdown of the constitutive equation at these processing conditions is possibly due to adiabatic temperature rise during high strain rate deformation.

Microstructures and mechanical properties of Al/Al2O3 surface nano-composite layer produced by friction stir processing

Abstract: In this study, a new processing technique, friction stir processing (FSP) was attempted to incorporate nano-sized Al 2 O 3 into 6082 aluminum alloy to form particulate composite surface layer. Samples were subjected to various numbers of FSP passes from one to four, with and without Al 2 O 3 powder. Microstructural observations were carried out by employing optical and scanning electron microscopy (SEM) of the cross sections both parallel and perpendicular to the tool traverse direction. Mechanical properties include microhardness and wear resistance, were evaluated in detail. The results show that the increasing in number of FSP passes causes a more uniform in distribution of nano-sized alumina particles. The microhardness of the surface improves by three times as compared to that of the as-received Al alloy. A significant improvement in wear resistance in the nano-composite surfaced Al is observed as compared to the as-received Al.

Enhanced properties of Mg-based nano-composites reinforced with Al2O3 nano-particles

Abstract: In this study, 0.5, 1 and 2 wt.% of alumina nano-particles were added to pure Mg and AZ31 magnesium alloy via a stir-casting method. A uniform distribution of the Al2O3 nano-particles with an average diameter of 100 nm, refined the grain structure of the cast materials and decreased the coefficient of thermal expansion (CTE), thus improving the dimensional stability of both pure magnesium and AZ31 alloy. The addition of 2 wt.% nano-Al2O3 particles showed great potential in the reduction of CTE from 27.9 to 25.9 × 10−6 K−1 in pure Mg, and from 26.4 to 25.2 × 10−6 K−1 in AZ31. Some of the cast samples were hot rolled and annealed to investigate the pinning effect of nano-particles on the recrystallization and subsequent mechanical property behavior. Characterization of mechanical properties revealed that the presence of nano-particles significantly increased yield stress and tensile strength but decreased the ductility of both pure magnesium and AZ31. The yield stress and tensile strength both increased by 40 MPa in the Mg–2Al2O3 nano-composite, whereas this improvement was about 65 MPa for AZ31–2Al2O3. The yield strength improvement was mostly due to the CTE mismatch between the matrix and the particles, and to a lesser extent to the Orowan and Hall-Petch strengthening mechanisms. The contribution of each of these mechanisms was used in a modified shear lag model to predict the total composite-strengthening achieved. Examination of fracture surfaces showed that the relatively ductile fracture of the monolithic materials changed to a more brittle mode due to the presence of nano-Al2O3 particles.

Effect of moisture absorption on the mechanical properties of randomly oriented natural fibers/polyester hybrid composite

Abstract: In this work, the variation of mechanical properties such as tensile, flexural, and impact strengths of roselle and sisal fibers hybrid polyester composite at dry and wet conditions were studied. The composites of roselle/sisal polyester-based hybrid composites with different weight% of fibers were prepared. Roselle and sisal fibers at a ratio of 1:1 had been incorporated in unsaturated polyester resin at various fiber lengths. When the fiber content and length of the roselle and sisal fibers were increased, the tensile and flexural strength of the composite increased. When the samples were subjected to moisture environment, decrease in tensile and flexural strength was observed. The maximum percentage of strength reductions in tensile and flexural strength were observed for the composites having the fiber length of 150 mm and 30 wt% fiber content. For impact strength, it was with the composites of 20 wt% and 150 mm at wet conditions compared to dry conditions. The percentage of strength reductions increased with fiber content and length in wet conditions. A scatter in the impact strength values was identified on both the conditions. The moisture absorption characteristics of the natural fibers are very important to produce the natural fiber hybrid composite materials with the positive hybrid effect. The experimental results are compared with theoretical and empirical or statistical results and found to be in good agreement.

