Aamir Mukhtar

EFFECT OF PROCESSING CONDITION AND COMPOSITION ON THE MICROHARDNESS OF Cu-(2.5-10)vol.%Al2O3 NANOCOMPOSITE POWDER PARTICLES PRODUCED BY HIGH ENERGY MECHANICAL MILLING

Nanostructured Cu-(2.5-10vol.%)Al2O3 nanocomposites were produced using high energy mechanical milling. For the as-milled Cu-Al2O3 composite powder particles having Al2O3 volume fractions of 2.5% and 5%, the increase in average microhardness is significant with the increase of milling time from 12 hours to 24 hours. With the increase of the content of Al2O3 nanoparticles the microhardness increases and in the range of 255HV-270HV. The milled nanocomposite powders were heat treated at 150, 300, 400 and 500°C for 1 hour, respectively, to determine the thermal stability of the powder particles as a function of annealing temperature. The average microhardness increased/decreased for the Cu-Al2O3 composites after annealing at 150°C due to the dislocation density, while increasing the annealing temperature to 300°C and 400°C the average microhardness almost remained mostly unchanged. Further increasing the annealing temperature to 500°C causes significant decrease in average microhardness due to reduction in dislocation density and coarsening of Cu grains of the Cu-Al2O3 composite powders produced after 24 hours of milling. This paper is to report and discuss the changes of the microhardness of the material, caused by the compositions and processing conditions, used to fabricate the Cu-(2.5-10)vol.%Al2O3 nanocomposite powders.

Synthesis and thermal stability of Cu-(2.5-10)vol.%Al2O3nanocomposite powders by high energy mechanical milling

High energy mechanical milling (HEMM) of a mixture of Cu powder and Al2O3 nanopowder has been used to produce Cu-(2.5-10)vol.%Al2O3 nanocomposite powders with a ultrafine grained or nanocrystalline Cu matrix. The microstructure and microhardness of the as-milled powder particles and the thermal stability and microhardness change of the nanocomposite powder particles caused by annealing at temperatures up to 500oC have been studied. It is shown that HEMM can be effectively used to disperse (2.5-10)vol.% Al2O3 nanoparticles into a ultrafine grained or nanocrystalline Cu matrix. Using larger diameter balls or increasing the volume fraction of Al2O3 nanoparticles to 7.5% or higher allows synthesis of Cu-Al2O3 nanocomposite powders with nanocrystalline Cu matrix. Refining the microstructure of the Cu matrix and increasing the volume fraction of Al2O3 nanoparticles in the nanocomposite both increase the thermal stability of the nanocomposite structure.

Thermal stability and microhardness of Cu-10vol.%Al2O3nanocomposite produced by high energy mechanical milling

Cu-10vol.%Al2O3 nanocomposite powders were produced using two high energy milling routes and heat treated at 150, 300, 400 and 500°C for 1 hour, respectively, to determine the thermal stability of the microstructure and the micohardness change of the materials as a function of the annealing temperature. Annealing of the as-milled powders at 150°C caused recovery and recrystallisation that leads to significant decrease in dislocation density and slight decrease of microhardness. Increasing the annealing temperature to 400°C causes slight coarsening of the Cu grains and corresponding slight decrease of microhardness. Further increasing the annealing temperature to 500°C causes significant coarsening of the Cu grains and cause significant decrease in microhardness. The effects of different factors on the thermal stability and micohardness change of the Cu-Al2O3 are discussed.

Microstructure and thermal stability of nanostructured Cu-7.5vol.%Al2O3 composite powders produced by high energy mechanical milling

Nanostructured Cu-7.5vol.%Al2O3 composite powders were produced from a mixture of Cu powder and Al2O3 nanopowder using two routes of high energy mechanical milling. The milled composite powders were heat treated at 150, 300, 400 and 500°C for 1 hour, respectively, to determine the thermal stability of the microstructure and corresponding micohardness change of the powder particles as a function of the annealing temperature. For the nanocomposite powder produced with 12 hours of milling (Route 1), heat treatment at 150°C did not change the microstructure, increasing the annealing temperature from 150 to 500°C caused the Cu grain sizes to increase due to grain coarsening. For the composite powder produced with 24 hours milling (Route 2), the grain sizes of the Cu matrix also increased due to grain coarsening with increasing the annealing temperature from 150 to 500°C. Average microhardness decreased significantly for both the nanocomposite powders but the degree of grain size increase was smaller with increasing the annealing temperature from 400 to 500°C.

