Progress in Powder Metallurgy

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Authors: Robert Wilson, Nigel A. Stone, Mark A. Gibson
Abstract: Extrusion is a way to produce near net shape components from CP grade titanium powders of optimum density with minimum porosity and acceptable mechanical properties. Chemically pure, hydride/dehydride titanium powders were cold pre-compacted and extruded at 850oC under an argon atmosphere. The extrusion stress required was ~450MPa. To characterize the extrusions, the porosity distribution, qualitative microstructure and tensile properties were evaluated and compared with conventional extruded wrought titanium. Extrusion occurred after the green billets were upset to ~100% of theoretical density and adequate lubrication was applied to the die. The resultant product was 100% dense with a narrow band of surface porosity and exhibited an equiaxed microstructure of similar magnitude to the starting material. The tensile properties of the bars were observed to be significantly superior to conventionally extruded CP titanium bar products, a result associated with the much finer average grain size. Outcomes from this study have assisted in the identification of a number of key characteristics important to the extrusion of titanium from pre-compacted CP titanium powders, allowing the elimination of canning and hot isostatic pressing (HIPping) of billets prior to extrusion as per conventional PM processes.
Authors: Brian Gabbitas, Peng Cao, Stiliana Raynova, De Liang Zhang
Abstract: The research involves the development of a powder metallurgical route for producing good quality TiAl targets for making physical vapour deposition (PVD) coatings. Mixtures of elemental titanium and aluminium powders were mechanically milled using a novel discus milling technique under various conditions. Hot isostatic pressing (HIP) was then employed for consolidation of the mechanically alloyed powders. A cathodic arc vapour deposition process was applied to produce a TiAlN coating. A microstructural examination was conducted on the target material and PVD coatings, using X-ray diffractometry (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). It has been found that combining mechanical alloying and HIP enable us to produce a fairly good quality of TiAl based target. The PVD coatings obtained from the TiAl target showed very high microhardness values.
Authors: Ryuzo Watanabe, Duk Sun Choi, Akira Kawasaki
Abstract: Gas release behavior from aluminum and Al 7075 alloy powders during heating in argon was investigated by in-situ gas chromatography. Water vapor, hydrogen, carbon mono-oxide were detected as individual evolution spectra against heating temperature and time. The mechanisms of water and hydrogen evolutions were studied in detail for the determination of effective degassing condition. The adsorbed water and aluminum hydrate on the particle surfaces were considered to be the sources for the released water. Hydrogen peaks were formed from released gases through the reaction of aluminum and magnesium with adsorbed and hydrated water, and from liberated hydrogen that would have been excessively occluded during atomization. For the alloy powder magnesium was found to lower the hydrogen evolution temperature to enhance overall hydrogen release.
Authors: P.G. Esteban, Elena Gordo, Elisa Maria Ruiz-Navas
Abstract: The present work studies the influence of high-energy milling (HEM) and sintering cycle of Ti and Al powders on the obtainment of TiAl. This study shows that HEM modifies the diffusion processes during the sintering stage. The ignition temperature of the exothermic reaction (SHS) that occurs between Ti and Al, was considered as the key parameter of the sintering process, leading to the study of sintering cycles that avoid uncontrolled processes caused by the SHS reaction. The samples were obtained by cold uniaxial and isostatic pressing, pre-sintered at different temperatures, and then heated up to the sintering temperature. This study also shows the effect of powder additions processed by HEM on the sintering behavior of blended elemental Ti and Al powder compacts.
Authors: F. Romero, Vicente Amigó, M.D. Salvador, A. Vicente
Abstract: Titanium metal matrix composites were produced. The powder metallurgy route applied was a conventional route consisting of blending titanium matrix powder with different percentages of various titanium compounds, as reinforcement particles, followed by cold compaction in a uniaxial press with a floating matrix and a sintering process in a vacuum furnace. This work studied the different interactions between the titanium matrix and the various titanium compounds added. To evaluate these interactions microscopic techniques are used principally, optical and electronic microscopy, with EDX techniques. By microstructural analysis the reactivity between reinforcement and matrix particles was investigated, and any new phases that formed during the sintering process were evaluated. In addition, microhardness test were conducted to study the mechanical properties associated with the new phases, and to evaluate the relative strength or weakness of the interfacial zones.