Electrical and mechanical properties of carbon nanotube reinforced copper nanocomposites fabricated by electroless deposition process

Abstract: Multiwalled carbon nanotube/copper (CNT/Cu) nanocomposite powders with different CNTs volume fractions were prepared by electroless Cu deposition on the CNTs. The CNTs underwent acid treatment, sensitization and electroless copper deposition on their surface respectively. The microstructure of the prepared CNT/Cu nanocomposites was investigated by SEM and HRTEM as well as by XRD analysis. Copper was deposited in a form of a layer on the CNTs surface. The CNT/Cu nanocomposite powders were sintered by spark plasma sintering. The microstructure of the sintered materials were investigated by SEM indicating that the CNTs were homogenous distributed in the copper matrix with good sinterability and porosity content lower than unity in case of 5 and 10 vol.% of CNT/Cu nanocomposites and 2.9 and 3.5% respectively for 15 and 20 vol.% CNT/Cu nanocomposites. The electrical conductivity, hardness and the tensile properties were measured for evaluating the sintered CNT/Cu nanocomposites. The electrical conductivity decreased by increasing CNTs volume fraction in copper matrix, but the hardness was increased by increasing CNTs volume fraction. The Young's modulus was increased and the elongation was decreased by increasing the volume fraction of CNTs in copper matrix. In addition, the yield strength of the sintered materials was increased by increasing CNTs volume fraction except in case of 20 vol.% CNT/Cu composite the material was fractured before yielding.

Mechanical properties of basalt fiber reinforced geopolymeric concrete under impact loading

Abstract: Impact mechanical properties of basalt fiber reinforced geopolymeric concrete (BFRGC), including dynamic compressive strength, deformation and energy absorption capacity, were studied using a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. For the valid SHPB tests on BFRGC specimens, the improved pulse shaping techniques were proposed to obtain dynamic stress equilibrium and nearly constant strain rate loading over most of the test durations. Impact properties of BFRGC exhibit strong strain rate dependency, and increase approximately linearly with the strain rate. The addition of basalt fiber can significantly improve deformation and energy absorption capacities of geopolymeric concrete (GC), while there is no notable improvement in dynamic compressive strength. In addition, the optimum volume fraction of basalt fiber was presented for BFRGC.

Effect of martensite distribution on damage behaviour in DP600 dual phase steels

Abstract: The effect of martensite morphology and distribution in a ferrite matrix on the mechanical properties and the damage accumulation in uniaxial tension was investigated in two different automotive-grade dual phase DP600 steels. The two sheet steels had roughly 20% volume fraction of martensite but dissimilar chemical composition. A detailed analysis of microstructure and damage accumulation has been conducted as a function of strain. SEM analysis revealed that voids nucleation occurs by martensite cracking, separation of adjacent martensite regions, or by decohesion at the ferrite/martensite interface. Martensite morphology and distribution had a significant influence in the accumulation of damage. The steel with a more uniform distribution of martensite showed a slower rate of damage growth and a continuous void nucleation during the deformation process, which resulted in a higher void density before fracture. On the other hand, the steel with a centre-line of martensite through the sheet thickness exhibited accelerated void growth and catastrophic coalescence in the transverse orientation to the applied load.

Designing the energy absorption capacity of functionally graded foam materials

Abstract: In this paper, a functionally graded foam model is proposed in order to improve the energy absorption characteristics offered by uniform foams. In this novel model, the characteristics of the foam (e.g., density) are varied through the thickness according to various gradient functions. The energy absorption ability of the novel foam is explored by performing finite element simulations of physical impact tests on flat specimens of the functionally graded foam materials. Energy absorbing capacity with respect to parameters including gradient functions, density difference, average density, and impact energy, is explored in detail. It is illustrated that the functionally graded foam is superior in energy absorption to the uniform foam and that convex gradients perform better than concave gradients. The performance of such foams can be improved more if the density difference is enlarged. These findings provide valuable suggestions in the design of high performance energy absorption polymeric foams.

The effect of intercritical heat treatment temperature on the tensile properties and work hardening behavior of ferrite–martensite dual phase steel sheets

Abstract: This paper aims to investigate tensile properties and work hardening behavior of dual phase (DP) steels A series of DP steels containing ferrite and martensite with different volume fractions of martensite ( V m ) were produced by intercritical heat treatment Microstructural investigations, hardness test and tensile test were carried out Hardness, yield strength, ultimate tensile strength, ductility and fracture energy were correlated to martensite volume fraction The experimental results showed that dual phase steels with an equal amount of ferrite and martensite have excellent mechanical properties in terms of tensile strength, ductility and fracture energy A further increase in V m was found to decrease tensile strengths and ductility The increasing and then decreasing trend in tensile strength is in contrast to the law of mixture These unusual behaviors are discussed and explained Work hardening behavior was analyzed in terms of Holloman analysis Results showed that in DP steels with less than 50% V m , the work hardening took place in one stage and the work hardening exponent increased with increasing V m By increasing the volume fraction of martensite ( V m > 50%) two work hardening stages were observed in the Hollomon analysis

