Key Engineering Materials Vol. 682

Abstract: An investigation of solidification process of as-cast ADC12 aluminum alloy in sand molds with different pouring conditions by temperature field was discussed in the present study. The ADC12 aluminum alloy was investigated in quarzt sand mold and chromite mold using sodium silicate binder with hardening by CO₂ gas under atmospheric and low vacuum conditions. Mechanical properties results indicated that a chromites mold under low vacuum condition has the optimal value. Thermal diffusivity was determined by finite difference method. The maximum thermal diffusivity was obtained in a chromite mold under low vacuum condition and correspondence to the minimum solidification time field.

Abstract: Rheo-continuous casting method is a combination of rheo- and continuous castings. In rheo-casting process the nucleation occurs on cooling slope with high rate in whole casting volume, so nuclei are numerous, resulting in very fine microstructure of nodular crystals. In this work the rheo-continuous process was carried out with a casting machine using 2 rollers of same size: diameter of 300 mm and width of 100 mm. The pouring temperature is near-liquidus. The microstructure obtained is fine (grain size < 40 μm), with nodular morphology. The mechanical properties of as-cast samples were high (the tensile strength is above 220 MPa).

Abstract: The studies regard analysis of variable Al and Fe additions on the chosen group of CuSn alloys. The group of tin bronzes known and applied for thousands of years has still found its application in many branches of industry, however it was observed that small amounts of aluminum and iron may affect the original properties strongly. The changes of mechanical properties and microstructure is discussed in detail. Aside of many advantageous properties of these bronzes, it is noted that the mentioned alloying additions have beneficial effect on the gas-induced shrinkage porosity. Simultaneously, the effect of the aluminum addition on the characteristic phase transformation points was determined basing on the analysis of calorimetric curves. The results are correlated with microstructure observations.

Abstract: Precious metals are widely used in a variety of different industry mainly for their inertness and their intrinsic value. This latter, is also the reason to pursue time and material saving technologies, reducing scraps and metal losses. Powder Metallurgy offers some interesting and effective alternative to traditional precious metals forming, aiming either to a performance improvement (in terms of cost, time, complexity) or to a cost reduction. The ever increasing availability of metal based alloys with advanced features, supplied in powder form, opens the doors to the use of alternative sintering techniques, borrowed from smelting sectors. Powder Metallurgy applied to precious metals processing makes it possible to create both unconventional shapes and so-far unexplored alloys, using techniques already introduced in other ferrous and non ferrous metals processing. In this paper Field Assisted Sintering Techniques (FAST), Metal Injection Moulding (MIM) and Additive Manufacturing of precious powders are examined as avant-garde processes to obtain near net shape metal parts.

Abstract: Mixed and preliminarily consolidated powders of aluminium and nickel (90 mass % Al and 10 mass % Ni) were hot extruded. As results the rod, 8 mm in diameter, was obtained. As-extruded material was subjected to the microstructural investigations using scanning electron microscopy (SEM/EDS) and X-ray analysis (XRD). The differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA) were also performed. The mechanical properties of as extruded material were determined by the tensile test and Vickers hardness measurements. In order to evaluate the thermal stability of PM alloy, samples were annealed at the temperature of 475 and 550 °C. After annealing Vickers hardness measurements and tensile tests were carried out. The plastic consolidation of powders during extrusion was found to be very effective, because no pores or voids were observed in the examined material. The detailed microstructural investigations and XRD analyses did not reveal the presence of the intermetallic phases in the as-extruded material. During annealing, the Al₃Ni intermetallic compound was formed as the result of chemical reaction between the alloy components. The hardness of the alloy after annealing at the temperature of 475°C was found to be comparable to the hardness in as-extruded state. Annealing of the material at the temperature of 550°C results in hardness decreasing by about 50%, as the consequence of porosity formation and Al₃Ni cracking.

Abstract: Attempts have been made to describe the influence of the process parameters, such as compaction pressure and sintering atmosphere on the microstructure and properties of PM Al4Cu alloy. Homogenous mixtures of Al4Cu elemental powders were achieved by tumbling powders for 30 minutes in the Turbula T2F mixer. The powders were subsequently cold compacted under pressures of: 200MPa, 300MPa and 400MPa in a rigid die on a single action press. The green compacts were sintered in two different atmosphere - nitrogen and vacuum at 600°C for one hour. After that, the samples sintered in nitrogen atmosphere were re-pressed and re-sintered (2p2s) under the same conditions. The green compact and as-sintered densities were measured using the geometric method. Additionally, the Brinell hardness and the bending strength in three point bend test were determined. The microstructure of the samples was also analyzed using both the light microscopy (LM) and scanning electron microscopy (SEM).The obtained results show, that optimal pressing pressure is 300MPa. Increasing pressure to 400 MPa has not a substantial effect on increase of the final sample density. Therefore applying higher compaction pressure (over 300MPa), from the economical point of view, is unnecessary.

