High strength Al–Al2O3p composites: Optimization of extrusion parameters

Composite aluminium alloys reinforced with Al₂O_3p particles have been produced by squeeze casting followed by hot extrusion and a precipitation hardening treatment. Good mechanical properties can be achieved, and in this paper we describe an optimization of the key processing parameters. The parameters investigated are the extrusion temperature, the extrusion rate and the extrusion ratio. The materials chosen are AA 2024 and AA 6061, each reinforced with 30 vol.% Al₂O₃ particles of diameter typically in the range from 0.15 to 0.3 μm. The extruded composites have been evaluated based on an investigation of their mechanical properties and microstructure, as well as on the surface quality of the extruded samples. The evaluation shows that material with good strength, though with limited ductility, can be reliably obtained using a production route of squeeze casting, followed by hot extrusion and a precipitation hardening treatment. For the extrusion step optimized processing parameters have been determined as: (i) extrusion temperature = 500 °C–560 °C; (ii) extrusion rate = 5 mm/s; (iii) extrusion ratio = 10:1.     

Microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion

The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.     

Effect of hot extrusion on wear properties of Al–15 wt.% Mg2Si in situ metal matrix composites

Al–15 wt.% Mg₂Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470 °C – varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500 °C for 5 h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg₂Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from 5.63 in the as-cast condition, to 0.47 at the extrusion ratio of 25. It was found that abrasion is the dominant wear mechanism in all extruded composites, whilst a combination of adhesion and delamination appears to be the governing mechanism for as-cast composites.     

Correlation between microstructure and tensile properties in powder metallurgy AZ91 alloys

The ultrafine-grained (0.3–1.3 μm) AZ91 alloys, which were fabricated by powder extrusion in the range of 200 to 350 °C and subsequent aging at 100 °C for 8 h, exhibit a remarkable yield stress of 360–478 MPa and moderate tensile elongations of 6–8%. A composite structure was developed after extrusion with uniform β (Mg₁₇Al₁₂) particles dispersed in magnesium matrix. The extrusion temperature has an indirect role on yield stress since partial dissolution of β particles induced by high extrusion temperature fails to retard grain growth. Moreover, the strength was further enhanced by the formation of nano-scale precipitates during artificial aging. The high strength could be attributed to a combination effect of grain refinement, particle reinforcement and precipitation hardening.     

Dry sliding tribological behavior and mechanical properties of Al2024–5 wt.%B4C nanocomposite produced by mechanical milling and hot extrusion

In this paper, tribological behavior and mechanical properties of nanostructured Al2024 alloy produced by mechanical milling and hot extrusion were investigated before and after adding B₄C particles. Mechanical milling was used to synthesize the nanostructured Al2024 in attrition mill under argon atmosphere up to 50 h. A similar process was used to produce Al2024–5 wt.%B₄C composite powder. The milled powders were formed by hot pressing and then were exposed to hot extrusion in 750 °C with extrusion ratio of 10:1. To study the microstructure of milled powders and hot extruded samples, optical microscopy, transmission electron microscopy and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) were used. The mechanical properties of samples were also compared together using tension, compression and hardness tests. The wear properties of samples were studied using pin-on-disk apparatus under a 20 N load. The results show that mechanical milling decreases the size of aluminum matrix grains to less than 100 nm. The results of mechanical and wear tests also indicate that mechanical milling and adding B₄C particles increase strength, hardness and wear resistance of Al2024 and decrease its ductility remarkably.     

Microstructure development and tensile mechanical properties of Mg–Zn–RE–Zr magnesium alloy

The Mg–5.3 wt.%Zn–1.13 wt.%Nd–0.51 wt.%La–0.28 wt.%Pr–0.79 wt.%Zr alloy prepared by direct chill casting is subjected to hot extrusion. The effects of extrusion ratio and temperature on microstructure and tensile mechanical properties have been studied. The results indicate coarse grains of as-cast alloys are refined with extrusion ratio increasing from 0 to 9. The eutectic constituents are elongated along extrusion direction. However, further increase of extrusion ratio has a little influence on grain refinement and the improvement of mechanical properties of the alloy. Dynamic recrystallisation is the main mechanism of grain refinement during hot extrusion. Raising extrusion temperature results in grain coarsening. Grain shape becomes more equiaxed-like with raising extrusion temperature. At the same time, mechanical properties decrease with the increase of extrusion temperature.     