Mechanical Behaviour of Gas Nitrided Ti6Al4V Bars Produced by Selective Laser Melting

Gas atomized Ti-6Al-4V (Ti64) alloy powder was used to prepare distinct designed geometries with different properties by selective laser melting (SLM). Several heat treatments were investigated to find suitable processing parameters to strengthen (specially to harden) these parts for different applications. The results showed significant differences between tabulated results for heat treated billet Ti64 and SLM produced Ti64 parts, while certain mechanical properties of SLM Ti64 parts could be improved by different heat treatments using different processing parameters. Most heat treatments performed followed the trends of a reduction in tensile strength while improving ductility compared with untreated SLM Ti64 parts.Gas nitriding [GN] (diffusion-based thermo-chemical treatment) has been combined with a selected heat treatment for interstitial hardening. Heat treatment was performed below β-transus temperature using minimum flow of nitrogen gas with a controlled low pressure. The surface of the SLM produced Ti64 parts after gas nitriding showed TiN and Ti2N phases (“compound layer”, XRD analysis) and α (N) – Ti diffusion zones as well as high values of micro-hardness as compared to untreated SLM produced Ti64 parts. The microhardness profiles on cross section of the gas nitrided SLM produced samples gave information about the i) microhardness behaviour of the material, and ii) thickness of the nitrided layer, which was investigated using energy dispersive spectroscopy (EDS) and x-ray elemental analysis. Tensile properties of the gas nitrided Ti64 bars produced by SLM under different conditions were also reported.

Microstructure and Mechanical Behaviour of Ultrafine Grained Al-4wt%Cu-(2.5-10) Vol.% SiC Metal Matrix Composites Produced by Powder Compact Forging

Ultrafine grained Al-4wt%Cu-(2.5-10) vol.% SiC metal matrix composite powders were produced from a mixture of Al, Cu and SiC powders using high energy mechanical milling (HEMM). The composite powders produced were first hot pressed at 300°C with a pressure of 240 MPa to produce cylindrical powder compacts with a relative density in the range of 80-94% which decreased with increasing the SiC volume fraction. Powder compact forging was utilized to consolidate the powder compacts into nearly fully dense forged disks. With increasing the volume fraction of SiC from 2.5% to 10%, the average microhardness of the forged disks increased from 73HV to 162HV. The fracture strength of the forged disks increased from 225 to 412 MPa with increasing the volume fraction of SiC particles from 2.5 to 10%. The Al-4wt%Cu-2.5vol.%SiC forged disk did not show any macroscopic plastic yielding, while the Al-4wt%Cu-(7.5 and 10)vol.% SiC forged disk showed macroscopic plastic yielding with a small plastic strain to fracture (~1%).

Mechanical Properties of Ti-6Al-4V Rods Produced by Powder Compact Extrusion

This paper describes the mechanical properties of Ti-6Al-4V alloy produced by consolidation of a blended powder mixture of elemental hydride-dehydride (HDH) titanium powder and master alloy (60Al-40V) powder. The warm pressed compacts of blended powders were sintered using a vacuum sintering furnace prior to the extrusion process. Impact toughness and micro-hardness were measured using as extruded Ti-6Al-4V alloy rod with an extrusion ratio 9:1. In addition, tensile testing was performed on extruded rod with an extrusion ratio 25:1. Detailed work was performed using optical microscopy and scanning electron microscopy to explore the microstructure and fracture surfaces of the tested material. The tensile properties of as extruded Ti-6Al-4V rod were found to be comparable with corresponding literature values of wrought material. However, impact toughness was found to be lower than published data. Some possible reasons contributing to the measured mechanical properties of the Ti-6Al-4V rods in this study are discussed.

