Papers by Keyword: Dendrites

Abstract: The flow behavior of semi-solid slurry determines the quality of the castings produced by the semi-solid forming process. Many studies have done to investigate the flow behavior of slurry under different conditions, and results show that the rheological behavior of slurry with dendritic structure is inappropriate for semi-solid forming. In this study, slurries with varying morphologies of grain for the same alloy with the same fraction solid have tested using a partial filling method. The SEED process was employed, and the pouring temperature adjusted to prepare semi-solid slurries with different grain morphologies. The flow pattern, entrapped air during the filling process, and also microstructure of the samples were examined to characterize the macro and micro flow behavior. The results show that a turbulent macro-flow, leading to entrapped air, and severe segregation appeared in the sample using slurry of T_pour ≥ 660 °C . For the slurry of T_pour < 660 °C, none of the three phenomena found in the sample. This investigation further showed that the detriment of dendrite on the semi-solid forming process, and implied that large size dendrite in semi-solid slurry must avoided.

Abstract: The paper considers the total effect of primary structure parameters on steel mechanical properties depending on the wall thickness of castings. It is shown that during the formation of mechanical properties with an increase in the thickness of the work-piece, the role of the sizes of primary grains decreases and the role of reducing the dispersion of dendritic crystals increases, due to the process of dendritic branches coarsening.

Abstract: Dendrites were observed in the failure of semiconductor sensor devices. EDX analysis showed that the dendrites grown from bare sensor dice consisted of tin metal. The tin dendrites exhibited massive and dense branches. Dendrites grown from mechanically decapped parts consisted of silver. The silver dendrites exhibited delicate, lace-like structure. Binary and grey scale images of dendrites were analyzed for fractal dimension number and branch density. The tin dendrites had a higher, statistically significant branch density number than silver, due to tin’s more intricate branching pattern. Fractal numbers can be used to differentiate between tin and silver dendrites, even in the absence of EDX analysis equipment.

Abstract: At present, the discussion about pores/ voids in large ingots is still actual. Researchers investigated the closing behavior during the open die forging process. Mostly, the approaches take the account into void closure with artificial pores. But artificial pores do not reflect the real closing behavior. This is caused in the surface/ structure of inner voids. During drilling and heating, the voids got a smooth surface and have contact to the atmosphere. Therefore, the material oxidizes on surface and the closing behavior is not similar to the real process. Real pores show a fractal surface with dendrites. And the inner of voids contains a vaccum. In the framework of a new approach, for void closure cast ingots with pores were generated and the structure and the closing behavior were investigated.The final goal is to find out a global closing function “Z”. This closing function should improve the understanding of void behavior and in the future the industrial process. The function depends on different parameters, like yield strength of material (P_kf), the pore size (A_Eq), the pore structure (P_struct) and process parameters (P_proc), such as tool geometry or bite ratio. Finally, there is a parameter P_disturb. This parameter works against the final closure and is important for the understanding of the process, because it represents the influence of dendrites. Furthermore the closing behavior is not comprehensible without the consideration of recrystallization.

Abstract: In this research it was examined the influence of crystallisation conditions binary leaded bronze on parametres of a received microstructure. Change of crystallisation conditions was carried out by change of cooling melt speed, through preliminary heating of casting moulds. Quantitative regularities of influence of cooling rate of explored bronze on parametres dendritic cells, grain size are presented. The data about formation of lead inclusions between dendrites of a copper matrix are published as well. It is shown that high rates of cooling of an order 100-150°С/c lead to dendritic structures formation containing only axes of the first and second order. Decrease of cooling rate at the moment of crystallisation to the values less 15°C/c leads to appearance and growth of axes of 3rd order at dendrites matrix.

