Formation Of Macroscopic Defects Research Articles

Material extrusion printing of WC-8%Co cemented carbide based on partly water-soluble binder and post-processing

In this paper, WC-8%Co cemented carbide parts were fabricated by material extrusion additive manufacturing. Water debinding, thermal debinding and sintering were carried out to obtain densified parts. An optimized binder system consists of 50 vol% polyethylene glycol, 15 vol% polyvinyl butyral, 25 vol% high density polyethylene, 5 vol% stearic acid and 5 vol% polyethylene-wax was developed to prepare printable feedstock. The results show that polyvinyl butyral in binder is contributed to tight bonding of powders, but excessive high polyvinyl butyral content caused separation of binder and powder, inefficiency of debinding and formation of macroscopic defects. Extraction of polyethylene glycol is found to be controlled by both dissolution and diffusion. The insoluble binder can be further removed by heating, the products from pyrolysis of insoluble binder included volatile alcohols and aldehydes, carbon dioxide, hydrocarbon and water. Residual carbon and decarburization during thermal debinding can be minimized by using N2/H2 atmosphere in furnace. The complete debinding body was almost absolutely densified by sintering beyond 1340 °C. The sintered body shows a relative density of 99.0%, HV30 hardness of 1360 ± 16 and transverse rupture strength of 1678 ± 125 MPa with optimal post-processing, which are slightly inferior to those obtained by the conventional pressing method. Extrusion printing method is an alternative method for additive manufacturing of WC-Co cemented carbide with good printability and shape retention in process.

An improved process for grain refinement of large 2219 Al alloy rings and its influence on mechanical properties

The large 2219 Al alloy rings used to connect propellant tank components of a satellite launch vehicle to each other are conventionally manufactured by radial-axial ring rolling at 460 °C with 50% deformation, but often suffer from coarse elongated grain and low ductility. An improved process (hot ring rolling at 460 °C with 30% deformation, then air cooling to 240 °C, followed by ring rolling at 240 °C with 20% deformation) was tested for ring manufacturing. The corresponding microstructure evolution and mechanical properties of the produced rings were studied. The results show that the improved process can successfully be applied to manufacture the large 2219 Al alloy rings without formation of macroscopic defects, resulting in a product with fine and uniform grains after heat treatment. The fracture mechanism of both rings was mainly intergranular fracture. With the resulting grain size refinement due to the improved process, more homogeneous slip occured and the crack propagation path became more tortuous during the tensile testing process. Thus, the elongation in all three orthogonal directions was greatly improved, and the axial elongation increased from 3.5% to 10.0%.

Effect of tool rotational speed and pin profile on microstructure and tensile strength of dissimilar friction stir welded AA5083-H111 and AA6351-T6 aluminum alloys

The relatively new welding process friction stir welding (FSW) was applied in this research work to join 6mm thick dissimilar aluminum alloys AA5083-H111 and AA6351-T6. The effect of tool rotational speed and pin profile on the microstructure and tensile strength of the joints were studied. Dissimilar joints were made using three different tool rotational speeds of 600rpm, 950rpm and 1300rpm and five different tool pin profiles of straight square (SS), straight hexagon (SH), straight octagon (SO), tapered square (TS), and tapered octagon (TO). Three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone. The tool rotational speed and pin profile considerably influenced the microstructure and tensile strength of the joints. The joint which was fabricated using tool rotational speed of 950rpm and straight square pin profile yielded highest tensile strength of 273MPa. The two process parameters affected the joint strength due to variations in material flow behavior, loss of cold work in the HAZ of AA5083 side, dissolution and over aging of precipitates of AA6351 side and formation of macroscopic defects in the weld zone.

Influence of the brazing process parameters on the microstructural characteristics of copper alloy jointsPaper Presented at the 4th National Welding Day, Workshop: “Brazing” Genova, 25–26 October 2007.

This paper reports the results of microstructural characterization studies conducted on copper alloy joints obtained by different brazing processes. In particular, tests have been conducted on Cu–Ni–Zn ternary alloys (nickel silvers), traditionally used in the spectacle industry, in order to highlight the merits and any metallurgical and functional defects in samples obtained using two different heat input methods: induction and LASER beam welding. Along with the metallographic analysis of joints made using the traditional induction brazing technique, the joints obtained by LASER brazing have been investigated. In particular, it has been evaluated as to how altering the process parameters pertaining to both the LASER beam (power and impact time) and base metal surface conditions (presence or absence of surface oxides) can significantly influence the formation of macroscopic defects in the joints or on deformation of the components themselves. Hence, the main scope of the paper has been an attempt to identify the optimal process parameters for LASER beam brazing, so that the joints thus obtained have acceptable mechanical properties, while maintaining good aesthetic appearance, a decidedly essential requirement in components for spectacles.

Relationship of properties of MBE grown GaAs layers with growth conditions

Reflection medium energy electron diffraction, scanning electron microscope and measurements of angular dependence of reflectivity in vacuum ultraviolet region, low-temperature photoluminescence and Hall effect were used to study surface structure and morphology and optical and electrical properties of GaAs layers grown by molecular beam epitaxy (MBE) and influence of growth conditions on their properties. The quality of MBE GaAs layers depends strongly on the growth conditions. The residual background impurities in MBE system and other contamination sources degrade the optical and electrical quality and can influence the formation of macroscopic defects. The incorporation of background impurities and the formation of vacancy-impurity complexes are dramatically affected by As4 to Ga flux ratio at a given growth temperature and suppressed strongly under growth conditions with slight As excess which ensures to maintain the As-stabilised surface. The growth under such conditions results in improved electrical and optical properties and satisfactory surface morphology.

The Size Distribution of Macroscopic Defects in Media Containing Gas Admixture

The influence of temperature fluctuations on the process of formation of macroscopic defects in a medium containing a gas is investigated. The equilibrium distribution function for macroscopic defect sizes is obtained and several of its important characteristics are analysed. The dependence of the main observed characteristics of this function are determined from both external parameters, such as temperature and gas admixture content, and from internal (microscopic) parameters, such as chemical potential and coefficient of surface tension. A comparison of the results obtained with experimental data is presented and good agreement is found. [Russian Text Ignored].

Formation of defects in zinc selenide crystals grown from the melt under argon pressure

The formation of macroscopic defects in ZnSe crystals is described. Crystals were grown by the Bridgman technique at the lowering rate of a crucible of 0.8–10.1 mm/h under an argon pressure of 100 kg/cm 2. The formation of the assembly of rod-like low angle grain boundaries was dependent upon the angle θ between the growth axis and the (111) twin plane normal. For 75° < θ < 90° the rod-like low angle grain boundaries occurred and for θ < 75° crystals free of the boundaries grew at the lowering rates used in our experiments. The formation of voids was dependent on the lowering rate of the crucible. Many crystals free of these defects were obtained in the growth range 0.8–2.5 mm/h.