Mechanical Behaviour of Ultrafine Grained Cu and Cu-(2.5 and 5) Vol.%Al2O3 Composites Produced by Powder Compact Forging

Nanostructured Cu-(2.5 and 5)vol.%Al2O3 composite powders were produced from a mixture of Cu powder and Al2O3 nanopowder using high energy mechanical milling, and then compacted by hot pressing. The Cu and Cu-Al2O3 composite powder compacts were then forged into disks at temperatures in the range of 500-800°C to consolidate the Cu and Cu-Al2O3 composite powders. Tensile testing of the specimens cut from the forged disks showed that the Cu forged disk had a good ductility (plastic strain to fracture: ~15%) and high yield strength of 320 MPa, and the Cu-(2.5 and 5)vol.%Al2O3 composite forged disks had a high fracture strength in range of 530-600 MPa, but low ductility.

MICROSTRUCTURAL EVOLUTION DURING HIGH ENERGY MECHANICAL MILLING AND HEAT TREATMENT OF Cu-(5,10)vol.%Al2O3 NANOCOMPOSITE POWDERS

Cu-(5-10)vol.%Al2O3 nanocomposite powders were produced from a mixture of Cu powder and Al2O3 nanopowder using a high energy mechanical milling (HEMM) route consisting of two stages. The microstructural evolution of the Cu–Al2O3 nanocomposite powder particles (or granules) produced after first and the second stages of milling was studied using scanning electron microscopy (SEM), transmission electron microcopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) mapping. The study confirmed that homogenous dispersion of Al2O3 nanoparticles in the Cu matrix was achieved after the first stage of milling and the relatively large Al2O3 particles were further broken into smaller nanoparticles after the second stage of milling. The milled nanocomposite powders were also heat treated at 150, 300, 400 and 500°C for 1 hour, respectively, to determine the microstructural changes of the powder particles as a function of annealing temperature. It was found that after heat treatment at 150°C, the Cu grain sizes decreased due to recrystallisation, and increasing the annealing temperature to 300°C causes slight coarsening of the Cu grains. Further increasing the annealing temperature to 500°C caused significant coarsening of the Cu grains and the Al2O3 nanoparticles. It also appeared that the coarsening of Cu grains in the composite powder particles after annealing at 500°C become less severe with increasing the volume fraction of Al2O3 particles.

Microstructural stability and microhardness of ultrafine grained and nanostructured Cu-5vol.%Al 2 O 3 composite lumps/powders produced by high energy mechanical milling

Ultrafine grained lumps and a nanostructured powder of Cu-5vol%Al2O3 composite were produced using two high energy mechanical milling routes respectively. The milled composite materials were heat treated at 150, 300 and 500 degC for 1 hour, respectively, to determine the microstructural stability and micohardness changes of the materials as a function of the heat treatment condition. For the Cu-5vol.%Al2O3 composite lumps produced using route 1 (12 hours milling), after heat treatment at 150 degC, the Cu grain sizes decreased from the range of 100-250 nm to the range of 50-180 nm due to recrystallisation, but its average microhardness also decreased from 224 HV to 212 HV due to reduction of dislocation density. For the 24 hours milled Cu-5vol%Al2O3 powder produced using Route 2, the Cu grain sizes increase slightly from the range of 40-180 nm to the range of 50-200 nm, and as the result of this grain coarsening and decrease of dislocation density, the average microhardness decreased from 270 HV to 257 HV respectively. Further increasing the annealing temperature to 300 degC caused the grain sizes of the 12 hours milled lumps to increase to the range of 50-350 nm, and those of the 24 hours milled powder to 60-300 nm, both resulting in a decrease in the average microhardness to 207 HV for the lumps and 248 HV for the powder. Increasing the annealing temperature from 300 to 500 degC caused a much more significant increase of the Cu grain sizes of both the lumps and the powder, and a significant decrease in the microhardness of the 24 hours milled powder particles to 216 HV. However, the microhardness of the lumps decreases very little to 196 HV, suggesting the significant reinforcement effect of the Al2O3 nanoparticles.