Microstructure and mechanical properties of CoCrFeNiTiAlx high-entropy alloys

Abstract: CoCrFeNiTiAl x ( x values in molar ratio, x = 0, 0.5, 1.0, 1.5 and 2.0) high-entropy alloys were prepared using a vacuum arc melting method. The effects of Al addition on microstructure and mechanical properties were investigated. The results show that only a face-centered cubic (FCC) crystal structure phase is observed in the CoCrFeNiTi alloy. The phase composition transforms to stabilized body-centered cubic (BCC) structure phases and typically cast dendrite structure appears when Al is added. The dendrite region is rich in Co, Ni, Ti and Al elements while the interdendrite region is rich in Fe and Cr elements. Subgrains and nanosized precipitates are observed in the as-cast CoCrFeNiTiAl alloy. These CoCrFeNiTiAl x high-entropy alloys exhibit excellent room-temperature mechanical properties. For CoCrFeNiTiAl 1.0 alloy, the compressive strength and elastic modulus reach as high as 2.28 GPa and 147.6 GPa, respectively. High density of dimple-like structure is observed from the fracture surfaces of the Al 0 alloy, while alloys with Al addition show typical cleavage fractures with river-like patterns and cleavage steps.

Aging effects on the microstructure and creep behavior of Inconel 718 superalloy

Abstract: Standard heat treatment (HT1) for Inconel 718 superalloy is solutionizing at 1095 °C, 1 h/AC, then aging at 955 °C, 1 h/AC + 720 °C, 8 h/FC 57 K/h to 620 °C, 8 h/AC. In order to study the aging effects of the δ phase, two more conditions HT2 (no aging condition 955 °C) and HT3 (955 °C, 3.5 h/AC) were studied in this research. Lever arm creep tests were performed at 650 °C under constant stress 625 MPa. Since HT2 produces no δ phase, the stress rupture life, creep elongation to failure and steady state creep rate of HT2 are largest among these three aging conditions. However, increasing the 955 °C aging time, the stress rupture life, creep elongation and steady state creep rate raise slightly as compared to HT1, because platelet δ phase is more uniformly nucleated and more direction oriented at grain boundaries. Fractographs show ductile fracture patterns mostly and, small portion of inter-granular fracture in the HT2 specimens. Generally only inter-granular fracture is observed in the other two cases of HT1 and HT3. Besides twinning and dislocation mechanisms, grain boundary sliding is also activated, so that creep elongation to failure of HT2 specimens could reach 5.6%, whereas 1% for the other two schemes.

Synthesis of multi-walled CNT reinforced aluminium alloy composite via friction stir processing

Abstract: Friction stir processing is used to produce an aluminium alloy reinforced with multi-walled carbon nanotubes. Microscopy by SEM and TEM indicates that the nanotubes are embedded into Al-alloy matrix produced in the stir zone, and their multi-walled microstructure survived the thermo-mechanical conditions imposed during processing. Increasing the tool rotation speed from 1500 and 2500 rpm and increasing the tool shoulder penetration depth improved homogeneity of nanotubes in the Al-alloy matrix, however a fully uniform distribution could not be achieved when regularly tangled nanotubes were used.

Age-hardening response of Mg-0.3 at.%Ca alloys with different Zn contents

Abstract: We have investigated the age-hardening responses and corresponding microstructures of Mg–0.3Ca– x Zn ( x = 0.0, 0.1, 0.3, 0.6, 1.0, 1.6 at.%) alloys by hardness test, transmission electron microscopy (TEM), and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Zn additions up to x = 0.6 lead to enhanced age-hardening responses with the highest peak hardness of HV = 69 for x = 0.6. Further addition of Zn degraded the age-hardening responses. HAADF-STEM images revealed that the finely dispersed monolayer G.P. zones with internal ordered structure are the major contributor to the age-hardening. Excess addition of Zn resulted in the formation of Ca 2 Mg 6 Zn 3 precipitates suppressing the formation of the ordered G.P. zones.

