Papers by Keyword: High Energy Mechanical Milling

Abstract: Abstract This paper presents and discusses the factors influencing the yield of Ti-Al alloy in the TiPro process which is a process developed at the University of Waikato for producing titanium alloy powders by mechanically activating Al/TiO2 powder mixtures and subsequently preheating the resultant composite powder in order to ignite a combustion synthesis reaction and separate the liquid Ti-Al alloy by extrusion. In this study, TiO2/Al composite powders with different powder particle microstructures have been produced and used to study the effects of starting composite powder particle microstructure on the solid/liquid separation of TiAl from solid Al2O3 by extrusion. Results obtained so far indicate that maximizing the time the Ti-Al alloy phase is maintained in the liquid state after the reaction between TiO2 and Al is one of the critical factors to increase the yield of Ti-Al alloy produced through the separation of liquid Ti-Al from the solid Al2O3 phase by extruding the mixture of liquid Ti-Al and Al2O3 formed through reactions and heating.

Abstract: 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%).

Abstract: 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.

Abstract: To provide reference for choosing technological parameters of subsequent powder densification, thermal stability of 3 h-milled 3 wt% graphite/copper nanocrystalline composite powders were investigated with such analytical methods as scanning electron microscopy (SEM), back-scattered electron images and X-ray diffraction (XRD). The results show that diffraction peak of graphite in XRD pattern is absent because of too small graphite particle. No major variation of grain size of Cu with annealing temperature is observed. Accumulation and growth of graphite phase aren’t seen. The microhardness is nearly constant for the annealed powders. Therefore, 3 h-milled 3 wt% graphite/copper nanocrystalline composite powders possess good thermal stability.

Abstract: Ultra-high molecular weight polyethylene (UHMWPE) is a polyethylene with a very long chain, which provides excellent features, however it makes the processing difficult due to high melt viscosity. Many studies intend to found out means to make its processing easier. Recently, the high-energy mechanical milling has been used for polymeric materials and it was detected that physical and chemical changes occur during milling. In such case, powder of UHMWPE was milled in three types of mills: SPEX, attritor e planetary, in different times of milling. The polymer was characterized by SEM and XRD. Thus, it was observed that the material processed in attritor mill showed larger phase transformation from orthorhombic to monoclinic. This is most likely due to the smaller milling temperature of attritor mill when compared with the other two mills and the high shear force generated during milling.

Abstract: Gamma TiAl based alloys are important materials with potential applications in aerospace and automotive applications due to their high specific strength and creep resistance. The major barrier for their applications is their limited ductility at room temperature and limited hot workability. One way of overcoming this barrier is to reduce the grain sizes to ultrafine grained (<500μm) or nanostructured (<100nm) level. In our present study, we attempt to produce bulk ultrafine grained Ti- 47Al-2Cr (at%) alloy using a combination of high energy mechanical milling of elemental powders to produce a very fine structured Ti/Al/Cr composite powder and consolidation of the powder using hot isostatic pressing (HIPping). It was confirmed that high energy ball milling using a planetary ball mill led to the formation of extremely fine Ti and Al layered composite structure. The thermal behaviour of the powder was studied using differential thermal analysis, and it was shown that the reactions between the Ti and Al phases in the fine structured composite powder occur at fairly low temperatures, below the melting point of the Al phase (660oC). The macrostructure and phase structure of the HIPped samples were also examined using optical and scanning electron microscopy and X-ray diffractometry (XRD). This paper is to report and discuss the results of this investigation.

Abstract: Alumina-iron nanocomposite powders containing 5vol.% of iron were fabricated by high-energy ball milling with different ball-to-powder weight ratios (BPRs) as part of the study of ceramic-metal nanocomposite magnetic materials. The microstructure and morphology of the composite powders were characterized using the X-ray diffraction, optical microscopy and scanning electron microscopy. XRD analysis and SEM examination in combination with energy dispersive X-ray spectrometry confirmed that the nanocomposite structure of the powder particles formed only after 8 hours milling for both BPRs used. With a higher BPR of 16:1, Fe-Cr alloy material was broken from the stainless steel balls and incorporated into the nanocomposite powder. However, such a problem did not occur with a lower BPR of 5:1. The mechanism for formation of the alumina matrix nanocomposite powder is found to be dependent on BPR and milling time.

Abstract: The aim of the present work is to produce new types of solid nanomaterials for different purposes (coatings, fillers, foams, bulk pieces, etc.). Technologies such as RS Al flake production, high energy mechanical milling and high energy rate forming technology (HERF) for compacting are used. The products are analyzed mainly by XRD, SEM and TEM methods. It was shown that the new-type of RS Al “flake” material is suitable not only for pigments but also for powder metallurgical purposes, i.e. Al based nanocomposites. By choosing suitable parameters for mechanical alloying with the Fritsch Planetary mill 4, very fine, alloyed and composited nanostructures can be produced (Al-4.5w%Cu- 10w%Al2O3, Al-15w%Pb) Dynamic compaction (HERF) using explosive techniques seems to offer a good way for the compaction of Al (metal) matrix nanostructured composites.

Abstract: Y- α-sialon (m=1.35, n=0.675) ceramics were prepared by high-energy mechanical milling followed by spark plasma sintering. The milling promoted not only liquid-phase sintering, but also phase transformation from β-Si3N4 to α-sialon. Under the same holding time of 5 min, milled powder could be completely densified at 1500oC, which is about 250oC lower than that required for as-received powder. The temperature where the phase transformation finished was 1600oC and 1750oC for milled and as-received powder, respectively. The grain size of obtained dense ceramics from milled powder was significantly decreased. Nano-sized dense ceramics have been obtained by sintering the milled powder at 1500oC for 5 min. Although 100 % α-sialon has not been achieved, the nano-sized ceramics can be used for superplastic deformation, taking advantage of small grain size and large amount of transient liquid phase.