Authors: Si Young Chang, Han Gyoung Cho, Jin Chun Kim, Seong Hee Lee, Byung Heum Song
Abstract: The bimodal-structured materials composed of Al-5wt%Mg alloy and its composites reinforced with SiC and Al2O3 particles were prepared by ball-milling and subsequent compaction under the pressure of 350MPa and sintering at temperatures ranging from 973K to 1173K for 1h, 3h and 5h. The macro-interface between Al-Mg and Al-Mg/ Al2O3 composite was macroscopically well-bonded compared to that between Al-Mg and Al-Mg/SiC composite under same sintering conditions. The bonding of macro-interface became better as the sintering time and temperature increased, resulting in that the bimodal-structured materials sintered at 1173K for 5h showed the relative density of nearly 100%. In addition, the higher sintering temperature was, the more irregular macro-interface was. The microhardness of macro-interface area was in between the Al- Mg and composites, which was independent of the sintering temperature.
Authors: F. Romero, Vicente Amigó, M.D. Salvador, E. Martinez
Abstract: Particulate reinforced titanium composites were produced by PM route. Different volumetric percentages of TiN reinforcements were used, 5,10,15 vol%. Samples were uniaxially pressed and vacuum sintered at different temperatures between 1200-1300°C. Density, porosity, shrinkage, mechanical properties and microstructure were studied. Elastic properties and strength resistance were analysed by flexural strength and tension tests, and after the test, fractured samples were analysed as well to obtain the correlation between the fracture, interparticular or intraparticular, and the level of reinforcement addition. Hardness and microhardness test were done to obtain a better understanding of its mechanical properties. In order to study wear resistance pinon- disc tests were conducted. In addition, the influence of temperature, the reactivity between matrix and reinforcement on microstructural development were observed by optical and electron microscopy.
Authors: Hai Yi Lou, Wei Lu, Lei Yang, Biao Yan
Abstract: Microstructure and mechanical properties of a newly developed Zn61Al34M5 (M=Cu, Si, RE, et al.) alloy obtained by warm-compacting sintering technique were studied using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) associated with measurements of mechanical properties. The results showed that the new alloy consisted of α-phase and η-phase and have good plasticity; its hardness increased by 10%~20% and density decreased by about 16% as compared with those of the traditional cast Zn-Al alloys.
Authors: J.K. Lee, Taek Soo Kim, Ha Guk Jeong, Jung Chan Bae
Abstract: The microstructure and mechanical properties of the Mg97Zn1Y2 alloy prepared by spark plasma sintering of gas atomized powders have been investigated. After consolidation, precipitates were observed to form in the α-Mg solid solution matrix of the Mg97Zn1Y2 alloy. These precipitates consisted of Mg12YZn and Mg24Y5 phases. The density of the consolidated bulk Mg-Zn-Y alloy was 1.86 g/cm3. The ultimate tensile strength and elongation were dependent on the consolidation temperature, which were in the ranges of 280 to 293 MPa and 8.5 to 20.8 %, respectively.
Authors: C. Zubizarreta, I. Arribas, S. Giménez, I. Iturriza
Abstract: Powder compacts from two different commercial aluminium powder grades have been densified by direct hot extrusion. The extrusion runs have been carried out at 425 °C, with an extrusion ratio of 1:16 and a ram speed of 1.5 mm/s. Prior to extrusion, some green compacts were presintered (500 °C). The evolution of the extrusion load during the process and the hardness of the final products have been investigated. Additionally, microstructural characterization by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD) was carried out in order to understand the softening-hardnening mechanisms taking place during the process. The obtained results evidence grain refinement. Additionally, inter-metallic precipitation, dynamic recovery and geometric dynamic recrystallization take place depending on the composition of the powder, the heat treatment before extrusion and the strain involved in the process.

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