Anisotropy of thermal conductivity and mechanical properties in Mg–5Zn–1Mn alloy

The microstructure, texture, thermal conductivity and mechanical properties of the as-extruded Mg–5Zn–1Mn (ZM51) magnesium alloy were investigated on specimens with the extrusion direction (ED), the transverse direction (TD) and the normal to the extrude plane (ND), respectively. The results indicated that the thermal conductivity of ZM51 alloy at room temperature is 125 (W/m K), almost twice as high as other conventional commercial magnesium alloys, such as Mg–Al series and Mg–RE series. The effect of texture on anisotropy of mechanical properties and thermal conductivity has been analyzed. The strong crystallographic texture typical of Mg alloys results in much higher yield strength and tensile strength (UTS) in the extrusion direction, but higher ductility and thermal conductivity in the transverse direction.     

Investigation of high-strength and superplastic Mg–Y–Gd–Zn alloy

The Mg–7Y–4Gd–1Zn (wt.%) alloy was prepared by hot extrusion technology, and the microstructure, tensile properties and superplastic behavior have been investigated. The extruded alloy possesses high tensile strength both at room temperature and 250 °C, and especially the yield strength can remain above 300 MPa at 250 °C. The outstanding microstructure, i.e. bent 18R long period stacking ordered (LPSO) strips and dynamic recrystallization (DRX) Mg grains containing fine lamellae with 14H LPSO or stacking fault structures, is responsible for the excellent mechanical properties, and it is considered that the integrated performance can be further improved by controlling the size of LPSO phase. The alloy shows the maximum elongation of 700% at 470 °C and 1.7 × 10⁻⁴ s⁻¹. The predominant superplastic mechanism is considered to be grain boundary sliding assisted by lattice diffusion. The fracture of superplastic deformation is related to the microstructure evolution, i.e. the disappearance of LPSO phase and the formation of cubic phase. Both high temperature and stress contribute to the phase transformation.     

An investigation to the hot deformation characteristics of AZ31 alloy through continuous cooling compression testing method

The design and control of thermomechanical processing (TMP) schedule are substantially facilitated by a thorough understanding of the dominant deformation mechanisms and phase transformations which occur in the alloy system under consideration. In this regards due to the effects of deformation history, the results of conventional methods such as dilatometry and thermal analysis are insufficient. In the present work, the hot deformation characteristics of AZ31 magnesium alloy has been studied through applying a series of continuous cooling compression (CCC) tests. The compression tests were conducted as the temperature was continuously reduced from 500 °C to 100 °C. The variations of true stress with true strain (or temperature) were extracted and the critical temperatures were determined. The derived true stress-true strain curves revealed five deviations at 440 ± 5 °C, 350 ± 5 °C, 300 ± 5 °C, 215 ± 5 °C and 150 ± 5 °C, which were properly addressed considering the related microstructural evolutions.     

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca(wt%) alloy containing W phase(Mg_3Y_2Zn_3) prepared by permanent mold direct-chill casting is indirectly extruded at 350?C and 400?C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized(DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2' precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350?C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400?C shows lower yield strength of 332 MPa,ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400?C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the un DRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of un DRXed regions which contributes to higher strength of the alloy extruded at 350?C.     

Hot deformation and processing map of an as-extruded Mg–Zn–Mn–Y alloy containing I and W phases

The hot deformation characteristics of an as-extruded ZM31 (Mg–Zn–Mn) magnesium alloy with an addition of 3.2 wt.% Y, namely ZM31 + 3.2Y, have been studied via isothermal compression testing in a temperature range of 300–400 °C and a strain rate range of 0.001–1 s^− 1. A constitutive model based on hyperbolic-sine equation along with processing maps was used to describe the dependence of flow stress on the strain, strain rate, and deformation temperature. The flow stress was observed to decrease with increasing deformation temperature and decreasing strain rate. The deformation activation energy of this alloy was obtained to be 241 kJ/mol. The processing maps at true strains of 0.1, 0.2, 0.3 and 0.4 were generated to determine the region of hot workability of the alloy, with the optimum hot working parameters being identified as deformation temperatures of 340–500 °C and strain rates of 0.001–0.03 s^− 1. EBSD examinations revealed that the dynamic recrystallization occurred more extensively and the volume fraction of dynamic recrystallization increased with increasing deformation temperature. The role of element Y and second-phase particles (I- and W-phases) during hot compressive deformation was discussed.     