Investigation of the effect of process parameters on the formation and characteristics of recast layer in wire-EDM of Inconel 718

Abstract: Inconel 718 is a high nickel content superalloy possessing high strength at elevated temperatures and resistance to oxidation and corrosion. The non-traditional manufacturing process of wire-electrical discharge machining (EDM) possesses many advantages over traditional machining during the manufacture of Inconel 718 parts. However, certain detrimental effects are also present and are due in large part to the formation of the recast layer. An experimental investigation was conducted to determine the main EDM parameters which contribute to recast layer formation in Inconel 718. It was found that average recast layer thickness increased primarily with energy per spark, peak discharge current, and current pulse duration. Over the range of parameters tested, the recast layer was observed to be between 5 and 9 μm in average thickness, although highly variable in nature. The recast material was found to possess in-plane tensile residual stresses, as well as lower hardness and elastic modulus than the bulk material.

Effect of particles on the formation of deformation twins in a magnesium-based alloy

Abstract: The deformation behaviour of the age hardenable alloy Mg–5%Zn after different precipitation treatments has been examined. It has been found that during compressive deformation, fine particles increase the number of twins that form, but reduce the size and total volume fraction of twins. Visco-plastic self-consistent modelling has been used to show that the presence of precipitates hardens the twin and prismatic slip systems more than the basal slip system. It is proposed that because the { 1 0 1 ¯ 2 } twin requires basal slip to accommodate the twinning shear, this twin type will always be hardened equal to, or more than, the basal slip system in response to precipitation.

Useful properties of twist extrusion

Abstract: We present an experimental study of the kinematics of twist extrusion (TE) and show that TE has the following properties: (i) as in equal-channel angular pressing (ECAP), the mode of deformation in twist extrusion is simple shear. Unlike in ECAP, there are two shear planes; one of them is perpendicular and the other is parallel to the specimen axis. (ii) The following processes are present during twist extrusion: vortex-like flow with large strain gradient, stretching and mixing of metal particles. We argue that, due to these properties, TE opens possibilities for investigating and forming new microstructures. It has already been successfully used to obtain ultrafine-grained microstructures with good properties in Al, Cu and Ti alloys.

Strain hardening behavior of a Fe―18Mn―0.6C―1.5Al TWIP steel

Abstract: The strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel was investigated through the modified Crussard–Jaoul (C–J) analysis and microstructural observations. The strain hardening rate obtained by modified C–J analysis was high up to the critical strain of 37% and then greatly decreased with further strain. The electron backscatter diffraction (EBSD) observation showed that the deformation twinning rate is greatly decreased beyond about 34% strain, indicating that the reduced strain hardening rate at the large strain region is attributed to the deceleration of deformation twinning rate. The volume fraction of twinned region was increased with tensile strain due to the increase in the number of deformation twins not to the lateral growth of each deformation twin.

Microstructural and mechanical characterization of Al–MWCNT composites produced by mechanical milling

Abstract: Novel Al-based nanocomposites reinforced with multi-walled carbon nanotubes were produced by mechanical milling. Next, pressure-less sintering at 823 K under vacuum and hot extrusion at 773 K were carried out. The interface between Al matrix and the multi-walled carbon nanotubes was examined using transmission electron microscopy. The values of yield strength (σy), maximum strength (σmax) and microhardness Vickers (HVN) of the composites were evaluated and reported as a function of carbon nanotubes content. The concentration of multi-walled carbon nanotubes has an important effect on the mechanical properties of the nanocomposite. Formation of aluminum carbide in the nanocomposites was observed. Possible strengthening mechanisms are presented and discussed.

Micro-compression testing: A critical discussion of experimental constraints

Abstract: Micro-compression testing is a promising technique for determining mechanical properties at small length scales since it has several benefits over nanoindentation. However, as for all new techniques, experimental constraints influencing the results of such a micro-mechanical test must be considered. Here we investigate constraints imposed by the sample geometry, the pile-up of dislocations at the sample top and base, and the lateral stiffness of the testing setup. Using a focused ion beam milling setup, single crystal Cu specimens with different geometries and crystal orientations were fabricated. Tapered samples served to investigate the influence of strain gradients, while stiff sample top coatings and undeformable substrates depict the influence of dislocation pile-ups at these interfaces. The lateral system stiffness was reduced by placing specimens on top of needles. Samples were loaded using an in situ indenter in a scanning electron microscope in load controlled or displacement controlled mode. The observed differences in the mechanical response with respect to the experimental imposed constraints are discussed and lead to the conclusion that controlling the lateral system stiffness is the most important point.