Abstract: A brief review of structural investigation and results of mechanical tests for mechanically alloyed AlMg-based composite reinforced with 9 wt.% addition of CeO₂ is presented. The as extruded and annealed samples were examined by means of SEM, TEM and X-ray analysis. Heavily refined matrix grains and particles of cerium oxides were observed in the as extruded material. Fine microstructure attained by mechanical alloying and high affinity of oxygen to aluminum-magnesium matrix results in promoted solid state reactions between the matrix and reinforcements at elevated temperatures. Consequently, Al₄Ce intermetallic grains and Al/Mg oxides are formed in the result of CeO₂ decomposition. Hot compression tests were performed at constant true strain rate of 5·10^-3 s^-1 within the temperature range of 293 – 823K. Highly refined structure of the material was found to result in high strength of the composite, particularly for samples tested at low and intermediate temperatures.

Abstract: Samples made of iron powder with addition of 1.5 and 2% of molybdenum and 0-0.6% of boron were compacted at 600 MPa and sintered at 1200°C for 60 minutes in hydrogen atmosphere after mixing in Turbula mixer. The samples were deformed in a tensile test till rupture. The effect of molybdenum and boron on topography of fracture is discussed. It is noted that the sintering mechanism changes upon addition of boron particles into Fe-Mo alloy. The fractures of the studied samples were observed by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The addition of Mo influences the change of fracture to ductile type. The type of fracture is brittle with Mo and borides segregating to grain boundaries. In the alloys with low concentrations of molybdenum boron induces brittle transgranular fracture.

Abstract: Copper based Metal Matrix Composites are promising materials for electrical and electrotechnical applications such as electronic packaging and contacts, resistance welding electrodes, heat exchangers etc. Introducing the ceramics particles into the copper matrix allows to achieve a higher mechanical properties comparing to pure copper. The literature shows the variety of reinforcement materials are used. The most commonly strengthening phase include: oxides Al2O3,Y2O3, SiO2, carbides SiC, WC, TiC, ZrC, borides TiB2, ZrB2 and others such us volcanic tuff, carbon or intermetalic phases Al-Fe. [1-7]. It is obvious that reinforcement material without TiN leads to decrease the electrical conductivity of copper. Preliminary investigations concerning nanoscale Cu-based composites with TiN particles were presented in papers [10, 11]. Powder metallurgy (PM) process leads to obtain uniform distribution of strengthening phase in matrix. In order to achieve uniform distribution the process parameters such as mixing and selection the sizes of particles must be appropriate selected. The another factor of decreasing the mechanical and electrical properties by using PM route is porosity. Conventional PM process includes pressing and sintering does not always allow to achieve the high density what is one of the main criterion for high electrical conductivity material. The hard ceramic particles in metal matrix which are not deformable make difficult the densification process. In some cases the use of more advanced methods of production is desirable. The use of titanium nitride particles is justified by their high electrical conductivity in compare to the other reinforcement materials.

Abstract: The Cyclic Extrusion – Compression - reciprocating extrusion process (CEC) is one of severe plastic deformation methods (SPD), which allow to produce bulk nanomaterials without changing the initial shape of deformed samples. The results are presented showing that the average grains size and microbands thickness in aluminium alloys decrease below 100 nm. The investigations revealed that the average grains size is about 250 nm and 200 nm in polycrystalline and monocrystalline copper, respectively.The Cyclic Extrusion Compression method is also used to produce bulk materials by powder consolidation. The subgrains/nanograins inside the silver powder particles after the consolidation processes achieved the mean size of about 100 nm. Moreover, it has been found that inside structure observed by TEM, the consolidated powder granules consisted from nanometric twins of about 10 – 20 nm. This silver based powder consolidated by CEC method were extruded by hydrostatic extrusion method. The final product were the wires with a diameter of 3 mm, which were used to electrical contacts production.