Extrusion textures in Al, 6061 alloy and 6061/SiCp nanocomposites

The 6061 alloy matrix composites reinforced with 10 wt.% and 15 wt.% of SiC nanoparticles with an average diameter of ~ 500 nm were hot extruded in strip shape from ball milled powders. The microstructures and textures of the hot extruded nanocomposites have been investigated by means of three dimensional orientation distribution functions and electron backscatter diffraction (EBSD) techniques. Pure Al and 6061 alloy extruded strips from atomised powders have been produced for comparison purposes. The results show that the non-deformable SiC particulates have a strong influence on the formation of extrusion textures in the matrix. Pure Al and 6061 alloy develop a typical β fibre texture after extrusion in strip shape. For 6061/SiC_p nanocomposites, the intensities of major texture components decrease with increasing amount of SiC particles. The total intensities of Brass, Dillamore and S components have decreased by 19% for 6061/10 wt.% SiC_p and 40% for 6061/15 wt.% SiC_p composites when compared with the 6061 alloy. EBSD analysis on local grain orientations shows limited Al grain rotations in SiC rich zones and decreased texture intensities.     

Experimental and prediction of sintered Cu–W composite by using artificial neural networks

High-energy mechanical milling was used to mix Cu and W powders. Cylindrical preforms with initial preform density of 85% were prepared using a die and punch assembly. The preforms were sintered in an electric muffle furnace at 750 °C, 800 °C, 850 °C, and subsequently furnace cooled and then the specimens are hot extruded to get 92% preform density. Scanning Electron Microscope and X-ray diffraction observations used to evaluate the characteristics. The pore size reduction during extrusion was studied using Auto CAD 2010. Neural networks are employed to study the tribological behavior of sintered Cu–W composites. The proposed neural network model has used the measured parameters namely the weight percentage of tungsten, sintering temperature, load and sliding distance to predict multiple material characteristics, hardness, specific wear rate, and coefficient of friction. The predicted values from the proposed networks coincide with the experimental values. In addition, a relative study between the regression analysis and the networks revealed that the artificial neural networks can predict the tribological characteristics of sintered Cu and W composites better than regression polynomials within a very few percent error.     

Superelastic anisotropy characteristics of columnar-grained Cu–Al–Mn shape memory alloys and its potential applications

The columnar-grained (CG) Cu–Al–Mn shape memory alloy samples possess a strong < 001>-oriented texture along the solidification direction (SD) and straight low-energy grain boundaries fabricated by unidirectional solidification technique. When the angle between tensile direction and the SD ranged from 0° to 90° at the tensile tests, the superelasticity of samples changed in a “V” shape and showed a large anisotropy. Meanwhile, the martensite transformation critical stress of the CG Cu–Al–Mn samples increased from 258.5 MPa for 0° to 521.9 MPa for 45°, and then decreased to 324.3 MPa. The large anisotropy of the superelasticity was attributed to the combined effects of grain orientation and grain boundaries, wherein the influence of the grain boundaries had an obvious dependence on orientation. The potential applications of CG Cu–Al–Mn alloys as anisotropic shock isolators and dampers in high rise buildings and precision instruments were also proposed.     

Effects of extrusion temperature on the microstructure and tensile properties of Al–16 wt% Al4Sr metal matrix composite

This study was undertaken to investigate the effect of extrusion temperature on the microstructure and tensile properties of Al metal matrix composite (MMC) containing 16 wt% Al₄Sr intermetallic. Microstructural examinations were assessed by the use of optical microscope, scanning electron microscope (SEM) and X-ray diffractometry (XRD). The results showed that hot extrusion with the ratio of 18:1 at 420 °C reduces the maximum length of Al₄Sr particles from 222 μm to 35 μm. It was found that by applying extrusion parameters in optimum conditions, uniform distribution of fine Al₄Sr intermetallic in Al matrix is obtained. Microstructural evolution also intensified the ultimate tensile strength (UTS) values of the MMC from 54 MPa to 145 MPa. Remarkable result of this study revealed that hot extrusion improves the ductility of the MMC significantly. Fractographic examinations of the composite in as-cast condition showed a complete cleavage fracture surface that changes to more homogenous dimples after hot extrusion process.     