Microstructure and mechanical properties of Ni–P coated Si3N4 reinforced Al6061 composites

Abstract: Silicon nitride is currently a popular choice as reinforcement to develop light alloy composites owing to its high hot hardness and excellent wear and corrosion resistance. However, meager information is available as regards the development of aluminum alloy–silicon nitride composites by stir cast technique. This route of processing metal matrix composites (MMCs) poses challenges in particular, poor wettability of the ceramic reinforcement in the matrix alloy. To overcome this problem, researchers are currently focusing on use of metallic coated ceramic reinforcement. In the light of the above, this paper discusses the development of Ni–P coated silicon nitride reinforced Al6061 composites by stir cast method. XRD, metallographic, EDAX studies, microhardness, and tensile strength tests of the developed composites have been carried out. A maximum of 10 wt% of Ni–P coated silicon nitride has been dispersed uniformly in the matrix alloy. A drastic improvement in both microhardness and tensile strength is observed.

Microstructural evolution in pure aluminum processed by high-pressure torsion

Abstract: Pure Al (99.99%) was processed using high-pressure torsion (HPT), and Vickers microhardness was measured along the diameter of the disk samples. When all hardness values were plotted as a function of equivalent strain, they fell on a single line having three distinctive regions. The hardness increases with the strain in the first region and, after taking a maximum at an equivalent strain of ∼2, the hardness decreases with further straining in the second region. The third region appears at an equivalent strain of ∼6 or higher as a steady state where the hardness remains unchanged. Electron backscatter diffraction analysis and transmission electron microscopy were conducted in the corresponding regions and microstructural evolution with straining was examined. Grain refining mechanism using HPT was discussed based on the change in the hardness and microstructures.

Precipitation hardening of high-strength low-alloy steels by nanometer-sized carbides

Abstract: The effects of Ti, Ti–Mo, and Ti–Nb microalloy additions on the precipitation strengthening in three experimental high-strength low-alloy steels have been investigated. The objective of this work was to study the carbide precipitation under the conditions of continuous cooling and interrupted cooling. It was found that titanium molybdenum complex carbide, (Ti, Mo)C, can strongly maintain nanometer-scaled sizes and has the largest contribution to the hardness as compared to titanium carbide, TiC, and titanium niobium complex carbide, (Ti, Nb)C. The result emphasizes that (Ti, Mo)C particles possess an excellent behavior of thermal stability.

Microstructural evolution in high purity aluminum processed by ECAP

Abstract: High purity (9999%) aluminum was processed by equal-channel angular pressing (ECAP) through 1–12 passes and examined using orientation imaging microscopy The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to an essentially equiaxed array of ultrafine grains and from 4 to 12 passes there is no measurable change in the average grain size and grain aspect ratio The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 12 passes Anomalously large grains were visible in the central region of the billet pressed through 12 passes due to dynamic recovery and grain growth The results suggest optimum processing is achieved by pressing through 4–8 passes

Processing and properties of porous titanium using space holder technique

Abstract: To satisfy the mechanical requirement of porous bone substitutes, the porous Ti with the porosity in the range of 55–75% was fabricated using the space-holder sintering process. The pore size is in the range of 200–500 μm, and the mean value is 410 μm. The mechanical properties were investigated by the compressive test. Results show that the plateau stress and Young's modulus are in the range of 10–35 MPa and 3–6.4 GPa, respectively. The relationship between the mechanical properties and the relative density of porous Ti is found to obey a power law relation. The strength of the porous Ti is mainly affected by the density. The typical rupture section of compressed samples has the V-shape.

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Abstract: Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary { 1 0 1 ¯ 2 } tensile twins and { 1 0 1 ¯ 1 } compression twins, and then { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg17Al12 phases at grain boundaries occurs together with the precipitation of discontinuous Mg17Al12 phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.

The arc characteristics and metal transfer behaviour of cold metal transfer and its use in joining aluminium to zinc-coated steel

Abstract: Cold metal transfer (CMT) is a modified metal inert gas welding process based on short-circuiting the transfer process, characterised by low heat input and no-spatter welding. The arc characteristics and its droplet transfer process have been studied by high-speed video photography. The process was used to join aluminium to zinc-coated steel. The results shows that no-spatter welding and low heat input during the welding process can be realized by CMT, and a dissimilar metal joint with good performance can be obtained by the CMT process.