Abstract: Surface modification by means of alloying is a vital process to improve the performance of a material surface which is subjected to wear and corrosion environments without altering the bulk properties. In this study, the surface of commercial purity-titanium (CP-Ti) material was alloyed using pre-placed powder mixture of iron, silicon and carbon at different weight ratios under TIG torch melting technique. The effects of energy input (1080 and 1350 J/mm) in TIG torch on the melt geometry, topography, microstructure and hardness were examined. The results showed that the TIG torch produced melt pools geometry with hemispherical in shape and different geometrical dimensions. Pores were seen to be concentrated at the edges where low melting energies are prominent to entrapped escaped gases upon fast melt solidification. The melt layers with the 1350 J/mm consisted of armed typed of TiC precipitation in the presence of longer solidification time. The sizes of dendrites observed at energy inputs of 1350 J/mm was found to be greater in population and larger in the middle of the melt pool compared to the energy input of 1080 J/mm. The alloyed layer exhibited a maximum hardness of ~ 810 HV which is about 4 times greater than the base hardness of 200 HV. The high hardness observed at particular areas is attributed to the higher population and larger sizes of dendritic microstructure produced using 1350 J/mm TIG arc source compared to 1080 J/mm which was dominated by lesser precipitated TiC resulting lower hardness values in the melt pool. Keywords: Surface modification; TiC; TIG; dendrites; microhardness

Abstract: In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.

Abstract: The study proves that by introducing the iron powder to low-sulphur cast iron still before the inoculation carried out with a conventional graphitising inoculant, the mechanical properties similar to those obtained during the inoculation treatment carried out on cast iron with the recommended high sulphur content are achieved. The said operation increases the number of crystallisation nuclei for of the primary austenite dendrites. In this case, the iron particles act as substrates for the nucleation of primary austenite due to a similar crystallographic behaviour of the regular face centered cubic lattice The more numerous are the dendrites of primary austenite, the less free space is available in the interdendritic spaces for the formation of graphite eutectic grains, which makes the mechanical properties higher.

Abstract: In this study Mg10GdxNd alloys are investigated by nanoindentation hardness measurements in several material conditions. Mg10GdxNd alloys with an average coarse grain size of 500 µm were cast by permanent mold direct chill casting. Hardness values vary due to the inhomogeneous microstructure formed during the solidification process consisting of dendrite arms with preferred orientation direction. The effect of dissolving particles during solution heat treatment (T4) and isothermal ageing (T6) was observed to a different extent depending on Nd content. Isothermal ageing promotes a duplex microstructure of coarse β₁ phase precipitates and regions containing much finer precipitates. Post processing by direct extrusion changes the microstructure dramatically to an average grain size of 15 µm. The microstructure after hot extrusion shows segregation of precipitates in the extrusion direction. Near this alignment of second phases hardness and plastic deformation differ from precipitates enriched in RE elements due to depleted regions of solid solution around them. This phenomenon is known from alloying element segregation to grain boundaries. Depending on the amount and location of second phases in the as-cast microstructure and degree of cold work, recrystallization leads to an inhomogeneous microstructure, consisting of fine grains (15 µm) and very fine grains, where second phases act as nuclei during the recrystallization process. Furthermore, mechanical testing (fatigue) causes an increase in dislocation density by work hardening and extensive twinning near the fractured surface. Here the hardening effect interferes with grain size strengthening.

Abstract: A flexible and efficient process of tin electrorefining has been devised, using aqueous solutions of relatively low HCl and tin chloride concentrations. Any additives have been avoided, which enhances purity but also incurs the difficulty of obtaining compact cathode deposits. This feature is however assisted by the natural inhibition in complexing chloride solutions and the modulation of the current. Due to the rough dendritic crystal growth, special provisions are required to avoid short circuiting. Using short pulses in the 50-250 μs range of periodically reversed or interrupted currents, the lengths of the usually large dendrites can be moderated. The periodic current with extremely short cycle times and special electrode arrangement can be utilized for averting short circuits and for achieving better coverage of the cathode surface. This technique can be applied efficiently to obtain pure tin from soldering waste materials.