Mechanical properties of hot pressed CoCrMo alloy compacts for biomedical applications

This study aimed at investigating the influence of the processing conditions on the mechanical properties of hot pressed compacts of a CoCrMo biomedical alloy.Several hot pressed CoCrMo compacts were processed in vacuum (10⁻² mbar), at a pressure of 60 MPa with different temperatures (900 °C, 1000 °C and 1100 °C) and different times (10 min, 30 min and 60 min). Compacts were examined by SEM/EDS. The transverse rupture strength, Young’s Moduli and hardness were determined. The fracture surface of compacts were also examined.The compacts hot pressed at 900 °C exhibited lower TRS than those processed at 1000 °C and 1100 °C, which showed similar strength values, regardless the sintering time. The 900 °C compacts showed also lower YM and higher porosity. Lower hardness values were registered for 900 °C compacts while 1000 °C compacts exhibited the highest values. The fracture surface analyses revealed fragile fracture for 900 °C compacts (10 min and 30 min) and 1000 °C (10 min). The remaining compacts exhibited ductile fracture.A full characterization of the mechanical properties of hot pressed CoCrMo compacts has been made and the selection of the processing parameters according to the desired mechanical properties is now possible.     

Development of high-performance quaternary LPSO Mg–Y–Zn–Al alloys by Disintegrated Melt Deposition technique

In the present study, new quaternary MgY_1.65Zn_0.74Al_0.53 and MgY_3.72Zn_1.96Al_0.45 alloys (wt.%) were synthesized employing the Disintegrated Melt Deposition (DMD) casting technique followed by hot extrusion. Microstructural characterization revealed the presence of 14H long-period stacking ordered structure (LPSO) and Mg₄Y₂ZnAl₃ phases aligned along the direction of extrusion in both alloys. Refined grains (⩽5 μm) due to the effect of dynamic recrystallization (DRX) were also observed to co-exist with larger worked grains (⩾20 μm) in the extruded microstructures. Compared to monolithic Mg, significant increase in the microhardness (∼67–88%), tensile yield strength (∼245–290%) and ultimate tensile strength (∼113–144%) were observed in the Mg–Y–Zn–Al alloys. Despite the significant increase in strength of materials, failure strains of both Mg–Y–Zn–Al alloys were comparable to monolithic Mg. Ignition temperatures of both Mg–Y–Zn–Al alloys were found to outperform commercially available AZ31, AZ80 and WE43 (high-temperature) Mg alloys, and the highest ignition temperature of 770 °C was achieved in the MgY_3.72Zn_1.96Al_0.45 alloy.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Workability characteristics and mechanical behavior modeling of severely deformed pure titanium at high temperatures

In the present study, compression tests were performed at temperatures of 600–900 °C and at strain rates of 0.001–0.1 s⁻¹ to study the deformation and workability characteristics of commercially pure titanium after severe plastic deformation (SPD). It was found that the effects of temperature and strain rate are significant in dictating the steady state flow stress levels and the strain values corresponding to peak flow stress. The strain rate sensitivity (m) during hot compression of severely deformed Ti was shown to be strongly temperature dependent, where m increased with the increase in deformation temperature up to 800 °C. High temperature workability was analyzed based on the flow localization parameter (FLP). According to the FLP values, deformation at and below 700 °C is prone to flow localization. The flow behavior was predicted using Arrhenius type and dislocation density based models. The validities of the models were demonstrated with reasonable agreement in comparison to the experimental stress–strain responses.     

Hot corrosion behavior of porous nickel-based alloys containing molybdenum in the presence of NaCl at 750 °C

The hot corrosion of porous Ni-23Cr-xMo (0%, 4.5%, 9.0%, 13.5%, mass fraction) alloys tested at 750 °C under cyclic procedure was investigated in order to elucidate the effect of Mo addition on hot corrosion in the presence of NaCl. The hot corrosion experiments were performed at 750 °C in air with 4 mg cm^− 2 NaCl deposit. The performance of the alloys was evaluated by the results of weight change kinetics. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the corrosion products. The results indicate that NaCl accelerated the oxidation of the alloys by chloridized elements Mo and Cr. Among the porous Ni-23Cr-xMo alloys, Ni-23Cr-9Mo alloy exhibited the best hot corrosion resistance due to the formation of NiO-NiCr₂O₄-Cr₂O₃ oxide scales. Furthermore, these oxide scales were confirmed more effective to protect the alloys after adding of Mo.